Rationalising reactivity: a Combined DFT and Hyperpolarisation Approach

The complexes Ru(CO)3(dpae) and Ru(CO)2(dpae)(PPh3) have been found experimentally to undergo various reactions with para-hydrogen and substrates. Reactions with para-hydrogen and diphenylacetylene led to the detection of hydrogenation products, confirming the complexes as hydrogenation catalysts. The catalytic behaviour was identified to be different to that of the equivalent phosphine containing complex. High level DFT investigations have revealed significant insight into the mechanism of reaction. The experimentally detected dihydride complex Ru(H)2(CO)(dpae) was calculated to be a viable reaction product, with various pathways modelled for rearrangement. In contrast, the rearrangements for the complex Ru(H)2(CO)dpae)(PPh3) were found to compete with the reductive elimination of dihydrogen. The routes of reaction by initiation method was examined, with the high energy 14-electron intermediates only accessible photochemically. Routes for the hydrogenation of diphenylacetylene were identified, alongside the mechanism of cis-trans scrambling of stilbene and formation of 1,2-diphenylethane. The formation of 1,2,3,4-tetraphenylbutadiene was also rationalised. The reaction of hydrogen with W(N2)2(dppe-κ2P)2 was shown theoretically to involve an intra-molecular ortho-metallation reaction from the reactive 14-electron intermediate W(dppe-κ2P)2. Low barriers were obtained from the 16-electron intermediate W(H)2(dppe-κ2P)2. This rationalised the formation of the experimentally proposed complex W(H)3(dppe-κ2P)(PPh(C6H4CH2CH2Ph2P)-κ2P). The 14-electron intermediate W(dppe-κ2P)2 was calculated to adopt a butterfly geometry in a singlet state, which than rearranges upon reaction to form 16-electron intermediates. The observation of PHIP in the end products confirms the involvement of an electronic singlet state. Limited solvation was predicted from THF despite its ability to coordinate to metal centres. In summary, the combination of high level DFT models and the use of para-hydrogen reactions is demonstrated to be a powerful tool for probing chemical processes and pathways, and contributes to achieving a greater understanding of reactivity in these metal complex systems