Thesis Title: Gas phase studies of metal complexes, isomeric carbanions and distonic radical anions under soft ionization mass spectral conditions

Gas phase studies of metal complexes, isomeric
carbanions and distonic radical anions under
soft ionization mass spectral conditions

THESIS
SUBMITTED TO
OSMANIA UNIVERSITY
FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
IN CHEMISTRY

By

M. Kiran Kumar, M.Sc.

NATIONAL CENTRE FOR MASS SPECTROMETRY
INDIAN INSTITUTE OF CHEMICAL TECHNOLOGY
Hyderabad -500 007, INDIA
April, 2007

Dedicated To
My Beloved Parents
and wife

DECLARATION

The research work presented in this thesis entitled “Gas phase studies

of metal complexes, isomeric carbanions and distonic radical anions

under soft ionization mass spectral conditions” was carried out by me

independently in this institute under the supervision of Dr. M. Vairamani,

Scientist-in-Charge, National Centre for Mass Spectrometry, Indian Institute of

Chemical Technology, Hyderabad. This work is original and has not been

submitted in part or full, for any degree or diploma of this or any other

university.

Dt : (M. Kiran Kumar)
)
National Center for Mass Spectrometry
Indian Institute of Chemical Technology
Hyderabad, AP-500 007.

Dr. M. Vairamani National Centre for Mass Spectrometry
Scientist ‘G’, Head Indian Institute of Chemical Technology
Analytical Division Council of Scientific & Industrial Research
Hyderabad-500 007, A.P., India

CERTIFICATE

This is to certify that the research work incorporated in this thesis

entitled “Gas phase studies of metal complexes, isomeric carbanions and

distonic radical anions under soft ionization mass spectral conditions”

submitted by Mr. M. Kiran Kumar was carried out by the candidate under my

supervision. This work is original and has not been submitted for any other

research degree or diploma of this or any other university.

Dt : (Dr. M. Vairamani)

Tel : +91-40-27193482
Fax : +91-40-27193156
e-mail : vairamani@iict.res.in

ACKNOWLEDGEMENTS
I am very much thankful to my guide and supervisor Dr. M Vairamani, Head Analytical
Chemistry Division, National Centre for Mass Spectrometry (NCMS), for welcoming me into his
research group and providing me enough impetus to carry out my work independently. My heartfelt
gratitude to him for his guidance and valuable suggestions throughout my research
My stepping into the arena of research remains incomplete without mentioning about Prof. G.
L. David Krupadanam and Dr. D. Sitha Ram, whose inspiration and guidance encouraged me to step
into the world of Mass Spectrometry.
Special thanks go to Dr. G Narahari Shastry for introducing me to the theoretical chemistry
and also for his constant encouragement.
I express my heartfelt gratitude to Dr. S. Prabhakar, for his timely help and valuable
suggestions throughout the course of my work. I am very thankful to him as he listened to all my
problems with utmost patience and suggested me solutions in an appropriate manner. I cannot imagine
anybody like him taking better care of me and he has been a great source of inspiration for me.
My sincere thanks to Dr. R. Srinivas, Mr. L. K. Rao, Dr. N. S. Swamy, Mr. R. Narsimha,
Dr. N. P. Raju,, Dr. U. V. R. V. Saradhi, V. V. S. Lakshmi and M. R. V. S. Murty for their
cooperation and encouragement.
My special thanks to Dr. N. P. Raju for reading my thesis with patience.
I would also like to thank my inter and degree college classmates, Ravi,, Nagi Reddy and
Hari, who stood beside me all the way to keep me in the right path. It is my pleasure to thank all my
past and present collegues, Shama, Veni, Srikanth, Jagadeshwar Reddy, Bhaskar, Ramu, Murali,
Shivaleela, Ramesh and Sangeeta, for making my stay at NCMS a pleasant experience. I thank my
colleagues Srinivasa Rao, Sateesh Kumar and Nagaraju.from Molecular Modeling Division.
I am grateful to my entire family for their support and encouragement throughout my studies.
There are many, many people who have helped me along the way. My regrets to those whom I
have forgotten if any, but one can be assured that, his help has been greatly appreciated!
Lastly, I would like to thank CSIR, New Delhi for the financial support in the form of
Research Fellowship (JRF/SRF). I take this opportunity to thank Dr. J. S. Yadav Director, IICT,
and Dr. K. V. Raghavan, former director for providing the facilities to carryout my research work.

-Morishetti Kiran Kumar

Abbrevations

ACN : Acetonitrile

BSSE : Basis Set Super position Error

CID : Collision-Induced Dissociation

∆E : Binding energy difference

EI : Electron Ionization

EPR : Electron Paramagnetic Resonance

ESI : Electrospray Ionization

ESR : Electron Spin Resonance

FA : Flowing Afterglow

FAB : Fast Atom Bombardment

FTICR : Fourier transform ion-cyclotron resonance

FTMS : Fourier transform mass spectrometer

FWHM : Full Width at Half Maximum

HF : Hartree-Fock

LAMMA : Laser Microprobe Mass Analysis

MALDI : Matrix Assisted Laser Desorption Ionization

MO : Molecular Orbital

NMR : Nuclear Magnetic Resonance

Pc/Pd : Precursor/Product ratio

QITMS : Quadrupole ion trap mass spectrometers

SORI : Sustained off-resonance irradiation

Teff : Effective Temperature

Index
Contents Page No

CHAPTER 1
Gas phase ion Chemistry of CrIII(Salen) complex under electrospray
ionization conditions

1. Prologue 1
1.1. Brief introduction of ESIMS 5
1.2. Metal-salen complexes analysis by ESIMS 7
2. Scope of the work 12
3. Results and discussion 12
3.1. Source experiments 13
3.2. Ligand-pickup experiments 20
3.3. Collision induced dissociation (CID) experiments 25
4. Conclusions 28
5. Experimental 29
6. References 31

CHAPTER 2
Proton and alkali metal ion affinities of bidentate bases: spacer
chain length effects

Part 1: Proton and alkali metal ion affinities of α,ω-Diamines: Spacer
chain length effects

1. Prologue 35
1.1. The Kinetic method 40
3.1. Li+ ion affinity ladder construction 45
3.2. Na+ and K+ ion affinity ladder construction 48
3.3. Proton affinity ladder construction 51
3.4. Relative alkali metal ion binding energy calculations 52
3.5. Comparison between the proton and alkali metal ion affinity
53
orders
3.6. Theoretical studies 54
4. Conclusions 59

Part 2: Proton and alkali metal ion affinities of α,ω-Diols: Spacer chain
length effects

1. Prologue 61
3.1. Proton affinity ladder construction 63
3.2. Li+, Na+ and K+ ion affinity ladder construction 67
4. Conclusions 72
5. Experimental 73
6. References 74

CHAPTER 3
Generation of regiospecific carbanions under electrospray
ionization conditions and characterization by ion-molecule
reactions with carbon dioxide

Part 1: Generation of regiospecific carbanions from aromatic hydroxy
acids and dicarboxylic acids

1. Prologue 79
1.1. The generation of carbanions in the gas phase 80
1.1.1. Proton abstraction method 81
1.1.2. Fluorodesilylation method 82
1.1.3. Collision induced decarboxylation method 83
1.2. Characterization of carbanions 84
1.3. Stability studies of carbanions 86
1.4. Generation and characterization of specific carbanions 88
3.1. Geometrical isomers 99
3.2. Positional isomers 102
3.2.1. Aromatic dicarboxylic acids 102
3.2.2. Aromatic hydroxy acids 106
3.3. Effect of desolvation temperature 110
3.4. Theoretical calculations 112
4. Conclusions 118

Part 2: Generation of regiospecific carbanions from sulfobenzoic acids

1. Prologue 119

3.1. Isomeric sulfobenzoic acids 122
3.2. Isomeric benzenedisulfonic acids 127
4. Conclusions 130
5. Experimental 131
6. References 133

Chapter 4
Generation of distonic dehydrophenoxide radical anions under
electrospray and atmospheric pressure chemical ionization
conditions

Part 1: Generation of distonic dehydrophenoxide radical anions from
substituted phenols under Electrospray ionization conditions

1. Prologue 140
1.1. Formation of radical anions in the gas phase 141
1.1.1. Electron attachment 141
1.1.2. Electron transfer 142
1.1.3. Ion-molecule reactions 144
1.2. Distonic radical anions 145
1.3. Characterization of radical anions 148
3. Results and discussions 150
3.1. Isomeric nitrobenzoic acids 150
3.2. Isomeric hydroxytoluenes 154
3.3. Isomeric nitrophenols and hydroxy benzaldehydes 157
3.3. Ion-molecule reactions in the collision cell with CO2 160
4. Conclusions 163

Part 2: Generation of distonic dehydrophenoxide radical anions from
substituted nitrobenzenes under atmospheric pressure chemical
ionization mass spectral conditions

1. Prologue 164
1.1. Atmospheric pressure chemical ionization 165
3. Results and discussions 167
3.1. Isomeric nitrobenzaldehydes 168
3.2. Isomeric nitroacetophenones 172
4. Conclusions 176
5. Experimental 177
6. References 179
ℵ Abstract

GAS PHASE ION CHEMISTRY OF CrIII(SALEN) COMPLEX UNDER

ELECTROSPRAY IONIZATION CONDITIONS

L

N N
III
Cr
O O

L

Chapter 1 Chemistry of CrIII-Salen complex…

CHAPTER 1
CHAPTER 1

Gas phase ion Chemistry of CrIII(Salen) complex under electrospray

ionization conditions

1. PROLOGUE

I
t is important to characterize the metal complexes and to identify the crucial

intermediates in metal-mediated reactions in order to understand the nature

and reactivity of metal complexes and their reaction pathways.1-4 Variety of

techniques based on X-ray diffraction, infrared spectra, nuclear magnetic

resonance (NMR), and electron paramagnetic resonance (EPR) have been used

to gather coordination structure information.5 For example, the use of NMR is

limited for characterization of metal complexes that contain a paramagnetic

metal atom; this technique is less applicable if metal complexes are present at

low concentrations or as complex mixtures.5 Consequently, researchers have

chosen the advantage of using mass spectrometry (MS) as a technique of

choice to gather coordination structure information of metal complexes. The

study of metal complex systems using MS (i.e., in the gas phase) is a rapidly

expanding field of research.1,2 As the mass spectrometer is operated in either

of the positive or negative ion mode, metal complexes can readily be isolated

and studied without interferences from counter ions, solvent or additional

complexes those are usually present in solution.1,2 These experimental

1


conditions are ideally suited for studying the intrinsic properties and reactivity

of various chemical entities may be clearly unrevealed.

Knowledge of the gas-phase structures of metal complexes is important

for analytical applications, as evidenced by several reviews.1,2,6,7 Recent mass

spectrometric experiments have drawn direct correlations to metal complex

mediated catalytic processes involved in various reactions.2,8-13 Mass

spectrometric investigations benefit from the ability to evaluate catalytically

active species in the gas phase that are too transient to study in solution. The

species with short lifetimes in solution do not pose a problem in the high-

vacuum environment of a mass spectrometer.

Complexes of N,N-bis(salicylidene)ethylenediamine, commonly known

as H2Salen (Figure 1), belong to a fundamental class of compounds in

coordination chemistry, known since 1933.15 This compound also belongs to the

class of Schiff base ligands, because of the preparation of this compound is by

the condensation of salicylaldehyde and ethylene diamine. Schiff base ligands

are able to coordinate metals through imine nitrogen and also the through the

hydroxyl oxygen in the case of Salen complexes. In fact, Schiff bases are able

to stabilize many different metals in various oxidation states, controlling the

performance of metals in a large variety of useful catalytic transformations. The

Salen type complexes have been extensively studied and more than 2500

complexes have been synthesized.16 Interest in Salen type complexes

intensified in 1990 when the groups of Jacobsen17 and Katsuki18 discovered the

2


enantioselective epoxidation of unfunctionalised alkenes using chiral

MnIII(Salen) complexes as catalysts (Scheme 1).

N N

OH HO

Figure 1: H2Salen.

III + O
[M (Salen)]
+ PhIO + PhI

Scheme 1: Epoxidation of olefins with Metal(Salen) complex.

Since that time, an extremely wide variety of reactions catalyzed by

Salen complexes have been investigated. These include oxidation of

hydrocarbons,19 aziridination of alkenes,20 Diels Alder reaction,21 hydrolytic

kinetic resolution of epoxides,22 alkylation of aldehydes23 and oxidation of

sulfides to sulfoxides.24 Different mass spectrometry techniques have been

used to characterize the Metal-Salen complexes in the gas phase.8,25-28 In

general, application of the traditional method of ionization i.e. electron

ionization (EI), the earlier ionization method of MS, was limited to some metal

complexes, because most of the metal complexes are non-volatile and

thermally labile.25,27 However, there are few reports on the EI studies on a few

metal Salen complexes.25 The reported complexes include Co, Ni and Cu Salen

complexes. The EI spectra of these complexes showed abundant molecular

3


ions and fragment ions. Rohly et al.27 compared the EI mass spectra of metal

Salen complexes with the laser microprobe mass analysis (LAMMA) spectra,

wherein they report positive ion LAMMA spectra failed to provide the

information that is obtained in EI and negative ion LAMMA spectra were

dominated with only the carbon cluster ions.

The soft ionization methods such as chemical ionization, field desorption,

plasma desorption, secondary ion and fast atom bombardment (FAB) have

been developed to overcome the drawbacks encountered in EIMS towards the

analysis of metal-complexes. The FAB ionization technique has been extended

to new areas of inorganic and organometallic chemistry.28-30 Zhao at al.28

analyzed metal Salen complexes by using positive ion FAB technique. They

found that among many solvents tried to dissolve the complexes, the use of

trifluoro acetic acid (TFA) was crucial for producing good FAB spectra. Though,

much of the researchers used the FAB technique to analyze various metal

complexes, still there are some problems, like complications arising from

recombination of fragments, or interactions with the matrix used.2

Recent developments of ionization methods like matrix assisted laser

desorption ionization (MALDI)31-34 and electrospray ionization (ESI),1,7-14,35-37

have also been applied for the characterization of the metal complexes. MALDI

technique, though often used for characterization of high molecular weight

compounds, is relatively not explored much in characterization of metal

complexes. The selection of a suitable matrix is crucial in MALDI experiments,

and the ligand exchange reactions by MALDI matrix are known to complicate

4


the spectra. Relatively more studies are available for the analysis of metal

complexes using the ESI technique. The major impact of ESIMS to date is that it

can be used in identification of metal complexes, because it has allowed

observation of mass spectra for low as well as high molecular weight

compounds of ionic and nonvolatile, such as salts. In addition to the ionization

of the analytes, ESI process also transfers pre-existing ions in solution, if any, to

the gas phase, and hence is ideal for inorganic and organometallic compounds.

Further, ESI has proven to be a soft ionization method that keeps intact any

weakly bound ligands in a complex ion.36,37 Moreover, the ionization

techniques like ESI need very small amounts of sample to generate reasonably

good spectra. Use of low level quantities of samples for ESI enables the

technique for the analysis of environmental or biological samples, where the

samples are precious. With these advantages, ESI has become increasingly

popular as an analytical tool in inorganic/organometallic chemistry. This

technique, in combination with tandem mass spectrometry (MS/MS), has been

employed to study mechanistic pathways of reactions.1,7-14,35-37

1.1. BRIEF INTRODUCTION OF ESIMS

ESI technique involves spraying of a solution of the sample through a

electrically charged needle the so-called capillary which is at atmospheric

pressure (Figure 2). The spraying process can be streamlined by using a

nebulizing gas. The charged droplets are produced where the positive or

negative ions are solvated with solvent molecules. Hot gas or a dry gas, usually

called as desolvation gas, is applied to the charged droplets to cause solvent

5


evaporation. The desolvation process decreases the droplet size, leads to the

columbic repulsion between the like charges present in the droplet and further

the droplet fission leads to the formation of individual gas phase analyte ions.

The charged ions are then focused into the mass analyzer.

Figure 2: Schematic diagram of a typical ESIMS source phenomenon
operating in positive mode. Solvent evaporation of the charged
droplet generated in the source can be clearly seen (courtesy
reference Gaskell SJ, J. of Mass Spectrom., 1997)38

Application of an electrostatic field in the region between the capillary

exit and cone causes collisional activation of the solvated analyte ions. The

electrostatic field can be easily varied and provides control over the amount of

6


collisional activation. At low levels of cone voltage, the generated ions can be

sampled without causing any fragmentation. At higher levels of cone voltages,

the generated ions can be induced to undergo dissociation to give structurally

informative fragment ions. Such fragmentation in the source is called ‘cone-

voltage fragmentation’ or ‘source fragmentation’.

1.2. METAL-SALEN COMPLEXES ANALYSIS BY ESIMS

It is well known that Mn- and Cr-Salen complexes catalyze the oxidation

of organic substrates through the formation of a high-valent metal-oxo species,

(Salen)M=O. Kochi et al. used metal-Salen complexes as versatile epoxidation

catalysts in the 1980s.39-41 A typical mechanism is shown in Scheme 2.

N N N O N
III V
M M
O O
+PhIO O O
-PhI
[MIII(Salen)]+ [O=MV(salen)]+

O
V + III +
[O=M (salen)] + + [M (salen)]

Scheme 2: The mechanism for olefin epoxidation with Metal(salen) complex.

Epoxidation of various alkenes was successfully carried out with

iodosylbenzene in the presence of catalytic amounts of CrIII(Salen), and the

epoxidation reaction failed in the absence of the chromium complex. They

successfully isolated the catalytically active oxo-chromium(V) (O=CrV)

complexes in the condensed phase by careful recrystalization and

characterized by X-Ray and ESR studies.39 The successful isolation and

7


characterization of O=CrV(Salen) revealed the basis for oxygen activation in

the O=Cr(V) functionality. Kochi et al.39 further developed the use of

MnIII(Salen) complexes as much more versatile oxidation catalysts. An

enatioselective version of the reaction was developed later by Jacobsen et al.17

and Katsuki et al.18 by using chiral MnIII(Salen) complexes (Schemes 1 and 2).

However, the mechanistic studies on the MnIII(Salen) systems were hampered

by the fact that the catalytically active oxomanganese (O=MnV) species appear

only as short-lived putative intermediates.40 At that time, it was suggested that

the concentration of MnV-oxo complex was regulated by an equilibrium

involving µ-oxo-manganese (V) as depicted in Scheme 2. However, the

reactive species were neither isolated nor characterized in the condensed

phase. Plattner et al. successfully applied ESI technique to give a direct proof

for the epoxidation reactions using MnIII(Salen) complexes.4 They used ESI

method in combination with tandem mass spectrometry to study the

mechanistic pathway for oxygen transfer to organic substrates in the gas phase.

The [MnIII(Salen)]+ salts with iodosylbenzene were electrosprayed and the

resulted ESI spectrum showed two oxidized species, i.e. [(Salen)Mn=O]+ at m/z

337, [PhIO(Salen)Mn–O–Mn(Salen)OIPh]2+ at m/z 549.4,11 The collision-induced

dissociation (CID) of the ion at m/z 549 resulted in the decomposition products

of MnIII and MnV-oxo derivatives [Scheme 3]. These findings represented the

first experimental evidence for the formation (conproportionation) and

decomposition (disproportionation) of a µ-oxo bridged MnIV dimer acting as

reservoir of the catalytically active species involved in the oxidation reaction.12

8


[PhIO(Salen)Mn-O-Mn(Salen)OIPh]2+
m/z 549
[PhIO(Salen)MnIII]++ [PhIO(Salen)MnV=O]+
m/z 541 m/z 557

Scheme 3

The capability of MnV-oxo species to transfer oxygen to suitable organic

substrates in the gas phase was also demonstrated by collision experiments.

When [(Salen)MnV=O]+ ions were mass selected and submitted to CID

experiments with either Ar or Xe as collision gases, no fragmentation could be

obtained, [Scheme 4]. However, if the inert gas was replaced with oxygen

acceptors like sulfides or electron-rich olefins, formation of [(Salen)MnIII]+ ions,

that is, the reduction product of the oxidation reaction, was detected, [Scheme

4].

Ar/Xe
[(Salen)MnV=O]+ gas
[(Salen)MnV=O]+ (1)
m/z 337 m/z 337

O

[(Salen)MnV=O]+ [(Salen)MnIII]+ + (2)
m/z 337 m/z 321

Scheme 4: Collision cell experiments for [(Salen)MnIII]+ ions with (1) Ar/Xe

gas (2) electron rich olefin gas.

Further, Plattner et al.12 evaluated the effects of various substituents in

the 5- and 5'-positions of the Salen and found that the electron-withdrawing

substituents enhance the reactivity of the Mn=O moiety. The importance of the

axial positions in MnIII(Salen) complexes was also demonstrated by the

9


enhancement of epoxidation reaction yields with addition of a donor ligand that

stabilizes the oxometal-Salen complex. Further, the significance of axial ligands

was demonstrated by their studies on the coordination chemistry of MnIII(Salen)

and oxo MnV(Salen) complexes by applying ion-molecule reactions in the

collision cell (ligand-pick up experiments).10 Thus, the role of axial ligation on

geometry and reactivity of the high-valent oxo complex appeared to be quite

drastic.

Study of solvent clusters with ionic species in the gas phase provides

basic insights into the chemical reactivity and dynamics of ions in the

condensed phase. Such studies also provide a wealth of information on

interaction between singly charged metal ions and small ligands such as water,

methanol, acetonitrile etc. Beauchamp et al.37 studied the evaporation kinetics

on hydrated Cr and Mn Salen complexes by using ESI technique in a “soft

sampling” mode. In this study, they observed that the kinetics of water

evaporation from solvated Salen complexes is highly dependent on the central

metal ion. The clusters of CrIII(Salen) ions with two water molecules attached

exhibit special stability, indicated by their prominence in the overall cluster

distribution. These results were in accord with the solution phase chemistry

and with the ligand field theory.

Madusudanan et al.36 studied the axial interactions of CrIII(Salprn), where

Salprn = N,N-bis(salicylidene)propanediamine complexes with nucleotides

and nucleosides using ESI-MS. The nucleosides formed 1:1 and 2:1 adducts

with [CrIII(Salprn)]+ and dinucleotides formed only the 1:1 adducts. The CID of

10


these adducts revealed the attachment of Cr+ ion to the bases in nucleosides

and to both the phosphate and base in nucleotides.

It is well known that the complexes of transition metal ions are known to

undergo redox reactions during the ESI process (Scheme 4).42-47 Within the

transition metal ions, only copper ion is shown as an oxidant in several

examples for peptides and amino acids.42-46 O’Hair et al.47 studied the redox

processes in various metal ions other than copper by taking the advantage of

vacant axial positions of the metal(Salen) complexes (metal = Cr, Mn, Fe and

Co). In this process they have generated singly charged metal(Salen) ternary

complexes with hexapeptides under ESI conditions. The CID experiments on

these ternary complexes produced peptide radical cations (P+.) by redox

process (Scheme 5). The authors suggested that the redox process occur

either by a homolytic cleavage or by a heterolytic process followed by

subsequent electron transfer. In the fragmentation reactions of ternary

complexes, produced P+. were found to be highly dependent on the metal ion

used. The redox pathway was favored with FeIII or MnIII complexes when Salen

ligand contained an electron withdrawing group. The resulting peptide radical

cations are odd electron species of nonvolatile precursors, and are not

typically available under ESI or MALDI processes.

[MII(Salen)] + P+.

[MIII(Salen)(P)]+ Redox
process
[MIII(Salen)]+ + P

Scheme 5: Redox process of [MIII(Salen)(P)]+ in the CID experiment.

11


2. SCOPE OF THE WORK

Since all of the metal-Salen complexes generally are used as a catalyst in

the reactions, which specifically give enantioselective products, here, in this

study we have selected [CrIII(Salen)]PF6 complex for gas phase chiral

discrimination of enantiomeric compounds. It is well known that, CrIII(Salen)

complex have two free axial positions, hence, we started to make use of these

positions towards chiral recognition by using the kinetic method.

Unfortunately, we are unsuccessful in achieving the chiral discrimination with

R- and S- phenyl ethyl amines and napthyl ethyl amines in the gas phase. In this

experiment, we found that the affinity of these amines towards the axial

positions of metal-Salen complexes is fair, hence, we attempted to check the

interaction among the mono and bidentate ligands. To the best of our

knowledge, detailed studies on the behavior of [CrIII(Salen)] complexes at its

axial positions and coordination chemistry in the gas phase are not available in

the literature. The use of CoIII-Schiff base complexes with two amines in the

axial positions as antimicrobial agents was reported earlier. Therefore we

employed in our work the ESI method in combination with tandem mass

spectrometry to study the coordination chemistry of axial positions on the

unsubstituted [CrIII(Salen)] complex with amines and diamines.

3. RESULTS AND DISCUSSION

In the process of analysis of [CrIII(Salen)]PF6 complex in acetonitrile

(ACN) under ESI conditions with mono and diamines; we applied different

12


types of experiments, i.e. i) Source, ii) Ligand pick-up, and iii) CID

experiments.

3.1. SOURCE EXPERIMENTS

The positive ion ESI mass spectrum of [CrIII(Salen)]+ complex in ACN

shows major ions at m/z 318, 359 and 400, corresponding to [CrIII(Salen)]+,

[CrIII(Salen)(ACN)]+ and [CrIII(Salen)(ACN)2]+ ions, respectively (Figure 3).

Such attachment of solvent molecules to a central metal atom in the ESI process

is well documented.5,48 However, it was found that the relative abundances of

these ions are very much dependent on experimental conditions, especially

the cone voltage. Hence, we recorded the spectra at cone voltages of 10, 20

and 30 V (Figure 3(a-c)) to help understand the effect of solvent co-ordination

in the gas-phase. The spectrum recorded at a cone voltage of 10 V showed

mainly two peaks at m/z 359 and 400, corresponding [CrIII(Salen)(ACN)]+ and

[CrIII(Salen)(ACN)2]+, respectively, in addition to a minor peak at m/z 318 that

corresponds to [CrIII(Salen)]+ ion. The [CrIII(Salen)(ACN)2]+ ion was found to be

the base peak in the spectrum under these conditions. It is interesting to note

that the [CrIII(Salen)]+ did not pick up more than two acetonitrile molecules. If

the ions at m/z 359 and 400 had resulted from simple solvation of the

[CrIII(Salen)]+ complex ion by acetonitrile, one would have expected a series of

ions corresponding to [CrIII(Salen)(ACN)n]+ where n = 1,2,3 etc. The absence of

such higher adducts (n > 2) indicates that the [CrIII(Salen)]+ complex is able to

accept only two acetonitrile molecules, and that the central metal ion can adopt

a maximum of six as co-ordination state in the gas-phase.

13


H2(Salen) is a tetra-dentate ligand that occupy four coordination sites of

the central metal ion through the N, N', O, O' atoms. The ions observed at m/z

359 and 400 represent the occupation of the axial positions of the CrIII(Salen)

complex by one and two acetonitrile molecules, respectively, proving the

capability of the [CrIII(Salen)]+ complex to form five- or six-coordinated species

in the gas-phase. Similar behavior has been reported for the [CrIII(Salprn)]+

complex under ESI conditions.36 It is also in good agreement with reported

crystallographic studies in which [MnIII(Salen)] complexes were shown to bind

with one or two solvent molecules such as acetone, ethanol etc., that were used

for recrystallization, usually in axial positions.49

The ESI spectrum recorded at a cone voltage of 20 V showed the ion at

m/z 318 as the base peak with the acetonitrile adducts present at reasonable

abundance. However, the spectrum obtained at cone voltage of 30 V contains

mainly the ions at m/z 318 (base peak) and 359, and the ion at m/z 400 is

absent. This demonstrates that, at higher energies (i.e. at high cone voltages, ≥

30V), the solvent molecules are dissociated from the complex to leave the

[CrIII(Salen)]+ ion. This result prompted us to study the coordination chemistry

of the [CrIII(Salen)]+ complex with mono- and bi-dentate ligands (amines and

diamines) in detail.

14


Figure 3: The ESI mass spectra of CrIII(Salen) at different cone voltages. a)

10, b) 20, c) 30V.

The ESI mass spectrum of the [CrIII(Salen)]+ complex in the presence of

propylamine (Pr-NH2) clearly demonstrates that the displacement of solvent

molecules present in the axial positions by the stronger ligand. At low cone

voltage (10V) the [CrIII(Salen)(Pr-NH2)2]+ ion at m/z 436 is dominant, that of the

[CrIII(Salen)(Pr-NH2)(ACN)]+ ion at m/z 418 is of significant abundance, and the

[CrIII(Salen)]+ ion is of negligible importance (Figure 4a).

15


[CrIII(Salen)(Pr-NH2)(ACN)2]+

Figure 4: The positive ion ESI mass spectra of [CrIII(Salen)]PF6 complex in the

presence of propylamine (Pr-NH2) at the cone voltage of a) 10 eV b) 20 eV and

c) 30 eV.

The ion at m/z 418 is dominant over the ion at m/z 436 in the spectrum

recorded at a cone voltage 20V (Figure 4b). It may be due to decomposition of

a fraction of [CrIII(Salen)(Pr-NH2)2]+ to [CrIII(Salen)(Pr-NH2)]+ that immediately

picks up one molecule of acetonitrile, the surrounding solvent molecule, to

result in a stable six-coordinated [CrIII(Salen)(Pr-NH2)(ACN)]+ ion (m/z 418).

The ion at m/z 418 is stable even at high cone voltage (30V) but abundant

16


[CrIII(Salen)]+ ions and its acetonitrile adduct ions at m/z 359 and 400 also are

observed in the spectrum as the result of the fragmentation of the

[CrIII(Salen)(Pr-NH2)2]+ ions (Figure 4c) (the abundance of the [CrIII(Salen)(Pr-

NH2)2]+ ion is considerably reduced at high cone voltages ≥30 V). Thus, the

relative abundances of the ions at m/z 318, 359 and 400 in this experiment can

be used as a measure of the stability of the [CrIII(Salen)(Pr-NH2)2]+ ion. These

experiments reveal the ability of Pr-NH2 to occupy both the axial positions of

the complex to form six-coordinate complex ions that survive at low cone

voltage. This prompted us to study the coordination of diamines which are

bidentate in nature and can occupy both axial positions of the [CrIII(Salen]+

complex. We carried out three types of experiments, i.e. source experiments,

ligand pick-up experiments and CID experiments, for this purpose.

A series of primary α,ω-diamines (DA) were selected to study not only

the effect of the bidentate nature of diamines but also the effect of chain length

of the ligand on the occupation of the axial positions of [CrIII(Salen)]+. We used

1,2-diaminoethane (1), 1,3-diaminopropane (2), 1,4-diaminobutane (3), 1,5-

diaminopentane (4), 1,6-diaminohexane (5), 1,7-diaminoheptane (6) and 1,8-

diaminooctane (7) as bidentate ligands. The ESI mass spectra of an equimolar

(100 µM) mixture of [CrIII(Salen)]+ and diamines were recorded at cone

voltages of 10 and 30 V. All the spectra recorded at 10V show mainly the

[CrIII(Salen)(DA)]+ ions, and other ions are negligible. However, the spectra

recorded at 30V still showed the [CrIII(Salen)(DA)]+ ion as the base peak but, in

addition, [CrIII(Salen)]+, [CrIII(Salen)(ACN)]+ and [CrIII(Salen)(ACN)2]+ ions are

17


also present in significant abundances. This indicates that the

[CrIII(Salen)(DA)]+ ion is stable at high cone voltage (30V), unlike

[CrIII(Salen)(ACN)2]+ and [CrIII(Salen)(Pr-NH2)2]+. Though the

[CrIII(Salen)(DA)]+ ion is dominant, the appearance of the other low mass ions

in the spectra recorded at cone voltage 30 V but not at 10 V implies partial

decomposition of [CrIII(Salen)(DA)]+ ion to give unbound [CrIII(Salen)]+ ion

(m/z 318) that picks up surrounding solvent molecules in the API interface

region to yield the ions at m/z 359 and 400 (Figure 4).

+
H2N
(CH2)n
N N
ESI N N
CrIII + H2N-(CH2)n-NH2 III
Cr
O O
PF6 O O
N
H2

Scheme 6: Reaction of [CrIII(Salen)]+ with a diamines (n=2-8, 1-7) to form a
bidentate complex.

The relative abundances of these ions were found to vary depending on

the size of the ligand used. Hence, we consider the spectra recorded at a cone

voltage of 30V for further discussion, as the relative abundances of the ions at

m/z 318, 359 and 400 are found to reflect the stability of the diamine co-

ordination complexes formed with [CrIII(Salen)]+. The ESI mass spectra of

[CrIII(Salen)] in the presence of the chosen diamines are listed in Table 1. It is

clear that ligands 1 and 2 are able to form the [CrIII(Salen)(DA)]+ ion with ease,

and the other ions due to loss of DA at m/z 318, 359 and 400 are less abundant

(<8%). [CrIII(Salen)(DA)2]+ and [CrIII(Salen)(DA)(ACN)]+ ions are absent,

18


consistent with the effective occupation of the two empty axial positions of

[CrIII(Salen)]+ by the two amino groups in the case of ligands 1 and 2 (Scheme

6).

Relative abundance (%)
Ion
1 2 3 4 5 6 7
[CrIII(Salen)]+
2.9 4.3 10 19 44 34 40
m/z 318
[CrIII(Salen)(ACN)]+
5.8 7.9 20 37 65 61 66
m/z 359
[CrIII(Salen)(ACN)2]+
4.4 7.1 18 35 56 54 48
m/z 400
[CrIII(Salen)(DA)]+ 100 100 100 100 100 100 100
III +
[Cr (Salen)(DA)(ACN)] - - 2.9 5.1 5.1 0.7 0.3

[CrIII(Salen)(DA)2]+ - - 1.5 4.4 5.1 8.7 13

Table 1: Positive ion ESI mass spectra (cone voltage 30 V) of mixtures of
[CrIII(Salen)]+ (as the PF6- salt) with diamines (DA) ligands (1-7) in
acetonitrile (ACN) solvent.

In the case of higher diamine homologues (3-6), the [CrIII(Salen)(DA)2]+

and [CrIII(Salen)(DA)(ACN)]+ ions are also present, reflecting the decreasing

ability of the higher homologues to yield stable bidentate complexes. It is

interesting to note that the relative abundances of the ions m/z 318, 359 and 400

gradually increase from ligand 1 to 5. In the case of 6 and 7, the relative

abundances of these ions are lower than those found for ligand 5. The gradual

increase of the abundances of the fragment ions at m/z 318, 359 and 400 from

ligand 1 to 5 reflects a gradual decrease in the stability of [CrIII(Salen)(DA)]+

ion from 1 to 5 under the ESI conditions used here. The stability of

19


[CrIII(Salen)(DA)]+ ion for ligands 6 and 7 seems to be higher than that of ligand

5 and lower than those from 1-4. There is a marginal but consistent increase in

the relative abundances of [CrIII(Salen)(DA)(ACN)]+ and [CrIII(Salen)(DA)2]+

ions from 3 to 5, reflecting that the ability of the diamine to occupy two axial

positions decreases from 1 to 5. In the case of 6 and 7, the abundance of the

[CrIII(Salen)(DA)(ACN)]+ ion is negligible and that of the [CrIII(Salen)(DA)2]+ ion

is higher than for the other ligands used. From these observations it can be

inferred that the bidentate nature decreases as the chain length increases from

1 to 5, while that of ligands 6 and 7 shows mixed behavior. In order to

understand the stability of these ionic species in the gas phase, we performed

ligand-pickup experiments in the collision cell.

3.2. LIGAND-PICKUP EXPERIMENTS

In these experiments the ions of interest were selected using MS1 and

allowed to undergo ion-molecule reactions in the collision cell with the ligand

of interest introduced into the collision cell. The resultant product ions were

then analyzed by MS2 (Figure 5). Experiments done on [CrIII(Salen)]+ as the

mass-selected precursor ion using acetonitrile as the collision gas showed the

pickup of one and two acetonitrile molecules by [CrIII(Salen)]+ to form five- and

six-coordinated species (ions at m/z 359 and 400, respectively, Figure 6). The

same behavior was observed when [CrIII(Salen)(ACN)]+ and [CrIII(Salen)

(ACN)2]+ are selected as precursor ions; the former ion showed addition of one

acetonitrile and the latter ion did not undergo any further addition of

acetonitrile (Figure 6).

20


Figure 5: Schematic diagram of a triple quadrupole instrument.

Figure 6: The spectra obtained from ligand-pickup experiments using
acetonitrile as the collision gas for precursor ions:
a) [CrIII(Salen)]+ ion, m/z 318
b) [CrIII(Salen)(ACN)]+ ion, m/z 359 and
c) [CrIII(Salen) (ACN)2]+ ion, m/z 400.

21


The displacement of weaker ligands (ACN) in the axial positions of

[CrIII(Salen)]+ by relatively stronger ligands (amines) was also observed in the

collision cell experiments using propylamine as collision gas. The propylamine

formed abundant [CrIII(Salen)(Pr-NH2)]+ and [CrIII(Salen)(Pr-NH2)2]+ ions with

[CrIII(Salen)]+ as the precursor ion; the same product ions are also observed

when [CrIII(Salen)(ACN)]+ and [CrIII(Salen)(ACN)2]+ are selected as precursors

(Figure 7). These experiments clearly indicate that the empty axial positions of

unsubstituted [CrIII(Salen)]+ ion are easily occupied by any ligand, and that the

displacement of weaker ligands by relatively stronger ligands occurs when a

complex with weaker ligands is selected as precursor ion. Similar ligand-

pickup experiments were reported previously by Plattner et al.10 for MnIII

(Salen) species; they showed that MnIII is able to form only five-coordinated

species unless there is an electron-deficient substituent on Salen. However, in

the present case, [CrIII (Salen)]+ is able to form six-coordinate complexes

easily, possibly due to differences in the electronic configurations of MnIII and

CrIII ions. Note that it is difficult to achieve equilibrium between the collision

(reactant) vapor and the selected ion species in the collision cell under the

mass spectral conditions used, as the experimental time window for the ion in

the collision cell is about 10-100 milliseconds. However, the results indicate

that it is possible to study the relative efficiencies of ligand exchange in the

collision cell.

22


propylamine as the collision gas for precursor ions:
a) [CrIII(Salen)]+ ion, m/z 318
b) [CrIII(Salen)(ACN)]+ ion, m/z 359 and
c) [CrIII(Salen) (ACN)2]+ ion, m/z 400.

We extended the ligand-pickup experiments to study of the bidentate

nature of diamines and the stability of diamine complexes, using acetonitrile in

the collision cell. From the ESI source mass spectra (Table 1) it is evident that

the five-coordinated complex [CrIII(Salen)(L)]+ (L = acetonitrile or

propylamine) exists in the gas-phase in addition to the stable six-coordinated

23


species. The observation of [CrIII(Salen)(DA)]+ ion in the ESI mass spectrum of

the mixture of [CrIII(Salen)]+ and diamine poses the question whether or not

both axial positions are occupied by the two amino groups of the diamine

ligand. When we selected a complex ion containing a monodentate ligand (Pr-

NH2) for ligand-pickup experiments, for example [CrIII(Salen)(Pr-NH2)]+, it

picked up one acetonitrile molecule in the collision cell to yield [CrIII(Salen)(Pr-

NH2)(ACN)]+ (Figure 8). This observation is as expected because one axial

position in [CrIII(Salen)(Pr-NH2)]+ is free and acetonitrile can readily occupy the

vacant axial position. When we selected [CrIII(Salen)(DA)]+ ion for ligands 1-4

in MS1 for ligand-pickup experiments with acetonitrile in the collision cell, no

addition of acetonitrile to the selected species was observed. The same

experiments for [CrIII(Salen)(DA)]+ ions from 5-7 resulted in

[CrIII(Salen)(DA)(ACN)]+ ions of low abundance (5.1, 1.7 and 0.5% for 5, 6 and

7, respectively); the spectrum for the case of ligand 5 is shown in Figure 8b as

an example. Similar results were obtained from ligand-pickup experiments

between [CrIII(Salen)(DA)]+ ions of 1-7 and propylamine in the collision cell.

From these experiments it can be concluded that [CrIII(Salen)(DA)]+ ions from

ligands 1-4 are stable in the collision cell under the present experimental

conditions. In the case of ligands 5-7, one of the axial coordinate bonds

between the central metal ion and the amino group of the ligand becomes

weaker, so that the DA can be displaced by acetonitrile in the collision cell.

Further, these experiments confirm that the [CrIII(Salen)(DA)]+ species is

24


relatively less stable for ligand 5 than for 6 and 7. These observations are

consistent with the results obtained from the ESI mass spectra (source).

acetonitrile as the collision gas for precursor ions:
a) [CrIII(Salen)(Pr-NH2)]+ ion, m/z 377 and
b) [CrIII(Salen)(hexd)]+ ion, m/z 434.
3.3. COLLISION INDUCED DISSOCIATION (CID) EXPERIMENTS

With a view to study the stability of diamine complexes with

[CrIII(Salen)]+, we also performed CID experiments on [CrIII(Salen)(DA)]+ ions

using argon as the collision gas at different collision energies (10, 12 and 14

25


eV). All the spectra resulted in only one product ion corresponding to

[CrIII(Salen)]+, and the relative abundance of this ion was found to depend on

the nature of the diamine used. It is demonstrated in the literature that the

precursor/product (Pc/Pd) abundance ratio can be used to measure the

relative stability of adduct ions, since a stable precursor ion undergoes less

decomposition.50 The Pc/Pd ratios, i.e. relative abundance ratio

[CrIII(Salen)(DA)]+/[CrIII(Salen)]+, obtained from the CID spectra of

[CrIII(Salen)(DA)]+ ions from 1-7 at different collision energies, are presented in

graphical form in Figure 9.

Figure 9: The plot of Pc/ Pd ratios ([Cr(III)(Salen)(DA)]+/ [Cr(III)(Salen)]+)
obtained at collision energies of 10, 12 and 14 eV from CID of
[Cr(III)(Salen)(DA)]+ ions for ligands (Diamines) 1-7.

The order of stabilities of [CrIII(Salen)(DA)]+ complexes for diamines 1-7

can be given as 2> 1> 3> 4 ≈ 7> 6> 5. This agrees well with the similar stability

order obtained from source experiments (mass spectra) already discussed,

26


except for ligands 1 and 2. The collision cell experiments show that ligand 2

forms a more stable complex with [CrIII(Salen)]+ compared to ligand 1.

In the source experiments the behavior of ligands 1 and 2 is reversed

but the difference is marginal. Hence, it can be concluded from both source

and collision cell experiments that the feasibility of complexation of diamines

with unsubstituted [CrIII(Salen)]+, by occupying the axial positions, decreases

as the chain length increases from ligand 1 to 5. We cannot offer an explanation

from the available experimental data for the marginal increase in the stability

of the complexes with 1 and 2 and similarly with ligands 6 and 7.

27


4. CONCLUSIONS

The positive ion ESI mass spectra for [CrIII(Salen)]+ complex in the

presence of amines as ligands (propylamine and a series of diamines (1-7))

were studied with a view to understand the coordination chemistry of the

complex in the gas phase. The ESI mass spectra of [CrIII(Salen)]+, either in

acetonitrile alone or in the presence of propylamine, showed ions

corresponding to five- and six-coordinated species, respectively. The

[CrIII(Salen)]+ in the presence of bidentate ligands (L = diamines) mainly

resulted in [CrIII(Salen)(L)]+ ions in which the two empty axial positions in

[CrIII(Salen)]+ species are occupied by the two amino groups of the diamine. In

addition to five- and six-coordinated complex ions, other ions corresponding to

[CrIII(Salen)]+ and its solvent adduct ions are also observed in the ESI mass

spectra, and the relative abundances of these ions were found to depend on the

cone voltage. However, the relative abundances of the above ions at constant

cone voltage reflected the stability of the [CrIII(Salen)(L)]+ ions. The

[CrIII(Salen)(L)]+ ion is most stable for 1,2-diaminoethane and 1,3-

diaminopropane ligands. The stability of the complex ion decreased from 1,4-

diaminobutane to 1,6-diaminohexane, and there is a slight increase for 1,7-

diaminoheptane and 1,8-diaminooctane. A similar trend was observed from the

ligand-pickup experiments in the collision cell using acetronitrile or

propylamine as collision gas, and from CID experiments on [CrIII(Salen)(L)]+

ions.

28


5. EXPERIMENTAL

The [CrIII(Salen)]PF6 was synthesized using a known procedure.51 All the

ligands (propylamine and diamines, 1-7) used in the present study were

purchased from Sigma-Aldrich (Steinheim, Germany) and were used without

further purification. The solvents (HPLC-grade) were purchased from Merck

(Mumbai, India). Stock (1mM) solutions of all ligands and of [CrIII(Salen)]PF6

were made in acetonitrile. The stock solutions of the ligand of choice and of

[CrIII(Salen)]PF6 were mixed in appropriate volumes (1:1) and diluted with

acetonitrile to achieve final concentrations of 100µM each.

All the mass spectra were recorded using a Quattro LC triple-

quadrupole mass spectrometer (Micromass, Manchester, UK) coupled with an

HP1100 series liquid chromatograph (Agilent, Palo Alto, USA); the data were

acquired using Masslynx software (version 3.2). The ESI capillary voltage was

maintained between 4 and 4.2 kV, and the cone voltage was kept at 30 V unless

otherwise stated. Nitrogen was used as desolvation and nebulization gas. The

source and desolvation temperatures were kept at 100o C. The ESI mass spectra

were recorded by scanning MS1 and the sample solutions were injected

through the Quattro LC injector with a Valco six-port valve with a 10 µL loop,

using acetonitrile at a flow rate of 100 µL/min using the HPLC pump. The CID

spectra and ligand-pickup experiments were obtained by selecting the

precursor ion of interest with MS1 and scanning MS2. For these experiments,

the sample solutions were introduced into the source of the mass spectrometer

29


using an infusion pump (Harvard Apparatus) at a flow rate of 10 µL/min. Argon

was used as the collision gas for CID experiments and the collision cell

pressure was maintained at 9x10-4 mbar. For ligand-pickup experiments,

acetonitrile or propylamine was used as the collision gas, maintaining the

collision cell pressure at 9x10-4 mbar. All the spectra reported here were

obtained as the averages of 20 scans.

30


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34

PROTON AND ALKALI METAL ION AFFINITIES OF BIDENTATE BASES:
SPACER CHAIN LENGTH EFFECTS

X

+
(CH2)n M

X

X = NH2, OH; M+ = H+, Li+, Na+ and K+

Chapter 2.1 Proton and alkali metal ion interactions of Diamines..

CHAPTER 2
CHAPTER 2
PART--1
PART 1

Proton and alkali metal ion affinities of α,ω -Diamines:
Spacer chain length effects

1. PROLOGUE

P
rotonated species are central to many chemical and biological processes,

such as acid-base phenomena, astrochemistry, radiation chemistry, mass

spectrometry, catalysis, surface chemistry, protein conformation, membrane

transport and enzyme catalysis.1 Since, the proton is having almost comparable

attributes with the alkali metal ions, the hydrogen has been placed in the first

group of the periodic table. Alkali metal ions are one of the most abundant ions

in biological systems, where they are involved in a variety of processes,

including osmotic balance, the stabilization of biomolecular conformations and

information transfer through ion pumps and ion channels.2-6 They interact with

poly functional molecules, like peptides and proteins to perform such

regulatory and structural functions.2,7 Thus, the knowledge of proton and alkali

metal ion binding interactions with polyfunctional biomolecules is an important

step in understanding the biochemical processes.8,9 Good correlations exist

between the metal ion and proton binding affinity to the bases, though the

proton affinities are much higher.10,11 Alkali metal ion binding interactions to

small model ligands bearing the heteroatom (oxygen or nitrogen) functional

group (binding sites) in the gas phase provides intrinsic information necessary

35


for better understanding of the interaction of the metal ions with biologically

active macromolecules.

The proton/alkali metal ion interactions decrease the space occupied in

three dimensional structures wherever possible, and they can opt to the

regulation of enzymatic activity, protein folding and functioning and stability of

biological systems.2,7 A common structural feature of the proton/alkali metal ion

bound complex is the presence of interactions between multiple functional

groups. For instance, in the protonated polyfunctional ions, protonated part of

the molecule may interact with an unprotonated group to form intramolecular

hydrogen bonds. Many organic reactions also proceed through protonated

intermediates or involve direct hydrogen bonding such as those involved in

protein or DNA complexes. Such hydrogen bondings greatly influence the

structure and the properties of organic compounds. In particular,

intramolecular hydrogen bonds are often responsible for determining the

predominant conformers in solution12 as well as in the gas phase.13-17

Intramolecular solvation of protonated functional groups influence the gas-

phase basicities of polyfunctional molecules. Occurrence of intramolecular

solvation in protonated species was characterized by several authors from a

series of di- and polyfunctional ions such as diols,17-21 diamines,10,13,14,16,20-28

diethers,15,16 diketones,15,16,29 amino acid derivatives,30 cyclic and acyclic

polyethers, and open chain and cyclic diols31 and amino alcohols using both

theoretical and experimental studies.14 The intramolecular hydrogen bond

36


stabilizes the ion by up to 20 kcal mol-1, and thereby increases the proton

affinity of several bi or polyfunctional compounds.20/21

In biological systems, especially in proteins, several basic motifs exist,

separated by varying chainlengths. Polyamines found to be present in the cells

of microorganisms and animal organisms, contribute to the stabilization of the

structure and activity of tRNA and DNA.32 It is well known that polyamines, such

as putrescine (1,4-diaminobutane), spermidine and spermine, are present in

millimolar concentrations in most tissues and microorganisms. Other

polyamine derivatives including cadaverine (1,5-diaminopentane) and 1,3-

diaminopropane are also found in some living cells. Although there were

several reports that describe the effects of the polyamines on the higher order

structure of DNA, the mechanism of the action of polyamines on DNA molecules

has not been clarified yet.33

α,ω-alkanediamines are compounds of interest in various domains of

organic and organometallic chemistry because these are bifunctional, can

cyclize after protonation (Scheme 1).13,22 These are also known as chelating

bidentate ligands in coordination chemistry, as reactants in industrial

polymerization processes, and as synthetic enzymes for complex formations

with target substrates through hydrogen bonding.34 Thermochemical

properties of α,ω-diamines have been studied by several

researchers.10,13,14,16,20-27

37


H2
N
H+
H2N (CH2)n NH2 (CH2)n H+
N
H2

Scheme 1

Estimation of thermo chemical properties to the monofunctional

molecules is straight forward, whereas to that of molecules with two or more

functional groups is intresting to investigate, because of possible internal

hydrogen bonding between the two like or unlike functional groups. Molecules

with two or more functional groups may have more proton affinity, greater than

that expected for either of the individual groups, due to the internal hydrogen

bond formation by favorable molecular geometry. Intramolecular hydrogen

bonding and the consequent chelating ring size were found to be the key

factors controlling the stability of the protonated complexes.10,13,14,16,20-28 The

first examples and interpretations of this phenomenon were explicated by Aue

et al.22 and followed by Yamdagni and Kebarle,13 who found that the proton

affinities of α,ω-diamines are significantly higher than those of monoamines

with the same alkyl chain length. The protonated diamines were proposed to

have cyclic structures, and the ring strain present in the structures was

evaluated with reference to the strain-free structures of proton bound dimers of

monoamines. Both groups of workers noted that the proton affinity of

H2NCH2CH2NH2 was substantially less than that of its higher diamino analogues,

which was attributed to the large strain energy expected for a five membered

38


ring (the assumption being that for maximum stability, the N--H+--N bond will

be linear as in proton-bound dimers of monoamines). Later, Bouchoux et al.

extensively studied protonation thermo chemistry of α,ω−diamines. Mass

spectrometric methods and computational techniques were extensively used

for the protonation studies on α,ω-diamines. 10,13,14,16,20-28

Though there were a numerous reports on the protonation

thermochemistry of α,ω-diamines, studies towards the alkali metal ion affinities

of the diamines are scarce. There was only one report on ab initio molecular

orbital (MO) calculations on the stabilities and binding energies of bidentate

ethylene diamine with alkali metal (Li+ and Na+) ions.10 The computed binding

energies of Li+ and Na+ ions with ethylene diamine are 66.3 and 42.3 kcal mol-1,

respectively. Thermochemical data obtained in the gas phase are of particular

value both for understanding the nature of metal ion-basic component

interactions in condensed phase and for explaining solvent phenomenon.35

The solvent-free environment of the mass spectrometer provides an

ideal medium for measuring the intrinsic properties such as proton/metal ion

affintity in the absence of interfering solvent effects. The kinetic method

developed by Cooks et al.36-39 has been used to estimate thermochemical data

for a wide range of organic and biological molecules for more than 25 years

and has often been reviewed.36

39


1.1. THE KINETIC METHOD

The best-known application of the kinetic method is for the

determination of proton affinities, gas phase acidities, metal and chloride ion

affinities, and electron affinities.20,21,36-48 This method37,40,41 is an effective

method for estimating the relative binding energies of two similar bases that

bind to a central ion, typically a proton/metal ion. Several series of bidentate

molecules, such as diols, diethers and diamines, have been studied by this

method for the determination of their proton affinities. The method starts with

the generation of proton/metal ion bound dimer between two bases and is

subjected to tandem mass spectrometric experiments to obtain the

corresponding proton/metal ion bound monomeric bases. The ratio of the

relative abundances associated with two competitive dissociation channels

(heterodimers) is then measured to estimate the relative binding energies. The

logarithmic value of the relative abundance is proportional to the logarithm of

the relative rate of dissociation of the two reaction channels.

For example, the dissociation of a proton/metal (M) bound heterodimer

of L1 and L2 leads to M+ bound monomers (equation 1 and 2)

k1 L1 + L2M+ (1)
[L1- - -M+- - -L2]
k2
L2 + L1M+ (2)

40


Here k1 and k2 are the rate constants for the competitive dissociations of

the cluster ion to yield L1M+ and L2M+, respectively. Based on transition state

theory, 49 the natural logarithm of rate constant ratio is given by the equation 3.

ln(k1/k2) = ln(Q2*/Q1*) + [εo(1) - εo(2)]/RTeff (3)

In which Q1* and Q2* are the partition functions of the activated

complexes of reaction 1 and 2, respectively; εo(1) and εo(2) are the

corresponding activation energies; R is the gas constant and Teff is the effective

temperature, a parameter in temperature units that reflects the internal energy

of the dissociating heterodimer. Assuming that the abundances reflect rate

constants37, 40,41 and that no reverse activation barriers exist equation 3 tends

to,

ln([L2M+]/[L1M+]) = ln(Q2*/Q1*) - ∆HML1/RTeff + ∆HML2/RTeff (4)

Where, ∆H°M is the ∆H of the dissociation reaction LM+ L + M+ or the

metal ion affinity of L. If L1 and L2 are structurally similar, as expected with used

ligands, ∆(∆S M+) should be close to zero, i.e. Q2* ≈ Q1* and fragmentation of

L1M and L2M proceed by simple bond cleavages from the loosely bound

complex L1--M+--L2, the reverse activation energies for channels L1M and L2M

should be negligible. In such a case, the difference in proton/metal ion

affinities between the two amino acids of interest would be nearly equals to the

binding energy (∆E) of those amino acids: then the above equation is simplified

further to (equation 5).

41


ln([L2M+]/[L1M+]) ~ (∆HML2 - ∆HML1)/RTeff ~ ∆EM/RTeff (5)

∆EM is the binding energy of central proton/metal ion between the two

heterodimers, where L1M and L2M are the relative abundances of the two

reactions from the fragmentation of the M+ bound heteodimer, and Teff is the

effective temperature of the activated precursor cluster ion. Thus, Teff is a

measuring parameter for internal energy of dissociating cluster ion and

primarily depends on the structure and lifetime of the ion. Several

investigations have shown that different dimer ions (of chemically similar

molecules), generated under identical experimental conditions and found to

have the same lifetime, also have fairly similar Teff, independent of the central

ion holding them. Hence, Teff of [L1--M+--L2] can be approximated by the

effective temperature of the corresponding H+-bound heterodimers.

Using the above assumptions Cerda and Wesdemiotis39 semi-

quantitatively evaluated the relative Cu+ ion binding energies of α-amino acids.

In these experiments, Teff values for [AA1-Cu-AA2]+ was approximated to the Teff

for H+- bound hetero amino acids, coproduced in the same sample. Application

of the equation 5 yielded Teff value, and this value was further used to convert

ln(k2/k1) values in the estimation of Cu+ binding energies of all the 20 common

amino acids. In a similar way, Lee et al.41 also constructed relative Ag+ ion

binding energy ladder for essential α-amino acids using the kinetic method.

These binding energies were compared with their relative H+ and Cu+ ion

binding energies. However, there is no systematic study on the alkali metal

42


affnities of homologues series of α,ω-diamines, and the effect of spacer chain

length on their binding efficiency.

2. SCOPE OF THE WORK

The literature reports clearly demonstrate the enhancement of the

proton affinities of α,ω-diamines with respect to the primary amines. This was

readily explained by the formation of a strong internal hydrogen bond in the

protonated form of the diamines. Among homologues series of α,ω-diamines,

1,4-butane diamine depicts highest proton affinity, owing to the seven

membered ring stabilized structure after protonation. However, there were no

systematic studies on the alkali metal ion affinities of α,ω-diamines. Hence, we

undertook a systematic experimental and computational study on the

measurement of relative gas phase affinity of alkali metal ions (Li+, Na+ and K+)

with a series of α,ω-diamines and compared them with the corresponding

proton affinities. In this part, the kinetic method and quantum chemical

calculations are employed to address the following points.

What are the variations in the relative binding affinities of proton and alkali

metal ions in the given series?

What is the nature of bridging interactions the alkali metal ion complexes

have?

What are the structural differences between the proton and alkali metal ion

complexes of diamines?

43


3. RESULTS AND DISCUSSION

It is well known that electrospray ionization technique is the best method

to study the interactions between the metal ions and various systems. We have

used the kinetic method for evaluating the relative alkali metal ion [Li+, Na+ and

K+] affinities for a series of seven homologues α,ω-diamines, namely 1,2-

diaminoethane (1), 1,3-diaminoproane (2), 1,4-diaminobutane (3), 1,5-

diaminopentane (4), 1,6-diaminohexane (5), 1,7-diaminoheptane (6), and 1,8-

diaminooctane (7).

[Li-2]+
81
100 [1-Li-2]+
CID with
Ar gas

[1-Li-2]+
141

%

[Li-1]+
67

0 m/z
40 60 80 100 120 140 160 180 200
+
Figure 1: CID mass spectra of Li bound heterodimer of compounds 1 and 2.

The study was initiated with Li+ ion binding of 1-7. The typical ESI mass

spectrum recorded for a methanol/water solution containing two different

diamines from 1-7 (DA1 and DA2) and lithium chloride show the H+ and Li+

bound mono and dimeric cluster ions. The spectrum recorded for a mixture of

1 and 2 in the presence of Li+ is shown in Figure 1 as an example. The Li+

44


bound heterodimeric ions, [DA1+Li+DA2]+ formed with various combinations of

diamines are mass selected by MS1, and dissociated in the collision cell under

similar experimental conditions. The heterodimeric ions dissociate by

competitive elimination of neutral diamines yielding two fragment ions

corresponding to [DA1+Li]+ and [DA2+Li]+ (equations 6 and 7). The relative

abundances of the resulted Li+ bound monomers, viz., I(Li+-DA1) and I(Li+-DA2)

vary and reflect the Li+ ion affinity of individual diamine. The diamine that has

more affinity results in higher abundance of its Li+ bound monomer than that of

with less affinity. For example, the CID spectrum of [1+Li++2] (Figure 1) shows

higher abundance of [1+Li+] than [2+Li+], which confirms higher Li+ ion affinity

of 1 when compared to that of 2.

DA2 + DA1Li+ (6)
[DA1--Li+--DA2]
DA2Li+ + DA1 (7)

3.1. Li+ ION AFFINITY LADDER CONSTRUCTION

The CID spectra were recorded for all possible lithiated heterodimers of

diamines (1-7). The spectra of fifteen out of twentyone heterodimers resulted in

both Li+ bound monomers, [Li-DA1]+ and [Li-DA2]+ with considerable

abundance. The other spectra are dominated with only one of the lithiated

monomer being the other monomer negligible due to large difference in their

Li+ ion affinities. Hence, we consider only those spectra, which resulted in both

45


fragment ions, for constructing the Li+ ion affinity order by the kinetic method.

The relative abundance ratio of two fragment ions, i.e., I(Li+-DA2)/I(Li+-DA1)

ratio values are calculated from the CID spectra of all possible heterodimers,

where the Li+ ion affinity of DA2 is higher than DA1. The natural logarithm of

I(Li+-DA2)/I(Li+-DA1) ratio values are used to construct relative Li+ ion affinity

ladder. A metal ion binding ladder can be constructed with ln[I(Li+-DA2)/I(Li+-

DA1)] values in which the ligand of lowest affinity is considered as reference.

The experimentally measured ln[I(Li+-DA2)/I(Li+-DA1)] values are summarized

in a relative Li+ affinity ladder shown in Figure 2. In this ladder construction,

most of the diamines are compared to at least three others.

Figure 2: Measured ln[I(Li+-DA2)/I(Li+-DA1)] values for Li+-bound heterodimers of
diamines (1–7). The data presented under the heading ln[I(Li+-DA2)/I(Li+-
1)] are average cumulative values expressed relative to ethylene diamine
(1). The numbers given in parentheses are estimated errors resulting from
the measurement of abundance ratios.

46


The ln[I(Li+-DA2)/I(Li+-DA1)] values calculated for the successful

combinations are found to be reproducible. The ln[I(Li+-DA2)/I(Li+-DA1)] values

are internally consistent for the Li+ bound heterodimers of diamines. For

example, the value for [4·Li·1]+ is 3.51, a very similar value is obtained by

adding the ln[(ILi+-DA2/I(Li+-DA1)] values of three intermediate steps, viz.

ln[I(Li+-3)/I(Li+-1)] + ln[I(Li+-2)/I(Li+-3)] + ln[I(Li+-4)/I(Li+-2)] = 1.91 + 0.08 +

1.58 = 3.57. The ln[I(Li+-DA2)/I(Li+-DA1)] values for other pairs are also

consistent internally with a difference not more than 0.2. Similarly results are

also obtained when the experiments were performed at different collision

energy values (2, 4, 6, and 8 eV). This accord confirms that entropic effects,

which tend to be non-additive, are indeed negligible with the diamines

studied.

From Li+ ion affinity ladder, the relative Li+ ion affinity order for α,ω-
+ + + + + + +
diamines can be drawn as, 1Li < 3Li ≤ 2Li < 4Li < 6Li < 5Li ≤ 7Li . In the

Li+ affinity order for α,ω-diamines, the deviation of compound 2 and 5 in the

order indirectly suggests that the structure of the lithiated diamine may be

playing a role. There are two possible structures for the resulted lithiated

species. One possibility is acyclic structure in which Li+ ion is bound with one

of the amine group. The other is a cyclic structure where both the amine groups

in diamine coordinate to the Li+ ion. If the lithiated diamines were acyclic, one

would expect gradually increase in Li+ ion affinity order as the chain length of

diamine increased due to increase of positive inductive effect with increase in

47


the number of methylene groups attached to the amine groups in diamine.

However, the observed Li+ ion affinity order shows the possibility of cyclic

structures. The formation of cyclic structures for protonated diamines was

studied in detail. As mentioned in the introduction, the high proton affinity for 3

among series of primary α,ω-diamines (1-7) is due to its stable cyclic structure

on protonation. Similarly, the higher Li+ ion affinity for compound 2 and 5 when

compared to their respective higher homologous diamine may also be due to

the stability of the resulted cyclic lithiated species. In fact, the formation of

bicoordinated lithium complexes is known in the literature.50,51

3.2. Na+ AND K+ ION AFFINITY LADDERS CONSTRUCTION

We have extended the experiments towards Na+ and K+ ion affinity order

determination for diamines (1-7) by performing similar experiments as we

applied to lithium, to study the effect of metal ion size in the stabilization of

metal bound diamines. For this purpose, we have generated all possible

heterodimers of Na+/K+ ion bound diamines, [DA1-M+-DA2], where M=Na or K.

The same fifteen pairs of diamines that are used for Li+ are also successful for

both the Na+ and K+ experiments. The CID spectra of these [DA1-M+-DA2] ions

are recorded, and the relative abundances of the Na+/K+ bound monomers

(i.e., M+-DA1 and M+-DA2 formed during the dissociation) correlated with the

relative Na+/K+ ion affinities of the two bases. The natural logarithm of

abundance ratio, ln[I(M+-DA2)/I(M+-DA1)] values are calculated from the CID

spectra of all possible heterodimers at similar experimental conditions, where

48


the M+ ion affinity of DA2 is higher than DA1, and are used to obtain the Na+ and

K+ ion affinity ladders. The relative Na+ and K+ affinity ladder is shown in

Figure 3 and Figure 4, respectively.

Figure 3: Measured ln[I(Na+-DA2)/I(Na+-DA1)] values for Li+-bound
heterodimers of diamines (1–7). The data presented under the
heading ln[I(Na+-DA2)/I(Na+-1)] are average cumulative values
expressed relative to ethylene diamine (1). The numbers given in
parentheses are estimated errors resulting from the measurement of
abundance ratios.

From these ladders, the relative Na+ ion affinity order can be given as

1Na+ < 2Na+ < 3Na+ < 4Na+ < 5Na+ < 6Na+ < 7Na+, and is not similar when

compared to that obtained for lithium. The sodium ion affinity towards diamines

increases as the number of methylene groups in diamine is increased. This

49


observation suggests that the resulted sodiated diamines either have linear

structure, or do not reflect the ring size effect on their stabilization if there are

cyclic. However, from the present data we are unable to propose the correct

structure for the sodiated diamines.

Figure 4: Measured ln[I(K+-DA2)/I(K+-DA1)] values for Li+-bound heterodimers
of diamines (1–7). The data presented under the heading ln[I(K+-
DA2)/I(K+-2)] are average cumulative values expressed relative to
propane diamine (2). The numbers given in parentheses are
estimated errors resulting from the measurement of abundance
ratios.

The relative K+ ion affinity orders of the diamines (1-7) were also

determined and can be given as 2K+ < 1K+ < 3K+ < 4K+ < 6K+ < 5K+ < 7K+. As in

the case of Li+ ion affinity ladder, the ln[I(M+-DA2)/I(M+-DA1)] values for Na+

50


and K+ ions calculated for successful combinations are found to be

reproducible and values are internally consistent for the Na+/K+ bound

heterodimers of diamines. The K+ ion affinity order is closely comparable to

that obtained for sodium ion, except 1 and 2 where the potassium ion affinity of

1 is higher than 2. The higher affinity of compound 1 compared to that of 2 may

be explained assuming cyclic structures for the potassiated diamines.

3.3. PROTON AFFINITY LADDER CONSTRUCTION

The present study on the relative affinity of a series of diamines towards

Li+, Na+ and K+ shows that the affinity order is affected by the size of metal atom

and diamines. The discrepancies in the Li+ ion affinity order of diamines may be

explained through cyclic structures and their stability. However, the sodium

and potassium ion affinity order of diamines cannot be explained in a similar

way. Though all the alkali metals used are known to be bi-dentate in binding

with ligands, the present experimental results does not give much information

on the structures of the ions.

With a view to understand the differences between the alkali metal ion

affinity order of the studied compounds and the proton affinity order, we have

also constructed proton affinity ladder. We applied similar method that was

followed for alkali metal ions, for construction of proton affinity ladder by

replacing alkali metal ion with proton. The obtained proton affinity ladder is

given in Figure 5. From the ladder, the relative proton affinity order can be

given as 1H+ < 2H+ < 7H+ < 6H+< 5H+ < 4H+ < 3H+. The proton affinity order

51


obtained in the present method is in good agreement with the literature

values.52

Figure 5: Measured ln[I(H+-DA2)/I(H+-DA1)] values for H+-bound heterodimers
of diamines (1–7). The data presented under the heading ln[I(H+-
DA2)/I(H+-1)] are average cumulative values expressed relative to
propane diamine (2). The numbers given in parentheses are
estimated errors resulting from the measurement of abundance
ratios.

3.4. RELATIVE ALKALINE METAL ION BINDING ENERGY CALCULATIONS

It is well known that, for chemically similar compounds, the natural

logarithm of abundance (I) ratio values are directly proportional to the alkali

metal ion binding energy difference (∆E) (equation 8) between the used

diamines, where the entropy term is close to zero.40,41,53,54

52


ln[I(M+- DA2) /I(M+- DA1)] ~ ∆E /RTeff
~ (8)

Attempts were made to convert relative alkali metal ion affinity orders into

relative binding energies by measuring the Teff of the dissociating cluster

ions.40,41,53,54 It was already shown in the literature that when experiments are

performed at identical conditions, Teff is fairly similar for dimeric ions of

chemically similar molecules, irrespective of the central ion holding the two

molecules. For the measurement of Teff, the dissociation of proton bound

heterodimers of diamines was studied at different collision energies (2, 4, 6, and

8 eV). For successful measurement of Teff value in this method, heterodimers of

each diamine with atleast three other diamines should be studied. The diamine 1

and 3 could not be used for this purpose because of their extreme low or high

proton affinity values when compared to the other diamines. The left out

diamines 2, 4, 5 and 6 also could not be used for this study because the

difference in the proton affinity values among the three diamines (4, 5 and 6) is

very less (± 0. 5 kcal mole-1). This restricts the number of good references

needed for the measurement of a reliable Teff value. Consequently, we could not

obtain reliable Teff values due to the non availability of enough number of

references among the studied diamines. Hence, the present study is limited to

the relative alkali metal ion affinity orders.

3.5. COMPARISON BETWEEN PROTON AND ALKALI METAL ION AFFINITY ORDERS

Inspection of the relative orders of proton affinities and alkali metal ion

affinities of primary α,ω-diamines (1-7) reveals that the proton affinity order is

53


substantially different from the alkali metal ion affinity order. In the case of

proton affinities of diamines, the diamine 3 has higher proton affinity due to its

stable seven membered cyclic structure for the protonated 3. The diamine 1

has the least proton affinity due to unstable five membered ring formation after

protonation, and the proton affinity of diamine 2 is a little higher than 1 with a

relatively stable six membered structure for the protonated 2. The proton

affinity of diamines 4-7 are in between 3 and 2, and gradually decrease from 4

to 7, which could be due to gradual increase in the ring strains. Whereas, the

relative alkali metal ion affinity is always high for diamine 7 for all the alkali

metal ions studied. Although there are few differences among the alkali metal

ion affinity orders i.e., between 2 and 3; 5 and 6 in Li+ order, and 1 and 2 in K+

order, overall the metal ion affinity order decreased from 7 to 1. It suggests that

the positive inductive effect is playing major role in stabilization of the

metallated diamine than those of ring strains. The minor differences among the

relative orders of alkali metal ions may be due to the size of alkali metal atom.

We seek to explain the observed contrasting order for H+ and Li+ ion

affinities of α,ω-diamines through quantum chemical calculations.

3.6. THEORETICAL STUDIES

The H+ and Li+ ion affinities are estimated using the equations 9 and 10,

respectively. B3LYP/6-31G* method is used for the geometry optimizations and

obtaining the thermochemical data. All the structures considered are

characterized as minima on the potential energy surface. This is followed by

single point calculations at MP2/6-311++G** level. Counterpoise method was

54


used to calculate the basis set super position error (BSSE). In our studies all the

calculations were done using the Gaussian 9855 suite of program.

Metal ion affinity (∆H298) = ∆Eele + ∆Ethermal + T∆S - BSSE (9)

Proton affinity (∆H298) = ∆Eele + ∆Ethermal + 5RT/2 (10)

The relative binding affinity orderings of the computed results are in

excellent agreement with the experimental observations for both proton and

Li+ ion affinities, except the change of proton affinity order between 4 and 5.

Theoretically obtained H+ and Li+ ion affinity orders can be given as 1H+ < 2H+

< 7H+ < 6H+ ≤ 4H+ < 5H+ < 3H+ and 1Li+ < 3Li+ ≤ 2Li+ < 4Li+ < 6Li+ < 5Li+ ≈ 7Li+

respectively. Figure 6 depicts the optimized geometries of the Li+ and

protonated complexes. All the Li+ complexes are virtually symmetrically

bridged, and as the length of the spacer chain increases Li+ is going into the

cavity of the molecule. In agreement with the previous studies,2 computations

reveal that the Li+ ion affinities are less than one third of the proton affinities to

the diamines. The non-linearity of the relative binding affinities of Li+ ions can

be clearly traced to the subtle and intricate conformational changes in the Li+

complexed cyclic structures. In addition, higher energy mono-dentate

minimum energy structures where the cation is bound to the acyclic isomers

are obtained. Systematic conformational analyses of neutral diamines reveal

that the open chain linear structures are global minima besides several other

local minima with warped on cyclic structures. The energy difference between

the acyclic and cyclic neutral conformation (∆E1), the conformation

55

Thesis Title: Gas phase studies of metal complexes, isomeric carbanions and distonic radical anions under soft ionization mass spectral conditions

Thesis Title: Gas phase studies of metal complexes, isomeric carbanions and distonic radical anions under soft ionization mass spectral conditions

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