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)nL (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 1H-NMR spectra are compared with those of the corresponding RCo(DH)2−n(DBPh2)nL and RCo(DBF2)2L 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. 1H-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)2H2O 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.