Nonheme Iron(III) Azide and Iron(III) Isothiocyanate Complexes: Radical Rebound Reactivity, Selectivity, and Catalysis

Abstract

The new nonheme iron complexes Fe^II(BNPA^Ph2O)(N₃) (1), Fe^III(BNPA^Ph2O)(OH)(N₃) (2), Fe^II(BNPA^Ph2O)(OH) (3), Fe^III(BNPA^Ph2O)(OH)(NCS) (4), Fe^II(BNPA^Ph2O)(NCS) (5), Fe^III(BNPA^Ph2O)(NCS)₂ (6), and Fe^III(BNPA^Ph2O)(N₃)₂ (7) (BNPA^Ph2O = 2-(bis((6-(neopentylamino)pyridin-2-yl) methyl)amino)-1,1-diphenylethanolate) were synthesized and characterized by single crystal X-ray diffraction (XRD), as well as by ¹H NMR, ⁵⁷Fe Mössbauer, and ATR-IR spectroscopies. Complex 2 was reacted with a series of carbon radicals, Ar^X₃C· (Ar^X = p-X-C₆H₄), analogous to the proposed radical rebound step for nonheme iron hydroxylases and halogenases. The results show that for Ar^X₃C· (X = Cl, H, ^tBu), only OH· transfer occurs to give Ar^X₃COH. However, when X = OMe, a mixture of alcohol (Ar^X₃COH) (30%) and azide (Ar^X₃CN₃) (40%) products was obtained. These data indicate that the rebound selectivity is influenced by the electron-rich nature of the carbon radicals for the azide complex. Reaction of 2 with Ph₃C· in the presence of Sc³⁺ or H⁺ reverses the selectivity, giving only the azide product. In contrast to the mixed selectivity seen for 2, the reactivity of cis-Fe^III(OH)(NCS) with the X = OMe radical derivative leads only to hydroxylation. Catalytic azidation was achieved with 1 as catalyst, λ³-azidoiodane as oxidant and azide source, and Ph₃CH as test substrate, giving Ph₃CN₃ in 84% (TON = 8). These studies show that hydroxylation is favored over azidation for nonheme iron(III) complexes, but the nature of the carbon radical can alter this selectivity. If an OH· transfer pathway can be avoided, the Fe^III(N₃) complexes are capable of mediating both stoichiometric and catalytic azidation.