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

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.