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

The new nonheme iron complexes Fe II (BNPA Ph2 O)(N 3 ) ( 1 ), Fe III (BNPA Ph2 O)(OH)(N 3 ) ( 2 ), Fe II (BNPA Ph2 O)(OH) ( 3 ), Fe III (BNPA Ph2 O)(OH)(NCS) ( 4 ), Fe II (BNPA Ph2 O)(NCS) ( 5 ), Fe III (BNPA Ph2 O)(NCS) 2 ( 6 ), and Fe III (BNPA Ph2 O)(N 3 ) 2 ( 7 ) (BNPA Ph2 O = 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 1 H NMR, 57 Fe Mössbauer, and ATR-IR spectroscopies. Complex 2 was reacted with a series of carbon radicals, Ar X 3 C· (Ar X = p -X-C 6 H 4 ), analogous to the proposed radical rebound step for nonheme iron hydroxylases and halogenases. The results show that for Ar X 3 C· (X = Cl, H, t Bu), only OH· transfer occurs to give Ar X 3 COH. However, when X = OMe, a mixture of alcohol (Ar X 3 COH) (30%) and azide (Ar X 3 CN 3 ) (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 Ph 3 C· in the presence of Sc 3+ 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, λ 3 -azidoiodane as oxidant and azide source, and Ph 3 CH as test substrate, giving Ph 3 CN 3 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 (N 3 ) complexes are capable of mediating both stoichiometric and catalytic azidation.