Trioxorhena(VII)carborane anion and its methyl-substituted analogue: Synthesis, structure, DFT, and catalytic studies

Article abstract of DOI:10.1021/om201222r

Synthesis and characterization of trioxorhena(VII)carborane [Bu ₄N][(η¹-C₂B₉H ₁₁)ReO₃] (1a) and its methyl-substituted analogue [Bu ₄N][(7,8-Me₂-η¹-C_2<

Full text of DOI:10.1021/om201222r

Article
pubs.acs.org/Organometallics
Trioxorhena(VII)carborane Anion and Its Methyl-Substituted
Analogue: Synthesis, Structure, DFT, and Catalytic Studies
Kothanda Rama Pichaandi, Phillip E. Fanwick, and Mahdi M. Abu-Omar*
Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: Synthesis and characterization of trioxorhena-
(VII)carborane [Bu4N][(η1-C2B9H11)ReO3] (1a) and its
methyl-substituted analogue [Bu4N][(7,8-Me2-η1-C2B9H11)-
ReO3] (1b) are reported. The single-crystal X-ray structures
of 1a and 1b display η1 coordination between Re and the
carborane cage. Density functional theory computations at
B3LYP/LANL2DZ (Gaussian 09 suite) level show that the
strong π donor character of the oxo ligands results in a weak
interaction between the d orbitals on Re and the π orbitals of
the boron cage, the consequence of which is favoring η1 Re−B
coordination. Complexes 1a and 1b exhibit reversible one-
electron reduction in cyclic voltammetry experiments at E1/2
potentials of −1.83 and −1.88 V versus Cp2Fe/Cp2Fe+, respectively. Complex 1a catalyzes the hydrosilylation of aldehydes and
ketones in excellent yields and with high tolerance to a variety of functional groups. Mechanistic investigation of the
hydrosilylation reaction revealed potential involvement of multirhenium−boron cluster.
biomass-derived compounds.11 We report herein the synthesis
INTRODUCTION
The catalytic diversity of methyltrioxorhenium (CH3ReO3,
MTO) has fueled significant research development in organo-
■
of trioxorhena(VII)carborane [Bu4N][(η1-C2B9H11)ReO3]
(1a) and its methyl-substituted analogue [Bu4N][(7,8-Me2-η1-
metallic oxorhenium chemistry.1 In addition to being an
epoxidation catalyst with hydrogen peroxide, MTO was shown
to catalyze aldehyde olefination and a myriad of oxygen atom
transfer (OAT) reactions.2 Upon immobilization, heteroge-
neous MTO also catalyzes olefin metathesis.2a,3 More recently
oxorhenium complexes have received attention as hydro-
silylation catalysts.4 Even though transition metal oxo
complexes are often employed in oxidation reactions, the
utility of oxorhenium compounds in reduction catalysis has
stimulated new mechanistic paradigms for high oxidation state
organometallic complexes.5 In comparison, the 18-electron
species Cp*ReO3 has been more limited in its catalytic
applications. One noteworthy reaction that has garnered
renewed interest in the context of biomass conversion is
deoxydehydration of vicinal diols.6 Gable et al. studied the
mechanism of alkene extrusion from rhenium diolate
complexes.7 While the carborane complex [Me4N][(η5-
C2B9H11)Re(CO)3] has been known for nearly half a century,8
the trioxorhenium(VII) carborane (an analogue of Cp*ReO3)
has not been prepared. Limited exchange of the CO ligand with
other π acceptor ligands such as nitrosyl and alkylidene has
been noted. Fischer and co-workers have reported exchange of
CO on rhenium with halides such as Cl− and I− using Me3NO
as a CO scavenger.9
C2B9H11)ReO3] (1b). Unlike Cp*ReO3, the carborane
complexes 1 are formally 14-electron complexes featuring η1
ligation to the carborane cage. Density functional theory
(DFT) studies were undertaken to shed light on the reason
behind the preference for η1 ligation versus η5. The latter is
observed for the rhenium(I) carbonyl precursor complexes.
Trioxorhenium carborane 1a was investigated as a catalyst for
the hydrosilylation of aldehydes and ketones, and it gave good
to excellent yields of the corresponding silyl ethers. It should be
noted that the only known trioxo complex of carborane is that
of molybdenum [Me4N]2[(η1-C2B9H11)MoO3], which was
reported 15 years ago.12 It also adopts η1 ligation to the
carborane cage.
RESULTS AND DISCUSSION
■
Synthesis. The [Bu4N][nido-7,8-C2B9H12] and its rhenium
tricarbonyl complex [Bu4N][(η5-C2B9H11)Re(CO)3] (2a) as
well as [Bu4N][7,8-Me2-nido-7,8-C2B9H10] were prepared
according to published procedures.8,13 The rhenium tricarbonyl
complex [Bu4N][(7,8-Me2-η5-C2B9H11)Re(CO)3] (2b) was
prepared similarly to the procedure described for 2a. Oxidation
of 2a by PhIO in the presence of Ph2S2 or PhS(O2)SPh, with
catalytic amounts of CH3COOH at 6−8 and 15−20 °C,
respectively, yielded 1a in 83% yield (Scheme 1). In the
Our research group has had a long interest in the reaction
chemistry of oxorhenium complexes in the context of OAT,10
catalytic hydrosilylation,4d and selective deoxygenation of
Received: December 8, 2011
Published: February 13, 2012
© 2012 American Chemical Society
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absence of Ph2S2 or PhS(O2)SPh, the oxidation was not
successful and gave a poor yield. Similar observations were
reported by Kim et al. during the direct oxidation of
[Me4N]2[(η5-C2B9H11)Mo(CO)3] to [Me4N]2[(η1-C2B9H11)-
MoO3] by PhIO.12 Ph2S2 as well as PhS(O2)SPh acts as CO
scavengers. The need for a CO scavenger for the conversion of
2a to 1a is not surprising since the oxidation of 2a to
[Me4N][(η5-C2B9H11)Re(CO)2Cl2]− by [N(C6H4Br-4)3]-
[SbCl6] took place only in the presence of Me3NO, a
recognized CO scavenger.9 Also, PhS(O2)SPh is used as a
CO transfer agent in the synthesis of thiol esters via a radical
mechanism.14 The possibility of making an oxidative-addition
product of 2a with Ph2S2 analogous to [Me4N]2[(η5-C2B9H11)-
Mo(CO)2(SPh)2]12 is unlikely because no reaction was
observed between 2a and Ph2S2 in the absence of an oxidizing
agent. Oxidation of 2a to 1a can also be carried out with H2O2/
CH3COOH in the presence of Ph2S2 in benzene under reflux.15
PhS(O2)SPh and [Bu4N][ReO4] were obtained as byproducts
in this reaction. However, the reaction with H2O2/CH3COOH
gave a lower yield of 1a (42%) due to decomposition of 1a at
higher temperature. Compound 2b was converted to its
Scheme 1. Synthesis of Trioxorhenium Carborane
Compounds 1
Table 1. Crystal Data and Structure Refinement for 2b, 1a, and 1b
2b
1a
1b
formula
fw
C31H35B9O3PRe
770.1
C18H47B9NO3Re
609.08
C20H51B9NO3Re
637.13
space group
a, Å
P 1 21/c 1 (No. 14)
15.6225(4)
7.4327(2)
28.7002(10)
90
P1 (No. 2)
P 1 21/n 1 (No. 14)
14.3733(10)
11.3361(7)
19.5318(14)
90
̅
11.3208(6)
12.0687(8)
12.5154(5)
66.350(4)
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
Z
95.433(3)
90
78.506(4)
95.052(5)
90
62.778(4)
3317.62(17)
4
1392.58(13)
2
3170.1(4)
4
d
calc, g cm−3
1.542
1.452
1.335
cryst dimens, mm
temperature, K
0.20 × 0.10 × 0.04
150
0.20 × 0.14 × 0.10
150
0.20 × 0.10 × 0.03
293
radiation (wavelength, Å)
monochromator
linear abs coef, mm−1
absorp corr applied
transmn factors: min., max.
diffractometer
Cu Kα (1.54184)
confocal optics
7.858
Cu Kα (1.54184)
confocal optics
8.661
Cu Kα (1.54184)
confocal optics
7.633
empirical
empirical
empirical
0.59, 0.73
0.30, 0.42
0.61, 0.80
Rigaku RAPID-II
0 to 19, 0 to 9, −31 to 33
3.09−145.18
0.62
Rigaku RAPID-II
0 to 13, −12 to 14, −14 to 15
7.71−144.38
1.3
Rigaku RAPID-II
0 to 16, 0 to 13, −21 to 20
4.54−126.76
0.72
h, k, l range
2θ range, deg
mosaicity, deg
programs used
SHELXTL
SHELXTL
SHELXTL
1288
F000
1520
612
1/[σ2(Fo ) + (0.0612P)2 + 9.4122P] where P = (Fo + 2Fc2)/3
2
2
weighting
data collected
34 553
6166
21 937
4882
23 105
3998
unique data
Rint
0.055
0.075
0.075
3998
data used in refinement
cutoff used in R-factor calculations
data with I > 2.0σ(I)
no. of variables
largest shift/esd in final cycle
R(Fo)
6166
4882
2
2
2
2
2
2
Fo > 2.0σ(Fo )
Fo > 2.0σ(Fo )
Fo > 2.0σ(Fo )
5963
444
4772
337
2874
347
0
0
0
0.039
0.109
1.101
0.05
0.138
1.114
0.063
0.194
1.058
2
Rw(Fo )
goodness of fit
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trioxorhenium carborane 1b using H2O2/CH3COOH in
refluxing benzene in the presence of Ph2S2 or PhS(O2)SPh
with 33% yield. Oxidation of 2b by PhIO was not successful,
resulting in partial oxidation and decomposition to boric acid.
Structural Characterization. Table 1 represents the
crystal data and structure refinement for 2b, 1a, and 1b. X-
ray quality crystals of 2b as the tetraphenylphosphonium salt
were obtained as needles belonging to the P21/c space group.
The structure of 2b (Figure 1) resembles that of 2a reported by
and steric effect of methyl substitution on the cage carbon
atoms.
Single-crystal X-ray structures of 1a and 1b are given in
Figure 2. Their comparison of bond lengths and bond angles, in
addition to the DFT-predicted structure for 1a, is given in
Table 2. Complexes 1a and 1b have η1 coordination and
distorted tetrahedral geometry around the rhenium center. The
Re−B bond lengths in 1a and 1b are comparable within the
error limits. All three Re−O bond lengths in 1a and 1b are also
comparable among themselves as well as with the Re−O bond
17
lengths in CpReO3 and Cp*ReO3.18
DFT Studies. Earlier DFT studies by Fisher et al. on 2a
were limited to explaining its nonemissive behavior.9 To shed
light on the η5 versus η1 bonding preference observed in 2 and
1, respectively, we carried out DFT calculations at the B3LYP/
LANL2DZ level (Gaussian 09 suite).19 Geometry optimizations
for 1a, 1b, and 2b were obtained from their respective crystal
structures reported in this work, and for 2a, the crystal structure
reported by Zalkin et al.16 Table 2 displays DFT-calculated
bond lengths and bond angles for 1a alongside experimental
values obtained from X-ray (for 1b, 2a, and 2b, see Supporting
Information). The bond lengths and angles of the optimized
structures are in reasonable agreement with experimental
values. As expected, computed complexes 2a and 2b feature
η5 bonding, whereas 1a and 1b have η1 bonding in their global
minimum structure, consistent with experimental structures.
When 1a and 1b were forced to have η5 coordination at the
onset of the geometry optimization, the calculation converged
to give η1 coordination, demonstrating that DFT was
successfully predicting the preferred geometry irrespective of
the starting structure.
The compositions of the molecular orbitals (MO)
responsible for bonding between Re and the boron cage were
examined. Representative MOs for 2a and 1a are compared in
Figure 3. In complex 2a, as expected for metal carbonyl
compounds, there is strong π back-donation from the d orbitals
of Re to the π* orbitals of CO. This creates an electron
deficiency on the Re center and favors the same d orbitals to
interact robustly with the in-plane π orbitals of the boron cage.
This synergy results in a covalent interaction of all the π orbitals
on the boron cage and the rhenium atom, hence, η5
coordination of the carborane. Ten more such molecular
orbitals (Supporting Information) are involved in this bonding
Figure 1. ORTEP drawing of compound 2b. Ellipsoids are shown at
50% probability. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å), angles (deg), and torsions (deg): Re1−B4 =
2.338(5), Re1−B3 = 2.300(6), Re1−C1 = 2.347(5), Re1−C5 =
1.919(5), B3−C2 = 1.733(7), C1−C2 = 1.664(7), C5−Re1−B4 =
126.1(3), Re1−B4−B5 = 66.8(2), C5−Re1−C7 = 89.4(2), C6−Re1−
C7 = 89.3(2), Re1−C1−B5−B4 = −57.1(3), C5−Re1−B4−B5 =
−87.6(3).
Zalkin et al.16 It has η5 coordination between Re and the boron
cage, and all three CO groups are near perpendicular to each
other. One notable difference between 2b and 2a is the
shortening of the Re−B bond by 0.027 Å and elongation of the
Re−C as well as the C−C bonds of the cage by 0.057 and 0.048
Å, respectively. This difference is attributed to the electronic
Figure 2. ORTEP drawing of compounds 1a and 1b. Ellipsoids are shown at 50% probability. Hydrogen atoms are omitted for clarity.
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Table 2. Comparison of X-ray (1a, 1b) and DFT (1a) Bond Lengths (Å) and Bond Angles (deg)
bond length
experimental (X-ray)
DFT
bond angle
experimental (X-ray)
DFT
(1b)
(1a)
(1a)
(1b)
(1a)
(1a)
Re1−B4
Re1−O1
Re1−O2
Re1−O3
B4−B5
2.16(1)
1.715(8)
1.72(1)
1.69(1)
1.77(2)
1.79(2)
1.67(2)
1.52(1)
2.59(1)
3.156
2.19(1)
1.727(3)
1.695(3)
1.730(5)
1.797(8)
1.81(1)
1.626(9)
1.545(7)
2.574(8)
3.104
2.162
1.743
1.737
1.743
1.838
1.838
1.619
1.566
2.738
3.361
Re1−B4−B5
Re1−B4−B3
O2−Re1−B4
O3−Re1−B4
O2−Re1−O1
O1−Re1−B4
B4−B5−C1
80.6(6)
81.6(6)
80.9(4)
79.5(4)
86.00
85.96
105.35(5)
103.3(8)
105.4(4)
126.7(4)
104.3(8)
126.1(3)
101.7(3)
109.1(2)
104.1(3)
104.9(5)
120.05
104.42
109.73
104.39
104.04
B4−B3
B5−C1
C1−C2
Re1−B3
Re1−C1
contributions from nonbonding π orbitals on the oxygen atoms
as well as dxy on Re. The frontier unoccupied molecular orbitals
(LUMO, LUMO+1, and LUMO+2) are predominantly π* and
d orbitals of the ReO bond and Re metal, respectively, as
well as a minor contribution from the π* orbitals of the boron
cage. The frontier orbitals responsible for the observed UV−vis
spectrum of 1a (Figure 5) and a comparison of the excitation
energy between experimental and DFT of 1a and 1b are given
in Table 3 and are discussed next.
UV−Vis, IR, and Electrochemistry. Compounds 1a and
1b have the longest wavelength absorptions at 344 and 385 nm
with extinction coefficients (ε) of 2306 and 2418 L mol−1 cm−1,
respectively in CH2Cl2 (Figure 5). This transition can be
assigned to the LMCT transition from HOMO to LUMO+1
and LUMO+2 (Figure 4 for 1a, Supporting Information for 1b)
from TD-DFT calculations. The excitation energy calculated
from the UV−vis spectra is in reasonable agreement with the
value obtained from TD-DFT calculations (Table 3). The red
shift of 41 nm for 1b versus 1a is due to the electron-donating
methyl substituents on the carborane cage and is comparable to
the red shift of 22 nm obtained from TD-DFT calculations.
Tables 4 and 5 represent the comparison of IR frequencies of
2 and 1 with their structural analogues reported in the
literature.17,20 Complexes 2a and 2b have lower stretching νCO
compared with their Cp and Cp* analogues. This implies that
the carborane cage upon η5 coordination is a better electron
donor compared to Cp and Cp*. The νRe−O stretching
frequencies of 1 are in between MTO, PhReO3, and η5-
coordinated Cp/Cp*ReO3, indicating that the Re−O bond
strength of 1 is lower than MTO and PhReO3 but higher than
Cp/Cp*ReO3. This implies that the carborane cage upon η1
coordination is a weaker donor compared to Cp and Cp*, but a
stronger electron donor than Ph or methyl. These results
support the DFT calculations that show the π acceptor and π
donor characteristics of CO and oxo ligands are majorly
responsible for η5 versus η1 bonding modes in 2 and 1,
respectively.
Figure 3. Representative MOs of 2a and 1a responsible for their
respective η5 and η1 bonding.
mode in addition to the HOMO and HOMO−1 illustrated in
Figure 3. This bonding synergy does not exist in compound 1a,
primarily because of the π donor character of the oxo ligands.
Here the bonding between Re and the boron cage revolves
around HOMO−2 and HOMO−12 (Figure 3). These involve
Compounds 1a and 1b show a one-electron (Re7+/Re6+)
reversible reduction wave in the CV with E1/2 values of −1.83
and −1.88 V versus Cp2Fe/Cp2Fe+ in CH2Cl2, respectively
(Figure 6). The shift in the reduction potential (50 mV) for 1b
versus 1a is attributed to the electron-donating methyl groups
on the carborane cage. The observed reduction potentials for
1a and 1b are more negative than the reported value for MTO
(−1.24 V) in CH2Cl2.21
2
a unidirectional weak interaction between dz and a small
portion of the π orbitals of the boron cage and p orbitals of the
boron atom attached to Re, thereby explaining the observed η1
coordination. A similar observation was found in comparing the
methyl-substituted carborane analogues 1b and 2b (Supporting
Information).
Catalytic Studies. To explore the catalytic potential of 1,
hydrosilylation of aldehydes and ketones with Et3SiH was
carried out using 1a as a catalyst (Scheme 2). The reaction was
Figure 4 represents the frontier MOs of 1a. The HOMO
orbital is dominated by π orbitals of the boron cage with minor
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Figure 4. Frontier MO orbitals of 1a based on TD-DFT calculations.
Table 4. Comparison of IR Frequencies (cm−1) of 2 with
Their Structural Analogues20a,b
CpRe(CO)3
Cp*Re(CO)3
2a
2b
νsym(CO)
νas(CO)
2035
1946
2005
1906
1999
1895
1991
1875
Table 5. Comparison of IR Frequencies (cm−1) of 1 with
Their Structural Analogues17
CpReO3 Cp*ReO3
1a
954,
1b
954,
PhReO3 MTO
νsym(Re−O)
νas(Re−O)
926
909
986
1005
930
916,
903
929
916,
901
886
878
956
953
Figure 5. UV−vis spectra of 1a and 1b in CH2Cl2.
carried out in CD2Cl2 under reflux. The yields of silyl ether
products were good to excellent (Table 6). Catalyst 1a is
tolerant of various functional groups such as OMe, NMe2, Cl,
and NO2 (entries 3, 2, 4, and 6 in Table 6). Chemoselective
reduction of bifunctional molecules (entries 9, 12, and 14 in
Table 6) ascertains the versatility of 1a. The selectivity of trans
over cis (25:1) for the corresponding ether formation with tert-
butylcyclohexanone showcases the catalyst’s stereoselectivity,
similar to the results obtained for the dioxorhenium(V) catalyst
(PPh3)2Re(O)2I.5a Longer time and higher mol % of catalyst
were required for 4-nitrobenzaldehyde and pyridine-2-carbox-
aldehyde (entries 6 and 7; Table 6) due to their coordinating
ability with the catalyst. Moderate yield with 2-cyanobenzalde-
hyde (entry 5) can be understood by competitive hydro-
silylation of the cyano group with the formation of
corresponding imines.22 This agrees with the lower yields
Table 3. Comparison of Excitation Energies from UV−Vis Experiment and TD-DFT
experimental
excitation energy (eV)
3.61
DFT
compound
λmax
λmax
excitation energy (eV)
orbital transitions
oscillator strength
% contribution
1a
344
400
353
422
366
3.10
3.52
2.94
3.39
HOMO−LUMO+1
HOMO−LUMO+2
HOMO−LUMO+1
HOMO−LUMO+2
0.053
0.013
0.047
0.010
92
98
93
98
1b
385
3.22
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Table 6. Hydrosilylation of Ketones and Aldehydes
a b
,
Catalyzed by Trioxorhenium 1a
Figure 6. Cyclic voltametry of 1a and 1b in CH2Cl2 at 100 mV scan
rate using Bu4NPF6 (0.1 M) as a supporting electrolyte.
Scheme 2. Hydrosilylation of Carbonyl Compounds Using
1a As Catalyst
obtained when acetonitrile was used as solvent for the
hydrosilylation of benzaldehyde with triethylsilane. When a
lower temperature (25 °C) was tried for hydrosilylation of
benzaldehyde with Et3SiH, there was no impact on the yield;
however, longer reaction times (48 h) were required. When
THF-d8 was used as a solvent for the hydrosilylation of
benzaldehyde, the result was the same as that of CD2Cl2.
However when CDCl3 was used as a solvent, a competitive Si−
H/C−Cl exchange reaction was observed with the formation of
CDHCl2 and Et3SiCl, in addition to the hydrosilylation
reaction.
The hydrosilylation of 2-butanone with Et3SiH (1:1) under
neat reaction conditions gave 55% of the corresponding ether.
The lower yield was due to catalyst precipitation as the solvent
polarity changes during the course of the reaction. The
precipitated catalyst can be reused with fresh reactants without
altering the yield or reaction time. Moderate to good yields
were obtained (Table 7) with various organosilanes. In the case
of PhSiH3, the lower yield of PhSiH2OCH2Ph was observed
due to consecutive hydrosilylation of the formed product with
the formation of PhSiH(OCH2Ph)2 and PhSi(OCH2Ph)3 as
observed by GC-MS. However, reduction with sterically
a
Reaction conditions: 1 mol % of 1a, substrate = 0.23 mmol, 1.8−2
b
c
d
equiv of Et3SiH, CD2Cl2, 40 °C. NMR yields. 10 mol % of 1a.
mol % of 1a.
6
that is responsible for catalysis. A common test for
heterogeneous catalyst activity is poisoning by metallic Hg.24
Catalytic hydrosilylation of benzaldehyde with 1a was carried
out in the presence of a 275-fold excess of Hg. The yields of
silyl ether and catalyst activity were unaffected by the presence
of Hg. This result rules out heterogeneous Re metal being the
catalyst.
23
i
encumbered Pr3SiH was not successful.
Observation of an induction period (ca. 1 h) in the
hydrosilylation of benzaldehye with Et3SiH indicated the
formation of an active intermediate(s) over prolonged reaction
times. Reaction of 1a with a 20-fold excess of Et3SiH in the
absence of benzaldehyde, under reflux, resulted in a dark brown
mixture that is sensitive toward moisture. Formation of
The catalytic mixture gave a complex EPR spectrum with a g
value of 2.3 (Supporting Information), indicating the presence
of paramagnetic species. The complexity and the hyperfine
splitting pattern indicated the presence of multiple rhenium
clusters.25 The broad 11B{1H} NMR spectrum that extends
from δ +60 to −60 in contrast to the sharp chemical shifts
observed for 1a is very typical for paramagnetic transition metal
dicarbollides and exhibits the characteristic boron cage having a
large paramagnetic contact and pseudocontact shift (Support-
ing Information).26 The presence of chemical shifts corre-
1
molecular H2 was observed by H NMR. Removal of the
solvent and excess silane under vacuum from the brown
reaction mixture afforded a solid residue that, after washing
with hexane, acted as an effective catalyst for the hydrosilylation
of benzaldehyde with Et3SiH without exhibiting an induction
period. Within 1 h, 43% of the corresponding silyl ether was
formed, in comparison with only 11% under similar conditions
starting with precatalyst 1a. The absence of an induction period
indicated that this mixture contained the active catalyst.
Another plausible interpretation of the long induction period
is formation of a heterogeneous Re metal (or nanoparticles)
sponding to OSiEt3 groups in the H and 13C{1H} NMR
1
indicated the possibility of active species having Et3SiO− ligands
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Re(CO)3] (2a), as well as [Bu4N][7,8-Me2-nido-7,8-C2B9H10] were
prepared according to published procedures.8,13,27 Rhenium tricar-
bonyl complex [Bu4N][(7,8-Me2-η5-C2B9H11)Re(CO)3] (2b) was
Table 7. Hydrosilylation of Benzaldehyde Using Different
Organosilanes Catalyzed by Trioxorhenium 1a
a b
,
1
prepared similarly to 2a. H NMR spectra were recorded on Varian
Inova 300 and 600 MHz and Bruker DRX 500 and 800 MHz
instruments. 13C{1H} NMR were recorded in Varian Inova 75 MHz
and Bruker DRX 125 MHz instruments. 11B{1H} NMR spectra were
recorded in Bruker DRX 165 MHz and Varian Inova 99 MHz
instruments. EPR spectra were recorded on a Bruker ESP 300E EPR
spectrometer equipped with an hp 5350B microwave frequency
counter, an Oxford ITC4 temperature controller, and a VC40 gas flow
controller (for liquid He) and a Eurotherm temperature control unit
(liquid N2). IR spectra and UV−vis were recorded using Thermo-
Nicolet Nexus FT-IR and Shimadzu 2501-A instruments, respectively.
A PARSTAT 2273 potentiostat-galvanostat was used in cyclic
voltammetric experiments in dichloromethane with Bu4NPF6 (0.1
M) as a supporting electrolyte with a scan rate of 100 mV/s. A silver
wire immersed in 0.010 M AgNO3/0.10 M Bu4NPF6/acetonitrile
served as a reference electrode. The working electrode was a glassy
carbon electrode with an area of 0.074 cm2. All potentials are reported
vs Cp2Fe/Cp2Fe+, and the E1/2 was calculated by the average of
cathodic and anodic peak potentials. All data fittings were carried out
using KaleidaGraph. Mass spectrometry was performed by the Purdue
University Campus Wide Mass Spectrometry Center using a Hewlett-
Packard Engine mass spectrometer (GC/MS). All electrospray
ionization analyses were carried out on a FinniganMAT LCQ Classic
(ThermoElectron Corp, San Jose, CA, USA) mass spectrometer
system. The electrospray needle voltage was set at 4.0 kV, the heated
capillary voltage was set to 10 V, and the capillary temperature was 207
°C. Typical background source pressure was 1.2 × 10−5 Torr as read
by an ion gauge. The sample flow rate was approximately 8 μL per
minute. The drying gas was nitrogen. The LCQ is typically scanned to
2000 amu for these experiments. The electrospray ionization high-
resolution mass measurements were obtained in the peak matching
mode using a FinniganMAT XL95 (FinniganMAT Corp., Bremen,
Germany) mass spectrometer. The instrument was calibrated to a
resolution of 10 000 with a 10% valley between peaks using the
appropriate polypropylene glycol standards.
a
Reaction conditions: 1 mol % of 1a, substrate = 0.23 mmol, 1.8−2
b
equiv of silane, CD2Cl2, 40 °C. NMR yields.
attached to Re. However no signals were observed in the 29Si
NMR. The IR spectrum showed characteristic bands for B−H
(2536 cm−1) and Re−O (906 cm−1) stretching modes. Efforts
to obtain single crystals from the reaction of 1a and Et3SiH for
further characterization were not successful. In the absence of
Et3SiH, 1a was inert toward benzaldehyde and showed no
significant reaction. No kinetic isotope effect was observed with
C6H5CDO and Et3SiD. In summary, we believe the active
catalytic mixture is composed of multinuclear paramagnetic
rhenium decorated with the carborane cage, oxo ligands, and
possibly Et3SiO−. In order to compare the catalytic activity of
compound 1b with 1a, a model reaction of hydrosilylation of
benzaldehyde with Et3SiH was carried out under the same
conditions used for 1a. The results were the same as those of
1a, except with a relatively longer reaction time of 24 h.
Characterization of [Bu4N][(7,8-Me2-η5-C2B9H11)Re(CO)3]
1
(2b). Yield: 0.82 g (70%). H NMR (500 MHz, CD2Cl2): δ 3.11 (t,
8H); 2.27 (s, 6H); 1.61 (m, 8H); 1.42 (m, 8H); 1.00 (t, 12H).
13C{1H} NMR (125 MHz, CD2Cl2): δ 200.4 (CO), 62.5 (C of
dicarbollide), 59.5 (Me2C of dicarbollide), 59.8, 24.7, 20.5, 14.2.
11B{1H} NMR (160 MHz, CD2Cl2): δ −6.3, −9.8, −15.4. IR ν (KBr):
2962, 2933, 2876, 2554, 2530, 1991, 1875. HRMS (m/z): calcd for
C5H15B9ReO3 [M−] 431.1388, found 431.1392. X-ray quality crystals
for 2b were obtained by dissolving its Li salt in acetone, precipitated
with water as its tetraphenylphosphonium (PPh4) salt by using
PPh4Br, and then vapor diffusion of pentane to the THF solution of
the PPh4 salt of 2b, in sequence.
CONCLUSION
■
We have reported herein the efficient synthesis of [Bu4N][(η1-
C2B9H11)ReO3] (1a) and its methyl-substituted analogue
[Bu4N][(1,2-Me2-η1-C2B9H11)ReO3] (1b) from their respec-
tive carbonyl precursors 2a and 2b in the presence of Ph2S2 or
PhS(O2)SPh. Through DFT calculations the coordination
mode of η5 versus η1 in compounds 2 and 1 was rationalized by
the strong π acceptor and π donor character of CO and oxo
ligands, respectively. The catalytic utility of 1a in hydro-
silylation of aldehydes and ketones was demonstrated. Complex
1a exhibits high chemoselectivity and tolerance of various
functional groups. The intermediates formed from the reaction
of Et3SiH with 1a act as the active homogeneous catalyst(s).
Extension of the catalytic activity of this family of
trioxorhenium carborane compounds to other organic reactions
is underway in our laboratory.
Synthesis of [Bu4N][(η1-C2B9H11)ReO3] (1a). Method 1. To a
suspension of iodosobenzene (0.5 g, 2.27 mmol) in 55 mL of
THF were added 2a (0.20 g, 0.31 mmol) and Ph2S2 (0.07 g,
0.31 mmol) with stirring, and the mixture was cooled to −5 to
0 °C in an ice salt bath. Acetic acid (67 μL) was added to the
above suspension, and the temperature was slowly raised to 6−
8 °C. The suspension became clear and turned a yellowish-
orange color. After stirring for 2 h at room temperature, the
solvent was removed under vacuum. The solid obtained was
washed with hexane (50 mL) twice to remove the iodobenzene
and Ph2S2. It was further dissolved in CH2Cl2 and filtered
through Celite to remove any boric acid formed during the
reaction and recrystallized by the solvent diffusion method by
using ether as diffusing solvent to give 1a as bright yellow
EXPERIMENTAL SECTION
■
Reactions were performed in a nitrogen-filled glovebox or using
standard Schlenk techniques using argon. Solvents were degassed and
purified with a solvent purification system developed by Anhydrous
Engineering Inc. prior to use. CD2Cl2 was dried with CaH2, distilled
under argon, and stored over molecular sieves. o-Carborane and
bromopentacarbonylrhenium(I) were purchased from Strem and used
as received. Organosilanes, aldehydes, and ketones were purchased
from Gelest or Aldrich and used as received. The [Bu4N][nido-7,8-
C2B9H12] and its rhenium tricarbonyl complex, [Bu4N][(η5-C2B9H11)-
1
crystals. Yield: 0.16 g (83%). H NMR (500 MHz, CD2Cl2): δ
3.11 (t, 8H); 2.83 (b, 2H); 1.61 (m, 8H); 1.42 (h, 8H); 1.00 (t,
12H). 13C{1H} NMR (125 MHz, CD2Cl2): δ 60.4 (CH of
dicarbollide), 59.8, 24.7, 20.5, 14.2. 11B{1H} NMR (160 MHz,
CD2Cl2): δ −2.4 (3B), −2.8 (1B), −8.8 (2B), −12.5 (2B),
−25.6 (1B). IR ν (KBr): 2962, 2933, 2876, 2554, 2530, 1469,
1894
dx.doi.org/10.1021/om201222r | Organometallics 2012, 31, 1888−1896

Organometallics
Article
954, 930, 916, 903. HRMS (m/z): calcd for C2H11B9ReO3 [M−]
367.1075, found 367.1080. When PhS(O2)SPh was used
instead of Ph2S2, the reaction mixture became clear at 15−20
°C and the remaining procedure and yield were the same.
Method 2. To a solution of 2a (0.20 g, 0.31 mmol) and Ph2S2 (0.07
g, 0.31 mmol) in 7.0 mL of benzene were added 7.0 mL of H2O2 (30%
solution) and 70 μL of acetic acid, and the solution was heated to
reflux for 3 h. The yellowish-orange organic phase was extracted with
methylene chloride (20 mL twice), dried with Na2SO4, and rotary
evaporated to remove the solvent. The residue was column
chromotographed starting with hexane and ethylacetate in a gradient
fashion up to 50%. Yield: 0.08 g (42%). PhS(O2)SPh and
[Bu4N][ReO4] were obtained as byproducts in this reaction.
Synthesis of [Bu4N][(7,8-Me2-η1-C2B9H11)ReO3] (1b). To a
solution of 2b (0.17 g, 0.25 mmol) and Ph2S2 (0.04 g, 0.25 mmol) in
11.0 mL of benzene were added 5.0 mL of H2O2 (30% solution) and
40 μL of acetic acid, and the solution was heated to reflux. After 5 and
7.5 h two lots of H2O2 (4.5 mL) and acetic acid (30 μL) were added,
and the reaction mixture was refluxed further until the disappearance
of peaks in the 11B{1H} NMR corresponding to 2b . The yellowish-
orange organic phase was extracted with methylene chloride (20 mL
twice), dried with Na2SO4, and rotary evaporated to remove the
solvent. The solid obtained was washed with hexane twice (50 mL
each) to remove the iodobenzene and PhS(O2)SPh. It was dissolved in
THF and recrystallized by the solvent diffusion method by using
pentane as diffusing solvent to give 1b as bright yellow crystals. Yield:
0.03 g (33%). 1H NMR (500 MHz, CD2Cl2): δ 3.11 (t, 8H); 1.61 (m,
8H); 1.51 (s, 6H); 1.42 (m, 8H); 1.00 (t, 12H). 13C{1H} NMR (125
MHz, CD2Cl2): δ 72.9 (C(CH3)2 of dicarbollide), 59.5, 24.4, 22.6
(C(CH3)2 of dicarbollide) 20.3, 13.9. 11B{1H} NMR (160 MHz,
CD2Cl2): δ −4.1 (1B), −7.2 (3B), −8.8 (2B), −12.8 (2B), −24.9
(1B). IR ν (Nujol): 2962, 2933, 2876, 2554, 2530, 1469, 954, 929,
916, 901. HRMS (m/z): calcd for C4H15B9ReO3 [M−] 397.1416,
found 397.1412.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
*E-mail: mabuomar@purdue.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding for this research was provided by DOE-BES, grant no.
DE-FG-02-06ER15794. We thank Dr. Alice Rene and Prof.
Dennis H. Evans of Purdue University for their help with the
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