Impact of counter anions on spin-state switching of manganese(III) complexes containing an azobenzene ligand.

Four mononuclear manganese(III) complexes coordinated with photo-active hexadentate azobenzene ligands, [Mn(5azo-sal 2 -323)](X) (X = Cl, 1; X = BF 4 , 2; X = ClO 4 , 3; X = PF 6 , 4), were prepared. The impact of various counter anions on the stabilization and switching of the spin state of the manganese(III) center was explored through detailed magneto-structural investigation using variable temperature single-crystal X-ray diffraction, magnetic, spectroscopic, and spectroelectrochemical studies, along with theoretical calculations. All four complexes consisted of an isostructural monocationic distorted octahedral MnN 4 O 2 coordination environment offered by the hexadentate ligand and Cl - , BF 4 - , ClO 4 - , and PF 6 - as counter anions respectively. Complex 1 with a spherical Cl - counter anion showed a reversible and gradual spin-state switching between low-spin (LS) ( S = 1) and high-spin (HS) ( S = 2) states above 400 K, where non-covalent cation-anion interactions played a significant role in stabilizing the LS state. While, irrespective of the shape of the counter anion, complexes 2-4 remained in the HS state throughout the measured temperature range of 300-2 K, where strong π-π interaction between the azobenzene motifs among cationic units played a substantial role in stabilizing the HS state. Furthermore, magnetic data analyses revealed significantly large zero-field splitting in the S = 1 state for 1 ( D = 19.4 cm -1 , E / D = 0.008) in comparison with that in the S = 2 state for 2-4 ( D = 3.99-4.97 cm -1 , E / D = 0.002-0.195). Spectroelectrochemical investigations revealed the quasi-reversible reduction and oxidation of the manganese(III) center to manganese(II) and manganese(IV), respectively. A detailed theoretical calculation at the DFT and CASSCF level of theory was carried out to better understand the magneto-structural correlation.