Dioxygen activation by a low-valent cobalt complex employing a flexible tripodal N-heterocyclic carbene ligand

Article abstract of DOI:10.1021/ja046048k

The novel versatile cobalt(I) tris-carbene complex [(TIMEN ^xyl)Co]Cl (1) (where TIMEN = (tris[2-(3-arylimidazol-2-ylidene)ethyl] amine) reacts with CO, one-electron oxidizers such as CH₂Cl ₂, and O₂ to yield the cobalt complexes [(TIMEN ^xyl)Co(CO)]Cl (2), [(TIMEN^xyl)Co(Cl)]Cl (3), and peroxo species [(TIMEN^xyl)Co(O₂)](BPh₄) (5). All new complexes were fully characterized by ¹H NMR, UV/vis, and IR spectroscopy as well as superconducting quantum interference device (SQUID) magnetization measurements and single-crystal X-ray crystallography. The nucleophilic character of the η²-bound dioxygen ligand in 5 was confirmed by density functional theory (DFT) studies and allows for oxygen-transfer reactions with electron-deficient organic substrates, such as benzoyl chloride.

Full text of DOI:10.1021/ja046048k

Published on Web 09/28/2004
Dioxygen Activation by a Low-Valent Cobalt Complex
Employing a Flexible Tripodal N-Heterocyclic Carbene Ligand
Xile Hu, Ingrid Castro-Rodriguez, and Karsten Meyer*
Contribution from the Department of Chemistry and Biochemistry, Mail Code 0358,
UniVersity of California, San Diego, 9500 Gilman DriVe, La Jolla, California 92093-0358
Received July 2, 2004; E-mail: kmeyer@ucsd.edu
Abstract: The novel versatile cobalt(I) tris-carbene complex [(TIMENxyl)Co]Cl (1) (where TIMEN ) (tris[2-
(3-arylimidazol-2-ylidene)ethyl]amine) reacts with CO, one-electron oxidizers such as CH2Cl2, and O2 to
yield the cobalt complexes [(TIMENxyl)Co(CO)]Cl (2), [(TIMENxyl)Co(Cl)]Cl (3), and peroxo species
[(TIMENxyl)Co(O2)](BPh4) (5). All new complexes were fully characterized by 1H NMR, UV/vis, and IR
spectroscopy as well as superconducting quantum interference device (SQUID) magnetization measure-
ments and single-crystal X-ray crystallography. The nucleophilic character of the η2-bound dioxygen ligand
in 5 was confirmed by density functional theory (DFT) studies and allows for oxygen-transfer reactions
with electron-deficient organic substrates, such as benzoyl chloride.
Introduction
tion chemistry are rare. Since the isolation of stable imidazol-
2-ylidene carbenes in 1991 by Arduengo et al.,17 N-heterocyclic
Ligands that enforce a tripodal topology on coordinated metal
ions are known to provide powerful platforms for small molecule
activation and functionalization. Recently, Schrock and co-
workers have developed such a ligand system based on the tris-
amidoamine [N3N]3- framework,1 utilizing it for catalytic
dinitrogen reduction.2 The tetradentate tris-amidoamine tripod
creates a protected cavity when coordinated to molybdenum,
thereby allowing for isolation and characterization of a variety
of intermediates of the reductive dinitrogen cleavage reaction.2-4
Tripodal ligands have also been shown to stabilize molecular
species with unusual reactive terminal functionalities. For
example, using an ureayl-derivatized tris-amidoamine chelator,
Borovik and co-workers were able to prepare an unprecedented
monomeric iron(III) oxo complex.5 Peters and co-workers have
demonstrated that, by employing zwitterionic tris(phosphine)-
borate ligands, remarkably reactive late transition metal com-
plexes with terminal imido6,7 and even nitrido ligands can be
synthesized.8
carbenes have emerged as a predominant ligand class in
organometallic coordination chemistry.18,19 This is mainly due
to their successful application in homogeneous catalysis,20
especially in C-C bond formation10,21,22 and olefin metathesis
reactions.23,24 We have recently developed a novel nitrogen-
anchored tripodal NHC ligand system, TIMENR (tris[2-(3-
alkylimidazol-2-ylidene)ethyl]amine with R ) tert-butyl, ben-
zyl), and synthesized the corresponding copper(I)/(II) complexes.15
The tris-carbene ligand TIMEN binds to transition metals
exclusively in a 1:1 fashion and supports controlled reactivity
at the metal center.
The xylene-derivatized TIMENxyl ligand described here-
in renders the low-valent cobalt(I) tris-carbene complex
[(TIMENxyl)Co]Cl (1) electronically and structurally flexible for
(11) Arnold, P. L.; Scarisbrick, A. C.; Blake, A. J.; Wilson, C. Chem. Commun.
2001, 2340-2341.
(12) Danopoulos, A. A.; Winston, S.; Motherwell, W. B. Chem. Commun. 2002,
1376-1377.
Due to the limited accessibility of well-defined polydentate
N-heterocyclic carbene (NHC) ligands,9-16 reports on chelating
NHC ligands designed for metal-assisted small-molecule activa-
(13) Hu, X.; Castro-Rodriguez, I.; Meyer, K. Organometallics 2003, 22, 3016-
3018.
(14) Hu, X.; Tang, Y.; Gantzel, P.; Meyer, K. Organometallics 2003, 22, 612-
614.
(15) Hu, X.; Castro-Rodriguez, I.; Meyer, K. J. Am. Chem. Soc. 2003, 125,
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13464
J. AM. CHEM. SOC. 2004, 126, 13464-13473
10.1021/ja046048k CCC: $27.50 © 2004 American Chemical Society

Dioxygen Activation by a Low-Valent Cobalt Complex
A R T I C L E S
Scheme 1. Synthesis of Complex [(TIMENxyl)Co]Cl (1)
additional ligand binding and various redox events such as
dioxygen activation. Dioxygen activation at transition metal
centers is a fundamentally important process due to its postulated
role in biological catalyses.25-30 Catalytic aerobic oxidation
assisted by transition metal complexes therefore remains an
attractive, yet challenging, synthetic task.31-33 There is long-
standing interest in the dioxygen chemistry of cobalt complexes
owing to their potential use as artificial oxygen carriers34 and
industrial oxidation catalysts.35-38 The majority of isolated 1:1
cobalt dioxygen adducts feature the oxygen molecule in an end-
on (η1) binding mode and a cobalt center supported by a
chelating polyamine ligand.39-45 Cobalt complexes with side-
on (η2) dioxygen ligands are uncommon.46-50
carbene ligand tris[2-(3-xylenylimidazol-2-ylidene)ethyl]amine
(TIMENxyl) was prepared by deprotonation of the corresponding
imidazolium salt with potassium tert-butoxide. Reaction of
TIMENxyl with a suitable cobalt(I) source, i.e., Co(PPh3)3Cl,
yields the [(TIMENxyl)Co]Cl (1) target complex, which pre-
cipitates from benzene as an analytically pure yellow powder
in ∼80% yield (Scheme 1). Complex 1 possesses saltlike solu-
bility and is only soluble in polar organic solvents such as aceto-
nitrile and DMSO. It is, however, sparsely soluble in tetrahy-
drofuran (THF) but insoluble in hydrocarbons and diethyl ether.
The 1H NMR spectrum of 1 in DMSO-d6 exhibits paramag-
netically shifted and broadened peaks and is consistent with
the proposed structural formula given in Scheme 1. In this
complex, the 3-fold symmetry of TIMENxyl is preserved, giving
rise to 10 distinctive resonances in the range of 75 to -25 ppm.
Although the assignment of these signals remains largely
equivocal, their positions are diagnostic for determination of
the purity and stability of 1 in solution.
Under an inert-gas atmosphere, compound 1 is stable in the
solid state but gradually oxidizes to yield the cobalt(II) species
[(TIMENxyl)CoCl]Cl (3) when stored in acetonitrile or THF
solution; replacement of the chloride anion with other counter-
ions does not increase the complexes’ stability in solution. This
instability in solution greatly hampered our attempt to obtain a
crystal structure of 1 in its pure form. In fact, slow ether
diffusion into an acetonitrile solution of 1 yielded red, plate-
shaped crystals that, according to their NMR spectrum, were a
mixture of about 35% 1 and 65% 3. An X-ray diffraction
analysis confirmed cocrystallization of 1 and 3. The overall
structure was modeled and refined with the two cocrystallizing
components of 1 (42%) and 3 (58%) (see Figure 1 and Figures
S1 and S2 in Supporting Information). In 1, the TIMENxyl ligand
is tetradentate-coordinated through the three carbenoid carbon
We here report a rare example of a side-on (η2) peroxo
complex with an NHC ligand.51,52 Density functional theory
(DFT) calculations and reactivity studies established the nu-
cleophilic character of the coordinated dioxygen ligand in
[(TIMENxyl)Co(O2)]+. Accordingly, the peroxo complex reacts
with strong electrophiles to transfer the O2 fragment. The
reactions of 1 with CO and CH2Cl2, yielding the corresponding
mononuclear Co(I) carbonyl and Co(II) chloro complexes are
also reported.
Results and Discussion
Synthesis of Complex [(TIMENxyl)Co]Cl (1). Following
our previously reported method,13,15 the aryl-substituted tris-
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1077-1133.
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Compounds; Academic Press: New York, 1981
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ActiVation by Transition Metals; Plenum Press: New York, 1988.
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Complexes; Kluwer Academic Publishers: Dordrecht, The Netherlands,
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(51) Dioxygen reactivity of a nickel(II) NHC complex has been reported; see
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with two monodentate NHCs as supporting ligands; see Konnick, M. M.;
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Figure 1. Overlay representation of the solid-state molecular structures of
cocrystallized 1 and 3 (see Figure S2 for independent structures of 1 and
3).
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A R T I C L E S
Hu et al.
Scheme 2. Synthesis of Complex [(TIMENxyl)Co(CO)]Cl (2)
atoms and the anchoring nitrogen atom to the cobalt ion; in 3,
the TIMENxyl binds only through the three carbene centers and
the axial coordination site of the cobalt ion is occupied by a
chloride anion. The cocrystallization of 3 prevents a detailed
discussion of the metric parameters of 1, but the structural data
still allow for confirmation of atom connectivities and the
cationic structure of 1. In addition, the structure of 3 deduced
from this experiment agrees well with that of an independently
synthesized, pure crystal of 3 (vide infra).
Figure 2. Solid-state molecular structure of [(TIMENxyl)Co(CO)]Cl‚2CH3CN
(2‚2CH3CN). Hydrogen atoms, anion, and solvent molecules are omitted
for clarity; thermal ellipsoids are shown at 50% probability. Selected bond
lengths (angstroms) and angles (degrees): Co(1)-C(3) 2.0347(17), Co(1)-
C(16) 2.0545(17), Co(1)-C(29) 2.0267(16), Co(1)-C(40) 1.8463(19),
Co(1)-N(1) 3.034, C(40)-O(1) 1.103(2), C(3)-Co(1)-C(16) 117.92(6),
C(3)-Co(1)-C(29) 113.02(6), C(16)-Co(1)-C(29) 118.33(6).
Synthesis and Structure of Complex [(TIMENxyl)Co(CO)]-
Cl (2). In complex 1 the xylene substituents of the functionalized
TIMENxyl ligand form a well-protected cavity with sufficient
access to the cobalt ion for additional ligand binding. To test
the reactivity and to further stabilize the electron-rich cobalt(I)
ion, complex 1 was reacted with an excess of CO gas (1 atm,
Scheme 2). The reaction proceeded smoothly within minutes
and the resulting green complex [(TIMENxyl)Co(CO)]Cl (2) was
isolated in 85% yield. The infrared vibrational spectrum of 2
shows an intense absorption band at 1927 cm-1, indicative of
a terminal carbonyl ligand.53 The 1H NMR spectrum of 2
exhibits paramagnetically shifted and broadened resonances
between 80 and -15 ppm, suggesting a d8 high-spin (S ) 1)
electronic configuration of the Co(I) ion.
The solid-state molecular structure of 2 was determined by
X-ray diffraction analysis (Figure 2). The average Co-Ccarbene
distance of 2.039(2) Å is slightly longer than those of other
cobalt(I) NHC carbonyl complexes [1.888(3)-1.949(11) Å],54-56
reflecting a greater degree of steric hindrance, most likely caused
by the xylene o-methyl groups of the arene substituents. The
CO ligand resides on the pseudo-C3 axis of the [(TIMEN)Co]
fragment and is centered within the C3 propellerlike arrangement
of the xylene substituents. The Co-Cco and carbonyl C-O bond
distances were determined to be 1.8463(19) Å and 1.101(3) Å,
respectively. The cobalt center is located +0.392 Å above the
idealized trigonal plane defined by the three carbenoid carbon
ligands. This, together with an average Ccarbene-Co-Ccarbene
bond angle of 116.42(6)°, suggests that the coordination
polyhedron of the cobalt ion in 2 is best described as distorted
trigonal pyramidal.
Synthesis and Structure of Complexes [(TIMENxyl)Co-
(Cl)]Cl (3) and [(TIMENxyl)Co(CH3CN)](BPh4)2 (4). On the
basis of our observation that, when stored in acetonitrile solution
over extended periods of time, 1 slowly oxidizes to form the
corresponding cobalt(II) species [(TIMENxyl)Co(Cl)]Cl (3), we
sought to develop an independent synthesis for 3. Access to 3
was achieved by treatment of 1 with benzyl chloride or
chlorinated solvents, such as dichloromethane or chloro-
form (Scheme 3), yielding blue microcrystals in high yield
(>85%). The paramagnetic 1H NMR spectrum of 3 in CD2Cl2
solution shows paramagnetic resonances between 70 and -4
ppm.
Blue crystals suitable for an X-ray diffraction study were
obtained by slow ether diffusion into a DMSO solution of 3.
The complex features a four-coordinate cobalt(II) center with a
distorted trigonal pyramidal geometry (Figure 3). The TIMENxyl
ligand coordinates in a tridentate fashion and the axial chloride
atom resides inside the pocket provided by the three xylene
substituents. The average Co-C distance of 2.077(5) Å and
C-Co-C angle of 115.5(2)° are similar to those found in the
Co(I) carbonyl complex 2. In 3, the displacement of the divalent
Co ion from the carbene C3-plane, however, was found to be
+0.448 Å and, thus, is considerably larger than that found in
the structure of 2.
Scheme 3. Synthesis of Complexes [(TIMENxyl)Co(Cl)]Cl (3) and [(TIMENxyl)Co(CH3CN)](BPh4)2 (4)
9
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Dioxygen Activation by a Low-Valent Cobalt Complex
A R T I C L E S
Figure 3. Solid-state molecular structure of [(TIMENxyl)Co(Cl)]Cl‚DMSO
(3‚DMSO). Hydrogen atoms, anion, and solvent molecules are omitted for
clarity; thermal ellipsoids are shown at 50% probability. Selected bond
lengths (angstroms) and angles (degrees): Co(1)-C(3) 2.078(5), Co(1)-
C(16) 2.080(5), Co(1)-C(29) 2.074(5), Co(1)-Cl(1) 2.2557(15), Co(1)-
N(1) 3.061, C(3)-Co(1)-C(16) 117.57(19), C(3)-Co(1)-C(29) 114.50(19),
C(16)-Co(1)-C(29) 114.4(2).
Figure 5. Plots of the effective magnetic moments, µeff, versus temperature
from temperature-dependent SQUID magnetization measurements for
complexes 1 (black 9), 2 (blue b), 3 (red [), and 4 (green 2).
Figure 4. Solid-state molecular structure of [(TIMENxyl)Co(CH3CN)]-
(BPh4)2‚CH3CN (4‚CH3CN). Hydrogen atoms, anions, and solvent mol-
ecules are omitted for clarity; thermal ellipsoids are shown at 50%
probability. Selected bond lengths (angstroms) and angles (degrees): Co(1)-
C(3) 2.028(3), Co(1)-C(16) 2.033(3), Co(1)-C(29) 2.047(3), Co(1)-N(8)
2.035(3), Co(1)-N(1) 3.146, C(3)-Co(1)-C(16) 111.71(13), C(3)-Co(1)-
C(29) 111.53(13), C(16)-Co(1)-C(29) 121.24(13), Co(1)-N(1)-C(40)
179.1(3).
Figure 6. UV/vis absorption spectra of complexes 1-4 recorded in
acetonitrile solutions.
acetonitrile molecule is essentially linearly coordinated, with a
Co-N-C angle of 179.1(3)°. The displacement of the Co(II)
ion from the trigonal plane of the carbene carbon ligands was
determined to be +0.469 Å and, thus, is very similar to that
found in 3.
Electronic Absorption Spectra and Magnetic Properties
of Complexes 1-4. Paramagnetic complexes 1-4 were char-
acterized by SQUID magnetization measurements and electronic
absorption spectroscopy (Figures 5 and 6).
At room temperature, cobalt(I) complexes 1 and 2 exhibit
magnetic moments of 3.65µB and 3.49µB, respectively. These
values are comparable to that of 3.8µB reported for a related
cobalt(I) complex [(Tp)Co(C2H4)]57 and exceed the calculated
spin-only value of 2.83µB for a high-spin d8 (S ) 1) system.
Such deviation is generally considered to be caused by a
substantial orbital angular momentum contribution from the
cobalt ion. In the temperature range between 300 and 15 K, the
magnetic moment of 1 is nearly constant at 3.65µB, but, likely
due to zero-field splitting, decreases rapidly below 15 K until
reaching a minimum of 2.60µB at 5 K. In comparison, the
magnetic moment of 2 shows little variation over the entire
temperature range from 300 to 5 K.
Chloro complex 3 in acetonitrile reacts with NaBPh4 to yield
the dicationic acetonitrile complex [(TIMENxyl)Co(CH3CN)]-
(BPh4)2 (4) (Scheme 3) and NaCl precipitation. After filtration
of the blue solution and storage at low temperatures, dark-blue
plate-shaped crystals of 4 were isolated in 80% yield. Alterna-
tively, an acetonitrile solution of 1 containing NaBPh4 will also
yield 4 within a few days. The 1H NMR spectrum of 4 exhibits
paramagnetically shifted and broadened signals spread over a
wider range (100 and -8 ppm) compared to complex 3.
The molecular structure of 4 displays features similar to those
of 2 and 3, namely, a four-coordinate cobalt ion and a tris-
chelating TIMENxyl ligand with distorted trigonal pyramidal
coordination geometry (Figure 4). The average Co-C distance
is 2.036(3) Å and the C-Co-C angle is 114.83(13)°. The axial
(53) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination
Compounds, 5th ed.; John Wiley & Sons: New York, 1997; Vol. 2.
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Organometallics 2003, 22, 5374-5377.
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A R T I C L E S
Hu et al.
Table 1
1
complex
absorption maxima λmax/nm (molar extinction coefficient ꢀ )
/cm-1 M-
1
2
3
4
220 (12426), 252 (6200), 384 (4080)
220 (16274), 254 (14300), 332 (6830)
220 (11262), 230 (12880), 308 (2890), 596 (782)
220 (78706), 324 (2801), 558 (201), 606 (581)
Scheme 4. Synthesis of Peroxo Complex [(TIMENxyl)Co(O2)]BPh4
(5)
Figure 7. Solid-state molecular structure of complex [(TIMENxyl)Co(O2)]-
BPh4‚Et2O (5‚Et2O). Hydrogen atoms, anion, and solvent molecules are
omitted for clarity; thermal ellipsoids are shown at 50% probability. Selected
bond lengths (angstroms) and angles (degrees): Co(1)-C(3) 2.003(4),
Co(1)-C(16) 1.934(4), Co(1)-C(29) 1.950(4), Co(1)-N(8) 2.137(3),
Co(1)-O(1) 1.855(2), Co(1)-O(2) 1.906(2), O(1)-O(2) 1.429(3), C(3)-
Co(1)-C(16) 169.67(14), C(3)-Co(1)-C(29) 98.65(14), C(16)-Co(1)-
C(29) 91.56(14), O(1)-Co(1)-O(2) 44.64(10), N(1)-Co(1)-C(3) 89.64(13),
N(1)-Co(1)-C(16) 91.56(14), N(1)-Co(1)-C(29) 89.27(13).
Cobalt(II) complexes 3 and 4 exhibit magnetic properties
3
characteristic for an ion with 3d7 high-spin (S ) /2) electron
configuration:58 Complex 3 exhibits a temperature-independent
magnetic moment of 4.23µB in the temperature range between
300 and 5 K. The magnetic moment of complex 4, on the other
hand, has a room-temperature value of 4.83µB, which decreases
gradually with decreasing temperature until it plateaus between
60 and 15 K. The magnetic moment decreases again at 10 K,
reaching a minimum value of 3.80µB at 5 K. The room-
temperature moments of both complexes 3 and 4 fall in the
range between 4.3µB and 4.8µB. This range is typically observed
for high-spin cobalt(II) complexes with pseudo-tetrahedral or
lower symmetry58-61 and is suggestive of a quartet magnetic
ground state for complexes 3 and 4.
cm-1) or superoxo (1200-1070 cm-1) species.62 In the infrared
vibrational spectrum of 5, the O-O stretching frequency νO-O
is found at 890 cm-1, and it shifts to 840 cm-1 in the corre-
sponding 18O-labeled compound [(TIMENxyl)Co(18O2)]BPh4.
The isotopic shift of 50 cm-1 agrees well with the 51 cm-1
calculated from a simple diatomic harmonic oscillator model.
Accordingly, the infrared spectra of 5 suggest the formation of
a side-on peroxo complex.
1
The H and 13C NMR spectra of 5 are consistent with a
diamagnetic cobalt(III) peroxo species. In the 13C NMR
spectrum, signals from the carbenoid carbons are not observed,
Figure 6 and Table 1 summarize the UV/vis spectral data
for complexes 1-4. All absorption spectra are dominated by
intense charge-transfer bands in the UV and near-visible regions.
The visible spectra of cobalt(II) complexes 3 and 4 show d-d
transitions at around 600 nm (ꢀ ) 782 and 581 M-1 cm-1),
characteristic for high-spin Co(II) complexes in a distorted
ligand field.59 The positions and intensities of these bands
correlate well with their distorted trigonal pyramidal geometry
(pseudo-C3V).
7
probably due to coupling with the cobalt ion (I ) /2, 100%).
Both 1H and 13C spectra, however, indicate a 3-fold symmetry
in the coordinated TIMENxyl ligand, suggesting a fluxional
behavior of 5 in solution. The UV/vis absorption spectrum of 5
exhibits two weak d-d transitions centered at 484 (ꢀ ) 100
cm-1 M-1) and 646 nm (ꢀ ) 35 cm-1 M-1) (see Figure S3,
Supporting Information). The low extinction coefficients are
consistent with the pseudo-octahedral geometry of the complex
(vide infra).
Synthesis and Structure of the Cobalt Dioxygen Complex
[(TIMENxyl)Co(O2)]BPh4 (5). The unusual molecular and
electronic structure of the coordinatively unsaturated complex
1 prompted us to investigate its reactivity toward dioxygen. We
found that a solution of 1 reacts cleanly with dioxygen at room
temperature to form a 1:1 cobalt dioxygen adduct [(TIMENxyl)-
Co(O2)]+ (Scheme 4). When the reaction is carried out in THF
and in the presence of 1 equiv of NaBPh4, the resulting complex
[(TIMENxyl)Co(O2)]BPh4 (5) precipitates from the reaction
mixture and is isolated as an analytically pure pale pink powder.
The molecular structure of 5 was established by X-ray
crystallography (Figure 7). The hexacoordinate cobalt ion is
situated in the plane defined by the two oxygen atoms of the
dioxygen ligand, the nitrogen atom on the nexus of the TIMEN
framework, and one carbene carbon from the TIMENxyl ligand,
thereby bisecting the tetradentate chelator. The pseudo-
octahedral coordination sphere is completed by the remaining
two carbene carbon pendant arms of TIMENxyl in the axial
positions. The dioxygen ligand coordinates side-on with an O-O
distance of 1.429(3) Å. This O-O bond distance falls well
within the range of typical peroxide complexes (1.4-1.5 Å)63
and is comparable with other rare cobalt(III) peroxo complexes
[1.414(12)-1.441(11) Å].46-49 This distance differs substan-
tially, however, from the cobalt dioxygen complex [Tp′Co(O2)]
with the structurally related hydro tris(pyrazolyl)borate sup-
On the basis of the O-O stretching frequency, dioxygen
complexes are generally classified as either peroxo (930-740
(58) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular
Paramagnetism; John Wiley & Sons: New York, 1976; pp P217-219.
(59) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999; pp
P820-821.
(60) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J.; Betley, T. A.; Peters, J. C. J.
Am. Chem. Soc. 2002, 124, 15336-15350.
(61) Drago, R. S. Physical Methods for Chemists, 2nd ed.; Surfside Scientific
Publishers: Gainesville, FL, 1992; p P486.
(62) Jones, R. D.; Summerville, D. A.; Basolo, F. Chem. ReV. 1979, 79, 139-
179.
(63) Cramer, C. J.; Tolman, W. B.; Theopold, K. H.; Rheingold, A. L. Proc.
Natl. Acad. Sci. U.S.A. 2003, 100, 3635-3640.
9
13468 J. AM. CHEM. SOC. VOL. 126, NO. 41, 2004

Dioxygen Activation by a Low-Valent Cobalt Complex
A R T I C L E S
porting ligand [1.355(3) Å at T ) -123 °C and 1.262(8) Å at
room temperature].50,63 It is noteworthy that the structurally
determined O-O bond distance correlates well with the
observed O-O stretching frequency.63 The Co-O bonds are
slightly asymmetric with bond distances of 1.855(2) and
1.906(2) Å. With an average value of 1.962(4) Å, the Co-C
distances are shorter than those of complexes 2-4, reflecting
the smaller radius of the cobalt(III) ion. In 5, two C-Co-C
angles were found to be close to 90° and a third C-Co-C angle
was close to 180° [C3-Co1-C29 ) 98.65(14), C16-Co1-
C29 ) 91.63(15), and C3-Co1-C16 ) 169.67(14)]. This,
together with an average N-Co-C angle of 90.2(1)°, demon-
strates the remarkable flexibility of the TIMEN ligand system,
stabilizing complexes with trigonal planar, trigonal pyramidal,
and octahedral coordination geometries.
Complex 5 represents a rare example of a metal NHC
complex with a coordinating dioxygen ligand.52 The ability of
TIMENxyl to stabilize a monomeric cobalt peroxo entity likely
arises from several factors: the tetradentate chelator is strongly
electron-donating and, thus, is able to accommodate the high-
valent Co(III) ion; the ligand backbone is very flexible and
enables TIMENxyl to form octahedral complexes; and the three
xylene substituents flank the activated dioxygen ligand, thereby
effectively blocking the commonly observed bimolecular de-
composition pathway to form µ-peroxo dimers.
Figure 8. Energies and relative positions of the frontier molecular orbitals
of [(TIMENxyl)Co(O2)]+. Calculations are performed at the BP86/ZORA/
TZP level with the ADF 2003.01 program suites.
DFT Study on Complex [(TIMENxyl)Co(O2)]+. DFT cal-
culations were carried out on the cationic part of the high-valent
peroxo complex 5. The calculation resulted in a geometry that
is in good agreement with the experimentally determined one.
The calculated O-O distance of 1.432 Å is very close to that
of 1.429(3) Å found in the solid-state molecular structure of 5,
and the Co-O distances of 1.852 and 1.905 Å are almost
identical to the experimental values of 1.855(2) and 1.906(2)
Å. The calculated Co-C and Co-N distances with 1.974 and
2.172 Å are also comparable to their experimental counterparts
[1.962(4) and 2.137(3) Å].
assessed by reaction with the extremely electron-deficient olefin
tetracyanoethylene (TCNE).68 While early transition metal per-
oxides, such as [(porphyrin)Ti(O2)],67 reportedly react as elec-
trophiles, late transition metal peroxides, like [(PPh3)2Pt(O2)]68
and [(porphyrin)Fe(O2)]-,67,69 generally are nucleophilic. As
suggested by the DFT study, peroxo complex 5 is nucleophilic
and, accordingly, only reacts with electron-deficient organic
substrates.
An acetonitrile solution of complex 5 reacts with benzoyl
chlorides, for instance, yielding complex 4 and phenyl benzoate
quantitatively. The NMR and absorption spectra of 4 isolated
from this reaction are identical to those of an independently
synthesized sample prepared as described above. The only
tractable organic product identified in this reaction was phenyl
benzoate, identified by NMR, IR, and gas chromatography-
mass spectrometry (GC-MS). A possible reaction mechanism
is shown in Scheme 5. In a first step, the coordinated dioxygen
attacks benzoyl chloride to form a cobalt(III) peroxyphenyl
Inspection of the frontier orbitals of complex 5 confirms that
the complex’s diamagnetism originates from a d6 low-spin (S
) 0) cobalt(III) center with a dative peroxo ligand (Figure 8).
The highest occupied molecular orbital (HOMO) is predomi-
nantly composed of dioxygen π* orbitals with some minor
contribution from the metal d orbital. Five nearby orbitals,
namely, lowest unoccupied molecular orbital (LUMO) + 1,
LUMO, HOMO - 1, HOMO - 2, and HOMO - 3, are mainly
metal d-orbital-based. The shapes and relative positions of these
acetate [(TIMEN)CoOOC(O)Ph]2+ 70
which undergoes ho-
,
molytic Co-O bond cleavage31 to give a cobalt(II) species and
2
2
2
20
20
2
orbitals indicate that the cobalt ion has a dxy dxz dyz dz dx -y
a •OOC(O)Ph radical. The organic radical then decomposes into
electronic configuration. The presence of a high-lying, filled
dioxygen π* orbital suggests the coordinated dioxygen ligand
to be nucleophilic.
Reactivity of [(TIMENxyl)Co(O2)]BPh4 (5). Transition metal
peroxo complexes are classified as either nucleophilic or
electrophilic according to their reactivity with organic sub-
strates.31,64-66 While reaction of triphenylphosphine to give
triphenylphosphineoxide is used to assess metal peroxides’
electrophilicity,67 the nucleophilicity of peroxo species is
•
CO2 and a OPh radical, which in turn reacts with another
molecule of benzoyl chloride to form the final PhOC(O)Ph prod-
uct.71 In support of this mechanism, we find that Ph18OC(O)Ph
is produced when an 18O2-labeled sample of 5 is employed in
the reaction. Unfortunately, we are unable to detect any CO2
evolution from the reaction mixture. This is likely due to the
small, stoichiometric quantities of CO2 released in this reaction.
(68) Sheldon, R. A.; Vandoorn, J. A. J. Organomet. Chem. 1975, 94, 115-
129.
(64) Ballistreri, F. P.; Tomaselli, G. A.; Toscano, R. M.; Conte, V.; Difuria, F.
J. Am. Chem. Soc. 1991, 113, 6209-6212.
(69) Sisemore, M. F.; Burstyn, J. N.; Valentine, J. S. Angew. Chem., Int. Ed.
Engl. 1996, 35, 206-208.
(70) An iron peroxyacetate complex was identified in the reacton of peroxo
complex [(porphrin)Fe(O2)]- with acylating agents; see Khenkin, A. M.;
Shteinman, A. A. J. Chem. Soc., Chem. Commun. 1984, 1219-1220.
(71) The fate of the chlorine radical generated in this reaction is not clear.
Formation of Co(II)-Cl species was not observed.
(65) Adam, W.; Haas, W.; Lohray, B. B. J. Am. Chem. Soc. 1991, 113, 6202-
6208.
(66) Kitajima, N.; Morooka, Y. Chem. ReV. 1994, 94, 737-757.
(67) Sisemore, M. F.; Selke, M.; Burstyn, J. N.; Valentine, J. S. Inorg. Chem.
1997, 36, 979-984.
9
J. AM. CHEM. SOC. VOL. 126, NO. 41, 2004 13469

A R T I C L E S
Hu et al.
forms the high-spin carbonyl species [(TIMEN)Co(CO)]+,
phosphine-based ligand systems NP3 and PhBP3 (P ) PPh2)
Scheme 5. Proposed Mechanism for the Reaction of 5 with
Benzoyl Chloride
-
form diamagnetic complexes [(NP3)Co(CO)]+ and [(PhBP3)-
Co(CO)2], respectively. The cobalt(II) halide [(TIMENxyl)-
CoCl]+ is four-coordinate, distorted trigonal pyramidal, and
high-spin; the equivalent phosphine species [(NP3)CoCl]+, how-
ever, is five-coordinate, trigonal-bipyramidal, and high-spin;75
[(PhBP3)CoI] is four-coordinate, pseudo-tetrahedral, and low-
spin.60
The steric properties of the TIMEN ligand system are also
unique and are generally beneficial for the stabilization of
reactive intermediates. While the sterics of coordinated poly-
phosphine ligands are regulated by substituents on the coordi-
nating phosphorus atom, the sterics of TIMEN are controlled
by substituents at the imidazole N3 position. Upon formation
of (distorted) tetrahedral metal phosphine complexes, the
substituents all point away from the metal center. As a result,
the axial binding site in these complexes is exposed and dimeric
compounds form readily via binuclear decomposition pathways.
Complex 5 also reacts with electron-deficient alkenes. Reac-
tion of 5 with benzylidenemalonitrile yields 4 and benzyl
aldehyde, and reaction of 5 with TCNE yields 4 and unidentified
organic compounds. The mechanisms of these reactions are
currently under investigation, but on the basis of the reaction
products, we suspect homolytic Co-O bond cleavage as part
of the mechanism. Palladium and platinum peroxo complexes
undergo similar reactions via a 2 + 2 cycloaddition pathway to
give cyclic peroxy adducts.68,72
We also treated 5 with R,â-unsaturated alkenes such as
2-cyclohexene-1-one, 1,4-naphthoquinone, and 2-methyl-1,4-
naphthoquinone, but no reaction was observed during an
extended period of time. Likewise, complex 5 also is inert
toward styrene, cyclohexene, and triphenylphosphine, confirm-
ing its lack of electrophilicity.
Valentine and co-workers have classified peroxo species into
class I and II nucleophiles according to their relative nucleo-
philicity.67 Class I nucleophiles react not only with highly
electron-deficient substrates such as TCNE and acyl halide but
also with enones and quinones. Class II nucleophiles, on the
other hand, merely react with TCNE and acyl halides. Accord-
ingly, the reactivity of complex 5 identifies it as class II
nucleophile. Other class II nucleophiles include group VIII metal
peroxides, such as [(PPh3)2Pd(O2)], [(PPh3)2Pt(O2)], and
[(PPh3)2Ir(CO)(O2)]I.68,72,73
-
Even the sterically encumbering and versatile PhB(iPr2P)3
ligand system does not prevent such dimerization as evidenced
by a unique but highly reactive terminal iron(IV) nitrido complex
[(PhB(iPr2P)3)Fe≡N] that forms a dinitrogen-bridged dinuclear
ferrous species.8 Due to constrains of the sp2-hybridized ring
nitrogen, aryl substituents in complexes of the TIMENR ligand
are oriented perpendicular to the trigonal tris-carbene metal
plane, thereby forming a deep, well-protected binding cavity.
As a result, in complexes of the type [(TIMEN)M(Lax)]n+
,
binuclear decomposition pathways are effectively suppressed.
This steric difference is similarly evident for TIMEN and
â-diketiminate ligand systems.80-85 It was reported that the
cobalt(I) complex of the xylene-functionalized â-diketiminate
ligand, namely, [(Me2NN)Co(η6-toluene)] (Me2NN ) 2,4-
bis(2,6-dimethylphenyllimido)pentane), reacts with dioxygen to
form a dinuclear [(Me2NN)Co(µ-O)2Co(NNMe2)] complex.
Monomeric dioxygen adducts could not be isolated from this
reaction.85 The differences in dioxygen reactivity of [(Me2NN)-
Co(η6-toluene)] and [(TIMENxyl)Co]+ may thus also be steric
in origin.
Conclusion
The low-valent and coordinatively unsaturated cobalt(I) tris-
carbene complex [(TIMENxyl)Co]Cl (1) has been synthesized
and characterized. This versatile starting material reacts with
CO, CH2Cl2, and O2 to yield the cobalt carbonyl [(TIMENxyl)-
Co(CO)]Cl (2), the chloro [(TIMENxyl)Co(Cl)]Cl (3), and peroxo
[(TIMENxyl)Co(O2)](BPh4) (5) complexes. Spectroscopic and
structural characterization of all complexes 1-5 (including the
acetonitrile complex [(TIMENxyl)Co(NCCH3)](BPh4)2 (4)) has
allowed important insights into structure/reactivity relations. The
X-ray analyses revealed the remarkable flexibility of the tripodal
ligand system TIMEN, stabilizing distorted trigonal pyramidal,
Comparison of the Tris-Carbene TIMEN and Analogous
Tris-Phosphine Ligand Systems. The unique electronic prop-
erties of NHCs render the N-anchored tris-carbene TIMEN
ligand system considerably diverse from known analogous
polydentate phosphine chelators,74 such as the neutral NP3 (P
) PPh2) ligand by Sacconi75-78 and the anionic PhBP3- system
(P ) PPh2, PiPr2) developed by Peters et al.6-8,60,79 These
differences are reflected in the coordination polyhedra and
electronic properties of the isolated Co-TIMEN complexes 1-4.
While, with cobalt(I) for instance, the TIMEN ligand system
(72) Mimoun, H. J. Mol. Catal. 1980, 7, 1-29.
(73) Regen, S. L.; Whitesides, G. M. J. Organomet. Chem. 1973, 59, 293-
297.
(80) Basuli, F.; Bailey, B. C.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J.
J. Am. Chem. Soc. 2003, 125, 6052-6053.
(74) Mayer, H. A.; Kaska, W. C. Chem. ReV. 1994, 94, 1239-1272.
(75) Sacconi, L.; Bertini, I. J. Am. Chem. Soc. 1968, 90, 5443-5446.
(76) Sacconi, L.; Ghilardi, C. A.; Mealli, C.; Zanobini, F. Inorg. Chem. 1975,
14, 1380-1386.
(81) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 1999, 121, 7270-7271.
(82) Spencer, D. J. E.; Aboelella, N. W.; Reynolds, A. M.; Holland, P. L.;
Tolman, W. B. J. Am. Chem. Soc. 2002, 124, 2108-2109.
(83) Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers,
G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222-
9223.
(77) Ghilardi, C. A.; Midollini, S.; Sacconi, L. Inorg. Chem. 1975, 14, 1790-
1795.
(78) Ghilardi, C. A.; Sabatini, A.; Sacconi, L. Inorg. Chem. 1976, 15, 2763-
(84) Dai, X. L.; Warren, T. H. Chem. Commun. 2001, 1998-1999.
(85) Dai, X. L.; Kapoor, P.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 4798-
4799.
2767.
(79) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074-5084.
9
13470 J. AM. CHEM. SOC. VOL. 126, NO. 41, 2004

Dioxygen Activation by a Low-Valent Cobalt Complex
A R T I C L E S
a set of d functions and frozen core 1s. C: triple ú-basis set augmented
with a set of d functions and frozen core 1s. O: triple ú-basis set
augmented with a set of d functions and frozen core 1s. H: triple-ú
basis set augmented with a set of p functions. This basis combination
is denoted TZP in the ADF program. No symmetry was specified in
the calculations starting from X-ray defined geometry of the cationic
portion of 5; the counteranion was not included in the calculation.
Molecular orbitals were visualized by use of the MOLDEN pro-
gram package (http://www.cmbi.kun.nl/∼schaft/molden/molden.html).
ADFrom program was used to convert the TAPE21 file from ADF
into a MOLDEN file.
Starting Materials. Tris(2-chloroethyl)-amine and 1-(2,6-xylyl)-
imidazole were prepared following literature procedures.93,94 Chloro-
tristriphenylphosphine cobalt(I) (Aldrich), tetracyanoethylene (Aldrich),
benzylidenemalonitrile (Aldrich), benzoyl chloride (Fischer), sodium
tetraphenylborate (Acros), and potassium tert-butoxide (Acros) were
obtained from commercial sources and used as received. Carbon
monoxide and dioxygen gases were purchased from Matheson Tri-
gas, and 95% 18O-labeled O2 was ordered from Cambridge Isotope
Laboratories, Inc. The imidazolium salt [H3TIMENxyl](PF6)3 and free
carbene TIMENxyl were prepared by modifying the previously described
methods.15
Synthesis: [H3TIMENxyl](PF6)3. A 50 mL flask was charged with
tris(2-chloroethyl)amine (1.58 g, 7.7 mmol) and 1-(2,6-xylyl)imidazole
(4.00 g, 60 mmol), and the mixture was heated to 150 °C for 2 days.
During this time, a brown solid precipitated from the solution. The
solid was filtered off and dissolved in 20 mL of methanol; this solution
was filtered and then evaporated to dryness to yield the crude product
[H3TIMENxyl]Cl3. The hygroscopic chloride salt was converted to the
corresponding, stable hexafluorophosphate salt, [H3TIMENxyl](PF6)3,
by addition of a solution of NaPF6 (3.88 g, 23.1 mmol) in 20 mL of
methanol. The white hexafluorophosphate salt precipitated immediately
and was collected by filtration, washed with small volumes of cold
methanol, redissolved in acetone, and filtered. The solvent was
evaporated to dryness and the resulting solid was dried in a vacuum
(6.5 g; yield 80%).
1H NMR (300 MHz, DMSO-d6, 20 °C): δ 9.46 (s, 3H), 8.09 [d,
3J(H,H) ) 1.1 Hz, 3H], 8.02 [d, 3J(H,H) ) 1.1 Hz, 3H], 7.49 [t, 3J(H,H)
) 7.8 Hz, 3H], 7.39 [d, 3J(H,H) ) 7.8 Hz, 6H], 4.48 [t, 3J(H,H) ) 6.0
Hz, 6H], 3.26 (s, 9H), and 2.12 ppm (s, 18H). 13C NMR (100 MHz,
DMSO-d6, 20 °C): δ 137.3, 136.6, 133.5, 130.6, 128.7, 123.6, 123.4,
52.2, 46.9, and 17.0 ppm.
four-coordinate cobalt(I) and cobalt(II) complexes as well as a
six-coordinate cobalt(III) complex with pseudo-octahedral ge-
ometry. This structural flexibility, allowing formation of a large
variety of 1:1 metal complexes,15 is provided by the anchoring
nitrogen atom of the carbene tripod. We have demonstrated that
this nitrogen atom additionally holds potential to serve as a two-
electron σ-donor ligand to aid stabilization of higher valent
complexes such as the cobalt(III) peroxo complex reported here.
The stability of the monomeric cobalt(III) peroxo complex
[(TIMENxyl)Co(O2)](BPh4) is increased by steric protection of
the bulky xylene substituents on the TIMEN imidazole rings.
The sterically well-protected coordinated dioxygen ligand reacts
with electron-deficient organic substrates. This experimentally
determined dioxygen reactivity correlates well with the theoreti-
cally predicted reactivity as established by DFT calculations.
One-electron reduction of complex 5 is expected to further
increase the nucleophilicity of the dioxygen ligand.
The ability of the polydentate carbene ligand TIMENR (R )
aryl) to stabilize both low- and high-valent metal ions and its
unique steric properties suggest that the [(TIMENR)M] scaffold
is well-suited for hosting other highly reactive terminal func-
tionalities, e.g., monodentate imido, nitrido, and oxo ligands.
Efforts in pursuing such species are currently underway.
Experimental Section
Methods and Procedures. Manipulations of air-sensitive compounds
were performed under a dry nitrogen atmosphere by standard Schlenk
techniques and inert-gas gloveboxes (MBraun Labmaster by M. Braun,
Inc.). Solvents were purified on a two-column solid-state purification
system (Glasscontour Systems, Joerg Meyer, Irvine, CA) and transferred
to the glovebox without exposure to air. NMR solvents were obtained
from Cambridge Isotope Laboratories, degassed, and stored over
nitrogen and activated molecular sieves prior to use. All NMR spectra
were recorded on Varian spectrometers operating at 400/300 MHz (1H
NMR) and 100 MHz (13C NMR) at room temperature (20 °C) in
benzene-d6, acetonitrile-d3, dichoromethane-d2, and DMSO-d6 solutions,
respectively. The signals were referenced to residual solvent peaks
unless noted otherwise (δ in parts per million, ppm). Solid-state
magnetization measurements of powdered samples were recorded on
a SQUID magnetometer (Quantum Design) at 10 kOe between 5 and
300 K. Magnetic susceptibility data were corrected for background and
underlying diamagnetic contributions by use of tabulated Pascal
constants.86 Data reproducibility was carefully checked in multiple
individual measurements of independently synthesized samples. Infrared
spectra (400-4000 cm-1) of solid samples were obtained on a Thermo
Nicolet Avatar 360 Fourier transform infrared (FT-IR) spectrophotom-
eter as KBr pellets. Electronic absorption spectra were recorded from
190 to 820 nm (HP 8452A diode-array UV/vis spectrophotometer).
Elemental analyses were performed by Kolbe Microanalytical Labora-
tory (Mu¨lheim a.d. Ruhr, Germany).
[TIMENxyl]. A solution of potassium tert-butoxide (0.428 g, 3.81
mmol) in THF was added dropwise to a suspension of [H3TIMENxyl]-
(PF6)3 (1.00 g, 0.95 mmol) in 5 mL of THF and stirred for 1 h. The
solution was then evaporated to dryness and the solid residue was
dissolved in 15 mL of diethyl ether. The resulting solution was filtered
and the filtrate was evaporated to dryness in a vacuum. The solid was
collected, washed with cold pentane, and dried in a vacuum (0.43 g;
yield 75%).
3
1H NMR (300 MHz, DMSO-d6, 20 °C): δ 7.32 [d, J(H,H) ) 1.1
Hz, 3H], 7.21 (m, 3H), 7.14 (m, 6H), 7.06 [d, 3J(H,H) ) 1.1 Hz, 3H],
3
4.08 [t, J(H,H) ) 6.0 Hz, 6H], 3.01 [t, J(H,H) ) 6.0 Hz, 6H], and
Computational Details. DFT calculations on complex 5 were carried
out with the Amsterdam Density Functional program package ADF,
release 2003.01.87 The Vosko, Wilk, and Nusair (VWN) local density
approximation,88 Becke’s exchange correlation,89 and Perdew correla-
tion90 were used. The calculation also included scalar relativistic effects
(ZORA)91 for all atoms. Uncontracted Slater-type orbitals (STOs)92 were
used as basis functions. Co: triple-ú basis set augmented with a set of
p functions and frozen core 2p. N: triple-ú basis set augmented with
1.94 ppm (s, 18H). 13C NMR (100 MHz, benzene-d6, 20 °C): δ 211.8,
141.4, 135.8, 121.3, 120.5, 120.3, 119.9, 119.6, 67.5, 56.6, 49.7, 32.2,
and 18.4 ppm.
[(TIMENxyl)Co]Cl (1). A solution of (PPh3)3CoCl (1.1 g, 1.24 mmol)
in 5 mL of benzene was added dropwise to a solution of TIMENxyl
(0.76 g, 1.24 mmol) in 5 mL of benzene. The reaction mixture was
allowed to stir for 1 h, during which a yellow-brown precipitate formed.
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Patent 6177575, 2001.
9
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A R T I C L E S
Hu et al.
The precipitate was filtered, washed with benzene and diethyl ether,
and dried in a vacuum (0.69 g; yield 79%).
was collected by filtration, washed with diethyl ether, and dried in a
vacuum (90 mg; yield 57%).
1H NMR (300 MHz, DMSO-d6, 20 °C): δ 72.1 (s, 3H, ∆V1/2 ) 62
Hz), 27.3 (s, 3H, ∆V1/2 ) 51 Hz), 15.5 (s, 3H, ∆V1/2 ) 46 Hz), 7.8 (s,
9H, ∆V1/2 ) 81 Hz), 7.0 (s, 6H, ∆V1/2 ) 37 Hz), 5.5 (s, 3H, ∆V1/2 ) 30
Hz), 4.4 (s, 3H, ∆V1/2 ) 32 Hz), -2.8 (s, 3H, ∆V1/2 ) 18 Hz), -5.0 (s,
9H, ∆V1/2 ) 93 Hz), and -24.0 ppm (s, 3H, ∆V1/2 ) 20 Hz). Elemental
analysis (%) calcd for C39H45N7CoCl: C 66.33, H 6.42, N 13.88.
Found: C 66.09, H 6.22, N 13.57.
1H NMR (300 MHz, DMSO-d6, 20 °C): δ 7.50 (s, 3H), 7.16 (br s,
3
8H), 6.92 (m, 14H), 6.78 [d, J(H,H) ) 6 Hz, 3H], 6.75 (s, 3H), 6.71
(s, 3H), 4.25 (br s, 3H), 3.60 (br s, 3H), 3.50(br s, 3H), 2.65 (br s, 3H),
1.64 (s, 9H), and 1.25 ppm (s, 9H). 13C NMR (100 MHz, DMSO-d6,
20 °C): δ 164.8, 164.3, 163.8, 163.3, 140.6, 136.2, 135.3, 127.6, 127.4,
126.1, 126.0, 125.9, 125.7, 123.5, 122.2, 47.7, 18.6, and 17.8 ppm.
IR(KBr): ν(OO) ) 890 cm-1. Elemental analysis (%) calcd for
C73H65N7BO2Co: C 76.77, H 5.70, N 8.59. Found: C 76.34, H 5.43, N
8.28.
[(TIMENxyl)Co(CO)]Cl (2). A yellow solution of [(TIMENxyl)-
Co]Cl (30 mg, 0.04 mmol) in acetonitrile was sparged with carbon
monoxide gas, resulting in an immediate color change to green. The
reaction mixture was stirred for 30 min, filtered through Celite, and
evaporated to give 2 as a green solid. The precipitate was collected by
filtration, washed with diethyl ether, and dried in a vacuum (25 mg;
yield 85%). Green crystals suitable for X-ray diffraction analysis were
grown by diffusion of diethyl ether into a saturated acetonitrile solution
of 2 at room temperature.
The 18O-labeled [(TIMENxyl)Co(18O2)]BPh4 complex was synthesized
employing 95% 18O2-labeled gas.
Reactions of [(TIMENxyl)Co(O2)]BPh4 with Benzoyl Chloride.
Benzoyl chloride (1 equiv) was added to a stirred solution of 5 (30
mg, 0.026 mmol) in acetonitrile. The reaction was allowed to stir for
1 day, and the resulting blue solution was evaporated to dryness. Diethyl
ether was added to extract the organic product, which, after workup,
was identified as phenyl benzoate by IR, NMR, and GC-MS. The solid
residue was also collected and identified as complex 4 by NMR
spectroscopy (30 mg; yield 79%). The same reaction products were
isolated when excess benzoyl chloride was used. NMR reaction showed
that 4 was the only tractable cobalt complex formed.
Reactions of [(TIMENxyl)Co(O2)]BPh4 with Benzylidenemalo-
nitrile. Benzylidenemalonitrile (1 equiv) was added to a solution of 5
(15 mg, 0.013 mmol) in acetonitrile under stirring. A blue solution
formed in 30 min. The solution was evaporated to dryness and diethyl
ether was added to extract the organic product, which contains benzyl
aldehyde as evidenced by NMR and GC-MS. The solid residue was
also collected and identified as complex 4 by its NMR spectrum (10
mg; yield 53%). NMR reaction showed that 4 is the only tractable cobalt
complex formed.
1H NMR (300 MHz, acetonitrile-d3, 20 °C): δ 76.0 (s, 3H, ∆V1/2
)
15 Hz), 36.1 (s, 3H, ∆V1/2 ) 53 Hz), 25.7 (s, 3H, ∆V1/2 ) 11 Hz), 11.7
(s, 3H, ∆V1/2 ) 17 Hz), 10.1 (s, 3H, ∆V1/2 ) 15 Hz), 5.8 (s, 3H, ∆V1/2
) 29 Hz), 3.9 (s, 3H, ∆V1/2 ) 37 Hz), 1.4 (s, 9H, ∆V1/2 ) 49 Hz),
-3.4 (s, 3H, ∆V1/2 ) 39 Hz), and -10.7 ppm (s, 9H, ∆V1/2 ) 106 Hz).
IR (KBr): ν(CO) ) 1927 cm-1. Elemental analysis (%) calcd for
C40H45N7OCoCl: C 65.43, H 6.17, N 13.35. Found: C 65.37, H 5.96,
N 13.28.
[(TIMENxyl)CoCl]Cl (3). [(TIMENxyl)Co]Cl (50 mg, 0.71 mmol)
was dissolved in 2 mL of dichloromethane to form a blue solution.
Diethyl ether was then diffused into the solution at room temperature
to give blue crystals of 3 overnight. The crystals were collected by
filtration, washed with diethyl ether, and dried in a vacuum (46 mg;
yield 87%). Crystals suitable for X-ray diffraction analysis were grown
by diffusion of diethyl ether into a solution of 3 in DMSO at room
temperature.
Reactions of [(TIMENxyl)Co(O2)]BPh4 with TCNE. Benzylidene-
malonitrile (1 equiv) was added to a stirred solution of 5 (15 mg, 0.013
mmol) in acetonitrile. The resulting blue solution was evaporated to
dryness and diethyl ether was added to extract any organic products.
The solid residue was collected and identified as complex 4 by NMR
spectroscopy (12 mg; yield 64%). An NMR reaction showed that 4 is
the only tractable cobalt complex formed.
1H NMR (300 MHz, dichloromethane-d2, 20 °C): δ 67.4 (s, 3H,
∆V1/2 ) 66 Hz), 33.3 (s, 3H, ∆V1/2 ) 94 Hz), 31.2 (s, 3H, ∆V1/2 ) 96
Hz), 9.5 (s, 6H, ∆V1/2 ) 24 Hz), 6.8 (s, 3H, ∆V1/2 ) 72 Hz), 6.4 (s, 3H,
∆V1/2 ) 53 Hz), 5.7 (s, 3H, ∆V1/2 ) 71 Hz), 1.9 (s, 9H, ∆V1/2 ) 114
Hz), 0.6 (s, 9H, ∆V1/2 ) 103 Hz), and -3.3 ppm (s, 3H, ∆V1/ ) 24
2
Reactions of [(TIMENxyl)Co(O2)]BPh4 with Alkenes and Tri-
phenylphosphine. The reactivity of 5 with 2-cyclohexen-1-one, 1,4-
naphthaquinone, 2-methyl-1,4-naphthaquinone, styrene, cyclohexene,
and triphenylphosphine was examined. In a typical reaction, an NMR
tube was loaded with equal molar amounts of 5 and alkene (or
phosphine) and the reaction was monitored by NMR spectroscopy. In
all cases, no change in the NMR spectrum was observed after 2 days.
Crystallographic Details for [(TIMENxyl)Co(CO)]Cl‚2CH3CN (2‚
2CH3CN). A crystal of dimensions 0.41 × 0.33 × 0.08 mm3 was
mounted on a glass fiber. A total of 17 360 reflections (-13 e h e
13, -13 e k e 13, - 27 e l e 26) were collected at T ) 100(2) K in
the range of 0.97-27.51°, of which 8901 were unique (Rint ) 0.0190);
Mo KR radiation (λ) 0.710 73 Å). The structure was solved by direct
methods (Shelxtl version 6.10, Bruker AXS, Inc., 2000). All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed in calculated idealized positions. The residual peak and hole of
electron densities were 0.740 and -0.344 eA-3. The absorption
coefficient was 0.527 mm-1. The least-squares refinement converged
with residuals of R(F) ) 0.0367, wR(F2) ) 0.0903, and a GOF ) 1.033
[I > 2σ(I)]. C44H51ClCoN9O, space group P1h, triclinic, a ) 10.5775(9),
b ) 10.6288(9), c ) 21.2014(9), R ) 94.2400(10)°, â ) 93.0000(10)°,
γ ) 119.2060(10)°, V ) 2064.1(10) A3, Z ) 2, Fcalcd ) 1.313 Mg/m3.
Crystallographic Details for [(TIMENxyl)Co(Cl)]Cl‚DMSO (3‚
DMSO). A crystal of dimensions 0.30 × 0.13 × 0.02 mm3 was
mounted on a glass fiber. A total of 11 898 reflections (-11 e h e
11, -11 e k e 11, - 22 e l e 22) were collected at T ) 100(2) K in
the range of 1.98-22.50° of which 5352 were unique (Rint ) 0.0259);
Hz). Elemental analysis (%) calcd for C39H45N7CoCl2: C 63.16, H 6.11,
N 13.22. Found: C 63.17, H 5.88, N 13.16.
[(TIMENxyl)Co(CH3CN)](BPh4)2 (4). (Method A) NaBPh4 (2 equiv)
was added to a blue solution of [(TIMENxyl)CoCl]Cl (120 mg, 0.16
mmol) in acetonitrile, and the mixture was filtered through Celite. Blue
crystals start to form from this filtrate within 30 min. The crystals are
collected by filtration, washed with diethyl ether, and dried in a vacuum
(168 mg; yield 80%).
(Method B) Excess NaBPh4 was added into a yellow solution of
[(TIMENxyl)Co]Cl (30 mg, 0.04 mmol) in acetonitrile. The solution
gradually turns blue, and blue needlelike crystals form within 1 day.
The crystals were collected by filtration, washed with diethyl ether,
and dried in a vacuum (20 mg; yield 38%).
1H NMR (300 MHz, DMSO-d3, 20 °C): δ 98.3 (s, 3H, ∆V1/2 ) 15
Hz), 30.7 (s, 3H, ∆V1/2 ) 42 Hz), 19.6 (s, 3H, ∆V1/2 ) 20 Hz), 12.5 (s,
9H, ∆V1/2 ) 15 Hz), 7.8 (s, 3H, ∆V1/2 ) 33 Hz), 7.16 (m, 16H), 6.92
(m, 16H), 6.80 (dd, 8 H), 4.3(s, 6H, ∆V1/2 ) 29 Hz), 1.3 (s, 9H, ∆V1/2
) 35 Hz), -3.2 (s, 3H, ∆V1/2 ) 8 Hz), and -6.0 ppm (s, 3H, ∆V1/ 2
)
7 Hz). Elemental analysis (%) calcd for C89H88N8B2Co: C 79.17, H
6.57, N 8.30. Found: C 80.69, H 6.11, N 7.53.
[(TIMENxyl)Co(O2)]BPh4 (5). NaBPh4 (1 equiv) was added to a
suspension of [(TIMENxyl)Co]Cl (100 mg, 1.4 mmol) in 3 mL of THF.
All solid dissolved immediately, and the resulting solution was filtered
and transferred to a Schlenk flask. The flask was evacuated and cooled
to -78 °C. Neat dioxygen gas was transferred to the flask and the
reaction mixture was allowed to warm to room temperature. A pale-
pink solid starts to precipitate from the solution within 1 h. The solid
9
13472 J. AM. CHEM. SOC. VOL. 126, NO. 41, 2004

Dioxygen Activation by a Low-Valent Cobalt Complex
A R T I C L E S
Mo KR radiation (λ ) 0.710 73 Å). The structure was solved by direct
methods (Shelxtl version 6.10, Bruker AXS, Inc., 2000). All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed in calculated idealized positions. The cocrystallized DMSO
solvent molecule was disordered and, therefore, modeled on two
positions. The residual peak and hole of electron densities were 1.682
and -0.429 eA-3. The absorption coefficient was 0.643 mm-1. The
least-squares refinement converged with residuals of R(F) ) 0.0618,
wR(F2) ) 0.1654, and a GOF ) 1.071 [I > 2σ(I)]. C41H45Cl2CoN7OS,
space group P1h, triclinic, a ) 10.667(2), b ) 10.702(2), c ) 20.707(4),
R ) 87.212(3)°, â ) 83.314(3)°, γ ) 60.477(3)°, V ) 2062.7(7) A3,
Z ) 2, Fcalcd ) 1.323 Mg/m3.
k e 17, - 19 e l e 19) were collected at T ) 100(2) K in the range
of 1.34-25.0° of which 10 206 were unique (Rint ) 0.065); Mo KR
radiation (λ ) 0.710 73 Å). The structure was solved by direct methods
(Shelxtl version 6.10, Bruker AXS, Inc., 2000). All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were placed in calculated
idealized positions. Two sites occupied by diethyl ether were identified
in the unit cell. The sites were considerably disordered and were treated
by SQUEEZE95 as a diffuse contribution. In the resulting void space,
a contribution of 115 e-/unit cell was found (required, 84 e-). The
residual peak and hole of electron densities were 0.670 and -0.355
eA-3. The absorption coefficient was 0.341 mm-1. The least-squares
refinement converged with residuals of R(F) ) 0.0650, wR(F2) )
0.1356, and a GOF ) 0.953 [I > 2σ(I)]. C63H65BCoN7O2, space group
P1h, triclinic, a ) 13.600(3), b ) 14.788(3), c ) 16.356(3), R ) 95.254-
(3)°, â ) 109.027(3)°, γ ) 106.392(3)°, V ) 2921.7(9) A3, Z ) 2,
Fcalcd ) 1.162 Mg/m3.
Crystallographic Details for [(TIMENxyl)Co(CH3CN)](BPh4)2‚
CH3CN (4‚CH3CN). A crystal of dimensions 0.40 × 0.10 × 0.05 mm3
was mounted on a glass fiber. A total of 42 407 reflections (-25 e h
e 25, -12 e k e 12, - 29 e l e 29) were collected at T ) 100(2)
K in the range of 1.98-22.50° of which 9860 were unique (Rint
)
Acknowledgment. This work was supported by the Univer-
sity of California, San Diego. We thank Professor Arnold L.
Rheingold for help with the crystallography. I.C.-R. is grateful
for an NIH fellowship. K.M. is an Alfred P. Sloan research
fellow.
0.0697); Mo KR radiation (λ ) 0.710 73 Å). The structure was solved
by direct methods (Shelxtl version 6.10, Bruker AXS, Inc., 2000). All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms were
placed in calculated idealized positions. The residual peak and hole of
electron densities were 0.476 and -0.249 eA-3. The absorption
coefficient was 0.282 mm-1. The least-squares refinement converged
with residuals of R(F) )0.0491, wR(F2) ) 0.1065, and a GOF ) 1.049
[I > 2σ(I)]. C91H91B2CoN9, space group P21/c, monoclinic, a )
23.7912(19), b ) 11.5916(9), c ) 27.394(2), R ) 90°, â )
94.8400(10)°, γ ) 90°, V ) 7527.6(10) A3, Z ) 4, Fcalcd ) 1.228 Mg/
m3.
Supporting Information Available: Crystallographic details
on a crystal containing a mixture of 1 and 3, and complete
listings of structural parameters for 2-5 (CIF and PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Crystallographic Details for [(TIMENxyl)Co(O3)]BPh4‚Et2O (5‚
Et2O). A crystal of dimensions 0.21 × 0.10 × 0.03 mm3 was mounted
on a glass fiber. A total of 20 992 reflections (-16 e h e 16, -17 e
JA046048K
(95) Spek, A. L. J. Appl. Cryst. 2003, 36, 7-13.
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J. AM. CHEM. SOC. VOL. 126, NO. 41, 2004 13473