Photochemistry of Transition Metal Hydrides

Abstract

Photochemical reactivity associated with metal-hydrogen bonds is widespread among metal hydride complexes and has played a critical part in opening up C-H bond activation. It has been exploited to design different types of photocatalytic reactions and to obtain NMR spectra of dilute solutions with a single pulse of an NMR spectrometer. Because photolysis can be performed on fast time scales and at low temperature, metal-hydride photochemistry has enabled determination of the molecular structure and rates of reaction of highly reactive intermediates. We identify five characteristic photoprocesses of metal monohydride complexes associated with the M-H bond, of which the most widespread are M-H homolysis and R-H reductive elimination. For metal dihydride complexes, the dominant photoprocess is reductive elimination of H2. Dihydrogen complexes typically lose H2 photochemically. The majority of photochemical reactions are likely to be dissociative, but hydride complexes may be designed with equilibrated excited states that undergo different photochemical reactions, including proton transfer or hydride transfer. The photochemical mechanisms of a few reactions have been analyzed by computational methods, including quantum dynamics. A section on specialist methods (time-resolved spectroscopy, matrix isolation, NMR, and computational methods) and a survey of transition metal hydride photochemistry organized by transition metal group complete the Review.

Hydride Conformers of the Nitrogenase FeMo-cofactor Two-Electron Reduced State E2(2H), Assigned Using Cryogenic Intra Electron Paramagnetic Resonance Cavity Photolysis

“…Our earlier conclusion that 1b is the E 2 (2H) state with its reducing equivalents stored as a bound hydride 4,6,9 is corroborated by the observation that the E 2 /1b state is photoactive, like inorganic hydride complexes, 17,18 the [Fe–H–Fe] bridging hydrides of the E 4 (4H) state of nitrogenase, 19,20 and the [Ni–H–Fe] bridging hydrides of the Ni–C state of NiFe hydrogenase. 21 As noted above, the earlier cryoannealing study of E 4 (4H) implied that the hydride of E 2 /1b adopts a [Fe–H–Fe] bridging structure, and it will be thus described in this report.…”

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

“…This effect could be understood by photoinduced reductive elimination of H 2 followed by oxidative addition of p-H 2 . Photochemical loss of phosphine competing with loss of H 2 was also observed for Ru(H) 2 (PMe 3 ) 4 . 27 The time scales available and the sensitivity of TRIR spectroscopy have improved greatly since our 1997 study, and there are no longer any gaps in the nanosecond time domain.…”

“…4 One of the earliest metal hydrides to be studied photochemically was Ru(H) 2 (CO)(PPh 3 ) 3 (1) (Scheme 1), a compound of great interest for its catalytic properties both under illumination and by thermal means. 5−13 The original studies of its photochemistry demonstrated loss of H 2 , but not CO, on irradiation in benzene.…”

A Non‐Heme Iron Photocatalyst for Light‐Driven Aerobic Oxidation of Methanol

“…The redox‐active photosensitizers available include organic dyes,3 inorganic clusters,4 and transition‐metal complexes, such as [Ru(bpy) 3 ] 2+ and its derivatives,5, 6 whose redox potentials can be fine‐tuned by ligand modification 7, 8, 9. Photoredox catalysis can bypass reactive stoichiometric oxidants, such as H 2 O 2 and ClO − , to generate high‐valent transition‐metal oxido species by electron‐transfer oxidation.…”

“…Photoredox catalysis can bypass reactive stoichiometric oxidants, such as H 2 O 2 and ClO − , to generate high‐valent transition‐metal oxido species by electron‐transfer oxidation. Non‐heme iron complexes that are well‐known catalysts for a wide range of oxidation reactions have been combined with photoredox catalysts, such as [Ru(bpy) 3 ] 2+ , for light‐driven oxidation reactions 9, 10, 11. In this multicatalyst strategy (Scheme 1 a), excitation of the photoredox sensitizer is followed by electron‐transfer oxidation of the catalyst to raise it to a higher oxidation state so that it can subsequently oxidize substrates.…”