A Synthetic Analogue of the Active Site of Fe-Containing Nitrile Hydratase with Carboxamido N and Thiolato S as Donors:  Synthesis, Structure, and Reactivities

As part of our work on models of the iron(III) site of Fe-containing nitrile hydratase, a designed ligand PyPSH4 with two carboxamide and two thiolate donor groups has been synthesized. Reaction of (Et4N)[FeCl4] with the deprotonated form of the ligand in DMF affords the mononuclear iron(III) complex (Et4N)[FeIII(PyPS)] (1) in high yield. The iron(III) center is in a trigonal bipyramidal geometry with two deprotonated carboxamido nitrogens, one pyridine nitrogen, and two thiolato sulfurs as donors. Complex 1 is stable in water and binds a variety of Lewis bases at the sixth site at low temperature to afford green solutions with a band around 700 nm. The iron(III) centers in these six-coordinate species are low-spin and exhibit EPR spectra much like the enzyme. The pKa of the water molecule in [FeIII(PyPS)(H2O)]- is 6.3 ± 0.4. The iron(III) site in 1 with ligated carboxamido nitrogens and thiolato sulfurs does not show any affinity toward nitriles. It thus appears that at physiological pH, a metal-bound hydroxide promotes hydration of nitriles nested in close proximity of the iron center in the enzyme. Redox measurements demonstrate that the carboxamido nitrogens prefer Fe(III) to Fe(II) centers. This fact explains the absence of any redox behavior at the iron site in nitrile hydratase. Upon exposure to limited amount of dioxygen, 1 is converted to the bis-sulfinic species. The structure of the more stable O-bonded sulfinato complex (Et4N)[FeIII(PyP{SO2}2)] (2) has been determined. Six-coordinated low-spin cyanide adducts of the S-bonded and the O-bonded sulfinato complexes, namely, Na2[FeIII(PyP{SO2}2)(CN)] (4) and (Et4N)2[FeIII(PyP{SO2}2)(CN)] (5), afford green solutions in water and other solvents. The iron(II) complex (Et4N)2[FeII(PyPS)] (3) has also been isolated and structurally characterized.