Effect of N 4-substituent choice on spin crossover in dinuclear iron(II) complexes of bis-terdentate 1,2,4-triazole-based ligands

Abstract

Seven new dinuclear iron(II) complexes of the general formula [Fe II2(PMRT)2](BF4)4· solvent, where PMRT is a 4-substituted-3,5-bis{[(2-pyridylmethyl)-amino]methyl}- 4H-1,2,4-triazole, have been prepared in order to investigate the substituent effect on the spin crossover event. Variable temperature magnetic susceptibility and 57Fe Mössbauer spectroscopy studies show that two of the complexes, [FeII2(PMPT)2](BF4) 4·H2O (N4 substituent is pyrrolyl) and [FeII2(PMPhAT)2](BF 4)4 (N4 is N,N-diphenylamine), are stabilized in the [HS-HS] state between 300 and 2 K with weak antiferromagnetic interactions between the iron(II) centers. Five of the complexes showed gradual half spin crossover, from [HS-HS] to [HS-LS], with the following T1/2 (K) values: 234 for [FeII2(PMibT)2](BF 4)4·3H2O (N4 is isobutyl), 147 for [FeII2(PMBzT)2](BF4) 4 (N4 is benzyl), 133 for [FeII2(PMCF3PhT)2](BF4) 4·DMF·H2O (N4 is 3,5-bis(trifluoromethyl)phenyl), 187 for [FeII2(PMPhT) 2](BF4)4 (N4 is phenyl), and 224 for [FeII2(PMC16T)2](BF 4)4 (N4 is hexadecyl). Structure determinations carried out for three complexes, [FeII2(PMPT) 2](BF4)4·4DMF, [FeII2(PMBzT)2](BF4)4·CH 3CN, and [FeII2(PMPhAT) 2](BF4)4·solvent, revealed that in all three complexes both iron(II) centers are stabilized in the high spin state at 90 K. A general and reliable 4-step route to PMRT ligands is also detailed. © 2013 American Chemical Society.