Effect of counteranion X on the spin crossover properties of a family of diiron(II) triazole complexes [Fe^II₂(PMAT) ₂](X)₄

Seven diiron(II) complexes, [Fe^II₂(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), CF₃SO ₃^- (5), B(PhF)₄^- (6), and C ₁₆H₃₃SO₃^- (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 T_1/2 ≈ 180 K. The B(PhF)₄^- and C ₁₆H₃₃SO₃^- complexes, [6] ·¹/₂MeCN and [7] ·2H₂O, are approximately [LS-HS] at all temperatures, with an onset of gradual SCO with T_1/2 > 300 K.