Reactivity of Five-Coordinate Models for the Thiolate-Ligated Fe Site of Nitrile Hydratase

To examine inhibitor binding to an iron site resembling that of the metalloenzyme nitrile hydratase (NHase), a coordinatively unsaturated, five-coordinate Fe III thiolate complex was synthesized, and its reactivity examined. Ferricinium hexafluorophosphate induced oxidation of gem -dimethyl-protected [Fe II S 2 Me2 N 3 (Pr,Pr)] affords the chiral, five-coordinate complex [Fe III S 2 Me2 N 3 (Pr,Pr)] + ( 2 ) in reasonable yields. The magnetic properties and EPR of 2 are consistent with an S = 1 / 2 ground state. This unusual spin state, in conjunction with the low coordination number, of 2 result in unusually short Fe−S bonds (2.15(2) Å). Ligand constraints distort the S−Fe−N angles in 2 and create an open (132.3(1)°) reactive site. Azide binds to this site to afford a model for the azide-inhibited form of NHase [Fe III S 2 Me2 N 3 (Pr,Pr)(N 3 )] ( 3 ). In MeOH azide binds reversibly, whereas in MeCN it binds irreversibly. This demonstrates that the secondary coordination sphere (i.e., the solvent, or possibly a protein binding pocket) can have a dramatic influence on the substrate binding properties of a metal complex. A variable-temperature equilibrium study in MeOH afforded thermodynamic parameters (Δ H = −5.2 ± 0.9 kcal/mol and Δ S = −12.4 ± 0.4 eu) for the binding of this inhibitor. The electronic spectrum of 3 displays an intense band at 708 (1600) nm similar to that (710 (∼1200) nm) of the pH = 7.3 form of NHase, and other six-coordinate cis -dithiolate ligated Fe III complexes synthesized by our group. EPR parameters for 3 ( g = 2.23, 2.16, 1.99) are nearly identical to those of the azide-inhibited form of NHase ( g = 2.23, 2.14, 1.99), suggesting that (1) the iron site of our model closely resembles that of the enzyme, and (2) azide binds directly to the metal ion in NHase. Reactivity is oxidation-state dependent, and the reduced analogue of 2, [Fe II S 2 N 3 (Pr,Pr)] ( 4 ), reversibly binds CO, but not azide, whereas oxidized 2 binds azide, but not CO.