Spin crossover in cis manganese(III) quinolylsalicylaldiminates

Two new MnII/III redox pairs, [Mn(qsal-Cl)2]0/+ 10/+ and [Mn(qsal-Br)2]0/+ 20/+ have been synthesized employing the tridentate Schiff base ligands, 5-X-N-(8-quinolyl)salicylaldimine (Hqsal-X, X = Cl and Br). The neutral MnII complexes 1 and 2 were prepared from MnCl2 and Hqsal-Cl and Hqsal-Br while oxidation of 1 or 2 with AgOTf yields the cationic MnIII complexes, [Mn(qsal-Cl)2]OTf, 1+, and [Mn(qsal-Br)2]OTf, 2+. 1 and 2 have been characterized by single-crystal X-ray diffraction as CH2Cl2 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, Mn2+/3+ and Mn3+/4+. Spectroscopic studies show the LMCT bands move to lower wavelengths by ca. 30 nm while the ?C=N 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 MnIII systems consistent with spin crossover and provide insight into the ligand design necessary for spin crossover in cis-N4O2 MnIII compounds. We report the redox pairs [Mn(qsal-X)2]0/+ in which the MnIII cations exhibit spin crossover despite the cis O donors.