Competing Pathways in the Photochemistry of Ru(H)<sub>2</sub>(CO)(PPh<sub>3</sub>)<sub>3</sub>

Published on 2018-02-13T05:31:00Z (GMT) by
The photochemistry of Ru­(H)<sub>2</sub>(CO)­(PPh<sub>3</sub>)<sub>3</sub> (<b>1</b>) has been reinvestigated employing laser and conventional light sources in conjunction with NMR spectroscopy and IR spectroscopy. The sensitivity of NMR experiments was enhanced by use of <i>p</i>-H<sub>2</sub>-induced polarization (PHIP), and a series of unexpected reactions were observed. The photoinduced reductive elimination of H<sub>2</sub> was demonstrated (a) via NMR spectroscopy by the observation of hyperpolarized <b>1</b> on pulsed laser photolysis in the presence of <i>p</i>-H<sub>2</sub> and (b) via nanosecond time-resolved infrared (TRIR) spectroscopy studies of the transient [Ru­(CO)­(PPh<sub>3</sub>)<sub>3</sub>]. Elimination of H<sub>2</sub> competes with photoinduced loss of PPh<sub>3</sub>, as demonstrated by formation of dihydrogen, triphenylarsine, and pyridine substitution products which are detected by NMR spectroscopy. The corresponding coordinatively unsaturated 16-electron intermediate [Ru­(H)<sub>2</sub>(CO)­(PPh<sub>3</sub>)<sub>2</sub>] exists in two isomeric forms according to TRIR spectroscopy that react with H<sub>2</sub> and with pyridine on a nanosecond time scale. These two pathways, reductive elimination of H<sub>2</sub> and PPh<sub>3</sub> loss, are shown to occur with approximately equal quantum yields upon 355 nm irradiation. Low-temperature photolysis in the presence of H<sub>2</sub> reveals the formation of the dihydrogen complex Ru­(H)<sub>2</sub>(η<sup>2</sup>-H<sub>2</sub>)­(CO)­(PPh<sub>3</sub>)<sub>2</sub>, which is detected by NMR and IR spectroscopy. This complex reacts further within seconds at room temperature, and its behavior provides a rationale to explain the PHIP results. Furthermore, photolysis in the presence of AsPh<sub>3</sub> and H<sub>2</sub> generates Ru­(H)<sub>2</sub>(AsPh<sub>3</sub>)­(CO)­(PPh<sub>3</sub>)<sub>2</sub>. Two isomers of Ru­(H)<sub>2</sub>(CO)­(PPh<sub>3</sub>)<sub>2</sub>(pyridine) are formed according to NMR spectroscopy on initial photolysis of <b>1</b> in the presence of pyridine under H<sub>2</sub>. Two further isomers are formed as minor products; the configuration of each isomer was identified by NMR spectroscopy. Laser pump-NMR probe spectroscopy was used to observe coherent oscillations in the magnetization of one of the isomers of the pyridine complex; the oscillation frequency corresponds to the difference in chemical shift between the hydride resonances. Pyridine substitution products were also detected by TRIR spectroscopy.

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Procacci, Barbara; Duckett, Simon B.; George, Michael W.; W. D. Hanson-Heine, Magnus; Horvath, Raphael; Perutz, Robin N.; et al. (2018): Competing Pathways in the Photochemistry of Ru(H)2(CO)(PPh3)3. ACS Publications. Collection.