Organometallics
Article
a Chemglass NMR tube reaction heating block or irradiated at 350
nm in a Rayonet RPR-1000 photochemical reactor for a total of 6 h
(not including breaks for acquiring NMR spectra). NMR spectra were
acquired approximately every hour (exact times were recorded and
used during analysis of the results). During processing, the integration
values of all signals were standardized relative to the cyclohexane
signal (the cyclohexane signal integration was set to 10). Integration
values were then normalized by multiplying the integral by a scalar
value to correct for the actual initial concentrations at t = 0.
Synthesis of NaCp·1/3 THF (24). The title compound was
prepared according to a procedure adapted from the literature.41
Sodium (2.00 g, 87.0 mmol) was added to dicyclopentadiene (160
mL), and the mixture was heated at 160 °C for 24 h until hydrogen
evolution ceased and no sodium remained. The copious white solid
was collected by vacuum filtration. This solid was heated in toluene
(60 mL) and THF (6 mL) at reflux for 1 h, after which it was
collected by vacuum filtration, washed with copious pentane, and
dried in vacuo to give a free-flowing off-white solid (6.78 g, 60.5
mmol, 69% yield). Spectral data are consistent with those previously
reported.41
Synthesis of Ta(NCMe3)Cl3(py)2 (25). The title compound was
prepared according to a procedure adapted from the literature.25c tert-
Butylamine (5.9 mL, 56 mmol) and pyridine (20.0 mL, 250 mmol)
were added sequentially to a stirred heterogeneous mixture of freshly
sublimed TaCl5 (10.00 g, 27.9 mmol) and Na2SiO3 (6.84 g, 56.0
mmol) in toluene (80 mL) and Et2O (20 mL). The mixture was
stirred for 18 h, after which it was filtered over Celite. The Celite plug
was washed with toluene, and the combined filtrates were
concentrated in vacuo to give an opaque yellow solid. Recrystallization
from dichloromethane/pentane at −25 °C gave large yellow crystals
after 5 d (8.04 g, 56% yield). An additional product can be obtained
from the same recrystallization liquor with additional time and
pentane additions. Spectral data are consistent with those previously
reported.25c
Synthesis of Cp2Ta(NCMe3)Cl (26). The title compound was
prepared according to a procedure adapted from the literature.25b
While cooling to −84 °C,42 a solution of NaCp·1/3 THF (1.88 g,
16.88 mmol, 2.1 equiv) in Et2O (60 mL) and THF (15 mL) was
added to a heterogeneous solution of Ta(NCMe3)Cl3(py)2 (4.13 g,
8.00 mmol, 1.0 equiv) in Et2O (270 mL). The mixture was allowed to
slowly warm to room temperature and stirred for 18 h, after which
solvent removal in vacuo gave an opaque red solid. Extraction into
pentane (3 × 50 mL) and concentration of the combined extracts
gave a deep red oily-to-waxy product (2.79 g, 83% yield). Spectral
data are consistent with those previously reported.25b
Synthesis of Cp2TaH3 (1). The title compound was prepared
according to a procedure adapted from the literature.25a While cooling
to −84 °C, a solution of Cp2Ta(NCMe3)Cl (2.79 g, 6.68 mmol, 1.0
equiv) in Et2O (65 mL) was added to a suspension of LiAlH4 (1.02 g,
26.8 mmol, 4.0 equiv) in Et2O (65 mL). The mixture was allowed to
slowly warm to room temperature and stirred for 18 h, after which it
was cooled to 0 °C and degassed deionized water was added dropwise
(1.96 mL, 108 mmol, 16 equiv). Extraction of the mixture into Et2O
(3 × 50 mL) and concentration of the combined extracts gave an off-
white solid (0.90 g, 43% yield). Spectral data are consistent with those
previously reported.25a
C6D6 at 100 °C for several hours. The results of DFT
calculations are consistent with a steric explanation for the lack
of reactivity of Cp2*TaH3. In particular, the [Cp2*Ta]
framework undergoes more extensive unfavorable distortion
upon π-coordination compared to the corresponding [Cp2Ta]
or [CpCp*Ta] scaffolds.
The use of early transition metals for hydrocarbon activation
is attractive because of their high reactivity toward inert bonds.
Although homogeneous tantalum hydrides are potential
candidates for catalytic hydrocarbon upgrading, strategies to
enhance the stability of such complexes without detracting
from their activity will be necessary in order to realize their
potential as C−H activation catalysts. Furthermore, activation
3
of C(sp )−H bonds by homogeneous tantalocene hydrides
remains elusive.
EXPERIMENTAL SECTION
■
General Materials and Methods. Sodium bis(trimethylsilyl)-
amide and tert-butylamine were obtained from Acros Organics and
used as received. Methyl iodide and pyridine were obtained from
Acros Organics in ACROSeal bottles and used as received.
Dicyclopentadiene was obtained from Acros Organics and passed
across a short silica plug to remove water and stabilizer prior to use.
Sodium metal (foil-wrapped sticks in mineral oil) was obtained from
Acros Organics and washed with copious hexanes prior to use. TaCl5
was obtained from Acros Organics and purified by sublimation prior
to use. Di-n-butyl ether, sodium metasilicate, and triphenyl phosphite
were obtained from Alfa Aesar and used as received. Potassium
hydride (30% w/w in mineral oil) was obtained from Alfa Aesar,
washed with copious hexanes, and then dried under high vacuum
before use. Isotopically labeled compounds (C6D6, CDCl3, D2,
DMSO-d6, and toluene-α,α,α-d3) were obtained from Cambridge
Isotopes. Dichloromethane, diethyl ether (Et2O), pentane, tetrahy-
drofuran (THF), and toluene were obtained from Fisher Chemical,
degassed, and dried on a JC Meyer solvent system prior to use.
Magnesium turnings and tri-n-butyltin chloride were obtained from
Oakwood Chemical and used as received. Trimethylphosphine was
either prepared according to a procedure adapted from the literature
(vide infra)39 or purchased from Strem and used as received. 1,2,3,4,5-
Pentamethylcyclopentadiene and lithium aluminum hydride were
obtained from TCI America and used as received. Unless otherwise
noted, all operations were performed in a nitrogen-filled glovebox or
using air-free techniques in oven-dried glassware. NMR spectra were
recorded at 298 K on a Bruker DRX 500 MHz (500.233 MHz for 1H)
spectrometer. Elevated-pressure 1H NMR experiments were per-
formed in 9 in. Wilmad Glass medium-wall high-pressure precision
NMR tubes. 1H NMR chemical shifts are reported in parts per million
(ppm) relative to TMS, with the residual solvent peak used as an
internal reference [CHCl3 (7.26 ppm), C6D5H (7.16 ppm), and
DMSO-d5 (2.50 ppm)].
General Procedure for Measuring H/D Exchange Rates by
1H NMR. In a 4 mL vial within a glovebox, tantalocene trihydride (0.2
mol % relative to arene) was fully dissolved in a solution of deuterated
arene (1.80 mL) containing cyclohexane (0.2 mol % relative to
arene). The resulting solution was divided equally (0.60 mL each)
into three medium-wall high-pressure precision NMR tubes (Wilmad,
9″ length, 3.46 mm internal diameter), and the tubes were sealed with
Teflon stopcocks and removed from the glovebox. One at a time, the
tubes were connected to a Schlenk line and pressurized to 60 psi (4.14
bar) with the desired gas. In experiments using H2 or D2, the tube was
immersed in liquid nitrogen until the contents were frozen (about 1
min) and then evacuated and refilled three times with the desired gas,
sealed, and warmed to room temperature in a water bath (about 2
min). In experiments using a nitrogen atmosphere, low-temperature
evacuation−refill cycles were unnecessary as the headspace of the tube
was already filled with nitrogen from the glovebox.40 Initial 1H NMR
spectra (500 MHz, 32 scans, D1 relaxation delay = 10 s) were
obtained of each sample (time = 0 h), and the samples were heated in
Synthesis of NaCp* (27). The title compound was prepared
according to a procedure adapted from the literature.43 1,2,3,4,5-
Pentamethylcyclopentadiene (7.43 g, 54.6 mmol, 1.0 equiv) was
added dropwise to a stirred solution of sodium bis(trimethylsilyl)-
amide (9.17 g, 50.0 mmol, 0.92 equiv) in Et2O (45 mL), and the
opaque suspension was allowed to stir for 18 h. After this time, the
solid product was collected by vacuum filtration, washed with Et2O (2
× 5 mL) and pentane (2 × 10 mL), and dried in vacuo to give a white
solid (5.69 g, 40.0 mmol, 72% yield). Spectral data are consistent with
those previously reported.43
Synthesis of (n-Bu)3SnCp* (28). The title compound was prepared
according to a procedure adapted from the literature.44 (n-Bu3)SnCl
(9.0 mL, 33.2 mmol, 1.0 equiv) was added dropwise to a stirred
suspension of NaCp* (5.25 g, 33.2 mmol, 1.0 equiv) in toluene (200
2673
Organometallics 2021, 40, 2666−2677