Modeling Features of the Non-Heme Diiron Cores in O2-Activating Enzymes through the Synthesis, Characterization, and Oxidation of 1,8Naphthyridine-Based Complexes

Multidentate naphthyridine-based ligands were used to prepare a series of diiron(II) complexes. The compound [Fe2(BPMAN)((mu)-O2CPh)2](OTf)2 (1), where BPMAN = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine, exhibits two reversible oxidation waves with E1/2 values at +310 and +733 mV vs Cp2Fe+/Cp2Fe, as revealed by cyclic voltammetry. Reaction with O2 or H2O2 affords a product with optical and Mossbauer properties that are characteristic of a ((mu)-oxo)diiron(III) species. The complexes [Fe2(BPMAN)((mu)-OH)((mu)-O2CArTol)](OTf)2 (2) and [Fe2(BPMAN)((mu)-OMe) ((mu)-O2CArTol)](OTf)2 (3) were synthesized, where ArTolCO2- is the sterically hindered ligand 2,6-di(p-tolyl)benzoate. Compound 2 has a reversible redox wave at +11 mV, and both 2 and 3 react with O2, via a mixed-valent Fe(II)Fe(III) intermediate, to give final products that are also consistent with ((mu)-oxo)diiron(III) species. The paddle-wheel compound [Fe2(BBAN)((mu)-O2CArTol)3](OTf) (4), where BBAN = 2,7-bis(N,N-dibenzylaminomethyl)-1,8 -naphthyridine, reacts with dioxygen to yield benzaldehyde via oxidative N-dealkylation of a benzyl group on BBAN, an internal substrate. In the presence of bis(4-methylbenzyl)amine, the reaction also produces p-tolualdehyde, revealing oxidation of an external substrate. A structurally related compound, [Fe2(BEAN)((mu)-O2CArTol)3](OTf) (5), where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine, does not undergo N-dealkylation, nor does it facilitate the oxidation of bis(4-methylbenzyl)amine. The contrast in reactivity of 4 and 5 is attributed to a difference in accessibility of the substrate to the diiron centers of the two compounds. The Mossbauer spectroscopic properties of the diiron(II) complexes were also investigated.