Ground Spin State Variation in Carboxylate-Bridged Tetranuclear [Fe2Mn2O2]8+ Cores and a Comparison with Their [Fe4O2]8+ and [Mn4O2]8+ Congeners

Syntheses of ten new tetranuclear complexes containing the [MA(μ3-O)2MB]8+ butterfly core [MA = MB = FeIII (1 and 2), MA = MB = MnIII (3 and 4) and MA = FeIII, MB = MnIII (5−10)] using 1,4,7-trimethyl-1,4,7-triazacyclononane (L) as the capping ligand, salicylaldoximato dianion (salox2−) 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 MIII ions disposed in a butterfly core where bridging between the MIII 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 MnIII ions. Complexes 1−10 have been characterised by variable-temperature (2−290 K) magnetic susceptibility measurements (1 T) and by Mossbauer spectroscopy. A “2J” model has been applied to simulate the experimental μeff 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 St = 0 and St = 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 St = 1 to St = 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. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)