Synthetic and structural studies on [Fe2(SR)2(CN)x(CO)6-x]x- as active site models for Fe-only hydrogenases

A series of models for the active site (H-cluster) of the iron-only hydrogenase enzymes (Fe-only H-2-ases) were prepared. Treatment of MeCN solutions of Fe-2(SR)(2)(CO)(6) with 2 equiv of Et4NCN gave [Fe-2(SR)(2)(CN)(2)(CO)(4)](2-) compounds. IR spectra of the dicyanides feature four nu (CO) bands between 1965 and 1870 cm(-1) and two nu (CN) bands at 2077 and 2033 cm(-1). For alkyl derivatives, both diequatorial and axial-equatorial isomers were observed by NMR analysis. Also prepared were a series of dithiolate derivatives (Et4N)(2)[Fe-2(SR)(2)(CN)(2)(CO)(4)] where (SR)(2) = S(CH2)(2)S, S(CH2)(3)S. Reaction of Et4NCN with Fe-2(S-t-Bu)(2)(CO)(6) gave initially [Fe-2(S-t-Bu)(2)(CN)(2)(CO)(4)](2-), which comproportionated to give [Fe-2(S-t-Bu)(2)(CN)(CO)(5)](-). The mechanism of the CN--for-CO substitution was probed as follows: (i) excess CN- with a 1:1 mixture of Fe-2(SMe)(2)(CO)(6) and Fe-2(SC6H4Me)(2)(CO)(6) gave no mixed thiolates, (ii) treatment of Fe-2(S2C3H6)(CO)(6) with Me3NO followed by Et4NCN gave (Et4N)[Fe-2(S2C3H6)(CN)(CO)(5)], which is a well-behaved salt, (iii) treatment of Fe-2(S2C3H6)(CO)(6) with Et4NCN in the presence of excess PMe3 gave (Et4N)[Fe-2(S2C3H6)(CN)(CO)(4)(PMe3)] much more rapidly than the reaction of PMe3 with (Et4N)[Fe-2(S2C3H6)(CN)(CO)(5)], and (iv) a competition experiment showed that Et4NCN reacts with Fe-2(S2C3H6)(CO)(6) more rapidly than with (Et4N)[Fe-2(S2C3H6)(CN)(CO)(5)]. Salts of [Fe-2(SR)(2)(CN)(2)(CO)(4)](2-) (for (SR)(2) = (SMe)(2) and S2C2H4) and the monocyanides [Fe-2(S2C3H6)(CN)(CO)(5)](-) and [Fe-2(S-t-Bu)(2)(CN)(CO)(5)](-) were characterized crystallographically: in each case, the Fe-CO distances were similar to 10% shorter than the Fe-CN distances. The oxidation potentials for Fe-2(S2C3H6)(CO)(4)L-2 become milder for L = CO, followed by MeNC, PMe3, and CN-; the range is similar to1.3 V. In water, oxidation of [Fe-2(S2C3H6)(CN)(2)(CO)(4)](2-) occurs irreversibly at -0.12 V (Ag\AgCl) and is coupled to a second oxidation.