Cooperativity in spin crossover materials as ligand's responsibility – investigations of the Fe(ii) – 1,3-bis((1H-tetrazol-1-yl)methyl)bicyclo[1.1.1]pentane system

Abstract

Criteria for a technologically relevant spin crossover (SCO) material include temperature and abruptness. A series of Fe(II) – 1,3-bis((1H-tetrazol-1-yl)methyl)bicyclo[1.1.1]pentane SCO complexes with various anions (BF₄⁻, ClO₄⁻, and PF₆⁻) designed using a structure–property based concept is reported. All complexes feature abrupt SCO-behavior with T_1/2 between 170 K and 187 K. These materials demonstrate that without stabilizing the effects of incorporated solvents or a hydrogen bond-network, the observed cooperativity during high-spin–low-spin transition is anion independent and originates only from the rigidity and internal strain of the propellane-moiety in the ligand. Spectroscopy and structural investigations of these materials are supported by quantum chemical calculations.