Ground Spin State Variation in Cardboxylate-Bridged Tetranuclear [Fe_2Mn_2O_2]~(8+) Cores and a Comparison with Their [Fe_2O_2]~(8+) and [Mn_2O_2]~(8+) Congeners

摘要

Syntheses of ten new tetrauclear complexes containing the [M_A(#mu#_3-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 [L_2Fe_2(#mu#_3-O)_2(salox)_2(diphenylglycolate)_3Fe_2](ClO_4) (1), [L_2Mn_2(#mu#_3-O)_2(salox)_2(diphenylglycolate)_3Mn_2](ClO_4) (3), [L_2Fe_2(#mu#_3-O)_2(salox)_2(acetate)_3Mn_2](ClO_4) (5), [L_2Fe_2(#mu#_3-O)_2(salox)_2(diphenylglycolate)_3Mn_2](ClO_4) (6), and [L_2Fe_2(#mu#_3-O)_2(salox)_2(benzoate)_3Mn_2](ClO_4) (8) were determined by X-ray crystallography. The complexes are isostructural, with M~(III) ions disposed in a butterfly core where birding between the M~(III) ions occurs via two #mu#_3-oxo anions. In complexes 5, 6 and 8 the "wing-tip" positions of the butterfly are occupied by the Lfe~(3+) units, whereas the "body" metal ions are d~4 high-spin Mn~(III) 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 #mu#_(eff) vs. T plots. Overall exchange interactions are antiferromagnetic in nature. Complexes with the Fe_4O_2 core (1 and 2) and the Mn_4O_2 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 [Fe_2Mn_2O_2] 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 Fe_2Mn_2O_2 butterfly arrangement clearly demonstrates the ground state variation as a function of the ratio of two competing coupling interaction.