Silicon–Heteroaromatic [FeFe] Hydrogenase Model Complexes: Insight into Protonation, Electrochemical Properties, and Molecular Structures

Abstract

To learn from Nature how to create an efficient hydrogen-producing catalyst, much attention has been paid to the investigation of structural and functional biomimics of the active site of [FeFe]-hydrogenase. To understand their catalytic activities, the μ-S atoms of the dithiolate bridge have been considered as possible basic sites during the catalytic processes. For this reason, a series of [FeFe]-H2 ase mimics have been synthesized and characterized. Different [FeFe]-hydrogenase model complexes containing bulky Si-heteroaromatic systems or fluorene directly attached to the dithiolate moiety as well as their mono-PPh3 -substituted derivatives have been prepared and investigated in detail by spectroscopic, electrochemical, X-ray diffraction, and computational methods. The assembly of the herein reported series of complexes shows that the μ-S atoms can be a favored basic site in the catalytic process. Small changes in the (hetero)-aromatic system of the dithiolate moiety are responsible for large differences in their structures. This was elucidated in detail by DFT calculations, which were consistent with the experimental results.

[FeFe]‐Hydrogenase H‐Cluster Mimics with Unique Planar μ‐(SCH2)2ER2 Linkers (E=Ge and Sn)

“…Furthermore, we have tested the electrochemical behavior of the models 3 and 4 a in the presence of the acids CF 3 CO 2 H (p K a MeCN =12.7) and CF 3 SO 3 H (p K a MeCN =0.7) . The presence of CF 3 CO 2 H in the solution of 3 (or 4 a ) shifts their primary cathodic peak to less negative potentials by approximately 110 mV, which is a typical observation when protonation of a reduced species takes place . As evident from Figure , complexes 3 and 4 a reveal catalytic behavior through two processes: process I at E p cat ≈−1.78 V for 3 and 4 a and process II at E p cat ≈−1.91 V or −1.89 V for 3 or 4 a , respectively, where E p cat is the peak potential of the catalytic waves in Figure .…”

“…The flap angle ( α ) formed from the intersection between the C 2 E and S 2 C 2 planes, as shown in Figure , lies within the range 118°> α >160° in various [FeFe]‐hydrogenase models. Only one complex, namely Fe 2 (CO) 6 {(μ‐SCH 2 ) 2 Si(1‐silafluorene)}, has a large angle α =171°. In the planar conformation, α =180° and the ‐SCECS‐ moiety is planar.…”

“…The synthetic models related to the structure of the H‐cluster may be divided into two categories: 1) bioinspired models with azadithiolato linkers, and 2) artificial models with abiological linkers μ‐(XCH 2 ) 2 Y in which the central atom/group Y is CR 2 , O, S, Se, SiR 2 , or Ph‐P=O and X could be S, Se, Te,, or PR . The use of these abiological linkers may provide valuable information and answers to basic questions concerning the role of the dithiolato bridge as well as the effect of the unusual Y on the structural and functional properties of such artificial H‐cluster mimics.…”

“…For example, Goy et al. recently reported a model with an especially bulky Si‐containing moiety, namely Fe 2 (CO) 5 (PPh 3 ){(μ‐SCH 2 ) 2 Si(1‐silafluorene)} . In this case, protonation of the sulfur atoms by triflic acid is favored by the interplay of the presence of the Si atom, the steric bulkiness of the Si‐containing moiety, and the presence of one σ‐donor PPh 3 ligand at the diiron core.…”

Photocatalytic Hydrogen Evolution Driven by [FeFe] Hydrogenase Models Tethered to Fluorene and Silafluorene Sensitizers

“…Herein, we report the complete characterization of the spectroscopic and photocatalytic properties of our photoactive [FeFe]‐H 2 ase mimics tethered to a fluorene or silafluorene sensitizer unit 1 a – 4 b (Scheme ) . Photocatalytic H 2 evolution experiments were performed in an organic environment and in aqueous solutions.…”