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Inorganica Chimica Acta 331 (2002) 246– 256

Remote ligand substituent effects on the properties of oxo-Mo(V)
centers with a single ene-1,2-dithiolate ligand
Frank E. Inscore, Hemant K. Joshi, Anne E. McElhaney, John H. Enemark *
Department of Chemistry, Uni6ersity of Arizona, Tucson, AZ 85721, USA

Received 25 August 2001; accepted 12 November 2001

Dedicated to Professor A.G. Skyes

Abstract

The oxomolybdenum mono-ene-1,2-dithiolate complex (Tp*)MoO(bdtCl2) (3) has been synthesized and characterized (Tp* is
hydrotris(3,5-dimethyl-1-pyrazolyl)borate; bdtCl2 is 3,6-dichloro-1,2-benzenedithiolate). The X-ray structural data show that 3
,
crystallizes in the monoclinic space group, P21/c, where a= 7.963 (3), b=26.272 (11), c= 14.016 (6) A, i= 105.352 (7). The
(Tp*)MoO(bdtCl2) molecule exhibits a distorted pseudo-octahedral coordination geometry, with the Mo atom ligated by a
terminal oxo atom, two sulfur donor atoms of the bdtCl2 ligand and three nitrogen atoms of the tridentate facially coordinated
Tp* ligand. The coordination environment about the Mo atom is similar to that of (Tp*)MoO(bdt) (1) (bdt is 1,2-benzenedithi-
olate), but the fold angle between the MoS2 plane and S2C2 plane of the bdtCl2 ligand (q =6.9°) is substantially smaller than the
feature in 1 (q=21.3°). The similar IR, EPR, and electronic absorption spectroscopic results for 1 and 3 indicate that the electron
withdrawing nature of the chlorine substituents of 3 does not significantly perturb the electronic structure of the Mo(V) center.
However, the solution redox potentials and the gas-phase ionization energies are sensitive to remote substituent effects. © 2002
Elsevier Science B.V. All rights reserved.

Keywords: Oxo-molybdenum ene-1,2-dithiolate complexes; Molybdenum dithiolate complexes; Pyranopterin molybdenum enzymes; Sulfite oxidase

1. Introduction atom transfer [1]. During enzymatic turnover, the
molybdenum center is proposed to shuttle through the
The syntheses and physical characterization of new Mo(VI)/Mo(V)/Mo(IV) oxidation states [1]. The X-ray
mono-oxo molybdenum complexes possessing various crystal structures are now known for a large number of
ene-1,2-dithiolate ligands coordinated to the metal cen- these enzymes, and all reveal a common active site
ter continues to be an important area of study in structural feature, which consists of at least one pyra-
connection with the pyranopterin molybdenum en- nopterin-ene-1,2-dithiolate (Fig. 1) coordinated to the
zymes. These molybdenum-containing enzymes, which molybdenum center [2–15]. At least one terminal oxo
are categorized into three families based on structure ligand is associated with the Mo active sites during
and reactivity (xanthine oxidase, DMSO reductase, and catalysis. The role of the pyranopterin ene-1,2-dithio-
sulfite oxidase), catalyze a variety of specific two-elec- late has been postulated to couple the Mo center into
tron redox reactions that are coupled to formal oxygen
efficient superexchange pathways for facilitating elec-
tron-transfer regeneration of the active site [16] and to
Abbre6iations: bdt, 1,2-benzenedithiolate; bdtCl2, 3,6-dichloroben-
modulate the Mo center reduction potential [1,16,17].
zenedithiolate; qdt, 2,3-dithioquinoxaline; tdt, 3,4-toluenedithiolate; The structural relevance and importance of the coor-
Tp*, hydrotris(3,5-dimethyl-1-pyrazolyl)borate; (S S), generic ene- dinated pyranopterin-ene-1,2-dithiolate on the reactiv-
1,2-dithiolate ligand that forms a five-membered chelate ring with the ity of these Mo containing enzymes have provided the
Mo atom.
* Corresponding author. Tel.: + 1-520-621 2245; fax: + 1-520-521
motivation to synthesize and characterize new oxo-Mo
8407. dithiolate complexes. The series of complexes shown in
E-mail address: jenemark@u.arizona.edu (J.H. Enemark). Fig. 2 possess a {(Tp*)MoVO}2 + center, which pro-

0020-1693/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 1 ) 0 0 8 1 7 - 9

F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256 247

vides an invariant structural unit that allows the funda- 2. Experimental
mental properties of oxo-Mo complexes with an ene-
1,2-dithiolate ligand to be investigated. These 2.1. General methods and materials
mono-oxo-Mo compounds represent minimal structural
models of the Mo(V) active site in the sulfite oxidase All reactions, synthetic operations and manipulations
family, which possess a single pyranopterin ene-1,2- followed strict anaerobic procedures and were per-
dithiolate coordinated to the Mo center throughout the formed under a dry blanket of pre-purified argon gas
catalytic cycle. The spectroscopic silence of the (Tp*) using standard Schlenk techniques, a high-vacuum/gas
ligand has allowed for detailed electronic structure double line manifold, and an inert atmospheric glove
studies to be initiated, which have defined the nature of bag. Synthetic operations were also carried out in an
oxo-Mo-S (dithiolate) interactions in this system [16– inert atmosphere glove box filled with pure dinitrogen
24]. Of particular importance, are the spectroscopic gas. The argon was predried by passing the high-purity-
studies of (Tp*)MoO(bdt) (1) and (Tp*)MoO(tdt) (2) grade inert gas through a series of drying towers.
that have provided insight into oxygen atom transfer Dinitrogen was obtained directly from a pressurized
reactivity and electron-transfer regeneration of the ac- liquid nitrogen cryogenic transfer/storage dewar. All
tive site in the pyranopterin Mo enzymes [16,19]. Subse- glassware was oven dried at 150 °C and Schlenk ware
quent studies on (Tp*)MoO(qdt) (4) have revealed that was further purged by repeated evacuation and inert
the nature of the ene-1,2-dithiolate ligand plays an gas flushes prior to use. Tetrahydrofuran (THF) and
important role in defining the spectroscopic differences toluene were distilled from Na/benzophenone; triethy-
observed within this series [17,23]. A continuing area of lamine was distilled from Na/K amalgam [25]. The
importance relates to the electron-donating ability of prepurified solvents were subsequently transferred and
the ene-1,2-dithiolate, and how this affects the geomet- stored under N2 over fresh drying agents. These sol-
ric and electronic structures of oxo-Mo systems possess- vents were freshly distilled under nitrogen prior to use,
ing mono-ene-1,2-dithiolate coordination. thoroughly degassed by repeated freeze–thaw–pump
We describe here the syntheses and physical charac- cycles, and transferred to reaction vessels via steel
terization of (3) [hydrotris(3,5-dimethyl-1-pyrazolyl)- cannulae techniques under a positive pressure of inert
borato](3,6-dichloro-benzene-1,2-dithiolato)-oxomolyb gas. Dichloromethane, 1,2-dichloroethane, cyclohexene,
denum(V), a new oxo-Mo(V) mono-ene-1,2-dithiolate toluene (EM Science, Omnisolv), n-hexane and n-pen-
complex that has electron withdrawing groups on the tane (Burdick and Jackson) were used as received and
aromatic ring. The X-ray crystal structure, electrochem- deoxygenated by argon saturation prior to use. Solvents
ical behavior, and spectroscopic data for 3 are com- employed in the spectroscopic characterization studies
pared with those for related (Tp*)MoO(S S) complexes were degassed by freeze–thaw– pump cycling before
(defined in Fig. 2). use. The 1,2-dichloroethane used in the electrochemical
studies was of anhydrous grade (EM Science; Drisolv)
and required no further purification. Reagents were
generally used as received. Molybdenum pentachloride
(MoCl5, Aldrich) was dried in vacuo and stored under
dinitrogen prior to use. Potassium hydrotris(3,5-
dimethyl-1-pyrazolyl)borate (KTp*) and the precursor
Fig. 1. Structure of the pyranopterin derived from protein crystallo-
complex, (Tp*)MoVOCl2, were prepared according to
graphic studies (shown in fully reduced form) [2 –15]. literature procedures [18]. The ligands H2bdt (1,2-ben-
zenedithiol) and H2bdtCl2 (3,6-dichloro-1,2-ben-
zenedithiol) employed in the syntheses of the
(Tp*)MoVO(S S) compounds (1, 3) were used as re-
ceived from Aldrich. The preparation of (Tp*)Mo-
O(bdt) (1) followed from published procedures [20,21].
The synthesis, isolation, purification and characteriza-
tion of (Tp*)MoO(bdtCl2) (3) is described below.

2.2. Preparation of compounds

Highly purified (Tp*)MoOCl2 (500 mg, 1.0 mmol)
was added to an evacuated Schlenk flask and dissolved
Fig. 2. Stereochemistry of the {(Tp*)MoVO}2 + system containing in 50 ml of dry degassed toluene. The mixture was
equatorial ene-1,2-dithiolate ligands (S S) coordinated to the Mo deoxygenated thoroughly with argon saturation while
center. Structures of the dithiolate dianions (S S) are shown. being stirred at 80 °C. Solid H2bdtCl2 (220 mg, 1.1

248 F.E. Inscore et al. / Inorganica Chimica Acta 331 (2002) 246–256

Table 1 n-pentane. The red–brown powder precipitate was col-
Crystal data and structure refinement parameters for
lected by filtration and washed with n-pentane until the
(Tp*)MoO(bdtCl2) (3)
eluant was clear. The powder was then dissolved in
Empirical formula C21H24BCl2MoN6OS2·1/2C6H14 dichloromethane, filtered to remove any insoluble mate-
Formula weight 661.33 rials, and evaporated to dryness in vacuo. The solid was
Temperature (K) 173(2) pumped on for several hours to ensure dryness and the
Wavelength (A) , 0.71073 complete removal of excess triethylamine (Et3N). The
Crystal system Monoclinic
Space group P2(2)/c
solid material was re-dissolved in dichloromethane,
Unit cell dimensions concentrated, and loaded on a silica gel chromato-
a (A) , 7.963(3) graphic column (230–400 mesh, pore diameter 60 A, ,
b (A) , 26.272(11) Merck) under a positive pressure of argon. A red–
,
c (A) 14.016(6) brown fraction (band c 2) eluted off the column using
h (°) 90
i (°) 105.352(7)
dichloromethane–cyclohexene (1:3) as the eluant. The
k (°) 90 purity of (Tp*)MoO(bdtCl2) was confirmed by TLC
,
V (A3) 2828(2) analysis. The red–brown powder was evaporated to
Z 4 dryness in vacuo. The compound was re-dissolved in
Dcalc (Mg m−3) 1.553 dichloromethane, and layered with n-pentane to yield a
Absorption coefficient (mm−1) 0.83
F(000) 1252
dark red–brown crystalline material. The crystalline
Crystal size (mm) 0.25×0.10×0.03 material was filtered, washed and then dried in vacuum.
Theta range for utilized data (°)1.69–27.78 The product was characterized by IR, UV–Vis, EPR
Limiting indices −105h59, −345k534, and mass spectroscopy. Suitable crystals for X-ray dif-
−185l517 fraction studies were obtained by a slow diffusion of
Reflections utilized 30 776
Independent reflections 6266 [Rint = 0.0539]
n-pentane (or n-hexane) into a concentrated
Completeness to theta =27.78° 93.7% dichloromethane solution of purified 3.
Absorption correction None
Max/min transmission 0.9757, 0.8203 2.3. X-ray crystal structure determinations
Refinement method Full-matrix least-squares on F 2
Data/restraints/parameters 6266/0/340
Goodness-of-fit on F 2 1.040
A burgundy plate of 3 was mounted on a glass fiber
Final R indices [I2sigma(I)] R1 = 0.0442, wR2 = 0.1005 for structure determination using a Bruker SMART
R indices (all data) R1 = 0.0764, wR2 = 0.1147 1000 CCD detector X-ray diffractometer. Key parame-
Largest difference peak and hole 0.636 and −0.432 ters for the structure determination are summarized in
,
(e A−3) Table 1. Empirical absorption and decay corrections
,
RMS difference density (e A−3) 0.098
were applied using the program SADABS. The structure
was solved by direct methods using SHELXS in the
mmol) was added in slight excess to the suspension Bruker SHELXTL (Version 5.0) software package. Refin-
under a positive pressure of argon. The resulting solu- ements were performed using SHELXL and illustrations
were made using XP. Hydrogen atoms were added at
tion was purged with argon for 20 min. Dry degassed
idealized positions, constrained to ride on the atom to
Et3N (0.40 ml, 2.8 mmol) was added slowly dropwise
which they are bonded and given thermal parameters
via gas tight syringe to this rigorously stirring solution.
equal to 1.2 or 1.5 times Uiso of that bonded atom. A
The mildly refluxing reaction solution was observed to
parameter describing extinction was included. Scatter-
change gradually from an emerald green to a dark
ing factors and anomalous dispersion were taken from
red – brown color after 4 h of stirring. The reaction International Tables Vol C, Tables 4.2.6.8 and 6.1.1.4
progress, and hence optimal yield, was monitored by [26]. The crystals contain one half molecule solvent
TLC analysis (silica gel 60 F254 plastic sheets, EM (hexane) of crystallization per asymmetric unit (Table
Science). The reaction was stopped upon observing the 1). The large values of thermal displacement parameters
near disappearance of the green (Tp*)MoOCl2 precur- of the carbon atoms of the hexane of crystallization can
sor concomitant with the maximal formation of the be attributed to the partial loss of solvent molecules
red – brown product. Upon completion of the reaction, during the time interval between crystal isolation and
the blue– green precipitate, primarily Et3N·HCl result- low temperature data collection.
ing from the hydrogen abstraction and ligand exchange
processes, was filtered off the hot solution under dry 2.4. Other physical measurements
argon. The filtrate was cooled to room temperature
(r.t.) and evaporated to dryness with a rotorary evapo- Mass spectra were recorded on a JEOL HX110 high-
rator. The solid red–brown residue was re-dissolved in resolution sector instrument utilizing fast atom bom-
toluene, concentrated under vacuum, and layered with bardment (FAB) ionization in a matrix of 3-nitrobenzyl


alcohol (NBA). Infrared (IR) vibrational spectroscopic 0.020–0.023 eV. All data were intensity corrected with
data were collected on a Nicolet Avatar ESP 360 FT-IR an experimentally determined instrument analyzer sen-
spectrophotometer. The IR spectra (4000– 400 cm − 1) sitivity function. The He I spectra were corrected for
were measured in KBr disks or as dichloromethane the presence of ionizations from other lines (He Ib line,
solutions (between NaCl plates) at r.t. Electronic ab- 1.9 eV higher in energy and 3% the intensity of the He
sorption spectra of samples solvated in 1,2- Ia line). All samples sublimed cleanly with no de-
dichloroethane solutions were recorded with a 1-cm tectable evidence of decomposition products in the gas
pathlength Helma quartz cell equipped with a Teflon phase or as a solid residue. The sublimation tempera-
stopper, on a modified Cary 14 (with OLIS interface, tures were monitored using a ‘‘K’’ type thermocouple
250 – 2600 nm) spectrophotometer. Quantitative absorp- passed through a vacuum feed and attached directly to
tion spectra were acquired at 2.0 nm resolution using a the aluminum ionization sample cell. The sublimation
dual-beam Hitachi U-3501 UV– Vis – NIR spectropho- temperatures (in °C, 10 − 4 Torr) were as follows:
tometer calibrated with known mercury lines and a 6% (Tp*)MoO(bdtCl2), 198 °C; and (Tp*)MoO(bdt),
neodymium doped laser glass standard (Schott Glass). 183 °C.
Absorption spectra were analyzed using Hitachi sup-
plied Grams software. Electron paramagnetic resonance
(EPR) spectra at X-band frequency ( 9.1 GHz) of 3. Results and discussion
solution (298 K) and frozen glasses (77 K) were ob-
tained on a Bruker ESP 300 spectrometer. The EPR 3.1. Syntheses, properties and physical characterization
samples were prepared as 1.0 or 2.0 mM solutions in of (Tp*)MoO(bdtCl2)
dry degassed toluene. Cyclic voltammetric (CV) data
were collected on a Bioanalytical Systems (BAS) CV-50 The synthesis of (Tp*)MoO(bdtCl2) was achieved by
W system. BAS supplied software provided scan acqui- a ligand exchange reaction between the precursor com-
sition control and data analysis/graphics capabilities. plex (Tp*)MoOCl2 and free ligand H2bdtCl2 in the
The electrochemical cell employed was based on a presence of a strong base (Et3N), as with other related
normal three-electrode configuration. This cell consists compounds [16,18,20,21,27,28]. The identity of the re-
of a platinum disk working electrode (1.6 mm diameter, action product was confirmed by its high resolution
BAS), a platinum wire counter electrode (BAS) and a mass spectrum, which shows an [M+H]+ peak that
NaCl saturated Ag/AgCl reference electrode (BAS). gives m/z= 619.0063 (calculated, 619.0059) and corre-
Prior to each experiment, the electrode was polished sponds to the formula [12C21H25N11B32S35Cl2O97Mo].
6 2
using 0.05 mm alumina (Buehler) and electrochemically The product is soluble in dichloromethane,
cleaned in dilute sulfuric acid. Cyclic voltammetric dichloroethane, toluene and benzene. This compound
measurements of (Tp*)MoO(bdtCl2) and related appeared to be relatively stable in air; however, to
(Tp*)MoO(S S) complexes were performed in dry de- ensure structural integrity and sample purity, the
gassed 1,2-dichloroethane solutions (10 ml, 1 mM, product was stored under argon prior to use.
25 °C) over a potential window of 91.5 V versus The solid-state IR spectrum of (Tp*)MoO(bdtCl2) in
Ag/AgCl with 0.1–0.2 M dried tetra-n-butylammonium KBr exhibited bands characteristic of the (Tp*) ligand
tetrafluoroborate [n-Bu4N][BF4] (Aldrich) as the sup- (w(B H) =2554 cm − 1) as well as a strong absorption at
porting electrolyte. The background scans of dry/de- 933 cm − 1 typical of {MoVO}3 + containing species
oxygenated DCE with the [n-Bu4N][BF4] supporting [16,18]. The solution IR spectrum of (Tp*)MoO(bdtCl2)
electrolyte exhibited no electroactive impurities or sol- in dichloromethane showed no significant frequency
vent decomposition within the potential window em- shifts or additional bands, indicating structural in-
ployed. Ferrocene was utilized as an internal standard, tegrity was maintained upon solvation. The absence of
and all potentials were referenced relative to the Fc/ a strong absorption at 961 cm − 1 associated with the
Fc+ couple. He I gas-phase photoelectron spectra w(Mo O) stretching mode of the (Tp*)MoOCl2 precur-
(PES) were collected on a spectrometer with a 36-cm sor was indicative of the relative purity of the
radius hemispherical analyzer (8 cm gap, McPherson), (Tp*)MoO(bdtCl2) compound. The frequencies for the
sample cells, excitation sources, and detection and con- Mo O vibration in the (Tp*)MoO(S S) complexes con-
trol electronics using methods that have been previously taining a monoene-1,2-dithiolate ligand span a narrow
described in detail [22]. The absolute ionization energy range (926–941 cm − 1), and are lower than that ob-
scale for the He I experiments was calibrated by using served for the (Tp*)MoOCl2 precursor [16,18,20]. The
the 2E1/2 ionization of methyl iodide (9.538 eV), with lower w(Mo O) observed in the structurally character-
the argon 2P3/2 ionization (15.759 eV) used as an inter- ized series, (Tp*)MoO(bdt) (931 cm − 1)B
−1
nal calibration lock during the experiment. During data (Tp*)MoO(bdtCl2) (933 cm )B (Tp*)MoO(qdt) (941
collection the instrument resolution (measured using cm − 1) relative to related halide complexes [18,29,30]
the FWHM of the argon 2P3/2 ionization peak) was can be attributed to the weaker p-donor properties of


Table 2
Summary of ground-state IR a and EPR b spectral data for (Tp*)MoO(S S)

Complex w(Mo O) g1 g2 g3 Žg c
ŽA d Reference

(1) (Tp*)MoO(bdt) 931 2.004 1.972 1.934 1.971 37.0 [20,21]
(2) (Tp*)MoO(tdt) 926 2.004 1.974 1.937 1.972 34.3 [18]
(3) (Tp*)MoO(bdtCl2) 933 2.002 1.974 1.938 1.973 35.6 This work
(4) (Tp*)MoO(qdt) 941 2.005 1.975 1.952 1.975 35.6 [34]

a
IR obtained in KBr disks (cm−1).
b
X-band frequencies, in toluene, anisotropic gi at 77 K.
c
Isotropic Žg values at 298 K.
d 95,97
Mo, ×10−4 cm−1, isotropic ŽA values at 298 K.

the halide atom versus the dithiolate donor ligands. Table 3 presents selected interatomic distances and
This trend in w(Mo O) stretching frequencies reflecting bond angles. The molecule exhibits a distorted pseudo-
the stronger p-donor ability of equatorial sulfur donor octahedral coordination geometry, where the Mo atom
atoms has been observed between a variety of is ligated by a terminal oxo atom, two sulfur donor
analogous thiolate and halide oxo-Mo systems [27,31]. atoms of the bdtCl2 ligand and three nitrogen atoms of
The X-band CW EPR spectra of (Tp*)MoVO- the tridentate facially coordinated Tp* ligand. No sym-
(bdtCl2), measured in both frozen (77 K) and fluid (298 metry is imposed on 3 by this space group, but the
K) toluene solutions, show that only one EPR-active effective symmetry is Cs. The terminal oxo ligand and
molybdenum species is present. The fluid solution EPR the two sulfur donor atoms of the ene-1,2-dithiolate
spectrum of (Tp*)MoO(bdtCl2) at room temperature chelate ligand (bdtCl2) are constrained to be mutually
displayed a single strong signal and six surrounding cis to each other by the fac stereochemistry imposed by
satellite lines that are characteristic of a mononuclear the tridentate Tp* ligand. This is the third such struc-
Mo(V) center with the naturally abundant distribution turally characterized six-coordinate oxo-Mo(V) com-
of isotopes and with Žg =1.973 and 95,97Mo ŽA = plex that contains a single ene-1,2-dithiolate ligand cis
38.1 ×10 − 4 cm − 1 [18,21,27,28,32 – 35]. The isotropic to the terminal oxo group. The structures of
Žg value obtained from the room temperature solution (Tp*)MoO(bdt) [20,21] and (Tp*)MoO(qdt) [36] have
spectrum agrees well with the mean of the anisotropic gi been previously reported.
values, gi/3= 1.971, measured directly from the frozen The structural parameters of the {(Tp*)MoO}2 +
solution EPR. Table 2 compares the EPR parameters core in (Tp*)MoO(bdtCl2) agree with those found for
for (Tp*)MoO(bdtCl2) to related (Tp*)MoO(S S) com- other oxo-Mo(V) complexes [18,20,21,29,32,33,36–41].
plexes. The rhombic frozen glass EPR spectrum of ,
The observed Mo O distance (1.679(3) A) is similar to
(Tp*)MoO(bdtCl2) exhibits parameters that are quite ,
that of (Tp*)MoO(bdt) (1.678(4) A) [20,21], (Tp*)Mo-
similar to related (Tp*)MoO(S S) complexes [18,21,35]. ,
O(qdt) (1.6865(18) A) [36], and (Tp*)MoO(SPh)2
However, (Tp*)MoO(qdt) (4), which contains a quinox- ,
(1.676(4) A) [18]. The terminal axial oxo ligand, as
aline dithiolate rather than a substituted benzene dithi- expected, exerts a strong trans influence, lengthening
olate has a somewhat larger g3 value (Table 2) [35]. , ,
the Mo N31 distance (2.391(3) A) by 0.214 A relative
Nonetheless, the EPR data of Table 2 further support
the view that the EPR parameters of oxo-Mo centers
are primarily determined by the inner coordination
environment of the molybdenum center [18]. For these
d1 (Tp*)MoO(S S) complexes containing monoene-1,2-
dithiolate coordination, these EPR results indicate that
the unpaired electron is localized in a HOMO that is
predominantly metal dxy in character, and whose or-
bital composition is relatively unperturbed by remote
substituents [18,19,28].

3.2. X-ray structure analysis of
(Tp*)MoO(bdtCl2) ·1 /2(C6H14)
Fig. 3. The ORTEP drawing of (Tp*)MoO(bdtCl2) (3). The atoms are
The structure of (Tp*)MoO(bdtCl2) (3), determined drawn as 50% probability ellipsoids. H-atoms have been made arbi-
by single-crystal X-ray diffraction, is shown in Fig. 3. trarily small for clarity.


Table 3 The most significant difference between the structures
Important bond lengths ,
(A) and bond angles (°) for
of (Tp*)MoO(bdtCl2) (3) and (Tp*)MoO(bdt) (1) is the
(Tp*)MoO(bdtCl2) (3)
angle (l) formed between the ene-1,2-dithiolate
Bond lengths Bond angles (S C C S) least squares plane and the S Mo S plane
along the line of intersection containing the S···S atoms
Mo(1) O(1) 1.679(3) O(1) Mo(1) N(11) 91.60(12) (see Fig. 4). This angle in 3 is 173.1°, and thus the
Mo(1) N(11) 2.166(3) O(1) Mo(1) N(21) 91.87(12)
S C C S plane is folded up towards the terminal oxo
Mo(1) N(21) 2.177(3) N(11) Mo(1) N(21) 86.17(11)
Mo(1) S(2) 2.3683(12) O(1) Mo(1) S(2) 100.99(10)
group by q= 180° –l= 6.9°. The (bdtCl2) ring structure
Mo(1) S(1) 2.3805(12) N(11) Mo(1) S(2) 94.92(8) itself is essentially planar. The fold angle for
Mo(1) N(31) 2.391(3) N(21) Mo(1) S(2) 167.05(8) (Tp*)MoO(bdt) is considerably greater with q=
S(2) C(2) 1.750(4) O(1) Mo(1) S(1) 101.81(10) 21.3(1)° [20]. This deformation in (Tp*)MoO(bdt) has
S(1) C(1) 1.753(4) N(11) Mo(1) S(1) 166.47(8)
been previously attributed to a steric interaction be-
C(2) C(1) 1.406(5) N(21) Mo(1) S(1) 91.54(8)
S(2) Mo(1) S(1) 84.38(4) tween the C36 methyl group of the trans N31 pyrazolyl
S···S bite 3.190 O(1) Mo(1) N(31) 166.79(11) ring and the C1 and C2 atoms of the chelate ring [21].
distance The C36···C1 and C36···C2 non-bonded contact dis-
Mo distance 0.258 N(11) Mo(1) N(31) 78.40(11) ,
tances in (Tp*)MoO(bdt) are 3.58 and 3.57 A, respec-
from N2S2
tively, and significantly greater than the corresponding
plane
,
distances of 3.375 and 3.345 A in (Tp*)MoO(bdtCl2).
The estimated van der Waals contact between a methyl
,
group and an aromatic ring is 3.72 A [42]. Thus,
non-bonded contact in (Tp*)MoO(bdtCl2) would seem-
ingly favor a larger fold angle, as was observed in
(Tp*)MoO(bdt). The packing of the molecules in the
unit cell for each of these systems was also examined
and it was determined for 3 that the C36 methyl groups
from neighboring molecules were well separated from
,
the C1 and C2 atoms (greater than 8 A). The packing
diagram showed that the closest Cl···Cl distance be-
Fig. 4. Definition of the S S fold angle (q). ,
tween neighboring molecules was 3.920 A, greater than
,
the van der Waals contact of 3.60 A for chlorines [43].
, ,
to Mo N21 (2.177(2) A) and 0.225 A relative to The results for the packing diagram analysis suggest
,
Mo N11 (2.166(3) A). This effect is slightly greater that non-bonded intermolecular contacts are not the
than that observed in the (Tp*)MoO(bdt) complex cause of the observed fold angle deviation. The origin
where the Mo N31 bond was found to be 2.372(4) A , and contributions to this fold angle in (Tp*)MoO(bdt),
[20]. The average Mo S distance (2.3744(12) A) in , (Tp*)MoO(bdtCl2), and (Tp*)MoO(qdt) are now being
(Tp*)MoO(bdtCl2) is indistinguishable from that ob- investigated through DFT calculations [44]. Compari-
,
served in (Tp*)MoO(bdt) (2.373(2) A). These Mo O son of the bond distances within the S C C S chelate
and Mo S bond length comparisons for (Tp*)Mo- rings of 1 and 3 shows that the C1 C2 distance of 1 of
O(bdtCl2) and (Tp*)MoO(bdt), clearly show that these ,
1.395(8) A and the distance in 3 of 1.406(5) A are ,
structural parameters are not significantly perturbed by similar. The average S C distances of 1.760(6) in 1 and
remote ligand substituent effects. The bond angles ,
1.751(4) A in 3 are also not significantly different from
about the six-coordinate Mo atom deviate significantly one another.
from that of octahedral geometry, as illustrated by In summary, the crystallographic structures for three
O Mo S1 =101.81(10)° and O Mo S2 = 100.99(10)°. different (Tp*)MoO(S S) systems (1, 3, 4) show that
The average O Mo S angle of 101.40° is slightly peripheral substituents on the ene-1,2-dithiolate chelate
greater than that reported for (Tp*)MoO(bdt) (100.95°) induce no significant structural changes of the inner
[20]. The molybdenum atom is displaced 0.258 A , coordination sphere of the molybdenum center. These
above the mean N2S2 equatorial plane in the direction results are consistent with other studies of Mo-ene-1,2-
of the apical oxo ligand. This is slightly less than dithiolates obtained by X-ray and EXAFS that show
,
observed for (Tp*)MoO(bdt) at 0.264 A [20]. The little differences in Mo S distance with changes of the
S Mo S chelate bite angle of 84.38(4)° is tighter by substituents on the ene-1,2-dithiolate [45]. However, the
0.74° relative to the corresponding angle (85.12(6)°) in substantial difference in the fold angle (q) of the ene-
(Tp*)MoO(bdt) [20]. The S S non-bonded contact dis- dithiolate chelate ring between (Tp*)MoO(bdtCl2) and
,
tance was calculated to be 3.190 A; this is only 0.021 A , (Tp*)MoO(bdt), warrants further investigation. The
smaller than that observed for (Tp*)MoO(bdt) (3.211 structures of 1 and 3 suggest that this geometric feature
,
A). may be related to subtle differences in the overall


electronic structure of the (Tp*)MoO(S S) systems. It is tral features observed for 1 and 3 provides the basis for
possible that this fold angle may play an important the same band assignments being transferred to 3. The
regulatory role in the reactivity of pyranopterin Mo transition energies, intensities and band assignments of
enzymes during catalytic turnover. DFT calculations are the Gaussian resolved absorption spectra of 3 and 1 are
in progress to determine the effects of this fold angle on presented in Table 4 for comparative purposes. The
electronic structure in these well characterized most important spectral feature to recognize is the band
(Tp*)MoO(S S) compounds [44]. centered at 19 000 cm − 1 in these (Tp*)MoO(S S)
compounds, which has been assigned as the in-plane (ip)
3.3. Electronic absorption spectroscopy pseudo-s (Sip “ Mo dxy ) CT transition [16].1 The inten-
sity of this Sip “ Mo dxy transition probes the dominant
The 298 K electronic absorption spectrum of 3 in covalency contributions between the Mo dxy redox ac-
dichloroethane is shown in Fig. 5, and is very similar to tive orbital and the dithiolate Sip molecular orbitals
those observed for other (Tp*)MoO(S S) complexes [16].2,3 The charge-transfer intensity of the Sip “Mo dxy
[16–21,23]. The observed band patterns appear to be transition is observed to be very similar between 1 and
characteristic of the (Tp*)MoO(S S) complexes re- 3, which indicates nearly equivalent pseudo-s mediated
ported to date, which possess a five-membered chelate charge donation (covalency) between the in-plane ene-
ring formed between the ene-1,2-dithiolate and molyb- 1,2-dithiolate donor orbitals of the bdt and bdtCl2
denum atom [16,19]. Detailed electronic structure stud- ligands and the oxo-Mo dxy acceptor orbital. The similar
ies of (Tp*)MoO(bdt) (1) and (Tp*)MoO(tdt) (2) have energies of this transition reflect that the strength of this
resulted in the assignment of observed absorption bands bonding interaction, and thus the covalent destabiliza-
below 26 000 cm − 1 as S“ Mo CT transitions [16,19]. tion of the Mo dxy redox orbital, remains essentially
invariant and is little perturbed by remote substituent
The absorption features below 26 000 cm − 1 in 3 are
effects on the benzene dithiolate ring. Another interest-
nearly identical in energies and intensities relative to the
ing aspect of the absorption spectra of 1 and 3 is that
observed bands in 1. The close correspondence in spec-
the energy and intensity for the Sop “ Mo dxz,yz CT
transitions that occur at higher energy are not signifi-
cantly different for the two compounds. The very similar
absorption spectra observed among 1, 3 and 4 suggests
these complexes possess nearly identical electronic struc-
tures that are not significantly perturbed by remote
substituents on the benzene ring. This conclusion is also
consistent with the near identity of their EPR parame-
ters (Table 2). However, the slight differences observed
in the spectra of 4 with respect to 1 are perhaps
reasonable because the qdt ligand possesses a benzene
ring fused with a quinoxaline ring containing nitrogens
[16,17,23]. The electron-donor properties of the ene-1,2-
dithiolate have been discussed in detail with respect to
the observed differences in the spectroscopic and electro-
chemical data between 2 and 4 [23]. The origin of these
Fig. 5. The 293 K electronic absorption spectrum of spectral differences and their relationship to reduction
(Tp*)MoO(bdtCl2) (3) in 1,2-dichloroethane ( 10 − 4 M). potentials and ionization potentials remain an important
area of research.
Table 4
Electronic absorption spectral data a
3.4. Solution reduction potentials
Band (Tp*)MoO(bdt) (1) b (Tp*)MoO(bdtCl2) (3)
assignment The electrochemical properties of 3 have been exam-
ined by cyclic voltammetry (CV) and are summarized in
Energy m Energy m
(cm−1) (M−1 cm−1) (cm−1) (M−1 cm−1)

Sop “ Mo dxy 9100 360 9700 440 1
The pseudo-s interaction refers to the three-center bonding inter-
Sop “ Mo dxy 13 100 270 13 200 330 action that occurs between the Mo dxy based orbital and the in-plane
Sip “ Mo dxy 19 400 sh, 1220 19 400 sh, 1080 Sps LCAO of the dithiolate chelate.
Sop “ Mo dxz,yz 22 100 sh, 2380 22 500 sh, 1370 2
Sip and Sop refer to primarily sulfur containing molecular orbitals
Sop “ Mo dxz,yz 25 100 5280 25 000 5530 of a dithiolate chelate oriented parallel and perpendicular to the plane
of the dithiolate chelate, respectively.
a 3
Gaussian resolved data, dichloroethane solutions, sh = shoulder. The intensity of a CT transition is proportional to the square of
b
Data taken from Ref. [16]. the overlap integral Ž€M

€L2 as described by E. Solomon [46].


Table 5
Summary of electrochemical data for (Tp*)MoO(S S) systems a,b

Complex Mo(VI/V) Mo(V/IV)

E1/2 (mV) DEp (mV) ipc/ipa E1/2 (mV) DEp (mV) ipc/ipa

(1) (Tp*)MoO(bdt) 534 74 1.160 −642 74 0.9654
(2) (Tp*)MoO(tdt) 462 76 1.051 −650 76 0.9516
(3) (Tp*)MoO(bdtCl2) 742 64 2.589 −508 77 0.9969
(4) (Tp*)MoO(qdt) c −422

a
Conditions: cyclic voltammetry, 100 mV s−1, 0.5–1.0 mM sample, 0.1–0.2 M Bu4NBF4 in 1,2-dichloroethane.
b
Potentials vs. ferrocene/ferrocenium couple.
c
Data from Ref. [23].

Table 5. The cyclic voltammograms of 3 and of related to the oxidation of the coordinated toluene-3,4-dithio-
(Tp*)MoO(S S) compounds listed in Table 5 exhibit late ligand in a separate study of 2 [49]. The more
both a quasi-reversible one-electron reduction wave and positive potential for the oxidation process of 3 relative
a quasi-reversible one-electron oxidation wave in to 1 is consistent with their relative reduction poten-
dichloroethane solution (25 °C) [47]. The chemical re- tials. This correlation suggests that 4 would be the most
versibility determined by peak current ratios are near difficult to oxidize and that the potential could be well
unity (ipa/ipc :1.0), but the anodic to cathodic peak outside of the accessible solvent window. A previous
potential separations (DEp in Table 5) are somewhat electrochemical study of 4 in 1,2-dichloroethane did not
larger than the ideal value of 59 mV expected for a exhibit a well-defined quasi-reversible oxidative wave
completely reversible process. The quasi-reversibility of within the potential window for this solvent [23]. Stud-
the reduction and oxidation couples indicates that min- ies are underway to investigate in greater detail the
imal structural rearrangement occurs during the electro- electrochemical properties of the (Tp*)MoO(S S) com-
chemical processes. plexes listed in Table 5 [47].
The well-defined reduction wave of 3 occurs at − 508 The electrochemical potentials in the (Tp*)MoVO-
mV versus the Fc/Fc+ reference couple and can be (S S) complexes listed in Table 5 shift significantly as
assigned to the Mo(V)/Mo(IV) redox couple that is a the nature of the ene-1,2-dithiolate (S S) ligand
characteristic feature typical of the {(Tp*)MoVO}2 + changes. The electrochemical reduction or oxidation of
complexes possessing various equatorial donor ligands 3 and related (Tp*)MoVO(S S) compounds results for-
[18]. The reduction potential of 3 is more positive than mally in the addition or removal of an electron from
that for 1 and 2, but less positive than that for 4. The the HOMO, primarily Mo (dxy )1. The redox potential
overall trend of Ered is 2 B1 B 3 B4. data of the (Tp*)MoVO(S S) series of complexes shows
The cyclic voltammograms of 1 and 2 are unusual that the ease of reduction and difficulty in oxidation
with respect to the majority of {(Tp*)MoVO}2 + com- increases as the ene-1,2-dithiolate ligand becomes in-
pounds investigated in that they also exhibit a well- creasingly more electron-withdrawing. The more posi-
defined quasi-reversible oxidation wave at a potential of tive reduction potential for 3 relative to 1 suggests a
+ 534 and +462 mV versus the Fc/Fc+ internal refer- decreased electron density on the Mo(V) center of 3 due
ence couple, respectively. A similar oxidative wave (+ to the electron-withdrawing nature of the bdtCl2 ligand,
732 mV) was observed for 3, but was ill-defined as it which decreases the electron density on the sulfur donor
occurred at the edge of the solvent window. The exact atoms coordinated to Mo, thereby stabilizing the
nature of this oxidative wave remains to be determined. HOMO and making reduction easier as compared to
One reasonable possibility is the formal one-electron the unsubstituted bdt ligand. Likewise, the relatively
Mo(V/VI) oxidation of the metal center. However, the greater density on the sulfur donor atoms in 1 destabi-
oxidation could also be centered on the sulfur donor lize its HOMO energy relative to 3 and make oxidation
atoms with no formal change in Mo oxidation state. A of 1 easier than 3. Similar remote ligand effects on the
third possibility is a coupled internal redox process in electron-donor properties of the ene-1,2-dithiolate lig-
which the S centers are effectively oxidized by two and have been discussed in previous spectroscopic stud-
electrons and the Mo reduced by one-electron to ies of 4 in order to understand the large positive ( 220
Mo(IV). Such internal redox reactions have been ob- mV) shift in its reduction potential relative to 2 [23].
served in other Mo S systems [48]. This oxidation wave However, it is not quite clear from the spectroscopic
was not observed in previous studies of 1 and 2 in data, why there is such a shift in reduction potential
acetonitrile solutions [18,20]. However, a reversible one- ( 150 mV) between 1 and 3. The similar absorption
electron oxidation at 1.00 V (in CH3CN) was assigned intensity observed for the Sip “ Mo dxy CT transition in


these two (Tp*)MoO(S S) complexes indicates that the and 3. Detailed studies employing DFT calculations are
nature of this in-plane bonding interaction is essentially underway to further probe the electronic structure con-
equivalent, and therefore cannot be the origin of the tributions to the reduction potentials and the nature of
significant reduction potential differences observed be- the donor/acceptor orbitals in 3 and related
tween these complexes. Unlike the absorption spectrum (Tp*)MoO(S S) complexes [44].
of 4, the intensities of the Sop “Mo dxz,yz CT transi-
tions in 1 and 3 are nearly the same, indicating no 3.5. Gas-phase photoelectron spectroscopy
significant change in the nature of Mo-dithiolate bond-
ing between 1 and 3. Thus, it appears that electronic The gas-phase He I valence photoelectron spectra
structure contributions to the valence ionization energy (5.7–15.7 eV) of 1 and 3 are presented in Fig. 6. The
(VIE) of the ground-state HOMO (covalent destabiliza- ionizations exhibited in the region above 8.2 eV are
tion of the redox orbital and reduction of the Mo characteristic of the {(Tp*)MoVO}2 + core observed in
effective nuclear charge via electron-donating ability of the PES of a variety of complexes possessing equatorial
the dithiolate) does not play a dominant role in the oxygen, halide and sulfur donor ligands studied previ-
observed reduction potential differences between 1 and ously [22,23,50,51]. The ionizations in this spectral re-
3. Solvation effects and changes in the reorganization gion for 1 and 3 are associated primarily with removal
energy of the complex and the solvent can also con- of electrons from the orbitals of the (Tp*) ligand, the
tribute to the potential differences observed between 1 benzene ring, and lone pairs of the oxo and chloride
atoms. The large number and highly overlapping nature
of these ionizations make unambiguous assignment of
the spectral features in this region difficult. However,
the spectral differences between 1 and 3 in this region
are mainly due to the chloride lone pair ionizations at
11 eV in 3, which are absent in the spectrum of 1.
The greatly reduced intensity of this particular ioniza-
tion in the He II spectrum of 3 supports this assignment
[44]. The spectra of 1 and 3 show two ionizations below
8.2 eV that are energetically isolated from the ligand-
based bands. The two spectral features in this region
are similar to those observed in 2 and 4 [23]. The
ionization energies for these two bands observed in 3
and related (Tp*)MoO(S S) compounds are presented
in Table 6. The energies of the first ionization band for
the (Tp*)MoO(S S) series increase in the order, 2B
1B 3B 4. The first band has been attributed to the
ionization of the Mo (dxy )1 electron from these formally
Mo(V) compounds [22,23]. Thus, the valence ionization
energy of the Mo (dxy ) orbital is of particular interest
as it is the active orbital associated with the metal-cen-
Fig. 6. He I photoelectron spectroscopy of (Tp*)MoO(bdtCl2) (3) tered redox processes occurring in these oxomolybde-
(top) and (Tp*)MoO(bdt) (1) (bottom). The first ionization, centered num monoene-1,2-dithiolate complexes. The ionization
around 7.2 eV, is associated with metal and sulfur based molecular energies show that the first band in 3 is stabilized by
orbitals. Complex 3 has ionizations at 11 eV assigned to chlorine
based orbitals. Both complexes show pyrazolyl ring based ionizations
0.29 and 0.16 eV relative to 2 and 1, respectively. The
centered around 9 eV, as previously assigned [50,51]. position of the first band in 3 has been found to be
destabilized by 0.48 eV from the first ionization energy
Table 6 of 4. These results clearly demonstrate the effects of
Comparison of He I gas-phase spectral data for (Tp*)MoO(S S) remote ligand substituents on the ionization energies of
systems the metal-based HOMO. The shifts in these gas-phase
Complex Ionization energy (eV, 9 0.2 eV)
ionization energies follow trends in redox potentials
measured in solution. Thus, the increasing electron
Band A Band B Reference withdrawing properties of the dithiolate ligand and ease
in reduction of 3 relative to 1, are consistent with the
(1) (Tp*)MoO(bdt) 7.08 7.57 This work, [44] increase in ionization energy (stabilization) of the
(2) (Tp*)MoO(tdt) 6.95 7.5 [22,23]
(3) (Tp*)MoO(bdtCl2) 7.24 7.66 This work, [44]
HOMO in 3 compared to that observed in 1. Fig. 7
(4) (Tp*)MoO(qdt) 7.72 8.04 [23] presents the equation for the least-squares line of com-
pounds 1, 2, and 3, where IE (first ionization energy,


olate. Solvation effects, reorganizational energy
changes, and covalent reduction of the effective nuclear
charge of the Mo ion due to charge transfer differences
involving higher energy acceptor orbitals are currently
being investigated as possible contributions to redox
potential differences. The crystal structures now avail-
able for compounds 1, 3, and 4 provide a framework
for high-level DFT calculations to be initiated and
evaluated within the context of existing spectroscopic
and electrochemical data [44]. Such calculations should
provide detailed insight into the geometric and elec-
tronic structures of these oxo-Mo monoene-1,2-dithio-
late centers and help to define structure/function
correlations for the active sites of the pyranopterin Mo
Fig. 7. Correlation of half-wave oxidation potential with lowest enzymes.
energy ionization potential for the (Tp*)MoO(S S) compounds of
Table 6.
4. Supplementary data
eV) = 0.976(Eox, V) +6.52 with a correlation coefficient
of 0.977. These data clearly show a correlation between Crystallographic data for the structural analysis have
the first gas-phase ionization energy and the solution been deposited with the Cambridge Crystallographic
oxidation potential, and indicates that the nature of Data Centre, CCDC No. 174598. Copies of this infor-
these processes is similar. The second ionization band mation may be obtained free of charge from The
in the (Tp*)MoO(S S) complexes has no counterpart in Director, CCDC, 12 Union Road, Cambridge, CB2
the spectra of analogous alkoxide or diolato com- 1EZ, UK (fax: +44-1223-336-033; e-mail: deposit@
pounds [50,51] and has been tentatively assigned to the ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
removal of electrons from dithiolate-based orbitals lo-
calized on the sulfur donor atoms of the ene-1,2-dithio-
late chelate. The PES data presented in Table 6 here for Acknowledgements
3 and other (Tp*)MoO(S S) complexes listed promise
considerable insight into Mo S covalency and the pos- The X-ray diffraction studies were carried out using
tulated role of the ene-1,2-dithiolate as an electronic the facilities of the Molecular Structure Laboratory,
buffer [22]. However, additional data are required from Department of Chemistry, University of Arizona under
multi-wavelength sources and high-level DFT calcula- the direction of Dr. Michael Carducci; mass spectra
tions in order to analyze in detail the observed spectral were recorded at the University of Arizona Mass Spec-
features and this work is currently in progress [44]. trometry Facility; EPR studies were carried out at the
University of Arizona under the direction of Dr.
3.6. Summary Arnold Raitsimring. PES studies were carried out at the
Center for Gas-Phase Electron Spectroscopy, Univer-
The synthesis and characterization of the new com- sity of Arizona under the direction of Dr. Nadine
plex, (Tp*)MoO(bdtCl2) (3) have provided an addi- Gruhn. We thank Professor Martin Kirk and Nick
tional oxo-Mo monoene-1,2-dithiolate system for direct Rubie at the University of New Mexico for helpful
comparison with the structurally characterized com- discussions and quantitative absorption spectral data.
pounds (Tp*)MoO(bdt) (1) and (Tp*)MoO(qdt) (4). We gratefully acknowledge support by the National
The most significant structural difference for 3 is the Institutes of Health (Grant GM-37773).
much smaller fold angle (q) between the MoS2 and
S C C S planes. Variations in q may modulate the
References
electronic structure of these systems and could play an
important regulatory role in the pyranopterin Mo en- [1] R. Hille, Chem. Rev. 96 (1996) 2757.
zymes during catalysis. The solution redox potentials [2] C. Kisker, H. Schindelin, A. Pacheco, W.A. Wehbi, R.M. Gar-
and the gas-phase ionization energies clearly demon- rett, K.V. Rajagopalan, J.H. Enemark, D.C. Rees, Cell 91 (1997)
strate the sensitivity of the (Tp*)MoO(S S) system to 973.
remote ligand effects. However, IR, EPR, and elec- [3] M.J. Romao, M. Archer, I. Moura, J.J.G. Moura, J. LeGall, R.
˜
Engh, M. Schneider, P. Hof, R. Huber, Science 270 (1995) 1170.
tronic absorption spectroscopies suggest that the elec- [4] R. Huber, P. Hof, R.O. Duarte, J.J.G. Moura, I. Moura, M.Y.
tronic structure of 1 and 3 remains relatively Liu, J. LeGall, R. Hille, M. Archer, M.J. Romao, Proc. Natl.
˜
unperturbed by peripheral ligation to the ene-1,2-dithi- Acad. Sci. USA 93 (1996) 8846.

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