ESI-MS Studies and Calculations on Second-Generation Grubbs and Hoveyda–Grubbs Ruthenium Olefin Metathesis Catalysts

Abstract

Electrospray ionization mass spectrometry (ESI-MS) and subsequent MS/MS methods were used to study second-generation Grubbs catalysts 2 and 3 and first- and second-generation Hoveyda–Grubbs catalysts 4 and 5, respectively, as well as the pyridine-tethered Ru carbene catalyst 6. Direct ESI-MS analyses of Ru catalysts 2–6 showed the corresponding radical cations 2•+–6•+ and the protonated ligand PCy3 and H2IMes, respectively. Alkali metal adduct ions 2·M+ and 3·M+ (M = Li, Na, K, Cs) and 4·M+–6·M+ (M = Li, K) could be easily obtained by mixing the CH2Cl2 solution of catalysts 2–6 with the CH3OH solution of alkali-metal chloride using an online microreactor coupled directly to the electrospray ion source of a quadrupole time-of-flight (Q-TOF) mass spectrometer and were studied by collision-induced dissociation (CID). Remarkably, the alkali metal cationized 14-electron Ru complexes 2a·M+ and 3a·M+ formed by dissociation of phosphine from 2 and 3, respectively, were detected directly from solution. The ratio [2a·M+]/[2·M+] increased with decreasing Lewis acidity of M+ from Li+ to Cs+. Moreover, theoretical computations were performed on Ru complexes 2, 5, and 6, showing good agreement with experimental X-ray diffraction data and providing more structural information about the alkali metal adduct ions 2·M+, 5·M+, and 6·M+ (M = Li, K) as well as about the 14-electron species 2a, 5a, and 6a and the respective alkali metal adduct ions.