Multiple Si-H bond activations in a bis(diethylphosphino)methane-bridged complex of rhodium and iridium: Synthesis of an unusual bis(silyl)/μ-silylene complex

Article abstract of DOI:10.1021/om300283k

The reaction of 1 equiv of a primary silane (PhSiH₃ or MesSiH₃; Mes = mesityl) with the bis(diethylphosphino)methane-bridged complex [RhIr(CO)₃(depm)₂] (1; depm = Et ₂PCH₂PEt₂) yields the silylene-bridged dihydride complexes [RhIr(H)₂(CO)₂(μ-SiHR)(depm) ₂] (2, R = Ph; 3, R = Mes) by double Si-H bond activation. Complex 2 reacts further with excess phenylsilane to yield a bis(silylene)-bridged product, [RhIr(CO)₂(μ-SiHPh)₂(depm)₂] (4), while complex 3 reacts with CO to produce a silylene- and carbonyl-bridged product, [RhIr(CO)₂(μ-CO)(μ-SiHMes)(depm)₂] (5). The reaction of 1 with 1 equiv of diphenylsilane leads to a silylene-bridged dihydride, [RhIr(H)₂(CO)₂(μ-SiPh₂)(depm) ₂] (6), while reaction with 2 equiv of diphenylsilane at 60 °C in the presence of CO yields the silyl/μ-silylene complex [RhIr(H)(SiPh ₂H)(CO)₃(κ¹-depm)(μ-SiPh ₂)(depm)] (7). The reactions of 1, 6, and 7 with excess silane at 80 °C under a CO purge yield a novel bis(silyl)/μ-silylene complex, [RhIr(SiPh₂H)₂(CO)₄(μ-SiPh ₂)(depm)] (8), with accompanying loss of a depm ligand.

Full text of DOI:10.1021/om300283k

Article
pubs.acs.org/Organometallics
Multiple Si−H Bond Activations in a Bis(diethylphosphino)methane-
Bridged Complex of Rhodium and Iridium: Synthesis of an Unusual
Bis(silyl)/μ-Silylene Complex
Md Hosnay Mobarok,† Michael J. Ferguson,†,‡ and Martin Cowie*,†
‡
†Department of Chemistry and X-ray Crystallography Laboratory, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
S
* Supporting Information
ABSTRACT: The reaction of 1 equiv of a primary silane (PhSiH3 or MesSiH3;
Mes = mesityl) with the bis(diethylphosphino)methane-bridged complex
[RhIr(CO)3(depm)2] (1; depm = Et2PCH2PEt2) yields the silylene-bridged
dihydride complexes [RhIr(H)2(CO)2(μ-SiHR)(depm)2] (2, R = Ph; 3, R =
Mes) by double Si−H bond activation. Complex 2 reacts further with excess
phenylsilane to yield a bis(silylene)-bridged product, [RhIr(CO)2(μ-
SiHPh)2(depm)2] (4), while complex 3 reacts with CO to produce a silylene-
and carbonyl-bridged product, [RhIr(CO)2(μ-CO)(μ-SiHMes)(depm)2] (5).
The reaction of 1 with 1 equiv of diphenylsilane leads to a silylene-bridged
dihydride, [RhIr(H)2(CO)2(μ-SiPh2)(depm)2] (6), while reaction with 2 equiv
of diphenylsilane at 60 °C in the presence of CO yields the silyl/μ-silylene
complex [RhIr(H)(SiPh2H)(CO)3(κ1-depm)(μ-SiPh2)(depm)] (7). The re-
actions of 1, 6, and 7 with excess silane at 80 °C under a CO purge yield a novel
bis(silyl)/μ-silylene complex, [RhIr(SiPh2H)2(CO)4(μ-SiPh2)(depm)] (8),
with accompanying loss of a depm ligand.
incorporates two bridging SiHR units in reactions involving
primary silanes to produce bis(silylene)-bridged complexes.13
In a recent report on the stepwise activation of Si−H bonds of
silanes by the heterobimetallic complex [RhIr(CO)3(dppm)2],
involving each of the metals (Rh, Ir) from the above previous
studies, we described an unusual example of a heterobimetallic
silyl/μ-silylene complex, [RhIr(H)(SiPh2H)(CO)3(κ1-dppm)-
(μ-SiPh2)(dppm)].14 An analogous germyl/μ-germylene com-
plex, [RhIr(H)(GePh2H)(CO)3(κ1-dppm)(μ-GePh2)(dppm)],
has also been subsequently reported.15 This type of bimetallic
complex, containing both bridging silylene and terminal silyl
groups, is not common.11,14−16 In our previous studies, we
proposed that dissociation of one end of a dppm ligand could
play a pivotal role in the incorporation of the second Si- or Ge-
containing fragment by generating the required coordinative
unsaturation for oxidative addition of the second silane or
germane and in easing the steric crowding at the metals. In the
case of disubstituted silyl/μ-silylene or germyl/μ-germylene
examples, the steric bulk of these groups combined with the
bulk of the dppm ligands prevented recoordination of the
pendant dppm group that resulted upon addition of the second
silane.14,15 We had also observed significant differences in
reactivity involving primary and secondary silanes or germanes,
in which bis(μ-silylene), bis(μ-germylene), or μ-silylene/μ-
germylene complexes could not be generated when both
groups were disubstituted.14,15,17 Clearly, steric factors play a
INTRODUCTION
■
Transition-metal complexes containing silyl or silylene ligands
are effective catalysts in a number of reactions, including those
involving industrially important silicon−carbon and silicon−
silicon bond formation.1−4 Recent investigations suggest that
monometallic complexes containing both silyl and silylene
ligands can act as intermediates in silicon−silicon bond
formation, silane oligomerization, isomerization, and redistrib-
ution.5−9 Among these transformations, Si−Si bond formation
during silane oligomerization is proposed to occur via facile 1,2-
silyl migration from the transition-metal center to the silylene
group within the coordination sphere of monometallic silyl/
silylene complexes.5 There are precedents for metal-to-metal
silyl migration in two heterobimetallic (Fe/Pt10 and Rh/Pt11)
complexes, and in both cases the silyl group migrates from the
more labile to the less labile metal. It is of interest to determine
whether such silyl migrations can take place within a bimetallic
core from either metal to a bridging silylene ligand, instead of
merely between metals; such reactivity has not been observed
to date.
The nature of the pair of metals in such bimetallic
compounds has been shown to exert a significant influence
on the reactivity. In a previous study on silane activation by a
dppm-bridged diiridium complex (dppm = Ph2PCH2PPh2), our
group demonstrated that one bridging SiR2 unit could be
incorporated through reactions of an Ir2 precursor with primary
and secondary silanes, but reaction with a second equivalent of
silane was not observed.12 In contrast, a related dirhodium
system, studied by Eisenberg and co-workers, readily
Received: April 8, 2012
Published: June 20, 2012
© 2012 American Chemical Society
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Organometallics
Article
dissolved in 2 mL of toluene. A 10.9 μL portion (0.065 mmol) of
C6H2Me3SiH3 was added to the stirred solution by a microliter syringe.
The reaction mixture was stirred for 3 h. Slow addition of 10 mL of
pentane yielded a brown powder of compound 3 in 73% isolated yield.
Anal. Calcd for C29H58IrO2P4RhSi: C, 39.32; H, 6.60. Found: C, 39.52;
H, 6.71. 31P{1H} NMR (27 °C; CD2Cl2, 161.8 MHz): δ 21.6 (Rh−P,
bm, 1P), 6.9 (Rh−P, bm, 1P), −19.3 (Ir−P, bm, 1P), −25.0 (Ir−P,
significant role in the incorporation of two or more Si- or Ge-
containing fragments into the bimetallic core. For this reason,
we set out to investigate related chemistry involving bis-
(diethylphosphino)methane (depm = Et2PCH2PEt2) instead of
dppm, in order to determine what role this smaller diphosphine
might play in this chemistry. In addition, in earlier studies we
had proposed the involvement of unobserved intermediates
containing three Si-containing fragments during isomerization
of bis(μ-silylene) complexes14 and set out to determine if such
species could be observed with the smaller depm group as a
bridging ligand. Finally, the incorporation of Si- and Ge-
containing fragments was brought about by the multiple
oxidative additions of Si−H and Ge−H bonds, which should
also be more favorable in complexes involving the more basic
depm ligand, adding further incentive for investigating this
chemistry, the initial results of which are reported herein.
1
bm, 1P). H NMR (27 °C; CD2Cl2, 399.8 MHz): δ −10.54 (Rh−H,
bm, 1H), −11.96 (Ir−H, bm, 1H). 31P{1H} NMR (−80 °C; CD2Cl2,
1
2
161.8 MHz): δ 23.2 (Rh−P, ddm, 1P, JRhP = 94 Hz, JPP = 120 Hz),
1
2
6.9 (Rh−P, ddm, 1P, JRhP = 104 Hz, JPP = 118 Hz), −17.3 (Ir−P,
2
2
1
dm, 1P, JPP = 120 Hz), −24.9 (Ir−P, dm, 1P, JPP = 118 Hz). H
1
NMR (−80 °C; CD2Cl2, 399.8 MHz): δ 7.22 (Si−H, bs, 1H, JSiH
=
180 Hz), 7.00 (Ar−H, s, 2H), 3.43 (o-CH3, bs, 6H), 2.94 (PCH2P, m,
1H), 2.54 (PCH2P, m, 1H), 2.31 (p-CH3, m, 3H), 2.02 (PCH2P, m,
1H), 1.94 (PCH2P, m, 1H), 1.92−0.78 (C2H5, m, 40 H), −10.60
2
1
(Rh−H, ddm, 1H, Jtrans‑PH = 155 Hz, JRhH = 11 Hz), −11.79 (Ir−H,
dm, 1H, Jtrans‑PH = 126 Hz). 13C{1H} NMR (27 °C; CD2Cl2, 100.5
2
MHz): δ 194.4 (Rh−CO, dm, 1C, 1JRhC = 65.0 Hz), 181.0 (Ir−CO, bs,
1C), 48.2 (PCH2P, m, 1C), 32.4 (PCH2P, m, 1C). 29Si{1H} NMR
(−80 °C; CD2Cl2, 78.5 MHz, DEPT): δ 104.0 (m).
EXPERIMENTAL SECTION
■
General Comments. All solvents were dried (using appropriate
drying agents), distilled before use, and stored under dinitrogen.
Reactions were performed under an argon atmosphere using standard
Schlenk techniques. Ph2SiH2 and PhSiH3 were purchased from Sigma-
Aldrich, while MesSiH3 (Mes = mesityl) was prepared according to the
literature method.18 Silanes were dried over CaH2 and distilled under
Ar before use. 13C-enriched CO (99.4%) was purchased from
Cambridge Isotope Laboratories, while [RhIr(CO)3(depm)2] (1)
was prepared as previously reported19 and kept as a benzene solution
under Ar at −20 °C (1 is unstable in CH2Cl2 over extended periods of
time even at −20 °C). In subsequent reactions carried out in other
solvents, benzene was removed under high vacuum followed by
dissolution of the solid in the appropriate solvent. NMR spectra were
recorded on Varian Inova-400 or Varian Unity-500 spectrometers
operating at the resonance frequencies for the NMR nuclei as given in
the spectral information. The 1H, 13C{1H}, and 29Si{1H} NMR spectra
were referenced internally to residual solvent proton signals relative to
tetramethylsilane, whereas 31P{1H} spectra were referenced relative to
external 85% H3PO4. All NMR spectra were recorded at 27 °C unless
otherwise noted. Selected NMR spectra for compounds 2 and 4−8 are
given as Supporting Information. Infrared spectra were obtained on a
Nicolet Avatar 370 DTGS spectrometer. The elemental analyses were
performed by the Microanalytical Laboratory in the Department of
Chemistry at the University of Alberta.
(c). [RhIr(CO)2(μ-SiHPh)2(depm)2] (4). Under an atmosphere of Ar,
1 mL of a benzene solution containing 60 mg (0.079 mmol) of
[RhIr(CO)3(depm)2] (1) was placed in a 10 mL Schlenk tube and
diluted to 5 mL by addition of benzene followed by the addition of 39
μL (0.32 mmol, 4 equiv) of phenylsilane by microliter syringe. The
tube was set in an oil bath at 60 °C for 2 h, during which time the dark
red solution turned orange. Addition of 5 mL of diethyl ether resulted
in the precipitation of a light brown powder in 53% (40 mg) isolated
yield. Single crystals of 4 were obtained by slow diffusion of ether into
a concentrated solution in benzene. However, due to extreme disorder,
this structure could not be refined acceptably. Anal. Calcd for
C32H56IrO2P4RhSi2: C, 40.54; H, 5.95. Found: C, 40.79; H, 5.27.
Compound 4 can also be prepared by reacting a benzene solution of 2
(45 mg, 0.059 mmol) with excess phenylsilane (2 equiv, 15 μL) at 60
°C for 1 h. 31P{1H} NMR (27 °C; C6D6, 161.8 MHz): δ 18.3 (Rh−P,
1
1
dm, 2P, JRhP = 102 Hz), −15.4 (Ir−P, m, 2P). H NMR (27 °C;
1
C6D6, 399.8 MHz): δ 6.48 (Si−H, m, 2H, JSiH = 161 Hz), 2.92
(PCH2P, m, 2H), 1.30 (PCH2P, m, 2H), 1.90 − 0.85 (C2H5, m, 40 H).
13C{1H} NMR (27 °C; CD2Cl2, 100.5 MHz): δ 197.7 (Rh−CO, dm,
1
1C, JRhC = 73 Hz), 184.9 (Ir−CO, bs, 1C), 33.4 (PCH2P, m, 2C).
29Si{1H} NMR (27 °C; CD2Cl2, 79.5 MHz, DEPT): δ 115.3 (m).
(d). [RhIr(CO)2(μ-CO)(μ-SiHMes)(depm)2] (5). Under an atmos-
phere of Ar, 60 mg (0.079 mmol) of [RhIr(CO)3(depm)2] (1) was
dissolved in 2 mL of toluene in a Schlenk tube followed by the
addition of 26 μL (0.16 mmol, 2 equiv) of mesitylsilane by microliter
syringe. The tube was set in an oil bath at 60 °C overnight under a
slow CO purge. During this time a light yellow precipitate settled from
the solution. The supernatant was removed by cannula, and the solid
was washed three times with 15 mL of pentane. The residual solvent
was removed under high vacuum, and analytically pure complex 5 was
obtained in 63% (45 mg) isolated yield. Anal. Calcd for
C30H56IrO3P4RhSi: C, 39.52; H, 6.19. Found: C, 39.61; H, 6.09.
Compound 5 can also be generated by purging CO through a toluene
solution of complex 3. 31P{1H} NMR (27 °C; CD2Cl2, 201.6 MHz): δ
Preparation of Compounds. (a). [RhIr(H)2(CO)2(μ-SiHPh)-
(depm)2] (2). A total of 53 mg of [RhIr(CO)3(depm)2] (1; 0.069
mmol) was placed in a 10 mL Schlenk tube in 2 mL of CH2Cl2
followed by the addition of 10 μL of phenylsilane (0.081 mmol) to the
solution by a microliter syringe. The solution changed from dark red
to orange within 30 min. After 1 h the 31P{1H} NMR spectrum
suggested quantitative formation of complex 2. The solvent was
removed under high vacuum, followed by dissolution of the compound
in 0.5 mL of toluene. Slow addition of 5 mL of pentane yielded a
brown powder of compound 2. 31P{1H} NMR (27 °C; C7D8, 161.8
MHz): δ 23.7 (Rh−P, bm, 1P), 8.2 (Rh−P, bm, 1P), −16.7 (Ir−P, bm,
1
1
2
2
1P), −23.9 (Ir−P, bm, 1P). H NMR (27 °C; C7D8, 399.8 MHz): δ
28.6 (Rh−P, ddd, 1P, JRhP = 102 Hz, JPP = 281 Hz, JPP = 24 Hz),
1
2
2
7.10 (Si−H, b, 1H), 2.92 (CH2, b, 2H), 2.40 (CH2, b, 2H), −10.70
23.2 (Rh−P, 1P, ddd, JRhP = 97 Hz, JPP = 185 Hz, JPP = 20 Hz),
(Rh−H, m, 1H), −11.96 (Ir−H, m, 1H). 31P{1H} NMR (−80 °C;
2
2
−12.0 (Ir−P, dd, 1P, JPP = 281 Hz, JPP = 24 Hz), −12.2 (Ir−P, dd,
C7D8, 161.8 MHz): δ 25.3 (Rh−P, bm, 1P), 8.3 (Rh−P, dd, 1P, 1JRhP
=
2
2
1
1P, JPP = 185 Hz, JPP = 20 Hz). H NMR (27 °C; CD2Cl2, 498.1
MHz): δ 6.77 (m-Ph−H, s, 2H), 5.17 (Si−H, m, 1H, 1JSiH = 167 Hz),
2.77 (PCH2P, m, 1H), 2.67 (o-CH3, m, 6H), 2.45 (PCH2P, m, 2H),
2.23 (p-CH3, m, 3H), 2.30−1.03 (C2H5, m, 40H), 0.70 (PCH2P, m,
1H). 13C{1H} NMR (27 °C; CD2Cl2, 125.7 MHz): δ 242.9 (μ-CO,
2
108 Hz, JPP = 138 Hz), −15.3 (Ir−P, m, 1P), −23.7 (Ir−P, m, 1P).
1H NMR (−80 °C; C7H8, 399.8 MHz): δ 7.30 (Si−H, bs, 1H, 1JSiH
=
180 Hz), 3.04 (PCH2P, m, 1H), 2.80 (PCH2P, m, 1H), 0.70−2.10
(C2H5, m, 40H), −10.63 (Rh−H, ddm, 1H, 2Jtrans‑PH = 154.0 Hz, 1JRhH
= 12 Hz), −11.85 (Ir−H, dm, 1H, 2Jtrans‑PH = 122 Hz). 13C{1H} NMR
1
1
dm, 1C, JRhC = 27 Hz), 194.4 (Rh−CO, dm, 1C, JRhC = 78 Hz),
182.5 (Ir−CO, bs, 1C), 32.1 (PCH2P, m, 1C), 24.2 (PCH2P, m, 1C).
29Si{1H} NMR (27 °C; CD2Cl2, 78.5 MHz, DEPT): δ 52.2 (m).
(e). [RhIr(H)2(CO)2(μ-SiPh2)(depm)2] (6). Under an atmosphere of
Ar, 52 mg (0.068 mmol) of [RhIr(CO)3(depm)2] (1) was dissolved in
1 mL of toluene in a Schlenk tube followed by the addition of 12.6 μL
(0.068 mmol) of diphenylsilane by microliter syringe. The reaction
1
(−80 °C; C7D8, 100.5 MHz): δ 194.4 (Rh−CO, dt, 1C, JRhC = 70.0
Hz), 181.4 (Ir−CO, t, 1C), 48.5 (PCH2P, m, 1C), 35.2 (PCH2P, m,
1C). 29Si{1H} NMR (−80 °C; C7D8, 78.5 MHz, DEPT): δ 134.3 (dt,
2
1JSiRh = 32 Hz, JPSi = 70 Hz).
(b). [RhIr(H)2(CO)2(μ-SiHMes)(depm)2] (3). A 50 mg portion (0.065
mmol) of [RhIr(CO)3(depm)2] (1) was placed in a Schlenk flask and
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Organometallics
Article
Scheme 1
2
1
1
mixture was stirred for 3 h. Layering the toluene solution with 2 mL of
pentane in an NMR tube yielded light yellow crystals (suitable for X-
ray analysis) in 29% (18 mg) isolated yield. 31P{1H} NMR (27 °C;
1JRhP = 92 Hz, JPP = 70 Hz), −30.5 (Ir−P, d, 1P, JPP = 70 Hz). H
NMR (27 °C; CD2Cl2, 498.1 MHz): δ 7.75−6.99 (Ph−H, m, 30 H),
5.95 (Ir−Si−H, d, 1H, 1JSiH = 182 Hz, 3JPH = 4 Hz), 5.74 (Rh−Si−H,
1
d, 1H, 1JSiH = 182 Hz, 3JPH = 5 Hz), 3.08 (PCH2P, dd, 2H, JPH = 11,
2
C7D8, 161.9 MHz): δ 17.7 (Rh−P, m, 2P), −19.4 (Ir−P, m, 2P). H
NMR (27 °C; C7D8, 399.8 MHz): δ −10.51 (Rh−H, m, 1H), −12.05
11 Hz), 1.79−0.93 (CH3−CH2, m, 20 H). 13C{1H} NMR (27 °C;
CD2Cl2, 125.7 MHz): δ 202.5 (Rh−CO, dd, 2C, 1JRhC = 54 Hz, 2JPC
=
(Ir−H, m, 1H). 31P{1H} NMR (−80 °C; C7D8, 161.8 MHz): δ 17.4
2
1
1
12 Hz), 187.7 (Ir−CO, d, 2C, JPC = 11 Hz), 45.5 (PCH2P, m, 1C).
(Rh−P, dm, 1P, JRhP = 101 Hz), 12.1 (Rh−P, dm, 1P, JRhP = 105
Hz), − 21.6 (Ir−P, m, 1P), − 23.3 (Ir−P, m, 1P). 1H NMR (−80 °C;
C7D8, 399.8 MHz): δ 7.25−7.06 (Ph−H, m, 10 H), 2.87 (PCH2P, m,
2H), 2.27 (PCH2P, m, 2H), 1.87−0.28 (C2H5, m, 40H), −10.61 (Rh−
29Si{1H} NMR (27 °C; CD2Cl2, 78.5 MHz, DEPT, gHSQC): δ 136.9
1
(μ-Si, m) −2.3 (Rh−Si, d, JRhSi = 23), −23.1 (Ir−Si, s).
X-ray Data Collection and Structure Determination. (a). Gen-
eral Considerations. Single crystals suitable for X-ray diffraction were
obtained by the slow diffusion of pentane into CH2Cl2 (5 and 8) and
toluene (6) solutions of the compounds. Data were collected on either
a Bruker D8/APEX II CCD diffractometer (5, 6) or a Bruker
PLATFORM/APEX II CCD (8) diffractometer at −100 °C using Mo
Kα radiation.20 Data were corrected for absorption through the use of
Gaussian integration from indexing of the crystal faces. All structures
were solved using direct methods (SHELXS-97).21 Refinement was
carried out using the program SHELXL-97.21 Hydrogen atoms
attached to carbons were assigned positions on the basis of the sp2
or sp3 hybridization geometries of their parent atoms and were given
isotropic displacement parameters 20% greater than the Ueq’s of their
parent carbons. Metal hydrides for 6 and Si-bound hydrogens for 5
and 8 were located from difference Fourier maps and treated as
detailed below. A listing of crystallographic experimental data is
provided for all structures as Supporting Information.
2
1
H, ddm, 1H, Jtrans‑PH = 152 Hz, JRhH = 11 Hz), −12.06 (Ir−H, dm,
1H, 2Jtrans‑PH = 120 Hz). 13C{1H} NMR (−80 °C; C7D8, 100.5 MHz):
1
δ 198.1 (Rh−CO, dm, 1C, JRhC = 67 Hz), 184.5 (Ir−CO, bs, 1C),
48.1 (PCH2P, m, 1C), 30.2 (PCH2P, m, 1C). 29Si{1H} NMR (27 °C;
C7D8, 78.5 MHz, DEPT): δ 148.1 (m).
(f). [RhIr(H)(SiPh2H)(CO)3(κ1-depm)(μ-SiPh2)(depm)] (7). Under an
atmosphere of Ar, 100 mg (0.131 mmol) of [RhIr(CO)3(depm)2] (1)
was dissolved in 5 mL of toluene in a Schlenk tube followed by the
addition of 97 μL (0.52 mmol) of diphenylsilane by microliter syringe.
The tube was set in an oil bath at 60 °C overnight under a slow CO
purge. During this time the solution changed from dark red to orange
accompanied by the precipitation of a light green solid. The
supernatant was separated by cannula, and the solvent was removed
to give a brown oil. The oil was further washed with 3 × 5 mL of
pentane and dried under high vacuum to give compound 7 in 47% (69
mg) isolated yield. This complex could not be obtained analytically
pure, as the oil contained some unidentified impurities. The solid was
identified as compound 8 (described below). 31P{1H} NMR (27 °C;
CD2Cl2, 161.8 MHz): δ −3.9 (Rh−P, ddd, 1P, 1JRhP = 94 Hz, 2JPP = 86
Hz, 3JPP = 4 Hz), −7.5 (Ir−P, dddd, 1P, 2JPP = 32 Hz, 2JPP = 9 Hz, 3JPP
= 11 Hz, 3JRhP = 5 Hz), −27.1 (Ir−P, ddd, 1P, 2JPP = 86 Hz, 3JPP = 11
(b). Special Refinement Conditions. i. Compound 5. One metal
atom site (labeled Ir) was refined with a site occupancy of 67% Ir and
33% Rh; the other site (labeled Rh) was refined as 67% Rh and 33% Ir.
The silicon-bound hydrogen atom was given an isotropic displacement
parameter 20% greater than that of the silicon atom, and its
coordinates were allowed to freely refine.
ii. Compound 6. Metal positions were refined with 50% site
occupancy of Ir and Rh. The Ir- and Rh-bound hydrides were
constrained to have a M−H distance of 1.60 Å and were given
isotropic displacement parameters 20% greater than those of the metal
atoms to which they are attached.
iii. Compound 8. The “IrRh(CO)4(depm)” fragment of the
molecule was found to be disordered about the crystallographic 2-
fold rotation axis that passes through Si(2) and approximately bisects
the Ir−Rh bond. Pairs of atoms (Ir and Rh, P(1) and P(2), O(1) and
O(3), O(2) and O(4), C(1) and C(3), and C(2) and C(4)) that are
pseudo-related by the rotation axis were refined with equivalent
anisotropic displacement parameters. Additionally, the P(1)−C(5) and
P(2)−C(5) distances were restrained to be approximately equal.
Hz, 4JPP = 5 Hz), −35.0 (pendant-P, dd, 1P, 2JPP = 32 Hz, 4JPP = 5 Hz).
1
1H NMR (27 °C; CD2Cl2, 399.8 MHz): δ 6.02 (Si−H, d, 1H, JSiH
=
3
177 Hz, JPH = 4 Hz), 2.60 (PCH2P, m, 1H), 1.92 (PCH2P, m, 1H),
1.84 (PCH2P, m, 1H), 1.14 (PCH2P, m, 1H), 2.15−0.55 (C2H5, m,
40H), −11.93 (Ir−H, dd, 1H, JPH = 20, 20 Hz). 13C{1H} NMR (27
2
1
°C; CD2Cl2, 100.5 MHz): δ 205.2 (Rh−CO, dd, 1C, JRhC = 56 Hz,
2JPC = 10 Hz), 202.4 (Rh−CO, dm, 1C, JRhC = 54 Hz), 187.1 (Ir−
1
CO, m, 1C), 54.3 (PCH2P, m, 1C), 38.1 (PCH2P, m, 1C). 29Si{1H}
NMR (27 °C; CD2Cl2 78.5 MHz, gHSQC): δ −3.4 (m), 143.3 (m).
(g). [RhIr(SiPh2H)2(CO)4(μ-SiPh2)(depm)] (8). Under an atmosphere
of Ar, 200 mg (0.262 mmol) of [RhIr(CO)3(depm)2] (1) was
dissolved in 7 mL of toluene in a Schlenk tube followed by the
addition of 194 μL (1.048 mmol, 4 equiv) of diphenylsilane by
microliter syringe. The tube was set in an oil bath at 80 °C overnight
under a slow CO purge. During this time a light green precipitate
settled from the solution. The supernatant was removed by cannula,
and the solid was washed three times with 15 mL of pentane. Removal
of residual solvents under high vacuum yielded analytically pure
compound 8 in 43% (123 mg) isolated yield. Anal. Calcd for
C49H54IrO4P2RhSi3: C, 51.25; H, 4.74. Found: C, 51.49; H, 4.74.
31P{1H} NMR (27 °C; CD2Cl2, 201.6 MHz): δ −1.9 (Rh−P, dd, 1P,
RESULTS AND COMPOUND CHARACTERIZATION
■
Reaction of the bis(diethylphosphino)methane-bridged com-
plex [RhIr(CO)3(depm)2] (1; depm = Et2PCH2PEt2) with 1
equiv of primary silanes (RSiH3, R = Ph, Mes) yields the
respective mono(silylene)-bridged dihydride complexes [RhIr-
(H)2(CO)2(μ-SiHR)(depm)2] (2, R = Ph; 3, R = Mes) by
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double Si−H bond activation (Scheme 1) much as that
reported for the dppm-bridged systems.12−15 The rate of
formation of 2 is somewhat slower than that for the previously
reported dppm analogue [RhIr(H)2(CO)2(μ-SiHPh)(dppm)2]
under similar reaction conditions.14 While the formation of
[RhIr(H)2(CO)2(μ-SiHPh)(dppm)2] was complete within 30
min, in the same time period conversion of 1 to 2 was only half
complete. This slight rate inhibition in the depm chemistry may
result from the lower tendency for carbonyl loss from the more
electron-rich depm species in comparison to the dppm
analogue. Both 2 and 3 display fluxional behavior at room
temperature, as is evident by the broad and unresolved peaks in
The 31P{1H} NMR spectrum of 5 displays four multiplets at
δ 28.6, 23.2, −12.0, and −12.2, for which the two downfield
peaks display Rh−P coupling of 102 and 97 Hz, respectively.
The 13C{1H} NMR spectrum displays three multiplets at δ
242.9 (1JRhC = 27 Hz), 194.4 (1JRhC = 78 Hz), and 182.5
corresponding to the bridging and Rh- and Ir-bound carbonyls,
respectively. The Si-bound proton appears as a multiplet at δ
5.17 in the 1H NMR spectrum, while a multiplet is observed at
δ 52.2 in the 29Si{1H} NMR spectrum for the bridging silylene
unit.
The structure of complex 5 is shown in Figure 1, confirming
the formulation described above, in which the bridging
1
the H NMR and 31P{1H} NMR spectra, which upon cooling
to −80 °C give rise to well-resolved peaks. This fluxionality
resembles that of previously characterized dppm-bridged
13
mono(μ-silylene) dihydride complexes of Rh2 and Rh/
Ir14,15 and so was not investigated in this study. In the
31P{1H} NMR spectra at −80 °C both complexes display four
multiplets, in which two downfield resonances are assigned to
the Rh-bound 31P nuclei on the basis of the observed Rh−P
coupling. The 1H NMR spectra display two multiplets between
δ −10.0 and −12.0 for the metal-bound hydrides, while the
silicon-bound proton in each compound appears as a broad
singlet at ca. δ 7.2. A multiplet is observed in the 29Si{1H}
NMR spectrum at δ 134.3 for 2 and δ 104.0 for 3, consistent
with a bridging silylene unit in each case.1b,22
Reaction of the phenylsilylene dihydride complex (2) with
excess phenylsilane (4 equiv) at 60 °C yields a bis(silylene)-
bridged complex, [RhIr(CO)2(μ-SiHPh)2(depm)2] (4), incor-
porating a second bridging silylene unit, accompanied by the
reductive elimination of 2 equiv of H2. This reaction requires
only 1 h for completion, in contrast to the formation of the
dppm analogue [RhIr(CO)2(μ-SiHPh)2(dppm)2],13 which
requires over 12 h to form under similar reaction conditions.
In this case, the observed rate increase might be attributed to
the lower steric demand and higher electron-donating ability of
the depm ligand, which allows better substrate access to the
metals and facilitates the oxidative addition of the Si−H bonds,
respectively. Owing to the higher symmetry of this complex in
comparison to that of 2, the 31P{1H} NMR spectrum of 4
displays only two multiplets in which the downfield resonance
is again assigned to the Rh-bound 31P nuclei (1JRhP = 102 Hz).
The two silicon-bound protons appear as a multiplet at δ 6.48
in the 1H NMR spectrum, while in the 29Si{1H} NMR
spectrum a multiplet is observed for the two chemically
equivalent bridging silylene units at δ 115.3. The 13C{1H}
NMR spectrum displays a doublet of multiplets (at δ 197.7)
and a singlet (at δ 184.9) for Rh- and Ir-bound carbonyls,
respectively. We were unable to obtain X-ray structural
information for compound 4, owing to extensive disorder in
the crystals.
The reaction of 3 with CO at elevated temperature (60 °C)
yields a carbonyl- and silylene-bridged complex, [RhIr(CO)2(μ-
CO)(μ-SiHMes)(depm)2] (5), in 63% yield (see Scheme 1).
Interestingly, the reaction of 2 with CO under similar reaction
conditions did not yield the phenylsilylene analogue, and
increasing the reaction temperature to 80 °C led only to
decomposition. Although the analogous phenylsilylene-bridged
dppm species [RhIr(H)2(CO)2(μ-SiHPh)(dppm)2] did react
with CO under similar conditions, the tricarbonyl product
[RhIr(CO)2(μ-CO)(μ-SiHR)(dppm)2] was only obtained in
very low yield (15%).14
Figure 1. Perspective view of compound 5 showing the numbering
scheme. Non-hydrogen atoms are represented by Gaussian ellipsoids
at the 20% probability level. Hydrogen atoms are shown arbitrarily
small. The metal atom position labeled Ir was refined with a site
occupancy of 67% Ir/33% Rh, while that labeled as Rh was refined as
67% Rh/33% Ir.
carbonyl is pseudo-trans to one bridging depm ligand while
the bridging mesitylsilylene unit is pseudo-trans to the other.
Although these types of silylene- and carbonyl-bridged species
13
were characterized by NMR in previously reported Rh2 and
RhIr systems,14 they were not crystallographically characterized.
The Rh−Ir distance (2.6961(2) Å) in 5 is significantly shorter
than those in the previously reported mono(μ-silylene), bis(μ-
silylene), and silyl/μ-silylene complexes of Rh/Ir, for which
separations of ca. 2.83−2.86 Å were typical.14 As a
consequence, the Ir−Si−Rh angle (68.95(2)°) is also more
acute than those in previous determinations (ca. 72.0°).14 This
shorter separation and accompanying acute Rh−Si−Ir angle
may reflect less crowded environments at the metals by virtue
of the smaller diphosphines or may be a function of the small
accompanying bridging carbonyl group requiring a closer metal
approach to optimize orbital overlap with the metals. However,
the significantly longer Rh−Ir separation in a related structure
(vide infra) appears to suggest that the shorter Rh−Ir
separation in 5 is not solely steric in origin. The carbonyl
and silylene groups both bridge symmetrically (Rh−C(3) =
2.061(2) Å, Ir−C(3) = 2.047(2) Å and Rh−Si = 2.3759(6) Å,
Ir−Si = 2.3874(6) Å).
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Reaction of 1 equiv of a secondary silane (Ph2SiH2) with
complex 1 again yields a mono(silylene)-bridged product,
[RhIr(H)2(CO)2(μ-SiPh2)(depm)2] (6; Scheme 2), closely
(depm)] (7), and a bis(silyl)/μ-silylene complex, [RhIr-
(SiPh2H)2(CO)4(μ-SiPh2)(depm)] (8), after 18 h in an
approximate 60/40 ratio (based on isolated yield). Increasing
the reaction temperature to 80 °C, but keeping all other
reaction conditions the same, led to the quantitative formation
of 8 after the same period. Complex 7 can be prepared by the
reaction of 6 with 1 equiv of Ph2SiH2 in the presence of CO at
60 °C. It is interesting to note that the formation of 7 requires a
temperature higher than that for the dppm analogue [RhIr-
(H)(SiPh2H)(CO)3(κ1-dppm)(μ-SiPh2)(dppm)],14 which
readily forms at ambient temperature. To determine whether
compounds 7 and 8 form by competing reaction pathways or
whether 7 is an intermediate during the formation of 8, the
former was reacted with 1 equiv of Ph2SiH2 with a CO purge at
80 °C. The clean formation of 8 with the elimination of 1 equiv
of depm (as evident from 31P NMR) and H2 in this reaction
suggests that that 7 is an intermediate in the formation of 8.
The NMR features of 7 are similar to those of the dppm
analogue,14 displaying four resonances in the 31P{1H} NMR
spectrum at δ −3.9, −7.5, −27.1, and −35.0, of which only the
resonance at δ −3.9 displays Rh−P coupling (94 Hz),
confirming that only one phosphine arm of the diphosphines
is coordinated to Rh. The most upfield resonance, showing the
lowest multiplicity (a doublet of doublets, arising due to
coupling to the two Ir-bound 31P nuclei), is assigned to the
Scheme 2
resembling complexes 2 and 3. This product has been fully
characterized by NMR and X-ray crystallography; owing to the
similarity of the NMR parameters to those of 2 and 3 these data
are not discussed further. The X-ray structure of this complex is
shown in Figure 2. The Rh−Ir distance of this complex
1
pendant end of depm. In the H NMR spectrum the silyl
hydrogen appears as a doublet at δ 6.02 (3JPH = 4 Hz); although
it displays no resolvable coupling to Rh, its collapse to a singlet
only upon irradiation of the Rh-bound 31P resonance indicates
that the silyl group is bound to Rh. The hydride ligand appears
as an apparent triplet (a doublet of doublets) at δ −11.93
showing equal coupling to both Ir-bound 31P nuclei (2JPH = 20
Hz), as established by selective 31P{1H} decoupling experi-
ments. The absence of Rh−H coupling further indicates that
the hydride is terminally bound to Ir. In the 13C{1H} NMR
spectrum two resonances, observed at δ 205.2 (1JRhC = 56 Hz)
and 202.4 Hz (1JRhC = 54 Hz), correspond to the Rh-bound
carbonyls, while the peak at δ 187.1, having no Rh coupling,
corresponds to the CO that is bound to Ir. Two multiplets were
observed for the silyl and silylene groups in the 29Si{1H} NMR
spectrum at δ −3.4 and 143.3, respectively.
Complex 8 has been fully characterized by multinuclear
NMR spectroscopy and X-ray analysis. The 31P{1H} NMR
spectrum displays a doublet of doublets at δ −1.9 (1JRhP = 92
Hz) and a doublet at δ −30.5 for Rh- and Ir-bound 31P nuclei,
respectively. Two doublet resonances are observed at δ 5.95
1
and 5.74 in the H NMR spectrum for the metal-bound silyl
protons (displaying three-bond couplings to phosphorus of 4
and 5 Hz, respectively); both signals collapse to a singlet upon
broad-band 31P decoupling. The depm ethyl protons display a
cluster of multiplets between δ 1.79 and 0.93, and interestingly,
integration of these signals indicates that only one depm is
coordinated. The 29Si{1H} NMR spectrum displays a doublet at
δ −2.3 (1JRhSi = 23 Hz) and a singlet at δ −23.1 for Rh-bound
and Ir-bound silyl groups, respectively, while the bridging
silylene group appears as a multiplet at δ 136.9. A doublet of
doublets at δ 202.5 (1JRhC = 54 Hz) and a doublet at δ 187.7 are
observed in the 13C{1H} NMR spectrum for Rh- and Ir-bound
carbonyls.
Figure 2. Perspective view of compound 6 showing the numbering
scheme. The atom-labeling scheme and thermal parameters are as
described in Figure 1. The structure was refined with each metal
position as a 50% mix of Rh and Ir.
(2.8829(3) Å) is significantly longer than in complex 5 but
agrees well with distances in previously characterized mono-
(silylene)-bridged complexes.13 Consistent with the longer
Rh−Ir distance, the Rh−Si−Ir angle (75.65(3)°) in this
complex is also significantly larger than that of complex 5.
These larger values in comparison to those in 5 suggest that the
lower steric demand of the depm groups is in fact not a
significant factor in determining the Rh−Ir separation.
The X-ray structure of complex 8 is shown in Figure 3,
confirming the bis(silyl)/μ-silylene formulation in which the
silylene-bridged bimetallic core is flanked by two terminal silyl
ligands. The bridging silylene ligand resides opposite the
Reaction of complex 1 with excess Ph2SiH2 (4 equiv) under a
CO purge at 60 °C yields a mixture of a silyl/μ-silylene
complex, [RhIr(H)(SiPh2H)(CO)3(κ1-depm)(μ-SiPh2)-
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of the pendant-depm species [RhIr(H)(SiPh2H)(CO)3(κ1-
depm)(μ-SiPh2)(depm)] (7) analogous to the previously
reported dppm species suggests a proclivity for phosphine
dissociation in both systems. In this study we see that although
the reactions of 1 with 1 equiv of primary silanes proceed with
the formation of the respective monosilylene-bridged dihydride
complexes, much as reported in the dppm chemistry, the rate of
formation of these complexes is found to be slightly slower.
This decrease in rate might be attributed to the slow
dissociation of CO, which is necessary for the double Si−H
bond activation of the first 1 equiv of silane. For the
incorporation of the second silylene group the relative
reactivities are reversed such that the rate in the depm system
is approximately 10 times faster than that for dppm, indicating
that for this transformation substrate accessibility and/or the
greater tendency for oxidative addition dominate. The high
yield of a carbonyl- and silylene-bridged complex (5) upon
reacting 3 with CO is also in contrast to the previously reported
dppm chemistry with the Rh/Ir metal combination, for which
the reaction of [RhIr(H)2(CO)2(μ-SiHPh)(dppm)2] with CO
under similar reaction conditions only yielded the carbonyl
adduct in low yield, and this product could not be isolated.14
The reason for this difference is not clear, particularly since a
“Rh2(dppm)2” analogue cleanly yielded the related carbonyl-
and silylene-bridged product.13a,b
The reactivity of 1 with diphenylsilane also displays
similarities with the dppm chemistry, initially yielding a
monosilylene-bridged dihydride product (6) followed by
dissociation of one end of a diphosphine ligand, elimination
of H2, and addition of a second equivalent of silane and a
carbonyl ligand to give the silyl/μ-silylene complex [RhIr(H)-
(SiPh2H)(CO)3(κ1-depm)(μ-SiPh2)(depm)] (7). However, in
contrast to the dppm analogue, complex 7 reacts further with a
third equivalent of diphenylsilane to produce the novel
bis(silyl)/μ-silylene complex 8, by loss of the pendant depm
ligand (see Scheme 2). Complete diphosphine loss was not
previously observed in related dppm chemistry, arguing against
a dissociative pathway, for which loss of the bulkier dppm
ligand should be favored. Possibly the lower steric bulk of depm
allows additional substrate access promoting phosphine
dissociation in this case. In a previous study we had observed
the isomerization of a bis(phenylsilylene)-bridged complex,
[RhIr(CO)2(μ-SiHPh)2(dppm)2], from the kinetic isomer in
which a phenyl ring on one silylene group was adjacent to a
hydrogen on the other to the thermodynamic product in which
the phenyl groups on each silylene unit were adjacent and
parallel.14 This isomerization occurred only in the presence of
excess phenylsilane, suggesting an unobserved intermediate in
which three Si-containing fragments were incorporated. The
observation of the bis(silyl)/μ-silylene product 8 confirms that
such species can exist.
Figure 3. Perspective view of compound 8, showing the numbering
scheme. The atom-labeling scheme and thermal parameters are as
described in Figure 1. Metal positions were refined with 50% site
occupancy of Rh and Ir. The primed atoms are related to the
unprimed atoms by the 2-fold rotation axis that passes through Si(2)
and bisects the Ir−Rh bond (see the Experimental Section).
bridging diphosphine, while the terminal silyl ligands lie
opposite the metal−metal bond. Structurally, complex 8 is
similar to the previously reported silane-bridged diruthenium
species [Ru2(SiTol2H)2(CO)2(μ-η2:η2-H2SiTol2)(dppm)].23
The Rh−Ir distance (2.897(1) Å) in 8 is slightly longer than
in complex 5 and other previously reported mono(μ-silylene)
and bis(μ-silylene) complexes,14 while the metal−silicon
distances in the terminal and bridging silicon moieties are
quite similar (Ir−Si(1) = 2.413(1) Å, Ir−Si(2) = 2.401(1) Å;
Rh−Si(1′) = 2.410(3) Å, Rh−Si(2′) = 2.399(2) Å). The
carbonyls on each metal are approximately mutually trans
(C(1)−Ir−C(2) = 171.9(9)° and C(3)−Rh−C(4) = 166.5(9)
°), with those on one metal being staggered slightly with
respect to those on the other (OC−Ir−Rh−CO torsion angles
between 13.3(9) and 19.0(10)°).
DISCUSSION
■
As noted in the Introduction, the purpose of this study was to
compare the reactivity of the depm-bridged complex [RhIr-
(CO)3(depm)2] (1) with silanes to that previously reported for
the dppm-bridged analogue. While we see many similarities in
the reactions of primary and secondary silanes in these two
systems, some reactivity differences are also evident. The
relative rates of these reactions involving depm and dppm can
be expected to depend on a number of factors. The smaller size
of depm favors silane access to the metals, while the greater
basicity of this diphosphine should favor silane oxidative
addition at both metals; both factors should lead to an
increased rate for the depm system. However, the smaller depm
should have less tendency for dissociation, which has been
proposed to be a factor in generating the required coordinative
unsaturation,14 while CO dissociation and H2 reductive
elimination should also be less facile for this more basic system
(due to greater π back-donation to CO and favoring of the
higher oxidation state, respectively); both of these factors
should lead to a slower rate for depm. It is also possible that
both systems (depm and dppm) generate coordinative
unsaturation by different mechanisms, although the observation
ASSOCIATED CONTENT
■
S
* Supporting Information
Tables of crystallographic experimental details and selected
bond distances and angles for compounds 5, 6 and 8, figures
giving selected NMR spectra for compounds 2 and 4−8, and
CIF files giving atomic coordinates, interatomic distances and
angles, anisotropic thermal parameters, and hydrogen param-
eters for 5, 6, and 8. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Article
(19) Slaney, M. E.; Anderson, D. J.; Ferguson, M. J.; McDonald, R.;
Cowie, M. Organometallics 2012, 31, 2286.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: martin.cowie@ualberta.ca. Fax: 01 7804928231. Tel:
01 7804925581.
(20) Programs for diffractometer operation, data collection, data
reduction, and absorption correction were those supplied by Bruker.
(21) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(22) Ogino, H.; Tobita, H. Adv. Organomet. Chem. 1998, 42, 223.
(23) Hashimoto, H.; Aratani, I.; Kabuto, C.; Kira, M. Organometallics
2003, 22, 2199.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the University of Alberta for
providing financial support for this research. We thank the
NSERC for funding the Bruker D8/APEX II CCD diffrac-
tometer and Nicolet Avatar FTIR spectrometer. M.H.M. thanks
the Bangladesh Council of Scientific and Industrial Research
(BCSIR) for granting him a study leave at the University of
Alberta and the Government of Alberta for funding support
(Q.E. II Scholarship). We thank the Department’s Analytical
and Instrumentation Laboratory and the NMR facility for their
outstanding support.
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