Using Vibrational Spectroscopy to understand the catalytic behaviour of iron carbonyl catalysts

Detailed understanding of the mechanistic pathways underpinning metal-catalysed reactions permit for rational development of complexes with efficiency and enhanced activity. Observing the bonding and formation events that underpin a catalytic reaction is necessary to truly be able to design better catalytic systems as it would be understood which species are short lived or if they are observable at low concentrations. The aim of this project is to investigate the mechanistic pathways controlling the behaviour of iron carbonyl complexes used as catalysts for transfer dehydrogenation on fast time scales. Ultrafast TRIR was performed, observations were made on timescales ranging from 1 ps to 1 ms using a pump wavelength of 330 nm on [1a] and [3a] in seven solvents. In all solvents, initial formation (< 1 ps) and relaxation (< 200 ps) of an excited state was observed. Followed (200 ps) by the observation of the photodissociation of a CO ligand and a rapid solvent coordination to the vacant site [2aSolvents] and [4aSolvents]. In five of the seven the aqua complex [2aOH2] and [4aOH2] formed and persisted for the rest of the experiment. In the case of isopropyl alcohol, transfer dehydrogenation occurs.

[1a] was also studied using IRPUMP-IRPROBE in three solvents to examine the vibrational dynamics and solvent interactions of the carbonyl ligand stretching modes. Ultrafast vibrational energy dissipation occurred in two solvent dependent relaxation pathways, rapid intramolecular vibrational redistribution (IVR ~ 0.2-7.6 ps) and relaxation to the ground vibrational state (T1 ~ 1.7-44.2 ps) different solvents affected the rate of the two pathways with THF having the quickest IVR and heptane the largest range of the T1. This study has provided a better understanding into the mechanistic pathways that underpin catalysis by iron carbonyl complexes, specifically, the fundamental processes underpinning CO-photodissociation and the nature of the activated iron catalyst.