para-Substituted diphenylborylated organocobaloximes:: effects of substituents on conformation and redox properties

Some new derivatives of organocobaloximes containing para-substituted diphenylboryl groups, RCo(DH)(2-n)(DB(p-XPh)(2))(n)L (R = alkyl or aryl group, L = N-MeIm, Py or H2O, X = OCH3, CH3 or Cl, n = 1 or 2) have been synthesized. The X-ray structures and the H-1-NMR spectra are compared with those of the corresponding RCo(DH)(2-n)(DBPh2)(n)L and RCo(DBF2)(2)L complexes. The insertion of X groups in the phenyl rings does not significantly affect the equatorial Co-N distances, whereas the Co-Py distances increase slightly in the order (DB(p-OCH3Ph)(2))(2) < (DB(p-ClPh)(2))(2) < (DBF2)(2). H-1-NMR spectra suggest that the conformational distribution in solution is similar to that observed in the corresponding BPh2 derivatives. Electrochemical studies on the corresponding MeCo(DB(p-XPh)(2))(2)H2O compounds show a mono-electron Co(III)/Co(II) transfer reaction followed by two parallel reactions: (a) mono-electron Co(II)/Co(I) transfer; (b) homolytic dissociation of the Co-C bond with the formation of Co(I) species, the relative rates of the two processes being dependent on X. As the electron-withdrawing power of the equatorial ligand increases, the reduction potentials associated with both Co(III)/Co(II) and Co(II)/Co(I) processes shift towards more positive values, indicating a decrease of electron density on the Co atom. The effects are comparable with those observed by changing the axial ligands. (C) 2000 Elsevier Science S.A. All rights reserved.