Iron Carbonyl Sulfides, Formaldehyde, and Amines Condense To Give the Proposed Azadithiolate Cofactor of the Fe-Only Hydrogenases

Abstract

The azadithiolate (SCH2NHCH2S) cofactor proposed to occur in the Fe-only hydrogenases forms efficiently by the condensation of Fe2(SH)2(CO)6 (1), formaldehyde, and ammonia (as (NH4)2CO3). The resulting Fe2[(SCH2)2NH](CO)6 reacts with Et4NCN to give (Et4N)2[Fe2[(SCH2)2NH](CO)4(CN)2], for which crystallographic characterization confirmed an axial N-H and an elongated C-S bond of 1.858(3) A. Primary amines RNH2 (R = Ph, t-Bu) also participate in the condensation reaction, and Fe3S2(CO)9 can be employed in place of 1. Mechanistically, the Fe2[(SCH2)2NH] moiety is shown to arise via two pathways: (i) via the intermediacy of Fe2[(SCH2OH)2](CO)6, which was detected and shown to react with amines, and (ii) via the reaction of 1 with cyclic imines (CH2)3(NR)3 (R = Ph, Me). The reaction of 1 with (CH2)6N4 (hexamethylenetetramine) gives Fe2[(SCH2)2NH](CO)6. Trace amounts of Fe2[(SCH2)2N-t-Bu](CO)6 arise via the reaction of aqueous FeSO4, formaldehyde, NaSH, and t-BuNH2 under an atmosphere of CO.

A comparative study of the electrochemical and proton-reduction behaviour of diphosphine-dithiolate complexes [M2(CO)4(μ-dppm){μ-S(CH2) n S}] (M = Fe, Ru; n = 2, 3)

“…This observation proves that the protonated 1 and 2 are indeed responsible for the catalytic waves seen at more positive potential than their first reduction wave. We speculate a CECE mechanism which is involved for this catalytic event as observed for related complexes [13][14][15]. However, an attempt to monitor this slow protonation process via NMR spectroscopy was unsuccessful.…”

“…Dithiolate-bridged diiron complexes of the type [Fe 2 (CO) 6 -(l-dithiolate)] have been intensely studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] due to their structural resemblance with the two-iron unit of the H-cluster active site of [FeFe]-hydrogenases, enzymes that catalyse the reversible interconversion of protons-electrons and hydrogen. A key step in electrocatalytic proton reduction is protonation of the diiron centre, but [Fe 2 (CO) 6 (l-dithiolate)] complexes are not basic enough to undergo protonation except by extremely strong acids [20][21][22].…”

Biomimetic hydrogen generation catalyzed by triironnonacarbonyl disulfide cluster

“…10 The reactivity of Fe 3 S 2 (CO) 9 has been explored and continues to be of great interest. 1,[11][12][13][14] Some important reactions of Fe 3 S 2 (CO) 9 are depicted in Scheme 1: (a) the reaction of Fe 3 S 2 (CO) 9 with formaldehyde and amines to form diiron clusters of the type, Fe 2 [(SCH 2 ) 2 NR](CO) 6 , models for the active site of [Fe-Fe] hydrogenase enzyme ( Figure 2) 14 and (b) the photochemical transformation of Fe 3 S 2 (CO) 9 in the presence of Fe(CO) 5 to the tetrairon cluster, Fe 4 S 2 (CO) 11 . Although the reactivity, electrochemical properties, and electronic structure of Fe 3 S 2 (CO) 9 have been investigated, to the best of our knowledge, its catalytic properties remain unexplored.…”

Design of Biomimetic Models Related to the Active Sites of Fe-Only Hydrogenase

“…If the aza nitrogen site is to assist the enzymatic H 2 production/uptake, availability of the vacant site on the nearby Fe center is a compliment necessity for the high catalytic turnover frequency of the entatic state. Isolation of the model complexes possessing the azadithiolate bridges has been successfully accomplished Li & Rauchfuss, 2002;Ott et al, 2004). On the contrary, difficulties have been encountered when the model compounds with the apical empty site are prepared albeit few unstable examples have been reported.…”