Contrasting Photochemical and Thermal Catalysis by Ruthenium Arsine Complexes Revealed by Parahydrogen Enhanced NMR Spectroscopy

The thermal and photochemical reactivity of Ru(CO)(3)(dpae) (1) and Ru(CO)(2)(dpae)(PPh3) (2) towards H-2 and diphenylacetylene is described. These reactions are monitored by NMR spectroscopy in conjunction with the parahydrogen induced polarisation (PHIP) effect, spatially resolved chemical shift imaging and the Only Parahydrogen Spectroscopy (OPSY) signal filtering method. The results are supported by DFT. The thermal and photochemical reactions of 1 with H-2 proceed by CO loss and form Ru(H)(2)(CO)(2)(dpae) (3). 1 catalyses the formation of 1,2 diphenylethane, cis- and trans-stilbene, and 1,2,3,4 tetraphenyl-butadiene under 325 nm irradiation at 295 K in a reaction where Ru(CO)(2)(dpae)(eta(2)-CHPh=CPhCPh=CHPh) forms. When the same reaction is monitored under thermal conditions at 333 K the eta(2)-diene complex is no longer detected but hydride containing Ru(CHPhCH2Ph)(H)(CO)(2)(dpae) and Ru(CO)(2)(dpae)(trans-stilbene) are seen. For 1, the photochemical promotion of hydrogenation through 325 nm irradiation results in an approximate 5.5-fold increase in turnover at 333 K when compared to no irradiation. In contrast, 2 reacts thermally with H-2 at 295 K through PPh3 and CO loss with both Ru(H)(2)(CO)(dpae)(solvent) and 3 being detected. Under irradiation, CO loss dominates and two isomers of Ru(H)(2)(CO)(dpae)(PPh3) form. While 2 forms the same 4 organic products at 295 K and a second isomer of Ru(CHPhCH2Ph)(H)(CO)(2)(dpae) alongside the diene complex no photochemical promotion of hydrogenation is observed.

Automated summary

This article describes the thermal and photochemical reactions of Ru metal complexes with H-2 and diphenylacetylene. The reactions were monitored and validated using various techniques including NMR spectroscopy. The results show that the reaction mechanisms and products vary with different complexes and reaction conditions.