Metal vs. ligand protonation and the alleged proton-shuttling role of the azadithiolate ligand in catalytic H2 formation with FeFe hydrogenase model complexes.

Alexander Aster,Shihuai Wang,Gustav Berggren,Leif Hammarström,Reiner Lomoth,Charlène Esmieu,Mohammad Mirmohades

doi:10.1039/c9sc00876d

Abstract

Electron and proton transfer reactions of diiron complexes [Fe2adt(CO)6] (1) and [Fe2adt(CO)4(PMe3)2] (4), with the biomimetic azadithiolate (adt) bridging ligand, have been investigated by real-time IR- and UV-vis-spectroscopic observation to elucidate the role of the adt-N as a potential proton shuttle in catalytic H2 formation. Protonation of the one-electron reduced complex, 1- , occurs on the adt-N yielding 1H and the same species is obtained by one-electron reduction of 1H+ . The preference for ligand vs. metal protonation in the Fe2(i,0) state is presumably kinetic but no evidence for tautomerization of 1H to the hydride 1Hy was observed. This shows that the adt ligand does not work as a proton relay in the formation of hydride intermediates in the reduced catalyst. A hydride intermediate 1HHy+ is formed only by protonation of 1H with stronger acid. Adt protonation results in reduction of the catalyst at much less negative potential, but subsequent protonation of the metal centers is not slowed down, as would be expected according to the decrease in basicity. Thus, the adtH+ complex retains a high turnover frequency at the lowered overpotential. Instead of proton shuttling, we propose that this gain in catalytic performance compared to the propyldithiolate analogue might be rationalized in terms of lower reorganization energy for hydride formation with bulk acid upon adt protonation.

Highlights

The adtH+ complex retains a high turnover frequency at the lowered overpotential. We propose that this gain in catalytic performance compared to the propyldithiolate analogue might be rationalized in terms of lower reorganization energy for hydride formation with bulk acid upon adt protonation
Spectrum of the protonation product proves that the adt ligand remains the preferred site of protonation, at least kinetically, upon one-electron reduction of 1. This conclusion is corroborated by the UV-vis spectrum of 1H (ESI†), which is similar to its precursor 1À, while metal protonation of 2À was previously shown to cause complete bleach of its visible absorption bands that arise from transitions involving predominantly metal-based orbitals
We could demonstrate by real-time spectroscopic observation that the initial protonation of hydrogenase model complex 1, in its one-electron reduced Fe2(I,0) state 1À, occurs on the adt ligand rather than the Fe2 core

Summary

Introduction

Proton binding sites in the second coordination sphere of catalytic metal centres are widely considered as an important design principle for facilitating proton coupled electron transfer in molecular catalysts for e.g. water splitting,[1,2,3,4,5] H2 formation and activation[6,7,8,9,10,11,12] or CO2 reduction.[13,14,15,16,17,18,19,20,21] For the directed design of synthetic catalysts it is, important to note that improvements to catalytic performance (TOF, overpotential) brought about by basic sites in the second coordination sphere are not necessarily arising from a proton relay activity. Edge Article analogue 2 in electrochemical H2 formation has been attributed to proton shuttling, involving speci cally terminal protonation of the metal centre by tautomerization of a adt-NH+ precursor upon one-electron reduction of the catalyst.[27] We were intrigued whether the proposed role of the adt ligand as proton shuttle and the formation of terminal hydride intermediates in these model complexes could be inferred from direct spectroscopic observation.

Results

Conclusion