Extension of Azine-Triazole Synthesis to Azole-Triazoles Reduces Ligand Field, Leading to Spin Crossover in Tris-L Fe(II).

The first examples of -triazole R ligands, bidentate (3-(1-methyl-1-imidazol-4-yl)-5-phenyl-4-(-tolyl)-4-1,2,4-triazole) and (4-(5-phenyl-4-(-tolyl)-4-1,2,4-triazol-3-yl)thiazole), have been prepared, by extension of the general synthesis used to access many examples of -triazoles. The tris-L Fe complexes of the azine-triazoles are consistently low spin (LS). As intended, these new -triazole ligands provide lower field strengths, resulting in high-spin (HS) [Fe()](BF) (·4HO) and spin crossover (SCO) active [Fe()](BF) (·0.5HO). Single-crystal structure determinations revealed that at 100 K ·solvents is HS whereas ·solvents is LS. Solid-state variable temperature magnetic studies of air-dried crystals showed that the methylimidazole-triazole complex ·4HO remains HS while the thiazole-triazole complex ·0.5HO undergoes a two-step gradual SCO ( approximately 275 and 350 K). Variable-temperature Evans method NMR studies of , in five different solvents (CDNO, CDCN, CDCOCD, CDCl, and CDCl) gave values in a relatively narrow range, 214-259 K. These values did not correlate with the solvent polarity index ' ( = 0.25) but did correlate with the solvent basicity parameter SB ( = 0.90). Variable-temperature UV-vis studies on a golden yellow CHCN solution of , with monitoring of the d-d transition at 530 nm (ε = 39 L mol cm at 293 K) while the solution was heated from 253 to 303 K, showed that the high-spin fraction increased from 0.51 to 0.77. Cyclic voltammetry studies in CHCN revealed a Fe(III)/Fe(II) redox process that was reversible for and irreversible for , with significant tuning of the value: the methylimidazole-triazole complex is significantly easier to oxidize (0.46 V) than the thiazole-triazole complex (0.68 V; both vs 0.01 M Ag/AgNO).