Spin transitions in a series of [Fe(pybox)2]2+ complexes modulated by ligand structures, counter anions, and solvents

Abstract

Herein, we investigate the spin transition properties in a series of mononuclear Fe(II) complexes constructed by two pybox ligands and four counter anions. 1(BF₄) undergoes an abrupt spin-transition at around 150 K, which proceeds to 50% completeness and is accompanied by a structural phase transition. 1(BPh₄)·MeCN, 2(ClO₄), and 2(BF₄) all display gradual and complete SCO behaviours around or above ambient temperature. The desolvated compound 1(BPh₄) exhibits a gradual but incomplete SCO. 2(PF₆)·MeCN, 2(PF₆) and 2(BPh₄) show a two-step SCO. The well-defined two-step SCO of 2(BPh₄) takes place at 100–200 K (T_1/2 = 150 K) and 400–500 K (T_1/2 = 446 K). In contrast, compounds 1(BF₄)·MeCN, 1(PF₆)·MeCN·Et₂O and 1(PF₆) remain high-spin at the measured temperatures. The LIESST (light induced excited spin state trapping) effect of several SCO compounds are also investigated, which display photo-conversion during light irradiation at low temperature at a relatively high level. Our results demonstrate that the [Fe(pybox)₂]²⁺ series of complexes will become a new versatile system in spin-crossover research.