Photoinduced bond oscillations in ironpentacarbonyl give delayed synchronous bursts of carbonmonoxide release

Ambar Banerjee,Michael Odelius,Philippe Wernet,Markus Kowalewski,Hampus Wikmark,Raphael M Jay,Michael R Coates

doi:10.1038/s41467-022-28997-z

Ambar Banerjee, Michael Odelius + Show 5 more

Open Access

https://doi.org/10.1038/s41467-022-28997-z

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Abstract

Early excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. However, time-resolved experiments have not yet revealed mechanistic details in the sub-picosecond regime. Hence, in this study the photoexcitation of ironpentacarbonyl Fe(CO)5 is simulated with semi-classical excited state molecular dynamics. We find that the bright metal-to-ligand charge-transfer (MLCT) transition induces synchronous Fe-C oscillations in the trigonal bipyramidal complex leading to periodically reoccurring release of predominantly axial CO. Metaphorically the photoactivated Fe(CO)5 acts as a CO geyser, as a result of dynamics in the potential energy landscape of the axial Fe-C distances and non-adiabatic transitions between manifolds of bound MLCT and dissociative metal-centered (MC) excited states. The predominant release of axial CO ligands and delayed release of equatorial CO ligands are explained in a unified mechanism based on the σ*(Fe-C) anti-bonding character of the receiving orbital in the dissociative MC states.

Highlights

Excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates
From this theoretical study of the mechanism of the photodissociation of ironpentacarbonyl, we can reveal a mechanistic pathway closely related to conceptual ideas in the previous studies[2,8]
Instead of non-adiabatic transitions between states with forces acting in different directions, as suggested in ref. 8, we observe oscillations of a Fe–C breathing mode in the metal-to-ligand charge-transfer (MLCT) state yielding regular bursts of CO release after nonadiabatic transitions to MC states

Summary

Introduction

Excited state dynamics in the photodissociation of transition metal carbonyls determines the chemical nature of short-lived catalytically active reaction intermediates. The choice of especially first-row transition metal carbonyls for these studies is motivated by their relative simplicity in terms of electronic structure and chemistry, due to the small number of electrons and limited relativistic effects Of these systems, ironpentacarbonyl Fe(CO)[5] has been one of the most thoroughly investigated systems[7], in particular the processes initiated by photoinduced metal-to-ligand chargetransfer (MLCT). Ramasesha and co-workers have very recently confirmed this singlet dissociation pathway using ultrafast IR spectroscopic techniques, and have estimated that triplet involvement and inter-system crossing happen in 15 ps time frame[11] From these studies, insights into the kinetics of the process and the chemical changes based on charge localization were obtained[12]. Analogous to the Fe(CO)[5] system, Cr(CO)[6] has been studied with excited state MD simulations using forces from time-dependent density functional theory (TDDFT) to follow dynamics in the lowest singlet excited states S1, S2, and S331

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