Abstract

Abstract Two new Mn^II/III redox pairs, [Mn(qsal-Cl)<inf>2</inf>]^0/+ 1^0/+ and [Mn(qsal-Br)<inf>2</inf>]^0/+ 2^0/+ have been synthesized employing the tridentate Schiff base ligands, 5-X-N-(8-quinolyl)salicylaldimine (Hqsal-X, X = Cl and Br). The neutral Mn^II complexes 1 and 2 were prepared from MnCl<inf>2</inf> and Hqsal-Cl and Hqsal-Br while oxidation of 1 or 2 with AgOTf yields the cationic Mn^III complexes, [Mn(qsal-Cl)<inf>2</inf>]OTf, 1⁺, and [Mn(qsal-Br)<inf>2</inf>]OTf, 2⁺. 1 and 2 have been characterized by single-crystal X-ray diffraction as CH<inf>2</inf>Cl<inf>2</inf> solvates. The Mn centres adopt a strongly distorted octahedral geometry with cis O donors due to the pair of meridionally bound qsal-X ligands. Electrochemical studies indicate two reversible one-electron redox processes, Mn^2+/3+ and Mn^3+/4+. Spectroscopic studies show the LMCT bands move to lower wavelengths by ca. 30 nm while the ν<inf>C=N</inf> stretches are little changed in the cationic complexes. Variable temperature magnetic susceptibility measurements indicate that 1⁺ and 2⁺ undergo gradual half spin crossover, despite having cis O donors. DFT calculations reveal a small HS-LS gap in the Mn^III systems consistent with spin crossover and provide insight into the ligand design necessary for spin crossover in cis-N<inf>4</inf>O<inf>2</inf> Mn^III compounds. We report the redox pairs [Mn(qsal-X)<inf>2</inf>]^0/+ in which the Mn^III cations exhibit spin crossover despite the cis O donors.