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

Abstract

Photochemical reactions of diruthenium tetrahydride complexes containing cyclopentadienyls as auxiliary ligands with carbon dioxide were studied for the effective fixation and reduction of CO 2 . Whereas the reactions of CpsRu(μ-H) 4 RuCps (Cps = Cp*, C 5 Me 5 ; Cp Et , C 5 Me 4 Et; Cp ‡ , 1,2,4-C 5 ( t Bu) 3 H 2 ) with CO 2 did not proceed under dark and mild conditions, the photochemical reactions under UV (365 nm) irradiation smoothly proceeded to afford two types of products, (i) a μ-formato complex and (ii) a μ-carbonyl-μ-oxo complex, according to the width of the space for the reaction stretching between the two auxiliary cyclopentadienyl ligands.T he photochemical behavior of mononuclear polyhydrido transition metal complexes has been intensively studied since the early 1970s. 1 Reductive elimination of H 2 is the most salient process observed in the initial stages of photolysis of mononuclear polyhydride species. 2 The coordinatively unsaturated, highly reactive species generated by dehydrogenation often promote bond activation of relatively inert hydrocarbons. 3 In contrast, polynuclear polyhydride-bridged complexes tend to be highly resistant toward reductive elimination of H 2 , because each hydride ligand is bound to multiple metal centers. 4 Initial reactions observed for the photolysis of mononuclear polyhydride complexes, i.e., the formation of vacant sites via liberation of H 2 , would, therefore, be impeded upon photoexcitation of a bridging polyhydride cluster. At the same time, preservation of the bridging hydrides would prevent fragmentation of the cluster framework.Recently, we reported photoinduced C−H bond cleavage of acetone, resulting in the formation of a dinuclear bridging oxatrimethylenemethane complex of ruthenium. 5 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. This result strongly stimulated us to examine the reaction of the polyhydride-bridged clusters with carbon dioxide under photoirradiation to activate less-reactive CO 2 in a bimetallic way.Carbon dioxide has long been considered to be an abundantly available feedstock for C 1 chemistry as well as carbon monoxide, methanol, and methane. 6 However, the chemical inertness of CO 2 , originating from its highly negative reduction potential, has limited its use in organic transformations. Consequently, activation of CO 2 via coordination to a transition metal has become a subject of great interest. 7 Coordination patterns are primarily controlled by the electronic situation at the metal center, and thus, electron-rich transition metal centers can perform a nucleophilic attack at the carbon atom of CO 2 . 8 In contrast, electron-poor metal centers act as electrophiles and react with CO 2 via one of the oxygen atoms. 9 We report herein that the reactions of CpsRu(μ-H) 4 RuCps with CO 2 smoothly proceed with preservation of dinuclear structure under UV irradiation to pro...

CO2Fixation and Catalytic Reduction by a Neutral Aluminum Double Bond

“…The central Al−CO−Al−O core in 3 is completely planar, with the NHC and silyl groups trans ‐orientated to the ring. Compound 3 represents the first dialuminacarbonyl compound and a rare example of a central M(μ‐CO)(μ‐O)M bridging motif . The retention of the carbonyl group is confirmed upon comparison of structural data.…”

Photoinduced Reactions of Diruthenium Tetrahydride Complexes: Carbon–Hydrogen Bond Cleavage of Tetrahydrofuran Leading to Bridging Cyclic Fischer-Type Carbene Complexes

“…To date, there have been numerous reports on the photochemical behavior of transition metal complexes containing polyhydride ligands terminally bonded to the metal atom. Photoinduced liberation of hydrogen to generate coordinative unsaturation is the most salient process occurring in the initial stage . In contrast, we studied the photochemical behavior of polyhydride-bridged complexes supported by cyclopentadienyl auxiliaries.…”

“…In recent years, we have studied the photochemical behavior of polyhydride-bridged transition metal complexes and reported innovative reactions of dinuclear tetrahydride-bridged complexes of ruthenium, CpsRu­(μ-H) 4 RuCps ( 1 ) (Cps = a : C 5 Me 5 (Cp*), b : C 5 EtMe 4 (Cp Et ), and c : 1,2,4-C 5 ( t -Bu) 3 H 2 (Cp ‡ )), with 2-alkanones and carbon dioxide (Scheme ). UV irradiation of 1a (365 nm) in acetone promotes a new reaction with retention of the dinuclear structure, leading to the selective formation of a dinuclear μ-oxatrimethylenemethane complex, {Cp*Ru­(μ-H)} 2 {μ-η 3 :η 1 -(CH 2 ) 2 CO} ( 2a ), as a result of hydrogen abstraction at the methyl groups of an acetone molecule. Complex 1 also reacts with CO 2 under photoirradiative conditions to produce a μ-formate complex, (Cp*Ru) 2 (μ-OCHO)­(μ-H) 3 ( 3a ), or a μ-carbonyl−μ-oxo complex, (Cp ‡ Ru) 2 (μ-CO)­(μ-O) ( 4c ), selectively according to the bulkiness of the auxiliary ligand, Cp* or Cp ‡ .…”

CO2Fixation and Catalytic Reduction by a Neutral Aluminum Double Bond

“…[14,31,32] Ther etention of the carbonyl group is confirmed upon comparison of structural data. Compound 3 represents the first dialuminacarbonyl compound and ar are example of ac entral M(m-CO)(m-O)M bridging motif.…”

“…Compound 3 represents the first dialuminacarbonyl compound and ar are example of ac entral M(m-CO)(m-O)M bridging motif. [14,31,32] Ther etention of the carbonyl group is confirmed upon comparison of structural data. TheC = O bond length is 1.194 (9) ,which fits well with average m-CO bridging motifs (1.17-1.22 ), and the notable downfield shift in 13 CNMR is also similar to those reported for M(m-CO)(m-O)M cores (M = B(d 278.7 ppm) and Fe (d 289.8 ppm)).…”