Catalytic hydrogenation activity and electronic structure determination of bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl and Hydride Complexes

Article abstract of DOI:10.1021/ja406608u

The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ( ^iPrCNC)CoCH₃, was evaluated for the catalytic hydrogenation of alkenes. At 22 C and 4 atm of H₂ pressure, ( ^iPrCNC)CoCH₃ is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (^iPrCNC)CoH, was accomplished by hydrogenation of (^iPrCNC)CoCH₃. Over the course of 3 h at 22 C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H₂-^iPrCNC)CoN₂. Similar alkyl migration was observed upon treatment of (^iPrCNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2- ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.

Full text of DOI:10.1021/ja406608u

Article
pubs.acs.org/JACS
Catalytic Hydrogenation Activity and Electronic Structure
Determination of Bis(arylimidazol-2-ylidene)pyridine Cobalt Alkyl
and Hydride Complexes
†
†
†
†
Renyuan Pony Yu, Jonathan M. Darmon, Carsten Milsmann, Grant W. Margulieux,
†
‡,§
,†
S. Chantal E. Stieber, Serena DeBeer, and Paul J. Chirik*
†
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
‡
Max-Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mu
̈
lheim an der Ruhr, Germany
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, United States
S Supporting Information
§
*
ABSTRACT: The bis(arylimidazol-2-ylidene)pyridine cobalt
iPr
methyl complex, ( CNC)CoCH , was evaluated for the
3
catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2
iPr
pressure, ( CNC)CoCH is an effective precatalyst for the
3
hydrogenation of sterically hindered, unactivated alkenes such
as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-
dimethyl-2-butene, representing one of the most active cobalt
hydrogenation catalysts reported to date. Preparation of the
iPr
cobalt hydride complex, ( CNC)CoH, was accomplished by
iPr
hydrogenation of ( CNC)CoCH . Over the course of 3 h at
3
iPr
2
2 °C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H - CNC)CoN . Similar alkyl migration
2 2
iPr
was observed upon treatment of ( CNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this
class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic,
and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in
reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting
for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.
7
,8
INTRODUCTION
of this approach. A second strategy employs redox-active
ligands, those that can engage in reversible one-electron
■
The hydrogenation of carbon−carbon multiple bonds catalyzed
by homogeneous transition metal complexes is one of the most
widely employed reactions in the synthesis of commodity and
9
−11
chemistry with the transition metal.
ative metal−ligand redox events, net two-electron chemistry
can be achieved while the metal engages in only one-electron
By enabling cooper-
1
fine chemicals. There has been renewed interest in developing
12
processes. This concept, important in the function of many
catalysts using earth abundant elements as alternatives or even
as improvements to more commonly used precious metal
1
3
metalloenzymes, has emerged as an effective strategy in base
2
a,10,12,14,15
2
,3
metal catalysis,
small molecule activation, and group
compounds. One potential challenge to the discovery of
catalysts based on first row metals is the difference in redox
properties compared with their second and third row
congeners. In catalytic hydrogenation and related processes
such as hydrosilylation and hydroboration, important bond-
breaking and making processes such as oxidative addition and
1
6,17
transfer chemistry.
Aryl-substituted bis(imino)pyridine iron dinitrogen com-
plexes have proven to be effective precatalysts for alkene
1
8,19
18,20,21
22−24
hydrogenation,
and hydroboration
and selectivity of these compounds rivals or surpasses
hydrosilylation,
cycloaddition,
2
5,26
reactions. In many cases, the activity
4
reductive elimination often rely on two-electron changes. First
row metals often engage in one-electron chemistry, offering
alternative pathways that disrupt desired transformations
required for catalytic turnover.
Two strategies have predominated in overcoming the
preference for one-electron redox chemistry associated with
base metals to achieve catalysis. The first and most established
method is the use of highly covalent, strong field ligands that
27
established precious metal catalysts. Notably, introduction
of electron donating substituents into the 4-position of the
pyridine ring increased the activity for alkene hydrogenation
and hydrosilylation.
N2)2 exhibited higher turnover frequencies than the first
generation precatalyst, ( PDI)Fe(N ) ( PDI = 2,6-(2,6- Pr -
C H -NCMe) C H N) in both reaction types (Scheme 1).
28
5
2
0b
iPr
For example, (4-Me N- PDI)Fe-
2
29
(
iPr
iPr
i
2 2
2
6
3
2
5
3
6
favor low spin configurations. The rich and continually
evolving catalytic chemistry observed with thermal and
photoactivated Fe(CO) and Fe (CO) supports the viability
Received: June 28, 2013
Published: August 5, 2013
5
2
9
©
2013 American Chemical Society
13168
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Journal of the American Chemical Society
Article
Scheme 1. Bis(imino)pyridine and Bis(arylimidazol-2-ylidene)pyridine Dicarbene Iron Dinitrogen Complexes That Are Active
a
Pre-Catalysts for Olefin Hydrogenation
a
R
The dimers, [( PDI)Fe(N )](μ -N ) (R = Me, Et), are depicted as monomers for simplicity.
2
2
2
These findings prompted study of even more electron rich iron
the catalytic hydrogenation of sterically hindered alkenes using
bis(arylimidazol-2-ylidene)pyridine cobalt complexes. The high
activity of the cobalt precatalysts raised the question as to the
electronic structure of the compounds and whether this class of
pincer ligand engages in radical chemistry with a reduced base
metal. A combined spectroscopic and computational study
clearly established the presence of ligand-centered radicals and
migration of hydrides and alkyls to the 4-position of the
pyridine, supporting this view of the electronic structure.
bis(dinitrogen) complexes. Danopoulos and co-workers
iPr
iPr
reported the synthesis of ( CNC)Fe(N ) ( CNC = 2,6-
(
2
2
i
30
2,6- Pr -C H -imadazol-2-ylidene) -C H N) and infrared
2 6 3 2 5 3
stretching frequencies of the N ligands establish a more
2
electron rich iron center than in the corresponding bis(imino)-
iPr
pyridine compounds. Accordingly, ( CNC)Fe(N ) and its
2
2
Me
less sterically protected variants, ( CNC)Fe(N ) and
2
2
Mes
(
CNC)Fe(N ) , are extremely active hydrogenation pre-
2 2
catalysts. The latter compounds exhibit rapid turnover for
unactivated tri- and tetrasubstituted alkenes, challenging
substrates for even the most active precious metal catalysts.
RESULTS AND DISCUSSION
31
■
Evaluation of Catalytic Performance. Our studies
commenced with evaluation of the bis(arylimidazol-2-ylidene)-
pyridine cobalt methyl complex, ( CNC)CoCH , for the
catalytic hydrogenation of a series of alkenes. A summary of the
observed hydrogenation activity is reported in Table 1. A
Cobalt compounds have also demonstrated considerable
promise for alkene hydrogenation. Historically, cobalt-catalyzed
alkene hydrogenation has been accomplished using borohy-
drides or hydrogen as the terminal reductant although activities
iPr
3
32
and selectivities are generally low. Budzelaar and co-workers
have since reported that four-coordinate, aryl- and alkyl-
substituted bis(imino)pyridine cobalt alkyl and hydride
iPr
comparison to previously reported ( CNC)Fe(N ) -catalyzed
2
2
28
alkene hydrogenation is also presented in the table. Each
33,34
complexes, (PDI)CoR,
are active for the hydrogenation
Table 1. Catalytic Hydrogenation of Alkenes with
Bis(arylimidazol-2-ylidene)pyridine Cobalt and Iron
Precatalysts
of simple, unactivated terminal and gem-alkenes; more sterically
35
hindered trisubstituted olefins are unreactive. Our laboratory
has prepared enantiopure, C -symmetric variants of these
1
compounds and observed asymmetric hydrogenation of
3
6
styrenyl-substituted olefins with high enantioselectivity.
Hanson and co-workers have recently reported a cobalt(II)
alkyl complex supported by a [PNP]-type pincer ligand that is
active for the hydrogenation of olefins, ketones, aldehydes, and
3
7
imines, as well as for the acceptorless dehydrogenation of
3
8
alcohols. This catalyst class offers tolerance to a range of
functional groups, although hydrogenation of sterically
hindered substrates such as tri- and tetrasubstituted olefins
39
remains a challenge.
The similarity between bis(imino)pyridines and the bis-
(arylimidazol-2-ylidene)pyridines and the improved catalytic
performance for alkene hydrogenation observed with iron
28
dinitrogen complexes of the latter pincer class prompted
evaluation of the corresponding cobalt alkyl compounds.
Danopoulos and co-workers have previously reported the
a
All catalytic reactions carried out with 5 mol % of the metal complex
(
0.032 mmol) in 0.916 M substrate benzene solution with 4 atm of H
2
b c
iPr
iPr
synthesis of ( CNC)CoCH by treatment of ( CNC)CoBr
3
at 22 °C. Conversions determined by GC-FID. Conversions
determined by H NMR spectroscopy. Substrate used as racemic
40
1
d
with methyllithium. While this compound was evaluated for
ethylene polymerization activity, catalytic alkene hydrogenation
studies, to our knowledge, were not reported. Here we describe
e
f
mixture. 32% conversion at 12 h. At 50 °C, conversion is 69% over
24 h.
1
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Journal of the American Chemical Society
Article
Scheme 2. Synthesis and Reactivity of a Bis(arylimidazol-2-ylidene)pyridine Cobalt Hydride
catalytic hydrogenation was conducted under standard
conditions employing 5 mol % of the metal precatalyst, 0.916
CoCH precatalyst prompted study of the corresponding cobalt
3
hydride, a likely component of the catalytic cycle. Exposure of a
iPr
M substrate in benzene with 4 atm of H at 22 °C. The
25 mM benzene-d solution of ( CNC)CoCH to 1 atm of
2
6
3
conversion to alkane was determined by gas chromatography
hydrogen gas resulted in rapid liberation of CH (as judged by
4
1
1
and H NMR spectroscopy.
H NMR spectroscopy) and formation of a new diamagnetic
The first substrate examined was ethyl 3,3-dimethylacrylate
C2v symmetric compound identified as the desired cobalt
iPr
(
entry 1) as this alkene has been used as a benchmark for iron-
hydride, ( CNC)CoH (Scheme 2).
1
9,28
1
catalyzed olefin hydrogenation.
The cobalt methyl
The benzene-d6 H NMR resonances observed for the
iPr
iPr
iPr
precatalyst, ( CNC)CoCH , exhibits comparable activity to
chelate in ( CNC)CoH are similar to those for ( CNC)-
3
iPr
(
CNC)Fe(N ) , completing the hydrogenation in 1 h under
CoCH with the hydrogen in the 4-position of the pyridine
2
2
3
standard conditions. Because of the high activity, more
challenging, unfunctionalized hindered alkenes were examined.
The trisubstituted alkenes, α,β-dimethylstyrene (entry 2), 2-
methyl-2-butene (entry 3), the isomers of β-methylstyrene
located downfield at 11.3 ppm. The Co−H resonance was
observed as a very broad (ν1 ≈ 148 Hz) signal whose
/2
chemical shift is dependent on the amount of excess
dihydrogen present. In a rigorously degassed sample, the
(
entry 5) and 2,6-dimethyl-2-octene (entry 6) were also readily
peak appears at −27 ppm, while in the presence of 1 atm of H
2
hydrogenated. Each substrate reached complete conversion
over the course of hours. Hydrogenation of 1-methyl-1-
cyclohexene (entry 7), a traditionally more challenging endo
cyclic trisubstituted olefin, was also successful. Approximately
this resonance shifts to −18 ppm with additional broadening. A
slight broadening of the 4-pyridine and imidazole positions
accompanies the downfield shift, consistent with rapid exchange
between the Co−H and free H on the NMR time scale.
2
3
2% hydrogenation to alkane was observed after 12 h, and
Addition of D2 to a 25 mM benzene-d6 solution of
iPr
complete conversion was achieved in 120 h. Unfortunately,
only minimal turnover was observed at 22 °C with the
tetrasubstituted olefin, 2,3-dimethyl-2-butene (entry 8). Heat-
ing the catalytic reaction to 50 °C resulted in 69% conversion
to product after 24 h. These results demonstrate that
(
corresponding iron dinitrogen complex, ( CNC)Fe(N ) ,
and is one of the most active base metal alkene hydrogenation
catalysts known, proving effective for the reduction of
unactivated, sterically hindered alkenes.
( CNC)CoCH resulted in rapid liberation of an 87:13 ratio
3
iPr
of CH D and CH along with formation of ( CNC)CoD. The
3
4
observation of CH likely arises from competitive isopropyl aryl
4
group cyclometalation. Continued exposure to excess D2
resulted in selective incorporation of the isotopic label into
the 4-position of the pyridine, one of the imidazolylidene C−H
bonds (adjacent to the pyridine), and one of the isopropyl
methyl groups (Scheme 2). The rate of deuterium incorpo-
ration into the isopropyl methyl and imidazolylidene positions
was significantly faster than the pyridine 4-position. Complete
deuteration of these positions was accomplished after
approximately 6 h with periodic agitation of the J. Young
NMR tube after which time the Co−H was no longer observed.
iPr
CNC)CoCH3 is a more active precatalyst than the
iPr
2
2
41
Synthesis, Characterization, and Reactivity Studies of
a Bis(arylimidazol-2-ylidene)pyridine Cobalt Hydride.
The high catalytic hydrogenation activity of the ( CNC)-
iPr
1
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iPr
Allowing a benzene-d solution of ( CNC)CoH to stand at
6
2
2 °C under a N atmosphere resulted in conversion to a new
2
C2v symmetric cobalt compound over the course of 3 h. The
toluene solution infrared spectrum exhibited a strong band
−
1
centered at 2048 cm signaling dinitrogen coordination.
1
Notable resonances in the benzene-d6 H NMR spectrum
include two triplets centered at 3.57 and 4.31 ppm signaling
formation of allylic and vinylic protons due to modification of
the pyridine ring. The spectroscopic data in conjunction with
structural characterization of related compounds (vida infra)
establish the identity of the purple-blue product as (4-
iPr
H - CNC)CoN , arising from migration of the cobalt hydride
2
2
Notably, there was no evidence for migration of the alkyl group
to the 4-position of the pyridine ring.
Insertion studies were also conducted with a more hindered
to the para-pyridine position of the bis(arylimidazol-2-
ylidene)pyridine chelate. The assignment of the allylic
resonances was also confirmed by isotopic labeling, whereby
iPr
1,1-disubstituted olefin. Addition of 1 equiv of 1,1-diphenyl-
(
CNC)CoD was synthesized and then converted to (4-
iPr
iPr
ethylene to a 25 mM benzene-d solution of ( CNC)CoH
6
HD- CNC)CoN . The migration of the cobalt hydride to the
,
2
under an N atmosphere resulted in a rapid color change from
2
4
-position of the pyridine ring is suppressed by excess
yellow-brown to deep blue. Analysis of a benzene-d solution of
6
dihydrogen (or dideuterium). In this manner, isotopic labeling
the product by IR spectroscopy revealed a strong band centered
experiments could be conducted to determine the fate of the
−1
iPr
at 2050 cm , again signaling dinitrogen coordination. The
Co-D during formation of (4-H,D- CNC)CoN . During the
2
1
benzene-d6 H NMR spectrum exhibited the number of
course of these investigations, we also discovered that (4-
resonances consistent with C symmetry. As with (4-
iPr
s
H - CNC)CoN could also be prepared by treatment of
2
2
iPr
H - CNC)CoN , a doublet and triplet were observed at 4.33
iPr
2
2
(
CNC)CoBr with NaBEt H.
In solution, (4-H - CNC)CoN proved to be unstable,
3
and 4.90 ppm, respectively, signaling modification of the
iPr
2
2
pyridine ring. The combined IR and NMR data support
slowly undergoing decomposition over the course of days even
at temperatures as low as −35 °C. This instability has thus far
inhibited the isolation of single crystals for X-ray diffraction.
iPr
formation of (4-CPh CH - CNC)CoN , arising from migra-
2
3
2
tion of the alkyl to the 4-position of the pyridine ring (eq 2).
Nevertheless, solutions of the compound could be generated by
iPr
hydrogenation of ( CNC)CoCH and used for subsequent
3
reactivity studies. It was of interest to determine whether the
newly formed C−H bond in the 4-position of the pyridine ring
was subject to abstraction. Addition of 1 equiv of Ph CCl to a
3
iPr
toluene solution of (4-H - CNC)CoN at −35 °C resulted in
an immediate color change from deep blue to brown. Analysis
of the product mixture by H NMR spectroscopy established
formation of ( CNC)CoCl and a stoichiometric quantity of
2
2
1
iPr
Ph CH (Scheme 2). The cobalt complex was isolated in 74%
3
yield and was also characterized by X-ray diffraction. The
overall molecular geometry and metrical parameters are similar
iPr
40
to those previously reported for ( CNC)CoBr.
A
representation of the molecular structure is presented in the
Supporting Information, and selected bond distances are
presented in the electronic structure section of the manuscript.
The modification of the pyridine ring in (4-
iPr
CPh CH - CNC)CoN was confirmed by X-ray diffraction.
2
3
2
A representation of the molecular structure is presented in
Figure 1, and selected bond distances and angles are reported in
Table 2. The overall molecular geometry about the metal is best
described as idealized planar, as anticipated for a formally
Importantly, the reactivity with Ph CCl demonstrates the
3
ability of the functionalized bis(arylimidazol-2-ylidene)pyridine
chelate to participate in cooperative metal−ligand reactivity
where removal of a C−H bond is coupled with formation of a
cobalt−chloride bond.
8
Co(I), d center. The aryl substituents are nearly orthogonal to
the idealized metal-chelate plane and the N(6)−N(7) distance
iPr
The reactivity of ( CNC)CoH with alkenes was explored to
of 1.112(2) Å signals weak activation of the N ligand. The 4-
2
gain insight into insertion behavior relevant to catalytic
position of the central pyridine ring is substituted with an alkyl
ligand resulting from a new bond between C(6) and C(36), the
more substituted carbon of the alkene forming a quaternary
center. The C(5)−C(6) and C(6)−C(7) distances are
elongated to 1.513(3) and 1.526(2) Å consistent with
formation of an sp3 hybridized carbon in the 4-position
although C(6) deviates little from the idealized plane of the
heterocycle.
hydrogenation. Addition of a slight excess (3.5 equiv) of 1-
iPr
butene or isobutene to benzene-d solutions of ( CNC)CoH
6
iPr
iPr
furnished ( CNC)CoCH (CH ) CH and ( CNC)-
2
2
2
3
CoCH CH(CH ) , respectively (eq 1). Both compounds
2
3 2
1
13
were characterized by H and C NMR spectroscopy and
established preference for the metal−alkyl derived from 1,2-
insertion of the alkene into the cobalt−hydride. Over the
course of 48 h under a dinitrogen atmosphere at 22 °C in
benzene-d6 solution both compounds decompose to an
intractable mixture of cobalt products. The rate of decom-
position is accelerated if the samples are stored under vacuum.
One possibility accounting for the formation of (4-
iPr
CPh CH - CNC)CoN is 2,1-insertion of the alkene into
2
3
2
the cobalt−hydrogen bond followed by migration of the alkyl
to the 4-position of the pyridine ring. Migration of alkyl groups
1
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Journal of the American Chemical Society
Article
45
from the cobalt hydride. Halpern proposed sequential HAT
events from in situ generated (CO) CoH during the reduction
4
46
of anthracene and in the hydrogenation of α-methylstyrene
with (CO) MnH. HAT pathways have also been proposed in
alkene reductions with chromium,
vanadium hydrides. Our current data do not allow for
distinction of these pathways. However, the observation of the
cobalt hydride and alkyl migration to the 4-position of the
pyridine raised the possibility of radical character in bis-
47
5
4
8,49
50
tungsten, and
5
1
(
arylimidazol-2-ylidene)pyridine chelates and prompted further
iPr
study of the electronic structure of ( CNC)CoX (X = H, alkyl,
halide) complexes.
Electronic Structure of Bis(arylimidazol-2-ylidene)-
pyridines Cobalt Complexes. The superior catalytic activity
iPr
iPr
of ( CNC)CoCH as compared with ( CNC)Fe(N ) and
3
2 2
related bis(imino)pyridine compounds raised the question as to
the electronic structure of the precatalyst and the role of the
bis(arylimidazol-2-ylidene)pyridine ligand. The observation of
hydride and alkyl migration to the 4-position of the chelate also
suggested the possibility of radical character in this ligand class.
iPr
Figure 1. Solid state molecular structure of (4-CPh CH - CNC)-
2
3
CoN at 30% probability ellipsoids. Hydrogen atoms, except for the
one attached to C(6), and one diethyl ether molecule were omitted for
2
iPr
1−
II
It is now well-established that diamagnetic ( PDI )Co CH
3
clarity.
is a low-spin Co(II) compound engaged in antiferromagnetic
5
2
coupling with a bis(imino)pyridine radical anion. The similar
Table 2. Selected Bond Distances (Å) and Angles (deg) for
iPr
1−
II
iPr
geometries between ( PDI )Co CH and ( CNC)CoCH
iPr
3
3
(
4-CPh CH - CNC)CoN
2
3
2
suggest the possibility of similar electronics. Determining the
oxidation state of both the metal and the pincer ligand is
essential for understanding the mechanism of turnover and
raises the broader question as to whether bis(arylimidazol-2-
ylidene)pyridine chelates can engage in radical chemistry with
an appropriately reducing base metal. To address these issues, a
series of spectroscopic, computational, and preparative studies
were carried out.
Co(1)−N(3)
Co(1)−C(3)
Co(1)−C(9)
Co(1)−N(6)
C(4)−C(5)
1.8553(16)
1.9015(19)
1.8980(19)
1.7500(17)
1.333(3)
C(7)−C(8)
1.335(3)
C(5)−C(6)
1.513(3)
C(6)−C(7)
N(1)−C(3)
C(3)−N(2)
N(4)−C(9)
1.526(2)
Evaluation of the Free Ligand. To determine whether
bis(arylimidazol-2-ylidene)pyridine chelates are redox-active,
studies with the free ligand were initially conducted. Our
laboratory has recently reported that cyclic voltammetry is one
of the most sensitive evaluators of the electronic properties of
1.357(3)
1.381(2)
1.377(2)
C(9)−N(5)
1.366(2)
53−55
N(6)−N(7)
N(1)−C(3)−N(2)
N(4)−C(9)−N(5)
1.112(2)
aryl-substituted bis(imino)pyridines.
Reduction potentials
+
103.67(15)
103.28(16)
between −2.35 and −2.83 V (vs Fc/Fc ) were observed in
THF solution depending on the specific substitution pattern of
the bis(imino)pyridine. Performing identical measurements on
iPr
56
to the 4-position of related bis(imino)pyridines has been
observed by Campora and co-workers in manganese chemistry
and has been evolved into a useful synthetic method for the
the free bis(arylimidazol-2-ylidene)pyridine, CNC, in THF
iPr
n
́
solution (1 mM CNC in 0.1 M [ Bu ]PF electrolyte) were
4 6
unsuccessful as no reversible redox waves were observed,
suggesting a less thermodynamically accessible radical anion for
this ligand class.
42
preparation of 4-alkyl substituted variants of these ligands.
The preference for 2,1 insertion of 1,1-diphenylethylene
contrasts the 1,2-insertion preference observed with 1-butene
and isobutene. In rhodium chemistry, Jones and co-workers
have rationalized the thermodynamic preference between linear
and branched metal alkyls as arising from the relative strengths
of the newly formed β-C−H bonds. It is likely that for the
cobalt complexes described here, the preference for formation
of a methyl C−H bond over a methine C−H is larger for 1,1-
diphenylethylene than for 1-butene (methylene) or isobutene.
The resulting tertiary cobalt alkyl is sufficiently sterically
pressured to undergo homolysis, and the resultant stabilized
radical then adds to the 4-position of the pyridine ring. Notably,
Despite the lack of success with electrochemical reduction,
access to the desired bis(arylimidazol-2-ylidene)pyridine radical
anion was explored chemically. For spectroscopic reference,
alkali metal salts of previously reported aryl-substituted
4
3
57
bis(imino)pyridine radical anions were initially synthesized
to evaluate various synthetic routes as well as to compare EPR
spectroscopic signatures. Stirring THF solutions of the free
chelates with excess sodium resulted in formation of red-purple
iPr
iPr
solutions identified as Na[ PDI] and Na[( BPDI)], respec-
iPr
i
tively ( BPDI = 2,6-(2,6- Pr -C H NCPh) C H N). After
2
6
3
2
5
3
dilution with toluene (toluene/THF ≈ 10:1), the samples were
44
1
,1-diphenylethylene undergoes catalytic hydrogenation and
was completely converted to the corresponding alkane in the
presence of 5 mol % ( CNC)CoCH and 1 atm of H over the
course of 24 h.
investigated by X-band EPR spectroscopy at 298 K (Figure 2).
iPr
The spectrum obtained for Na[( BPDI)] exhibits a sharp
iPr
isotropic signal centered at giso = 2.003 with 10 resolved lines
due to hyperfine coupling. Examples of EPR spectra of
bis(imino)pyridine radicals coordinated to diamagnetic metal
ions have been reported previously by Gambarotta and co-
3
2
An alternative pathway to account for formation of (4-
iPr
CPh CH - CNC)CoN involves H-atom transfer (HAT)
2
3
2
1
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lines. Indeed, the recorded isotropic spectrum centered at g
=
iso
iPr
2
.003 for Na[( PDI)] exhibits a pattern that is similar to the
iPr
spectrum of Na[( BPDI)] split by additional hyperfine
features. Unfortunately, we have not been able to obtain a
satisfactory fit of the data due to the high complexity of the
hyperfine pattern.
Similar studies were conducted with the bis(arylimidazol-2-
iPr
ylidene)pyridine, CNC. Stirring the free dicarbene with
excess sodium metal in THF immediately produced an orange
solution. Analysis by EPR spectroscopy established the
iPr
formation of a paramagnetic product assigned as Na[ CNC]
62
(
Figure 3). An alternative route, whereby CNC(HBr)2 was
iPr
Figure 2. THF/toluene solution EPR spectrum of Na[( BPDI)]
iPr
(
top) and Na[ PDI] (bottom) recorded at 298 K. Conditions for
iPr
Na[( BPDI)], microwave frequency = 9.3749 GHz, power = 0.63
mW, modulation amplitude = 0.05 mT/100 kHz; conditions for
Na[( PDI)], microwave frequency = 9.3746 GHz, power = 0.63 mW,
iPr
modulation amplitude = 0.05 mT/100 kHz.
iPr
58
iPr
workers for ( PDI)AlMe , our laboratory for ( BPDI)VE
2
2
59
iPr
60
(
E = O, S, NPh) and ( PDI)CoN . These studies
2
established that the major contributions to the hyperfine
coupling pattern arise from the nitrogen and hydrogen nuclei
constituting the plane of the bis(imino)pyridine ligand with
additional contributions from the metal center as expected for a
radical delocalized over the conjugated π-system of the chelate.
iPr
iPr
Figure 3. Toluene solution EPR spectrum of Na[ CNC] (top) and
Given these precedents, the EPR spectrum of Na[( BPDI)]
iPr
MgBr[( CNC)] (bottom) recorded at 298 K. Conditions for
Na[ CNC], microwave frequency = 9.3783 GHz, power = 2.00
mW, modulation amplitude = 0.02 mT/100 kHz; conditions for
MgBr[( CNC)], microwave frequency = 9.3710 GHz, power = 2.00
was readily simulated assuming hyperfine coupling to the
iPr
1
4
pyridine and imine nitrogen nuclei (A ( N ) = 6.2 MHz,
iso
py
14
1
A ( N ) = 3.3 MHz), the para-pyridine proton (A ( Hpara)
iPr
iso
im
iso
2
3
=
14.0 MHz), and the sodium cation (A ( Na) = 8.4 MHz).
iso
mW, modulation amplitude = 0.1 mT/100 kHz.
In agreement with the previous studies and related efforts with
61
pyridine radical anions, the largest contributions to the
hyperfine coupling arise from coupling to the proton in the 4-
position of the pyridine ring, while contributions from the
protons in the 3-position are small and not resolved in the
treated with excess sodium, proved less reliable and generated
unidentified byproducts. Similar to the bis(imino)pyridine
radicals, the X-band EPR spectrum of Na[ CNC] centered at
giso = 2.003 exhibits the complexity expected for an organic
radical delocalized over the π-system of the pyridine and NHC
rings. Again assuming hyperfine contributions from the in-plane
N, H, and Na nuclei of the complex, a maximum number of
16200 resonances are possible, giving rise to the observed
multiline pattern. Computational studies (see below) reveal
iPr
iPr
recorded spectrum of Na[( BPDI)]. It should be noted that
the observed 10 line pattern is merely the envelope of a more
complex hyperfine pattern that is not further resolved due to
line broadening. Assuming only contributions from the H, N,
and Na nuclei of the chelate plane, a total of 360 resonances is
expected for a complex with C symmetry or higher. This
s
iPr
iPr
number increases for the EPR spectrum of Na[( PDI)] due to
that the spin density in the [ CNC] radical anion are mostly
contributions from the in-plane methyl substituents of the
chelate backbone producing a theoretical maximum of 2520
localized on the pyridine ring suggesting that hyperfine
contributions from the imidazole nuclei are small compared
1
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to contributions from the pyridine ring and the metal cation.
However, the multitude of small contributions adds to the line
broadening of the spectrum, thus lowering the resolution and
complicating simulations of the data.
The compound was characterized by single crystal X-ray
diffraction and a representation of the molecular structure is
presented in Figure 4. Selected metrical parameters for this and
In an attempt to simplify the hyperfine structure of the
[iPrCNC] radical anion, alternative reductants were explored to
2
3
replace the sodium (100% Na, I = 3/2) with a nonmagnetic
cation. Stirring a THF slurry of CNC(HBr) with excess
2
magnesium powder in the presence of catalytic amounts of
6
3
anthracene generated an orange solution identified as
iPr
MgBr[( CNC)] (Figure 3, bottom). The X-band EPR
spectrum exhibits an isotropic signal (giso = 2.003) consistent
with an S = 1/2 organic radical. As expected, the introduction
of the predominantly nonmagnetic magnesium nucleus (90%
2
4/26
25
Mg, I = 0; 10% Mg, I = 5/2) simplifies the data
significantly producing a spectrum with a 14-line main feature
and additional small signals visible at the edges of the spectrum
2
5
arising from coupling to Mg. Despite the smaller number of
potential hyperfine lines (4050), the spectrum of MgBr-
iPr
[
( CNC)] covers a slightly wider field range, which is most
iPr
F
Figure 4. Solid state molecular structure of [( CNC)CoN ][BAr ]
2
4
likely due to small changes in the spin distribution over the π-
system induced by Mg as compared with Na resulting in
slightly different coupling constants for the N and H nuclei.
Unfortunately, a satisfactory simulation could not be obtained
even for the simplified spectrum of the magnesium derivative.
Synthesis of Additional Bis(arylimidazol-2-ylidene)-
pyridine Cobalt Complexes. Additional bis(arylimidazol-2-
ylidene)pyridine cobalt compounds were synthesized as
references for structural and spectroscopic measurements.
Previous studies with bis(imino)pyridine cobalt complexes
at 30% probability ellipsoids. Hydrogen atoms, one molecule of
pentane, and BArF anion were omitted for clarity. See Supporting
4
Information for full structural representation.
all of the bis(arylimidazol-2-ylidene)pyridine cobalt compounds
crystallographically characterized for electronic structure
elucidation are presented in Table 3. The crystallographic
data establishes a planar cobalt dinitrogen complex with no
close contacts between the cation and anion (see Supporting
Information).
34
served as a guide for specific cobalt derivatives. Gibson and
A second example of a compound expected to have a neutral,
redox-innocent bis(arylimidazol-2-ylidene)pyridine was
5
9,64
our laboratory
(
have established that the cationic bis-
iPr
+
imino)pyridine cobalt dinitrogen complex, [( PDI)CoN ] , is
a low-spin Co(I), d compound with a neutral, redox-innocent
chelate. Likewise, one-electron oxidation of ( PDI )Co CH
is ligand-based, furnishing [( PDI )Co CH ] , an overall S =
iPr
F
iPr
2
[
( CNC)CoCH ][BAr ]. Oxidation of ( CNC)CoCH
3
4
3
8
F
with [Cp Fe][BAr ] in fluorobenzene furnished blue-green
2
4
iPr
1−
II
3
crystals identified as the desired cobalt methyl cation,
iPr
0
II
+
iPr
F
3
[
( CNC)CoCH ][BAr ], in 15% isolated yield (eq 4).
3 4
1
/2 compound best described as low-spin Co(II) with a neutral
65
bis(imino)pyridine. EPR studies on the latter support this
view with identification of a cobalt-centered spin. Inspired by
these findings, the synthesis of the analogous bis(arylimidazol-
2
-ylidene)pyridine cobalt cations was explored.
Addition of NaBArF to a diethyl ether solution of
4
(iPrCNC)CoBr under a dinitrogen atmosphere followed by
recrystallization from pentane−fluorobenzene yielded a bright
iPr
F
green diamagnetic solid identified as [( CNC)CoN ][BAr ]
(
2
4
F
BAr 4 = B(3,5-(CF ) C H ) ) in 92% yield (eq 3). The
3 2 6 3 4
A solution (Evans method) magnetic moment of 1.8(5) μB
was measured at 22 °C, establishing an S = 1/2 ground state.
Accordingly, the EPR spectrum (Figure 5) recorded on a solid
powder sample at 10 K exhibits a rhombic signal, consistent
with the observed doublet state. The spectrum was successfully
simulated assuming g values of g = 3.36, g = 2.43, g = 1.85
x
y
z
and hyperfine coupling to 59Co (100%, I = 7/2, Axx = 1062
MHz, Ayy = 581 MHz, Azz = 494 MHz). The remarkable
anisotropy of the g tensor and the large hyperfine coupling
constants demonstrate a cobalt-centered spin consistent with
previous reports on the EPR spectra of square-planar Co(II)
iPr
F
toluene solution infrared spectrum of [( CNC)CoN ][BAr ]
2
4
−
1
66
exhibits a strong band at 2141 cm , signaling dinitrogen
complexes.
1
13
coordination. The H and C NMR spectra recorded in
The solid-state structure was determined by X-ray diffraction
fluorobenzene-d are consistent with a C symmetric molecule.
and a representation of the molecule is presented in Figure 6.
5
2v
1
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iPr
iPr
F
iPr
F
iPr
Table 3. Selected Bond Distances (Å) for ( CNC), [( CNC)CoCH ][BAr ], [( CNC)CoN ][BAr ], ( CNC)CoCl,
3
4
2
4
iPr
iPr
(
CNC)CoBr, and ( CNC)CoCH3
(iPrCNC)
a
(iPrCNC)CoMe+
(iPrCNC)CoN +
(iPrCNC)CoCl
(iPrCNC)CoBrb
(iPrCNC)CoMeb
2
Co−Npyr
Co−Ccarbene
1.9193(13)
1.9479(17)
1.871(2)
1.913(3)
1.903(3)
2.2004(8)
1.832(2)
1.904(3)
1.907(3)
2.3169(8)
1.840(5)
1.918(6)
1.909(6)
1.951(4)
1.865(3)
1.914(4)
1.898(4)
1
.9432(17)
Co−X
Co−N2
Npyr−Cipso
1.9542(18)
1.801(3)
1.327(4)
1.333(2)
1.335(2)
1.382(3)
1.381(2)
1.388(3)
1.389(3)
1.399(2)
1.403(2)
1.384(2)
1.382(2)
1.346(2)
1.347(2)
1.340(4)
1.339(4)
1.375(4)
1.383(4)
1.384(5)
1.389(5)
1.396(4)
1.404(4)
1.373(3)
1.379(4)
1.352(4)
1.348(4)
1.357(4)
1.369(4)
1.367(4)
1.371(5)
1.399(5)
1.392(5)
1.393(4)
1.400(4)
1.383(4)
1.380(4)
1.364(4)
1.366(4)
1.353(7)
1.371(7)
1.379(7)
1.378(8)
1.397(9)
1.410(9)
1.386(7)
1.387(7)
1.388(6)
1.384(7)
1.360(7)
1.364(7)
1.358(5)
1.357(5)
1.373(7)
1.374(6)
1.397(7)
1.416(6)
1.394(5)
1.397(5)
1.399(5)
1.400(5)
1.375(6)
1.386(6)
1
.345(4)
1.362(5)
.388(5)
1.387(5)
.392(5)
1.413(4)
.427(4)
1.372(4)
.378(4)
1.349(4)
.359(4)
Cipso−Cm‑pyr
1
Cm‑pyr−Cp‑pyr
Cipso−Ncarbene
Ncarbene−Ccarbene
Ccarbene−Naryl
1
1
1
1
a
b
Data taken from ref 56. Data taken from ref 40.
iPr
F
iPr
Figure 6. Solid state molecular structure of [( CNC)CoCH ][BAr ]
3 4
Figure 5. Solid state powder EPR spectrum of [( CNC)CoCH ]-
3
F
F
at 30% probability ellipsoids. Hydrogen atoms and BAr anion were
omitted for clarity. See Supporting Information for full structure
representation.
4
[
BAr ] recorded at 10 K. Conditions: microwave frequency = 9.3825
GHz, power = 0.5 mW, modulation amplitude = 1.0 mT/100 kHz.
4
The crystallographic data confirm the identity of the compound
iPr
F
4
as [( CNC)CoCH ][BAr ], and there are no close contacts
3
iPr
between the cation and anion (see Supporting Information).
The sum of the angles about the metal center is 359.96(8)°,
indicating an idealized planar compound.
Examples of compounds with the potential of adopting one-
electron reduced chelates were of particular interest to
determine whether bis(arylimidazol-2-ylidene)pyridines are
redox-active. Previous studies with bis(imino)pyridine com-
pounds identified ( PDI)CoN2 as a low-spin Co(I) d8
compound with a one-electron reduced chelate accounting
59
for the observed S = 1/2 ground state. The toluene solution
EPR spectrum of the compound recorded at 298 K
experimentally verified the ligand-centered radical as the
SOMO of the molecule thereby providing definitive evidence
for the redox activity of this ligand class. Accordingly, the
1
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iPr
bis(arylimidazol-2-ylidene)pyridine analog, ( CNC)CoN , was
the ligand radical to the cobalt center, as observed for the
2
iPr
60
targeted for synthesis and electronic structure determination.
Attempts to prepare the desired neutral cobalt dinitrogen
compound by reduction of ( CNC)CoBr with sodium
amalgam yielded a mixture of cobalt products with (4-
H - CNC)CoN as the only identifiable compound. Perform-
ing the reduction with sodium metal in the presence of 5 mol %
of naphthalene also furnished a mixture of products from which
4-H - CNC)CoN and Na[ CNC] were identified. By
contrast, stirring a toluene or benzene solution of [( CNC)-
CoN ][BAr ] in the presence of 1 equiv of (η -C Me ) Co for
PDI analog. The A is similar to that observed in
iso
iPr
previously reported ( PDI)CoN (24 MHz), supporting an
2
iPr
electronic structure description of a low-spin Co(I) ion with a
iPr
1−
iPr
[ CNC] chelate radical anion. Unfortunately, ( CNC)-
CoN2 underwent rapid decomposition in solution to an
unidentified S = 1/2 cobalt compound. The spectrum was
simulated with the presence of an S = 1/2 species centered at
giso = 2.00 at a 40% ratio. A time course of the decomposition as
measured by EPR spectroscopy is reported in the Supporting
Information.
iPr
2
2
iPr
iPr
(
2 2
iPr
F
5
2
4
5
5 2
1
0 min yielded a blue-green solution. Monitoring the reaction
Analysis of the Metrical Parameters of Structurally
Characterized Bis(arylimidazol-2-ylidene)pyridine Cobalt
Complexes. Distortions to the metrical parameters of a
iPr
by infrared spectroscopy established that all of the [( CNC)-
CoN ][BAr ] is consumed with concomitant appearance of a
band centered at 2047 cm , signaling formation of the neutral
cobalt dinitrogen complex, ( CNC)CoN (eq 5).
F
2
4
−1
chelating ligand have been established as a diagnostic tool for
iPr
67,68
establishing redox activity.
Supporting these observations
2
with computational and spectroscopic studies is useful for
determining the spectroscopic oxidation state of redox-active
transition metal compounds. Because several bis(arylimidazol-
2
-ylidene)pyridine cobalt complexes have now been structurally
characterized, meaningful analysis can be carried out to
establish the bond distances associated with the neutral and
one-electron reduced forms of the ligand. As will be presented
in a subsequent section of the manuscript, the established
ranges have also been computationally verified.
Selected bond distances for structurally characterized bis-
(
arylimidazol-2-ylidene)pyridine cobalt complexes are pre-
sented in Table 3. Also presented in Table 3 are the metrical
parameters of the free chelate as a reference and comparison.
Computational studies (vide infra) and resonance structures
allow prediction of the distortions to the chelate. Illustrated in
Analysis of the in situ generated product by EPR spectros-
copy in benzene solution at 298 K established an S = 1/2
iPr
the table is the DFT computed b molecular orbital of the
compound, similar to ( PDI)CoN . The observed isotropic
2
2
bis(arylimidazol-2-ylidene)pyridine radical anion, which high-
lights the expected bond elogations and contractions.
Reduction of the bis(arylimidazol-2-ylidene)pyridine pincer
results in elongations and contractions to the central pyridine
ring as well as an elongation of Ccarbene−Nimid distances. Within
signal at giso = 1.998 exhibits eight partially overlapped
hyperfine lines, arising from coupling of the principally
5
9
ligand-centered radical to the I = 7/2 Co nucleus (Figure
). Simulation of the data established an isotropic hyperfine
7
coupling constant, Aiso = 36 MHz. No hyperfine coupling to
hydrogen or nitrogen atoms was observed, which can be
attributed to increased line broadening due to coordination of
the pyridine ring, the N −C and Cmeta−C distances are
pyr
ipso
para
expected to elongate upon addition of an electron, while Cipso−
Cmeta bonds contract.
iPr
In the free bis(arylimidazol-2-ylidene)pyridine ( CNC),
iPr
F
iPr
F
[
( CNC)CoCH ][BAr ], and [( CNC)CoN ][BAr ], the
3 4 2 4
N −C distances range between 1.327(4) and 1.345(5) Å.
pyr
ipso
In the cobalt monohalide and methyl complexes, this length
range expands to 1.353(7)−1.371(7) Å, consistent with one-
electron reduction of the chelate. The C −C
distances
meta
para
also exhibit distortions, albeit slightly less pronounced, ranging
between 1.384(5) and 1.392(5) Å in the redox-innocent
compounds that elongate to 1.392(5)−1.416(6) Å in the
iPr
1−
II
(
CNC )Co X derivatives. The anticipated contraction of
the Cipso−C
distances is apparent but less definitive. In the
redox-innocent compounds, values range between 1.362(5) and
meta
1
(
.388(5) Å and are altered to 1.367(4)−1.378(8) Å in the
iPr
1−
II
CNC )Co X examples. It should be noted that the lower
limit of the redox-innocent range is skewed by one anomalous
value in the structure of the free bis(arylimidazol-2-ylidene)-
pyridine. If this value is discarded, the range for the neutral
iPr
Figure 7. Benzene solution EPR spectrum of ( CNC)CoN recorded
2
ligand becomes 1.375(4)−1.388(5) Å. The C
−N
carbene
aryl
at 298 K. Conditions: microwave frequency = 9.3735 GHz, power =
0.0 mW, modulation amplitude = 0.10 mT/100 kHz. Simulation was
performed with the presence of an unidentified S = 1/2 species at g =
.000. The relative ratio of ( CNC)CoN and the unidentified species
in this spectrum is approximately 3:2. See Supporting Information for
subspectra from the simulation and decomposition time course.
distances also exhibit distortions, ranging between 1.346(2)
and 1.359(4) Å in redox-innocent compounds and elongating
to 1.360(7)−1.386(6) Å in the reduced chelate. The distortions
in bond distances were also anticipated from examination of the
calculated b2 orbital of [CNC]1 chelate, which clearly
2
iPr
2
2
−
1
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iPr
iPr
Figure 8. Normalized Co K-edge X-ray absorption spectra of the rising edge (left) and pre-edge (right) of ( CNC)CoBr and ( CNC)CoCH
3
iPr
iPr
recorded at 10 K. Also included are the data for ( PDI)CoCl and ( PDI)CoCH .
3
Table 4. Cobalt K-Edge Parameters for Bis(arylimidazol-2-ylidene)pyridine and Bis(imino)pyridine Cobalt Halide and Methyl
Compounds
a
b
compound
(iPrCNC)CoBr
pre-edge energy (eV)
7709.8
pre-edge intensity A
pre-edge intensity B
temp (K)
33(3)
17(4)
42(3)
7(1)
10
10
10
10
10
10
7
711.8
7709.7
711.2
(iPrCNC)CoCH3
29.8(6)
5.8(8)
14.1(9)
13(1)
37.6(8)
7(2)
7
iPr
(
(
PDI)CoCl
7709.3
7709.3
18(1)
16(2)
iPr
PDI)CoCH3
a
b
Fit using method described in ref 72. Fit using method described in ref 73.
establishes antibonding character at elongated positions. Taken
together, the crystallographic data clearly establish that the
results support low-spin cobalt(II) compounds with bis-
(arylimidazol-2-ylidene)pyridine radical anions.
H NMR Spectroscopic Studies. The NMR spectra of
diamagnetic transition metal complexes bearing redox-active
iPr
1−
II
1
(
CNC )Co X family of compounds, much like their
bis(imino)pyridine counterparts, contain radical bis-
ligands has proven valuable in determination of their electronic
(
(
arylimidazol-2-ylidene)pyridine chelates and low-spin cobalt-
II) centers.
5
1,74
structure. Budzelaar and co-workers
laboratory
and subsequently our
5
9,69,75
1
have demonstrated the utility of H NMR
X-ray Absorption Studies. The single crystal X-ray
iPr
iPr
chemical shifts of the in-plane hydrogens of bis(imino)pyridine
chelates to be diagnostic of radical chemistry and participation
in the overall electronic structure. Deviations of the in-plane
chelate hydrogens from their diamagnetic reference values have
been attributed to either thermal population of a triplet excited
diffraction data for both ( CNC)CoBr and ( CNC)CoCH
3
are consistent with low-spin Co(II) complexes with bis-
arylimidazol-2-ylidene)pyridine radical anions. To further
(
support this view of the electronic structure, X-ray absorption
spectroscopic (XAS) studies were conducted (Figure 8). This
5
1
74
iPr
1−
II
iPr
1−
II
state or temperature-independent paramagnetism.
Because of the simplicity and insightfulness of the technique,
study also included ( PDI )Co Br and ( PDI )Co CH ,
3
established low-spin Co(II) compounds with chelate radical
anions. The rising edge and pre-edge region of all four
compounds fall in a similar energy range (Table 4), supporting
the same cobalt oxidation state for all members of the series.
1
iPr
the H NMR spectrum of ( CNC)CoCH was examined in
3
40
1
detail. As reported by Danopoulos, the benzene-d6 H NMR
spectrum of ( CNC)CoCH at 22 °C exhibits a downfield
iPr
3
shifted resonance at 10.65 ppm for the para-pyridine proton.
iPr
iPr
For ( CNC)CoBr and ( PDI)CoCl, the first pre-edge
features appear at 7709.8 and 7709.3 eV, respectively, values
consistent with those previously reported for Co(II) com-
This shift is similar to the value of 10.19 ppm reported for
iPr
76
(
PDI)CoCH3. The spectra of the two compounds are
presented in Figure 9. 1
iPr
6
9
pounds. The 0.5 eV shift for the bis(arylimidazol-2-ylidene)-
pyridine derivative is likely due to the greater π-acidity of this
chelate relative to the bis(imino)pyridine. The replacement of
Cl by Br in the first coordination sphere, however, may also
contribute to a decrease in edge energy. Likewise, the pre-edges
The similarity in the H NMR shifts of ( CNC)CoCH and
3
(iPrPDI)CoCH is consistent with the other spectroscopic and
3
metrical data that indicate that the former is a low-spin
cobalt(II) compound with a bis(arylimidazol-2-ylidene)-
pyridine radical anion. Because thermal population of a triplet
excited state has been invoked to explain the anomalous
iPr
iPr
for ( CNC)CoCH and ( PDI)CoCH were observed at
3
3
iPr
51
1
7709.7 eV, and at 7709.3 eV, respectively. Both of the
chemical shifts of ( PDI)CoCH , variable-temperature H
3
iPr
bis(arylimidazol-2-ylidene)pyridine cobalt compounds exhibit
an additional pre-edge feature at slightly higher energy, likely
NMR experiments were performed on a series of ( CNC)CoX
(Br, CH , H) compounds in toluene-d . Stack plots of the
3
8
70,71
due to a 1s to ligand π* transition.
Collectively, the XAS
spectra are reported in the Supporting Information. For both
1
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Journal of the American Chemical Society
Article
Figure 9. Benzene-d6 1H NMR spectra of ( PDI)CoCH (top) and ( CNC)CoCH (bottom). The data for ( PDI)CoCH was previously
iPr
iPr
iPr
3
3
3
iPr
reported in ref 75 and that for ( CNC)CoCH in ref 40.
3
(iPrCNC)CoBr and( CNC)CoCH , the chemical shifts are
iPr
If thermal population of a triplet excited state is indeed the
3
1
invariant with temperature, changing <0.1 ppm from −70 to 22
C. Thus, no change in population of the triplet state was
detectable by H NMR spectroscopy. The hydride compound,
origin of the anomalous H NMR shifts observed at 22 °C, a
−4
°
(4.7 × 10 )% population of the triplet excited state is
responsible for shifting the imine methyl groups approximately
3−4 ppm from their diamagnetic reference values. Cooling the
sample should decrease this population significantly and
likewise induce a significant shift of the imine methyl group
and other in-plane hydrogens toward their diamagnetic
reference values. Similarly, heating the sample should result
in a greater dispersion of these chemical shifts. Because no
change was observed over a 110 °C temperature range, we
conclude that thermal population of a triplet excited state is not
the origin of the anomalous shifts. The more likely explanation
is temperature-independent paramagnetism, whereby a triplet
excited state mixes into the singlet ground state by spin orbit
1
iPr
(
CNC)CoH, is anomalous. Almost all of the resonances are
invariant with temperature with the exception of the para-
pyridine, which broadens significantly from −40 to 22 °C. This
broadening is accompanied by an upfield shift of approximately
.5 ppm upon warming. Recall that this compound undergoes
migration of the metal hydride to the 4-position of the pyridine
and exchange with free H , and thus we are unable to
distinguish the changes in the NMR spectrum resulting from a
chemical process or an unusual electronic structure.
The observation of temperature-independent chemical shifts
in the bis(arylimidazol-2-ylidene)pyridine cobalt bromide and
methyl compounds raised the question of similar behavior in
0
2
1
coupling as the resulting H NMR chemical shifts are invariant
iPr
(
PDI)CoCH . To our knowledge, the variable-temperature
with temperature. Such behavior has been observed in
isoelectronic neutral ligand complexes of bis(imino)pyridine
3
NMR data for this compound has not been reported. Over the
temperature range of −70 to 40 °C, no significant change in
chemical shifts was observed (see Supporting Information for
stack plots). Budzelaar and co-workers previously calculated a
difference of 7.2 kcal for the energy between the singlet and
7
4,77
iron.
78
Computational Studies. Broken-symmetry DFT computa-
tional studies were conducted to corroborate the experimental
results and firmly establish the electronic structures of various
bis(arylimidazol-2-ylidene)pyridine cobalt compounds. Full
molecule calculations and geometry optimizations were
conducted in each case at the B3LYP level of DFT. Broken
symmetry calculations were performed to describe the open
shell singlet configuration of the redox-active cobalt com-
pounds. In the broken symmetry notation, BS(m, n) describes a
state in which there are m unpaired spin-up electrons and n
iPr
51
triplet states for ( PDI)CoCH . Assuming a Boltzman
3
distribution between the states, the abundance of the triplet
state at 22 °C is (4.7 × 10 )%. Cooling the sample to −70 °C
decreases this value to (1.8 × 10 )%, while warming to 40 °C
increases the population to (9.4 × 10 )%. That is to say, the
triplet state of ( PDI)CoCH is 260 times more abundant at
2 °C than at −70 °C and 526 times more abundant at 40 °C
−4
−6
−4
iPr
3
2
7
7,79,80
than at −70 °C.
unpaired spin-down electrons on separate fragments.
1
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Journal of the American Chemical Society
Article
qualitative molecular orbital diagram is presented in Figure 10
and clearly establishes a low-spin Co(I) complex with a neutral,
redox-innocent bis(arylimidazol-2-ylidene)pyridine, consistent
with the experimental data. The highest occupied molecular
2
orbital is exclusively cobalt d , as expected for a square planar,
z
8
d metal complex. To provide additional support for the
accuracy of the DFT result, a vibrational frequency calculation
of the N stretch was performed. This calculation was carried
2
out with the BP86 functional for computational expediency.
This calculation predicts the N stretching frequency at 2173
2
−1
cm , in reasonable agreement with the experimental value of
141 cm . Thus both the experimental and computational data
support [( CNC )Co N ] as the best description of the
−1
2
iPr
0
I
+
2
electronic structure.
Computations were also performed on [( CNC)CoCH ]
iPr
+
3
using a doublet ground state as established experimentally.
Applying a spin-unrestricted formalism, the calculations
converged to an open-shell solution, and a comparison of
computed and experimental bond distances is reported in the
Supporting Information. Presented in Figure 11 is a truncated
qualitative molecular orbital diagram along with a Mulliken spin
density plot. The computational results clearly support a low-
spin Co(II) complex with a redox neutral bis(arylimidazol-2-
ylidene)pyridine. The ligand oxidation state is similar to that
iPr
+
Figure 10. Qualitative molecular orbital diagram for [( PDICoN ]
from a geometry optimized B3LYP calculation.
iPr
+
iPr
+
2
observed with [( CNC)CoN ] . For [( CNC)CoCH ] , the
2
3
2
SOMO of the molecule is a cobalt dz molecular orbital,
consistent with the observed EPR spectrum for the compound.
Having corroborated the electronic structures of compounds
with neutral bis(arylimidazol-2-ylidene)pyridines, cobalt com-
The first class of compounds examined were those expected
to have neutral bis(arylimidazol-2-ylidene)pyridine chelates.
The two cationic cobalt complexes, [( CNC)CoN ] and
( CNC)CoCH ] , were initially explored, and only the
cationic portion of the molecule was calculated with S = 0 and S
1/2 ground states, respectively. The crystallographic data
suggest no participation of the chelate in the electronic
structure of the compounds. A comparison between the
experimental and computational bond distances for both
compounds is reported in the Supporting Information.
iPr
+
iPr
plexes with one-electron reduced chelates (i.e., ( CNC)CoX)
2
iPr
+
[
were studied. Geometry optimizations were performed with
RKS and BS(1,1) inputs at the B3LYP level of DFT. The
BS(1,1) solution was found to be 7 kcal/mol lower in energy
than the RKS alternative, supporting a redox-active description.
The metrical parameters from both calculations are in excellent
agreement with the experimentally determined values and are
summarized in the Supporting Information.
Examination of the computed molecular orbitals from the
open shell singlet solution (Figure 12) revealed a low-spin
Co(II) complex with three doubly occupied metal d orbitals
and a SOMO of primarily dxz parentage. The SOMO is
3
=
iPr
+
Calculations on [( CNC)CoN ] were performed using
2
both BS(1,1) and restricted Kohn−Sham (RKS) formalisms.
The BS(1,1) input converged to the close-shell solution
identical to the results from the RKS input. A truncated,
iPr
+
Figure 11. Qualitative molecular orbital diagram (left) for [( CNC)CoCH ] from an UKS calculation at B3LYP level. Spin density plot (right)
3
was obtained from Mulliken population analysis (red, positive spin density; yellow, negative spin density).
1
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Article
iPr
Figure 12. Qualitative molecular orbital diagram (left) for ( CNC)CoBr from a BS(1,1) calculation at B3LYP level. Spin density plot (right) was
obtained from Mulliken population analysis (red, positive spin density; yellow, negative spin density).
iPr
Figure 13. Qualitative molecular orbital diagram for ( CNC)CoCH obtained from a BS(1,1) DFT calculation at the B3LYP level of DFT (left).
3
iPr
Spin density plot for ( CNC)CoCH was obtained from a Mulliken population analysis (right).
3
engaged in an antiferromagnetic coupling interaction (S = 0.58)
an antiferromagnetic coupling interaction (S = 0.48) with an
iPr
iPr
with an CNC b molecular orbital. Examination of the
CNC b molecular orbital. As with the bromide, the ligand-
2
2
Mulliken spin population analysis also corroborates this view of
the electronic structure.
centered radical is largely pyridine localized.
Presented in Figure 14 is a comparison of the Mulliken spin
density of bis(imino)pyridine versus bis(arylimidazol-2-
ylidene)pyridine cobalt methyl complexes. It is notable that
the radical character on the bis(arylimidazol-2-ylidene)pyridine
chelate is largely localized in the pyridine ring with little
contribution from the N-heterocyclic carbenes. This feature
likely accounts for the observed reactivity where metal hydrides
and stabilized alkyl radicals migrate to the 4-position of the
ring. In bis(imino)pyridine compounds, significant delocaliza-
Similar studies were also conducted on the cobalt methyl
iPr
iPr
complex, ( CNC)CoCH . As with ( CNC)CoBr, a BS(1,1)
3
solution was preferred corresponding to a low-spin cobalt(II)
complex and a bis(arylimidazol-2-ylidene)pyridine radical
anion. A truncated, qualitative molecular orbital diagram and
Mulliken spin population analysis are presented in Figure 13.
Three doubly occupied metal d orbitals and a SOMO of
primarily d character are observed. The SOMO is engaged in
xz
1
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Article
iPr
iPr
Figure 14. Comparison of the spin density plots of ( PDI)CoCH (left) and ( CNC)CoCH (right).
3
3
tion over the entire π-system of the chelate is observed and
hence analogous migration processes have not been observed in
reduced cobalt chemistry (Figure 14).
catalytic performance. The hydrogenation of trans-methylstil-
bene was used as a representative catalytic reaction. Each trial
was conducted with 5 mol % of the metal complex in a 0.916 M
solution of the substrate in the desired solvent under 4 atm of
One final set of time-dependent DFT calculations was
81
conducted, using methods previously described, to further
understand the observed XAS spectra. Using 50 roots (Figure
H at 22 °C for 1 h. The results of these studies are reported in
2
Table 5.
15), we evaluated both closed-shell (RKS) and BS(1,1) inputs
Table 5. Comparison of Various Bis(arylimidazol-2-
ylidene)pyridine Cobalt Precursors for the Catalytic for the
Hydrogenation of trans-Methylstilbene
a
cobalt compound
CNC)CoCH3
solvent
C H
conversion (%)
iPr
iPr
(
(
(
50
14
27
27
6
6
6
CNC)CoCH3
C H F
6
5
6
6
iPr
4-H - CNC)CoN
C H
6
2
2
iPr
F
b
[
[
( CNC)CoN ][BAr ]
C H
6
2
4
iPr
F
4
( CNC)CoN ][BAr ]
C H F
6 5
2
a
All catalytic reactions carried out with 5 mol % of the metal complex
0.032 mmol) in 0.916 M substrate solution in the desired solvents
with 4 atm of H at 22 °C. Conversions determined by GC-FID.
(
2
b
iPr
F
Generated in situ using ( CNC)CoBr and 1.05 equiv NaBAr .
4
iPr
F
4
Because of the poor solubility of [( CNC)CoN ][BAr ] in
2
hydrocarbon solvents such as benzene, the catalytic perform-
ance of the compound was initially evaluated in fluorobenzene.
Only 6% conversion to alkane was observed under these
iPr
iPr
Figure 15. Calculated XAS spectra of ( CNC)CoBr and ( CNC)-
CoCH using the BP86 functional. A broadening of 1.5 eV has been
applied to the computed spectra, which have not been energy
3
iPr
conditions. For comparison, the catalytic activity of ( CNC)-
normalized.
CoCH3 was also evaluated in fluorobenzene, and the
conversion was reduced to 14% compared with 50% in
benzene. Accordingly, a method was developed to generate
iPr
for ( CNC)CoCH to determine whether XAS could
3
iPr
F
distinguish redox-innocent and redox-active possibilities. The
closed shell solution resulted in no calculated pre-edge
transition while the BS(1,1) solution reproduced one pre-
edge transition dominated by a transition to the cobalt dxz
orbital. These further corroborate the BS(1,1) electronic
structure description for ( CNC)CoCH and firmly establish
bis(arylimidazol-2-ylidene)pyridine as a redox-active ligand.
Evaluation of the Catalytic Alkene Hydrogenation
Activity of Other Bis(arylimidazol-2-ylidene)pyridine
Cobalt Precursors. Determination of the electronic structure
of various bis(arylimidazol-2-ylidene)pyridine cobalt com-
pounds prompted evaluation of their catalytic alkene hydro-
genation activity. Such studies may ultimately allow for a
correlation between redox activity of the supporting ligand and
[( CNC)CoN ][BAr ] in situ in hydrocarbon solvents. A
2 4
iPr
benzene solution of ( CNC)CoBr was stirred with a slight
excess of NaBAr , and the catalytic hydrogenation of trans-
F
4
methyl stilbene was conducted under standard conditions.
Using this method, an improved conversion of 27% was
iPr
observed after 1 h, but the observed activity remains inferior to
3
iPr
the neutral cobalt methyl compound, ( CNC)CoCH . One
3
final example, the neutral cobalt(I) dinitrogen compound, (4-
iPr
H - CNC)CoN , was studied. Only 27% conversion to alkane
2
2
was observed after 1 h under standard conditions, again inferior
to the neutral cobalt methyl compound. While it is tempting to
correlate electronic structure with catalytic performance, more
detailed studies are required to make such assertions because
the kinetics of the reaction, number of active sites, and stability
1
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Article
Preparation of [(iPrCNC)CoCH
][BArF
] (BArF
= B(3,5-
of the active species may be the origin of the observed
differences.
3
4
4
(
CF ) C H ) ). A 20 mL scintillation vial equipped with a magnetic
3 2 6 3 4
iPr
stir bar was charged with ( CNC)CoCH (0.036 g, 0.059 mmol, 1
3
equiv) and approximately 2 mL of fluorobenzene. The solution was
CONCLUDING REMARKS
chilled in a −35 °C freezer for 10 min, then a diethyl ether solution of
■
F
4
[
Cp Fe][BAr ] (0.062 g, 0.059 mmol, 1 equiv, in 1 mL of diethyl
2
The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex,
iPr
ether) was added dropwise with rapid magnetic stirring. An immediate
color change from yellowish-brown to blue-green was observed. The
mixture was stirred at 22 °C for 5 min, after which the mixture was
filtered through Celite and subsequently stored at −15 °C. Over a
period of 2 h, a brown-colored oil condensed on the bottom of the
glass container and was separated from the supernatant by carefully
decanting the solution into a clean 20 mL scintillation vial. After three
repeated decanting processes over a period of 6 h, the solution
eventually resumed a teal color, with no further formation of a brown
oil. Layering the solution with pentane (∼ 5 mL) and storing the
mixture at −15 °C resulted in formation of bright green crystals
(
CNC)CoCH , was found to be one of the most active base
3
metal catalysts for the hydrogenation of unactivated, sterically
hindered olefins such as trans-methylstilbene, 1-methyl-1-
cyclohexene, and 2,3-dimethyl-2-butene. Spectroscopic obser-
vation of the catalytically relevant hydride compound,
CNC)CoH, established hydride migration to the 4-position
of the pyridine ring of the chelate. This reactivity was extended
to alkyls upon addition of 1,1-diphenylethylene to ( CNC)-
CoH. The chemical participation of the ligand signaled the
possibility of chelate radicals in the electronic structure of
iPr
(
iPr
iPr
F
4
identified as [( CNC)CoCH ][BAr ] (0.013 g, 0.009 mmol, 15%
iPr
3
(
CNC)CoX (X = halide, alkyl, H) compounds. Accordingly,
yield). Once crystalline, the solid was no longer soluble in benzene,
toluene, or light hydrocarbons but was highly soluble in diethyl ether
and fluorobenzene. Analysis Calcd for C H BCoF N : C, 55.60; H,
a combination of structural, spectroscopic, and computational
studies established bis(arylimidazol-2-ylidene)pyridines as
redox-active, undergoing one-electron reduction in reduced
cobalt chemistry. Unlike redox-active bis(imino)pyridines,
which have a more extended π system, the ligand-centered
radical in the bis(arylimidazol-2-ylidene)pyridine is essentially
pyridine localized, likely contributing to the observed hydride
and alkyl migration chemistry. These studies establish the utility
of bis(arylimidazol-2-ylidene)pyridines as excellent support
ligands in base metal catalysis and also add these pincers to
the growing number of examples of redox-active tridentate
68
56
24
5
3.82; N, 4.77. Found: C, 55.45; H, 4.11; N, 4.49. μ (Evans method,
eff
1
22 °C) = 1.8(5) μ
. H NMR (benzene-d , 22 °C): δ = −9.98 (182
B
6
Hz, 12H, CH(CH ) ), 4.39 (22 Hz, 12H, CH(CH ) ), 6.57 (12 Hz,
3
2
3 2
F
F
4
H, m-BAr ), 7.85 (9 Hz, 10H, o-BAr and p-aryl), 8.84 (20 Hz, 4H,
4
4
CH(CH ) ), 9.35 (17 Hz, 2H, 3-pyr), 12.64 (474 Hz, 3H, CoCH ),
3
2
3
1
5.61 (76 Hz, 2H, imidazolylidene backbone), 16.07 (94 Hz, 2H,
imidazolylidene backbone), 26.25 (33 Hz, 1H, 4-pyr), m-aryl not
located.
8
2
ASSOCIATED CONTENT
■
chelates. These results also suggest that other pyridine-based
pincer ligands may be redox-active when coordinated to an
appropriately reducing base metal. This question and its
broader impact on base metal catalysis are currently under
scrutiny in our laboratory.
*
S
Supporting Information
Crystallographic data for (i P r CNC)CoCl, (4-
iPr
iPr
F
CPh CH - CNC)CoN , [( CNC)CoN ][BAr ], and
2
3
2
2
4
iPr
F
[( CNC)CoCH ][BAr ] in cif format and additional
3
4
experimental procedures, structural data, spectroscopic data,
and computational data. This material is available free of charge
via the Internet at http://pubs.acs.org.
8
3
EXPERIMENTAL SECTION
■
iPr
Preparation of (4-CPh CH - CNC)CoN . A thick-walled glass
vessel was charged with 0.100 g (0.160 mmol) of ( CNC)CoCH and
approximately 10 mL of toluene. The vessel was submerged in a −78
C bath, and the headspace was evacuated and replaced with an
2
3
2
iPr
3
AUTHOR INFORMATION
■
°
Corresponding Author
atmosphere of H . The contents of the vessel were warmed to ambient
2
pchirik@princeton.edu
temperature and stirred for 15 min. The excess dihydrogen was
removed at −78 °C, and 0.030 g (0.170 mmol) of 1,1-diphenyl-
ethylene was added at 22 °C under an atmosphere of dinitrogen. A
rapid color change to deep blue was observed, and the reaction
mixture was stirred for an additional 15 min. The solution was then
filtered through Celite and concentrated to dryness. The residue was
dissolved in diethyl ether and stored at −35 °C to afford 0.082 g
0.010 mmol, 62% yield) of blue solid identified as (4-
CPh CH - CNC)CoN . Analysis Calcd for C H N Co: C, 73.57;
H, 6.80; N, 12.26. Found: C, 73.52; H, 6.66; N, 11.93. H NMR
benzene-d , 22 °C): δ = 0.99 (d, 7 Hz, 6H, CH(CH ) ), 1.03 (d, 7
Hz, 6H, CH(CH ) ), 1.37 (d, 7 Hz, 12H, CH(CH ) ), 1. 77 (s, 3H,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.J.C. thanks the U.S. National Science Foundation and
Deutsche Forschungsgemeinschaft for a Cooperative Activities
in Chemistry between U.S. and German Investigators Grant
(
iPr
2
3
2
49 54
7
1
(
CHE-1026084). J.M.D. also acknowledges support from the
(
6
3 2
National Institute of General Medical Sciences (Award Number
T32GM008500). S.C.E.S. thanks NSF for a graduate research
fellowship (DGE-0646086) for support. Portions of the
research were carried out at the Stanford Synchrotron
Radiation Lightsource, a national user facility operated by
Stanford University on behalf of the DOE, BES. The SSRL
SMB Program is supported by DOE, BER, and NIH, NCRR,
BMTP. The simulations presented in this article were
performed on computational resources supported by the
Princeton Institute for Computational Science and Engineering
3
2
3 2
Ph CCH ), 2.87 (spt, 7 Hz, 2H, CH(CH ) ), 2.91 (spt, 7 Hz, 2H,
CH(CH ) ), 4.33 (d, 4 Hz, 2H, vinyl H), 4.90 (t, 4 Hz, 1H, allyl H),
.03 (d, 2 Hz, 2H, imidazolylidene backbone), 6.30 (d, 2 Hz, 2H,
2
3
3 2
3
2
6
imidazolylidene backbone), 6.96 (d, 7 Hz, 4H, aryl or phenyl), 7.06 (t, 7
Hz, 4H, aryl or phenyl), 7.18 (d, 7 Hz, 4H, aryl or phenyl), 7.35 (d, 7
Hz, 4H, aryl or phenyl). 13C { H} NMR (benzene-d , 22 °C): δ = 22.4
1
6
(
2
5
(
1
1
(
2
Ph CCH ), 23.9 (CH(CH ) ), 24.0 (CH(CH ) ), 24.4 (CH(CH ) ),
2 3 3 2 3 2 3 2
4.5 (CH(CH ) ), 28.5 (CH(CH ) ), 28.6 (CH(CH ) ), 45.2 (allyl),
3 2 3 2 3 2
3.1 (Ph CCH ), 77.5 (vinyl), 113.3 (imidazolylidene backbone), 123.0
imidazolylidene backbone), 123.7 (aryl), 123.8 (phenyl), 125.6 (aryl),
28.3 (phenyl), 129.8 (phenyl), 136.0 (aryl), 145.9 (aryl or 2-pyr),
46.23 (aryl or 2-pyr), 146.25 (aryl or 2-pyr), 149.0 (phenyl), 194.1
carbene), one resonance not located. IR (benzene-d , 22 °C): ν
2
3
(
PICSciE) and the Office of Information Technology’s High
Performance Computing Center and Visualization Laboratory
at Princeton University. We also thank Scott Semproni for
assistance with X-ray diffraction.
=
6
(N2)
−1
050 cm
.
1
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Journal of the American Chemical Society
Article
(
18) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
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