Organometallics
Article
metal trityl complexes have also proven to be good sources of
metal-centered radicals, including Re(CO)4(η3-CPh3) and
LMeFe(η1-CPh3) (LMe = {(2,6-iPr2C6H3)NC(Me)}2CH),
providing further support for this concept.4 However, Ni-
(CPh3)2 was not fully characterized, and its structure was never
determined by X-ray crystallography. The latter point is
significant because there is some uncertainty concerning the
binding mode of the trityl ligand in Ni(CPh3)2, with Wilke
suggesting an η1 binding mode and other authors suggesting an
η3 binding mode.5
Herein, we report the synthesis and characterization of
bis(η3-trityl)nickel (1) and demonstrate its utility as a Ni(0)
synthon during the preparation the Ni phosphinidene cluster,
[Ni3(μ3-PPh)(μ-PPh2)2(PPh3)3] (3).
RESULTS AND DISCUSSION
Figure 1. ORTEP drawing of one independent molecule of 1. Shown
with 50% probability thermal ellipsoids. All hydrogen atoms are
omitted for clarity. Selected bond lengths (Å): Ni1−C1 = 2.05(2),
Ni1−C2 = 2.01(2), Ni1−C3 = 2.10(2), Ni1−C20 = 2.08(2), Ni1−
C21 = 2.03(2), Ni1−C22 = 2.04(2).
■
Reaction of a suspension of NiCl2 in THF with 2 equiv of
Li(CPh3) at −25 °C resulted in formation of Ni(η3-CPh3)2
(1), which was isolated as a purple powder in 46% yield after
work-up (Scheme 1). While similar yields were reported by
Scheme 1. Synthesis of Ni(η3-CPh3)2 (1)
have also been structurally characterized, including recently
LMeFe(η1-CPh3), which features an η1 trityl binding mode,4b
and [Ti(CO)4(η5-CPh3)]−, which features an η5 trityl binding
mode.8
The 1H NMR spectrum of 1 in CD2Cl2 at room temperature
exhibits several very broad resonances in the aromatic region,
tion, Figure S1).5b On cooling the NMR sample to −40 °C,
however, these resonances sharpen considerably, decoalescing
and S3). Doublets at 6.24 and 6.57 ppm are assignable to the
ortho protons of the Ni-coordinated and free phenyl groups,
respectively. A triplet at 6.99 ppm is assignable to the para
protons of the Ni-coordinated phenyl groups, a multiplet at
7.05 ppm is assignable to overlapping meta resonances of free
and bound phenyl groups, and a triplet at 7.23 ppm is
assignable to the para protons of the free phenyl groups. The
pattern and relative ratios of these peaks imply that
interconversion of coordinated and free phenyl groups via an
η1-trityl intermediate is slower than the NMR time scale at this
temperature.4a,9 However, the observation that the two ortho
protons of the Ni-coordinated phenyl group are still magneti-
cally equivalent at this temperature suggests that suprafacial
exchange of the η3-trityl ligand is still occurring.5b,9,10 Cooling
the sample to −79 °C led to only slight broadening of this
resonance, suggesting that the barrier to suprafacial exchange is
very low.
To test the efficacy of 1 as a Ni(0) synthon, we probed its
reaction with CO. Thus, addition of CO (1 atm) to a CD2Cl2
solution of 1 resulted in an immediate color change from
purple to colorless (Scheme 2). A solution-phase IR spectrum
of this sample revealed the presence of a strong absorption
which is assignable to the T2 stretching mode of Ni(CO)4 (cf.
ν(CO) = 2043 cm−1).11 Moreover, a 13C{1H} NMR
spectrum of this sample featured a resonance at 192 ppm,
matching the reported value for Ni(CO)4.12 Finally, examina-
tion of 1H NMR spectrum confirmed the presence of
Gomberg’s dimer, as indicated by the presence of diagnostic
multiplets at 6.22 and 5.96 ppm, along with a singlet at 5.11
Wilke and Schott in their preparation of 1, we found it
necessary to modify the reported work-up. In particular, the
low solubility of 1 in benzene (see below) made the reported
work-up (recrystallization from benzene) impractical. Instead,
we found that washing the crude reaction mixture with
hexanes, followed by washing with a 1:1 (v:v) solution of
MeCN and H2O, gave suitably pure material in acceptable
yields.
As a solid, 1 is stable in air for several months. However,
solutions of 1 in THF or CH2Cl2 are quite sensitive, quickly
losing their purple hue on exposure to air. Complex 1 is
insoluble in hexanes, acetonitrile, diethyl ether, methanol, and
water, and very sparingly soluble in ethyl acetate and acetone.
It is somewhat more soluble in benzene, toluene, and THF
(∼1 mg/mL in each). The highest solubility of 1 is observed in
CH2Cl2 and CHCl3 (∼2 mg/mL). However, it does slowly
decompose in these solvents, concomitant with formation of
Gomberg’s dimer and small amounts of triphenylmethane.
Complex 1 crystallizes in the monoclinic space group Pn,
with two independent molecules in the asymmetric unit
(Figure 1). The crystal was twinned and weakly diffracting,
which reduced the precision of the resulting metrical
parameters; nonetheless, the connectivity of 3 was unambig-
uously confirmed. Each trityl ligand in 1 adopts an η3 binding
mode, resulting in a structure that is reminiscent of the
archetypal Ni(II) organometallic, bis(η3-allyl)nickel.6 Its Ni−C
distances range from 2.01(2) to 2.10(1) Å (av. Ni−C = 2.05
Å), while the C−C bonds within the allyl unit range from
1.40(2) to 1.48(2) Å (av. Callyl−Callyl = 1.44 Å). These values
are similar to those reported for the only other structurally
characterized Ni(trityl) complex, CpNi(η3-trityl), as well as
(acac)Pd(η3-trityl).5b,7 A handful of other trityl complexes
B
Organometallics XXXX, XXX, XXX−XXX