This document discusses the synthesis of novel weakly coordinating anions derived from closo-heteroboranes. It proposes halogenating and methylating the 12-vertex closo-heteroboranes [PB11H11]- and [NB11H11]- to obtain [PB11Me11]-, [PB11X11]- (X = Cl, Br), [NB11X11]- (X = Cl, Br) and [NB11Me11]-. These anions will be characterized using various analytical techniques and their stability explored. Additionally, [PB11X11]- and [NB11X11]- will be used to synthesize [AlEt2][PB11X11]- and [Al

1. 1
Statement
Weakly coordinating anions play a very important role in Lewis acid/base chemistry.
Carborane [CH12B11]-
and its derivatives are examples of some of the best weakly
coordinating anions. However, there are several limitations, mainly in the synthesis of the
carborane. To tackle these problems, the syntheses of weakly coordinating anions [PB11Me11]-
,
[PB11X11]-
(X = Cl, Br), [NB11X11]-
(X = Cl, Br), and [NB11Me11]-
will be attempted.
Thermogravimetric analysis, UV-vis, infrared spectroscopy, mass spectrometry analysis,
X-ray diffraction and nuclear magnetic resonance spectroscopy will be used for detecting the
stability and structures of these anions. The anions [PB11X11]-
(X = Cl, Br) and [NB11X11]-
(X
= Cl, Br) are applied to synthesize [AlEt2][PB11X11]-
(X = Cl, Br) and [AlEt2][NB11X11]-
(X =
Cl, Br) and these compounds will be used for catalyzing CO2 reduction.

2. 2
1. Introduction
In the past 50 years, several very weakly coordinating anions have been used to stabilize
reactive cations (e.g. [R3C]+
, [S8]2+
,[HCO]+
, [N5]+
, [AuXe4]2+
, etc)[1]
in the electrophilic
chemistry field. Triflate [OSO2CF3]-
, tetrakis(3,5-bis(trifluoromethyl)phenyl)borate [BArF4]-
,
and perfluorotetraphenylborate [F20BPh4]-
are prime examples of weakly coordinating
anions[2]
. They show low nucleophilicity, chemical inertness, good solubility, leaving group
lability and weak coordination.
In the past decade, the carborane anion has been recognized as a new class of weakly
coordinating anions. They are based on the stable boron cluster framework such as [CB11H12]-
,
[CB11H6X6]-
(X = Cl, Br, I),[2]
[CB11X11]-
(X = Cl, Br, I) (Scheme 1) and [CH3-CB11X11]-
(X =
Cl, Br, I).[3]
The cage [CB11H12]-
, is basically an electron-deficient structure. According to
Wade’s rule, [CB11H12]-
has 50 valance
electrons, and 26 of the 50 valence electrons
belong to the skeletal bonds. This
icosahedron structure (Scheme 1) has 30
skeletal bonds. It suggests that there are not
enough valence electrons for the skeletal bond to make 2-center-2-electron bonds.[4]
Therefore, these 13 pairs of valence electrons are delocalized in the cluster, and the borons
are highly connected together. As a result, [CB11H12]-
is non-nucleophilic and shows low
electron density in the skeletal bonds. It also shows very good thermal and chemical
stabilities. Especially [CB11H6X6]-
and [CB11X11]-
(X =Cl, Br, I), are much larger, more
chemical inert and less nucleophilic compared to [CB11H12]-
. Two common ways to
BB
B
B
B
B
B
B
B
B
H
C
B
XX
XX
X X
X
X
X
XX X= H, Cl, Br, I
Scheme 1. Structure of [CB11X11]-

3. 3
Sn
I II2, -78 0
C
[SnB11H11]2-
2-
Scheme 2.
synthesize [CB11H12]-
are given in the literature procedures[4]
. One is boron insertion. This
way synthesizes CB10 precursor from B10H14 and NaCN. The CB10 precursor is then reacted
with Et3NBH3 to afford [CB11H12]-
. Another way is carbon insertion, where [B11H14]-
is
reacted with CHCl3 to afford [CB11H12]-
. There are several limitations to the synthesis of
[CB11H12]-
. Generally, the carbon insertion step is the most common way and can get a 40%
yield on a small scale, but only 20~25% yield for a 5-50 g scale.[4][5]
Boron insertion can
approach a ~65% yield, but it is an expensive synthesis either as alternative precursors for
[CB11H12]-
or as alternative WAC’s.[4][6]
As a result, the focus has been on other 12-vertex closo-heteroboranes. Metallaboranes, a
type of 12-vertex closo-heteroboranes [EB11H11]-
(E = AlCH3, PbCH3, As, Bi, Sb)[7][8][9]
, have
been reported for a while. My group was interested in [CH3SnB11H11]-
and [CH3GeB11H11]-
.[7]
These two anions have been prepared in good yield, up to 80 %, starting with the [B11H14]-
anion, SnCl2 or GeI2, and n-butyllithium. The yields of the related anions [EB11H11]-
(E = As,
Sb, Bi) are generally lower than 25%. Unfortunately, the first two anions are not as stable as
the [CHB11H11]-
anion in acidic condition, and they decompose in dilute HCl acid (0.5~1 M).
Therefore, [CH3SnB11H11]-
and [CH3GeB11H11]-
have only limited use as weakly coordinating
anions. Halogenation[3][6][10]
or methylation,[11]
which can further balance the electron density
of the carborane, is a proposed way of solving this problem. One report shows the
halogenation of metallaboranes
by iodine (Scheme 2)[12]
, but no
reference reports a high grade of
halogenations of the

4. 4
metallaboranes (e.g. from [MeSnB11H11]-
to [MeSnB11Hal11]-
(Hal= Cl, Br, I)).
Stabilization and yield of the metallaboranes should be a very good point of interest. The
methylation of the carborane anion was reported in 1996.[11]
It is also a good way to minimize
the nucleophilicity and to stabilize the carborane anions. For example, the anion [CB11Me12]-
shows high stability in basic conditions, and dilute acids. In addition, the Li+
salt of
[CB11Me12]-
is highly soluble in chloroform, carbon tetrachloride and toluene. [CB11Me12]-
has been reported in some catalytic applications such as pericyclic rearrangements and radical
grafting reactions.[13][14]
For these reasons, SO2Cl2 has been tried to chlorinate [CH3SnB11H11]-
and
[CH3GeB11H11]-
. Unfortunately, [CH3SnB11H11]-
and [CH3GeB11H11]-
are not as stable as
[CB11H12]-
. The CH3Sn- and CH3Ge- groups are cleaved off before the reaction is finished. It
is proposed that [CH3SnB11H11]-
and [CH3GeB11H11]-
are more reactive than [CB11H12]-
, and
sensitive to acidic conditions. Oxalyl chloride seemed like another way to chlorinate the
metallaboranes, since it is a milder chlorinating agent. However, [CH3SnB11H11]-
and
[CH3GeB11H11]-
also decomposed. The Sn-B bonds (2.288~2.306Å)[7]
and Ge-B bonds
(2.160~2.231)[15]
are significantly longer than C-B bonds (1.723~1.725Å) from [CB11H12]- [5]
.
As a result, it is easier for the metals to be attacked by HCl that is generated by the
chlorination reaction. Bromination and iodation will also generate strong acid (HBr and HI)
that will decompose the [CH3SnB11H11]-
and [CH3GeB11H11]-
anions. The other
metallaboranes [EB11H11]-
(E = AlCH3-, PbCH3-, As, Bi, Sb) are most likely also not good
candidates to undergo a halogenation. Their M-B (M = Al, Pb, As, Bi, Sb) bonds distances
are long and the bond weak. For example, the Al-B bonds in [CH3AlB11H11]-
range from

5. 5
2.131~2.140Å[8]
. [CH3AlB11H11]-
is also sensitive to air and decomposes in water.
Another type of 12-vertex closo-heteroboranes for group 15 elements has been reported,
namely [PB11H11]-
(1993)[16]
and [NB11H11]-
(1991)[17]
which attracted my attention.
[PB11H11]-
is synthesized from [B11H14]-
, n-butyllithium and PCl3, but the yield is about 29%.
[NB11H11]-
is synthesized from nido-NB10H13 and Et3NBH3 and the yield is 47%. For
application as a weakly coordinating anion, it should be chemically inert. Since the 12-vertex
elements P and N have lone pair electrons that can donate electrons easily, they are not
suitable candidates to be weakly coordinating anions that are very stable and
non-nucleophilic. Halogenation or methylation is a key to stabilize and neutralize
nucleophilicity of these anions. There is an example in the literature: [HNB11Cl5I6]-
. The
X-ray structure and space filling model of [NB11Cl5I6]-
(Figure 1 and Figure 2)[17]
indicate
that the lone pair of the N atom can be hidden and protected perfectly. Therefore, the lone
pair of the P atom in the boron cage may also be protected by halogens
Figure 1 ORTEP drawing of [HNB11Cl5I6]

6. 6
Figure 2 Space filling of [NB11Cl5I6]-
Compared with metallaboranes, these anions should be undergo halogenation or
methylation more easily. Their crystal structures show that the distances for P-B (2.048
Å)[16]
and N-B (1.686 Å)[17]
bonds are relatively short. For HNB11H11 and MeNB11H11, the
structures are very stable. Some halogenation derivatives have been reported such as
HNB11Cl5I6, MeNB11H10Br and HNB11H10I. It has not been fully chlorinated, brominated, or
methylated. The [PB11H11]-
anion is another example of a 12-vertex substituted metallaborane
cage, the properties of which have not been investigated. However, the crystal structure
shows that the P-B bond is short, and it should be able to be halogenated or methylated.
The proposal is interested in halogenating and methylating the [PB11H11]-
, [NB11H11]-
12-vertex closo-heteroboranes. [PB11Me11]-
, [PB11X11]-
(X = Cl, Br), [NB11X11]-
(X = Cl, Br)
and [NB11Me11]-
will be planed to synthesize. Meanwhile, these anions’ stability and
solubility are going to be explored. Furthermore, [PB11X11]-
(X = Cl, Br) and [NB11X11]-
(X =
Cl, Br) will be used to synthesize [AlEt2][PB11X11]-
(X = Cl, Br) and [AlEt2][NB11X11]-
(X =
Cl, Br). Then those compounds will be used to catalyze CO2 reduction with hydrosilanes such
as Et3SiH.[19]

7. 7
2. Project Description
2.1 Experiment Design
2.1.1 General Procedures.
All work has to be performed under anaerobic and anhydrous conditions by using either
modified Schlenk techniques or a Vacuum Atmosphere glove-box. Solvents were dispensed
from a commercial solvent purification system. The compounds [Me3NH][B11H14],
Cs[PB11H11] and Cs[NB11H11] have been prepared according to the literature
procedures.[8][16][17]
All other reagents were obtained from commercial supplies and used as
received. All these reactions can be monitored by 11
B NMR spectroscopy. NMR spectra were
recorded on a Bruker Avance 400 MHz spectrometer. 11
B NMR chemical shift values were
determined by the standard reference BF3·OEt2 in CDCl3. 1
H NMR chemical shift values
were determined relative to the residual protons in acetone-D6 or DMSO-D6 as internal
reference (δ 2.05 (5) or 2.49(5) ppm). 13
C NMR spectra were referenced to the solvent signal
of acetone-D6 at δ 29.84, 206.26 ppm or DMSO-D6 at δ 39.52 ppm. The ATR-FTIR spectra
were collected on a Nicolet IR200 FT-IR spectrometer with ATR attachment (ATR =
attenuated total reflection). For environmental concerns, all reactions will be performed
below a 500 mg scale. If reliable compounds are obtained, thermogravimetry, UV-vis
spectroscopy, infrared spectrometry, mass spectroscopy, X-ray diffraction and nuclear
magnetic resonance spectroscopy will be used for detecting the stability, mass and structure
of these anions.
2.1.2 Chlorination of Cs[PB11H11], Cs[NB11H11]
(1) 1 mmol Cs[PB11H11] or Cs[NB11H11] refluxed with 15 mL SbCl5.[7]

8. 8
(2) 1 mmol Cs[PB11H11] or Cs[NB11H11] refluxed with 15 mL SO2Cl2.[7]
(3) 1 mmol Cs[PB11H11] or Cs[NB11H11] with Cl2 in acetic acid (15 mL) at 85~90 ℃ while
stirring for a week.[3]
(4) 1 mmol Cs[PB11H11] or Cs[NB11H11] with 15 mL SO2Cl2 in UV light.
(5) 1 mmol Cs[PB11H11] or Cs[NB11H11] with 25 mL (COCl)2, 5% DMF.[11]
For reactions (2) and (4), SO2Cl2 can be substituted with a MeCN and SO2Cl2 mixture to
improve the solubility of [PB11H11]-
and [NB11H11]-
species in the reaction.
2.1.3 Bromination of Cs[PB11H11], Cs[NB11H11]
(6) 1 mmol Cs[PB11H11] or Cs[NB11H11] mixed with Br2 followed by SbCl5 and heated at
150 ℃.[7]
(7) 1 mmol Cs[PB11H11] or Cs[NB11H11] with excess Br2 and triflic acid at 200 ℃ for 4
days.[3]
(8) 1 mmol Cs[PB11H11] or Cs[NB11H11] with excess Br2 and catalytic amounts of AlBr3 at
20 ℃ for 12 h or 40 ℃ for 12 h or 60 ℃ for 12 h. The solvent can be CH2Br2.
[17]
2.1.4 Methylation of Cs[PB11H11], Cs[NB11H11]
(9) 1 mmol Cs[PB11H11] or Cs[NB11H11] with excess methyl triflate in the presence of
2,6-di-tert-butylpyridine and CaH2. The mixture would be stirred under N2 at 0 °C for 10 h
and then at 25 °C for 36 h.[11]
2.1.5 Application of these synthesized anions: (A) Synthesis of [Et2Al][PB11X11] (X = Cl,
Br) and [Et2Al][NB11X11] (X = Cl, Br) and testing the CO2 activation ability of these
compounds in the presence of Et3SiH. (B) Li[PB11Me11] and Li[NB11Me11] catalysts for
pericyclic rearrangements reactions[13]

9. 9
The synthetic route to [Et2Al][PB11X11] (X = Cl, Br) and [Et2Al][NB11X11] (X = Cl, Br) and
the CO2 activation test will be performed according to literature procedures.[19][20]
HCOOSiEt3
[Et2Al]+
2Et3SiH
-(Et3Si)2O
CH3OSiEt3
Et3SiH
-(Et3Si)2O
CH4
Cs[EB11X11] [Ph3C][EB11X11]
E= N,P
X= Cl, Br
[Et3Al] + [Ph3C][EB11X11] [Et2Al][EB11X11] + Ph3CH + C2H4
85 o
C
2 days
Ag[EB11X11]AgNO3
hot water
[Ph3C]Br
acetonitrile
[Et2Al]+
C6D6 C6D6
CO2 + Et3SiH
cat.
HCOOH + Et3SiH
Pd/C
The synthetic route to Li[PB11Me11] and Li[NB11Me11] would be analogous to the reported
procedure.[13]
Li[PB11Me11] and Li[NB11Me11] are planned to use to catalyze some pericyclic
rearrangement reactions.
Cs[NB11Me11]
or Cs[PB11Me11]
LiCl(aq) (15%)
Li[NB11Me11]
or Li[PB11Me11]70/30 Et2O/acetone
mixture
O
HO
Li[NB11Me11] or Li[PB11Me11]
C6D6
Li[NB11Me11] or Li[PB11Me11]
C6D6, 67 o
C

10. 10
2.2 Discussion
2.2.1 Chlorination
Chlorination is a known way to make the carborane less nucleophilic. The anions
[NB11H11]-
seems to be possible to react with SbCl5 and SO2Cl2 in refluxing condition or high
temperature condition. Previous reports suggest that the chlorination agent SbCl5 and SO2Cl2
should be strong enough to chlorinate [NB11H11]-
.[7][17]
Therefore, the key to the chlorination
reactions is if the anions are stable or not in acidic conditions and at high temperatures of up
to 200 ℃. It is because the chlorination reaction will generate a lot of HCl that may destroy
the anions. The N-B bond of [NB11H11]-
is short and strong. This suggests that [NB11H11]-
is
stable in acidic conditions. At this point, the anion [PB11H11]-
may not be as stable as
[NB11H11]-
because the P-B bond is weaker than the N-B bond. In this case, the HCl
generated by the chlorination reaction could attack the P-B bond and may destroy the anions.
Our group found out a new chlorination method (4) to synthesize [CHB11Cl11]-
from
[CHB11H11]-
under UV light.[18]
It could be a very efficient way to chlorinate [PB11H11]-
and
[NB11H11]-
. The reaction in this way will be completed in just a couple of hours. The reaction
condition is mild. It is a good way to approach the chlorinated products. UV irradiation is a
high efficiency reaction method in chemistry. The reaction in this way just takes a few hours
rather than a couple days. Meanwhile, some papers[11]
show that oxalyl chloride in the
presence of DMF is also a good chlorination agent in numbers of organic reactions for the
preparation of the carboxylic acid chlorides and alkyl chlorides. The plan is that if the B-H
groups in the anions [PB11H11]-
and [NB11H11]-
are reactive, oxalyl chloride can be used as a
weak chlorination agent to chlorinate these two anions. NMR and IR spectroscopy will be

11. 11
mainly used for monitoring these reactions. Meanwhile, mass spectrometry is another way to
know whether the compound decomposes or not and how they decompose.
2.2.2 Bromination
The bromination of Cs[PB11H11] and Cs[NB11H11] will be performed according to the
literature procedure[7][11][17]
. The reaction (6) seems that it is the most likely way for
bromination. The reaction condition is milder than the reaction (7) whose reaction condition
is up to 200 ℃ for 4 days. The problem with this method is that the reaction temperature is
very high and bromination will generate HBr. Both conditions may decompose the compound
Cs[PB11H11], because the P-B bonds are long and weak compared with the [CHB11H11]-
and
[NB11H11]-
. The plan for better results is to use different controlled temperatures and times for
the reaction, rather than directly heat it to 150 ℃. From the literature procedure (8)[17]
,
[HNB11H10Br], [MeNB11H9Br2] and [MeNB11H8Br3] can be synthesized from [NB11H11]-
.
However, because the literature does not show any high grade bromination product in this
procedure, [PB11HxBry]-
(x=11-y, y=1-5) may be synthesized rather than [PB11H5Br6]-
or
[PB11Br11]-
from [PB11H11]-
. The product also can be easily tracked by 11
B NMR, IR
spectroscopy and mass spectrometry.
2.2.3 Methylation
The methylation reaction (9) is analogous to the literature procedure.[11]
This method may
synthesize [PB11Me11]-
and [NB11Me11]-
. It shows that all the mixtures should react under
basic conditions, where the chlorination and bromination reactions will generate acid
byproducts in the process. Previous work shows that many 12-vertex closo-heteroboranes are
more sensitive in acid than in base. For example, [MeSnB11H11]-
and [MeGeB11H11]-
can exist

12. 12
in 1 M NaOH for a long time, but will decompose in dilute acid. Especially, [MeGeB11H11]-
cannot exist in water/acetone mixture for a long time and will decompose to boric acid. In
addition, the reaction temperature for methylation is just about 0~25℃. The product can be
easily detected by 11
B NMR.
Another plan is to methylate the halogenated anions [EB11H5Hal6]-
(E = P, N; Hal= Cl, Br,
I). From the literature[21]
, this plan assumes that [EB11H5Hal6]-
(Hal= Cl, Br, I) can be easily
methylated to get [EB11Me5X6]-
(E = P, N; X = Cl, Br, I). This reaction can be monitored by
11
B NMR.
2.2.4 Application of the synthesized anions: (A) Synthesis of [Et2Al][PB11X11] (X = Cl,
Br), [Et2Al][NB11X11] (X = Cl, Br) and testing the CO2 activation ability of these
compounds in presence of Et3SiH. (B) Li[PB11Me11] and Li[NB11Me11] catalyze
pericyclic rearrangements reactions[13]
From the procedure, the preparations of [Et2Al][PB11X11] (X = Cl, Br) and
[Et2Al][NB11X11] (X = Cl, Br) are easily approachable as soon as the anions have been
synthesized.
The CO2 activation mechanism is shown below (Scheme 3).[19]
Et3SiH is used as a
reducing agent it is slightly polar and the Si-H bond is weak compared to the H-H bond.
Therefore, it is easy to be activated. The cation [Et2Al]+
is used as the lewis acid center.

13. 13
Et3SiH + CO2 HCOOSiEt3
Et3SiH
H2C(OSiEt3)2
Et3SiH
H3COSiEt3
CH4
Et3SiH
(Et3Si)2O
(Et3Si)2O
cat.
cat.
cat. = [Et2Al]+
Scheme 3. Mechanism for the CO2 reduction in present of Et3SiH
[Et2Al]+
is very strong Lewis acid and very reactive. The interaction of the Et2Al-WCA
(WCA is weakly coordination anion) can affect the efficiency of CO2 activation. The new
anions [PB11X11]-
(X = Cl, Br) and [NB11X11]-
(X = Cl, Br) may weakly coordinate to [Et2Al]+
and just balance the charge of cation. Since the anions are chemical inert and barely
nucleophilic, the interactions between [Et2Al]+
and anions should be too weak to form
covalent bonds. As a result, the compounds [Et2Al][PB11X11] (X = Cl, Br) and
[Et2Al][NB11X11] (X = Cl, Br) dissolve in the solvent and form [Et2Al]+
which can catalyze
the reduction of CO2. The anions in the this reaction are used to balance [Et2Al]+
.
Li[NB11Me11] and Li[PB11Me11] can be used in pericyclic rearrangements reactions.[13]
The
plan assumes that the anions in this reaction are weakly coordinating and lipophilic.
Therefore, it can enhance the Li+
catalyst’s efficiency.

14. 14
Time Line
The proposal will start in the summer 2015 and begin with some preparation of the
reaction including synthesizing the precursors[22]
of [PB11H11]-
and [NB11H11]-
. After the
preparation of precursors, the anions [PB11H11]-
and [NB11H11]-
will be synthesized. In fall
2015, these anions will tried to be chlorinated. The experience from my previous work can
help me find a better way to finish this part and get some useful chlorinated anions. Starting
in spring of 2016, bromination and methylation of the [PB11H11]-
and [NB11H11]-
anions will
be attempted. If I can deal with the problems that come out of the chlorination attempts, it
should be easy for me to solve the problems in the bromination attempts because bromination
is similar to the chlorination reaction. The methylation of anions is the most important work
in this period. Methylation of the anions can be optimized in the future plan. For the rest of
2016, the CO2 reduction experiment and pericyclic rearrangements reactions will be
attempted.

15. 15
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[18]. Saled, M. unpublished result
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