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Effect of N4‑Substituent Choice on Spin Crossover in Dinuclear Iron(II) Complexes of Bis-Terdentate 1,2,4-Triazole-Based Ligands

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posted on 07.10.2013, 00:00 by Jonathan A. Kitchen, Juan Olguín, Rafal Kulmaczewski, Nicholas G. White, Victoria A. Milway, Guy N. L. Jameson, Jeffery L. Tallon, Sally Brooker
Seven new dinuclear iron­(II) complexes of the general formula [FeII2(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]­(BF4)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]­(BF4)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]­(BF4)4 (N4 is hexadecyl). Structure determinations carried out for three complexes, [FeII2(PMPT)2]­(BF4)4·4DMF, [FeII2(PMBzT)2]­(BF4)4·CH3CN, 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.