Modeling features of the non-heme diiron cores in O2-activating enzymes through the synthesis, characterization, and oxidation of 1,8-naphthyridine-based complexes

Multidentate naphthyridine-based ligands were used to prepare a series of diiron(II) complexes. The compound [Fe-2(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 E-1/2 values at +310 and +733 MV VS Cp2Fe+/CP2Fe, as revealed by cyclic voltammetry. Reaction with O-2 or H2O2 affords a product with optical and Mossbauer properties that are characteristic of a (mu-oxo)diiron(III) species. The complexes [Fe-2(BPMAN)(mu-OH)(mu-O2CAr (Tol))](OTf)(2) (2) and [Fe-2(BPMAN)(mu-OMe)(mu-O2CArTol)](OTf)(2) (3) were synthesized, where (ArCO2-)-C-Tol 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 02, 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 [Fe-2(BBAN)(mu-O2CArTol)(3)](OTf) (4), where BBAN = 2,7-bis(NN-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, [Fe-2(BEAN)(mu-O2CArTol)(3)](OTf) (5), where BEAN = 2,7-bis(NN-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.