Effect of N4-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 Fe(II)2(PMRT)24·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 (57)Fe Mössbauer spectroscopy studies show that two of the complexes, Fe(II)2(PMPT)24·H2O (N(4) substituent is pyrrolyl) and Fe(II)2(PM(Ph)AT)24 (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 Fe(II)2(PMibT)24·3H2O (N(4) is isobutyl), 147 for Fe(II)2(PMBzT)24 (N(4) is benzyl), 133 for Fe(II)2(PM(CF3)PhT)24·DMF·H2O (N(4) is 3,5-bis(trifluoromethyl)phenyl), 187 for Fe(II)2(PMPhT)24 (N(4) is phenyl), and 224 for Fe(II)2(PMC16T)24 (N(4) is hexadecyl). Structure determinations carried out for three complexes, Fe(II)2(PMPT)24·4DMF, Fe(II)2(PMBzT)24·CH3CN, and Fe(II)2(PM(Ph)AT)24·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.