Synthesis, Crystal Structures and Magnetic Characterization of Four β-diketonate-alkoxide Iron(III) Dimers. Dependence of the Magnetic Properties on Geometrical and Electronic Parameters

Synthesis, crystallog. characterization and magnetic properties of the dinuclear Fe(III) complexes [Fe(OMe)(dbm)2]2 (1), [Fe(OMe)(dpm)2]2 (2), [Fe(OEt)(bpm)2]2 (3) and [Fe(OPr)(npm)2]2 (4) are reported. 1 And 2 have sym. β-diketonate ligands dbm (dibenzoylmethanate) and dpm (dipivaloylmethanate), resp., whereas complexes 3 and 4 contain asym. β-diketonate ligands bpm (benzoylpivaloylmethanate) and npm (naphthoylpivaloylmethanate), resp. 1 Crystallizes in the triclinic system, space group P-1 a = 9.634(1), b = 10.946(2), c 13.079(1) Å, α = 79.95(1), β = 88.01(1), γ = 82.57(1)°, Z = 1. 2 Crystallizes in the triclinic system, space group P-1, a = 10.980(2), b = 14.255(2), c = 17.979(1)Å, α = 85.70(1), β = 89.63(1), γ = 71.65(1)°, Z = 2. 3 Crystallizes in the monoclinic system, space group P21/c, a = 11.546(2), b = 18.539(1), c = 13.595(2)Å, β = 113.18(1)°, Z = 2. 4 Crystallizes in the monoclinic system, space group P21/c, a = 13.746(2), b = 18.933(2), c = 14.158(2)Å, β = 117.37(1)°, Z = 2. Each complex consists of two Fe(III) ions that are sym. bridged by two alkoxide groups. The antiferromagnetic couplings between the S = 5/2 Fe centers were fitted by using the Hamiltonian H = JS1·S2 to give values of J = 15.4(1) cm-1 with g = 1.96(1) for 1, J = 19.0(6) cm-1 with g = 1.98(1) for 2, J = 14.8(5) cm-1 with g = 1.98(1) for 3 and J = 18.0(5) with g = 2 for 4. The structural and magnetic parameters of 1-4 and other Fe(III)-alkoxo clusters synthesized in the authors' lab. are compared. No simple correlation exists between the isotropic exchange-coupling const. J and the av. Fe-O(bridge) bond distance, whereas an approx. linear correlation is found between the J value and the Fe-O-Fe angle. The obsd. trend and the predictions of Extended Hueckel calcns. on the model [Fe(OH)H4]24- are compared and discussed.