Competing Pathways in the Photochemistry of Ru(H)₂(CO)(PPh₃)₃

The photochemistry of Ru­(H)₂(CO)­(PPh₃)₃ (1) 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 p-H₂-induced polarization (PHIP), and a series of unexpected reactions were observed. The photoinduced reductive elimination of H₂ was demonstrated (a) via NMR spectroscopy by the observation of hyperpolarized 1 on pulsed laser photolysis in the presence of p-H₂ and (b) via nanosecond time-resolved infrared (TRIR) spectroscopy studies of the transient [Ru­(CO)­(PPh₃)₃]. Elimination of H₂ competes with photoinduced loss of PPh₃, 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)₂(CO)­(PPh₃)₂] exists in two isomeric forms according to TRIR spectroscopy that react with H₂ and with pyridine on a nanosecond time scale. These two pathways, reductive elimination of H₂ and PPh₃ loss, are shown to occur with approximately equal quantum yields upon 355 nm irradiation. Low-temperature photolysis in the presence of H₂ reveals the formation of the dihydrogen complex Ru­(H)₂(η²-H₂)­(CO)­(PPh₃)₂, 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 AsPh₃ and H₂ generates Ru­(H)₂(AsPh₃)­(CO)­(PPh₃)₂. Two isomers of Ru­(H)₂(CO)­(PPh₃)₂(pyridine) are formed according to NMR spectroscopy on initial photolysis of 1 in the presence of pyridine under H₂. 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.