Novel Chemistry of Carbon Bound to Cobalt in Organocobalt Complexes Related to B12

A rare Co−N−C three-membered ring has been formed by a novel
pathway involving a facile interligand
nucleophilic addition of an equatorial nitrogen donor to a ligated
axial carbon in some organocobalt complexes.
The highly distorted structure of a typical resulting complex
provides clear experimental evidence useful in assessing
hypotheses and computations concerning Co−C bond activation in B12
chemistry. Treatment of R = −CH2X (X
= halogen) derivatives of imine/oxime B12 models of the type
[LCo((DO)(DOH)pn)R]+ with base afforded
major
products with striking NMR spectral features in common, e.g., two
one-proton olefinic doublets (J ≈ 3 Hz) and
two
one-proton singlets. The X-ray structure for the pyridine (py)
derivative
[pyCo(N-CH2-CHEL)]+
(1) reveals two
unexpected features:  a three-membered metallocycle containing an
η2-aminomethylene group, and an enamine (N−CCH2) in place of one imine moiety
(NC−CH3). The C−Co−N ring angle is acute
(43.7°) with the distortion
of the coordination sphere concentrated in the Co−C and the Co−N
ring linkages, which move away from the
normal pseudooctahedral positions. Studies of the formation of the
aqua analog of 1,
[H2OCo(N-CH2-CHEL)]+
(3),
in aqueous solution suggest that the initial intermediate formed is
deprotonated at one imine methyl. Ring closure
in the short-lived deprotonated intermediate is facile. Published
calculations predict that such ring closure could
occur and that, of two processes which could facilitate Co−C bond
cleavage (θ-bending of the N−Co−C angle and
φ-bending of the Co−C−C angle), the latter factor should be more
significant in weakening the Co−C bond. There
is large θ-bending in 1; thus, the new metallocyclic B12
analogs afford the first experimental test of these
calculations.
Our findings that the Co−C bond in 1 is short and
that the bond did not readily cleave support the predictions of
the
calculations.