Nitrile hydratase is the first and only current example of a metalloenzyme containing a single non-heme Fe in a mixed N/Sligated coordination site.'-3 Its function is to hydrolyze nitriles to the corresponding amides in organisms which can live on R-CN as their sole C and N source. On the basis of EPR? MCD,5 resonance Raman, EXAFS,* and ENDOR' studies, the active site of nitrile hydratase is proposed to contain a six-coordinate, low-spin (S = ' / 2 ) Fe3+ ion ligated by two ciscysteinates, three N's, and a water: N The spectroscopic properties of nitrile hydratase are pHdependent, and two distinct forms (pH -7 and pH = 9) have been ~haracterized.~.' The high-pH form6 appears to be identical to the "substrate-bound'' form on the basis of its spectroscopic properties.4 Two impediments to the synthetic modeling of this enzyme have been the ease with which ferric ions oxidize thiolates to disulfides and the propensity of metal thiolates to o l i g~m e r i z e .~.~ The small number of reported monomeric Fe-(111) thiolate complexes attests to this.I0-l3 Mixed N-/SR--(1) Jin, H.; Tumer, I. M., Jr.; Nelson, M. J.; Gurbiel, R. J.; Doan, P. E.; Hoffman, B. M. J . Am. Chem. Soc. 1993, 115, 5290-1. (2) Nelson, M. J.; Jin, H.; Turner. I. M., Jr.; Grove, G.; Scarrow, R. C.; Brennan. B. A.: Que, L., Jr. J. Am. Chem. Soc. 1991, 113, 7072-3. (3) Nagasawa. T.; Ryuno, K.; Yamada, H. Biochem. Biophys. Res. Commun. 1986, 139, 1305-12. (4) Sugiura, Y.; Kuwahara, J.; Nagasawa, T.; Yamada, H. J. Am. Chem. (5) Johnson, M. K.: et al. Manuscript in preparation. (6) Jin. H.; Brennan, B. A,; Nelson, M. J.; et. al. Manuscript in preparation. (7) Honda, J.; Kandori, H.; Okada, T.; Nagamune, T.; Shichida. Y.; Sasabe, H.; Endo, I. ligated Fe(II1) systems are even rarer.I4-l6 Low-spin complexes of this type are extremely rare; most Fe(II1) compounds are S = 5 / 2 h i g h -~p i n '~. '~ or, at best, exist in a S = 5 / 2 -' / 2 spin equi1ibri~m.I~ Previously reported model compounds approximate the coordination geometry and metric parameters of the nitrile hydratase active site but do not reproduce the spin state and electronic spectral proper tie^.'^ Herein we report the synthesis, structure, and properties of a stable low-spin Fe(II1) thiolate complex which is remarkably similar to nitrile hydratase in terms of its electronic and geometric structure.Thiolate-ligated [Fe*11(AMIT)2]Cl (1) was synthesizedz0 via the Schiff-base condensation of ethylenediamine with a-mercaptoacetone at an Fe3+ template. This approach has provided access to a variety of mixed aminekhiolate S,Nyligated transition-metal complexes by our g r o~p .~' -*~ Although we have not yet obtained X-ray-quality crystals of l, (13) Sellmann, D.; Geck, M.; Knoch, F.; Ritter, G.; Dengler, J. J. Am. Chem. (14) Beissel, T.; Buerger, K. S.; Voigt, G.; Wieghardt, K.; Butzlaff, C.: (15) Marini, P. J.; Murray, K. S.; West, B. 0. J. Chem. Soc., Dalron Trans. (16) Fallon. G. D.; Gatehouse, B. M. J. Chem. Soc., Dalton Trans. 1975, (17) Greenwood, N. N.; Eamshaw, A. In Chemisrv of the Elemenrs; ...