Département de chimie moléculaire

With the aim of establishing correlations between the ligand structure and the oxidn. site in nickel complexes from Schiff base ligands, derived from 2-picolylamine/aminoquinoline and 2-tert-butyl-6-formyl-4-methoxyphenol/2,4-di(tert-butyl)-6-formyl-phenol, five ligands and their nickel complexes were synthesized. The prototypical asym. Schiff base ligand HL1 contains both phenol and pyridine pendant arms with a pivotal imine nitrogen atom. Ligands HL2-5 differ from HL1 by either their phenolate para substituent, the hybridization of the pivotal nitrogen atom, and/or the N-donor properties of the pyridine moiety. The five complexes [Ni(L1-5)2] were obtained by treating the corresponding ligands with 0.5 equiv of Ni(OAc)2·4H2O in the presence of NEt3. X-ray crystal-structure diffraction studies as well as DFT calcns. reveal that [Ni(L1-5)2] involves a high-spin nickel(II) ion within a pseudo-octahedral geometry. The two ligands are arranged in a meridional fashion when the pivotal nitrogen atom is an imine {\{}as in [Ni(L1-2)2] and [Ni(L4-5)2]{\}}, while the fac isomer is preferred in [Ni(L3)2] (amino pivotal nitrogen atom). [Ni(L1)2] was characterized by an oxidn. potential at -0.17 V vs. Fc+/Fc. The 1-electron-oxidized species [Ni(L1)2]+ exhibits an EPR signal at g = 2.21 attributed to a phenoxyl radical that is antiferromagnetically coupled to a high-spin NiII ion. [Ni(L2)2] differs from [Ni(L1)2] by the phenolate para substituent (a tert-Bu instead of the methoxyl group) and exhibits an oxidn. potential that is ∼0.16 V higher. Compared to [Ni(L1)2]+, [Ni(L2)2]+ exhibits a SOMO that is more localized on the metal atom. The EPR and electrochem. signatures of [Ni(L3)2]+ are similar to those of [Ni(L1)2]+, thus showing that an imino to amino substitution compensates for a methoxy to tert-Bu one. Replacement of the pyridine by a quinoline group in [Ni(L4-5)2] makes the complexes slightly harder to oxidize. The EPR signatures of [Ni(L4-5)2]+ are roughly similar to those of the pyridine analogs [Ni(L1-2)2]+. The oxidn. site is thus not significantly affected by changes in the N-donor properties of the terminal imino nitrogen atom. [on SciFinder(R)]