Catalytic Reduction of Carbon Dioxide Using Cationic Organoaluminum and -Gallium Compounds

Abstract

Ethide abstraction from Et3M (M = Al and Ga), (2,6-Ph2C6H3)­AlEt2, 1, and (2,6-Dipp2C6H3)­GaEt2, 2 (Dipp = 2,6-iPr2C6H3), using the silylium ion [Et3Si]­[CHB11Cl11] afforded crystalline ion-like compounds [Et2M]­[CHB11Cl11] (M = Al, 3; Ga, 5) and [(2,6-Ph2C6H3)­AlEt]­[CHB11Cl11], 4, and the likely solvent-separated ion pair [(2,6-Dipp2C6H3)­GaEt]­[CHB11Cl11], 6. Crystalline compounds 3–5 feature cation···anion contacts in the solid state, and their solubility in low polarity benzene indicates that these contacts are maintained in solution. All compounds catalyze the reduction of CO2 with Et3SiH, but the activity of the gallium compound 5 is significantly lower due to its polymeric structure and the lower Lewis acidity of gallium. Whereas the reduction products from the reactions catalyzed by compounds 3–5 are mostly methane and toluene (from Friedel–Crafts alkylation of the benzene solvent), catalysis by 6 led mostly to Et3SiOCH3.

Oxygen‐Bridged Ga2(Et)3(OTeF5)3 and the Weakly Coordinating Anions [Ga(Et)(OTeF5)3]− and [Ga(OTeF5)4]−

“…The compound Ga 2 (Et) 3 [31] In this case, the gallium atom is sixfold coordinated by two ethyl groups and four chlorine atoms. As a result,t he solid-state structure of [Ga(Et) 2 ][CHB 11 Cl 11 ]c onsists of one-dimensional chains.…”

“…[32] Those low-coordinated cationico rganogallium compounds are candidates fort ransition-metal-free polymerization catalysts [32,33] or for the catalytic reduction of CO 2 . [31] To understand the nature of the bonds in Ga 2 (Et) 3 (OTeF 5 ) 3 ,q uantum-chemicalR I-DFT [34] (Resolution of Identity,D ensity Functional Theory) calculations with the B3LYP [35][36][37] functional and with def2-TZVPP basis sets [38,39] have been performed. The QTAIM (Quantum Theory of Atoms in Molecules) analysis [40] (Figure 3) shows bond critical points (BCPs) between both Ga atoms and their directly attached Ca nd Oa toms.…”

Recent Developments on the Use of Group 13 Metal Complexes in Catalysis

“…Given that Lewis acids catalyzet he hydrosilylationa nd hydroborationo fc arbonyl groups, the catalytic reduction of CO 2 using Al-based catalysts was recently developed. Strong cationic Lewis acids such as [Et 2 Al][CH 6 B 11 I 6 ], [50] [Et 2 Al][CHB 11 Cl 11 ] (11), [2,6-Ph 2 C 6 H 3 AlEt][CHB 11 Cl 11 ]( 12), [51] [(2,6-Ar*C 6 H 3 O) 2 Al] [CHB 11 Cl 11 ]( Ar* = 2,4,6-Me 3 C 6 H 2 , 13,S cheme 7) [52] catalyze the reduction of CO 2 with Et 3 SiH to methane and toluene.T he latter forms by Friedel-Craftsa lkylation of the benzene solvent with the methoxysilane Et 3 SiOMe. The strongn eutral Lewis acid Al(C 6 F 5 ) 3 performs only sluggishly,b ut is quite active in tandemw ith B(C 6 F 5 ) 3 .…”

“…[53] Interestingly,c atalysis by the cationicg allium compound [Dipp*GaEt][CHB 11 Cl 11 ]( Dipp* = 2,6-Dipp 2 C 6 H 3 ,D ipp = 2,6-iPr 2 C 6 H 3 )stops at the methoxysilane step, thus allowing in principle reduction of CO 2 to methanola fter hydrolysis of Et 3 SiOMe. [51] The less Lewis acidic gallium compound [Ga(H)t-Bu(nacnac)] (14,S cheme 8), which features af our-coordinate gallium center, requires the more reactive boron hydride pinacol borane HBpin for CO 2 reduction catalysis, to produce the methoxy product MeOBpin. [54] Essentially ar everser eaction to the CO 2 reduction discussed above is the recently reported aluminum-catalyzed dehydro-genationo ff ormic acid to H 2 and CO 2 using Al catalyst 15 (Scheme 9).…”

“…Interestingly, catalysis by the cationic gallium compound [Dipp*GaEt][CHB 11 Cl 11 ] (Dipp*=2,6‐Dipp 2 C 6 H 3 , Dipp=2,6‐ i Pr 2 C 6 H 3 ) stops at the methoxysilane step, thus allowing in principle reduction of CO 2 to methanol after hydrolysis of Et 3 SiOMe . The less Lewis acidic gallium compound [Ga(H) t Bu(nacnac)] ( 14 , Scheme ), which features a four‐coordinate gallium center, requires the more reactive boron hydride pinacol borane HBpin for CO 2 reduction catalysis, to produce the methoxy product MeOBpin …”