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

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(center dot+)-6(center dot+) and the protonated ligand PCy3 and H(2)IMes, 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.