Chemistry and reactivity of dinuclear iron oxamate complexes: alkane oxidation with hydrogen peroxide catalysed by an oxo-bridged diiron(III) complex with amide and carboxylate ligation

[EN] A new dinuclear iron(III) complex with the tetradentate ligand N,N'-o-phenylenebis(oxamate) (opba) has been synthesised, and structurally, magnetically and electrochemically characterised. It possesses an unprecedented triply bridged Fe-2(mu-O)(mu-RCO2...H2O...O2CR)(2) core, whereby two N-amides from the opba ligand complete the square-pyramidal coordination sphere of the O-carboxylate rich iron site (Fe-N = 2.053 Angstrom and Fe-O = 2.015 Angstrom), The antiferromagnetic exchange interaction between the two high-spin Fe-III ions through the oxo bridge (J = -190 cm(-1); H = -JS(1)(.)S(2)) is weaker than that found in related mu-oxo singly bridged diiron(III) complexes. The lessened antiferromagnetic coupling correlates to the remarkably long Fe-O distance (R = 1.808 Angstrom), which is associated to the relatively bent Fe-O-Fe angle (alpha = 131.8degrees) in the Fe2O unit. It experiences an irreversible one-electron oxidation process in acetonitrile at 25degreesC (E = 0.63 V versus SCE) to give the putative mixed-valent diiron(III, IV) species which is unstable under the experimental conditions. The reactivity of this unique oxo-bridged diiron(III) oxamate complex toward hydrogen peroxide activation and hydrocarbon oxidation in the presence of dioxygen has been examined. It catalyses the oxidation of alkanes like cyclohexane and adamantane to the corresponding alcohols and ketones by H2O2 and O-2 in acetonitrile at room temperature with limited catalytic activities (total yields of ca. 5% after 24 h) and modest selectivities (A/K = 0.9, KIE = 2.4, tert/sec = 3.0). Overall, these results are more in line with a mechanism involving alkyl radicals as transient intermediates, although they do not exclude the possibility that a metal-based active oxidant is also involved in this C-H bond oxidation chemistry.
This work was supported by the Ministerio de Ciencia y Tecnologı́a (MCYT) (Spain) through the Plan Nacional de Investigación Cientı́fica, Desarrollo e Innovación Tecnológica (I + D + I) (BQU2001 2928) and the Ramón y Cajal programme. Further support from the Oficina de Ciencia i Tecnologı́a de la Generalitat Valenciana (Spain) through the Plan Valenciano de Investigación Cientı́fica, Desarrollo Tecnológico e Innovación (PVIDI) (CTIDIA2002 131) is also acknowledged. E.P. thanks the Ministerio de Educación, Cultura y Deporte (Spain) for a FPU grant. We are specially thankful to Dr. E. Anxolabèhere-Mallart for the assistance with the electrochemical measurements and Dr. J. Cano for very stimulating talks and unselfish help during the realisation of this work.