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.