Facile Si-H bond activation and hydrosilylation catalysis mediated by a nickel-borane complex

Article abstract of DOI:10.1039/c3sc52626g

Metal-borane complexes are emerging as promising systems for study in the context of bifunctional catalysis. Herein we describe diphosphineborane nickel complexes that activate Si-H bonds and catalyze the hydrosilylation of aldehydes. Treatment of [^MesDPB^Ph]Ni (1) ([ ^MesDPB^Ph] = MesB(o-Ph₂PC₆H ₄)₂) with organosilanes affords the complexes [ ^MesDPB^Ph](μ-H)NiE (E = SiH₂Ph (3), SiHPh₂ (4)). Complex 4 is in solution equilibrium with 1 and the thermodynamic and kinetic parameters of their exchange have been characterized by NMR spectroscopy. Complex 1 is a catalyst for the hydrosilylation of a range of para-substituted benzaldehydes. Mechanistic studies on this reaction via multinuclear NMR spectroscopy are consistent with the intermediacy of a borohydrido-Ni-siloxyalkyl species.

Full text of DOI:10.1039/c3sc52626g

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Facile Si–H bond activation and hydrosilylation
catalysis mediated by a nickel–borane complex†
Cite this: Chem. Sci., 2014, 5, 590
a
b
a
*
Samantha N. MacMillan, W. Hill Harman and Jonas C. Peters
Metal–borane complexes are emerging as promising systems for study in the context of bifunctional
catalysis. Herein we describe diphosphineborane nickel complexes that activate Si–H bonds and catalyze
the hydrosilylation of aldehydes. Treatment of [MesDPBPh]Ni (1) ([MesDPBPh] ¼ MesB(o-Ph2PC6H4)2) with
organosilanes affords the complexes [MesDPBPh](m-H)NiE (E ¼ SiH2Ph (3), SiHPh2 (4)). Complex 4 is in
solution equilibrium with 1 and the thermodynamic and kinetic parameters of their exchange have been
characterized by NMR spectroscopy. Complex 1 is a catalyst for the hydrosilylation of a range of para-
substituted benzaldehydes. Mechanistic studies on this reaction via multinuclear NMR spectroscopy are
consistent with the intermediacy of a borohydrido-Ni-siloxyalkyl species.
Received 18th September 2013
Accepted 22nd October 2013
DOI: 10.1039/c3sc52626g
www.rsc.org/chemicalscience
Introduction
The metal-mediated activation of Si–H bonds provides a direct
and efficient strategy for the functionalization of organic
substrates.1 In these hydrosilylation processes, the oxidative
addition of an Si–H bond to the metal center is invoked as one
of the rst steps of the catalytic cycle.2 While Si–H oxidative
additions are known for metals of the nickel triad,1b,3,4,5 H–Ni–
SiR3 species are less well-studied in comparison to their Pt
counterparts, presumably due to their instability and high
degree of reactivity.1b The use of bulky or chelating ligands has
allowed for the isolation of a number of mononuclear Ni silyl
(Ni–SiR3)3a,6 and Ni silane6a,7 complexes. However, well-
characterized mononuclear H–Ni–SiR3 complexes, which
would be expected from the oxidative addition of an Si–H bond
at Ni, are rare.7b,8
Scheme 1
complexes derived from the oxidative addition of Si–H
substrates to a Ni–B core and probe the mechanism of benz-
aldehyde hydrosilylation by 1.
Results and discussion
Preparation and solid-state molecular structures of
We recently reported the preparation of a nickel–borane
complex, [MesDPBPh]Ni (1), that reversibly activates H2 to
borohydrido-Ni-silyl complexes
Treatment of 1 at room temperature with one equivalent of
phenylsilane or diphenylsilane results in immediate Si–H
bond cleavage to afford the borohydrido-silyl complexes
generate the borohydrido-hydride species
[
MesDPBPh](m-H)
Ni(H) (2, Scheme 1), and catalyzes olen hydrogenation.9
Intrigued by this bifunctional reactivity and its promise for
realizing 2-electron catalytic processes using late rst-row
transition metals, we sought to broaden our knowledge of
the reaction chemistry of 1. We reasoned that analogous
H–Ni–SiR3 species might be accessible as a consequence of the
stabilizing interaction of the boron center. Herein we report
the preparation of well-characterized borohydrido-Ni-silyl
[
MesDPBPh](m-H)NiSiH2Ph (3) and [MesDPBPh](m-H)NiSiHPh2 (4),
respectively (Scheme 1). Yellow-orange 3 and orange 4 are
diamagnetic, air-sensitive solids and are reliably isolated in ca.
75% yield. Although analytically pure samples of 4 can be iso-
lated in the solid state by crystallization, upon dissolution an
equilibrium mixture of 4, 1 and free H2SiPh2 results (vide infra).
1
The H NMR spectra of 3 and 4 each feature two signature
resonances, one corresponding to the Si–H protons and one
broad, upeld shied resonance corresponding to the bridging
hydride. The Si–H resonance of 3 is a sharp triplet centered at
aDivision of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA 91125, USA. E-mail: jpeters@caltech.edu; Tel: +1 626 395 4036
bDepartment of Chemistry, University of California, Riverside, CA 92521, USA
† Electronic supplementary information (ESI) available: Experimental procedures
and crystallographic data. CCDC 961588, 961589 and 961590. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c3sc52626g
1
4.30 ppm (3JHP ¼ 9.2 Hz, JHSi ¼ 138 Hz), while the bridging
borohydride resonance appears at ꢀ2.47 ppm; as expected,
1
these peaks integrate in a 2 : 1 ratio (Fig. 1). Similarly, the H
NMR spectrum of 4 displays two resonances that integrate in a
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asymmetric unit, one of which is shown in Fig. 2. Both 3 and 4
feature a distorted square planar geometry at nickel with two
trans-disposed phosphine donors, a terminal silyl group, and a
hydride bridging the nickel and boron centers (Fig. 2). The
nickel centers of 3 and 4 lie slightly outside the P–Si–P plane (3:
˚
˚
˚
0.036 A; 4: 0.047 and 0.085 A). The Ni–Si distance of 2.2379(7) A
˚
for 3 and 2.2435(7) A (avg) for 4 is in the range of other mono-
meric nickel complexes containing a monodentate silyl frag-
6a–c,7b,8,10
˚
˚
ment (2.19–2.37 A),
for example 2.239(8) A in the
unusual hydrido-silyl complex [Ni(AlCp*)3(H)(SiEt3)] reported
by Fischer.10c
Fig. 1 Signature resonances in the (a) 1H, (b) 31P{1H}, (c) 11B{1H} and (d)
29Si{1H} NMR spectra for 3.
Solution studies of [MesDPBPh](H)NiSiHPh2 (4)
The well-dened solution equilibrium of 1 and 4 (Scheme 1)
provided the opportunity to study both the thermodynamics
and kinetics of Si–H bond activation by 1. Dissolution of pure 4
in C6D6 gives an equilibrium mixture of 1, 4 and H2SiPh2, cor-
responding to a Keq z 960 Mꢀ1. A van’t Hoff analysis over a 50 K
range yielded thermal parameters of DH ¼ ꢀ12 ꢁ 1 kcal molꢀ1
and DS ¼ ꢀ27 ꢁ 3 eu. Repeating the experiment with D2SiPh2
yielded thermal parameters of DH ¼ ꢀ20 ꢁ 1 kcal molꢀ1 and
DS ¼ ꢀ47 ꢁ 3 eu, with an inverse equilibrium isotope effect of
EIE328 ¼ 0.18.11
1 : 1 ratio (5.01 ppm, t, Si–H, 3JHP ¼ 14.7 Hz, 1JHSi ¼ 88 Hz and
ꢀ2.61 ppm, s, B–H–Ni). The 31P{1H} NMR spectra each display a
singlet at ca. 40 ppm, similar to the borohydrido-hydride
2
complex 2; no JHP was resolved at room temperature for 3 or
4. The 11B{1H} NMR resonances of ꢀ2.2 ppm for complex 3 and
ꢀ1.7 ppm for complex 4 are sharpened and shied upeld
relative to 1 (21.6 ppm), indicative of increased electron density
at boron. The 29Si{1H} NMR spectra each display a triplet
centered at ꢀ17.0 ppm (2JSiP ¼ 38.7 Hz) and 3.1 ppm (2JSiP ¼ 37.9
Hz) for 3 and 4, respectively. The upeld shi of the 29Si{1H}
NMR resonance for the secondary versus primary silyl species is
consistent with that observed in the literature.8b The Si–H
stretches in the IR spectra for 3 and 4 are observed at 2060 and
2058 cmꢀ1, respectively.
The kinetics of the Si–H bond activation process dened in
Scheme 1 were studied by two-dimensional exchange spec-
troscopy (2D EXSY).12 1H–1H EXSY spectra were recorded in
toluene-d8 over a 40 K range with mixing times (Tm) of 0 ms
and 700 ms. A representative spectrum is shown in Fig. 3. The
forward (k1) and reverse (kꢀ1) rate constants were extracted
from the EXSY data and the results are shown in Table 1.
Eyring analysis of these data gave DH‡ ¼ 3.9 ꢁ 1 kcal molꢀ1
The previously reported equilibrium between 1 and the
borohydrido-hydride complex 2 could be studied in solution.
However, removal of the H2 atmosphere resulted in rapid H2
loss from 2 to regenerate 1, and the equilibrium process frus-
trated the growth of single crystals of 2 and the acquisition of
X-ray structural data. Gratifyingly, the molecular structures of 3
and 4 could be determined by single-crystal X-ray diffraction
and support our previous structural assignment of 2. The solid-
state structure of 4 features two independent molecules in the
and DS‡ ¼ ꢀ39 ꢁ 3 eu for the forward reaction and DH‡
¼
17 ꢁ 1 kcal molꢀ1 and DS‡ ¼ ꢀ9.6 ꢁ 3 eu for the reverse
process. The analogous reaction of 1 with D2SiPh2 proved too
slow to be studied by EXSY.
Fig. 2 Solid-state structures of 3 (left) and 4 (right). Thermal ellipsoids
are drawn at 50% probability. Hydrogen atoms bound to carbon and a
molecule of ether (3) and benzene (4) are omitted for clarity. Selected
bond lengths (A˚) and angles (ꢂ) for 3: Ni–Si 2.2379(4); Ni–H 1.58(2); Ni–
1
B 2.440(2); B–H 1.31(2); H–Ni–Si 168.3(6); 4 (avg): Ni–Si 2.2479(7); Ni– Fig. 3 Representative 2D H–1H EXSY spectrum of 4 taken at 348 K
H 1.64(3); Ni–B 2.506(3); B–H 1.27(3); H–Ni–Si 163.36(9).
with Tm ¼ 700 ms in toluene-d8.
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Table 1 Summary of rate data extracted from 2D 1H–1H EXSY spectra
of 4
T (K)
327
337
347
357
k1 (Mꢀ1 sꢀ1
)
43
0.16
270
51
0.36
140
67
0.79
85
73
1.4
50
k
(sꢀ1
)
ꢀ1
k1/k (Mꢀ1
)
ꢀ1
Catalytic hydrosilylation reactivity
In an effort to incorporate this Si–H bond activation process
into a useful catalytic cycle, we examined the hydrosilylation of
para-substituted benzaldehydes. Catalytic hydrosilylation of
benzaldehyde with 5 mol% 1 proved facile and was accom-
plished in 99% yield within 6 h at room temperature (Table 2,
entry 1). Catalyst 1 tolerated the electron-donating and electron-
withdrawing groups shown in Table 2. In contrast to the iron
pyridinediimine system recently reported by Chirik,13 the rate of
hydrosilylation by 1 increased as a function of placement of
more electron-donating substituents at the para position
(entry 4). The hydrosilylation transformation was found to be
zero order in both benzaldehyde and H2SiPh2 and rst order in
catalyst (Fig. 4). The reaction solution remained transparent
throughout the catalysis and mercury had no effect on the
catalytic rate or the products of the reaction, consistent with a
homogeneous catalytic system.
Fig. 4 Plot of aldehyde concentration versus time for the hydro-
silylation of p-dimethylaminobenzaldehyde by 1.25, 2.5, and 5 mol% 1.
Competition experiments were carried out to explore
substituent effects on the rate of hydrosilylation. For each
experiment, the hydrosilylation of two equimolar substrates was
monitored (1 eq. benzaldehyde A: 1 eq. benzaldehyde B: 1 eq.
H2SiPh2) using 5 mol% of 1. The Hammett correlation of the
relative rates of hydrosilylation is illustrated in Fig. 5 and shows
that electron-donating substituents in the para position accel-
erate the rate of hydrosilylation.
Catalytic hydrosilylation mechanistic studies
Typically, the transition metal-catalyzed reduction of carbonyl
functionalities is mediated by Rh, Pt and other precious
metals.14 In recent years, however, there have been numerous
efforts to incorporate more environmentally benign and less
expensive rst row metals, such as Ti,15 Fe,16 Ni,17 and Cu.18
Mechanistic proposals for these metal catalysts generally invoke
a hydride mechanism in which a metal hydride species is
generated, followed by carbonyl insertion into the M–H bond to
give an alkoxide complex.
Table 2 Summary of results of catalytic hydrosilylation of para-
substituted benzaldehydes by 1
Entrya
1
Substrate
Conversion; time
>98%; 5.9 h
Chemical yieldb
99%
Three plausible mechanisms we have considered for the
hydrosilylation of benzaldehydes catalyzed by 1 are outlined in
2
3
4
>98%; 2.5 h
>98%; 2.7 h
>98%; 44 min
97%
98%
98%
5
>98%; 4.6 days
92%
a
Fig. 5 Hammett correlation diagram for the relative rates of hydro-
silylation of para-substituted benzaldehydes by 1 and H2SiPh2.
1 mmol substrate, 1 mmol H2SiPh2, 5 mol% 1, 500 mL C6D6.
Determined by NMR integration against an internal standard.
b
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Fig. 6 Synthesis of 5 (left) and solid-state structure of 5 (right).
Thermal ellipsoids are drawn at 50% probability. Solvent molecules and
hydrogen atoms (except for that of the benzaldehyde carbon) are
omitted for clarity. Selected bond lengths (A˚) and angles (ꢂ) for 5: Ni–O
1.876(2); Ni–C 1.990(2); C–O 1.312(2); P1–Ni–P2 111.1(1); P2–Ni–C
104.6(1); P1–Ni–O 103.2(1).
Scheme 2
Scheme 2 (cycles A, B, and C). In each mechanism shown,
H2SiPh2 adds to 1 to generate 4. From here, benzaldehyde can
insert into the Ni–H bond as described above, generating a Ni-
Upon treatment of an equimolar solution of 1 and [a-13C]-
alkoxide intermediate [A] depicted in cycle A, which can then benzaldehyde with stoichiometric H2SiPh2, a new interme-
undergo reductive elimination of the organic product to diate species forms. This species has a half-life of ca. 15 minutes
regenerate 1. Alternatively, in an Ojima-type mechanism,19 at room temperature. The room temperature 13C{1H} NMR
benzaldehyde can insert into the Ni–Si bond to generate a spectrum of the solution features two singlets at 76.9 ppm and
borohydrido-siloxylalkyl intermediate [B] depicted in cycle B, 66.4 ppm, corresponding to the intermediate and the nal
followed by reductive elimination of the hydrosilylated product. hydrosilylated product, respectively. In the proton-coupled 13C
Finally, in a Zheng–Chan-type mechanism,20 benzaldehyde can NMR spectrum, the peak at 76.9 ppm splits into a doublet (JCH
¼
insert into the Si–H bond to generate the intermediate species 159 Hz) and the peak at 66.4 ppm splits into a triplet (JCH ¼ 142
[C] shown in cycle C, which can then lose the organic product Hz). Over the course of the reaction, the resonance associated
via reductive elimination to close the catalytic cycle.
with the intermediate decays concomitant with growth of the
1
To better understand the hydrosilylation process, stoichio- product peak. The intermediate resonance is coupled to a H
metric reactions were carried out using [a-13C]-benzaldehyde. NMR signal at 5.18 ppm, as determined by 2D HSQC NMR data.
Treatment of a toluene-d8 solution of 1 with a stoichiometric In addition, there is a new hydride resonance at ꢀ11.4 ppm in
amount of [a-13C]-benzaldehyde yields a red solution with a the H NMR spectrum. The 31P{1H} NMR spectrum exhibits a
1
singlet in the 31P{1H} NMR spectrum at 38.6 ppm. Upon cooling singlet at 38.0 ppm, corresponding to 5 and two doublets
1
to ꢀ60 C, a H NMR resonance at 5.99 ppm is present that is centered at 34.9 (2JPP ¼ 211 Hz) and 28.0 ppm (2JPP ¼ 211 Hz),
ꢂ
coupled to a 13C{1H} NMR resonance at 91.3 ppm (HSQC). In the corresponding to the intermediate species. These three sets of
proton-coupled 13C NMR spectrum, the peak at 91.3 ppm splits peaks decay over the course of the reaction and at the end of the
into a doublet with JCH ¼ 174 Hz (free aldehyde: 190.9 ppm, d, reaction the only 31P{1H} NMR resonance is that of 1. No other
JCH ¼ 174 Hz). The upeld shis of the 1H and 13C resonances 31P{1H} NMR resonances are observed during the reaction,
are indicative of substantial d / p* back donation,21 consistent indicating that MesDPBPh remains bound to Ni throughout. No
with other Ni(h2-aldehyde) species reported in the literature.22
C– P coupling was resolved, even upon cooling to ꢀ80 C.
Repeating the reaction under catalytic conditions (5 mol% 1)
13
31
ꢂ
Single crystals obtained from this reaction mixture were
analyzed by X-ray diffraction to reveal [MesDPBPh]Ni(h2-benzal- yielded spectra with signals analogous to those described above.
dehyde) (5, Fig. 6). The nickel atom is formally three-coordinate Fig. 7 shows representative proton-coupled 13C NMR spectra of
with the benzaldehyde bound in an h2 fashion. The C]O the catalytic hydrosilylation of [a-13C]-benzaldehyde by 1 and
ꢂ
linkage lies in the trigonal coordination plane; the sum of the H2SiPh2. At ꢀ60 C, two resonances are observed immediately
angles around the benzaldehyde carbon is 354ꢂ and the phenyl aer thawing the reaction mixture, corresponding to free alde-
ring is essentially coplanar to the C]O bond. The C–O bond hyde (192.1 ppm, d, JCH ¼ 174 Hz) and complex 5 (91.3 ppm, d,
˚
length of 1.312(2) A is slightly shorter than those reported for JCH ¼ 174 Hz) (Fig. 7, top). Aer 5 min at room temperature and
related Ni(h2-carbonyl) complexes (1.32–1.34 A),
for cooling to ꢀ60 ꢂC, the proton-coupled 13C NMR spectrum
22b,23
˚
2
˚
example, 1.345(2) A in [Ni(h -HPhCO)(dippe)] (dippe ¼ 1,2- features three resonances at 192.1 (d, JCH ¼ 174 Hz), 80.3 (d,
22a
´
bis(diisopropylphosphino)ethane), reported by Campora.
JCH ¼ 159 Hz) and 65.6 ppm (t, JCH ¼ 142 Hz), corresponding to
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have been studied by multinuclear NMR and single crystal X-ray
diffraction. The equilibrium process of 4 was studied by 1H–1H
2D EXSY spectroscopy and its kinetic and thermal parameters
established. Complex 1 was found to be a competent catalyst for
the hydrosilylation of para-substituted benzaldehydes and the
reaction was determined to be zero order in both aldehyde and
silane and rst order in catalyst. Stoichiometric reactions with
[a-13C]-benzaldehyde were undertaken to probe the identity of a
reaction intermediate and to reveal a possible catalytic reaction
mechanism; based on multinuclear 1D and 2D NMR experi-
ments, a scenario invoking a borohydrido-Ni-siloxyalkyl species
is proposed. Further studies will map the scope of these and
other E–H bond cleavage processes and probe their application
toward organometallic two-electron chemistries better known
for noble metal catalysts.
Fig. 7 Representative spectra for catalytic hydrosilylation of [a-13C]-
benzaldehyde by 1 and H2SiPh2, recorded in toluene-d8. (Top) Proton-
coupled 13C NMR spectrum recorded at ꢀ60 ꢂC after thawing the
reaction mixture. (Bottom) Proton-coupled 13C NMR spectrum
recorded at ꢀ60 ꢂC after removing from the probe and shaking at
room temperature for 5 min.
Experimental section
General methods
free aldehyde, the intermediate species and the nal hydro-
silylated product, respectively (Fig. 7, bottom). Under these
conditions, a small amount of a presumably off-path, uniden-
tied Ni complex is also observed by 31P{1H} NMR spectroscopy.
In addition to the peaks corresponding to 5 and the interme-
diate species, there is another set of doublets centered at 12.7
(2JPP ¼ 294.8 Hz) and 8.3 ppm (2JPP ¼ 294.8 Hz); this species
does not decay over the course of the reaction and accounts for
6% of the total phosphorus content in solution, as determined
by integration at the end of the reaction.
Unless otherwise noted, all manipulations were carried out
using standard Schlenk or glovebox techniques under a dini-
trogen atmosphere. Solvents were dried and deoxygenated by
sparging with dinitrogen and passing through activated
alumina in a solvent purication system from SG Waters USA,
LLC. Non-halogenated solvents were tested with a standard
purple solution of sodium benzophenone ketyl in tetrahydro-
furan in order to conrm effective oxygen and moisture
removal. All reagents were purchased from commercial
suppliers and used without further purication unless other-
wise noted. [MesDPBPh]Ni (1) and [a-13C]-benzaldehyde were
synthesized by previously reported procedures.9,25 D2SiPh2 was
prepared via LAD reduction of Cl2SiPh2 in OEt2. Elemental
analyses were performed by Midwest Microlab, LLC, Indian-
apolis, IN and Robertson Microlit Laboratories, Inc., Ledge-
wood, NJ. Deuterated solvents were purchased from Cambridge
In evaluating the mechanisms and intermediates outlined
in Scheme 2, the following considerations are of particular
importance:
(1) A hydride resonance is observed in the 1H NMR spectrum,
ruling out [A] as the intermediate species.
(2) In the proton-coupled 13C NMR spectrum, the peak cor-
responding to the intermediate species splits into a doublet,
indicating that only one proton is directly attached to the 13C-
labeled carbon atom, thereby ruling out both [A] and [C] as
the intermediate species.
Isotope Laboratories, Inc., degassed, and dried over activated 3
1
A molecular sieves prior to use. H and 13C chemical shis are
˚
reported in ppm relative to SiMe4 using residual solvent 1H and
13C resonances as internal standards. 31P, 11B and 29Si chemical
shis are reported in ppm relative to 85% aqueous H3PO4,
BF3$Et2O and SiMe4, respectively. GC-MS analyses were per-
formed on an Agilent Technologies 6890N GC system with an
HP-5MS column.
(3) Siloxyalkyl complexes of various transition metals have
been reported in the literature24 and the 13C NMR shis of the
metal-bound carbon atoms range from 69 ppm to 76 ppm; the
13C NMR shi of the intermediate species for this system (76.9
ppm) is just slightly downeld of this range.
We therefore believe the data to be most consistent with a
borohydrido-Ni-siloxyalkyl intermediate species (cycle B,
Scheme 2), as in an Ojima-type mechanism. While the aldehyde
binds 1 to generate the observable adduct species 5, this species
is off path and productive catalysis can proceed through the
borohydrido-Ni-silyl complex 4 via the intermediate [B] in cycle
B. We nevertheless caution that our ability to observe interme-
diate [B] does not preclude the kinetic competence of the other
mechanistic cycles shown.
Preparation of [MesDPBPh](m-H)NiSiH2Ph (3)
To a solution of 1 (44.5 mg, 62.6 mmol) in benzene (1 mL) was
added neat H3SiPh (7.8 mL, 63.2 mmol). The solution was stirred
2 h at room temperature and pentane was added to precipitate
an orange powder. The slurry was stirred for another 30 min
and the solids collected on a glass-sintered frit. The solids were
washed with pentane and dried in vacuo (36.6 mg, 71%). Crys-
tals suitable for X-ray analysis were grown from a concentrated
1
ether solution. H NMR (400 MHz, C6D6) d 7.63 (q, J ¼ 5.8 Hz,
Conclusions
4H), 7.52 (d, J ¼ 7.4 Hz, 2H), 7.33–7.22 (m, 6H), 7.14–7.07 (m,
3
The nickel complex 1 undergoes facile Si–H bond activation to 4H), 7.06–6.82 (m, 17H), 6.43 (s, 2H), 4.30 (t, JHP ¼ 9.2 Hz,
generate new borohydrido-Ni-silyl species. These complexes 1JHSi ¼ 138 Hz, 2H), 2.14 (s, 3H), 1.93 (s, 6H), ꢀ2.47 (s, 1H). 13
C
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{1H} NMR (101 MHz, C6D6) d 141.2, 136.5, 135.0, 133.8, 133.5,
130.6, 130.0, 129.8, 129.6, 128.9, 127.0, 126.3, 25.2, 20.6. 31P{1H}
NMR (162 MHz, C6D6) d 41.1 (s). 29Si{1H} (79 MHz, C6D6) d ꢀ17.0
(t, J ¼ 38.7 Hz). 11B{1H} (128 MHz, C6D6) d ꢀ2.2. IR (KBr, cmꢀ1):
2060. Anal. calcd for C51H47BNiP2Si$OEt2: C, 73.93; H, 6.41.
Found: C, 74.05; H, 6.13.
Mercury tests for homogeneity
Method 1. A solution of 1 (30 mL, 0.061 M, 5 mol%) in C6D6
was stirred with excess Hg (72.6 mg, ca. 200 eq. per Ni atom) for
15 min. A solution of p-dimethylaminobenzaldehyde (5.2 mg,
34.8 mmol) and H2SiPh2 (6.5 mL, 35.0 mmol) in C6D6 was added
and the mixture stirred at 23 ꢂC for 2 h. The solution was then
1
ltered through Celite and analyzed by H NMR spectroscopy.
Preparation of [MesDPBPh](m-H)NiSiHPh2 (4)
Complete hydrosilylation was observed in the time expected.
Method 2. 1 (30 mL, 0.061 M, 5 mol%) was added to a C6D6
solution of p-dimethylaminobenzaldehyde (5.2 mg, 34.8 mmol)
and H2SiPh2 (6.5 mL, 35.0 mmol). The resulting solution was
stirred for 5 min at 23 ꢂC, then added to Hg (46.5 mg, ca. 125 eq.
per Ni atom) and stirred for 2 h at 23 ꢂC. The solution was then
To a solution of 1 (59.4 mg, 83.5 mmol) in benzene (1 mL) was
added neat H2SiPh2 (16.4 mL, 88.1 mmol). The solution was
stirred 2 h at room temperature and pentane was added to
precipitate a yellow powder. The slurry was stirred for another
30 min and the solids collected on a glass-sintered frit. The
solids were washed with pentane and dried in vacuo (56.8 mg,
1
ltered through Celite and analyzed by H NMR spectroscopy.
Complete hydrosilylation was observed in the time expected.
1
76%). H NMR (400 MHz, tol-d8) d 7.69 (s, 4H), 7.43 (d, J ¼ 7.5
Hz, 2H), 7.18 (s, 2H), 7.15–6.91 (m, 20H), 6.89–6.76 (m, 6H),
6.71–6.60 (m, 4H), 6.33 (s, 2H), 5.01 (t, 3JHP ¼ 14.7 Hz, 1JHSi ¼ 88
Hz, 1H), 2.11 (s, 3H), 1.84 (s, 6H), ꢀ2.61 (s, 1H). 13C{1H} NMR
(101 MHz, C6D6) d 141.2, 139.3, 137.1, 136.3, 135.6, 134.1, 133.8,
133.1, 130.5, 129.9, 129.8, 129.3, 128.9, 127.5, 127.4, 127.0,
126.3, 124.5, 25.4, 20.6. 31P{1H} NMR (162 MHz, C6D6) d 40.9 (s).
29Si{1H} (79 MHz, C6D6) d 3.1 (t, J ¼ 37.9 Hz). 11B{1H} (128 MHz,
Variable temperature solution NMR studies
Equilibrium of 1 and 4. Ferrocene and 4 were dissolved in
toluene-d8 and transferred to a J. Young NMR tube. The tube
was inserted into a temperature controlled NMR probe and H
1
NMR spectra were collected at 10 K intervals from 328 K to 368
K, allowing 20 min for equilibration at each temperature. The
concentration of 1 and 4 were determined by integration of the
mesityl aryl C–H resonance for the respective complexes and
silane concentration was assumed to be equal to that of 1.
Details concerning the calculation of the thermal parameters
are given in the ESI.†
C6D6)
d
ꢀ1.7. IR (KBr, cmꢀ1): 2058. Anal. calcd for
C
57H51BNiP2Si: C, 76.45; H, 5.78. Found: C, 76.03; H, 5.99.
Preparation of [MesDPBPh]Ni(h2-benzaldehyde) (5)
To a slurry of 1 (53.2 mg, 74.8 mmol) in pentane (15 mL) was
added neat benzaldehyde (7.7 mL, 75.5 mmol). The slurry was
stirred 2 h at room temperature, during which time brown-red
microcrystalline material precipitated. The solids were
collected on a glass-sintered frit and washed with pentane, then
dried in vacuo (48.2 mg, 78%). 1H NMR (500 MHz, C6D6) d 7.77–
7.59 (m, 4H), 7.07–6.87 (m, 14H), 6.87–6.71 (m, 7H), 6.68 (d, J ¼
4.7 Hz, 4H), 6.50 (s, 2H), 6.48–6.38 (m, 4H), 6.25 (s, 1H), 2.35 (s,
6H), 2.07 (s, 3H). 13C{1H} NMR (126 MHz, C6D6) d 161.9, 145.7,
144.0, 140.0, 134.2, 132.5, 132.3, 139.6, 128.7, 126.5, 125.2,
124.7, 84.1, 26.9, 21.0. 31P{1H} NMR (202 MHz, C6D6) d 38.6 (s).
Equilibrium of 1 and [MesDPBPh](D)NiSiDPh2 (4-D). Ferro-
cene and an equimolar amount of D2SiPh2 and 1 were dissolved
in toluene-d8 and transferred to a J. Young NMR tube. The tube
1
was inserted into a temperature controlled NMR probe and H
NMR spectra were collected at 10 K intervals from 298 K to 358
K, allowing 20 min for equilibration at each temperature.
Concentrations of 1 and 4-D were determined by integration of
the mesityl aryl C–H resonance for the respective complexes and
silane concentration was assumed to be equal to that of 1.
Details concerning the calculation of the thermal parameters
are given in the ESI.†
11B{1H} (128 MHz, C6D6)
52H45BNiOP2: C, 76.41; H, 5.55. Found: C, 76.03; H, 5.99.
d 58.2 (br). Anal. calcd for
C
1H–1H EXSY study of the equilibrium of 1 and 4. 2D H–1H
1
EXSY spectra were collected using the samples prepared for the
variable temperature experiments outline above. To determine
the exchange rates of the equilibrium between 1 and 4, two 2D
EXSY experiments were acquired (mixing times (Tm) of 0 ms and
700 ms) at 10 K intervals from 328 K to 358 K, allowing 30 min
for equilibration at each temperature. Details and a discussion
concerning the extraction of the kinetic parameters are given in
the ESI.†
Hydrosilylation studies
In a typical experiment, a J. Young NMR tube was charged with 1
(100 mL, 0.039 M in C6D6), substrate (20 eq. relative to 1),
ferrocene (83 mL, 0.236 M in C6D6), C6D6 and frozen. Once
frozen, H2SiPh2 (20 eq. relative to 1) was added and the tube
frozen (total volume ¼ 600 mL). Once at the spectrometer, the
solution was thawed and spectra collected at regular intervals.
Aer completion, the reaction mixture was then exposed to air,
diluted with toluene, ltered through a plug of silica and
analyzed by GC-MS. Hydrosilylation products were identied
Crystallographic information
using 1H NMR and GC-MS data in comparison to the literature Low-temperature single crystal X-ray diffraction studies were
data.13,26 To aid characterization, (1-(4-triuoromethylphenyl)- carried out at the Beckman Institute Crystallography Facility on
methoxy)diphenylsilane was converted to 4-(triuoromethyl)- a Bruker Kappa Apex II diffractometer with Mo Ka radiation (l ¼
1
˚
phenyl methanol by treatment with 1 M HCl; the H NMR and 0.71073 A). Structures were solved by direct or Patterson
GC-MS data obtained agree with the literature data.27
methods using SHELXS28 and rened against F2 on all data by
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full-matrix least squares with SHELXL-97 (ref. 29) using estab-
lished techniques.30 Full details are given in the ESI.†
9 W. H. Harman and J. C. Peters, J. Am. Chem. Soc., 2012, 134,
5080.
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10 (a) F. Hoffmann, U. Bohme and G. Roewer, Z. Naturforsch., B:
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This work was supported by the National Science Foundation
Center for Chemical Innovation on Solar Fuels (CCI Solar, Grant
CHE-1305124) and the Gordon and Betty Moore Foundation.
We thank David VanderVelde for assistance with NMR
experiments.
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