Synthesis, crystal structures and magnetic characterization of four beta-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 beta-diketonate ligands dbm (dibenzoylmethanate) and dpm (dipivaloylmethanate), respectively, whereas complexes 3 and 4 contain asymmetric beta-diketonate ligands bpm (benzoylpivaloylmethanate) and npm (naphthoylpivaloylmethanate), respectively. Complex 1 crystallizes in the triclinic system, space group P (1) over bar (No. 2), a = 9.634(1), b = 10.946(2), c = 13.079(1) Angstrom, alpha = 79.95(1), beta = 88.01(1), gamma = 82.57(1)degrees, Z = 1. Complex 2 crystallizes in the triclinic system, space group P (1) over bar (No. 2), a = 10.980(2), b = 14.255(2), c = 17.979(1) Angstrom, alpha = 85.70(1), beta = 89.63(1), gamma = 71.65(1)degrees, Z=2. Compound 3 crystallizes in the monoclinic system, space group P2(1)/c (No. 14), a = 11.546(2), b = 18.539(1), c = 13.595(2) Angstrom, beta = 113.18(1)degrees, Z = 2. Compound 4 crystallizes in the monoclinic system, P2(1)/c (No. 14) space group, a = 13.746(2), b = 18.933(2), c = 14.158(2) Angstrom, beta = 117.37(1)degrees, 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 = JS(1).S-2 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 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 Huckel calculations on the model [Fe(OH)H-4](2)(4-) are compared and discussed.