Influence of Hydrogen Bonding on the Hysteresis Width in Iron(II) Spin‐Crossover Complexes

A detailed investigation on the properties of the high-temperature and low-temperature modifications of the iron(II) spin-crossover complex [FeL1(HIm)(2)] (1) and its isotopic deuterium-labelled analogue [FeL1(DIm)(2)] (1D), and the pair [FeL2(azpy)]center dot MeOH/[FeL2(azpy)]center dot CD3OD/[FeL2(azpy)] (2 center dot MeOH/2 center dot CD3OD/2), in which L1 and L2 are tetradentate N2O22--coordinating Schiff base like ligands, L1 = {diethyl (E, E)-2,2'-[1,2-phenylbis(iminomethylidyne)]bis[3-oxobutanoate](2-)-N,N',O-3,O-3'}, L2 = {2,2'-[1,2-phenylbis(iminomethylidyne)] bis[1-phenylbutane-1,3-dione](2-)-N,N',O-3,O-3'}, HIm = imidazole, and azpy = 4,4'-azopyridine, is presented. All complexes except 2 show a cooperative spin transition with hysteresis widths between approximately 5 K (1(LT) and 1(LT)D), 70 K (1(HT[9]) and 1(HT)D, around room temperature), and 80 K (2 center dot MeOH and 2 center dot CD3OD). In all cases, an influence of the H/D exchange on the transition temperature and the hysteresis width is observed. For 1(HT), first-order reversal curves (FORCs) have been recorded, and a statistical analysis gives the interaction parameter J = 560 K, indicating strong intermolecular interactions. X-ray structural analysis of the different samples (1(HT[9]) and 1(HT)D: HS; 1(LT) and 1(LT)D: LS; and 2 center dot MeOH and 2 center dot CD3OD: HS) gives a deeper insight into the molecular packing in the crystals and helps explain the increase of cooperative interactions during the spin transition. In all cases, one hydrogen bond involves an oxygen atom of the Schiff base like ligand that serves as a donor for the iron centre. The influence of this hydrogen bond on the ligand field strength of the iron centre is discussed and a new model is developed to explain the observed connection between hydrogen bonds and exceptionally wide hysteresis loops for the complexes presented in this work and other examples from the literature.