Bimetallic Activation of 2‐Alkanones through Photo‐Induced α‐Hydrogen Abstraction Mediated by a Dinuclear Ruthenium Tetrahydride Complex

Abstract

The photochemical behavior of transition-metal complexes has long been a fascinating subject that is related to electron transfer processes, specific organotransformations, photosynthesis, and electrochemical conversion of solar energy. Although there are a vast number of studies on the photochemistry of mononuclear complexes, multimetallic systems remain unexplored. [1] In general, in cluster complexes with metal-metal bonds, the lowest unoccupied orbital is an antibonding orbital that is delocalized over the metal framework. Whereas mononuclear polyhydride complexes often eliminate hydride ligands as dihydrogen, the elimination of dihydrogen upon photoirradation is hindered in hydride-bridged clusters. As a result, fragmentation of the metal framework of polyhydridebridged clusters upon photoinduced single electron excitation is suppressed. Therefore, cluster effects, that is, multiple interactions between the substrate and metal sites, are still anticipated, even in the photoexcited state of the polyhydride cluster. These characteristic features of polyhydride-bridged clusters may result in rather exotic photochemical behavior.We have studied the synthesis of homo-and heterometallic polyhydride cluters and have reported their reaction chemistry under thermal conditions. [2] Some remarkable reactions performed on the reaction sites of these polyhydride clusters were recently highlighted by Lavigne. [3] To extend our research to a new area, we began a project on the photochemistry of a polyhydride cluster which we anticipated to possess increased structural stability.We report herein an unusual photochemical reaction of a diruthenium tetrahydride complex, [(Cp*Ru)(m-H) 4 -(RuCp*)] (1) (Cp* = h 5 -C 5 Me 5 ), with 2-alkanones, which produces dinuclear oxatrimethylenemethane (OTMM) complexes.Upon irradiation with UV light (365 nm) at 10 8C for 48 h under an argon atmosphere, the red suspension of diruthenium tetrahydride (46.1 mg, 96.7 mmol) in acetone (15 mL) became a blue-purple solution. Removal of the solvent under reduced pressure, followed by purification of residual solids by column chromatography on alumina (Merck Aluminium Oxide 90 standardized) with a toluene/tetrahydrofuran (5:1) mixture afforded a dinuclear OTMM complex of ruthenium, [(Cp*Ru){m-h 3 :h 1 -(CH 2 ) 2 CO}(m-H) 2 (RuCp*)] (2 a), in 91 % yield (46.8 mg, 88.2 mmol; Scheme 1).The 1 H NMR spectrum of 2 a recorded in [D 6 ]benzene at 25 8C exhibited two singlet signals at d = 1.67 and 1.80 ppm, which were assignable to the Cp* groups. Two singlet-like signals for the methylene protons of the OTMM ligand appeared at d = 1.22 (2 H, w1 = 2 = 1.4 Hz, anti to the O atom) and 3.07 ppm (2 H, w1 = 2 = 2.1 Hz, syn to the O atom). A signal corresponding to the chemically equivalent bridging hydride ligands was observed at d = À12.18 ppm (s, 2 H, w1 = 2 = 13.5 Hz). A COSY and selective decoupling experiment showed the spin-coupling interactions among the anti, syn, and Ru-bound protons. [4] The 13 C NMR data fully supported the structure of 2 a: the signa...

Photochemistry of Transition Metal Hydrides

“…Much more recently, the photochemistry of (Cp*Ru) 2 (μ-H) 4 has been examined. This complex absorbs at 371 nm (ε = 2200 dm 3 mol –1 cm –1 ) and was photochemically inert in THF, but reacted on irradiation at 365 nm with methyl ketones to form (Cp*Ru) 2 (μ-H) 2 (μ–η 4 -CH 2 C­(O)­CHR) + 2H 2 where the bridging ligand is described as oxatrimethylenemethane . The authors propose that the reaction proceeds via an exciplex, although no further evidence is provided.…”

“…It also reacted photochemically with CpNi­(CO) 2 , CpCo­(CO) 2 , and [CpFe­(CO) 2 ] 2 to yield heterobimetallic clusters with different geometries. Although no mention was made of the primary photoprocess by the authors, elimination of H 2 from the tetrahydrido Ru-species is required for the observed product to be formed. ,, The only example of a polyhydride of osmium involved in a photochemical reaction is the photochemical reductive elimination of H 2 from Cp*Os­(H) 3 (CO) to form the dimer [Cp*Os­(CO)] 2 (μ-H) 2 …”

“…Most recently, photochemistry has been used to generate metal dihydride complexes in selected nuclear spin states formed synchronously with a pulsed laser and to study reaction intermediates in nitrogenase . Another important development is the recognition that equilibrated excited states of metal hydride polypyridine complexes can act as both strong acids and strong hydride donors, opening up new photocatalytic reactions. , The past few years have also seen important progress in the photochemistry of complexes with bridging hydrides. , …”

Synthesis, Structure, and Reactivity of Mixed-Ligand Dinuclear Ruthenium Polyhydrido Complexes Supported by 1,4,7-Trimethyl-1,4,7-triazacyclononane and Bulky Phosphine Ligands

“…Furthermore, we recently reported on the photoinduced bimetallic activation of 2-alkanones to afford dinuclear oxatrimethylenemethane complexes. 5 Our initial studies involved the synthesis of mixed-metal clusters with identical ligands on each metal atom to examine the effect of the metal atoms on the structure and reactivity of the cluster complexes. Thus far, we have synthesized a series of heterometallic polyhydrido clusters containing both the [Cp*Ru] moiety and Mo, W, 6 Re, 7 Os, 8 Co, 9 Rh, and Ir fragments.…”

Photochemical Reaction of Diruthenium Tetrahydride-Bridged Complexes with Carbon Dioxide: Insertion of CO2 into a Ru–H Bond versus C═O Double-Bond Cleavage

“…Recently, we reported photoinduced C–H bond cleavage of acetone, resulting in the formation of a dinuclear bridging oxatrimethylenemethane complex of ruthenium . This reaction is considered to be very typical of polyhydride-bridged cluster complexes, because the reaction mechanism does not involve either initial H 2 elimination or fragmentation of the cluster framework.…”