Cooperative effects of ligands and Counteranions in mononuclear Manganese(III) Schiff-base spin crossover complexes.

• A series of three new mononuclear manganese(III) Schiff-base complexes. • The change of substituents or anions affects the spin crossover (SCO) properties. • Magnetic measurements provided firm evidence for the cooperative SCO effects in complexes 2 and 3 , while complex 1 persists in the HS state. • Complex 3 exhibits a surprisingly gradual SCO process with a hysteresis width of 6 K at lower temperature. The flexible hexadentate Schiff-base ligands (R-sal-N-1,5,8,12)2− can be easily obtained by modifying the different substituents at the 5 or 3,5 positions on the phenyl rings of the salicylaldehyde. We report in this paper the syntheses, crystal structures and magnetic properties of three new mononuclear manganese(III) complexes [Mn(R-sal-N-1,5,8,12)]Y (R = 5-H, Y = BF 4 (1); R = 3,5-diCl, Y = SbF 6 (2); R = 5- t Bu, Y = ClO 4 (3)). All the complexes have an analogous monocationic MnN 4 O 2 surrounding offered by the flexible hexadentate ligand in a distorted octahedral geometry. Magnetic susceptibility measurements demonstrated that complex 1 maintains a high-spin (HS) state between 2 and 400 K due to the closing packing, while complex 2 shows a cooperative and complete one-step thermo-induced spin state switching with T 1/2 of 194 K. As for complex 3 , single-crystral X-ray structural studies show the existence of a large amount of HS manganese(III) ions at 100 K and it exhibits a surprisingly gradual SCO process with a hysteresis width of 6 K at lower temperature. [ABSTRACT FROM AUTHOR]