Co-C bond homolysis: Reactivity difference between alkyl- and benzylcobaloximes

Article abstract of DOI:10.1021/om070053q

The Co-C bond is activated toward homolysis due to the interactions between axial and equatorial dioxime ligands. Benzylcobaloxime gives an oxygen-inserted product, whereas the alky I derivative forms air-stable cobalt(II). The stabilization of the axial

Full text of DOI:10.1021/om070053q

© Copyright 2007
Volume 26, Number 11, May 21, 2007
American Chemical Society
Communications
Co-C Bond Homolysis: Reactivity Difference between Alkyl- and
Benzylcobaloximes
Debaprasad Mandal, Mouchumi Bhuyan, Moitree Laskar, and B. D. Gupta*
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
ReceiVed January 18, 2007
Summary: The Co-C bond is actiVated toward homolysis due
to the interactions between axial and equatorial dioxime ligands.
Benzylcobaloxime giVes an oxygen-inserted product, whereas
the alkyl deriVatiVe forms air-stable cobalt(II). The stabilization
of the axial R group due to the interaction between axial and
equatorial ligands causes the reactiVity difference.
to be too unstable for X-ray structural characterization.4 Modi-
fied cobaloximes have been studied with the aim of modeling
more successfully some specific properties of the B12 coenzyme,
such as reversible homolysis of the Co-C bond when it binds
the apoenzyme5 or the interaction of the corrin side chains with
the axial ligands,6 or avoiding undesired side reactions, such as
autoxidation of the metal center, when CoII complexes are
studied as oxygen carriers.7 However, such modifications are
limited to a few cases only. Busch et al.7 have successfully
designed various cobalt(II) dioximes as oxygen carriers by
electronic as well as steric modifications to stop the autoxidation
process. Electronic modification was done by BF2 bridging,
while steric modification was achieved by using bulky dioximes.
CoII(dmestgBF2)2 is the poorest example of an oxygen carrier
among various bulky dioximes, as it does not even take up
dioxygen (Supporting Information, Scheme S1).
Coenzyme B12 has long fascinated chemists, and its unique
property arises from the different catalytic activities of two
different coenzymes. How the Co-C bond is activated toward
homolysis or heterolysis is an enduring subject of research.1,2
Studies on model compounds have continued to comple-
ment those on the more complex cobalamin- and B12-based
proteins.
Steric factors are known to be important in weakening the
Co-C bond, and the Co-C bond length does indeed respond
to steric rather than electronic effects in the model compounds,
the organocobaloximes RCo(dmgH)2B (trans-bis(dimethylgly-
oximato)pyridine(organo)cobalt(III)).2 The bond lengths in
structurally characterized complexes vary over a remarkably
broad range of 0.2 Å from methyl to adamantyl.3 Spectroscopic
evidence has been presented that even longer bonds occur in
more sterically hindered systems which have thus far proved
We have recently shown that the dimesitylglyoxime com-
plexes RCo(dmestgH)2Py (R ) alkyl; dmestgH ) dimesityl-
glyoxime) have the maximum cobalt anisotropy and the highest
steric cis influence among the commonly studied dioximes.8
The crystal structures show that both the axial positions are very
(4) (a) Gupta, B. D.; Vijaikanth, V.; Singh, V. Organometallics 2004,
23, 2069. (b) Trommel, J. S.; Warncke, K.; Marzilli, L. G. J. Am. Chem.
Soc. 2001, 123, 3358. (b) Siega, P.; Randaccio, L.; Marzilli, P. A.; Marzilli,
L. G. Inorg. Chem. 2006, 45, 3359.
* To whom correspondence should be addressed. Tel: +91-512-2597046.
Fax: +91-512-2597436. E-mail: bdg@iitk.ac.in.
(1) Brown, K. L. Dalton Trans. 2006, 1123.
(5) Ko¨hler, B.; Kramer, M.; Clegg, W.; Elsegood, M. R. J.; Golding, B.
T.; Retey, J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2389 and references
therein.
(2) Randaccio, L. Comments Inorg. Chem. 1999, 21, 327.
(3) (a) Randaccio, L.; Bresciani-Pahor, N.; Toscano, P. J.; Marzilli, L.
G. J. Am. Chem. Soc. 1980, 102, 7372. (b) Marzilli, L. G.; Toscano, P. J.;
Randaccio, L.; Bresciani-Pahor, N.; Calligaris, M. J. Am. Chem. Soc. 1979,
101, 6754.
(6) Dreos, R.; Tauzher, G.; Vuano, S.; Asaro, F.; Pellizer, G.; Nardin,
G.; Randaccio, L.; Geremia, S. J. Organomet. Chem. 1995, 505, 135.
(7) Lance, K. A.; Goldsby, K. A.; Busch, D. H. Inorg. Chem. 1990, 29,
4537.
(8) Mandal, D.; Gupta, B. D. Organometallics 2005, 24, 1501.
10.1021/om070053q CCC: $37.00 © 2007 American Chemical Society
Publication on Web 04/18/2007

2796 Organometallics, Vol. 26, No. 11, 2007
Communications
Figure 1. Molecular structure of CoII(dmetgH)2(Py)2.
much crowded and laterally compressed by the methyl groups
of mesityl and, due to this steric crowding, pyridine is puckered
(strained) (Supporting Information, Figure S1). The strain is even
greater when R ) Et, Pr, Bu than it is for the methyl analogue,
as observed in 1H NMR; for example, the 2-Me of the mesityl
group is shifted upfield in the higher alkyl chain as compared
to the case for methyl.8 It seems that increasing the alkyl chain
length increases the bending angle (R) and the 2-Me moves
closer to pyridine and becomes affected by its ring current.9
While studying the dmestgH complexes, we have made an
important observation. During crystallization in a dichlo-
romethane-methanol mixture, n-BuCo(dmestgH)2Py, decom-
posed and gave nice orange crystals; the crystal data showed it
to be CoII(dmestgH)2Py2 (Figure 1).10 Benzyl analogues, on the
other hand, are highly unstable in solution. The workup must
be carried out rapidly under an argon atmosphere; otherwise,
the product is contaminated with the oxygen-inserted prod-
uct. A solution of 4-CN-C6H4CH2Co(dmestgH)2Py kept for
crystallization in air gave crystals which on X-ray analysis
showed it to be the oxygen-inserted product 4-CN-C6H4CH2-
(O2)Co(dmestgH)2Py (Figure 2).10 A clear reactivity difference
between alkyl- and benzylcobaloximes is observed. The results
are remarkable, since the alkyl cobaloximes, in general, are air
stable and do not readily decompose in solution. Also the
decomposition, if any, does not result in air-stable CoII. The
question is, how does this reactivity difference arise? Does the
weak interaction between the benzyl and the dioxime play any
role in the weakening of the Co-C bond?
Figure 2. Molecular structure of 4-CN-C6H4CH2(O2)Co-
(dmestgH)2Py.
∼17 kJ/mol on going from Me to benzyl;12 the thermal
decomposition shows different product formations in alkyl- and
benzylcobaloximes.13 Oxygen insertion, studied as a reliable
process to test the Co-C bond stability/reactivity, is much more
facile in benzylcobaloxime than in alkylcobaloxime.14 Ther-
molysis of benzyl-[Co] shows an intermolecular reaction
between freely diffusing radicals (PhCH2 and CoII[macrocycle]•)
•
and has revealed a selective recombination of these radicals
(99.999%) over bibenzyl (0.001%) formation.15 This is attributed
to the persistent radical effect; while the benzyl radical can
couple in a chain terminating step, the CoII radicals cannot. This
leads to buildup of the persistent radicals in solution and steers
the reaction to a single pathway in a highly selective fashion
(cage mechanism).
Recently, the benzyl group was shown to have a π-interaction
with the equatorial dioxime in many crystal structures, and it
is oriented over one of the dioxime wings and not over the
O-H‚‚‚O.16 Also, these interactions cause the nonequivalence
of the dioxime protons and the CH2 protons become diaste-
reotopic in 2-substituted benzylcobaloximes.16 The π interactions
can have a significant effect on the structure; for example, the
pyrazine-bridged alkyldicobaloxime has a staggered conforma-
tion, whereas the conformation switches over to eclipsed in the
benzyl analogue.17
In this context the present results showing the reactivity
difference between alkyl- and benzylcobaloximes are very
important. The tight interaction of the axial ligand with the
dioxime moiety activates the Co-C bond homolysis (steric cis
influence), and the reaction is accelerated by destabilization of
the reactant or electronic stabilization of the CoII homolysis
product by the dmestgH ligand.1
Schrauzer et al.11 in 1981 reported that benzylcobalamin
undergoes decomposition faster than bulky neopentylcobalamin
in solution and that this is not solely due to steric reasons; there
is an additional force that makes the benzyl-Co bond weaker.
There are many more instances where benzylcobaloximes are
shown to behave differently from the alkylcobaloximes: for
example, the Co-C bond dissociation energy (BDE) decreases
(9) Mandal, D.; Chadha, P.; Laskar, M.; Bhuyan, M.; Gupta, B. D.
Tetrahedron Lett. 2007, 48, 2377.
(10) Crystal data for Co(dmetgH)2(Py)2 (from DCM and methanol;
CCDC 251399): C50H56CoN6O4, Mr ) 863.94; monoclinic; space group
I2/a; a ) 12.128(2), b ) 31.999(9), c ) 12.130(2) Å; â ) 98.012(15)°; V
) 4662(3) Å3; Z ) 4; F ) 1.231 g cm-3; T ) 293(2) K; reflections
measured/unique 3213/3046; final R ) 0.0621 (Rw ) 0.1260); Enraf-Nonius
CAD4 Mach2 instrument using θ-2θ scan. Crystal data for 4-CN-
C6H4CH2(O2)Co(dmestgH)2Py‚C6H6‚2CH3CN (from DCM, acetonitrile
and benzene; CCDC 607226): C63H69CoN8O6, Mr ) 1093.19; triclinic;
space group P1h; a ) 14.2170(11), b ) 14.3381(11), c ) 16.6229(13) Å; R
) 112.3520(10), â ) 95.6900(10), γ ) 111.5940(10)°; V ) 2799.0(4) Å3;
Z ) 2; F ) 1.297 g cm-3; T ) 100(2) K; reflections measured/unique
18 867/13 342; final R ) 0.0748 (Rw ) 0.1041); Bruker CCD Detector
using æ-ω scan.
(12) (a) Toscano, P. J.; Brand, H.; Liu, S.; Zubieta, J. Inorg. Chem. 1990,
29, 2101. (b) Ng, F. T. T.; Rempel, G. L.; Mancuso, C.; Halpern, J.
Organometallics 1990, 9, 2762.
(13) (a) Brown, T. M.; Cooksey, C. J.; Dronsfield, A. T.; Fowler, J. H.
Inorg. Chim. Acta 1999, 288, 112. (b) Brown, T. M.; Dronsfield, A. T.;
Wilkinson, A. S. Inorg. Chim. Acta 1997, 262, 97. (c) Brown, T.; Dronsfield,
A.; Jablonski, A.; Wilkinson, A. S. Tetrahedron Lett. 1996, 37, 5413.
(14) Gupta, B. D.; Vijaikanth, V.; Singh, V. J. Organomet. Chem. 1998,
570, 1.
(15) Daikh, B. E.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 2938.
(16) (a) Mandal, D.; Gupta, B. D. Organometallics 2007, 26, 658. (b)
Mandal, D.; Gupta, B. D. Organometallics 2006, 25, 3305.
(17) Mandal, D.; Gupta, B. D. J. Organomet. Chem. 2005, 690, 3746.
(11) Schrauzer, G. N.; Grate, J. H. J. Am. Chem. Soc. 1981, 103, 541.

Communications
Organometallics, Vol. 26, No. 11, 2007 2797
Scheme 1. Plausible Cage Effect Mechanism of Oxygen Insertion Reaction
The formation of CoII as the end product in the alkylco-
baloximes points to its stabilization by the macrocyclic ligand.
The oxygen-inserted product is formed in the benzylcobaloxime
due to the stabilization of CoII by the macrocyclic ligand and
the formation of a stable benzyl radical, which remains intact
inside the cage by the interaction with the macrocyclic ligand.
This leads to buildup of the persistent radicals in solution and
steers the reaction in a highly selective manner. The very fact
that the dioxy complex is formed indicates that the benzyl group
is in the vicinity of the reaction center [CoII(O2)]. This can be
seen as a cage effect. However, there is a possibility that the
difference in reactivity may partly arise due to the difference
in the stability of the benzyl and alkyl radicals.
d is negative, R is very high, and τ deviates greatly from 90°.21
In contrast, d is always postive, R is low, and τ ≈ 90° in
cobaloximes with other dioximes (gH, dmgH, dpgH) (Support-
ing Information, Table S1). Benzyl analogues of dmestgH are
even more strained, as they are highly unstable in solution.
Interestingly, the strain in the molecule is released after the
oxygen insertion, as R (3.17°) is low and d (+0.011 Å) becomes
similar to those of other cobaloximes.
The crystal structure of 4-CN-C6H4CH2(O2)Co-(dmestgH)2-
Py is important, since only three structures of peroxoco-
baloximes have been reported in the literature.22 This is the first
crystal structure of a peroxo complex with a dioxime other than
dmgH that also has the Co(O2) unit attached to a primary carbon
(all of the three structures reported earlier are with dmgH and
Structural Aspects. CoII(dmetgH)2(Py)2. This is the first
crystal structure of an air-stable CoII(dioxime) complex. The
have Co(O2) bound to a secondary or tertiary carbon).22
A
earlier reported crystal structure of CoII(dmgH)2(Py)2 was
18
comparison of the molecular structure of 4-CN-C6H4CH2-
(O2)Co(dmestgH)2Py with that of cumyl(O2)Co(dmgH)2Py22a
shows that the Co-N (1.995(3) vs 1.994 Å) and Co-O
distances (1.896(2) vs 1.897 Å) are identical. The C-H‚‚‚π
interaction and orientation of Bn-O-O group in these two
systems are similar but not identical. Similar orientations of the
benzyl group and C-H‚‚‚π interactions have been observed in
the benzylcobaloximes ArCH2Co(dmgH)2Py (Supporting In-
formation, Figure S6 and S7). Such interactions should have
implications for the mechanism of the oxygen insertion in
organocobaloximes. The oxygen insertion rate data for Me,
n-Bu, and Bn Co(dmestgH)2Py complexes show kobs ) 2.5 ×
10-4, 4.5 × 10-4, and 5.0 × 10-2 s-1, respectively. The rates
were measured at 0 °C, and the insertion was over within 2
min in the case of benzyl. This suggests that the difference in
the rates in the methyl and butyl complexes is due to the
difference in the Co-C bond dissociation energies (BDE). In
contrast, the differences in the kobs values and in the product
formations in the butyl and benzyl complexes suggest that not
only are the BDE’s different but also the recombination step
must have some influence. This reactivity difference is similar
to that for the AdoCbl and MeCbl. Moreover, this is an
important input, since many mechanisms have been proposed
but no conclusive mechanism exists.14 Since the activation due
to the interactions between the equatorial and axial ligands and
substrate (O2) binding are the key factors for the homolysis of
the Co-C bond, in view of the stabilization of axial organic
radical a plausible mechanism can be written as in Scheme 1.
found to be very reactive toward molecular oxygen. The crystal
structure shows that both of the axial positions are occupied
by pyridine. The Co-Npy bond distance (2.050(4) Å) in
CoII(dmestgH)2 is considerably shorter as compared to that in
CoII(dmgH)2(Py)2 (2.25 Å). We are, at present, unable to provide
any explanation for this difference. The formation of the bis-
(pyridyl) complex, CoII(Py)2, indicates the rupture of the Co-C
bond followed by dissociation of base (pyridine). This seems
plausible, since pyridine is already in a strained position and is
loosely bound to cobalt in MeCo(dmetgH)2Py.8 This is very
much similar to the Co-C bond homolysis in AdoCbl, where
it shows a large geometric effect in the most flexible part of
the system Co-NIm bond (base-on and base-off).19
CoII(dmestgH)2(Py)2 has also been characterized by EPR, and
in the presence of air its solution shows an EPR spectrum similar
to that of the [CoII-O2]• radical and its oxygen binding is
reversible (Supporting Information, Figures S2 and S3). The
autoxidation is stopped due to the electronic and steric demands
of dimesitylglyoxime. The CoII(dmestgH)2(Py)2 catalyzes the
aerial oxidation of PPh3 to P(O)Ph3.
Cobalt(II) is a free radical initiator and has several important
applications.7,20 In general, CoII low-spin complexes are highly
air sensitive: for example, CoII(dmgH)2(Py)2 takes up oxygen
and is instantly autoxidized to CoIII, in contrast to the case for
CoII(dmestgH)2(Py)2. The autoxidation can be stopped by
modifying the dioxime moiety, CoII(dmgBF2)2.7
4-CN-C6H4CH2(O2)Co(dmestgH)2Py. In the previously
reported molecular structures of (Me/Cl/Br)Co(dmestgH)2Py,8
(18) Fallon, G. D.; Gatehouse, B. M. Cryst. Struct. Commun. 1978, 7,
263.
(21) d is the deviation of the cobalt atom from the mean equatorial N4
plane; the butterfly bending angle R is the dihedral angle between two
dioxime planes, and τ is the torsion angle between two planes, the axial
base pyridine and the plane that bisects the dioxime C-C bonds through
the cobalt atom. Positive signs for R and d indicate bending toward R and
displacement toward base and vice versa.
(22) (a) Giannotti, C.; Fontaine, C.; Chiaroni, A.; Riche, C. J. Organomet.
Chem. 1976, 113, 57. (b) Alcock, N. W.; Golding, B. T.; Mwesigye-
Kibende, S. J. Chem. Soc., Dalton Trans. 1985, 1997. (c) Chiaroni, A.;
Pascard-Billy, C. Bull. Soc. Chim. Fr. 1973, 781.
(19) Jensen, K. P.; Ryde, U. J. Am. Chem. Soc. 2005, 127, 9117.
(20) (a) Simandi, L. I.; Simandi, T. M.; May, Z.; Besenyei, G. Coord.
Chem. ReV. 2003, 245, 85. (b) Hu, X.; Cossairt, B. M.; Brunschwig, B. S.;
Lewis, N. S.; Peters, J. C. Chem. Commun. 2005, 4723. (c) Roberts, G. E.;
Barner-Kowollik, C.; Davis, T. P.; Heuts, J. P. A. Macromolecules 2003,
36, 1054. (d) Brown, T. M.; Cooksey, C. J.; Dronsfield, A. T.; Wilkinson,
A. S. Appl. Organomet. Chem. 1996, 10, 415 and references therein. (e)
Giese, B.; Hartung, J.; He, J.; Hu¨ter, O.; Koch, A. Angew. Chem., Int. Ed.
Engl. 1989, 28, 325.

2798 Organometallics, Vol. 26, No. 11, 2007
Communications
and CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org. Crystal data are also available from
the Cambridge Crystallographic Database as CCDC Nos. 251399
and 607226.
Acknowledgment. This work was supported by a grant from
the DST (SR/S1/IC-12/2004), New Delhi, India.
Supporting Information Available: Text, tables, and figures
giving EPR, crystal data comparison, supporting notes and figures,
OM070053Q