Synthetic and Structural Studies on [Fe2(SR)2(CN)x(CO)6-x]x- as Active Site Models for Fe-Only Hydrogenases
F Gloaguen, JD Lawrence, M Schmidt… - Journal of the …, 2001 - ACS Publications
F Gloaguen, JD Lawrence, M Schmidt, SR Wilson, TB Rauchfuss
Journal of the American Chemical Society, 2001•ACS PublicationsA series of models for the active site (H-cluster) of the iron-only hydrogenase enzymes (Fe-
only H2-ases) were prepared. Treatment of MeCN solutions of Fe2 (SR) 2 (CO) 6 with 2
equiv of Et4NCN gave [Fe2 (SR) 2 (CN) 2 (CO) 4] 2-compounds. IR spectra of the
dicyanides feature four νCO bands between 1965 and 1870 cm-1 and two νCN 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 …
only H2-ases) were prepared. Treatment of MeCN solutions of Fe2 (SR) 2 (CO) 6 with 2
equiv of Et4NCN gave [Fe2 (SR) 2 (CN) 2 (CO) 4] 2-compounds. IR spectra of the
dicyanides feature four νCO bands between 1965 and 1870 cm-1 and two νCN 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 …
A series of models for the active site (H-cluster) of the iron-only hydrogenase enzymes (Fe-only H2-ases) were prepared. Treatment of MeCN solutions of Fe2(SR)2(CO)6 with 2 equiv of Et4NCN gave [Fe2(SR)2(CN)2(CO)4]2- compounds. IR spectra of the dicyanides feature four νCO bands between 1965 and 1870 cm-1 and two νCN 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[Fe2(SR)2(CN)2(CO)4], where (SR)2 = S(CH2)2S, S(CH2)3S. Reaction of Et4NCN with Fe2(S-t-Bu)2(CO)6 gave initially [Fe2(S-t-Bu)2(CN)2(CO)4]2-, which comproportionated to give [Fe2(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 Fe2(SMe)2(CO)6 and Fe2(SC6H4Me)2(CO)6 gave no mixed thiolates, (ii) treatment of Fe2(S2C3H6)(CO)6 with Me3NO followed by Et4NCN gave (Et4N)[Fe2(S2C3H6)(CN)(CO)5], which is a well-behaved salt, (iii) treatment of Fe2(S2C3H6)(CO)6 with Et4NCN in the presence of excess PMe3 gave (Et4N)[Fe2(S2C3H6)(CN)(CO)4(PMe3)] much more rapidly than the reaction of PMe3 with (Et4N)[Fe2(S2C3H6)(CN)(CO)5], and (iv) a competition experiment showed that Et4NCN reacts with Fe2(S2C3H6)(CO)6 more rapidly than with (Et4N)[Fe2(S2C3H6)(CN)(CO)5]. Salts of [Fe2(SR)2(CN)2(CO)4]2- (for (SR)2 = (SMe)2 and S2C2H4) and the monocyanides [Fe2(S2C3H6)(CN)(CO)5]- and [Fe2(S-t-Bu)2(CN)(CO)5]- were characterized crystallographically; in each case, the Fe−CO distances were ∼10% shorter than the Fe−CN distances. The oxidation potentials for Fe2(S2C3H6)(CO)4L2 become milder for L = CO, followed by MeNC, PMe3, and CN-; the range is ∼1.3 V. In water, oxidation of [Fe2(S2C3H6)(CN)2(CO)4]2- occurs irreversibly at −0.12 V (Ag|AgCl) and is coupled to a second oxidation.
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