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 = -CH(2)X (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 approximate to 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 eta(2)-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 degrees) 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 (theta-bending of the N-Co-C angle and phi-bending of the Co-C-C angle), the latter factor should be more significant in weakening the Co-C bond. There is large phi-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.