Thermal and light-induced spin-crossover in salts of the heptadentate complex [tris(4-{pyrazol-3-yl}-3-aza-3-butenyl)amine]iron(ii)

Abstract

The syntheses of [FeL][BF₄]₂·H₂O, [FeL][ClO₄]₂·H₂O, [FeL][NO₃]₂·CH₃NO₂ and [FeL][CF₃SO₃]₂ (L = tris(4-{pyrazol-3-yl}-3-aza-3-butenyl)amine) are described. The isostructural BF₄⁻ and ClO₄⁻ salts are high-spin between 5–300 K, while the other two compounds are high-spin at room temperature but undergo gradual high→low spin transitions upon cooling. For [FeL][NO₃]₂ this transition is centred at 139 K and proceeds to near-completeness, while for [FeL][CF₃SO₃]₂ it is centred at 144 K and only proceeds to 50% conversion. The CF₃SO₃⁻ salt also undergoes spin-crossover centred at 200 K in (CD₃)₂CO solution, and exhibits dynamic inversion of its helical ligand conformation. All these compounds except the triflate salt have been crystallographically characterised, and show capped trigonal antiprismatic [6 + 1] coordination geometries. The NO₃⁻ and CF₃SO₃⁻ salts undergo quantitative conversion to trapped, high-spin excited states upon irradiation with a green laser at 10 K (the LIESST effect; LIESST = Light-Induced Excited Spin State Trapping). The thermal stabilities of their LIESST excited states (T(LIESST) = 80–82 K) resemble those found for iron(II) complexes of bidentate N-heterocyclic ligands. Hence, the LIESST properties of [FeL]²⁺ are those of a complex of three rigid bidentate domains linked by a flexible spacer, rather than of a single encapsulating podand.