Impact of the 2Fe2P core geometry on the reduction chemistry of phosphido-bridged diiron hexacarbonyl compounds

Abstract

The effect of core geometry constraints of hydrogenase H-cluster analogues on reduction chemistry have been explored by a combination of structural, electrochemical and IR spectroelectrochemical (IR-SEC) studies. A series of phosphido-bridged diiron hexacarbonyl complexes, Fe 2 (µ 2 -PPh 2 (CH 2 ) x PPh 2 )(CO) 6 , x = 2 (2P) and 4 (4P) and previously reported with x = 3 (3P) and the unlinked bis-diphenylphosphido (DP) analogues were investigated. The X-ray structures of the neutral complexes demonstrate the effect of the linking group on the Fe2P2 core geometry with P-Fe-Fe-P torsion angles of 95 (2P), 101 (3P), 108 (4P) and 109° (DP) and a twisting of the Fe(CO) 3 fragments from an eclipsed geometry (2P, 3P and DP) for 4P. For all four compounds the primary reduction process involves two close-spaced one-electron reactions (E 1 and E 2 ) with a systematic trend to more negative reduction potentials with a shorter link between the bridging phosphorus atoms. This reflects the greater constraint that the bridging group places on the adoption of a planar 2Fe2P geometry. The sensitivity of the core geometry is greater for E 2 than E 1 and this impacts the stability of the monoanion with respect to disproportion (K disp (298 K) = 0.02 (2P), 2.4 (3P) and 3540 (4P and DP)). 4P has a stable dianion and gives reversible cyclic voltammetry at 298 K and is quasi-reversible at 253 K, whereas the response of 2P is irreversible at 298 K, with two distinct daughter products, but becomes quasi-reversible at 253 K. IR-SEC measurements enabled elucidation of the spectra and time evolution of the reduction products. These results are consistent with a bimolecular reaction giving a distinct reduced product modelled as a dimeric, 4Fe species. The sensitivity of the reduction chemistry of the bridged diiron compounds underpins their utility as catalytic proton reduction catalysts and the systematic trends delineated in this investigation provide the framework for charting the path of their redox-coupled chemical reactions.

Normal vs. Inverted Ordering of Reduction Potentials in [FeFe]‐Hydrogenases Biomimetics: Effect of the Dithiolate Bulk

“…Combined sequential one‐electron transfers and ECE process have been also reported [50] . Single step two‐electron reduction processes due to potential inversion have been observed in several instances for hexacarbonyl diiron dithiolate, as well as for diselenolate or diphosphide, complexes [49–77] …”

Aza-Diphosphido-Bridged Di-Iron Complexes Related to the [FeFe]-Hydrogenases

“…In recent decades, the chemistry of carbonyl iron complexes has undergone intensive development within the field of biomimicry. Alongside numerous reports on carbonyl dithiolato-bridged di-iron complexes structurally close to the active site of [FeFe]hydrogenases (H-cluster) (Scheme 1a) [1-5], phosphido-bridged analogues (Scheme 1b,c) were studied in order to compare the electronic and stereochemical effects of the replacement of the µ-S 2 R bridge with the µ-PR 2 group on the reactivity and redox properties of such carbonyl di-iron systems [6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The chemistry of bis-phosphido-bridged di-iron complexes of general formula [Fe 2 (CO) 6 (µ-PR 2 ) 2 ] and linked-diphosphido [Fe 2 (CO) 6 (µ-(PR) 2 R')] dates back much earlier .…”

“…The chemistry of bis-phosphido-bridged di-iron complexes of general formula [Fe 2 (CO) 6 (µ-PR 2 ) 2 ] and linked-diphosphido [Fe 2 (CO) 6 (µ-(PR) 2 R')] dates back much earlier . Electrochemical and theoretical calculations on robust complexes [Fe 2 (CO) 6 (µ-PRR')(µ-PRR ′′ )] were carried out because most of these compounds present a reversible two-electron reduction involving a potential inversion [15,16,44] depending on R, R' and R ′′ groups which affect Fe-P distances and the geometry of the {Fe 2 P 2 } core. Among these complexes with a {Fe 2 P 2 } core, diphosphido compounds featuring a µ-η 2 :η 2 -(RP-CH 2 -R'-CH 2 -PR ′′ ) bridge were more recently the subject of interest (Scheme 1c) [11][12][13][14][15][16][17].…”