Syntheses, structures, and reactivities of Cobalt(III)-alkylperoxo complexes and their role in stoichiometric and catalytic oxidation of hydrocarbons

Article abstract of DOI:10.1021/ja9814873

Although Co(III)-alkyl peroxo species have often been implicated as intermediates in industrial oxidation of hydrocarbons with cobalt catalysts, examples of discrete [LCo(III)-OOR] complexes and studies on their oxidizing capacities have been scarce. In this work, twelve such complexes with two different ligands, L, and various primary, secondary, and tertiary R groups have been synthesized, and seven of them have been characterized by X-ray crystallography. The dianion (L^2-) of the two ligands N,N-bis[2-(2- pyridyl)ethyl]-pyridine-2,6-dicarboxamide (Py₃PH₂, 1) and N-N-bis[2-(1- pyrazolyl)ethyl]pyridine-2,6-dicarboxamide (PyPz₂-PH₂, 2) bind Co(III) centers in pentadentate fashion with two deprotonated carboxamido nitrogens in addition to three pyridine or one pyridine and two pyrazole nitrogens to afford complexes of the type [LCo(III)(H₂O)] and [LCo(III)(OH)]. Reactions of the [LCo(III)(OH)] complexes with ROOH in aprotic solvents of low polarity readily afford the [LCo(III)-OOR] complexes in high yields. This report includes syntheses of [Co(Py₃P)(OOR)] complexes with R = (t)Bu (7, (t)Bu) = CMe₃), Cm (8, Cm = CMe₂Ph), CMe₂CH₂Ph (9), Cy (10, Cy = c-C₆H₁₁), (i)Pr (11, (i)Pr = CHMe₂) or (n)Pr (12, (n)Pr = CH₂CH₂CH₃), and [Co(PyPz₂P)(OOR) complexes with R = (t)Bu (13), Cm (14), CMe₂CH₂Ph (15), Cy (16), (i)Pr (17) or (n)Pr (18). The structures of 8-12 and 16 have been established by X-ray crystallography. Complexes 10 and 16 are the first examples of structurally characterized compounds containing the [Co-OOCy] unit, proposed as a key intermediate in cobalt-catalyzed oxidation of cyclohexane. The metric parameters of 7-12 and 16 have been compared with those of other reported [LCo(III)-OOR] complexes. When these [LCo(III)-OOR] complexes are warmed (60-80 °C) in dichloromethane in the presence of cyclohexane (CyH), cyclohexanol (CyOH) and cyclohexanone (CyO) are obtained in good yields. Studies on such reactions (referred to as stoichiometric oxidations) indicate that homolysis of the O-O bond in the [LCo(III)-OOR] complexes generates RO· radicals, in the reaction mixtures which are the actual agents for alkane oxidation. [LCo-O·], the other product of homolysis, does not promote any oxidation. A mechanism for alkane oxidation by [LCo(III)-OOR] complexes has been proposed on the basis of the kinetic isotope effect (KIE) value (5 at 80 °C), the requirement of dioxygen for oxidation, the dependence of yields on the stability of the RO· radicals, and the distribution of products with different substrates. Both L and R modulate the capacity for alkane oxidation of the [LCo(III)-OOR] complexes. The extent of oxidation is noticeably higher in solvents of low polarity, while the presence of water invariably lowers the yields of the oxidized products. Since [LCo(III)-OOR] complexes are converted into the [LCo(III)(OH)] complexes at the end of single turnover in stoichiometric oxidation reactions, it is possible to convert these systems into catalytic ones by the addition of excess ROOH to the reaction mixtures. The catalytic oxidation reactions proceed at respectable speed at moderate temperatures and involve [LCo(III)-OOR] species as a key intermediate. Turnover numbers over 100 and ~10% conversion of CyH to CyOH and CyO within 4 h have been noted in most catalytic oxidations. The same catalyst can be used for the oxidation of many substrates. The results of the present work indicate that [LCo(III)- OOR] complexes can promote oxidation of hydrocarbons under mild conditions and are viable intermediates in the catalytic oxidation of hydrocarbons with ROOH in the presence of cobalt catalysts.

Full text of DOI:10.1021/ja9814873

J. Am. Chem. Soc. 1998, 120, 9015-9027
9015
Syntheses, Structures, and Reactivities of Cobalt(III)-Alkylperoxo
Complexes and Their Role in Stoichiometric and Catalytic Oxidation
of Hydrocarbons
Ferman A. Chavez, John M. Rowland, Marilyn M. Olmstead,† and Pradip K. Mascharak*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of California,
Santa Cruz, California 95064, and Department of Chemistry, UniVersity of California,
DaVis, California 95616
ReceiVed April 30, 1998
Abstract: Although Co(III)-alkyl peroxo species have often been implicated as intermediates in industrial
oxidation of hydrocarbons with cobalt catalysts, examples of discrete [LCoIII-OOR] complexes and studies
on their oxidizing capacities have been scarce. In this work, twelve such complexes with two different ligands,
L, and various primary, secondary, and tertiary R groups have been synthesized, and seven of them have been
characterized by X-ray crystallography. The dianion (L2-) of the two ligands N,N-bis[2-(2-pyridyl)ethyl]-
pyridine-2,6-dicarboxamide (Py3PH2, 1) and N,N-bis[2-(1-pyrazolyl)ethyl]pyridine-2,6-dicarboxamide (PyPz2-
PH2, 2) bind Co(III) centers in pentadentate fashion with two deprotonated carboxamido nitrogens in addition
to three pyridine or one pyridine and two pyrazole nitrogens to afford complexes of the type [LCoIII(H2O)]
and [LCoIII(OH)]. Reactions of the [LCoIII(OH)] complexes with ROOH in aprotic solvents of low polarity
readily afford the [LCoIII-OOR] complexes in high yields. This report includes syntheses of [Co(Py3P)(OOR)]
complexes with R ) tBu (7, tBu ) CMe3), Cm (8, Cm ) CMe2Ph), CMe2CH2Ph (9), Cy (10, Cy ) c-C6H11),
i
n
n
t
iPr (11, Pr ) CHMe2) or Pr (12, Pr ) CH2CH2CH3), and [Co(PyPz2P)(OOR)] complexes with R ) Bu
i
n
(13), Cm (14), CMe2CH2Ph (15), Cy (16), Pr (17) or Pr (18). The structures of 8-12 and 16 have been
established by X-ray crystallography. Complexes 10 and 16 are the first examples of structurally characterized
compounds containing the [Co-OOCy] unit, proposed as a key intermediate in cobalt-catalyzed oxidation of
cyclohexane. The metric parameters of 7-12 and 16 have been compared with those of other reported [LCoIII-
OOR] complexes. When these [LCoIII-OOR] complexes are warmed (60-80 °C) in dichloromethane in the
presence of cyclohexane (CyH), cyclohexanol (CyOH) and cyclohexanone (CyO) are obtained in good yields.
Studies on such reactions (referred to as stoichiometric oxidations) indicate that homolysis of the O-O bond
in the [LCoIII-OOR] complexes generates RO• radicals in the reaction mixtures which are the actual agents
for alkane oxidation. [LCo-O•], the other product of homolysis, does not promote any oxidation. A mechanism
for alkane oxidation by [LCoIII-OOR] complexes has been proposed on the basis of the kinetic isotope effect
(KIE) value (5 at 80 °C), the requirement of dioxygen for oxidation, the dependence of yields on the stability
of the RO• radicals, and the distribution of products with different substrates. Both L and R modulate the
capacity for alkane oxidation of the [LCoIII-OOR] complexes. The extent of oxidation is noticeably higher
in solvents of low polarity, while the presence of water invariably lowers the yields of the oxidized products.
Since [LCoIII-OOR] complexes are converted into the [LCoIII(OH)] complexes at the end of single turnover
in stoichiometric oxidation reactions, it is possible to convert these systems into catalytic ones by the addition
of excess ROOH to the reaction mixtures. The catalytic oxidation reactions proceed at respectable speed at
moderate temperatures and involve [LCoIII-OOR] species as a key intermediate. Turnover numbers over
100 and ∼10% conversion of CyH to CyOH and CyO within 4 h have been noted in most catalytic oxidations.
The same catalyst can be used for the oxidation of many substrates. The results of the present work indicate
that [LCoIII-OOR] complexes can promote oxidation of hydrocarbons under mild conditions and are viable
intermediates in the catalytic oxidation of hydrocarbons with ROOH in the presence of cobalt catalysts.
Introduction
PhCMe2OOH) is an important goal in such pursuits. Currently,
Co(II), Mn(II), and Cu(II) salts are employed for these oxidation
processes, and products are removed continusouly at low
conversions.1a,d,f Other transition metal salts are added to the
reaction mixtures to control product distribution. Although it
is commonly accepted that the metals have a direct role in the
generation of radical species responsible for initiating the
oxidation reactions and in the decomposition of the alkyl
hydroperoxides formed in the oxidation processes, complete
mechanistic understanding in most cases is missing. Such
Recently there has been much interest in catalytic oxidation
of inexpensive hydrocarbon substrates by transition metal salts
(or complexes) to commercially important products related to
pharmaceuticals, flavors, fragrances, plasticizers, and polymers.1
The development of catalysts which carry out the oxidation
processes under mild conditions and with inexpensive and
t
environmentally benign oxidants (i.e., O2, H2O2, BuOOH, or
† University of California, Davis.
S0002-7863(98)01487-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/20/1998

9016 J. Am. Chem. Soc., Vol. 120, No. 35, 1998
ChaVez et al.
information is crucial from a fundamental standpoint and also
for the improvement of the existing as well as future catalytic
systems.
Synthetic methods which have been employed in the syntheses
of such [LCoIII-OOR] complexes include (1) addition of excess
ROOH to the [LCoII] precursor,6,12,15 (2) dioxygen insertion into
metal-alkyl bonds,10,11,14 and (3) dioxygen insertion into metal
ligand bonds under irradiation.7-9,13 Only a few of these Co-
(III)-alkylperoxo complexes have been used to oxidize hydro-
carbons6,12,15 and the results of at least one study have indicated
that the rates of decomposition of the [LCoIII-OOR] complexes
and the extent of alkane oxidation depend on L. It thus appears
that a systematic study with structurally characterized [LCoIII-
OOR] complexes with different L and R will be very useful in
determining how such variations affect the stability of the
[LCoIII-OOR] species and their capacities of hydrocarbon
oxidation under specific conditions.
Catalytic oxidation is the preferred method for hydrocarbon
oxidation since traditional stoichiometric oxidants such as
permanganate and dichromate generate large amounts of toxic
inorganic effluents which are difficult to dispose of.2 Examples
of industrial processes that utilize catalytic oxidation of
hydrocarbons in large quantities include the Mid-Century/
Amoco process where p-xylene is oxidized to terephthalic acid,
an important precursor in the production of polyester fibers,
films, and plasticizers. Another example is the Du Pont adipic
acid synthesis in which cyclohexane is first oxidzed to cyclo-
hexanol and cyclohexanone. The “ol and one” mixture is then
oxidized to adipic acid3 which is the precursor for the synthesis
of nylon 6,6 and other polymers. Both of these methods employ
cobalt catalysts and are performed at high pressures and
temperatures. Kinetic data indicate that ROOH (R ) alkyl),
formed in the initial stages of these processes, reacts with the
cobalt catalysts to form Co(III) alkyl hydroperoxide interme-
diates.1a,d,4 The oxidation of Co(II) to Co(III) is easily observed
by a change in color from pale violet or pink to dark green due
to the formation of Co(III)-OOR species. Coordination of alkyl
hydroperoxide to cobalt weakens the O-O bond and assists in
O-O bond scission to form RO• radicals under the reaction
conditions. Since the oxidation of the hydrocarbons is carried
out by the radicals, the industrial processes utilize different
solvents and other factors (ligands) in the reaction mixtures to
modulate the stability of the [LCoIII -OOR] intermediates, the
overall yields, and the product distribution.1d,5
In our attempt to address these issues, we have employed
two ligands, Py3PH2 (1, Py3PH2 ) N,N-bis[2-(2-pyridyl)ethyl]-
pyridine-2,6-dicarboxamide) and PyPz2PH2 (2, PyPz2PH2 )
N,N-bis[2-(1-pyrazolyl)ethyl]pyridine-2,6-dicarboxamide, H’s
denote dissociable amido H’s in both cases) in synthesizing
several Co(III) complexes of the type [LCoIII-OOR]. These
alkylperoxo complexes are conveniently synthesized from
[LCoIII(OH)] precursors which in turn are obtained from [LCoIII-
(H2O)]+ complexes upon addition of 1 equiv of a base. In
earlier reports, we have described the syntheses and charac-
terization of [Co(Py3P)(H2O)]ClO4 (3),15 [Co(Py3P)(OH)] (4),15
[Co(PyPz2P)(H2O)][(H3O)(PF6)2]- (5),16 and [Co(PyPz2P)(OH)]
(6).16 In 3 and 4, Py3P2- (deprotonated 1) binds cobalt in a
pentadentate fashion with three pyridine nitrogens and two
deprotonated amido nitrogens serving as the donors, while in 5
and 6, PyPz2P2- (deprotonated 2) employs one pyridine nitrogen,
two 2-pyrazole nitrogens, and two deprotonated amido nitrogens
to bind cobalt in a similar manner. The sixth site on the Co-
(III) centers of these complexes is occupied by either a water
molecule or a hydroxide ligand.
To date, a handful of discrete Co(III)-alkylperoxo complexes
of the formula [LCoIII-OOR] bearing certain ligands L have
been isolated and characterized.6-15 The structures of a few
these complexes have been determined by X-ray crystallography.
(1) (a) Sheldon, R. A.; Kochi, J. K. In Metal-Catalyzed Oxidations of
Organic Compounds; Academic: New York, 1981. (b) ActiVation and
Functionalization of Alkanes; Hill, C. L., Ed.; Wiley-Interscience: New
York, 1989. (c) SelectiVe Hydrocarbon ActiVation, Principles and Progress;
Davies, J. A., Watson, P. L., Liebman, J. F., Greenberg, A., Eds.; VCH:
New York, 1990. (d) Parshall, G. W.; Ittel, S. D. In Homogeneous Catalysis.
The Applications and Chemistry of Catalysis by Soluble Transition Metal
Complexes, 2nd ed.; Wiley-Interscience: New York, 1992. (e) The
ActiVation of Dioxygen and Homogeneous Catalytic Oxidations; Barton,
D. H. R., Martell, A. E.; Sawyer, D. T., Eds.; Plenum Press: New York,
1993. (f) Ebner, J.; Riley, D. In ActiVe Oxygen in Chemistry; Foote, C. S.,
Valentine, J. S., Greenberg, A., Liebman, J. F., Eds; Chapman & Hall:
London, 1995; pp 205-248. (g) Shilov, A. E.; Shulpin, G. B. Chem. ReV.
1997, 97, 2879.
(2) Sheldon, R. A. Top. Curr. Chem. 1993, 164, 22.
The utility of the alkylperoxo adduct of such Co(III)
complexes in promoting alkane oxidation under mild conditions
has already been demonstrated by us in our preliminary work
with the complex [Co(Py3P)(OOtBu)] (7, tBu ) CMe3).15 This
structurally characterized complex of the type [LCoIII-OOR]
initiates heat-induced (70 °C) stoichiometric oxidation of
cyclohexane (CyH) in dichloromethane to form cyclohexanol
and cyclohexanone. In this paper, we describe the synthesis
and reactivity of a series of Co(III)-alkylperoxo complexes of
the type [LCoIII-OOR]. The complexes are [Co(Py3P)(OOR)]
with R ) Cm, (8, Cm ) CMe2Ph), CMe2CH2Ph (9), Cy (10,
(3) This oxidation is performed with HNO3 and a V5+ catalyst.
(4) (a) Bands, G. L.; Chalk, J. E.; Smith, J. F. Nature 1954, 174, 274.
(b) Black, J. F. J. Am. Chem. Soc. 1978, 100, 527.
(5) Druliner, J. D.; Ittel, S. D.; Krusic, P. J.; Tolman, C. A. US Patent
4,326,084, 1982.
(6) Saussine L.; Brazi, E.; Robine, A.; Mimoun, H.; Fischer, J.; Weiss,
R. J. Am. Chem. Soc. 1985, 107, 3534.
(7) Fontaine, C.; Duong, K. N. V.; Meriene, C.; Gaudemer, A.; Gianotti,
C. J. Organomet. Chem. 1972, 38, 167.
(8) Giannotti, C.; Fontaine, C.; Chiaroni, A.; Riche, C. J. Organomet.
Chem. 1976, 113, 57.
(9) Chiaroni, A.; Pascard-Billy, C. Bull. Soc. Chim. Fr. 1973, 781.
(10) Nishinaga, A.; Tomita, H.; Ohara, H. Chem. Lett. 1983, 1751.
(11) Nishinaga, A.; Tomita, H.; Nishizawa, K.; Matsuura, T.; Ooi, S.;
Hirotsu, K. J. Chem. Soc., Dalton Trans. 1981, 1505.
(12) Talsi, E. P.; Chinakov, V. D.; Babenko, V. P.; Sidelnikov, V. N.;
Zamaraev, K. I. J. Mol. Catal. 1993, 81, 215.
(13) (a) Kojima, M.; Akhter, F. M. D.; Nakajima, K.; Yoshikawa, Y.
Bull. Chem. Soc. Jpn. 1996, 69, 2889. (b) Akhter, F. M. D.; Kojima, M.;
Nakajima, K.; Yoshikawa, Y. Chem. Lett. 1996, 81.
(14) Mikolaiski, W.; Baum, Gerhard, B.; Massa, W.; Hoffman, R. W. J.
Organomet. Chem. 1989, 376, 397.
i
i
n
n
Cy ) c-C6H11), Pr (11, Pr ) CHMe2) or Pr (12, Pr ) CH2-
CH2CH3), and [Co(PyPz2P)(OOR)] with R ) tBu (13), Cm (14),
CMe2CH2Ph (15), Cy (16), iPr (17), or nPr (18). The structures
of 8-12 and 16 have been established by X-ray crystallography.
It must be emphasized here that complexes 10 and 16 are the
first examples of structurally characterized compounds contain-
(15) Chavez, F. A.; Nguyen, C. V.; Olmstead, M. M.; Mascharak, P. K.
Inorg. Chem. 1996, 35, 6282.
(16) Chavez, F. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem.
1997, 36, 6323.

Cobalt(III)-Alkylperoxo Complexes
J. Am. Chem. Soc., Vol. 120, No. 35, 1998 9017
General Procedure for the Synthesis of [LCoIII-OOR] Com-
plexes. A batch of 0.5 mmol of either [Co(Py3P)(OH)] or [Co(PyPz2P)-
(OH)] was dissolved in 10 mL of dichloromethane under ambient
conditions and to this was added approximately 2-5 mmol of ROOH.
The mixture was stirred at room temperature in the dark in a stoppered
50-mL round-bottom flask. After 2 h of stirring, 15 mL of diethyl
ether was added, and the solution was filtered. The filtrate was then
stored at 4 °C. Crystals of the [LCoIII-OOR] complex appeared within
48 h. The crystals were collected and washed with anhydrous diethyl
ether. A typical percent yield was 85% based on [LCoIII(OH)]. All
the [LCoIII-OOR] complexes gave satisfactory elemental analyses. The
complexes for which crystal structures are presently missing were
identified by their NMR spectra. These clean NMR spectra (vide infra)
exhibited only the expected resonances, and the proper intensities were
recorded for all the peaks.
ing the [Co-OOCy] unit, proposed as a key intermediate in
the cobalt-catalyzed oxidation of cyclohexane.1a,d,g,17 The metric
parameters of 7-12 and 16 have been compared with those of
other reported [LCoIII-OOR] complexes. The extent of sto-
ichiometric oxidation of CyH by 7-18 under selected conditions
has been determined, and the mechananism of hydrocarbon
oxidation has been discussed in detail. Variations in the
oxidizing capacities of the [LCoIII-OOR] complexes with
different L and R have been investigated; the effects of solvents
on the yields and the distribution of the oxidation products have
been explored with selected [LCoIII-OOR] complexes; and
finally, the results of catalytic oxidation of CyH by the [LCoIII-
OOR] complexes in the presence of excess alkyl hydroperoxide
are presented.
Selected spectroscopic data for the [LCoIII-OOR] complexes are
listed below. The complete list has been submitted as part of the
Supporting Information.
Experimental Section
Materials. Synthetic protocols for the ligands 118 and 219 and the
two precursors 415 and 616 have been reported by us previously.
Tetramethylammonium hydroxide pentahydrate, tert-butyl hydroper-
oxide (TBHP) 70%, cumene hydroperoxide 80%, hydrogen peroxide
50%, silver trifluoroacetate, cyclohexyl iodide, 2-propyl iodide, 1-propyl
iodide, 2-methyl-1-phenyl-2-propanol, lithium bromide, HBr 48%, and
anhydrous ethylene glycol were procured from Aldrich Chemical Co.
The tert-butyl hydroperoxide, cumene hydroperoxide, and hydrogen
peroxide were further dried over anhydrous MgSO4. All other reagents
were used without further purification. All solvents were dried and
distilled before use.
Spectroscopic Data: νCO (KBr, cm-1); electronic absorption
spectrum, λmax, nm (ꢀ, M-1 cm-1) in dichloromethane. [Co(Py3P)-
(OOCm)] (8): 1600; 640 (sh, 141), 250 (21 000). [Co(Py3P)(OOC(CH3)2-
CH2Ph)] (9): 1598; 630 (175), 315 (sh, 10 000), 270 (sh, 16 700), 248
(21 000). [Co(Py3P)(OOCy)] (10): 1594; 625 (sh, 76), 335 (sh, 7000),
270 (sh, 12 000), 248 (sh, 16 500). [Co(Py3P)(OOiPr)] (11): 1599; 635
(sh, 86), 340 (sh, 8600), 300 (sh, 9600), 270 (sh, 13 000), 245 (sh,
15 400). [Co(Py3P)(OOnPr)] (12): 1603; 630 (sh, 90), 540 (sh, 244),
318 (5800), 250 (sh, 18 700). [Co(PyPz2P)(OOtBu)] (13): 1594; 635
(165), 305 (sh, 5800), 250 (sh, 10 600). [Co(PyPz2P)(OOCm)] (14):
1597; 630 (sh, 149), 320 (sh, 7000), 280 (sh, 10 600), 250 (sh, 16 500).
[Co(PyPz2P)(OOCMe2CH2Ph)] (15): 1602; 625 (204), 545 (sh, 216),
320 (14 000), 250 (sh, 28 000). [Co(PyPz2P)(OOCy)] (16): 1601; 600
(sh, 85), 325 (5500), 250 (sh, 7900). [Co(PyPz2P)(OOiPr)] (17): 1602;
610 (sh, 109), 535 (sh, 180), 325 (5700), 250 (sh, 10 160), 225 (26 000).
[Co(PyPz2P)(OOnPr)] (18): 1602; 600 (sh, 86), 330 (sh, 4900), 250
(sh, 9500).
Physical Measurements. Infrared spectra were obtained with a
Perkin-Elmer 1600 FTIR spectrophotometer. Absorption spectra were
measured on a Perkin-Elmer Lambda 9 spectrophotometer. 1H NMR
spectra were monitored on a Varian 500 MHz Unity Plus instrument
interfaced with a Sun OS 4.1.3 computer. Standard organic product
analyses were performed on a Hewlett-Packard 5890 series II plus gas
chromatograph equipped with a flame-ionization detector (FID) and
heliflex AT-1701 (30 m × 0.25 mm o.d.; film thickness, 0.25 µ)
capillary column (Alltech).
Oxidation of Hydrocarbon Substrates. Stoichiometric oxidation
proceeded as follows: In a typical reaction, ∼5 mg (8 µmol) of the
[LCoIII-OOR] complex was dissolved in 1 mL of 1:1 CH2Cl2/CyH in
a 2-dram screwcap vial with a Teflon seal, and the mixture was kept
in a constant-temperature bath for a certain number of hours. Following
a rapid cooling period, the mixture was filtered through a 0.45-µm
Acrodisc nylon filter, and the products were analyzed by gas chroma-
tography. Toluene or cyclooctane (internal standard) and excess
triphenyl phosphine were added to the products before GC analyses.
Products were identified and quantitated by comparison with authentic
samples.
Catalytic oxidation proceeded as follows: The mixtures of [LCoIII-
OOR] complexes, ROOH, and cyclohexane were placed in thick-walled
screwcap glass bottles (volume: 115 mL) stoppered with Teflon seals
and kept in a constant-temperature bath for a certain number of hours.
After rapid cooling, the mixtures were filtered through 0.25-µm
Acrodisc nylon filters, and triphenylphosphine was added to the filtrates
to decompose unreacted peroxides. The oxidation products were then
analyzed as above.
Synthesis. Safety Notes. Caution! Although no problems were
encountered handling the alkyl hydroperoxides employed in this study,
alkyl hydroperoxides are potentially explosive and should be handled
in small amounts with great care.
Preparation of Compounds. Syntheses of Alkyl Hydroperoxides
(ROOH). Cyclohexyl hydroperoxide (CyOOH), 2-propyl hydroper-
oxide (iPrOOH), 1-propyl hydroperoxide (nPrOOH), and 2-methyl-1-
phenyl-2-propyl hydroperoxide (PhCH2CMe2OOH) were synthesized
from cyclohexyl iodide, 2-propyl iodide, 1-propyl iodide, and 2-methyl-
1-phenyl-2-propyl bromide,20 respectively, by following the method
of Cookson and Davies.21 The general procedure is as follows.
A
batch of 11.32 mmol of alkyl halide was mixed with 22.64 mmol of
22
dry H2O2 in 40 mL of anhydrous dichloromethane at 0 °C. The
addition of 11.32 mmol of silver trifluoroacetate to this mixture resulted
in the precipitation of the corresponding silver halide. The mixture
was stirred at 0 °C for 2 h, and then it was filtered. The filtrate was
washed three times with 40 mL of 7.5% (w/v) aqueous sodium
bicarbonate. For CyOOH, iPrOOH, and nPrOOH, the dichloromethane
layer was collected and evaporated to 5 mL. The solution of the alkyl
hydroperoxide in dichloromethane thus obtained was then used in the
synthesis of the [LCoOOR] complex (vide infra) without further
purification. In the case of PhCH2CMe2OOH, the solution of the alkyl
hydroperoxide in dichloromethane was evaporated, and the crude
product was purified by chromatography on silica column using 5%
ethyl acetate/hexanes as the eluant. The final product in this case was
a white crystalline solid. The concentrations of the other hydroper-
oxides in solution were determined by the method of Banerjee and
Budke.23 Typical percent yields for the alkyl hydroperoxides were as
follows: CyOOH (23%), iPrOOH (25%), nPrOOH (28%), and PhCH2-
CMe2OOH (80%).
(17) Goldstein, A. S.; Drago, R. S. Inorg. Chem. 1991, 30, 4506.
(18) Chavez, F. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem.
1996, 35, 1410.
(19) Chavez, F. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chim.
Acta 1998, 269, 269.
X-ray Data Collection and Structure Solution and Refinement.
Brown plates of 8 were obtained by slow cooling of a dichloromethane/
diethyl ether solution of 8 at 4 °C. There are no solvent molecules in
the unit cell, and the O3-O4 bond is well ordered. Light brown-red
dichroic needles of [Co(Py3P)(OOCMe2CH2Ph)]‚0.6CH2Cl2, 9‚0.6CH2-
Cl2, were obtained by slow cooling of a dichloromethane/diethyl ether
solution of 9. In the crystal lattice, the dichloromethane molecule is
disordered. Two dichloromethanes occupying the same site were
(20) Masada, H.; Murotani, Y. Bull. Chem. Soc. Jpn. 1980, 53, 1181.
(21) Cookson, P. G.; Davies, A. G.; Roberts, B. P. J. Chem. Soc., Chem.
Commun. 1976, 1022.
(22) In a 5-dram snap-cap vial was stored 15 mL of 50% H2O2 over 8
g of anhydrous magnesium sulfate at -20 °C. After 24 h the supernatant
was decanted into another vial, and 8 more grams of anhydrous magnesium
sulfate was added. The H2O2 was used after storage at -20 °C for an
additional 24 h (estimated purity, 80%).
(23) Banerjee, D. K.; Budke, C. C. Anal. Chem. 1964, 36, 792.

9018 J. Am. Chem. Soc., Vol. 120, No. 35, 1998
ChaVez et al.
Table 1. Summary of Crystal Data and Intensity Collection and Structure Refinement Parameters for [Co(Py3P)(OOCMe2Ph)] (8), for
[Co(Py3P)(OOCy)]‚0.5Et2O‚0.2CH2Cl2 (10‚0.5Et2O‚0.2CH2Cl2), Co(Py3P)(OOiPr)]‚H2O(11‚H2O), and [Co(PyPz2P)(OOCy)]‚2CHCl3
(16‚2CHCl3)
complex 8
complex 10
complex 11
complex 16
formula
(mol wt)
cryst color, habit
T, K
cryst system
space group
a, Å
C30H30N5O4Co
(583.52)
brown plate
130 (2)
orthorhombic
P212121
9.714 (2)
15.593 (3)
17.391 (3)
90
C29.2H37.4N5O5.5Cl0.4Co
(619.55)
red-brown plate
130 (2)
triclinic
P1h
10.6894 (13)
12.1421 (13)
13.2428 (9)
65.004 (8)
66.415 (8)
69.388 (9)
1392.6 (2)
2
C24H28N5O5Co
(525.44)
red-brown needle
130 (2)
triclinic
P1h
9.5636 (9)
11.6891 (10)
12.1806 (13)
90.502 (8)
110.190 (7)
107.581 (7)
1208.5 (2)
2
C25H30N7O4Cl6Co
(764.21)
red block
169 (2)
triclinic
P1h
8.902 (3)
10.133 (2)
19.135 (4)
99.42 (2)
102.57 (2)
99.38 (2)
1626.2 (8)
2
b, Å
c, Å
R, deg
â, deg
90
90
γ, deg
V, Å3
2634.3 (9)
4
1.471
5.496
1.033
4.70
9.15
Z
d
calcd, g cm-1
1.478
5.60
1.052
6.48
1.444
5.951
1.045
6.94
1.557
1.063
1.110
5.69
abs coeff, µ, mm-1
GOFa on F2
R1,b %
wR2,c %
16.74
15.00
13.67
a GOF ) [∑[w(Fo2 - Fc2)2]/(M - N)]1/2 (M ) no. of reflections, N ) no. of parameters refined). b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑[w(Fo
2
- Fc2)2]/∑[w(Fo )2]]1/2
.
2
located. These were refined with site occupancy of 0.37 and 0.23 for
sets “A” and “B” of the atoms Cl(1), Cl(2), C(32), and hydrogen atoms
at calculated postitions. Thermal parameters for these atoms were kept
isotropic. All other non-hydrogen atoms were refined with anisotropic
thermal parameters. Red-brown plates of [Co(Py3P)(OOCy)]‚H2O‚
0.5Et2O‚0.2CH2Cl2, 10‚H2O‚0.5Et2O‚0.2CH2Cl2, were obtained when
a solution of 10 in a mixture of dichloromethane and regular (not
anhydrous) diethyl ether was stored at 4 °C for 48 h. The diethyl ether
solvent of crystallization could not be completely resolved partly
because it shares a site with a dichloromethane and partly because of
a center of symmetry. The overall shape, however, indicates that it is
a diethyl ether molecule. The molecule of water in the structure is
hydrogen bonded to the complex. It is well-behaved, and the hydrogen
atoms show up on a difference map. The other region which is a mix
of 0.2 dichloromethane and 0.5 diethyl ether is best described as
disordered. The two chlorine atoms have the right separation for a
molecule of CH2Cl2; however, the carbon atom cannot be observed.
Red-brown needles of [Co(Py3P)(OOiPr)]‚H2O, 11‚H2O, were obtained
by slow cooling of a dichloromethane/diethyl ether solution of 11 which
also contained some water. Red plates of [Co(Py3P)(OOnPr)]‚1.5CH2-
Cl2‚0.5H2O, 12‚1.5CH2Cl2‚0.5H2O, moderately suitable for X-ray
analysis, were obtained by slow cooling of a solution of 12 in
dichloromethane/diethyl ether/water. Although the structure was refined
to R ) 13%, reliable metric parameters were obtained for this complex.
In the asymmetric unit, there are two molecules of the complex, three
molecules of dichloromethane, and one water molecule. The two
complexes differ mainly in the orientation of the peroxyl groups. One
of the three molecules of dichloromethane is badly disordered and could
not be modeled except as an ad hoc mixture of chlorine atoms. The U
values of these atoms were refined after their location on a final
difference map. Occupancies were as follows: Cl (5A), 0.40; Cl (5B),
0.40; Cl (5C), 0.40; Cl (5D), 0.25; Cl (5E), 0.25; Cl (5F), 0.15; Cl
(5G), 0.15. Red blocks of [Co(PyPz2P)(OOCy)]‚2CHCl3, 16‚2CHCl3,
were obtained by slow cooling of a chloroform/diethyl ether solution
of 16. There are two chloroform molecules in the unit cell, one being
slightly disordered. The chloroform molecules are hydrogen bonded
to a carbonyl oxygen and to the oxygen of the coordinated peroxo group
that is directly bonded to the Co(III) center (H25‚‚‚O1 ) 1.97 Å and
H26A‚‚‚O4 ) 2.32 Å).
Only random fluctuations of <1% in the intensities of two standard
reflections were observed during data collection for all of the complexes.
The structures were solved by direct methods (SHEXTL 5, XS,
Sheldrick, 1994). All calculations were carried out on a 486/DX50
computer using SHELXTL Version 5.03 program.24 The data were
corrected for absorption effects by the use of the program XABS2.25
Hydrogen atoms bonded to carbon were added geometrically and refined
with the use of a riding model.
Machine parameters, crystal data, and data collection parameters are
summarized in Table 1. Selected bond distances and angles are listed
in Table 2. The rest of the crystallographic data has been submitted
as Supporting Information.
Results and Discussion
In our earlier synthetic work,15 we synthesized [Co(Py3P)(OOt-
Bu)] (7) by reacting isolated [CoII(Py3P)] with excess tBuOOH
in dichloromethane under dry dinitrogen at room temperature.
Soon we discovered that this method also works for the synthesis
of [Co(Py3P)(OOCm)] (8) when tBuOOH is replaced by
CmOOH. Addition of diethyl ether to the reaction mixture and
cooling at 4 °C affords crystalline 8 within 48 h. Although
this procedure affords 7, 8, and other [LCoIII-OOR] com-
plexes,6,12 it soon became clear to us that it is a suitable
procedure for synthesizing [Co(Py3P)(OOR)] complexes with
tertiary R groups only. For secondary or primary R groups,
this method is not as useful since secondary and primary alkyl
hydroperoxides are less stable and decompose during the
synthetic procedures. In addition, this method was found to be
unsuitable for the syntheses of [Co(PyPz2P)(OOR)] complexes
even when tertiary R groups were employed; the desired
products were not obtained in most cases.26 Quite in contrast,
all of the [Co(Py3P)(OOR)] and [Co(PyPz2P)(OOR)] complexes
with tertiary, secondary or primary R groups can be conveniently
(24) Sheldrick, G. SHELXTL, Version 5.03; Siemens Industrial Automa-
tion, Inc.: Madison, WI, 1994. Tables of neutral atom scattering factors, f′
and f′′, and absorption coefficients were taken from: International Tables
for Crystallography; Wilson, A. J. C., Ed.; Kluwer: Dordrecht, The
Netherlands, 1992; Vol. C.
For 8-12, diffraction data were collected at 130 (2) K on a Siemens
P4 diffractometer equipped with a Siemens rotating anode, a nickel
filter, and Enraf Nonius low-temperature apparatus. Data for 16 were
collected at 169 (2) K on a Siemens R3m/V diffractometer equipped
with a normal-focus sealed tube and Enraf nonius low-temperature
appartatus. Cu KR radiation (λ ) 1.541 78 Å) radiation was used for
8-12 while Mo KR radiation (λ ) 0.710 73 Å) was employed for 16.
(25) Parkins, S.; Moezzi, B.; Hope, H. J. Appl. Crystallogr. 1995, 28,
53.
(26) In the best case, reaction of [CoII(PyPz2P)] with TBHP affords a
crystalline solid, the NMR spectrum of which indicates that the product
contains ∼70% 13 along with other hitherto unknown Co(III) com-
plex(es).

Cobalt(III)-Alkylperoxo Complexes
J. Am. Chem. Soc., Vol. 120, No. 35, 1998 9019
Table 2. Selected Bond Distances (Å) and Angles (deg)
[Co(Py3P)(OOCm)] (8)
Bond Distances
Co-N1
Co-N2
Co-N3
Co-N4
Co-N5
Co-O3
N1-C5
N2-C8
N3-C9
1.966(5)
1.917(5)
1.859(5)
1.947(5)
2.010(5)
1.883(4)
1.353(8)
1.337(8)
1.329(7)
N4-C14
N4-C15
N5-C17
O1-C8
1.346(8)
1.459(8)
1.364(8)
1.250(7)
1.485(5)
1.242(7)
1.443(7)
1.549(9)
1.517(9)
O3-O4
O2-C14
O4-C22
C22-C23
C22-C24
Bond Angles
N1-Co-N2
N1-Co-N3
N1-Co-N4
N1-Co-N5
N2-Co-N3
N2-Co-N4
N2-Co-N5
N3-Co-N4
N3-Co-N5
93.1(2)
170.0(2)
103. 1(2)
95.4(2)
82.2(2)
163.7(2)
92.6(2)
82. 1(2)
93.6(2)
N1-Co-O3
Co-N2-C7
Co-N3-C9
Co-N4-C14
Co-O3-O4
N2-C8-O1
N4-C14-C13
O3-O4-C22
O1-C8-C9
86.3(2)
124.5(4)
117.1(4)
115.1(4)
109.2(3)
128.1(6)
111.7(6)
107.1(4)
121.6(6)
Figure 1. Thermal ellipsoid (probability level 50%) plot of [Co(Py3P)-
(OOCm)] (8) with the atom-labeling scheme. H atoms are omitted for
the sake of clarity.
[LCoIII(OH)] + ROOH f [LCoIII-OOR] + H2O (I)
synthesized by reacting the [LCoIII(OH)] complexes with excess
ROOH. This method of synthesis has several advantages over
other methods previously used to synthesize [LCoIII-OOR]
complexes.6-15 They are as follows: There is no change in
the oxidation state of the cobalt in reaction I and hence, the
transformation can be followed by 1H NMR spectroscopy. The
reaction involves exchange of OH- with ROO-, and the only
byproduct is water. Thus, there is no need to purify the final
product by column chromatography or other means, and
crystalline materials are obtained readily. The synthesis is also
a very general one, and any ROOH can be used. Also, one
does not require rigorously pure ROOH for reaction I to work;
solutions of partially purified ROOH in dichloromethane afford
analytically pure [LCoIII-OOR] complexes.
[Co(Py3P)(OOCy)]‚H2O‚0.5Et2O‚0.2CH2Cl2
(10‚H2O‚0.5Et2O‚0.2CH2Cl2)
Bond Distances
Co-N1
Co-N2
Co-N3
Co-N4
Co-N5
1.990(4)
1.937(4)
1.857(4)
1.966(4)
2.029(4)
Co-O3
O1-C8
O3-O4
O2-C14
O4-C22
1.891(3)
1.254(6)
1.460(4)
1.259(6)
1.425(6)
Bond Angles
N1-Co-N2
N1-Co-N3
N1-Co-N4
N1-Co-N5
N2-Co-N3
N2-Co-N4
93.2(2)
168.8(2)
103.8(2)
94.9(2)
81.7(2)
162.9(2)
N2-Co-N5
N3-Co-N4
N3-Co-N5
N4-Co-N5
Co-O3-O4
O3-O4-C22
94.1(2)
81. 7(2)
95.4(2)
83.6(2)
113.6(2)
108.5(3)
[Co(Py3P)(OOiPr)]‚H2O (11‚H2O)
In the solid phase, the [LCoIII-OOR] complexes 7-18 are
stable for months when kept in a desiccator at room temperature.
They can be recrystallized from a variety of halogenated
solvents, diethyl ether, or pentane. Dissolution of the [LCoIII-
OOR] complexes in hydrocarbon substrates such as cyclohexane
can be achieved by the addition of dichloromethane, acetonitrile,
or excess ROOH.
Bond Distances
Co-N1
Co-N2
Co-N3
Co-N4
Co-N5
1.950(4)
1.911(4)
1.887(4)
1.958(4)
2.024(4)
Co-O3
O1-C8
O3-O4
O2-C14
O4-C22
1.887(4)
1.251(6)
1.446(4)
1.249(6)
1.423(7)
Bond Angles
N1-Co-N2
N1-Co-N3
N1-Co-N4
N1-Co-N5
N2-Co-N3
N2-Co-N4
N2-Co-N5
N3-Co-N4
92.9(2)
171.5(2)
103. 2(2)
94.7(2)
82.0(2)
163.7(2)
94.4(2)
82. 2(2)
N3-Co-N5
N4-Co-N5
N1-Co-O3
N2-Co-O3
N5-Co-O3
Co-O3-O4
O3-O4-C22
C23-C22-C24
92.5(2)
82.2(2)
86.7(2)
97.3(2)
168.1(2)
115.3(2)
109.2(4)
112.9(5)
Structure of [Co(Py3P)(OOCm)] (8) and [Co(Py3P)-
(OOCMe2CH2Ph)]‚0.6CH2Cl2 (9‚0.6CH2Cl2). Structural pa-
rameters of these two [Co(Py3P)(OOR)] complexes containing
a tertiary alkylperoxo group as R are listed in Table 2. In
complexes 8 (Figure 1) and 9 (Figure S1, Supporting Informa-
tion), the cobalt(III) center resides in a slightly distorted
octahedral environment. In both complexes, the Py3P2- ligand
is bound to the Co(III) center in a pentadentate square pyramidal
fashion with three pyridine nitrogens and two deprotonated
amido nitrogens as donors. This mode of coordination is similar
to that of other structurally characterized metal complexes of
this ligand15,19 (also see below). The structure of 8 is well-
ordered and devoid of any solvent molecule(s) of crystallization
in the unit cell. The structure of 9, however, has 0.6 molecules
of CH2Cl2 in the unit cell. Complex 9 is the first example of
a metal complex with a ligated PhCH2CMe2OO- group of the
mechanistic probe PhCH2CMe2OOH (MPPH)27 as R (vide
infra), that has been characterized by X-ray crystallography. The
nearly identical Co-O3 bond distances of 8 and 9,1.883(4) Å
[Co(PyPz2P)(OOCy)]‚2CHCl3 (16‚2CHCl3)
Bond Distances
Co-N1
Co-N3
Co-N4
Co-N5
Co-N7
Co-O1
1.944(3)
1.930(3)
1.844(3)
1.955(3)
1.971(3)
1.892(3)
N1-N2
N6-N7
O3-C6
O1-O2
O4-C12
C22-C23
1.362(4)
1.364(4)
1.241(4)
1.456(3)
1.254(4)
1.528(5)
Bond Angles
N1-Co-N3
N1-Co-N4
N1-Co-N5
N1-Co-N7
N3-Co-N4
N3-Co-N5
N3-Co-N7
N4-Co-N5
N4-Co-N7
N5-Co-N7
93.02(12)
171.26(13)
102.82(12)
93.32(13)
82.22(12)
163.52(12)
90.83(14)
82. 62(12)
94.07(13)
83.85(13)
N1-Co-O1
N3-Co-O1
N4-Co-O1
N7-Co-O1
Co-N1-N2
Co-O1-O2
N3-C6-O3
O1-O2-C18
O2-C18-C19
O2-C18-C23
87.05(12)
100.08(12)
86.57(12)
169.05(11)
122.6(2)
117.0(2)
127.7(3)
107.8(2)
113.2(3)
106.6(3)
(27) (a) Arends, I. W. C. E.; Ingold, K. U.; Wayner, D. D. M. J. Am.
Chem. Soc. 1995, 117, 4710. (b) Snelgrove, D. W.; MacFaul, P. A.; Ingold
K. U.; Wayner, D. M. Tetrahedron Lett. 1996, 37, 823. (c) MacFaul, P.
A.; Arends, I. W. C. E.; Ingold, K. U.; Wayner, K. U. J. Chem. Soc., Perkin
Trans. 2 1997, 135.

9020 J. Am. Chem. Soc., Vol. 120, No. 35, 1998
ChaVez et al.
Table 3. Comparison of Structural Parameters for Crystallographically Characterized [LCoIII-OOR] Complexes. Bond Distances Are in Å
and Bond Angles Are in deg
complexa
Co-O
Ltrans-Co
O-O
O-R
Co-O-O
O-O-R
ref
R ) tertiary
7
1.905 (7)
1.883 (4)
1.882 (5)
1.909 (3)
1.838 (5)
1.854 (9)
2.024 (3)
2.010 (5)
2.007 (7)
1.994 (3)
2.045 (5)
1.929 (11)
1.492 (8)
1.485 (5)
1.488 (8)
1.455 (3)
1.444 (6)
1.50 (1)
1.462 (7)
1.443 (7)
1.433 (10)
1.462 (5)
1.423 (9)
1.42 (1)
111.4 (6)
109.2 (3)
112.3 (4)
111.7 (2)
114.6 (3)
114.3 (6)
107.6 (5)
107.1 (4)
108.0 (6)
110.3 (2)
111.2 (6)
105.6 (8)
15
b
b
7
6
8
9
19
21
22
11
R ) secondary
10
1.891 (3)
1.887 (4)
1.892 (3)
1.897
2.029 (4)
2.024 (4)
1.971 (3)
2.013
1.460 (4)
1.446 (4)
1.456 (3)
1.455
1.425 (6)
1.423 (7)
1.437 (4)
1.451
113.6 (2)
115.3 (2)
117.0 (2)
112.6
108.5 (3)
109.2 (4)
107.8 (2)
107.2
b
b
b
11
16
20
9
23
1.856 (6)
1.973 (6)
1.437 (8)
1.42 (1)
108.7 (5)
110.0 (6)
13
R ) primary
12
12
24
1.855 (10)
1.856 (11)
1.92 (1)
1.999 (13)
1.994 (13)
2.04 (8)
1.451 (14)
1.46 (2)
1.40 (2)
1.40 (2)
1.40 (2)
1.41 (3)
118.2 (8)
114.0 (9)
120.1 (9)
107.1 (13)
106.2 (14)
117 (2)
b
b
14
a 19 ) [Co(dmgH)(py)(OOCm)], 20 ) [Co(dmgH)(py)OOCHMePhMe)], 21 ) [Co(BPI)(OCOPh)(OOtBu)], 22 ) [Co(Salpr)(4-OO-(2,4,6-
tBu-C6H2O)] (peroxy-p-quinolatocobalt(III) complex), 23 ) trans(t-N,O(C))-[Co{OOCH(CH3)COO-O,O}(tren)]BPh4, 24 ) [Co(TPP)(py)(OO-
CH2CHdCH2)]. b This work.
and 1.882(5) Å, respectively, are slightly shorter than the same
bond in 7 (1.914 Å) and [Co(dmgH)2(py)(OOCm)] (19)7 (1.909-
(3) Å, dmgH- ) monoanion of dimethylglyoxime) but longer
than the same bond in other structurally characterized [LCoIII-
OOR] complexes with tertiary R groups (See Table 3).6,11,13
Both 8 and 9 possess a short and a long Co-N(amido) bond
(Co-N2, 1.917(5) Å and 1.925(8) Å, respectively; Co-N4,
1.947(5) Å and 1.977(8) Å, respectively). Such nonequivalent
Co-N(amido) bond distances are also observed in 7 (1.921(3)
and 1.959(3) Å respectively)15 and other [Co(Py3P)(OOR)]
complexes (vide infra) but not in the parent aqua complex 3
(1.930(6) and 1.929(6) Å).15 The Co-N(amido) bond lengths
of 8 and 9 are however within the range of the Co-N(amido)
bond distances for similar Co(III)-peptido complexes (1.915-
1.968 Å).15,16,28-31 The Co-N5 bond distance (the bond trans
to the peroxide ligand -OOR with R ) tertiary) is 2.010(5) Å
in 8 and 2.007(7) Å in 9. As discussed in a latter section, this
distance is comparatively shorter when R ) secondary or
primary (Table 3). The O-O moiety in both 8 and 9 is ordered,
and the O-O distances are very comparable (1.485(5) Å and
1.488(8) Å, respectively) in these two complexes with tertiary
R groups. However, this O-O bond distance is longer than
the O-O bond distance of 19 (1.455(3) Å) which also includes
a CmOO- coordinated to a Co(III) center. Since dmgH- is a
relatively weak σ-electron donor compared to Py3P2-, the metal
center in 19 does not back-donate as much electron density to
the π* orbitals of the O-O moiety of the CmOO- ligand. As
a result, the O-O bond distance in 19 is shorter than that in 8.
The Co-O3-O4 bond angle (109.2(3)°) of 8 indicates that the
proximal oxygen is sp3 hybridized and the O-O bond is close
to a single bond. In 9, the same Co-O3-O4 bond angle is
Figure 2. Thermal ellipsoid (probability level 50%) plot of [Co(Py3P)-
(OOCy)] (10) with the atom-labeling scheme. H atoms are omitted for
the sake of clarity.
Figure 3. Thermal ellipsoid (probability level 50%) plot of [Co-
(Py3P)(OOiPr)] (11) with the atom-labeling scheme. H atoms are omitted
for the sake of clarity.
(28) (a) Farinas, E. F.; Tan, J. D.; Baidya, N.; Mascharak, P. K. J. Am.
Chem. Soc. 1993, 115, 2996. (b) Tan, J. D.; Hudson, S. E.; Brown, S. J.;
Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 1992, 114, 3841.
(c) Muetterties, M.; Cox, M. B.; Arora, S. K.; Mascharak, P. K. Inorg.
Chim. Acta 1989, 160, 123. (d) Brown, S. J.; Hudson, S. E.; Olmstead, M.
M.; Mascharak, P. K. J. Am. Chem. Soc. 1989, 111, 6446. (e) Delany, K.;
Arora, S. K.; Mascharak, P. K. Inorg. Chem. 1988, 27, 705 and references
cited therein.
(29) Collins, T. J.; Powell, R. D.; Slebodnick, C.; Uffelman, E. S. J.
Am. Chem. Soc. 1991, 113, 8419.
(30) Mak, S.-T.; Wong, W.-T.; Yam, V. W.-W.; Lai, T.-F.; Che, C.-M.
J. Chem. Soc., Dalton Trans. 1991, 1915.
slightly larger (112.3(4)°) presumably due to more steric
interactions from the OOR ligand.
Structure of [Co(Py3P)(OOCy)]‚H2O‚0.5Et2O‚0.2CH2Cl2
(10‚H2O‚0.5Et2O‚0.2CH2Cl2), and [Co(Py3P)(OOiPr)]‚H2O
(11‚H2O). Structural parameters of these two [Co(Py3P)(OOR)]
complexes containing secondary alkylperoxo groups as R are
listed in Table 2. The geometry around the cobalt (III) center
in both 10 (Figure 2) and 11 (Figure 3) is distorted octahedral.
An interesting feature of the structure of 10 is the presence of
(31) Ray, M.; Ghosh, D.; Shirin, A.; Mukherjee, R. Inorg. Chem. 1997,
36, 3568.

Cobalt(III)-Alkylperoxo Complexes
J. Am. Chem. Soc., Vol. 120, No. 35, 1998 9021
water molecules that are hydrogen bonded to the carbonyl
oxygens of the complex. Such hydrogen bonding brings two
molecules of the complex close to each other to form a dimeric
unit with two water molecules acting as bridges (Figure S2,
Supporting Information). X-ray quality crystals of 10 could
only be grown from media containing small amounts of water.
Rigorously dry solutions of 10 invariably afford microcrystalline
materials. The molecular structure of 10 provides the metric
parameters of the coordinated CyOO- moiety for the first time
since, prior to this report, no metal complex with coordinated
CyOO- group has been characterized by X-ray crystallography.
Along the same line, complex 11 is the first example of a
structurally characterized cobalt(III) complex containing a
ligated PrOO- group.32
i
Figure 4. Thermal ellipsoid (probability level 50%) plot of [Co-
(PyPz2P)(OOCy)] (16) with the atom-labeling scheme. H atoms are
omitted for the sake of clarity.
In 10, the cyclohexyl group exists in the chair conformation.
The O-O bond distance (1.460(4) Å) of the coordinated CyOO-
group is similar to that observed with 11 (1.446(4) Å), and the
oximato complex [Co(dmgH)2(OOCHMePh-p-Me)] (1.455 Å)
(Table 3) reported previously.9 In these three [LCoIII-OOR]
complexes with secondary R groups, the O-O distances fall in
the range of 1.44-1.46 Å. These distances are slightly shorter
than the O-O distances noted for [LCoIII-OOR] complexes
with tertiary R groups (Table 3). The metric parameters of the
Py3P2- ligand frames in 10 and 11 are very similar to those
observed with 8 and 9.
1H NMR Spectra of the [LCoIII-OOR] Complexes. The
purity and integrity of the [LCoIII-OOR] complexes in solutions
1
are readily evident from their H NMR spectra. For example,
1
the H NMR spectra of [Co(Py3P)(OOtBu)] (7), [Co(Py3P)-
(OOCm)] (8), and [Co(Py3P)(OOiPr)] (11) (Figure S4, Sup-
porting Information) display all of the characteristic peaks of
the coordinated Py3P2- ligand15 in addition to peaks associated
with the coordinated ROO- moiety. The resonance patterns
for the R groups of the ROO- ligands in these complexes
provide information related to the possible orientations of these
groups. For example, the tBu group of 7 resonates as a singlet
(free rotation), while the two methyl groups on the isopropyl
moiety (iPr) of 11 appear as two doublets, indicating fixed
disposition of the two methyl groups. The two methyl groups
of the coordinated CmOO- ligand of 8 also appear as two
distinct peaks at 0.77 and 0.81 ppm. This nonequivalency,
which is not present in PhCMe2OOH, indicates that the two
methyl groups are held fixed in space or move very slowly
relative to the NMR time scale. The NMR spectra of 8 and 11
in CDBr3 indicate that the inequivalency of the methyl groups
of the cumyl moiety of 8 and the isopropyl moiety of 11 is
preserved at 70 °C (temperature for the oxidation reactions, vide
infra).
Structure of [Co(Py3P)(OOnPr)]‚1.5CH2Cl2‚0.5H2O (12‚
1.5CH2Cl2‚0.5H2O). Selected bond distances and angles for
the complex 12 containing a ROO- ligand with primary R group
are listed in Table 2. In the asymmetric unit, there are two
molecules of 12 with slightly different orientations of the
nPrOO- moiety (Figure S3, Supporting Information). Two
slightly different O-O bond lengths (1.451(14) Å and 1.46(2)
Å) are observed for these two molecules. These O-O distances
are longer than the O-O distance (1.40(2) Å, Table 3) of [Co-
(TPP)(py)(OOCH2CHdCH2)]14 (TPP ) tetraphenylporphyrin
dianion) which also contains a primary alkylperoxo group.33
The two slightly different orientations of the nPrOO- group in
the two molecules of 12 in the asymmetric unit are characterized
by (a) the Co-O-O bond angles (118.2(8)° and 114.0(9)°),
(b) the O-C-C bond angles (111(2)° and 106(2)°), and (c)
the C-C-C bond angles (121(3)° and 113(2)°).
Structure of [Co(PyPz2P)(OOCy)]‚2CHCl3 (16‚2CHCl3).
The structure of one [LCoIII-OOR] complex with L )
PyPz2P2-, namely 16, was determined to find out the effect of
the ligand L on the metric parameters of the Co-OOR moiety.
The structure of 16 is shown in Figure 4, and selected bond
distances and bond angles are included in Table 2. In 16, the
PyPz2P2- ligand binds the cobalt(III) center in a pentadentate
square pyramidal fashion with two 2-pyrazole nitrogens, one
pyridine nitrogen, and two deprotonated amido nitrogens acting
as donors. A similar mode of binding of this ligand has been
noted in [Co(PyPz2P)(H2O)]+ and [Cu(PyPz2P)].16,20 The
cyclohexyl group of the coordinated CyOO- ligand in 16 is in
the chair conformation, and the O-O bond distance (1.456(3)
Å) is slightly shorter than the O-O bond distance of 10 (1.460-
(4) Å). Also, the Co-O-O bond angle (117.0(2)°) of the Co-
OOR moiety in 16 is somewhat larger than the same bond angle
in 10 (113.6(2)°).
The majority of the [LCoIII-OOR] complexes with L )
PyPz2P2- ligand has been characterized by their clean 1H NMR
spectra. All of these complexes (13-18) exhibit resonances
for the PyPz2P2- ligand16 as well as peaks for the coordinated
ROO- groups. Here also one notices similar peak patterns for
the R groups of the coordinated peroxo units. As an example,
the 1H NMR spectra of [Co(Py3P)(OOCy)] (10) and [Co-
(PyPz2P)(OOCy)] (16) are compared in Figure 5. In both cases,
the resonances for the cyclohexyl group appear as complicated
patterns between 0.7 and 2.3 ppm.
Formation of [LCoIII-OOR] Complexes. That the [LCoIII-
OOR] complexes are readily obtained from the [LCoIII(OH)]
complexes upon addition of excess ROOH is apparent from the
changes in the spectrum of [Co(Py3P)(OH)] (4) upon addition
of excess TBHP (Figure 6). In CD2Cl2 at 25 °C, addition of 5
equiv TBHP (dried over anhydrous MgSO4) to 4 results in
quantitative formation of [Co(Py3P)(OOtBu)] (7). The reaction
is clean, and there is no side product other than water. Since
no broadening of any peak is noticed during the transformation,
it is evident that no Co(II) species or radical(s) is generated in
the reaction mixture.
(32) For the only other structurally characterized complex containing a
iPrOO- group, a platinum complex, see Ferguson, G.; Parvez, M.;
Monaghan, P. K.; Puddephatt, R. J. J. Chem. Soc., Chem. Commun. 1983,
267.
(33) Interestingly, the O-O bond distance in [Ge(TPP)(OOCH2CH3)2],
a complex that has two primary alkylperoxo ligands bonded to germanium,
is 1.478(2) Å. See Balch, A. L.; Cornman, C. R.; Olmstead, M. M. J. Am.
Chem. Soc. 1990, 112, 2963.
Oxidation of Hydrocarbons with [LCoIII-OOR] Com-
plexes. The [LCoIII-OOR] complexes (7-18) decompose
when warmed in CH2Cl2 (or CHCl3) above 50 °C. The presence

9022 J. Am. Chem. Soc., Vol. 120, No. 35, 1998
ChaVez et al.
Table 4. Stoichiometric Oxidation of Cyclohexane by
[Co(PyPz2P)(OOtBu)] (13)a
Time Profileb,c
time, h
% A
% K
% total
0.5
1.0
2.0
3.0
4.0
5.0
6.0
4
13
37
51
70
82
91
4
12
43
50
66
76
89
8
25
80
101
136
159
180
Temperature Profileb,d
T, °C
% A
% K
% total
50
60
70
80
90
0
5
37
82
81
71
2
6
43
81
39
44
2
11
80
163
120
115
Figure 5. 1H NMR spectra (500 MHz, 298 K) of [Co(Py3P)(OOCy)]
(10) (top) and [Co(PyPz2P)(OOCy)] (16) (bottom) in CDCl3.
100
a Yields based on 13 (8 µmol used). b Reaction mixtures consisted
of 0.5 mL of CH2Cl2, 0.5 mL of CyH and 13. c Reaction temperature
was 70 °C. d Reaction time was 2 h.
a function of time. The results (Table 4) reveal that oxidation
of CyH is continued for over 6 h under this condition and up to
180% oxidized products (based on 13) are obtained after 6 h.
When the same reaction mixture was heated for 2 h at different
temperatures, the yields of the oxidized products (Table 4)
afforded a temperature profile of the oxidation reaction. No
oxidation is observed below 50 °C. At 50 °C, the extent of
oxidation is minimal as evidenced by the small amounts of the
oxidized products (A ) 0% and K ) 2%). The amounts of
oxidized products, however, increase significantly as the tem-
perature is raised further and reach a maximum at 80 °C (A )
82% and K ) 81%). Further increase in temperature results in
diminished yields which may result from overoxidation of the
oxidized products. Since all oxidation reactions by [LCoIII-
OOR] complexes afford products derived from the OOR groups
Figure 6. 1H NMR spectra (500 MHz, 298 K) showing conversion of
[Co(Py3P)(OH)] (4) (top) to [Co(Py3P)(OOtBu)] (7) (bottom) upon
addition of excess TBHP in CD2Cl2.
t
(viz, BuOH in the case of 13) in addition to A and K, it is
of hydrocarbons in such solutions results in the oxidation of
C-H bonds to produce the corresponding alcohols and ketones
(or aldehydes). In the present study, we have determined the
extent of oxidation of cyclohexane (CyH) and other hydrocarbon
substrates by the [LCoIII-OOR] complexes. In accordance with
the terminology of Mimoun,6 these oxidations will be referred
to as stoichiometric oxidations hereafter. We have also
performed oxidations of CyH with the [LCoIII-OOR] complexes
in the presence of additional ROOH. These oxidation reactions
will be referred to as catalytic oxidations. In the following
sections, we will first provide the results of stoichiometric
oxidation of CyH by the [LCoIII-OOR] complexes. The
discussion thereafter will focus on the general mechanism of
such stoichiometric oxidations. Results of stoichiometric oxida-
tion of hydrocarbons other than CyH will then be discussed to
compare the oxidizing capacities of these complexes. And
finally, we will discuss the results and mechanism of catalytic
oxidation of CyH by the [LCoIII-OOR] complexes.
Stoichiometric Oxidation of Cyclohexane. When solutions
of [LCoIII-OOR] complexes in CyH/CH2Cl2 are warmed above
60 °C, one obtains cyclohexanol (A) and cyclohexanone (K)
in moderate yields within hours. The results of stoichiometric
oxidations of CyH by [Co(PyPz2P)(OOtBu)] (13) are shown in
Table 4. To obtain the time profile of the oxidation reaction,
a solution of 8 µmol of 13 in 1 mL of 1:1 CyH/CH2Cl2 was
heated at 70 °C, and the yields of A and K were monitored as
evident that the oxidation of hydrocarbons is carried out by
species formed during decomposition of the [LCoIII-OOR]
complexes. This is further indicated by the fact that the rates
of formation of the OOR-derived product(s) and the oxidized
products from the hydrocarbon both increase with the increase
in temperature. For example, when 14 (8 µmol) is used to
oxidize CyH, the yields of A, K, 2-phenyl-2-propanol, and
acetophenone (the last two products are derived from the
[CoOOCMe2Ph] moiety of 14) for a 2 h period increase with
temperature as follows: at 50 °C, A (2%), K (2%), 2-phenyl-
2-propanol (3%), and acetophenone (4%); at 60 °C, A (10%),
K (8%), 2-phenyl-2-propanol (22%), and acetophenone (10%);
at 70 °C, A (36%), K (25%), 2-phenyl-2-propanol (46%), and
acetophenone (15%).
Mechanism of Oxidation by [LCoIII-OOR] Complexes.
In our preliminary work,15 we suggested that [Co(Py3P)(OOtBu)]
(7) undergoes exclusive homolysis of the O-O bond when
heated in a CH2Cl2/CyH mixture. The tBuO• radicals generated
in the reaction mixture then abstract H atoms from CyH to
produce cyclohexyl radicals (Cy•)34 which in the presence of
dioxygen afford A and K. In the present work, we have studied
(34) (a) Avila, D. V.; Ingold, K. U.; Lusztyk, J.; Green, W. H.; Procopio,
D. R. J. Am. Chem. Soc. 1995, 117, 2929. (b) Almarsson, O.; Bruice, T. C.
J. Am. Chem. Soc. 1995, 117, 4533 and references therein. (c) Minisci, F.;
Fontana, F.; Araneo, S.; Recupero, F.; Banfi, S.; Quici, S. J. Am. Chem.
Soc. 1995, 117, 226.

Cobalt(III)-Alkylperoxo Complexes
J. Am. Chem. Soc., Vol. 120, No. 35, 1998 9023
Table 5. Influence of the Alkyl Group R on Stoichiometric
the oxidation of CyH and other hydrocarbons by several
[LCoIII-OOR] complexes to collect more support for our earlier
suggestion and establish the general mechanism of alkane
oxidation by such species. The somewhat superior capacity of
CyH oxidation by a second [LCo(OOtBu)] complex namely,
[Co(PyPz2P)(OOtBu)] (13), provided the first support to our
suggestion that the tBuO• radical, the common product in thermal
decomposition of 7 and 13, is a key participant in the overall
oxidation process. Since the “radical strength”35 of tBuO• radical
(105 kcal/mol) is sufficient to abstract H atoms from C-H bonds
of higher molecular weight (>C4) alkanes including CyH (95
kcal/mol),36 the earlier suggestion of H abstraction from CyH
by the tBuO• radical appears quite reasonable. This H abstrac-
tion step is a crucial one since it allows the substrate hydro-
carbon to enter a radical-driven oxidation pathway (vide infra).
To determine if [Co(Py3P)(O•)], the other product of homoly-
sis, is also capable of initiating oxidation of CyH by H atom
abstraction, we employed [Co(Py3P)(OOCMe2CH2Ph)] (9) as
the starting oxidant. Since our results indicate that O-O bond
homolysis is the preferred mode of decomposition of the [Co-
(Py3P)(OOR)] complexes, one would expect formation of [Co-
(Py3P)(O•)] and PhCH2CMe2O• in this reaction mixture. Since
the PhCH2CMe2O• radical undergoes â-scission very rapidly
(PhCH2CMe2O• f PhCH2• + Me2CO, k ) 2.2 × 108 s-1 at 25
t
Hydrocarbon Oxidation with [Co(Py3P)(OOR)] (R ) Bu (7), Cm
i
n
(8), Cy (10), Pr (11), and Pr (12))a
Toluene
% benzaldehyde
% benzyl alcohol
% total
79
53
15
10
7
8
10
11
12
73
50
15
10
8
6
3
-
-
-
8
Cyclododecane
% cyclododecanol
% cyclododecanone
7
8
10
11
12
21
10
5
4
2
35
26
8
8
7
56
36
13
12
9
Adamantane
%1-adamantanol % 2-adamantanol % 2-adamantanone
7
55
50
13
20
16
8
7
2
4
2
5
1
2
2
2
67
58
17
26
20
8
10
11
12
a Yields based on the amount of the Co(III) complex. Reaction
mixtures consisted of Co(III) complex (8 µmol), 0.5 mL of CH2Cl2,
and 0.5 mL of substrate (toluene) or 200 mg (adamantane or
cyclododecane). Reaction time was 2 h, and reaction temperature was
70 °C.
•
°C) to produce the PhCH2 radical in the reaction mixture and
•
H atom abstraction from CyH by PhCH2 is very slow compared
to other radical termination reactions, only small amounts of
oxidation products of CyH are expected from the action of the
RO• fragment in the case of 9.27 On the other hand, if [Co-
(Py3P)(O‚)] is capable of H atom abstraction from CyH, one
would expect a more significant extent of oxidation to occur.
In the actual experiment, when 30 µmol of 9 in the presence of
1 mL of 1:1 CyH/CH2Cl2 was heated to 80 °C for 2 h, mostly
PhCHO (62%) was obtained along with PhCH2Cl (4%), K (3%),
PhCH2CMe2OH (6%), and a trace of PhCH2OH. This result
clearly indicates that [Co(Py3P)(O‚)] is not strong enough to
abstract H atoms from CyH.
The kinetic isotope effect (kH/kD, KIE) on CyH oxidation by
the [LCoIII-OOR] complexes indicates that H atom abstraction
from CyH is an important step. In one experiment, a solution
of 30 µmol of 7 in 1 mL of 0.25:0.25:0.5 C6D12/C6H12/CH2Cl2
was heated to 80 °C in a sealed vial. After 2 h, the reaction
was rapidly cooled to room temperature, and the oxidized
products were quantitated by gas chromatography. The KIE
value, calculated by dividing the total amount of undeuterated
oxidized products (C6H11OH + C6H10O) by the amount of
deuterated oxidized products (C6D11OH + C6D10O), came out
to be 5. Interestingly, the expected kH/kD value at 80 °C for a
system in which the C-H bond stretching in the transition state
is rate limiting is 4.9.37 Although production of A and K from
CyH by the [LCoIII-OOR] complexes may involve several steps
with more than one incident of C-H bond breaking (vide infra),
a KIE value of 5 strongly suggests that H atom abstraction from
CyH is a crucial step and the oxidation reactions are radical-
driven. In support, Que and co-workers have observed a KIE
value of 4.9 in the oxidation of cyclohexane by tBuO• radicals
at room temperature.38
Oxidation of CyH by the [LCoIII-OOR] complexes afford
A and K only in the presence of dioxygen. When reaction
mixtures are prepared and heated in the absence of dioxygen,
no O-containing product(s) is observed. For example, when a
solution of 32 µmol of 7 in 1 mL of 1:1 CyH/CH2Cl2 was heated
at 70 °C in the absence of dioxygen, the only product derived
from CyH was a small amount of cyclohexyl chloride (C).15
C
presumably arises from trapping of the cyclohexyl radicals (Cy•)
by the dichloromethane solvent.39 Along the same line, neither
benzaldehyde nor any O-containing product was obtained from
the solution of 9 in 1 mL of 1:1 CyH/CH2Cl2 following
anaerobic heating at 80 °C for 2 h. The products in this case
were PhCH2Cl (3%), PhCH2CMe2OH (2%), and a trace amount
of PhCH2CH2Ph. The PhCH2CMe2OH presumably arises from
H atom abstraction from cyclohexane by PHCH2CMe2O• while
PhCH2CH2Ph results from combination of two PhCH2• radicals
(â-scission product). These results clearly indicate that the
presence of dioxygen is necessary for the oxidation of CyH to
A and K by the [LCoIII-OOR] complexes.
Collectively, the results of CyH oxidation by various [LCoIII-
OOR] complexes confirm that upon heating, homolysis of the
O-O bond of [LCoIII-OOR] affords RO• radicals in the
reaction mixtures. Since most RO• radicals can initiate oxidation
of saturated hydrocarbons including CyH, one expects that the
present [LCoIII-OOR] complexes should promote oxidations
of many saturated hydrocarbons at elevated temperatures. In
the present study, two [LCoIII-OOtBu] complexes have been
used to oxidize five additional hydrocarbon substrates, and the
results are shown in Table 6. That the [LCoIII-OOR] com-
plexes in general are good oxidizing agents for saturated
hydrocarbons is evident from this table.
(35) Barton, D. J. R.; Sawyer, D. T. In The ActiVation of Dioxygen and
Homogeneous Catalytic Oxidation; Barton, D. J. R., Martell, A. E., Sawyer,
D. T., Eds.; Plenum Press: New York, 1993; p 4.
(36) Handbook of Chemistry and Physics, 67th ed.; Weast, R. C., Ed.;
CRC Press: Boca Raton, FL, 1987; pp F178-179.
(37) Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
Chemistry; Haper and Row: New York, 1987; pp 233-235.
(38) MacFaul, P. A.; Ingold, K. U.; Wayner, D. D. M.; Que, L., Jr. J.
Am. Chem. Soc. 1997, 119, 10594.
(39) Trapping of cyclohexyl radicals by halogenated hydrocarbons has
been noted in cyclohexane oxidation: (a) Leising, R. A.; Kojima, T.; Que,
L., Jr.. In The ActiVation of Dioxygen and Homogeneous Catalytic
Oxidation; Barton, D. H. R., Martell, A. E., Sawyer, D. T., Eds.; Plenum
Press: New York, 1993; pp 321-331. (b) Kim, J.; Larka, E.; Wilkinson,
E. C.; Que, L., Jr. Angew. Chem., Int. Ed. Engl. 1995, 34, 2048.

9024 J. Am. Chem. Soc., Vol. 120, No. 35, 1998
ChaVez et al.
Table 6. Stoichiometric Oxidation of Hydrocarbons by
[Co(Py3P)(OOtBu)] (7) and [Co(PyPz2P)(OOtBu)] (13)a: Influence
of the Ligand L on the Capacity for Oxidation
Scheme 1
Initiation
Cyclohexane
RO• + R′H f ROH + R′•
(1)
complex
% cyclohexanol
% cyclohexanone
% total
52
80
7
13
18
37
34
43
Propagation
Toluene
R′• + O2 f R′OO•
(2)
(3)
% benzaldehyde
% benzyl alcohol
7
13
73
127
6
14
79
141
R′OO• + R′H f R′• + R′OOH
Cyclooctane
Termination
% cyclooctanol
% cyclooctanone
7
13
16
20
66
114
82
134
RO• + RO• f ROOR
R′OO• + R• f R′OOR
(4)
(5)
Adamantane
% 1-adamantanol % 2-adamantanol % 2-adamantanone
7
13
55
91
8
15
5
8
67
114
R′OO• + R′OO• f oxidized products
(6)
Cyclododecane
products result from a radical termination reaction like 6 of
Scheme 1 and/or from metal-catalyzed decomposition of R′OOH
that accumulates in the reaction mixture (reaction 3, Scheme
1). Thus, when CyH is used as the substrate, CyOO• radicals
are produced via reactions 1 and 2 and terminate via reaction 6
to give A and K as the major products of oxidation. The A:K
ratio in most cases deviates from the typical 1:1 value in a
Russell-type recombination42 due to the overoxidation of A
under the reaction conditions as well as the contribution from
metal-catalyzed decomposition of CyOOH.
% cyclododecanol
% cyclododecanone
7
13
21
29
35
56
56
85
n-Heptane
% 2-heptanol
% 1-heptanol
% 3-heptanol
7
13
1
1
6
11
24
39
31
51
a Yields based on the amount of the Co(III) complex. Reaction
mixtures consisted of 8 µmol of 7 or 13, 0.5 mL of CH2Cl2, and 0.5
mL of substrate (in the cases of cyclohexane, toluene, cyclooctane,
and n-heptane) or 0.2 g of substrate (in the cases of adamantane and
cyclododecane). Reaction time was 2 h, and reaction temperature was
70 °C.
Finally, to confirm that the Co-O bond of the [LCoIII-OOR]
complexes is not homolyzed upon heating, we studied the
products of reactions in which the [LCoIII-OOR] complexes
were heated in the presence of substrates such as 1,4-cyclo-
hexadiene and 2,6-di-tert-butylphenol that easily afford H atoms.
Without exception, these reactions afforded ROH as essentially
the only product. For example, when 20 µmol of [Co(Py3P)-
(OOCMe2Ph)] (8) was heated at 70 °C for 2 h in 1 mL of 9:1
1,4-cyclohexadiene/MeCN under dinitrogen, PhMe2COH (78%)
and PhC(O)Me (2%) were formed along with PhH (530%).
These products are obtained from PhCMe2O• via reactions II
When analyzed in the light of what is known about the
chemistry of the RO• radicals,40 the results of stoichiometric
oxidation of hydrocarbons by the [LCoIII-OOR] complexes lead
one to consider a general mechanism that we have suggested
in our previous report.15 In this mechanism, oxidation of hy-
drocarbons occurs through several steps involving different
radical species (Scheme 1). The first step in this mechanism is
the production of RO• radicals from the [LCoIII-OOR] com-
plexes upon heating. The RO• radicals then initiate oxidation
of the hydrocarbon substrate (R′H) by removing H atoms from
R′H to produce R′• radicals (Scheme 1, reaction 1). In the
presence of dioxygen, R′• reacts at diffusion-controlled rates to
form peroxyl (R′OO•) radicals (Scheme 1, reaction 2).40 At
elevated temperatures, these peroxyl radicals can abstract an H
atom from another R′H to begin a chain process that results in
the accumulation of R′OOH (reactions 2 and 3).1d Termination
of these radicals can also occur through reactions 4 through 6.
Thermal decomposition of the [LCoIII-OOR] complexes in the
presence of R′H thus promotes oxidation of R′H via an
autocatalytic process involving RO• and R′OO•. The fact that
one obtains greater than 100% yields in some cases (Table 4)
supports the autocatalytic nature of the oxidation reactions by
[LCoIII-OOR] complexes.41 The final substrate-based oxidized
[Co(Py3P)(OOCMe2PH)] f [Co(Py3P)(O•)] + PhCMe2O•
(II)
2PhCMe2O• + 1,4-cyclohexadiene f 2PhCMe2OH + PhH
(III)
and III. If heating of 9 had promoted homolysis of the Co-O
bond, then one would have expected PhCMe2OO• radicals in
the reaction mixture which, in the presence of excess 1,4-
cyclohexadiene, would have produced PhCMe2OOH as the final
product. Along the same line, no CyOOH is detected when 10
and 16 were heated with 1,4-cyclohexadiene under dinitrogen
atmosphere.43 Collectively, these results confirm that homolysis
of the Co-O bond does not occur when the [LCoIII-OOR]
complexes are heated.
(40) (a) Walling, C. In Free Radicals in Solution; John Wiley & Sons:
New York, 1957. (b) Howard, J. A. AdV. Free Radical Chem. 1972, 4, 49.
(c) Ingold, K. U. In Free Radicals, Vol. 1; Kochi, J. K., Ed.; John Wiley
& Sons: New York, 1973; p 37.
(41) The partly autocatalytic nature of these oxidation reactions is further
indicated by the fact that the yields of the oxidized products depend on the
amount of the [LCoIII-OOR] complexes used in the reactions. The use of
greater amounts of the [LCoIII-OOR] oxidants often results in higher
amounts of the oxidized products but diminished percent of yields with
respect to the amounts of the metal complexes. For this reason, we have
employed the same equivalents of [LCoIII-OOR] complexes whenever
comparisons have been made to establish trends in such oxidation reactions.
Role of the R Group on the Oxidizing Capacity of
[LCoIII-OOR] Complexes. To investigate the effect of the
R group on the ability of [LCoIII-OOR] complexes to promote
hydrocarbon oxidation, we carried out a set of reactions in which
t
i
n
8 µmol of each [Co(Py3P)(OOR)] (R ) Bu, Cm, Cy, Pr, Pr)
complex was dissolved in a total volume of 1 mL of RH/CH2-
(42) Russell, G. A. J. Am. Chem. Soc. 1957, 79, 3871.
(43) The reaction conditions and detection procedures described here
allow one to clearly identify ROOH, if present in the reaction mixtures.

Cobalt(III)-Alkylperoxo Complexes
J. Am. Chem. Soc., Vol. 120, No. 35, 1998 9025
Table 7. Influence of Solvent on the Capacity for Stoichiometric
Oxidation of Cyclohexane by [Co(Py3P)(OOtBu)] (7)a
Cl2 and heated to 70 °C for 2 h. The results of these experiments
are shown in Table 5. Upon close scrutiny of Table 5, it
becomes apparent that for all of the hydrocarbons, [Co-
(Py3P)(OOtBu)] is the most effective oxidant, while [LCoIII-
OOR] complexes with R ) primary alkyl groups are the least
effective. Since this trend parallels the stability of the RO•
radicals, once more it becomes evident that the actual oxidants
in all cases are RO• radicals. The lifetime of the tBuO• radical
is longer than the CmO• radical due to the slower rate of
decomposition via â-scission.27b Secondary and primary alkoxyl
radicals are susceptible to 1,2-hydrogen-atom shifts, bimolecular
self-decomposition, and/or â-scission and hence have relatively
shorter radical lifetimes.44 As a result, a [LCoIII-OOR]
complex serves as an efficient oxidant only when the RO•
radicals derived from it bring about H atom abstraction from
the substrate (or the solvent) at a rate faster than the rate(s) of
reactions that remove them from the reaction mixture in parallel
pathways.
Role of Ligand L on the Oxidizing Capacity of the
[LCoIII-OOtBu] Complexes. Shown in Table 6 are the results
of stoichiometric oxidations of cyclohexane, toluene, cyclo-
octane, adamantane, cyclododecane, and n-heptane by 7 and
13. In all cases, 8 µmol of 7 or 13 was mixed with a substrate
in dichloromethane, and the solution was heated to 70 °C for
2h. The distribution of the oxidation products in each case
supports Scheme 1. Close examination of the product distribu-
tions and yields of these reactions (Table 6) reveals that in all
oxidation reactions, complex 13 gave more total oxidized
products (alcohol, aldehyde, or ketone) than 7. This trend
suggests that the ligand L has definite influence on the overall
ability of the [LCoIII-OOR] complexes to induce oxidation of
hydrocarbon substrates.
dielectric
constant (ꢀ) % A % K
solvent
% other
% total
dibromomethane
acetic acid
dichloromethane
pyridine
acetone
ethanol
4.78
6.15
8.93
12.40
20.70
24.55
17
52
18
3
3
6
19
40
34
18
3
CyBr (30)
66
92
52
10
6
3
9
a Yields based on the amount of the Co(III) complex. Reaction
Conditions: 8 µmol of 7 in 1 mL of 1:1 CyH/solvent, 70 °C, 2 h
reaction time.
momethane, considerable amount of cyclohexyl bromide (CyBr)
is formed in the reaction mixture due to trapping of the Cy•
radicals by Br atoms, and hence, lower yields of A and K are
obtained. In contrast, only a trace amount of cyclohexyl chloride
(CyCl) is produced when dichloromethane is used as the solvent.
The larger amount of CyBr (relative to that of CyCl) in the
final product presumably arises from weaker C-Br bonds in
dibromomethane. Pyridine, acetone, and ethanol, the solvents
with higher dielectric constants, all afford low yields of the
oxidized products. Collectively, these results indicate that an
increase in solvent polarity lowers the yields of the oxidized
products. In general, the [LCoIII-OOR] complexes are more
stable in solvents with low dielectric constants and hence are
expected to decompose slowly and promote more oxidation in
such media. Indeed, the rate of [Co(Py3P)(OOtBu)] decomposi-
tion at 70 °C in CD2Br2 (monitored by 1H NMR spectroscopy)
decreases considerably when cyclohexane is added. Polar
solvents, particularly those capable of H bonding, are also known
t
to greatly accelerate the rate of â-scission of the BuO• radical
To identify the ways L could influence the oxidative capacity
of the [LCoIII-OOR] complexes, decomposition of 7 and 13
in the absence of substrate (in CD2Br2, 70 °C) was followed by
1H NMR spectroscopy. The process of decomposition of the
two complexes was monitored by the disappearance of the
by stabilizing the transition state and decreasing the rate of H
atom abstraction via solvation of the tBuO• radical.40b,46,47 Since
t
both phenomena lower the extent of oxidation by the BuO•
radical, it is expected that oxidation by [Co(Py3P)(OOtBu)] will
afford higher yields of the oxidized products only in solvents
of low polarity, and this is exactly what one observes in Table
7. We conclude that oxidation of hydrocarbons by the [LCoIII-
OOR] complexes in general is facilitated in solvents of low
polarity.
t
t
resonance of the Bu group of the coordinated BuOO ligand.
The results indicate that 7 decomposes at a faster rate than 13.
Since tBuO• radicals are responsible for H atom abstraction from
the hydrocarbon substrate, it is evident that 7 affords more
oxidation products, at least in the initial stage. The high
Effect of Water. When aqueous solutions of the [LCoIII-
OOR] complexes are allowed to sit at room temperature or
heated, they are converted to the corresponding [LCoIII(OH)]
complexes. This process can be followed by monitoring the
electronic absorption spectra of the metal complexes. The
conversion also generates free ROOH which is easily detected
by gas chromatography.48 Decomposition of the [LCoIII-OOR]
complexes is therefore reaction IV which is the reverse of
t
concentration of the BuO• radicals, however, promotes more
radical terminations, and hence, in the long run, rapid decom-
position of 7 results in lower yields of the oxidized products.
t
Quite in contrast, slower realease of BuO• radicals into the
reaction mixture from 13 allows the autocatalytic oxidation
reaction to proceed with a lower incidence of termination, and
one obtains higher yields of the products in heat-induced
oxidation reactions by 13. The superior ability of 13 to oxidize
hydrocarbon substrates is therefore derived from its greater
thermal stability.
[LCoIII-OOR] + H2O f [LCoIII(OH)] + ROOH (IV)
Effect of Solvent. To examine the effects of solvent polarity
and the ability of the solvent to hydrogen bond on stoichiometric
oxidations of cyclohexane, oxidations by 7 were carried out in
1 mL of 1:1 CyH/solv mixtures with solv ) dibromomethane,
acetic acid, dichloromethane, pyridine, acetone, or ethanol.
Results of these oxidations are shown in Table 7. The highest
yields of the oxidized products are obtained in acetic acid, a
solvent with low dielectric constant.45 When solv ) dibro-
reaction I. One immediate consequence of reaction IV is the
failure of the [LCoIII-OOR] oxidants to initiate oxidation of
any substrate if the reaction mixture contains a significant
amount of water.
Effect of Added Co(II) Salts. In industrial oxidation
processes, divalent metal salts are often added to control product
distribution.1a,d,f To investigate the effect of Co(II) salts on
oxidations by the [LCoIII-OOR] complexes, we carried out
(44) Fossey, J.; Lefort, D.; Sorba, J. In Free Radicals in Organic
Chemistry; John Wiley & Sons: New York, 1995.
(45) Acetic acid also accelerates metal-catalyzed decomposition of
CyOOH formed in the reaction mixture1a,d,f and increases the yields of A
and K.
(46) Walling, C.; Padwa, A. J. Am. Chem. Soc. 1963, 85, 1593.
(47) Walling, C.; Wagner, P. J. J. Am. Chem. Soc. 1964, 86, 3368.
(48) The presence of ROOH in such solutions was also confirmed by
the use of aqueous Na2SO3 which is known to decompose ROOH to the
corresponding alcohol (see Hock, H.; Lang, S. Chem. Ber. 1942, 75, 313).

9026 J. Am. Chem. Soc., Vol. 120, No. 35, 1998
ChaVez et al.
Table 8. Catalytic Oxidation of Cyclohexane with Cobalt Complexes and Alkyl Hydroperoxidesa,b,c
Reaction Mixture Composition
Yield of Productsd
complex mass
ROOH
(mmol)
CyH
(mmol)
CyOH
(µmol)
CyO
(µmol)
Pe
(µmol)
total
(µmol)
complex
(µmol)
t.n.f
TBHP
4
6
7
13
4
6
25
25
25
10
10
10
10
30
30
30
30
60
60
60
1428
1518
1331
1452
5032
4992
5272
6720
1661
1824
1518
1763
4890
5706
3994
5522
174
169
145
174
570
930
942
628
3263
3511
2994
131
140
120
136
87
97
85
25
60
3389
120
120
120
120
200
200
200
200
10492
11628
10208
12870
7
13
114
CmOOHg
1617
4
6
4
6
7
25
25
120
120
120
120
10
10
30
30
30
30
60
60
200
200
200
200
1100
1121
2781
3413
3165
3023
2717
2680
8482
9453
8696
8866
109
107
71
79
72
1558
5701
6040
5531
5843
13
74
a Reaction mixtures were prepared under ambient conditions. b Reaction conditions: 80 °C, 4 h. c For each product, the identity and yield were
determined by comparison with those of an authentic sample and with cyclooctane as internal standard. d Yields of three runs of each reaction were
f
within (5%. e P ) CyOOtBu. Turnover numbers were obtained by dividing the total mass of the oxidized products by the catalyst mass. g PhCMe2OH
and PhC(O)CH3 are obtained in all of these reactions.
stoichiometric oxidation of CyH by 7 and 13 in the presence
and absence of Co2+. When 8 µmol 7 was dissolved in 1 mL
of 1:1 CyH/CH2Cl2 and heated at 70 °C for 2 h, A (18%) and
K (44%) are formed with an A/K ratio of 0.41. When the same
reaction was carried out in the presence of 20 µmol of Co-
(OAc)2‚4H2O, A (88%) and K (57%) are formed in higher
amounts and the A/K ratio changed noticeably to 1.63.
Similarly, oxidation of CyH by 13 (8 µmol) in 1 mL of 1:1
CyH/CH2Cl2 at 70 °C for 2 h afforded A (40%) and K (57%)
with an A/K ratio of 0.7. Addition of 20 µmol Co(OAc)2‚4H2O
to the same reaction mixture resulted in formation of A (80%)
and K (48%) in the ratio of 1.64. Thus in both cases, addition
of the Co(II) salt results in more oxidized products and an
enhancement in the formation of A relative to K. This result
strongly suggests that CyOOH is produced in the reaction
mixture (reactions 2 and 3 of Scheme 1). When a Co(II) salt
is added, it reacts with CyOOH to form CyO• and Co(III)OH.
Abstraction of H atoms from the substrate (CyH) by CyO• then
affords CyOH, and the new Cy• radical generated in this step
promotes more oxidation to improve the overall yields of the
oxidized products.
OOR] complexes in the presence of hydrocarbon substrate
affords the [LCoIII(OH)] species as the final product.
Catalytic Oxidation of Hydrocarbon Substrates by [LCoIII-
OOR] Complexes. The fact that the [LCoIII-OOR] complexes
are converted into the [LCoIII(OH)] complexes after single
turnover in oxidation reactions prompted us to design catalytic
systems based on these complexes in conjunction with excess
ROOH. That this goal has been accomplished in the present
study is indicated by the results included in Table 8. Catalytic
oxidations have been observed with [LCoIII(OH)] (4,6) and
[LCoIII-OOR] (7,13) complexes in the presence of excess
TBHP and PhCMe2OOH (CmOOH). Scale-ups of the catalytic
oxidations have been achieved by the use of more catalysts and/
or more ROOH with little loss in efficiencies. More than a
gram of total oxidized products can be obtained in such reactions
within 4 h and at a moderate temperature of 80 °C. The
catalysts remain active for over 10 h, and as high as 150
turnovers are obtained at longer (6-8 h) periods of time.
Control reactions with soluble Co(II) salts afford much smaller
amounts of oxidized products under identical conditions.
Interestingly, when the catalytic oxidation of CyH with [Co-
(PyPz2P)(OH)] and TBHP is allowed to continue for 12 h, adipic
acid (∼10% of the total oxidized product, quantitated as
dimethyl adipate) is obtained as one of the products. This
indicates that A and K, produced initially in these catalytic
oxidations, could be further oxidized in the same reaction vessel.
The results of the catalytic oxidation reactions by the [LCoIII-
OOR] complexes can be rationalized on the basis of Scheme 1.
Here the RO• radicals formed during initial homolysis of the
O-O bond of the [LCoIII-OOR] complexes abstract H atoms
from both CyH and excess TBHP (or CmOOH). The reaction
mixtures in these cases therefore contain an abundance of
tBuOO• (or CmOO•) radicals that serve two purposes, namely,
(a) they act as secondary sources for RO• radicals (reaction V)
Fate of [LCoIII-OOR] after Single Turnover in Stoichio-
metric Oxidation. Stoichiometric oxidation reactions with the
[LCoIII-OOR] complexes are not as messy as one expects in a
typical radical-driven autoxidation process. The colors of the
reaction mixtures indicate that rapid degradation of the cobalt
complexes does not occur in most cases. To determine the fate
of the [LCoIII-OOR] complexes in stoichiometric oxidations,
we dissolved 7 in 1 mL of 1:1 d12-cyclohexane/d2-dichlo-
romethane at 70 °C, and the decomposition of the complex was
t
followed by monitoring the disappearance of the Bu peak at
0.56 ppm. At the end of decomposition, as evidenced by the
complete disappearance of the peak for the coordinated OOtBu
group, diethyl ether was added to the reaction mixture to
precipitate the cobalt-containing end product. The electronic
absorption and 1H NMR spectrum of this precipitate were
identical to those of authentic [Co(Py3P)(OH)] (4). A similar
experiment with 13 afforded [Co(PyPz2P)(OH)] (6) at the end
of stoichiometric oxidation of CyH. In both cases, the [LCoIII-
(OH)] species were isolated in nearly quantitative amounts.
These results clearly indicate that decomposition of the [LCoIII-
tBuOO• f tBuO• + 1/2 O2
(V)
tBuO• + tBuOO• f tBuOOtBu + 1/2 O2
(VI)
2 tBuOO• f tBuOOOOtBu f tBuOOOtBu + 1/2 O2 (VII)
which in turn promote formation of more A and K via formation

Cobalt(III)-Alkylperoxo Complexes
J. Am. Chem. Soc., Vol. 120, No. 35, 1998 9027
of Cy• radicals (reaction 1 of Scheme 1) in the reaction mixtures,
and (b) they provide additional O2 in the reaction mixture
(reactions V-VII) and increase yields of A and K via reactions
2 and 6 of Scheme 1.34,49
(ii) The structures of several [LCoIII-OOR] complexes
indicate, for the first time, that the O-O bond distances in these
complexes do not change significantly with R groups.
(iii) When warmed (60-80 °C) in aprotic solvents of low
polarity, [LCoIII-OOR] complexes undergo O-O bond ho-
molysis and afford RO• radicals in solution. If alkanes are
present in such solutions, oxidation of alkane C-H bonds is
observed. These oxidation reactions are promoted by RO•
radicals and are autocatalytic to some extent. The [LCoIII-O•]
unit, the other product of homolysis, is not capable of promoting
alkane oxidation. The results of hydrocarbon oxidation by
various [LCoIII-OOR] complexes point toward a mechanism
of oxidation that is also supported by the known reactions of
RO• and ROO• radicals.
The catalytic oxidation reactions with [LCoIII(OH)] and
[LCoIII-OOR] complexes are noteworthy for more than one
reason. Inspection of Table 8 reveals that the overall conversion
of CyH to CyOH and CyO is about 10% within 4 h in all cases.
The colors of the reaction mixtures indicate that the [LCoIII-
OOR] species are regenerated during these oxidations. At
longer reaction times, slow decomposition of the cobalt
complexes to black residue(s) is also noted. Unlike the Du Pont
process, corrosive constituents such as acetic acid are not
required in such oxidations, and the same catalyst can be used
for oxidation of many substrates. Also, the oxidation reactions
proceed at respectable speed at moderate temperatures. It will
be interesting to find out whether unreacted CyH could be
separated from the reaction mixture and then recycled to design
an efficient homogeneous catalytic system for alkane oxidation
based on these cobalt complexes. Such studies are in progress
in this laboratory.
(iv) In stoichiometric oxidation of alkanes, more oxidation
is observed with [LCoIII-OOR] complexes containing tertiary
R groups. The extent of oxidation also depends on L. Thus,
[LCoIII-OOR] complexes with L ) PyPz2P2- afford higher
yields of the oxidized products compared to the yields of similar
complexes with L ) Py3P2-
.
(v) Stoichiometric oxidation of alkanes by [LCoIII-OOR]
complexes proceeds to a significant extent in aprotic solvents
of low polarity. The presence of water in the oxidation medium
impedes oxidation via conversion of [LCoIII-OOR] to [LCoIII-
(OH)].
To date, several groups have reported oxidation of hydro-
carbons by ROOH in the presence of cobalt salts.1 In most
cases, the results have been rationalized in terms of some or all
of the reactions included in Scheme 1 and the intermediacy of
[LnCoIII-OOR] species. However, prior to this report, very few
[LnCoIII-OOR] species had been structurally characterized and
their exact role(s) in the overall oxidation remained somewhat
speculative. In the present work, we have employed a series
of structurally characterized [LCoIII-OOR] complexes with two
different L’s and many different R groups in the oxidation of
several hydrocarbons and identified the various factors that
govern the decomposition of these intermediates and their
capacity of alkane oxidation. No other group has undertaken
this task before. The results of the present work clearly indicate
that [LnCoIII-OOR] intermediates, if formed in the reaction
mixture, break down quite readily to provide RO• radicals and
continue the oxidation process. The notion that such complexes
are dead-end species is therefore not correct. Also, the overall
activities of the [LCoIII-OOR] complexes described in this
account suggest that such complexes can be utilized in carrying
out oxidation of C-H bonds under mild conditions.
(vi) In stoichiometric oxidation reactions, [LCoIII-OOR]
complexes are converted to [LCoIII(OH)] at the end of one
turnover.
(vii) Oxidation of cyclohexane by [LCoIII-OOR] or [LCoIII-
(OH)] can be made catalytic by the addition of excess ROOH
to the reaction mixtures. Grams of oxidized products are
obtained in such oxidation reactions within hours at 80 °C.
Acknowledgment. This research was supported by a grant
from the donors of the Petroleum Research Fund, administered
by the American Chemical Society. Experimental assistance by
Jorge Briones is also acknowledged.
Supporting Information Available: Computer-generated
thermal ellipsoid plot (50% probability level) of 9 (Figure S1),
hydrogen-bonding scheme in the structure of 10 (Figure S2),
molecular structure of 12 (Figure S3), 1H NMR spectra of 7, 8,
and 11 in CDCl3 (Figure S4); spectroscopic data for 8-18,
crystal structure data for 8-12 and 16 including atomic
coordinates and isotropic thermal parameters, bond distances
and angles, anisotropic thermal parameters, and H-atom coor-
dinates (64 pages, print/PDF). See any current masthead page
for ordering information and Web access instructions.
Summary and Conclusions
The following are the principal results and conclusions of
this investigation.
(i) Reactions of [LCoIII(OH)] with ROOH in aprotic solvents
of low polarity afford [LCoIII-OOR] complexes in nearly
quantitative yields. This procedure is clean, convenient, and
works for primary, secondary, and tertiary R groups.
(49) Milas, N. A.; Plesnicar, B. J. Am. Chem. Soc. 1968, 90, 4450.
JA9814873