Photolysis Secondary Products of Cobaloximes
Organometallics, Vol. 24, No. 14, 2005 3501
in a series of alkylcobaloximes and imino/oxime com-
pounds, with nitrogen4,17 and phosphorus donor lig-
ands.4b,18 Most of this work has been performed with
cobalt(III) compounds, and much less information exists
on the structure and reactivity of the cobalt(II) species
formed upon homolysis of the cobalt-carbon bond, a fact
that can be traced to (a) the lability of cobalt(II) com-
pounds, which renders greater complexity to their chem-
istry, and (b) the great difficulty in obtaining crystals
of diffractometric quality for cobalt(II) compounds with
relevance to model systems in B12 chemistry.
intermediates of cyanocobalamin and adenosylcobal-
amin observed during catalysis. EPR and UV/vis spec-
troscopies have been used to get information on the
electronic structure of cobalt(II) intermediates.26
Homolysis of the cobalt-carbon bond in naturally
occurring cobalamines and in model compounds is
induced by photolysis and thermolysis, and it is believed
that the products of these reactions are identical to those
found during the enzymatic processes in which cobal-
amines are present.3,4a Accordingly, information on the
structure and reactions in which the cobalt(II) species
are involved may be achieved by studying photolysis and
thermolysis of B12 compounds and related models.21,27-33
For B12 model compounds several studies concerning
anaerobic photolysis in protic and aprotic solvents of
alkylcobaloximes with nitrogen18-32 and phosphorus33
donor bases have appeared in the literature and more
recently a study of photolysis of methylaquacobinamide
in the presence of N-donor ligands, which impose
variable electronic and steric effects,21 although no work
on photolysis of imino/oxime compounds has been
reported.
An X-ray structure of Co(II) B12 coenzyme has been
reported,25 and XAFS solution studies have provided
chemical and structural information on Co(II) and Co(I)
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The results obtained point, in general, to light-induced
homolytic cleavage of the cobalt-carbon bond, although
exceptions have been found for some methyl and benzyl
cobaloxime derivatives for which homolysis of the
cobalt-carbon bond does not occur at all irradiation
temperatures, reflecting the stronger Co-C bonds present
in those componds.31 Photolysis products of alkylco-
baloximes with symmetric phosphorus donor bases have
been shown to be independent of the alkyl group and of
the temperature of irradiation. Photolysis induces in all
cases homolytic cleavage of the cobalt-carbon bond and
originates as products alkyl radicals and cobalt(II) five-
coordinate species, which are not involved in subsequent
reactions in the temperature range 77-300 K.33 For
alkylcobaloximes with nitrogen bases, the same is not
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The purpose of this work is to characterize the
products of anaerobic photolysis in aprotic solvents of
alkylpyridinecobaloximes and similar imino/oxime com-
pounds (Figure 1). We have included imino/oxime
compounds with triphenylphosphine to be compared
with previous work on cobaloximes and also to confirm
deformations induced by bulky ligands.
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