Stoichiometric C=O bond oxidative addition of benzophenone by a discrete radical intermediate to form a cobalt(I) carbene

Article abstract of DOI:10.1021/ja4022683

Single electron transfer from the Zr^IIICo⁰ heterobimetallic complex (THF)Zr(MesNPⁱPr₂) ₃Co-N₂ (1) to benzophenone was previously shown to result in the isobenzopinacol product [(Ph₂CO)Zr(MesNPⁱPr ₂)₃Co-N₂]₂ (2) via coupling of two ketyl radicals. In this work, thermolysis of 2 in an attempt to favor a monomeric ketyl radical species unexpectedly led to cleavage of the C-O bond to generate a Zr/Co μ-oxo species featuring an unusual terminal Co=CPh ₂ carbene linkage, (η²-MesNPⁱPr ₂)Zr(μ-O)(MesNPⁱPr₂)₂Co=CPh ₂ (3). This complex was characterized structurally and spectroscopically, and its electronic structure is discussed in the context of density functional theory calculations. Complex 3 was also shown to be active toward carbene group transfer (cyclopropanation), and silane addition to 3 leads to PhSiH₂O-Zr(MesNPⁱPr₂)₃Co-N ₂ (5) via a proposed Co-alkyl bond homolysis route.

Full text of DOI:10.1021/ja4022683

Communication
pubs.acs.org/JACS
Stoichiometric CO Bond Oxidative Addition of Benzophenone by a
Discrete Radical Intermediate To Form a Cobalt(I) Carbene
Seth L. Marquard, Mark W. Bezpalko, Bruce M. Foxman, and Christine M. Thomas*
Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02454, United States
*
S Supporting Information
24
agents, possibly through carbenoid intermediates. Tungsten
complexes have been shown to undergo oxidative addition of
the CO bond of ketone substrates to afford W(oxo)-
III
0
2
ABSTRACT: Single electron transfer from the Zr Co
i
heterobimetallic complex (THF)Zr(MesNP Pr ) Co−N
1) to benzophenone was previously shown to result in
the isobenzopinacol product [(Ph CO)Zr-
2
3
2
5,26
(
(
carbene) species,
and in another example, the CO bond
2
in a cyclopropenone derivative was added across a Zr−Fe bond,
i
19
(
MesNP Pr ) Co−N ] (2) via coupling of two ketyl
2
3
2 2
resulting in the formation of an iron carbene. A key step in
radicals. In this work, thermolysis of 2 in an attempt to
favor a monomeric ketyl radical species unexpectedly led
to cleavage of the C−O bond to generate a Zr/Co μ-oxo
the reduction of CO to CO is CO bond cleavage, and our
2
group previously reported the facile cleavage of CO by a Zr/
2
Co heterobimetallic complex, resulting in a Zr(μ-O)Co(CO)
27
species featuring an unusual terminal CoCPh carbene
2
complex.
2
i
i
linkage, (η -MesNP Pr )Zr(μ-O)(MesNP Pr ) CoCPh
2
2 2
2
In a recent publication on ketone hydrosilylation catalyzed by
(
3). This complex was characterized structurally and
i
the heterobimetallic complex (THF)Zr(MesNP Pr ) Co−N
2
3
2
2
8
spectroscopically, and its electronic structure is discussed
in the context of density functional theory calculations.
Complex 3 was also shown to be active toward carbene
group transfer (cyclopropanation), and silane addition to 3
leads to PhSiH O−Zr(MesNP Pr ) Co−N (5) via a
(
1) (Mes = 2,4,6-trimethylphenyl), we identified the
tetrametallic isobenzopinacol-bridged complex 2 as the product
29
of stoichiometric addition of benzophenone to 1 (Scheme 1).
In an attempt to probe for an equilibrium between 2 and a ketyl
radical monomer, solutions of 2 were thermolyzed, leading to a
CO bond cleavage event.
i
2
2 3
2
proposed Co−alkyl bond homolysis route.
Scheme 1
lthough implicated as intermediates in a variety of C−C
bond-forming reactions such as cyclopropanation, C−H
A
1
−7
functionalization, and Fischer−Tropsch (FT) synthesis,
late
first-row transition metal terminal carbene complexes without
stabilizing heteroatom substituents are rare. First-row transition
metal complexes featuring bridging alkylidene ligands are well-
8
known, in contrast to their terminal counterparts. Structurally
characterized terminal first-row transition metal carbenes
9
include a nickel diphenylcarbene, copper diphenylcar-
1
0,11
12−16
benes,
and several examples of iron carbenes,
17
18
including cycloheptatrienylidene, cyclobutenylidene, and
cyclopropenylidene complexes.
1
9,20
Noticeably absent from
this list are cobalt carbenes, for which the only examples are the
difluoro- and fluoro(perfluoroalkyl)carbenes recently reported
21
by Baker and co-workers and those in which the carbene
2
2,23
carbon is part of a metal-bound cyclopentadienyl ligand.
Herein we report a rare example of a terminal cobalt carbene
complex and its unusual synthesis via reductive CO bond
cleavage.
Thermolysis of a benzene solution of 2 at 70 °C for 1 h
under N afforded a red-brown solution containing two new
2
paramagnetic complexes in a 4:1 ratio (Scheme 1). Fractional
crystallization was used to separate these two compounds, and
single-crystal X-ray diffraction determined the connectivity of
The cleavage of C−O multiple bonds is a relatively
uncommon reaction despite being a proposed step in the FT
process. Most proposed FT mechanisms proceed by cleavage of
the CO bond of carbon monoxide on metal surfaces
2
i
these structural isomers to be (η -MesNP Pr )Zr(μ-O)-
2
i
i
(
MesNP Pr ) CoCPh (3) and Ph ( Pr P)CO−Zr(μ-
2
2
2
2
2
i
NMes)(MesNP Pr ) Co (4). The structure of the minor red-
(
typically Fe or Co) followed by hydrogenation to afford
2 2
orange complex 4 reveals this product to be the result of
metal−carbene intermediates that give rise to long-chain
5
−7
hydrocarbon products.
Another example from organic
synthesis is the McMurry reaction, which involves the reductive
Received: March 4, 2013
Published: April 5, 2013
cleavage of ketones to olefins by mixtures of TiCl and reducing
3
©
2013 American Chemical Society
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Journal of the American Chemical Society
Communication
2
i
i
homolysis of one of the amidophosphine ligands (see the
Supporting Information).
analogous to (η -MesNP Pr )Zr(μ-O)(MesNP Pr ) Co−CO,
2
2 2
the product of carbon dioxide CO bond oxidative addition
27
The major product 3 has a characteristic paramagnetically
to 1. This observation permits speculation that a similar CO -
2
1
broadened H NMR spectrum with 18 distinct resonances,
radical-based intermediate may be formed on the CO2
activation reaction pathway.
Despite the absence of heteroatom donors to stabilize the
carbene carbon, it was tempting to describe complex 3 as a
indicative of a complex with three inequivalent phosphinoa-
mide ligands. Complex 3 was structurally identified as a
terminal Co−diphenylcarbene complex featuring a μ-oxo ligand
bridging Zr and Co (Figure 1A). This product presumably
IV
I
Zr Co complex featuring a neutral singlet carbene by analogy
2
i
i
27
to (η -MesNP Pr )Zr(μ-O)(MesNP Pr ) Co−CO. Further
2
2 2
examination of 3 using density functional theory calculations
revealed this to be the most accurate description. The geometry
of 3 was optimized to a minimum, and the computed structure
compared quite well with that determined experimentally using
X-ray crystallography (see the Supporting Information). The
distribution of the Mulliken spin density of 3 reveals little
radical character on the carbene, in contrast to porphyrin-based
3
2,33
cobalt carbenes reported in the literature.
Complex 3 was
unable to abstract a hydrogen atom from cyclohexadiene,
providing experimental evidence for the lack of radical character
on the carbene ligand. Natural bond orbital (NBO) calculations
(Figure 1B) revealed a Co−C bond with significantly more
carbon character (66.9%) than Co character (33.1%), and the
Figure 1. (A) ORTEP drawing of complex 3 with 50% probability
ellipsoids. H atoms have been omitted for clarity. Selected bond
lengths (Å): Co−C46, 1.906(2); Co−Zr, 3.0667(4); Zr−O1,
1.8459(16); Co−O1, 1.9710(16). (B) Computed NBO depicting
2
the Co−C interaction in complex 3 (BP86/LANL2TZ(f)/6-
carbene carbon’s orbital contribution resembles that of an sp -
311+G(d)/D95V).
hybridized carbon atom (27.7% s, 72.3% p), suggesting that a
dative donation of two electrons from carbon to cobalt is the
best way to describe this interaction. Furthermore, the NBO
calculations found no significant π interactions between the
carbene carbon and cobalt. Consistent with this observation,
results from cleavage of 2 into a radical monomer species
followed by oxidative addition of the C−O bond. Complex 3
represents a rare structurally characterized non-heteroatom-
stabilized cobalt carbene complex. The geometry about the
the Wiberg bond index (WBI) for the Co−C
bond was
carbene
2
computed to be 0.77, compared with average WBIs of 0.46 and
carbene carbon (C46) is planar, consistent with sp hybrid-
0
.49 for Co−P and Zr−N bonds, respectively, indicating a Co−
ization. The Co−C46 distance in 3 [1.906(2) Å] falls in the
range of previously reported Fischer-type carbenes on cobalt
C bond order between a single and double bond.
Metal carbenes generated via diazoalkane decomposition
pathways are often proposed as intermediates in olefin
cyclopropanation reactions, and cobalt-based catalysts using
(
based on a 2013 search of the Cambridge Structural Database)
but is considerably longer than the M−C distances in
diphenylcarbene complexes of most other mid-to-late first-
32−34
35
9
−11,13,14,30,31
porphyrin
and salen ligand frameworks have been
row transition metals (1.77−1.88 Å).
Floriani’s
developed. To probe whether heterobimetallic complex 3
reacts with olefins in a similar manner, 3 was treated with
styrene. After the reaction mixture was heated at 110 °C for 30
min, (1,2-diphenylcyclopropyl)benzene was observed by GC/
MS in 84% yield (Scheme 2). To probe whether this reaction
calix[4]arene-based FeCPh systems are the only exceptions,
2
although the long Fe−C distances in that case (∼1.95 Å) were
15
attributed to steric constraints. In the case of 3, we postulate
that the long Co−C46 distance might be attributed to the
paramagnetic nature of the Co center, which inevitably leads to
population of the metal−ligand antibonding orbitals. Indeed,
1
Scheme 2
complex 3 has a broad, paramagnetically shifted H NMR
spectrum and a solution magnetic moment of 3.1μ , consistent
B
with an S = 1 complex.
Several attempts were made to generate 3 via addition of
diphenyldiazomethane to coordinatively unsaturated Zr/Co
starting materials in an effort to circumvent the formation of
byproduct 4, but none of these routes proved successful.
1
However, we found that 3 was formed exclusively (by H NMR
spectroscopy) when the labile N ligand in 2 was removed in
2
i
vacuo (generating [(Ph CO)Zr(MesNP Pr ) Co] in situ)
2
2 3
2
prior to thermolysis. The isolated yield of 3 was still fairly
low because of the solubility of the complex in a wide variety of
organic solvents, including hydrocarbons. Conversely, we
proceeds via carbene extrusion from 3 at elevated temperatures,
the thermal stability of 3 was investigated. Upon thermolysis at
110 °C in the absence of styrene, complex 3 decomposed to
produce a mixture of tetraphenylethane, tetraphenylethylene,
considered that thermolysis of 2 under higher N pressure
2
might result in the exclusive formation of complex 4. However,
heating a benzene solution of 2 under 5 atm N at 75 °C for 2 h
2
did not result in any conversion to either 3 or 4. The results of
and diphenylmethane, suggesting that the :CPh fragment is
2
these reactions imply that N dissociation is necessary for the
ejected at elevated temperatures. Moreover, monitoring the
disappearance of carbene complex 3 by H NMR spectroscopy
in the absence or presence of styrene at 90 °C revealed similar
rates of reaction. Therefore, it is likely that cyclopropanation
2
1
formation of both species. While no mechanistic data are
available to suggest how 3 is formed from 2, the eventual
product of ketone CO bond oxidative addition to 1 is
6
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Journal of the American Chemical Society
Communication
proceeds via trapping of the extruded free diphenylcarbene
fragment by styrene.
ASSOCIATED CONTENT
■
*
S
Supporting Information
To explore the possibility that carbene complex 3 is a
possible intermediate in our previously reported hydrosilylation
1
Experimental procedures, H NMR spectra of 3−5, computa-
tional details, X-ray crystallographic data collection and
refinement details, and crystallographic data in CIF format.
This material is available free of charge via the Internet at
http://pubs.acs.org.
2
9
chemistry, the reaction of 3 with a silane was investigated.
Allowing a benzene solution of complex 3 to react with
phenylsilane at room temperature under N afforded a new S =
2
1
i
/2
complex identified as PhSiH O−Zr(MesNP Pr ) Co−N
5) (Scheme 3). H NMR spectroscopy of 5 revealed nine
2
2 3
2
1
(
AUTHOR INFORMATION
Corresponding Author
thomasc@brandeis.edu
■
Scheme 3
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We acknowledge financial support from the U.S. Department of
Energy under Award DE-SC0004019. C.M.T. is grateful for a
■
2
011 Sloan Research Fellowship. S.L.M. is grateful for a Camille
and Henry Dreyfus Environmental Chemistry Fellowship.
REFERENCES
■
(
1) Harvey, D. F.; Sigano, D. M. Chem. Rev. 1996, 96, 271−288.
paramagnetically broadened yet assignable resonances in the
2 to 8 ppm range, and the solution magnetic moment (μ =
(2) Doyle, M. P. Acc. Chem. Res. 1986, 19, 348−356.
(
(
3) Davies, H. M. L.; Manning, J. R. Nature 2008, 451, 417−424.
−
eff
4) Warren, T. H.; Dai, X. Diketiminato Cu(I) and Co(I) Carbene
1
.8μ ) is consistent with a single unpaired electron. Single-
B
Catalysts, and Cyclopropanation Methods Using Them. Pat. Appl.
WO/2005/007659, 2005.
crystal X-ray diffraction (see the Supporting Information)
confirmed the structural connectivity and identity of 5 as a
(5) James, O. O.; Chowdhury, B.; Mesubi, M. A.; Maity, S. RSC Adv.
zirconium siloxide species analogous to the PhSiH O−
2
2
012, 2, 7347−7366.
i
Zr(MesNP Pr ) Co(H)(CO) complex generated upon addition
2
3
(6) Corral Valero, M.; Raybaud, P. Catal. Lett. 2013, 143, 1−17.
(7) Cheng, J.; Hu, P.; Ellis, P.; French, S.; Kelly, G.; Lok, C. M. Top.
Catal. 2010, 53, 326−337.
2
i
i
27
of PhSiH to (η -MesNP Pr )Zr(μ-O)(MesNP Pr ) Co−CO.
Analysis of the crude reaction mixture by GC/MS indicated the
formation of tetraphenylethane as the carbene-derived by-
3
2
2 2
(8) Hahn, J. E. Prog. Inorg. Chem. 2007, 31, 205−264.
(9) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124,
product of this reaction. We previously reported that the CO
2
2
i
i
9
(
1
976−9977.
10) Dai, X.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 10085−
0094.
activation product (η -MesNP Pr )Zr(μ-O)(MesNP Pr ) Co−
2
2 2
CO reacts readily with PhSiH via Si−H addition across the
3
Co−O bond to generate the hydrido/carbonyl complex
i
27
(11) Hofmann, P.; Shishkov, I. V.; Rominger, F. Inorg. Chem. 2008,
7, 11755−11762.
PhSiH O−Zr(MesNP Pr ) Co(H)(CO). By extension, we
2
2 3
4
(
propose that complex 3 initially reacts with phenylsilane to
form a cobalt hydrido/carbene species. Subsequent insertion
would afford a diphenylmethylcobalt complex, and homolysis of
the Co −alkyl bond and dimerization of the resulting
benzhydryl radical would form tetraphenylethane. Indeed,
12) Klimes, J.; Weiss, E. Angew. Chem., Int. Ed. Engl. 1982, 21, 205.
(13) Li, Y.; Huang, J.-S.; Zhou, Z.-Y.; Che, C.-M.; You, X.-Z. J. Am.
Chem. Soc. 2002, 124, 13185−13193.
I
(14) Klose, A.; Hesschenbrouck, J.; Solari, E.; Latronico, M.; Floriani,
C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. J. Organomet. Chem. 1999, 591,
45−62.
36
homolysis of cobalt−alkyl bonds has been studied extensively
37
(
15) Esposito, V.; Solari, E.; Floriani, C.; Re, N.; Rizzoli, C.; Chiesi-
Villa, A. Inorg. Chem. 2000, 39, 2604−2613.
16) Mahias, V.; Cron, S.; Toupet, L.; Lapinte, C. Organometallics
996, 15, 5399−5408.
17) Riley, P. E.; Davis, R. E.; Allison, N. T.; Jones, W. M. J. Am.
Chem. Soc. 1980, 102, 2458−2460.
18) Bauer, D.; Harter, P.; Herdtweck, E. J. Chem. Soc., Chem.
and is known to occur in coenzyme B , cobalt-catalyzed
39
1
2
38
polymerization methods, and cross-coupling reactions.
(
1
(
From the reaction of carbene complex 3 with phenylsilane,
we can conclude that 3 does not play a role in the
hydrosilylation of benzophenone, since the C−O bond cleavage
appears to be irreversible.
(
In summary, we have shown that the highly polar metal−
metal multiple bond in the reduced Zr/Co complex 1 facilitates
the oxidative addition of the CO bond in ketones. This
particular route to a terminal carbene through a ketyl radical
intermediate appears to be unique due to the inability to
synthesize 3 via more traditional routes. Further study is
required to determine whether this CO bond cleavage is
unique to benzophenone and other diaryl ketones and whether
the CO bond cleavage reaction reported herein is relevant to
Commun. 1991, 829−830.
(19) Lutz, M.; Haukka, M.; Pakkanen, T. A.; Gade, L. H.
Organometallics 2001, 20, 2631−2634.
(20) Dettlaf, G.; Hu
Chem. 1982, 229, 63−75.
21) Harrison, D. J.; Gorelsky, S. I.; Lee, G. M.; Korobkov, I.; Baker,
R. T. Organometallics 2013, 32, 12−15.
22) Wadepohl, H.; Galm, W.; Pritzkow, H.; Wolf, A. Chem.Eur. J.
996, 2, 1453−1465.
23) Wadepohl, H.; Pritzkow, H. Angew. Chem., Int. Ed. Engl. 1987,
26, 127−128.
̈
bener, P.; Klimes, J.; Weiss, E. J. Organomet.
(
(
1
(
the mechanism of CO activation. Future studies will also focus
2
on further exploration of the reactivity and properties of
carbene complex 3 and potential catalytic applications of this
unique ketone activation.
(
24) Ephritikhine, M. Chem. Commun. 1998, 2549−2554.
(25) Chisholm, M. H.; Folting, K.; Klang, J. A. Organometallics 1990,
9, 602−606.
6
020
dx.doi.org/10.1021/ja4022683 | J. Am. Chem. Soc. 2013, 135, 6018−6021

Journal of the American Chemical Society
Communication
(
26) Bryan, J. C.; Mayer, J. M. J. Am. Chem. Soc. 1987, 109, 7213−
214.
27) Krogman, J. P.; Foxman, B. M.; Thomas, C. M. J. Am. Chem. Soc.
011, 133, 14582−14585.
28) Greenwood, B. P.; Rowe, G. T.; Chen, C.-H.; Foxman, B. M.;
Thomas, C. M. J. Am. Chem. Soc. 2010, 132, 44−45.
29) Zhou, W.; Marquard, S. L.; Bezpalko, M. W.; Foxman, B. M.;
Thomas, C. M. Organometallics 2013, 32, 1766−1772.
30) Herrmann, W. A.; Plank, J.; Kriechbaum, G. W.; Ziegler, M. L.;
Pfisterer, H.; Atwood, J. L.; Rogers, R. D. J. Organomet. Chem. 1984,
64, 327−352.
31) Herrmann, W. A.; Hubbard, J. L.; Bernal, I.; Korp, J. D.;
Haymore, B. L.; Hillhouse, G. L. Inorg. Chem. 1984, 23, 2978−2983.
32) Belof, J. L.; Cioce, C. R.; Xu, X.; Zhang, X. P.; Space, B.;
Woodcock, H. L. Organometallics 2011, 30, 2739−2746.
33) Dzik, W. I.; Xu, X.; Zhang, X. P.; Reek, J. N. H.; de Bruin, B. J.
Am. Chem. Soc. 2010, 132, 10891−10902.
34) Intrieri, D.; Caselli, A.; Gallo, E. Eur. J. Inorg. Chem. 2011,
071−5081.
35) Niimi, T.; Uchida, T.; Irie, R.; Katsuki, T. Adv. Synth. Catal.
7
(
2
(
(
(
2
(
(
(
(
5
(
2
(
(
(
001, 343, 79−88.
36) Halpern, J. Acc. Chem. Res. 1982, 15, 238−244.
37) Brown, K. L. Chem. Rev. 2005, 105, 2075−2150.
38) Smith, K. M.; McNeil, W. S.; Abd-El-Aziz, A. S. Macromol.
Chem. Phys. 2010, 211, 10−16.
39) Ford, L.; Jahn, U. Angew. Chem., Int. Ed. 2009, 48, 6386−6389.
(
6
021
dx.doi.org/10.1021/ja4022683 | J. Am. Chem. Soc. 2013, 135, 6018−6021