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

Seven diiron(II) complexes, Fe(II)(2)(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 = BF(4)(-) (1), Cl(-) (2), PF(6)(-) (3), SbF(6)(-) (4), CF(3)SO(3)(-) (5), B(PhF)(4)(-) (6), and C(16)H(33)SO(3)(-) (7). Most were isolated as solvates, and the microcrystalline (3], 4]·2H(2)O, 5]·H(2)O, and 6]·?MeCN) or powder (2]·4H(2)O, and 7]·2H(2)O) samples obtained were studied by variable-temperature magnetic susceptibility and Mo?ssbauer methods. A structure determination on a crystal of 2]·2MeOH·H(2)O, 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]·?IPA·H(2)O and 7]·1.6MeOH·0.4H(2)O showed them to be HS-HS] at 90 K, whereas magnetic and M?ssbauer studies on 5]·H(2)O and 7]·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 PF(6)(-) and SbF(6)(-) complexes, 3 and 4]·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 CF(3)SO(3)(-) complex 5]·H(2)O 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(16)H(33)SO(3)(-) complexes, 6]·(1)/(2)MeCN and 7]·2H(2)O, are approximately LS-HS] at all temperatures, with an onset of gradual SCO with T(1/2) > 300 K.