Synthesis, electronic structure, and ethylene polymerization activity of bis(imino)pyridine cobalt alkyl cations

Article abstract of DOI:10.1002/anie.201102825

A new spin on polymers: The title cations comprise low-spin Co^II centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single-component ethylene polymerization catalysts (see scheme) and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.

Full text of DOI:10.1002/anie.201102825

DOI: 10.1002/anie.201102825
Polymerization Catalysts
Synthesis, Electronic Structure, and Ethylene Polymerization Activity
of Bis(imino)pyridine Cobalt Alkyl Cations**
Crisita Carmen Hojilla Atienza, Carsten Milsmann, Emil Lobkovsky, and Paul J. Chirik*
The independent discovery of ethylene polymerization by
MAO-activated (MAO = methylaluminoxane) aryl-substi-
tuted bis(imino)pyridine iron and cobalt dihalide complexes
(ArPDI)MCl2 by Brookhart,[1] Gibson,[2] and their respective
co-workers has attracted both academic and industrial
attention and renewed interest in base-metal catalysis.[3]
Despite the numerous studies on catalyst optimization by
ligand substituent modification,[4] much remains to be learned
about the active species formed upon treatment with MAO,
and the propagating species responsible for monomer
enchainment. The possibility of different oxidation and spin
states of both iron and cobalt, coupled with the ability of the
bis(imino)pyridine ligand to directly participate in the
electronic structure[5] and undergo chemical modification
such as alkylation,[6] presents challenges in understanding the
mechanism of polymerization and the identity of the prop-
agating species.
[(iPrPDI)FeCl2] with excess MAO, suggest that high-spin FeII
complexes are the active propagating species in ethylene
polymerization catalyzed by bis(imino)pyridine iron.[9]
For the corresponding cobalt catalysts, the nature of the
active species remains a subject of controversy.[9] Seminal,
independent studies from Gal,[10] Gibson,[11,12] and their
respective co-workers, have reported that treatment of
[(iPrPDI)CoCl2] with two equivalents of LiR (R = Me,
CH2Ph, CH2SiMe3) resulted in reduction to the [(iPrPDI)CoR]
derivatives, thus raising the possibility that complexes that
contain formally CoI centers are candidates for the propagat-
ing species. However, these cobalt alkyl complexes were
inactive for ethylene polymerization. To activate these
complexes, [(iPrPDI)CoCH3] was treated with B(C6F5)3 in
=
the presence of N2 or CH2 CH2 and furnished the cationic
cobalt complexes with neutral ligands, [(iPrPDI)CoL][MeB-
=
(C6F5)3] (L = N2, CH2 CH2), which were reported to slowly
Considerable effort has been devoted to elucidating the
active species of bis(imino)pyridine iron and cobalt ethylene
polymerization catalysts. Our research group recently
reported the synthesis of bis(imino)pyridine iron alkyl cations
polymerize ethylene (Scheme 1).[12] Erker and co-workers
iPr
[(iPrPDI)FeR]+
(
PDI = 2,6-(2,6-iPr2-C6H3N CMe)2C5H3N;
=
R = CH2SiMe3, CH2CMe3, CH3), and demonstrated the utility
of these species as single-component ethylene polymerization
catalysts without the need for an activator.[7,8] Oxidation of
the corresponding neutral bis(imino)pyridine iron alkyl com-
plexes [(iPrPDI)FeR] proved to be the most reliable and
versatile synthetic route to the desired cationic derivatives.[8]
The combination of metrical parameters from X-ray diffrac-
tion, magnetochemistry, 57Fe Mçssbauer spectroscopy, and
DFT calculations established the electronic structure of these
overall S = 2 complexes as high-spin FeII derivatives with a
neutral bis(imino)pyridine chelate. Thus, oxidation of the
neutral iron alkyl species [(iPrPDI1À)FeIIR] to the correspond-
ing cation is ligand-based. These results, together with Talsi
and co-workersꢀ observation of [(iPrPDI)Fe(m2-Me2AlMe2)]-
[MeMAO] by NMR spectroscopy following treatment of
Scheme 1. Bis(imino)pyridine cobalt complexes proposed as active
species in ethylene polymerization.
[*] C. C. Hojilla Atienza, Dr. C. Milsmann, Prof. P. J. Chirik
Department of Chemistry, Princeton University
Princeton, NJ 08544 (USA)
have since prepared bis(imino)pyridine cobalt cations with
various neutral ligands with the [B(C6F5)4]À anion and also
observed slow ethylene polymerization. The absence of
transferable alkyl groups led to the suggestion of a metal
“alkyl-less” polymerization as a mechanistic possibility in
bis(imino)pyridine cobalt catalyzed ethylene polymeri-
zation.[13,14]
E-mail: pchirik@princeton.edu
Dr. E. Lobkovsky
Department of Chemistry and Chemical Biology
Baker Laboratory, Cornell University, Ithaca, NY 14853 (USA)
[**] We thank the US National Science Foundation and the Deutsche
Forschungsgemeinschaft for a Cooperative Activities in Chemistry
between US and German Investigators grant.
A
more traditional pathway is that activation of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102825.
[(ArPDI)CoCl2] with excess MAO generates cationic bis-
Angew. Chem. Int. Ed. 2011, 50, 8143 –8147
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8143

Communications
(imino)pyridine cobalt alkyl species that are similar to the
magnetometry and EPR spectroscopy (a plot of meff versus
temperature is presented in Figure S1 in the Supporting
Information). At temperatures above 10 K, the effective
magnetic moment meff is constant at 2.08 mB. Below this
temperature, the meff value decreases because of field satu-
ration and weak intermolecular interactions. This magnetic
behavior is consistent with an S = 1/2 ground state, even
though the meff value is higher than the expected spin-only
value of 1.73 mB. To account for this deviation, the data were
simulated[18] using a g value of 2.55, which is obtained from
the simulation of the EPR data (see below). In addition, the
best fit required 11.5% diamagnetic impurity and a small
Weiss temperature qW of À0.19 K to account for intermolec-
ular interactions.
iron derivatives. Experimental support for this activation
pathway has been provided by Talsi and co-workers[15] by
1
2
using H and H NMR spectroscopy to monitor the addition
of MAO to [(ArPDI)CoCl2] complexes. In a more recent
report, the spectroscopic data were correlated with kinetic
studies and support formation of the ion pairs
[(ArPDI)CoII(m2-Me2AlMe2)][A],
[(ArPDI)CoII(Me)S][A],
and [(ArPDI)CoIS][A] (A = [MeMAO]À or [B(C6F5)4]À, S =
solvent or vacancy, Scheme 1).[16] Notably, addition of ethyl-
ene resulted in rapid reduction of the metal from formally
CoII to CoI. However, none of these complexes were isolated
and fully characterized, and the redox activity of the bis-
(imino)pyridine chelate was not established.
Inspired by these reports and our recent observations on
iron complexes,[7,8] we targeted the preparation, isolation, and
full characterization of well-defined, single-component
[(iPrPDI)CoR]+ complexes. Such species could provide insight
into the oxidation and spin state of the active cobalt complex,
establish the participation, if any, of the bis(imino)pyridine,
and possibly aid future catalyst design. Moreover, these
studies will allow direct comparison of the activity to the
The EPR spectrum recorded in toluene glass at 77 K
shows a rhombic signal consistent with an S = 1/2 ground state
with large g anisotropy (Figure 1). Simulation of the data
previously isolated “alkyl-less” bis(imino)pyridine cobalt
Ar
À
cations [( PDI)Co L][A].
Given its success and versatility in the synthesis of the
corresponding iron complexes,[8] oxidation of the neutral
bis(imino)pyridine cobalt alkyl complexes [(iPrPDI)CoR]
(R = Me, Et) was initially explored as a route to the desired
cations. Budzelaar and co-workers have previously estab-
lished that these planar, formally CoI complexes are in fact
CoII species that are antiferromagnetically coupled to a
bis(imino)pyridine radical anion.[17] This result raises the
possibility that electron loss could either be metal- or ligand-
based. Treatment of a solution of (iPrPDI)CoR (R = CH3, Et)
in benzene with either [Cp2Fe][BPh4] or [Cp2Fe][BArF24]
resulted in oxidation and formation of the corresponding
bis(imino)pyridine cobalt alkyl cations [(iPrPDI)CoR][A]
(A = BPh4, BArF24) in high yields [Eq. (1); Cp = cyclopenta-
dienyl, BArF24 = B(3,5-(CF3)2C6H3)4)]. Other examples,
namely [(iPrPDI)CoR][BArF24] (R = CH2CMe3, CH2SiMe3)
and [(MesPDI)CoR][BArF24] (R = CH3, CH2CMe3), were also
prepared using this method.
Figure 1. EPR spectrum of [(iPrPDI)CoCH3][BArF24] recorded in toluene
glass at 77 K (microwave frequency 9.43 GHz, power 1.00 mW, modu-
lation frequency 2.5 mT).
resulted in g1 = 3.42, g2 = 2.26, and g3 = 2.04, and A1 = 360 ꢁ
10À4 cmÀ1, A2 = 100 ꢁ 10À4 cmÀ1, and A3 = 225 ꢁ 10À4 cmÀ1
(I 59Co = 7/2). The highly anisotropic g values and large
hyperfine coupling constants are consistent with a cobalt-
centered spin. The high g values indicate significant orbital
contributions that result from mixing of low-lying excited
states by spin–orbit coupling. In a square-planar ligand field,
the difference in energies of the occupied d orbitals is small,
thus allowing efficient mixing of states.
The solid-state structure of [(iPrPDI)CoCH3][BArF24] was
determined by X-ray diffraction (Figure 2). The molecular
geometry of the cobalt center is idealized-planar with the sum
of the angles around the cobalt totaling 360.06(22)8; the aryl
substituents are oriented nearly perpendicular to the metal–
chelate plane. There are no obvious close contacts between
the cation and the anion (see the Supporting Information). As
is now well-established in bis(imino)pyridine chemistry,
distortions to the ligand bond lengths are diagnostic of
redox activity and are useful for assigning the oxidation state
Each of the isolated [(iPrPDI)CoR][A] complexes is
paramagnetic. Characterization and elucidation of the elec-
tronic structure of [(iPrPDI)CoCH3][BArF24] will be presented
as a representative example (complete characterization of
every cationic bis(imino)pyridine cobalt alkyl cation prepared
in this work is given in the Supporting Information). The spin
state of [(iPrPDI)CoCH3][BArF24] was determined by SQUID
8144
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8143 –8147

Figure 2. Solid-state molecular structure of the cationic portion of
[(iPrPDI)CoCH3][BArF24]. Thermal ellipsoids set at 30% probability,
hydrogen atoms in the cation are omitted for clarity.
of both the metal and the supporting chelate.[5,19] The Cimine
–
À
À
N
imine distances of 1.298(4) (C8 N1) and 1.305(4) ꢂ (C2 N1)
are in the range established for a neutral bis(imino)pyridine.
Figure 3. a) Qualitative molecular orbital diagram for [(iPrPDI)CoCH3]+
from a geometry-optimized B3LYP DFT calculation. b) Spin density
plot obtained from Mulliken population analysis (dark, positive spin
density; light, negative spin density).
À
Accordingly, the Cimine–Cipso bond lengths of 1.472(4) (C7
C8 > ) and 1.469(4) ꢂ (C2 C3) are relatively long and also
diagnostic of a neutral chelate. The Co–Npyr distance of
À
À
1.851(2) ꢂ and the Co–Nimine distances of 1.971(2) (Co1 N3)
À
and 1.990(2) ꢂ (Co1 N1)are contracted and consistent with
the magnetic measurements and EPR data that establish an
S = 1/2 complex.
presence of a neutral supporting ligand is consistent with the
related iron complexes [(iPrPDI)FeR]+, the cobalt derivatives
have low-spin rather than high-spin electronic configurations
at the metal center. The electronic structure of the cobalt
alkyl cation also demonstrates that oxidation of the neutral
precursor [(iPrPDI1À)CoIICH3] is ligand- rather than metal-
based.
With a family of cationic bis(imino)pyridine cobalt alkyl
complexes in hand, their ethylene polymerization activity was
evaluated. Each polymerization experiment was conducted in
10 mL of toluene with 1 bar of ethylene at 238C (Table 1).
The results of a control experiment in which ethylene
polymerization was conducted with [(iPrPDI)CoCl2] and
1000 equivalents of MAO are also shown. In each case
examined, the polymer melting temperatures are consistent
with linear polyethylene and the polydispersity indices
The electronic structure of [(iPrPDI)CoCH3][BArF24] was
also studied by density functional theory using the B3LYP
functional. Only the cationic portion of the molecule
[(iPrPDI)CoCH3]+ was computed without further truncation.
The optimized structure, based on the observed S = 1/2
ground state, successfully reproduced the metrical parameters
determined by X-ray diffraction (see Table S2). A qualitative
molecular orbital diagram and spin-density plot derived from
these results are presented in Figure 3. This solution clearly
establishes a low-spin CoII complex with a neutral bis-
(imino)pyridine chelate, and is consistent with the exper-
imental diffraction, spectroscopic, and magnetochemical
data. The qualitative molecular-orbital diagram is as expected
for a square-planar complex with three doubly occupied
cloverleaf d orbitals and dz2 (the z axis being defined as
perpendicular to the chelate plane)
as the singly occupied molecular
Table 1: Data for the polymerization of ethylene.[a]
orbital (SOMO). An orbital princi-
pally of dx2Ày2 origin is found in the
unoccupied set. A closer analysis of
the orbital manifold reveals that the
dxz and dyz orbitals are nearly
degenerate. These two orbitals are
likely involved in the strong mixing
of states by spin–orbit coupling.
The combined experimental
and computational studies clearly
establish the electronic structure of
the bis(imino)pyridine cobalt alkyl
cations [(iPrPDI)CoR]+ as low-spin
CoII complexes with a neutral bis-
Catalyst
Yield
[g]
Reaction
time [min]
Produc-
tivity[b]
Mw
PI[c]
m.p.
[8C]
[gmolÀ1 [c]
]
iPrPDICoCl2/1000 MAO
[iPrPDICoCH3][BPh4]
[iPrPDICoEt][BPh4]
1.191
0.153
0.268
0.556
0.388
0.220
0.302
0.285
0.239
0.005
30
26
30
15
20
10
10
13
8
240
30
50
220
120
130
180
130
170
2
29600
30300
33600
24800
35800
36000
38100
2100
1.98
1.83
1.94
1.92
2.11
1.94
2.13
1.70
4.15
–
131
131
132
132
132
133
133
124
131
–
[iPrPDICoCH3][BArF24]
[iPrPDICoEt][BArF24]
[iPrPDICoNp][BArF24]
[iPrPDICoNs][BArF24]
[
[
[
MesPDICoCH3][BArF24]
MesPDICoNp][BArF24]
MesPDICoN2][MeB(C6F5)3]
17500
–
15
[a] Polymerization was carried out using 0.010 mmol of catalyst in 10 mL toluene at 238C with 1 bar of
ethylene. [b] Average values in mmolÀ1 hÀ1 barÀ1. [c] Polydispersity index determined by high-temper-
(imino)pyridine chelate. While the ature GPC in 1,2,4-trichlorobenzene. [d] Np=CH2CMe3; Ns=CH2SiMe3.
Angew. Chem. Int. Ed. 2011, 50, 8143 –8147
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8145

Communications
indicate single-site catalysts. Both [(iPrPDI)CoCH3][BPh4] and
[(iPrPDI)CoEt][BPh4] are active for ethylene polymerization,
albeit with relatively modest productivity. Use of the less
coordinating anion, [BArF24]À, increased productivity with
[(iPrPDI)CoCH3]+ yielding the most polymer per unit time.
Importantly, each of the cationic bis(imino)pyridine cobalt
alkyl cations [(ArPDI)CoR]+ is significantly more active than
the corresponding cationic cobalt dinitrogen complex
[(MesPDI)CoN2][MeB(C6F5)3].[11] However, most of the
single-component catalysts are less active than the MAO
system; this behavior is likely a result of the high sensitivity of
these catalysts. Anion effects are also likely to be important.
As expected for a metal-based initiator, changing the identity
of the cobalt alkyl complex did not result in significant
differences in the productivity of the catalysts or the proper-
ties of the polymer produced. However, a substantial change
in the weight-average molecular weight Mw of the polymer
was observed when the aryl group on the chelate was changed
from 2,6-diisopropylphenyl to a less sterically demanding
mesityl group. This effect has also been observed by
Brookhart and co-workers for NiII and PdII polymerization
catalysts,[20] and is consistent with the calculations by Ziegler
and co-workers,[21] whereby steric pressure promotes olefin
insertion and disfavors b-hydrogen elimination.
Figure 4. Solid-state molecular structure of the cationic portion of
[(MesPDI)Co(OEt2)][BArF24]. Thermal ellipsoids set at 30% probability,
hydrogen atoms in the cation are omitted for clarity.
addition of excess ethylene to this complex produced only
trace amounts of polymer, similar to the results obtained with
[(MesPDI)Co(N2)]+, thus supporting the conclusion that cat-
ionic cobalt alkyl complexes are more active polymerization
initiators than the corresponding neutral ligand analogues.
A crossover experiment was conducted to gain additional
insight into the mechanism of decomposition of
[(ArPDI)CoCH3]+ complexes in the presence of diethyl
ether to form [(ArPDI)Co(OEt2)]+. Addition of excess diethyl
ether to a solution containing an equimolar mixture of
[(MesPDI)CoCH3]+ and [(MesPDI)CoCD3]+ in [D6]benzene
resulted in formation of a mixture of CH3CH3, CH3CD3,
and CD3CD3, as shown by a combination of 1H and 2H NMR
spectroscopy. The observation of a near statistical mixture of
ethane isotopologues is consistent with a bimolecular reduc-
tive coupling pathway. One possibility is homolysis of the
cobalt–alkyl bond followed by attack of the alkyl radical on
the residual starting material. However, the absence of
methane formation raises questions about this pathway and
demonstrates the need for additional experiments.
The synthesis of well-defined, single-component cationic
bis(imino)pyridine cobalt alkyl complexes also allows the
opportunity to identify and to study catalyst deactivation
pathways. Such information is essential for future catalyst
design, and may help reconcile the controversy about the
active species. Addition of a slight excess of diethyl ether to a
solution of [(MesPDI)CoCH3][BArF24] in [D6]benzene at 238C
1
resulted in the formation of ethane (detected by H NMR
spectroscopy) over the course of 12 h and isolation of
[(MesPDI)Co(OEt2)][BArF24] [Eq. (2)]. No methane was
A similar experiment was conducted with the correspond-
ing cationic cobalt ethyl complex [(MesPDI)CoEt][BArF24].
Addition of excess diethyl ether to a solution of the complex
in [D6]benzene also produced [(MesPDI)Co(OEt2)][BArF24].
1
Analysis of the volatile products of the reaction by H NMR
spectroscopy established formation of a mixture of ethane,
ethylene, and butane. The ethylene likely arises from b-
hydrogen elimination from [(MesPDI)CoEt]+ to form
[(MesPDI)CoH]+. This species can undergo bimolecular
reductive coupling with the remaining [(MesPDI)CoEt]+ to
account for the experimentally observed ethane. Butane is
formed by coupling two cobalt ethyl fragments. It should be
noted that for both the cobalt methyl and ethyl complexes,
decomposition was only observed when diethyl ether was
added.
detected.
A
similar reaction was observed with
[(iPrPDI)CoCH3][BArF24], although the reaction required
3 days rather than hours to reach completion at 238C and is
suggestive of an associative process. For experimental con-
venience, all subsequent studies were carried out with
[(MesPDI)CoCH3][BArF24] or its [D3] isotopologue.
Diamagnetic [(MesPDI)Co(OEt2)][BArF24] was character-
ized by NMR spectroscopy, combustion analysis, and X-ray
diffraction. A representation of the solid-state structure is
shown in Figure 4. Unlike other structurally characterized
bis(imino)pyridine cobalt cations with neutral ligands,[12,22]
[(MesPDI)Co(OEt2)]+ is an idealized-planar complex with a
monoanionic bis(imino)pyridine chelate that is antiferromag-
netically coupled to a low-spin CoII center. Notably, the
In summary, a family of [(ArPDI)CoR]+ complexes have
been synthesized and their electronic structures established as
overall S = 1/2 with low-spin CoII centers and neutral bis-
(imino)pyridine chelates. Each of the cationic alkyl com-
plexes is active for ethylene polymerization, and is consistent
with observations made by NMR spectroscopy for traditional
8146
www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8143 –8147

(ArPDI)CoCl2/MAO catalyst mixtures. Conversion to cationic
complexes likely by cobalt–alkyl bond homolysis was also
identified as an important catalyst deactivation pathway to
form inactive cationic bis(imino)pyridine cobalt complexes
with a diethyl ether ligand.
[9] I. E. Soshnikov, N. V. Semikolenova, A. N. Bushmelev, K. P.
Bryliakov, O. Y. Lyakin, C. Redshaw, V. A. Zakharov, E. P. Talsi,
Organometallics 2009, 28, 6003.
[10] T. M. Kooistra, Q. Knijnenburg, J. M. M. Smits, A. D. Horton,
P. H. M. Budzelaar, A. W. Gal, Angew. Chem. 2001, 113, 4855;
Angew. Chem. Int. Ed. 2001, 40, 4719.
[11] M. J. Humphries, K. P. Tellmann, V. C. Gibson, A. J. P. White,
D. J. Williams, Organometallics 2005, 24, 2039.
[12] V. C. Gibson, M. J. Humphries, K. P. Tellmann, D. F. Wass,
A. J. P. White, D. J. Williams, Chem. Commun. 2001, 2252.
[13] W. Steffen, T. Blçmker, N. Kleigrewe, G. Kehr, R. Frçhlich, G.
Erker, Chem. Commun. 2004, 1188.
[14] N. Kleigrewe, W. Steffen, T. Blçmker, G. Kehr, R. Frçhlich, B.
Wibbeling, G. Erker, J.-C. Wasilke, G. Wu, G. C. Bazan, J. Am.
Chem. Soc. 2005, 127, 13955.
Received: April 23, 2011
Published online: June 20, 2011
Keywords: cobalt · ethylene · homogeneous catalysis ·
.
polymerization · redox chemistry
[1] a) B. L. Small, M. Brookhart, J. Am. Chem. Soc. 1998, 120, 7143;
b) B. L. Small, M. Brookhart, A. M. A. Bennett, J. Am. Chem.
Soc. 1998, 120, 4049; c) A. M. A. Bennett (Dupont), WO 98/
27124, 1998; [Chem. Abstr. 1998, 129, 122973x].
[2] a) G. J. P. Britovsek, V. C. Gibson, B. S. Kimberely, P. J. Maddox,
S. J. McTavish, G. A. Solan, A. J. P. White, D. J. Williams, Chem.
Commun. 1998, 849; b) G. J. P. M. Bruce, V. C. Gibson, B. S.
Kimberely, P. J. Maddox, S. Mastroianni, S. J. McTavish, C.
Redhsaw, G. A. Solan, S. Stromberg, A. J. P. White, D. J.
Williams, J. Am. Chem. Soc. 1999, 121, 8728.
[15] N. V. Semikolenova, V. A. Zakharov, E. P. Talsi, D. E. Babush-
kin, A. P. Sobolev, L. G. Echevskaja, M. M. Khysniyarov, J. Mol.
Catal. A 2002, 182 – 183, 283.
[16] I. E. Soshnikov, N. V. Semikolenova, A. N. Bushmelev, K. P.
Bryliakov, O. Y. Lyakin, C. Redshaw, V. A. Zakharov, E. P. Talsi,
Organometallics 2009, 28, 6003.
[17] Q. Knijnenburg, D. Hetterscheid, T. M. Kooistra, P. H. M.
Budzelaar, Eur. J. Inorg. Chem. 2004, 1204.
[18] E. Bill, http://www.mpibac.mpg.de/bac/index_en.php/logins/bill/
jul-x_en.php.
[3] P. J. Chirik, K. Wieghardt, Science 2010, 327, 794.
[4] a) V. C. Gibson, C. Redshaw, G. A. Solan, Chem. Rev. 2007, 107,
1745; b) C. Bianchini, G. Giambastiani, I. G. Rios, G. Mantovani,
A. Meli, A. M. Segarra, Coord. Chem. Rev. 2006, 250, 1391.
[5] a) Q. Knijnenburg, S. Gambarotta, P. H. M. Budzelaar, Dalton
Trans. 2006, 5442; b) S. C. Bart, K. Chlopek, E. Bill, M. W.
Bouwkamp, E. Lobkovsky, F. Neese, K. Wieghardt, P. J. Chirik,
J. Am. Chem. Soc. 2006, 128, 13901.
[6] J. Scott, S. Gambarotta, I. Korobkov, P. H. M. Budzelaar, J. Am.
Chem. Soc. 2005, 127, 13019.
[7] M. W. Bouwkamp, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc.
2005, 127, 9660.
[19] a) B. de Bruin, E. Bill, E. Bothe, T. Weyhermꢃller, K. Wieghardt,
Inorg. Chem. 2000, 39, 2936; b) P. H. M. Budzelaar, B. de Bruin,
A. W. Gal, K. Wieghardt, J. H. Lenthe, Inorg. Chem. 2001, 40,
4649.
[20] a) C. M. Killian, D. J. Tempel, L. K. Johnson, M. J. Brookhart, J.
Am. Chem. Soc. 1996, 118, 11664; b) F. C. Rix, M. L. Brookhart,
P. S. White, J. Am. Chem. Soc. 1996, 118, 4746.
[21] P. Margl, L. Deng, T. Zieger, Organometallics 1999, 18, 5701.
[22] A. C. Bowman, C. Milsmann, C. C. H. Atienza, E. Lobkovsky,
K. Wieghardt, P. J. Chirik, J. Am. Chem. Soc. 2010, 132, 1676.
[23] CCDC 821611
([(iPrPDI)CoCH3][BArF24])
and
821612
[(MesPDI)Co(OEt2)][BArF24] contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[8] A. M. Tondreau, C. Milsmann, A. D. Patrick, H. M. Hoyt, E.
Lobkovsky, K. Wieghardt, P. J. Chirik, J. Am. Chem. Soc. 2010,
132, 15046.
Angew. Chem. Int. Ed. 2011, 50, 8143 –8147
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8147