Solvent-Induced Polymorphism of Iron(II) Spin Crossover Complexes

摘要

Two new mononuclear iron(II) compounds ( 1 ) and ( 2 ) of the general formula [Fe( L ) 2 ](BF 4 ) 2 ·nCH 3 CN ( L = 4-(2-bromoethyn-1-yl)-2,6-bis(pyrazol-1-yl)pyridine, n = 1 for ( 1 ) and n = 2 for compound ( 2 )), were synthesized. The room temperature crystallization afforded concomitant formation of two different solvent analogues: compound ( 1 ) exhibiting triclinic P-1 and compound ( 2 ) monoclinic C2/c symmetry. Single-crystal X-ray studies confirmed the presence of the LS (low-spin) state for both compounds at 180 K and of the HS (high-spin) state for compound ( 2 ) at 293 K, in full agreement with the magnetic investigations for both solvent polymorphs. Compound ( 1 ) exhibits spin transition above 293 K followed by subsequent solvent liberation, while the spin transition of ( 2 ) takes already place at 237 K. After complete solvent removal from the crystal lattice, compound ( 1d ) (the desolvated polymorph derived from ( 1 )) exhibits spin transition centered at 342 K accompanied by a thermal hysteresis loop, while the analogous compound ( 2d ) (the desolvated derivate of compound ( 2 )) remains blocked in the HS state over all the investigated temperature range.