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 compounds1, 2 and3 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 compound1 were compared with the magnetic susceptibility measurements.