The influence of cyanide on the carbonylation of iron(II): synthesis of Fe-Sr-Cn-Co centers related to the hydrogenase active sites.

Recently iron sulfides have been proposed as being central to the emergence of life.1 For example, Huber and Wächtershäuser showed that iron sulfides catalyze carbonylation reactions leading to the formation of peptides and thioesters.2 The two best characterized Fe-S-CO enzymes (the hydrogenases) also feature cyanide, and it is likely that cyanide has a decisive stabilizing effect on the CO binding. Cyanide has been previously considered in the prebiotic context,3 but the influence of cyanide on the carbonyl chemistry of iron has received scant attention. In this report, we show that cyanide has a major effect on the carbonylation of ferrous salts, especially in the presence of sulfur ligands. CO-saturated MeCN slurries of FeCl2 were treated sequentially with NaSAr and Et4NCN to give good yields of (Et4N)2[Fe(SAr)2(CN)2(CO)2] (1a, Ar ) Ph; 1b, Ar ) p-tol) (Scheme 1). These same species also form in low yield upon treatment of Fe3(SPh)6(CO)6 with CN-.4 NMR and IR5 spectroscopic studies established that 1a and 1b exist in solution as both the trans,cisand cis,cis-isomers. The molecular structure of trans,cis-1a was determined crystallographically (Figure 1). In solution 1 is configurationally stable under a CO atmosphere, although in the absence of CO it suffers ligand redistribution to give (Et4N)2[Fe(SPh)4] and trans-[Fe(CN)4(CO)2] (2) (vide infra). Markó had previously demonstrated the carbonylation of Fe(II) thiolate solutions in the presence of chelating donor ligands, e.g. bipyridine, ethylenediamine, and Ph2PCH2CH2PPh2. Carbonylation of nonaqueous Fe(II)/EtSsolutions (as described for the preparation of 1) in the presence of CNafforded 2, not analogues of 1. IR spectra of fresh reaction solutions indicate that [Fe(SEt)2(CN)2(CO)2] is in fact formed, but that this species redistributes readily to the tetracyanide. In the absence of CN-, carbonylation of Fe(II)/PhSsolutions gives the ferrous derivatives Fe3(SPh)6(CO)6 and [Fe(SPh)3(CO)3]. The carbonylation of Fe(II)/NaSEt solutions affords the subferrous species Fe2(SEt)2(CO)6 as first reported by Reihlen.8 The yields are low (3-6%), and pyrrophoric iron metal is also formed in substantial amounts, but the formation of low valent iron may be significant in view of the likely role of subferrous species in the iron-only hydrogenases.9 Interestingly, the reductive nature of this carbonylation is quenched by the presence of cyanide. We showed that Fe2(SEt)2(CO)6 reacts with CNto give [Fe2(SEt)2(CN)2(CO)4], but such subferrous species are not observed when the Fe(II)/NaSEt solutions are carbonylated in the presence of CN(Scheme 1),10 i.e., our studies do not support the spontaneous assembly of hydrogenase-like subferrous species from Fe(II)/CNsolutions. In analogy to the preparation of 1, we examined the carbonylation of Fe(II) solutions in the presence of benzenedithiolate dianion (bdt2) C6H4S2). This reaction afforded complexes (Et4N)2[Fe(bdt)(CN)2(CO)] (3) and (Et4N)2[Fe(bdt)(CN)2(CO)2] (4). Initially complex 4 is observed spectroscopically; however, purging N2 through the reaction solution gave 3, which was characterized crystallographically as being pentacoordinate (Figure 2). The Fe-CN and Fe-CO distances differ by 0.2 Å, consistent with the strong π-bonding role of the CO vs the primary σ-interaction for the CNligand. Several related 16 epentacoordinate Fe(II) dithiolenes are known, e.g., Fe(bdt)(PMe3)3 and Fe[S2C2(SMe)2](CO)(PR3)2, but 3 is distinctive because it very closely simulates the Fe site in the [NiFe]-hydrogenases, which also feature (SR)2(CN)2(CO) coordination.12 Previously the best models for this site included the octahedral complex [Fe(SR)3(PR3)(CN)(CO)], the Fe unit in [{Fe(NS3)(1) Cody, G. D.; Boctor, N. Z.; Filley, T. R.; Hazen, R. M.; Scott, J. H.; Sharma, A.; Yoder, H. S., Jr. Science 2000, 289, 1337-1340. (2) Huber, C.; Wächtershäuser, G. Science 1997, 276, 245-247. Huber, C.; Wächtershäuser, G. Science 1998, 281, 670-672. (3) Levy, M.; Miller, S. L.; Oro, J. J. Mol. EVol. 1999, 49, 165-168 and references therein. (4) Walters, M. A.; Dewan, J. C. Inorg. Chem. 1986, 25, 4889-4893. (5) Selected IR spectra in MeCN for 1-5 (cm-1). 1a: νCN 2103(w), 2094(w), 2075(vw); νCO 2007(s), 1976(sh) 1953(s). 2: νCN 2103 (s); νCO 1999 (s). 3: νCN 2080(w), 2075(vw); νCO 1897(s). 4: νCN 2104(vw), 2096(w); νCO 2006(s), 1949(s). 5: νCN 2196(s), 2159(s); νCO 2005(s), 1949(s). (6) (a) Takács, J.; Markó, L. Trans. Met. Chem. 1984, 9, 10-12. (b) Takács, J.; Markó, L.; Párkányi, L. J. Organomet. Chem. 1989, 361, 109-116. (c) Takács, J.; Soós, E.; Nagy-Magos, Z.; Markó, L.; Gervasio, G.; Hoffmann, T. Inorg. Chim. Acta 1989, 166, 39-46. (7) Nagy-Magos, Z.; Markó, L.; Szakács-Schmidt, A.; Gervasio, G.; Belluso, E.; Kettle, S. F. Bull. Soc. Chim. Belg., 1991, 100, 445-458. (8) Reihlen, H.; Friedolsheim, A. v.; Ostwald, W. Justus Liebigs Ann. Chem. 1928, 465, 72-96. (9) De Lacey, A. L.; Stadler, C.; Cavazza, C.; Hatchikian, E. C.; Fernandez, V. M. J. Am. Chem. Soc. 2000, 122, 11232-11233. (10) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121, 9736-9737. (11) (a) Sellmann, D.; Kleine-Kleffmann, U.; Zapf, L.; Huttner, G.; Zsolnai, L. J. Organomet. Chem. 1984, 263, 321-331. (b) Ghilardi, C. A.; Laschi, F.; Midollini, S.; Orlandini, A.; Scapacci, G.; Zanello, P. J. Chem. Soc., Dalton Trans. 1995, 531-540. (c) Touchard, D.; Fillaut, J.-L.; Khasnis, D. V.; Dixneuf, P. H.; Mealli, C.; Masi, D.; Toupet, L. Organometallics 1988, 7, 67-75. (12) (a) Volbeda, A.; Garcin, E.; Piras, C.; de Lacey, A. L.; Fernandez, V. M.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 1996, 118, 12989-12996. (b) Fontecilla-Camps, J. C.; Ragsdale, S. W. AdV. Inorg. Chem. 1999, 47, 283-333. Figure 1. Structure of the dianion in (Et4N)2[Fe(SC6H5)2(CN)2(CO)2] (1a) with thermal ellipsoids set at the 50% probability level. Selected distances (Å) and angles (deg): Fe-C1, 1.935(4); Fe-C2, 1.928(4); FeC3, 1.782(4); Fe-C4, 1.805(4); Fe-S1, 2.3479(10); Fe-S2, 2.3489(10); S(1)-Fe(1)-S(2), 82.43(3).