Effect of the counter-anion on the spin-transition properties of a family of Fe(II) tetrazole complexes, [Fe(i4tz)6]X2 (X = ClO4-, PF6-, SbF6-, BF4-).

The monodentate ligand 1-isobutyl-1H-tetrazole (i4tz) was used to synthesize a series of iron(II) spin-crossover (SCO) complexes with the general formula [Fe(i4tz)6]X2, where X = ClO4(-) (1), PF6(-) (2), SbF6(-) (3), or BF4(-) (4). Upon cooling, magnetic susceptibility measurements of compounds 1, 2 and 3 show a reversible one-step spin crossover (SCO) behaviour between HS (S = 2) and LS (S = 0) states without hysteresis. The ClO4(-) compound shows a gradual spin transition at T(1/2) = 208 K at a considerably higher temperature than the other compounds. The larger anions PF6(-) (T(1/2) = 114 K) and SbF6(-) (T(1/2) = 116 K) show more gradual transitions, and a "frozen-in effect" was observed after fast cooling. The crystal structure of [Fe(i4tz)6](SbF6)2 (3) was determined at 296, 200 and 100 K, revealing a reversible first-order phase transition between 200 and 100 K, accompanied by a triplication of the unit cell volume at 100 K. Temperature-dependent vibration and electronic spectroscopic measurements of compound 1 were compared with the magnetic susceptibility measurements.