Structure–function correlations in mononuclear manganese(iii) spin crossover systems with a big conjugated hexadentate Schiff-base ligand

Abstract

This paper reports the syntheses, crystal structures and magnetic properties of spin crossover (SCO) salts of formulae [Mn(naphth-sal-N-1,5,8,12)]SbF₆ (1), [Mn(naphth-sal-N-1,5,8,12)]AsF₆ (2), [Mn(naphth-sal-N-1,5,8,12)]PF₆·1/2CH₃OH (3) and [Mn(naphth-sal-N-1,5,8,12)]ClO₄ (4), where (naphth-sal-N-1,5,8,12)²⁻ (2,2′-((1E,14E)-2,6,10,14-tetraazapentadeca-1,14-diene-1,15-diyl)diphenolate) is a big conjugated hexadentate Schiff-base ligand. Magnetic susceptibility measurements demonstrated that complexes 1 and 2 showed a gradual one-step SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis. The transition temperatures T_1/2 of the SbF₆ (1) and AsF₆ (2) salts estimated from the magnetic susceptibility measurements are 164 and 171 K, respectively. The existence of the crystal solvent of complex 3 changes the supramolecular packing, leading to close π⋯π stacking interactions between the phenyl groups of the ligands. These close stacking interactions hinder the flexibility of the whole ligand, precluding the spin transformation of complex 3 and leading to its stabilization in the HS state in the temperature range of 2–300 K. For 4, crystal structure analysis indicates that the reduction in the anion size leads to close contacts between the naphthalene rings. These C–H⋯π interactions provide a means for preventing the spin crossover occurring at low temperatures.