Reduction of CO₂ by Hydrosilanes in the Presence of Formamidinates of Group 13 and 12 Elements

Abstract

Homoleptic complexes 1-M of groups 13 and 12 elements (M = B−In and M = Zn, respectively) incorporating electron-withdrawing formamidinate ligands {(C 6 F 5 )N=CH−N(C 6 F 5 )} − ({NCN} − ) were synthesized and isolated in high yields. The compounds were characterized by X-ray crystallography, NMR spectroscopy and elemental analysis. While single-component 1-M appeared to be sluggishly active or inactive in reduction of CO 2 with hydrosilanes, a good catalytic performance was achieved with the two-component systems derived from combinations of 1-M and E(C 6 F 5 ) 3 (E = B, Al). In particular, the binary combination 1-Al/B(C 6 F 5 ) 3 showed the best performance within the whole series, thus providing quantitative hydrosilane (Et 3 SiH) conversions under a range of conditions (P CO2 , temperature, benzene or bromobenzene solvents) and affording mainly CH 2 (OSiEt 3 ) 2 and CH 4 as products. Kinetic and mechanistic studies revealed that at the initiation step the binary catalytic systems undergo a complex transformation in the presence of CO 2 /Et 3 SiH affording the products of 1-Al decomposition, namely, (C 6 F 5 )N(H)SiEt 3 , (C 6 F 5 )N(Me)SiEt 3 , {NCN}-SiEt 3 and also some unidentified aluminum species. Thus, the overall process of the reduction of CO 2 with hydrosilanes is presumed to be catalyzed by complex multi-site systems, evolved from the formamidinate precursor 1-Al, implicating different tandem combinations of N-base/B(C 6 F 5 ) 3 with putative Al-containing species.

More Than Just a Reagent: The Rise of Renewable Organohydrides for Catalytic Reduction of Carbon Dioxide

“…It is worth to mention that wide range of hydrosilanes are commercially available and environmentally benign [26,27] . However, it is a known fact that reduction of CO 2 by hydrosilanes requires catalysts such as metal complexes (transition metal, [26,28–41] main group metal‐based catalysts [41–51] ), or organocatalysts [27,52,55–67] or a combination of a metal and a strong Lewis acid‐like systems [68–82] (Scheme 1). Indeed, the reduction occurs at milder reaction conditions with the advantage of high chemoselectivity in the product.…”

“…Various non‐metallic hydride donors have been used to reduce CO 2 . Considering non‐metallic hydrides, various hydrosilanes, [26–83] hydroboranes, [85–96] ammonia borane (AB type), [6,97–100] showed great promise towards the reduction of CO 2 to highly reduced products such as formic acid (2e − ), formaldehyde (4e − ), methanol (6e − ) or even most reduced methane (8e − ) in the presence of appropriate catalysts. However, CO 2 reduction by these organohydrides are not catalytic.…”

Partnering a Three‐Coordinate Gallium Cation with a Hydroborate Counter‐Ion for the Catalytic Hydrosilylation of CO2

“…Thus, a variety of homogenous catalytic systems based on metals (Pd/Pt, [14–17] Rh, [18] Re, [19, 20] Ru, [21–26] Ir, [8, 27–30] Co, [13, 31] Mn, [12] Zr, [32, 33] Cu, [34–38] Ni, [39–42] Sc, [43–46] Zn, [47–57] Mg [57, 58] and Sr [59] ), Lewis acids, [60–66] frustrated Lewis pairs (FLPs), [67, 68] organo‐catalysts, [69–75] alkali metal carbonates, [76, 77] and metal borohydrides [78] have been investigated for this process. Many of these catalytic systems feature reactive metal hydride or alkyl groups, which effect the initial activation of CO 2 via insertion.…”

“…Thus, an emerging approach to enhance catalytic performance in reductive hydrosilylation is to couple the catalyst with a strong organo‐Lewis acid such as BEt 3 , [42] BPh 3 , [14, 55] or, more commonly, B(C 6 F 5 ) 3 [79] . Examples of such combinations include Pd, Pt/BR 3 (R=Ph, C 6 F 5 ), [14] Re/B(C 6 F 5 ) 3 , [20] Zr/B(C 6 F 5 ) 3 , [32, 33] Ni/BR 3 (R=C 6 F 5 , [39, 40] Et [42] ), Sc/B(C 6 F 5 ) 3 , [43–45] Zn/BR 3 (R=Ph, [55] C 6 F 5 [56, 57] ), Mg/B(C 6 F 5 ) 3 , [57, 58] and Al/B(C 6 F 5 ) 3 [60, 61] …”

“…[79] Exampleso fs uch combinations include Pd, Pt/BR 3 (R = Ph, C 6 F 5 ), [14] Re/B(C 6 F 5 ) 3 , [20] Zr/B(C 6 F 5 ) 3 , [32,33] Ni/BR 3 (R = C 6 F 5 , [39,40] Et [42] ), Sc/B(C 6 F 5 ) 3 , [43][44][45] Zn/BR 3 (R = Ph, [55] C 6 F 5 [56,57] ), Mg/B(C 6 F 5 ) 3 , [57,58] and Al/B(C 6 F 5 ) 3 . [60,61] Many metal complexes employedi nt his tandemc atalytic approacha re based on reactive M-H moieties which,i nc ombinationw ith B(C 6 F 5 ) 3 ,f orm an ion pair of the type [L n M][HB(C 6 F 5 ) 3 ]. [14,20,39,43,57] The Lewis acidicc ation activates carbond ioxide towards hydride transfer from the [HB(C 6 F 5 ) 3 ] À anion,g enerating af ormatoborate complex.T he B(C 6 F 5 ) 3 -activated silane, R 3 Si-H•••B(C 6 F 5 ) 3 ,t hen transfers a[ SiR 3 ] + silylium ion to the formatef unctionality,f orming as ilyl formatea nd regenerating the metalh ydridec omplex.…”