Homochiral Mn³⁺ Spin-Crossover Complexes: A Structural and Spectroscopic Study

Abstract

Structural, magnetic, and spectroscopic data on a Mn³⁺ spin-crossover complex with Schiff base ligand 4-OMe-Sal₂323, isolated in crystal lattices with five different counteranions, are reported. Complexes of [Mn(4-OMe-Sal₂323)]X where X = ClO₄^- (<b>1</b>), BF₄^- (<b>2</b>), NO₃^- (<b>3</b>), Br^- (<b>4</b>), and I^- (<b>5</b>) crystallize isotypically in the chiral orthorhombic space group <i>P</i>2₁2₁2 with a range of spin state preferences for the [Mn(4-OMe-Sal₂323)]⁺ complex cation over the temperature range 5-300 K. Complexes <b>1</b> and <b>2</b> are high-spin, complex <b>4</b> undergoes a gradual and complete thermal spin crossover, while complexes <b>3</b> and <b>5</b> 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 <b>4</b> and <b>2</b> with respective values: <i>D</i>_<i>S</i>₌₁ = +23.38(1) cm^-1, <i>E</i>_<i>S</i>₌₁ = +2.79(1) cm^-1, and <i>D</i>_<i>S</i>₌₂ = +6.9(3) cm^-1, with a distribution of <i>E</i> parameters for the <i>S</i> = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex <b>1</b> 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 <b>1</b> and mixed <i>S</i> = 1/<i>S</i> = 2 sample <b>5</b> reveal different intensities at higher energies, in line with the different electronic composition. The statistical prevalence of homochiral crystallization of [Mn(4-OMe-Sal₂323)]⁺ in five lattices with different achiral counterions suggests that the chirality may be directed by the 4-OMe-Sal₂323 ligand.