Investigating the addition of hydrogen to transition metal centres in conjunction with NMR spectroscopy and photochemistry within an NMR probe

Strategy: The aim in this project is to investigate the addition of hydrogen to transition metal centres in conjunction with NMR spectroscopy and photochemistry within an NMR probe. This approach will employ parahydrogen to enable the detection of true reaction intermediates by harnessing the increase in NMR signal strength by factors which approach 30,000 that is available at 9.4 T. This gain will be used to enable the monitoring of reactivity through magnetisation transfer studies in conjunction the observation of new reaction intermediates, and ultimately help to probe the role of the detected reaction intermediates in catalysis.1

Background: We have demonstrated that the generation of a reaction product with an essentially pure spin state is possible using p-H2 (Fig 1). This reaction was initiated by a pulse of UV light from a XeCl excimer laser that was triggered by the NMR spectrometer and featured Ru(CO)2(L2) which forms Ru(H)2(CO)2(L2) with H2. This result, illustrated in Figure 1, shows how we can easily detect photoproducts using the signal gain that is delivered with p-H2.

When a continuous UV irradiation source is employed, it is possible to build up appreciable amounts of a photoproduct at low temperature and then characterise it. However, when this continuous approach is used with p-H2, it also allows the continuous formation of NMR enhancing molecules. We seek to harness this route in conjunction with advanced NMR methods to characterise the reaction intermediates we detect. Furthermore, by varying the temperature, we can control reactivity and thereby their role in catalysis. Catalysis can also be regulated by adding a weakly binding co-ligand which has the potential reduce activity by trapping active species and forming materials that are amenable to characterisation before using photochemistry to reactivate the system.1

Metal dihydrides in catalysis: Our studies on Ru(CO)3L(H)2 and Ru(CO)2(L)2(H)2 have revealed that they can be used to form highly reactive species such as Ru(CO)4, Ru(CO)3 and Ru(CO)2(L) depending on the identity of L. By detecting intermediates such Ru(CO)2(L)(alkyne)(H)2 through trapping with hydrogen and an alkyne we hope to be able to probe their precise role in hydroformylation and hydrogenation. We have already detected related species during thermal studies with the cation Pd(L)2H+; these include vinyl, alkyl and acyl complexes such as Pd(L)2(CPh=CHPh)+ which can be stabilised by adding a passivating two-electron donor such a pyridine to facilitate detection. An array of palladium and ruthenium complexes, in addition to some isoelectronic rhodium spices, will be examined as part of this work.

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