Synthesis, Structure, and Magnetic Properties of Valence Ambiguous Dinuclear Antiferromagnetically Coupled Cobalt and Ferromagnetically Coupled Iron Complexes Containing the Chloranilate(2−) and the Significantly Stronger Coupling Chloranilate(•3−) Radical Trianion

Abstract

Dinuclear [(TPyA)M^II(CA^2-)M^II(TPyA)]²⁺ [TPyA = tris(2-pyridylmethyl)amine; CA^2- = chloranilate dianion; M = Co (1²⁺), Fe (2²⁺)] complexes have been prepared by the reaction of M(BF₄)₂·6H₂O, TPyA, H₂CA, and triethylamine in MeOH solution. Their reduced forms [(TPyA)M^II(CA^•3-)M^II(TPyA)]⁺ [M = Co(1⁺), Fe (2⁺)] have been synthesized by using cobaltocene, and oxidized forms of 1, [(TPyA)Co^III(CAⁿ)Co^III(TPyA)]^z+ [z = 3, n = 3− (1³⁺); z = 4, n = 2− (1⁴⁺)], have been obtained by using FcBF₄ and ThianBF₄ (Fc = ferrocenium; Thian = thianthrinium), respectively. The dinuclear compound bridged chloranilates (CA^2- or CA^•3-) were isolated and characterized by X-ray crystallography, electrochemistry, magnetism, and EPR spectroscopy. Unlike the other redox products, valence ambiguous 1³⁺ forms via a complex redox-induced valence electron rearrangement whereby the one-electron oxidation of the [Co^IICA^2-Co^II]²⁺ core forms [Co^IIICA^•3-Co^III]³⁺, not the expected simple 1-e^- transfer mixed-valent [Co^IICA^2-Co^III]³⁺ core. The M ions in 1 and 2 have a distorted octahedral geometry by coordination with four nitrogens of a TPyA, two oxygens of a chloranilate. Due to the interdimer offset face-to-face π−π and/or herringbone interactions, all complexes show extended 1-D and/or 2-D supramolecular structures. The existence of CA^•3- in 1³⁺ is confirmed from both solid-state magnetic and solution EPR data. Co-based 1ⁿ⁺ exhibit antiferromagnetic interactions [1²⁺: g = 2.24, J/k_B = −0.65 K (−0.45 cm^-1); 1⁺: g = 2.36, J/k_B = −75 K (52 cm^-1)], while Fe-based 2ⁿ⁺ exhibit ferromagnetic interactions [2²⁺: g = 2.08, J/k_B = 1.0 K (0.70 cm^-1); 2⁺: g = 2.03, J/k_B = 28 K (19 cm^-1)] [H = −2JS₁·S₂ for 1²⁺ and 2²⁺; H = −2J(S₁·S₂ + S₂·S₃) for 1⁺ and 2⁺]. Thus, due to direct spin exchange CA^•3- is a much strong spin coupling linkage than the superexchange spin-coupling pathway provided by CA^2-.