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, crystallographic characterization and magnetic properties of the dinuclear iron(III) complexes [Fe(OMe)(dbm)2]2(1), [Fe(OMe)(dpm)2]2(2), [Fe(OEt)(bpm)2]2(3) and [Fe(OProp)(npm)2]2(4) are reported. Complexes 1 and 2 have symmetric β-diketonate ligands dbm (dibenzoylmethanate) and dpm (dipivaloylmethanate), respectively, whereas complexes 3 and 4 contain asymmetric β-diketonate ligands bpm (benzoylpivaloylmethanate) and npm (naphthoylpivaloylmethanate), respectively. Complex 1 crystallizes in the triclinic system, space group P1̄ (No. 2), 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. Complex 2 crystallizes in the triclinic system, space group P1̄ (No. 2), 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. Compound 3 crystallizes in the monoclinic system, space group P21/c (No. 14), a=11.546(2), b=18.539(1), c=13.595(2) Å, β=113.18(1)°, Z=2. Compound 4 crystallizes in the monoclinic system, P21/c (No. 14) space group, a=13.746(2), b=18.933(2), c=14.158(2) Å, β=117.37(1)°, Z=2. Each complex consists of two iron(III) ions that are symmetrically bridged by two alkoxide groups. The antiferromagnetic couplings between the S=5/2 iron centers have been fitted by using the Hamiltonian H=JS1·S2to give values of J=15.4(1) cm-1with g=1.96(1) for 1, J=19.0(6) cm-1with g=1.98(1) for 2, J=14.8(5) cm-1with 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 iron(III)-alkoxo clusters synthesized in our laboratory are compared. No simple correlation exists between the isotropic exchange-coupling constant J and the average Fe-O(bridge) bond distance, whereas an approximately linear correlation is found between the J value and the Fe-O-Fe angle. The observed trend and the predictions of Extended Hückel calculations on the model [Fe(OH)H4]24-are compared and discussed.