Pseudo‐Octahedral Schiff Base Nickel(II) Complexes: Does Single Oxidation Always Lead to the Nickel(III) Valence Tautomer?

Abstract

AbstractWith the aim of establishing correlations between the ligand structure and the oxidation site in nickel complexes from Schiff base ligands, five ligands and their nickel complexes have been synthesized. The prototypical asymmetric 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] are 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 calculations 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] is characterized by an oxidation potential at –0.17 V vs. Fc+/Fc. The one‐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‐butyl instead of the methoxyl group) and exhibits an oxidation potential that is ca. 0.16 V higher. Compared to [Ni(L1)2]+ the cation [Ni(L2)2]+ exhibits a SOMO that is more localized on the metal atom. The EPR and electrochemical 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‐butyl 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 the cations [Ni(L4–5)2]+ are roughly similar to those of the pyridine analogs [Ni(L1–2)2]+. The oxidation site is thus not significantly affected by changes in the N‐donor properties of the terminal imino nitrogen atom.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)