Excited-state dynamics of a ruthenium(II) catalyst studied by transient photofragmentation in gas phase and transient absorption in solution

- Ultrafast dynamics of new Ru(II) catalysts investigated in gas phase and solution.
- Catalyst activation (HCl loss) achieved in ion trap by UV photoexcitation.
- Electronic relaxation proceeds by IVR and IC followed by ground state dissociation.
- No triplet formation in contrast to other Ru-polypyridine complexes.
- Solvent prohibits catalyst activation in solution by fast vibrational cooling.

We report studies on the excited state dynamics of new ruthenium(II) complexes [(η6-cymene)RuCl(apypm)]PF6 (apypm2-NR2-4-(pyridine-2-yl)-pyrimidine, RCH3 (1)/H (2)) which, in their active form [1+-HCl] and [2+-HCl], catalyze the transfer hydrogenation of arylalkyl ketones in the absence of a base. The investigations encompass femtosecond pump-probe transient mass spectrometry under isolated conditions and transient absorption spectroscopy in acetonitrile solution, both on the cations [(η6-cymene)RuCl(apypm)]+ (1+, 2+). Gas phase studies on mass selected ions were performed in an ESI ion trap mass spectrometer by transient photofragmentation, unambiguously proving the formation of the activated catalyst species [1+-HCl] or [2+-HCl] after photoexcitation being the only fragmentation channel. The primary excited state dynamics in the gas phase could be fitted to a biexponential decay, yielding time constants of <100 fs and 1-3 ps. Transient absorption spectroscopy performed in acetonitrile solution using femtosecond UV/Vis and IR probe laser pulses revealed additional deactivation processes on longer time scales (∼7-12 ps). However, the formation of the active catalyst species after photoexcitation could not be observed in solution. The results from both studies are compared to former CID investigations and DFT calculations concerning the activation mechanism.

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