om7b00802_si_001.pdf (1.45 MB)
Competing Pathways in the Photochemistry of Ru(H)2(CO)(PPh3)3
journal contribution
posted on 2018-01-17, 17:21 authored by Barbara Procacci, Simon B. Duckett, Michael W. George, Magnus W. D. Hanson-Heine, Raphael Horvath, Robin N. Perutz, Xue-Zhong Sun, Khuong Q. Vuong, Janet A. WelchThe 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.
