Abstract
Seven diiron(II) complexes, [Fe-2(II)(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]center dot 2H(2)O, [5]center dot H2O, and [6]center dot 1/2MeCN) or powder ([2]center dot 4H(2)O, and [7]center dot 2H(2)O) samples obtained were studied by variable-temperature magnetic susceptibility and Mossbauer methods. A structure determination on a crystal of [2]center dot 2MeOH center dot 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]center dot 3/4IPA center dot H2O and [7]center dot 1.6MeOH center dot 0.4H(2)O showed them to be [HS-HS] at 90 K, whereas magnetic and Mossbauer studies on [5]center dot H2O and [7]center dot 2H(2)O 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]center dot 2H(2)O, 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]center dot H2O undergoes gradual, partial SCO from [HS-LS] to a mixture of [HS-LS] and [HS-HS] at T-1/2 approximate to 180 K. The B(PhF)(4)(-) and C16H33SO3- complexes, [6]center dot 1/2MeCN and [7]center dot 2H(2)O, are approximately [LS-HS] at all temperatures, with an onset of gradual SCO with T-1/2>300 K.