Impact of the 2Fe2P core geometry on the reduction chemistry of phosphido-bridged diiron hexacarbonyl compounds†
Odi Th. E. Selan A B , Mun Hon Cheah
A C * , Brendan F. Abrahams A , Robert W. Gable A and Stephen P. Best A *
+ Author Affiliations
- Author Affiliations
A School of Chemistry, The University of Melbourne, Parkville, Melbourne, 3010, Vic., Australia.
B Department of Chemistry, Faculty of Science and Engineering, Nusa Cendana University, Kupang – NTT, 85001, Indonesia.
C Department of Chemistry, Molecular Biometics, Ångström Laboratory, Uppsala University, SE 75237 Uppsala, Sweden.
* Correspondence to: spbest@unimelb.edu.au, michael.cheah@kemi.uu.se
Handling Editor: George Koutsantonis
Australian Journal of Chemistry 75(9) 649-659 https://doi.org/10.1071/CH21309
Submitted: 30 November 2021 Accepted: 19 January 2022 Published: 26 February 2022
© 2022 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)
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, Fe2(µ2-PPh2(CH2)xPPh2)(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 (E1 and E2) 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 E2 than E1 and this impacts the stability of the monoanion with respect to disproportion (Kdisp(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.
Keywords: [Fe–Fe]-hydrogenase H cluster, bridged diiron carbonyl compounds, electrochemistry of transition metal complexes, IR spectroelectrochemistry, IR spectroscopy, phosphido-bridged diiron compounds, redox-coupled chemical reactions, transition metal carbonyl compounds.
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