Formation and Redox Interconversion of Niobium Methylidene and Methylidyne Complexes

Article abstract of DOI:10.1002/anie.201511867

The niobium methylidene [{(Ar′O)₂Nb}₂(μ₂-Cl)₂(μ₂-CH₂)] (2) can be cleanly prepared via thermolysis or photolysis of [(Ar′O)₂Nb(CH₃)₂Cl] (1) (OAr′=2,6-bis(diphenylmethyl)-4-tert-butylphenoxide). Reduction of 2 with two equivalents of KC₈ results in formation of the first niobium methylidyne [K][{(Ar′O)₂Nb}₂(μ₂-CH)(μ₂-H)(μ₂-Cl)] (3) via a binuclear α-hydrogen elimination. Oxidation of 3 with two equiv of ClCPh₃ reforms 2. In addition to solid state X-ray analysis, all these complexes were elucidated via multinuclear NMR experiments and isotopic labelling studies, including a crossover experiment, support the notion for a radical mechanism as well as a binuclear α-hydrogen abstraction pathway being operative in the formation of 2 from 1. Redox switch: Reduction of the niobium methylidene 1 with KC₈ results in formation of the first niobium methylidyne 2 through binuclear α-hydrogen elimination (see scheme). Oxidation of 2 with ClCPh₃ reforms 1 by virtue of a hydride migration. When 1 is prepared from [(Ar′O)₂Nb(CH₃)₂Cl], isotopic labeling studies suggest a radical mechanism, and a binuclear α-hydrogen abstraction being the most likely operative pathway.

Full text of DOI:10.1002/anie.201511867

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International Edition: DOI: 10.1002/anie.201511867
German Edition: DOI: 10.1002/ange.201511867
Niobium Methylidyne
Formation and Redox Interconversion of Niobium Methylidene and
Methylidyne Complexes
Keith Searles, Kyle T. Smith, Takashi Kurogi, Chun-Hsing Chen, Patrick J. Carroll, and
Daniel J. Mindiola*
Abstract: The niobium methylidene [{(Ar’O) Nb} (m -Cl) -
redox process. Both of these species have been characterized
in solution and by single crystal X-ray diffraction analysis
(XRD).
2
2
2
2
(
m -CH )] (2) can be cleanly prepared via thermolysis or
2 2
photolysis of [(Ar’O) Nb(CH ) Cl] (1) (OAr’ = 2,6-bis(diphe-
2
3 2
nylmethyl)-4-tert-butylphenoxide). Reduction of 2 with two
On a multigram scale the precursor [(Ar’O) Nb(CH ) Cl]
2
3 2
equivalents of KC results in formation of the first niobium
(1) (Scheme 1) can be readily prepared from the trans-
metallation of 2 equivalents of NaOAr’’ with [Nb-
(CH ) Cl ]. When treated with a strong Brønsted base
3 2 3
8
methylidyne [K][{(Ar’O) Nb} (m -CH)(m -H)(m -Cl)] (3) via
2
2
2
2
2
[7]
a binuclear a-hydrogen elimination. Oxidation of 3 with two
equiv of ClCPh reforms 2. In addition to solid state X-ray
such as the phosphorus ylide, H CPPh (2 equivalents), a rare
2 3
3
analysis, all these complexes were elucidated via multinuclear
NMR experiments and isotopic labelling studies, including
a crossover experiment, support the notion for a radical
mechanism as well as a binuclear a-hydrogen abstraction
pathway being operative in the formation of 2 from 1.
A
lthough methylidynes are known ligands for group 6 and 7
[
1,2]
transition metals,
there are no reported methylidyne
complexes with group 5 transition metals. This contrasts
examples of substituted alkylidynes, where the hydrogen is
[3,4]
replaced with a more sterically encumbering group.
Our
group is particularly interested in this archetypal moiety since
tantalum methylidenes and methylidynes, supported on
a silicon oxide surface, have been implicated in the dehy-
[
5]
drocoupling of methane to ethane and ethylene. Therefore,
we decided to explore niobium systems since this metal has
more accessible low-valent states than tantalum and could
allow us to probe redox processes involving the methylidyne
ligand—such as a-elimination and a-migration. Herein, we
report the synthesis and characterization of dinuclear com-
plexes of niobium having bridging methylidene and methyl-
idyne ligands. These dinuclear species are supported by the
aryloxide ligand OAr’’ (OAr’ = 2,6-bis(diphenylmethyl)-4-
Scheme 1. Synthesis of the bridging methylidene complex 2 from
thermolysis of 1 or 1-DMAP.
example of a terminally bound methylidene can be cleanly
[7]
prepared, namely [(Ar’O) Nb=CH (CH )(H CPPh )].
2
2
3
2
3
However, when complex 1 is thermalized at 808C, in the
absence of a Brønsted base, a very slow reaction takes place
over the duration of 5 days in benzene to cleanly form the
bridging methylidene [{(Ar’O) Nb} (m -Cl) (m -CH )] (2) in
[
6]
tert-butylphenoxide). We propose the methylidene ligand
2
2
2
2
2
2
IV,IV
to form via a radical mechanism pathway involving a Nb2
90% isolated yield as a green-yellow product (Scheme 1).
Formation of complex 2 can also be achieved via photo-
irradiation of complex 1 with a Xe lamp in benzene over
dimer, combined with a binuclear a-hydrogen abstraction
step. Formation of the first group 5 methylidyne complex
occurs by a reductively promoted a-hydrogen elimination
1
13
1
a much shorter time period of 18 h. The H and C{ H} NMR
spectra of complex 2 indicate formation of a diamagnetic
molecule where the methylidene ligand can be unambigu-
ously identified. The methylidene resonance is unique in that
III
pathway involving two Nb centers. The methylidene and
methylidyne moieties can interconvert via a two-electron
2
À
it is the only CH2 moiety in complex 2. Hence, a DEPT-135
NMR experiment clearly identifies the methylidene carbon
resonance at 172.48 ppm as the only negative peak in the
DEPT trace of the spectrum. An HSQC NMR experiment
[
*] K. Searles, K. T. Smith, T. Kurogi, P. J. Carroll, Prof. Dr. D. J. Mindiola
Department of Chemistry, University of Pennsylvania
Philadelphia, PA 19104 (USA)
E-mail: mindiola@sas.upenn.edu
13
1
1
correlates the former C{ H} NMR resonance to a H NMR
resonance at 5.78 ppm, integrating to two hydrogens. A solid-
state single crystal structure of 2 further confirms the
Dr. C.-H. Chen
Molecular Structure Center, Indiana University
Bloomington, IN 47405 (USA)
IV
presence of two Nb centers bridged by a methylidene
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
ligand (Nb1ÀC145, 2.1194(16) Š; Nb2ÀC145, 2.1306(17) Š;
1
002/anie.201511867.
Figure 1), where the hydrogens of the methylidene carbon
6
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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) Š). 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) Š 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.
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III,III
The reduction of 2 to 3 implies that a transient Nb2
methylidene, “[K][{(Ar’O) Nb} (m -CH )(m -Cl)] (C)”, is
2
2
2
2
2
likely being formed, and that a binuclear a-hydrogen
elimination involving the methylidene CÀH bond takes
IV,IV
place to reoxidize each metal back to a Nb2
core
(Scheme 3). Notably, we can chemically promote migration
of the hydride back to the methylidyne in 3 to reform 2 by
addition of two equivalents of the oxidant ClCPh . This
3
reaction resulted in formation of KCl along with one
equivalent of Gombergꢀs dimer, the latter which we can
1
observe by H NMR spectroscopy (Scheme 3). Attempts to
react 3 with only one equivalents of ClCPh results in only
3
5
0% conversion to 2. The formation of 2 from 3 mostly likely
involves two one-electron oxidation steps and a hydrogen
migration step (in no specific order).
Figure 2. Expanded regions of the coupled (left) and decoupled (right)
HSQC spectra of complex 3 showing the m -CH (left) and m -H (right)
resonances.
In conclusion, we have reported a bridging methylidene
complex of niobium via a NbÀCH3 bond homolysis step
2
2
followed by a binuclear a-hydrogen abstraction. Reduction of
IV,IV
the Nb2
core by two electrons resulted in a-hydrogen
elimination to give rise to the first example of a group 5
methylidyne complex, which also contains a hydride ligand.
Hydride migration back to the methylidyne can be readily
accomplished by two electron oxidation. Isotopic labelling
studies point towards a mechanism involving methyl radicals
as well as a binuclear a-hydrogen abstraction process. We are
now exploring the chemistry of these methylidyne with small
molecules, in particular hydrocarbons since these species
could model silicon oxide surfaces implicated in alkane CÀH
bond activation and dehydrocoupling reactions. Future work
will also focus on the mechanism involving formation of 2
from 1.
Acknowledgements
Figure 3. Molecular structure of complex 3 showing thermal ellipsoids
t
at the 50% probability level. Carbons of the aryl CHPh and Bu
For funding, we thank the University of Pennsylvania, the
2
groups, and hydrogen atoms (with the exception of the methylidyne
m2-CH) have been omitted for clarity. The chloride is disordered over
National
CHE31152123), and the JSPS.
Science
Foundation
(CHE30848248
and
two positions and the hydride ligand could not be located from the
XRD data.
Keywords: aryloxide · dinuclear complexes · methylidene ·
methylidyne · niobium
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6642–6645
Angew. Chem. 2016, 128, 6754–6757
2
.018(4) Š. Further confirmation of the m -CH ligand is the
2
lone hydrogen located in the Fourier map for the methylidyne
+
carbon. The methylidyne also interacts with a K (C145ÀK1,
3
.237(5) Š), which is further encapsulated by three aryl
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3
Supporting Information for details.
Received: December 23, 2015
Revised: February 16, 2016
Published online: April 25, 2016
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