Homochiral Mn Spin-Crossover Complexes: A Structural and Spectroscopic Study.

Structural, magnetic, and spectroscopic data on a Mn spin-crossover complex with Schiff base ligand 4-OMe-Sal323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal323)]X where X = ClO (), BF (), NO (), Br (), and I () crystallize isotypically in the chiral orthorhombic space group 222 with a range of spin state preferences for the [Mn(4-OMe-Sal323)] complex cation over the temperature range 5-300 K. Complexes and are high-spin, complex undergoes a gradual and complete thermal spin crossover, while complexes and show stepped crossovers with different ratios of spin triplet and quintet forms in the intermediate temperature range. High-field electron paramagnetic resonance was used to measure the zero-field splitting parameters associated with the spin triplet and quintet states at temperatures below 10 K for complexes and with respective values: = +23.38(1) cm, = +2.79(1) cm, and = +6.9(3) cm, with a distribution of parameters for the = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex at room temperature reveal a 2:1 ratio of enantiomers in the chiral conglomerate, and solution CD measurements on the same sample in methanol show that it is stable toward racemization. Solid-state UV-vis absorption spectra on high-spin complex and mixed = 1/ = 2 sample reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal323)] in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal323 ligand.