Ground spin state variation in carboxylate-bridged tetranuclear [Fe₂Mn₂O₂]⁸⁺ cores and a comparison with their [Fe₄O₂]⁸⁺ and [Mn₄O₂]¹⁸⁺ congeners

Syntheses of ten new tetranuclear complexes containing the [M_A(μ₃-O)₂M_B]⁸⁺ butterfly core [M_A = M_B = Fe^III (1 and 2), M_A = M_B = Mn^III (3 and 4) and M_A = Fe^III, M_B = Mn^III (5-10)] using 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligand, salicylaldoximato dianion (salox^2-) and six different carboxylate anions as bridging ligands, are described. The crystal structures of [L₂Fe₂(μ₃-O)₂ (salox)₂(diphenylglycolate)₃Fe₂] (ClO₄) (1), [L₂Mn₂(μ₃-O)₂ (salox)₂(diphenylglycolate)₃Mn₂] (ClO₄) (3), [L₂Fe₂(μ₃-O)₂ (salox)₂(acetate)₃Mn₂]- (ClO₄) (5), [L₂Fe₂(μ₃-O)₂ (salox)₂(diphenylglycolate)₃Mn₂]- (ClO₄) (6), and [L₂Fe₂(μ₃-O)₂ (salox)₂(benzoate)₃Mn₂]- (ClO₄) (8) were determined by X-ray crystallography. The complexes are isostructural, with M^III ions disposed in a butterfly core where bridging between the M^III ions occurs via two μ₃-oxo anions. In complexes 5, 6 and 8 the "wing-tip" positions of the butterfly are occupied by the LFe³⁺ units, whereas the "body" metal ions are d⁴ high-spin Mn^III ions. Complexes 1-10 have been characterised by variable-temperature (2-290 K) magnetic susceptibility measurements (1 T) and by Mössbauer spectroscopy. A "2J" model has been applied to simulate the experimental μ_eff vs. T plots. Overall exchange interactions are antiferromagnetic in nature. Complexes with the Fe₄O₂ core (1 and 2) and the Mn₄O₂ core (3 and 4) possess an S_t = 0 and S_t = 3 ground state, respectively, regardless of the nature of the bridging carboxylate ligand. In contrast, changing the bridging carboxylates in the [Fe₂Mn₂O₂] core from acetate (5) to diphenylglycolate (6), triphenyl acetate (7), benzoate (8), chloroacetate (9) and propionate (10), results in a variation of the ground state from S_t = 1 to S_t = 3, due to spin frustration of the "body" manganese centres. The spin-correlation diagram for the Fe₂Mn₂O₂ butterfly arrangement clearly demonstrates the ground state variation as a function of the ratio of two competing coupling interactions.