Model Studies of α-Keto Acid-Dependent Nonheme Iron Enzymes: Nitric Oxide Adducts of [Fe^II(L)(O₂CCOPh)](ClO₄) Complexest

To model the putative dioxygen-bound active-site structure of α-keto acid-dependent nonheme iron enzymes, the nitrosyl adducts of iron(II)-α-keto carboxylate complexes were prepared by the treatment of [Fe^II(L)(BF)]⁺ complexes with NO, where L is tris((6-methyl-2-pyridyl)methyl)amine (6TLA) or tris(2-pyridylmethyl)amine (TPA) and BF is benzoylformate. [Fe^II(6TLA)(BF)](ClO₄) (1) reversibly binds NO to afford the nitrosyl adduct [Fe(6TLA)(BF)(NO)](ClO₄) (3), while [Fe^II(TPA)(BF)](ClO₄) (2) reacts with NO irreversibly to form [Fe(TPA)-(BF)(NO)](ClO₄) (4). The difference in the binding affinities of NO for 1 and 2 correlates with the higher redox potential of 1 (+870 mV vs NHE) relative to that of 2 (+340 mV) and is reflected in the ¹H NMR properties of 3 and 4. Both nitrosyl adducts have an {FeNO}⁷ configuration based on the Enemark-Feltham nomenclature and exhibit characteristic S = ³/₂ EPR signals with axial g values of 4 and 2. An X-ray diffraction study of 4 reveals a six-coordinate iron-TP A complex in which the nitrosyl ligand binds trans to the amine nitrogen and cis to the monodentate BF ligand. The Fe-N-O angles in the two crystallographically independent conformational isomers of 4 are 162(2) and 155(2)° with Fe-NO bond lengths of 1.70(2) and 1.74(2) A, respectively. Crystal data are as follows: triclinic space group PI; a = 8.47(1) Å,b= 11.746(8) Å, c = 15.20(1) Å; α = 90.60(7)°, β = 99.20 (9)°, γ = 104.30 (7)°; V = 1447(5) ų; Z = 1; R = 0.084 and R_w = 0.076. Complexes 3 and 4 exhibit spectroscopic features comparable to those of nitrosyl adducts of several mononuclear nonheme iron proteins. Thus, the title compounds serve as models for the proposed nitrosyl (or dioxygen) adducts for the iron-cofactor complexes in a-keto acid-dependent nonheme iron enzymes.