The first example of a nitrile hydratase model complex that reversibly binds nitriles

Nitrile hydratase (NHase) is an iron-containing metalloenzyme that converts nitriles to amides. The mechanism by which this biochemical reaction occurs is unknown. One mechanism that has been proposed involves nucleophilic attack of an Fe-bound nitrile by water (or hydroxide). Reported herein is a five-coordinate model compound (Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+)) containing Fe(III) in an environment resembling that of NHase, which reversibly binds a variety of nitriles, alcohols, amines, and thiocyanate. XAS shows that five-coordinate Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) reacts with both methanol and acetonitrile to afford a six-coordinate solvent-bound complex. Competitive binding studies demonstrate that MeCN preferentially binds over ROH, suggesting that nitriles would be capable of displacing the H(2)O coordinated to the iron site of NHase. Thermodynamic parameters were determined for acetonitrile (DeltaH = -6.2(+/-0.2) kcal/mol, DeltaS = -29.4(+/-0.8) eu), benzonitrile (-4.2(+/-0.6) kcal/mol, DeltaS = -18(+/-3) eu), and pyridine (DeltaH = -8(+/-1) kcal/mol, DeltaS = -41(+/-6) eu) binding to Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) using variable-temperature electronic absorption spectroscopy. Ligand exchange kinetics were examined for acetonitrile, iso-propylnitrile, benzonitrile, and 4-tert-butylpyridine using (13)C NMR line-broadening analysis, at a variety of temperatures. Activation parameters for ligand exchange were determined to be DeltaH(+ +) = 7.1(+/-0.8) kcal/mol, DeltaS(+ +) = -10(+/-1) eu (acetonitrile), DeltaH(+ +) = 5.4(+/-0.6) kcal/mol, DeltaS(+ +) = -17(+/-2) eu (iso-propionitrile), DeltaH(+ +) = 4.9(+/-0.8) kcal/mol, DeltaS(+ +) = -20(+/-3) eu (benzonitrile), and DeltaH(+ +) = 4.7(+/-1.4) kcal/mol DeltaS(+ +) = -18(+/-2) eu (4-tert-butylpyridine). The thermodynamic parameters for pyridine binding to a related complex, Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) (DeltaH = -5.9(+/-0.8) kcal/mol, DeltaS = -24(+/-3) eu), are also reported, as well as kinetic parameters for 4-tert-butylpyridine exchange (DeltaH(+ +) = 3.1(+/-0.8) kcal/mol, DeltaS(+ +) = -25(+/-3) eu). These data show for the first time that, when it is contained in a ligand environment similar to that of NHase, Fe(III) is capable of forming a stable complex with nitriles. Also, the rates of ligand exchange demonstrate that low-spin Fe(III) in this ligand environment is more labile than expected. Furthermore, comparison of Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) and Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) demonstrates how minor distortions induced by ligand constraints can dramatically alter the reactivity of a metal complex.