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

Abstract

We report the syntheses, characterisations, and spin state behaviours of salts of the tripodal-ligated Fe(II) complex [FeL^6-OH]X₂ (L^6-OH = tris{4-[(6-methanol)-2-pyridyl]-3-aza-3-butenyl}amine, X = OTf⁻ (1), Br⁻ (2), I⁻ (3), BPh₄⁻ (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 (L² = 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.