Effect of N 4‑Substituent Choice on Spin Crossover in Dinuclear Iron(II) Complexes of Bis-Terdentate 1,2,4-Triazole-Based Ligands

Seven new dinuclear iron­(II) complexes of the general formula [FeII 2(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, [FeII 2(PMPT)2]­(BF4)4·H2O (N 4 substituent is pyrrolyl) and [FeII 2(PM Ph AT)2]­(BF4)4 (N 4 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 T 1/2 (K) values: 234 for [FeII 2(PMibT)2]­(BF4)4·3H2O (N 4 is isobutyl), 147 for [FeII 2(PMBzT)2]­(BF4)4 (N 4 is benzyl), 133 for [FeII 2(PM CF3 PhT)2]­(BF4)4·DMF·H2O (N 4 is 3,5-bis­(trifluoromethyl)­phenyl), 187 for [FeII 2(PMPhT)2]­(BF4)4 (N 4 is phenyl), and 224 for [FeII 2(PMC 16 T)2]­(BF4)4 (N 4 is hexadecyl). Structure determinations carried out for three complexes, [FeII 2(PMPT)2]­(BF4)4·4DMF, [FeII 2(PMBzT)2]­(BF4)4·CH3CN, and [FeII 2(PM Ph AT)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.