Effect of Counteranion X on the Spin Crossover Properties of a Family of Diiron(II) Triazole Complexes [FeII2(PMAT)2](X)4

Seven diiron(II) complexes, [FeII2(PMAT)2](X)4, varying only in the anion X, have been prepared, where PMAT is 4-amino-3,5-bis{[(2-pyridylmethyl)-amino]methyl}-4H-1,2,4-triazole and X = BF4– (1), Cl– (2), PF6– (3), SbF6– (4), CF3SO3– (5), B(PhF)4– (6), and C16H33SO3– (7). Most were isolated as solvates, and the microcrystalline ([3], [4]·2H2O, [5]·H2O, and [6]·1/2MeCN) or powder ([2]·4H2O, and [7]·2H2O) samples obtained were studied by variable-temperature magnetic susceptibility and Mössbauer methods. A structure determination on a crystal of [2]·2MeOH·H2O, revealed it to be a [LS-HS] mixed low spin (LS)-high spin (HS) state dinuclear complex at 90 K, but fully high spin, [HS-HS], at 293 K. In contrast, structures of both [5]·3/4IPA·H2O and [7]·1.6MeOH·0.4H2O showed them to be [HS-HS] at 90 K, whereas magnetic and Mössbauer studies on [5]·H2O and [7]·2H2O revealed a different spin state, [LS-HS], at 90 K, presumably because of the difference in solvation. None of these complexes undergo thermal spin crossover (SCO) to the fully LS form, [LS-LS]. The PF6– and SbF6– complexes, 3 and [4]·2H2O, appear to be a mixture of [HS-LS] and [HS-HS] at low temperature, and undergo gradual SCO to [HS-HS] on warming. The CF3SO3– complex [5]·H2O undergoes gradual, partial SCO from [HS-LS] to a mixture of [HS-LS] and [HS-HS] at T1/2 ≈ 180 K. The B(PhF)4– and C16H33SO3– complexes, [6]·1/2MeCN and [7]·2H2O, are approximately [LS-HS] at all temperatures, with an onset of gradual SCO with T1/2 > 300 K.