Steric effect of the adamantyl ligand on crystal structure and dissociation rates in organocobalt B12 models

The synthesis and crystal structure analyses of two new complexes of the type [Co(Hdmg)2(ad) L], where Hdmg = monoanion of dimethylglyoxime and ad = adamantyl have been performed, with L =(NH2Ph)(1) or 4-dimethylaminopyridine (2). Complex (1) crystallizes in the triclinic space group P with a= 11.196(2), b= 15.616(4), c= 16.129(4)A, α= 70.28(2), β= 83.53(2), γ= 79.58(2)°, Z= 4, and R= 0.054 for 6 133 independent reflections. Complex (2) crystallizes in the monoclinic space group P21/c with a= 9.268(3), b= 15.178(2), c= 20.286(4)A, β= 90.31(2)°, Z= 4, and R= 0.055 for 3 595 independent reflections. Both compounds are characterized by long axial Co–N bonds 2.215(4)(1) and 2.102(3)A(2) and Co–C bonds 2.159(4)(1) and 2.160(4)A(2). The steric effect of the ad ligand is evident in the ‘butterfly’ bending of the Hdmg ligands toward L. In order to determine how such steric effects could be reflected in the rates of L dissociation and in Co–L distances, log k1 and Co–L bond lengths in series of the type [Co(Hdmg)2R(L)] with L = NH2Ph, 1,5,6-trimethylbenzimidazole, 4-cyanopyridine, or P(OMe)3 and R = Me, Et, Pri, or ad were analysed as a function of an electronic parameter for R derived from 13C n.m.r. data in a previous study. This analysis suggests that the steric effect of adamantyl on rates is mainly reflected in the transition state.