Anion dependence in the spin-crossover properties of a Fe(ii) podand complex

摘要

We report the syntheses, characterisations, and spin state behaviours of salts of the tripodal-ligated Fe(ii) complex [FeL ~(6-OH)]X _2 (L ~(6-OH) = tris{4-[(6-methanol)-2-pyridyl]-3-aza-3-butenyl}amine, X = OTf ~- (1), Br ~- (2), I ~- (3), BPh _4 ~- (4)). Covalent linking of the ligand arms is imperative as a high-spin bis(tridentate) complex (5) is formed when a non-tethered ethyl iminopyridine ligand (L ~2 = 4-[(6-methanol)-2-pyridyl]-3-aza-3- butenyl) is used. For salts 1-4, thermally-induced spin-crossover (SCO) is observed in the solid state, with dependence on anion and solvate molecules. Salts with larger anions show more complete SCO centred at higher temperatures (1 > 3 > 2); the triflate salt 1 (T _(1/2) = 173 K) also shows the strongest cooperativity of the compounds examined. Hydrogen bonding appears to be critical to SCO in this family of salts: limiting interactions by use of tetraphenylborate produces a high-spin complex down to 5 K. In protic solvents such as methanol, spectra of [FeL ~(6-OH)] ~(2+) are largely unchanged over a period of three days, but dissociate when interrogated with strong field bidentate ligands. Compounds 1-3, and 5 remain high spin in solution down to 180 K, consistent with the data obtained in the solid state.