Spin Crossover in cis Manganese(III) Quinolylsalicylaldiminates

Two new Mn-II/III redox pairs, [Mn(qsal-Cl)(2)](0/+) 1(0/+) and [Mn(qsal-Br)(2)](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 MnCl2 and Hqsal-Cl and Hqsal-Br while oxidation of 1 or 2 with AgOTf yields the cationic Mn-III 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 Mn-III systems consistent with spin crossover and provide insight into the ligand design necessary for spin crossover in cis-N4O2 Mn-III compounds.