Angewandte
Communications
Chemie
mechanism to formation of 2 we prepared the isotopologue
[
[
{(Ar’O) Nb} (m -Cl) (m -CD )] (2-d ) from the photolysis of
2 2 2 2 2 2 2
(Ar’O) Nb(CD ) Cl] (1-d , prepared from [Nb(CH ) Cl ]
2
3
2
6
3
2
3
and 2 equivalents of NaOAr’) in C D and examined the
6
6
[11]
organic products formed. Not surprisingly, in C D , pho-
6
6
tolysis of 1-d produces the only isotope of methane, CD .
6
4
More importantly, carrying out the reaction with a 50%
mixture of 1 and 1-d in C D revealed the formation of CH ,
6
6
6
4
1
CH D, CD and CD H (confirmed from a combination of H
3
4
3
2
and H NMR spectroscopy). The formation of CD H is only
3
detected in the cross-over experiment and strongly supports
a binuclear process involving an a-hydrogen abstraction. The
crossover experiment resulted only in the formation of the
Figure 1. Molecular structure of complex 2 showing thermal ellipsoids
t
at the 50% probability level. Carbons of the aryl CHPh groups, Bu
2
isotopomers 2 and 2-d , thus also consistent with a binuclear
groups, and hydrogen atoms (with the exception of the methylidene
2
m -CH ) have been omitted for clarity.
process taking place.
2
2
In attempts to explore the accessibility of low-valent states
of niobium possessing a methylidene, the chemical reduction
of complex 2 was investigated. When compound 2 was
were located in the Fourier map and refined isotropically. In
addition to a bridging methylidene, two chlorides are also
shared between the two niobium centers. Each metal center
retains two aryloxides resulting in a closed-shell species most
likely having a Nb-Nb interaction (2.8329(2) ). Although
reduced with 2 equivalents of KC in toluene, a new dia-
8
magnetic species was formed in 63% isolated yield as green
crystals after removal of graphite and crystallization from
a concentrated toluene solution layered with pentane
[
7,8]
1
few examples of methylidenes exist for niobium,
the
(Scheme 3). The H NMR spectrum of this new complex
incorporation of a bridging methylidene from a Nb-methyl
moiety has not been documented.
Monitoring the formation of 2 by thermolysis in C D
intermittently via H NMR spectroscopy identifies only CH4
6
6
1
and toluene-d as by-products in the reaction mixture. The
5
same products, in addition to detectable amounts of CH D,
3
[9]
are formed when 1 is photoirradiated in C D to produce 2.
6
6
These observations suggest that 1 does not originate from the
Scheme 3. Synthesis of the niobium methylidyne 3 and oxidation back
to 2.
III
comproportionation of a Nb fragment [(Ar’O) NbCl] with
2
V
a four-coordinate Nb methylidene [(Ar’O) Nb=CH (Cl)]
2
2
since we fail to observe any formation of ethane. Instead our
preferred proposed pathway for the formation of 2 involves
a binuclear reaction where 1 undergoes NbÀC bond homol-
suggested formation of a highly asymmetric molecule with at
least four different aryloxide environments when judged by
t
ysis to form the intermediate [(Ar’O) Nb(CH )Cl)] (A) and
the different para- Bu groups for each OAr’. The most
2
3
1
a methyl radical. The identification of trace amounts of
intriguing feature of the H NMR spectrum was a highly
toluene-d5 in the reaction mixture is consistent with the
deshielded resonance at 13.18 ppm (1H), which was corre-
[10]
13
formation of methyl radicals. Subsequent dimerization of
lated to a highly deshielded C NMR doublet resonance at
IV
1
the Nb fragments A would result in the intermediate
335.18 ppm ( J = 150 Hz) via an HSQC NMR experiment
CH
[
{(Ar’O) Nb(CH )} (m -Cl) ] (B), which would then undergo
(Figure 2, left). It should be noted that the methylidyne
C NMR resonance could be directly detected in relatively
2
3
2
2
2
13
a binuclear a-hydrogen abstraction resulting in 2 along with
methane (Scheme 2). Attempts to prevent dimerization or
trap an intermediate in the reaction using a Lewis base such as
short collection times via use of a DCH CryoProbe. The
HSQC NMR spectrum also revealed a fairly broad resonance
1
4
-dimethylaminopyridine (DMAP) did not affect the out-
come of the reaction since thermolysis of 1-DMAP also yields
under identical conditions (Scheme 1). To further probe the
in the H NMR spectrum at 5.07 ppm (1H) which did not
correlate to any carbon resonances (Figure 2, right). These
spectroscopic features suggest the formation of a dinuclear
Nb complex having both a methylidyne and hydride ligand.
To conclusively establish the degree of aggregation and
connectivity involving these reactive ligand types we per-
formed an XRD study on a green single crystal. Shown in
Figure 3 is the molecular structure of the first niobium
methylidyne, namely the ate-complex [K{(Ar’O) Nb} -
2
2
2
(
m -CH)(m -H)(m -Cl)] (3). Complex 3 crystallizes in the
2 2 2
highly symmetric cubic I23 space group where the 101867.2-
3
(
19) volume of the unit cell is comprised of 24 dinuclear
molecules. The most notable features are the short and
asymmetric niobium-methylidyne bonds at 1.977(4) and
Scheme 2. Proposed mechanism to formation of 2.
Angew. Chem. Int. Ed. 2016, 55, 6642 –6645
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim