Organometallics
Article
Synthesis of 3. Solid 1 (34 mg, 0.078 mmol) was suspended in
pentane (6.5 mL) in a 20 mL scintillation vial with a stir bar. PhSiH3
(0.96 mL, 7.8 mmol) was added, and the resulting solution was stirred
for 5 min. The solution was filtered through Celite and was stored at
−40 °C overnight. Red-orange X-ray diffraction quality crystals (5.5
mg, 15% yield) were obtained. A second crop of crystals (6.1 mg, 17%
yield) was grown by leaving the mother liquor for 1 day more at −40
°C to give an overall yield of 32%. 1H NMR (400 MHz, C6D12 + drop
of PhSiH3): δ 29.3 (3H, Si−H), 20.6 (4H, m-H), 16.1 (12H, o-H),
15.7 (6H, p-H), and −94.6 (1H, dialdiminate ligand backbone) ppm.
Peaks corresponding to remaining protons are apparently either
broadened or obscured under peaks for free PhSiH3. Anal. Calcd for
C27H33N2SiCo: C, 68.62; H, 7.04; N, 5.93. Found: C, 68.69; H, 6.91;
N, 6.10 (first crop). C, 68.30; H, 6.96; N, 5.78 (second crop).
X-ray Crystallography. Low-temperature diffraction data (ω
scans) were collected at the Advanced Light Source, Lawrence
Berkeley National Laboratory, on a Bruker D8 goniometer coupled to
a PHOTON 100 detector with synchrotron radiation (λ = 0.7749 Å)
for the structure of 3. The data were integrated with the APEX2
software package, and an absorption correction was applied with
SADABS.36 The structure was solved using SHELXT and was refined
against F2 on all data by full-matrix least squares with SHELXL.37 All
non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were included in the model at geometrically calculated
positions and refined using a riding model. The isotropic displace-
ment parameters of all hydrogen atoms were fixed to 1.2 times the U
value of the atoms to which they are linked (1.5 times for methyl
groups). The only exceptions were the Si-bound H14a, H14b, and
H14c which were found in the difference map and freely refined. The
full numbering scheme of 3 can be found in the full details of the X-
ray structure determination, which is included as Supporting
reminiscent of our earlier mechanistic studies on β-hydride
elimination with β-diketiminate complexes of iron and
cobalt,32 where the mechanisms have the same elementary
steps that are familiar from low-spin complexes. Thus, high-
spin electronic configurations and weak ligand fields are not a
hindrance to well-controlled organometallic catalysis.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations except the syntheses
of alkene substrates and PhSiD3 were performed in an argon-filled
MBraun glovebox maintained below 1 ppm of O2 and H2O. Syntheses
of alkene substrates and PhSiD3 were performed under an N2
atmosphere using standard Schlenk techniques. Glassware was oven-
dried at 150 °C for at least 12 h prior to use. Celite and molecular
sieves were dried above 200 °C under vacuum for at least 12 h.
Pentane, hexane, benzene, toluene, and diethyl ether were purified by
passage through activated alumina and Q5 columns from Glass
Contour Co., under argon. Cyclohexane was dried over CaH2,
vacuum-transferred, and stored over 4 Å molecular sieves. THF was
distilled under argon from a potassium benzophenone ketyl still.
Benzene-d6 was dried over activated alumina and stored over 4 Å
molecular sieves. C6D12 was vacuum-transferred from a solution of
potassium benzophenone ketyl and was stored over 4 Å molecular
sieves. 1-Hexene and PhSiH3 were obtained from TCI and used as
received after checking their purity by 1H NMR spectroscopy. 1-
Hexene was vacuum-transferred from CaH2, passed through activated
alumina, and stored over 4 Å molecular sieves. PhSiH3 was dried over
4 Å molecular sieves prior to use. The hydrosilylation catalyst 1 and
PhSiH2hex were prepared by following a previously reported
procedure.6p PhSiD3 was synthesized by modifying a known
procedure (reduction of PhSiCl3 was done in tetraglyme instead of
THF).33 Substrates 534 and 635 were synthesized following previously
reported procedures. NMR data were collected on Agilent 400 or 500
MHz spectrometers. Chemical shifts in 1H NMR spectra are
referenced to the residual protiated solvent peaks of C6D5H (δ 7.16
ppm) and C6D11H (δ 1.38 ppm).
Monitoring the Catalyst Resting State. A stock solution of 1
(11 mM) was made by dissolving 1 (20 mg, 0.045 mmol) in C6D12
(4.0 mL). A J. Young NMR tube was charged with C6D12 (0.40 mL),
PhSiH3 (28 μL, 0.23 mmol), and 1-hexene (28 μL, 0.23 mmol), and
the resulting solution was frozen in the cold well at −78 °C. Then, an
aliquot of the stock solution of 1 (0.10 mL, 1.1 μmol of 1) was placed
in the J. Young tube, and it was frozen in the cold well as well. The
NMR tube was thawed immediately before inserting the sample into
NMR-Scale Alkene Hydrosilylation. A 20 mL scintillation vial
was charged with the solvent (C6D6 or C6D12, 0.80 mL), 1 (5.0 mg,
0.011 mmol), PhSiH3 (0.14 mL, 1.1 mmol), and 1-hexene (0.14 mL,
1.1 mmol). The solution was transferred to a J. Young NMR tube, and
a 1,3,5-trimethoxybenzene internal standard capillary was added. The
1H NMR spectrum was measured periodically, and the product
PhSiH2hex was identified on the basis of previously reported NMR
1
the spectrometer. An H NMR spectrum was recorded every 5 min
1
for 8 h. H NMR assignments of the Co species used in the Keq
determination were used.
Hydrosilylation of 2-Phenyl-1-Methylenecyclopropane (5).
In a 1 dram vial equipped with a stir bar in an N2 glovebox were
placed 1 (200 μL of a 12.5 mM C6D6 stock solution, 2.5 μmol of 1),
PhSiH3 (125 μL, 1.0 mmol), 5 (63 mg, 0.48 mmol), and C6D6 (175
μL). The vial was capped with a resealable valve and then brought out
of the glovebox and heated at 80 °C for 8 h. Then, the reaction
mixture was exposed to air. Diethyl ether (2.0 mL) was added, and
the solution was filtered through a short silica gel pad (1 cm,
prewashed with diethyl ether) in a disposable pipet. The filtrate was
concentrated under reduced pressure. The hydrosilylation product
was isolated via flash chromatography (hexane) as a clear oil (108 mg,
assignments in C6D6.6p PhSiH2hex: H NMR (400 MHz, C6D12) δ
1
7.47−7.51 (m, 2H), 7.19−7.26 (m, 3H), 4.34 (t, 2H, J = 3.6 Hz),
1.42−1.50 (m, 2H), 1.34−1.41 (m, 2H), 1.24−1.32 (m, 4H), 0.86−
0.94 (m, 5H) ppm.
Determination of Keq for Arene/Alkene Exchange. The Keq
measurement of benzene and PhSiH2hex exchange is shown as a
representative example (other exchange Keq measurements were
performed in the same fashion at a comparable scale). Solid 1 (10.0
mg, 22.6 μmol) was suspended in C6D12 (0.53 mL) in a 20 mL
scintillation vial. An equimolar solution of benzene (0.32 mL, 3.6
mmol) and PhSiH2hex (0.80 mL, 3.6 mmol) was prepared in a
separate vial. The benzene/PhSiH2hex solution (74 uL, 0.24 mmol of
both benzene and PhSiH2hex) was added to 1, and the suspension
became homogeneous. The resulting solution was transferred to a J.
Young NMR tube with a NiCp2 internal standard capillary, and the
1H NMR spectrum was recorded at 10 °C. Additional C6H6 (7.5 μL,
0.084 mmol) was added to the NMR tube, and the 1H NMR
spectrum was recorded at 10 °C. This process was repeated again by
adding more benzene (15 μL, 0.17 mmol). Keq was calculated by
1
82% yield). Integration of the peaks in the H NMR spectrum, along
with COSY, HSQC, and HMBC spectra, identified 5 (see the
1
2H), 7.01−7.14 (m, 8H), 4.42 (m, 2H, JSi−H = 96.5 Hz), 1.87 (dt, 1H,
J = 8, 6 Hz), 0.89−0.97 (m, 1H), 0.69−0.78 (m, 2H), 0.46 (dt, 1H, J
= 5, 5 Hz), 0.34−0.42 (m, 1H) ppm. 13C NMR (125 MHz, C6D6): δ
139.3, 135.6, 132.7, 129.8, 129.6, 128.34, 128.3, 126.1, 22.5, 15.4,
11.4, 9.7 ppm.25
Hydrosilylation of Dimethyl 2,2-Diallylmalonate (6). The
reaction mixture was prepared as above using 1 (200 μL of a 25 mM
C6D6 stock solution, 5.0 μmol of 1), PhSiH3 (125 μL, 1.0 mmol), 6
(106 mg, 0.51 mmol), and C6D6 (75 μL). The reaction mixture was
brought out of the glovebox and stirred at room temperature for 1 h.
Then, the reaction mixture was exposed to air. Diethyl ether (2.0 mL)
was added, and the solution was filtered through a short silica gel pad
(1 cm, prewashed with diethyl ether) in a disposable pipet. The
1
details). 2: H NMR (500 MHz, C6D12) δ 21.1 (4H, m-H), −97.8
1
(1H, dialdiminate ligand backbone) ppm. 4: H NMR (500 MHz,
C6D12) δ 273.2 (1H, vinyl H), 25.7. (1H, dialdiminate ligand
backbone), 13.7 (4H, m-H), −34.6 (6H, p-H) ppm. Full assignment
of 2 was not feasible due to the peaks overlapping with those of 1, and
for 4, only the observed paramagnetic peaks were assigned.
G
Organometallics XXXX, XXX, XXX−XXX