Inorganic Chemistry
Communication
this paramagnetic complex (Figure S1), the identity of 1 was
confirmed using X-ray crystallography.
4 as a green powder in near-quantitative yield (Scheme 1). The
Crystals of 1 obtained from a saturated tetrahydrofuran
THF) solution were suitable for X-ray diffraction, and the
(
solid-state molecular structure is depicted (Scheme 1).
Dimeric complex 1 is C -symmetric, with each V being
2
seven-coordinate and having distorted pentagonal-bipyramidal
2
geometry. All six pyridonates are κ -bound, with the two
central ligands also bridging through the O to the adjacent V
2
1
37−39
center (μ -O), exhibiting a κ :κ bridging mode.
The μ -O
2
2
interaction is unsymmetrical [V2−O3, 2.184(3) Å; V1−O3,
2
.078(3) Å]. The oxidation state of V in 1 can be definitively
III
40−42
assigned as V , as verified using Evan’s method,
where
paramagnetic 1 gives μeff = 2.89 μ (C D solution, 25 °C),
B
6
6
consistent with two unpaired d electrons (μspin‑only = 2.83 μB).
Thus, complex 1 is primed for the reductive coupling of
alcohols.
Figure 1. ORTEP representations of complexes 4 (left) and 5 (right)
with ellipsoids shown at 50% probability and H atoms omitted for
clarity. Selected bond lengths for 4 (Å): O4−H3, 1.914 (calcd).
Selected bond lengths for 5 (Å): V1−O1, 1.5931(14); V1−O3,
Next, the reductive coupling of benzhydrol was attempted
using 1 as the catalyst. Reaction conditions similar to those
21
employed by Nicholas and co-workers were used. Gratify-
ingly, heating a C D solution of benzhydrol with 5 mol % of
2.4520(14); V2−O3, 2.0039(15); V2−O4, 1.5954(15).
6
6
dimer 1 provides full conversion to the desired product with
concomitant formation of benzophenone (Table 1, entry 1).
obtained by X-ray crystallography using dark-green crystals
grown from a saturated toluene solution. This structure shows
that the hemilability of the pyridonate ligand is advantageous
to promote coordination of the alcohol substrate and facilitate
deprotonation of the alcohol. A short hydrogen-bonding
interaction with the alkoxide [O4−H3, 1.914 Å (calcd)] is
present, giving a six-membered vanadacycle within the
Table 1. Catalytic Reductive Coupling of Alcohols
a
a
distorted octahedral complex. Paramagnetic complex 4 has
entry
R
time (h)
product yield (%)
carbonyl yield (%)
1
1
1
2
3
Ph
Me
H
24
48
48
>99
65
34
88
66
31
b
NMR spectrum of 4 (Figure S2) shows an increased number
of signals compared to that of the more symmetric complex 1.
Most importantly, 4 provides experimental evidence for the
a
1
Yields determined by H NMR using 1,3,5-trimethoxybenzene as an
23
analogous species predicted computationally. Notably,
b
internal standard. A 1:1 mixture of meso/racemic diastereomers was
deprotonation of an alcohol by a pyridonate ligand is the
observed.
first step in iridium-catalyzed acceptorless alcohol dehydrogen-
2
9
ation, and here we show that similar metal−ligand
1
This is confirmed by the H NMR spectrum after the reaction
cooperativity is relevant to early-transition-metal reactivity.
2
(Figure S4), in which a singlet is observed at 4.68 ppm and a
Evan’s method of 4 confirms a d metal (μ = 2.62 μ ; C D
eff
B
6
6
doublet is observed at 7.71 ppm; these signals are diagnostic
for the methine H atoms of 1,1,2,2-tetraphenylethane and the
solution, 25 °C), as expected for this redox-neutral reaction.
Complex 4 was then heated in toluene to achieve reductive
coupling of the bound alcohol (Scheme 1). After 4 h, the dark-
green solution turned turquoise. Following removal of the
volatiles and subsequent recrystallization from toluene, blue
crystals of 5 suitable for X-ray diffraction studies (Figure 1,
right) and some colorless crystals (vide infra) were obtained.
21
o-H atoms of benzophenone, respectively. Observation of the
ketone byproduct suggests that the reaction proceeds through
the previously reported mechanism in which the alcohol acts as
a reductant. This reaction can be successfully extended to
Dimeric terminal oxo complex 5 is C -symmetric, with each V
2
43,44
derived from lignin biomass.
In the case of 1-phenyl-
atom being six-coordinate and having distorted octahedral
2
ethanol, a 1:1 mixture of a racemic/meso product is observed.
No ether byproducts were observed by gas chromatography
geometry. Similar to 1, all pyridonates bind κ , with two having
2
1
the κ :κ bridging mode through a μ -O to the other V center.
2
(
GC)−MS.
The μ -O−V interaction is more unsymmetrical in 5 compared
2
Previous work has shown that 1,3-N,O chelating ligands can
to 1 [V1−O3, 2.4520(14) Å; V2−O3, 2.0039(15) Å] due to
the strong trans influence of the oxo ligand. Additionally, the
V−O distances [V1−O1, 1.5931(14) Å; V2−O4, 1.5954(15)
Å] are in good agreement with a reported pyridine-2-thiolate
be used to promote the isolation and characterization of
reactive intermediates in catalysis.
studies of vanadium-catalyzed reductive coupling have used
density functional theory to predict intermediates in the
catalytic cycle. Here we can use the vanadium pyridonate
complex to our advantage to complete stoichiometric studies
45−49
Reported mechanistic
5
0
1
oxo
2
3
spectrum of 5 (Figure S3) gives a diagnostic signal at 13.91
ppm, consistent with increased symmetry relative to 4.
23
IV
1
and gain complementary mechanistic information.
Paramagnetic 5 contains two V
d metal centers, as
First, the interaction of bimetallic complex 1 with 2 equiv of
benzhydrol was investigated. Within 21 h at room temperature,
the reaction in THF or toluene produced a green solution,
which upon removal of volatiles afforded the alkoxide complex
predicted from the solid-state molecular structure. Thus, each
V atom is oxidized by one electron upon going from 4 to 5,
consistent with reductive coupling. Indeed, 1,1,2,2-tetraphenyl-
1
ethane was observed by H NMR spectroscopy when the
B
Inorg. Chem. XXXX, XXX, XXX−XXX
Griffin, Samuel E.
