Reaction mechanism of silicon-hydrogen bond activation studied using femtosecond to nanosecond IR spectroscopy and ab initio methods

The SI-H bond activation reactions by group VIIB, d(6) organometallic compounds eta(5)-CpM(CO)(3) (M = Mn. Re; Cp = Cs Hs) were studied in neat triethylsilane under ambient conditions. Utilizing femtosecond and nanosecond pump-probe spectroscopic methods, the spectral evolution of the CO stretching bands was monitored from 300 fs to tens of microseconds following UV photolysis. The reactive intermediates observed on the ultrafast time scale were also studied using ab initio quantum chemical modeling. It was found that photolysis of the manganese tricarbonyl resulted in dicarbonyls in their singlet or triplet electronic states, whereas photolysis of the rhenium complex led only to the singlet dicarbonyl. The branching ratio of the two manganese intermediates was measured and was related to different electronic excited states. For both the Mn and Re complexes, the reactions were found to be divided into two pathways of distinct time scales by the initial solvation of the dicarbonyls through the Si-H bond or an ethyl group of the solvent molecule. The time scale for the SI-H bond-breaking process was, for the first time, experimentally derived to be 4.4 ps, compared to 230 ns for breaking an alkane C-H bond. Knowledge of the elementary reaction steps including changes in molecular morphology and electronic multiplicity allowed a comprehensive description of the reaction mechanisms for these reactions.