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

Irina A Kühne,Stephen Hill,Paul Wix,Helge Müller-Bunz,Mooneerah Heerah-Booluck,Anthony B Carter,Grace G Morgan,Andrew Ozarowski,Solveig Felton,Aoife Casey,Tony D Keene,Aizuddin Sultan,Kane Esien

doi:10.1021/acs.inorgchem.1c03379

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– (1), BF4– (2), NO3– (3), Br– (4), and I– (5) crystallize isotypically in the chiral orthorhombic space group P21212 with a range of spin state preferences for the [Mn(4-OMe-Sal2323)]+ complex cation over the temperature range 5–300 K. Complexes 1 and 2 are high-spin, complex 4 undergoes a gradual and complete thermal spin crossover, while complexes 3 and 5 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 4 and 2 with respective values: DS=1 = +23.38(1) cm–1, ES=1 = +2.79(1) cm–1, and DS=2 = +6.9(3) cm–1, with a distribution of E parameters for the S = 2 state. Solid-state circular dichroism (CD) spectra on high-spin complex 1 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 1 and mixed S = 1/S = 2 sample 5 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.

Highlights

Manipulation of the internal electronic arrangement in spincrossover (SCO) complexes,1−5 with the attendant changes in magnetic,6−8 optical,9−14 and electric properties,7,15−22 constitutes one of the most versatile ways to build switchable molecular magnets
High-field EPR (HFEPR) spectra were recorded at low temperatures (∼10 K) on polycrystalline powder samples of compounds 2 and 4 in order to characterize the zero-field splitting (ZFS) parameters associated with the HS and LS species, respectively
HFEPR was used to estimate the magnitude and sign of the axial D parameter in both spin states by recording low-temperature variable-frequency spectra on complexes 2 and 4. This confirmed that the spin quintet form is axially compressed with a D value of +6.9(3) cm−1 which increased to D = +23.38(1) cm−1 in the spin triplet form

Summary

Introduction

Manipulation of the internal electronic arrangement in spincrossover (SCO) complexes,1−5 with the attendant changes in magnetic,6−8 optical,9−14 and electric properties,7,15−22 constitutes one of the most versatile ways to build switchable molecular magnets. We use low-temperature multifrequency EPR spectroscopy to establish the magnitude and sign of the axial D parameter in the spin quintet and triplet forms of the [Mn(4OMe-Sal2323]+ complex cation105 when it is crystallized in BF4− and Br− lattices, respectively complexes 2 and 4 in Scheme 1.

Results

Conclusion