Coordination versatility of tridentate pyridyl aroylhydrazones towards iron: tracking down the elusive aroylhydrazono-based ferric spin-crossover molecular materials

Abstract

The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL¹) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL²) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe^II(HL)₂](ClO₄)₂, [Fe^IIL₂] and [Fe^IIIL₂]ClO₄ (HL = HL¹ or HL²) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe^III(L¹)₂]ClO₄ is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe^III(L¹)₂]ClO₄ and [Fe^III(L²)₂]ClO₄ are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the ⁶A₁ ↔ ²T₂ transition curve is sigmoidal with T_½ ∼280 K and the SCO is virtually complete. As regards [Fe^III(L¹)₂]ClO₄, Mössbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.