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 L4NMeIm(3 (1 methyl 1H imidazol 4 yl) 5 phenyl 4 (p tolyl) 4H 1,2,4 triazole) and L4SIm (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 FeII 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) [FeII(L4NMeIm)3](BF4)2 (1*4H2O) and spin crossover (SCO) active [FeII(L4SIm)3](BF4)2 (2*0.5H2O) Single crystal structure determinations revealed that at 100 K 1*solvents is HS whereas 2*solvents is LS. Solid state variable temperature magnetic studies of air dried crystals showed that the methylimidazole triazole complex 1*4H2O remains HS while the thiazole triazole complex 2*0.5H2O undergoes a two step gradual SCO (T1/2 approximately 275 and 350 K) Variable temperature Evans method NMR studies of 2, in five different solvents (CD3NO2, CD3CN, CD3COCD3, CD2Cl2, and CDCl3) gave T1/2 values in a relatively narrow range, 214 259 K. These T1/2 values did not correlate with the solvent polarity index P' (R2 0.25) but did correlate with the solvent basicity parameter SB (R2 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 (e 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 Epa 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)
Inorganic chemistry