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

The first examples of azole-triazole Rat ligands, bidentate L4(NMeIm)(3-(1-methyl-1H-imidazol-4-yl)-5-phenyl-4-(p-tolyl)-4H-1,2,4-triazole) and L-4sIm (4-(5-phenyl-4-(p-tolyl)-4H-1,2,4-triazol-3-yl)thiazole), have been prepared, by extension of the general synthesis used to access many examples of azine-triazoles. The tris-L Fe-II complexes of the azine-triazoles are consistently low spin (LS). As intended, these new azole-triazole ligands provide lower field strengths, resulting in high-spin (HS) [Fe-II(L-4NMeIm)(3)] (BF4)(2) (1 center dot 4H(2)O) and spin crossover (SCO) active [Fe-II(L-4sIm)(3)](BF4)(2) (2 center dot 0.5H(2)O). Single-crystal structure determinations revealed that at 100 K 1 center dot solvents is HS whereas 2 center dot solvents is LS. Solid-state variable temperature magnetic studies of air-dried crystals showed that the methylimidazole-triazole complex 1 center dot 4H(2)O remains HS while the thiazole-triazole complex 2 center dot 0.5H(2)O undergoes a two-step gradual SCO (T-1/2 approximately 275 and 350 K). Variable-temperature Evans method NMR studies of 2, in five different solvents (CD3NO2, CD3CN, CD3COCD3, CD2Cl2, and CDCl3) gave T-1/2 values in a relatively narrow range, 214-259 K. These T-1/2 values did not correlate with the solvent polarity index P' (R-2 = 0.25) but did correlate with the solvent basicity parameter SB (R-2 = 0.90). Variable-temperature UV-vis studies on a golden yellow CH3CN solution of 2, with monitoring of the d-d transition at 530 nm (epsilon = 39 L mol(-1) cm(-1) 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 CH3CN revealed a Fe(III)/Fe(II) redox process that was reversible for 1 and irreversible for 2, with significant tuning of the E-pa value: the methylimidazole-triazole complex 1 is significantly easier to oxidize (0.46 V) than the thiazole-triazole complex 2 (0.68 V; both vs 0.01 M Ag/AgNO3).