Efficient CO2 Insertion and Reduction Catalyzed by a Terminal Zinc Hydride Complex

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

Stoichiometric carbon dioxide reduction to highly reduced C1 molecules, such as formic acid (2e^- ), formaldehyde (4e^- ), methanol (6e^- ) or even most-reduced methane (8e^- ), has been successfully achieved by using organosilanes, organoboranes, and frustrated Lewis Pairs (FLPs) in the presence of suitable catalyst. The development of renewable organohydride compounds could be the best alternative in this regard as they have shown promise for the transfer of hydride directly to CO₂ . Reduction of CO₂ by two electrons and two protons to afford formic acid by using renewable organohydride molecules has recently been investigated by various groups. However, catalytic CO₂ reduction to ≥2e^- -reduced products by using renewable organohydride-based molecules has rarely been explored. This Minireview summarizes important findings in this regard, encompassing both stoichiometric and catalytic CO₂ reduction.

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

A novel β-diketiminate stabilized gallium hydride, (^Dipp L)Ga(Ad)H (where (^Dipp L)={HC(MeCDippN)₂ }, Dipp=2,6-diisopropylphenyl and Ad=1-adamantyl), has been synthesized and shown to undergo insertion of carbon dioxide into the Ga-H bond under mild conditions. In this case, treatment of the resulting κ¹ -formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C₆ F₅ )₃ , (^Dipp L)Ga(Ad)H catalyses the reductive hydrosilylation of CO₂ . Under stoichiometric conditions, the addition of one equivalent of B(C₆ F₅ )₃ to (^Dipp L)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, [(^Dipp L)Ga(Ad)][HB(C₆ F₅ )₃ ]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO₂ to the formaldehyde oxidation level at 60 °C in the presence of Et₃ SiH (yielding H₂ C(OSiEt₃ )₂ ). When catalysis is undertaken in the presence of excess B(C₆ F₅ )₃ , appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H₂ C(OSiEt₃ )₂ , CH₄ and O(SiEt₃ )₂ . While this system represents proof-of-concept in CO₂ hydrosilylation by a gallium hydride system, the TOF values obtained are relatively modest (max. 10 h^-1 ). This is attributed to the strength of binding of the formatoborate anion to the gallium centre in the catalytic intermediate (^Dipp L)Ga(Ad){OC(H)OB(C₆ F₅ )₃ }, and the correspondingly slow rate of the turnover-limiting hydrosilylation step. In turn, this strength of binding can be related to the relatively high Lewis acidity measured for the [(^Dipp L)Ga(Ad)]⁺ cation (AN=69.8).

Molecular Zinc Hydride Cations [ZnH]+ : Synthesis, Structure, and CO2 Hydrosilylation Catalysis

Protonolysis of [ZnH2 ]n with the conjugated Brønsted acid of the bidentate diamine TMEDA (N,N,N',N'-tetramethylethane-1,2-diamine) and TEEDA (N,N,N',N'-tetraethylethane-1,2-diamine) gave the zinc hydride cation [(L2 )ZnH]+ , isolable either as the mononuclear THF adduct [(L2 )ZnH(thf)]+ [BArF 4 ]- (L2 =TMEDA; BArF 4 - =[B(3,5-(CF3 )2 -C6 H3 )4 ]- ) or as the dimer [{(L2 )Zn)}2 (μ-H)2 ]2+ [BArF 4 ]- 2 (L2 =TEEDA). In contrast to [ZnH2 ]n , the cationic zinc hydrides are thermally stable and soluble in THF. [(L2 )ZnH]+ was also shown to form di- and trinuclear adducts of the elusive neutral [(L2 )ZnH2 ]. All hydride-containing cations readily inserted CO2 to give the corresponding formate complexes. [(TMEDA)ZnH]+ [BArF 4 ]- catalyzed the hydrosilylation of CO2 with tertiary hydrosilanes to give stepwise formoxy silane, methyl formate, and methoxy silane. The unexpected formation of methyl formate was shown to result from the zinc-catalyzed transesterification of methoxy silane with formoxy silane, which was eventually converted into methoxy silane as well.

Structural Mimics of Acetylene Hydratase: Tungsten Complexes Capable of Intramolecular Nucleophilic Attack on Acetylene

Bioinspired complexes employing the ligands 6-tert-butylpyridazine-3-thione (SPn) and pyridine-2-thione (SPy) were synthesized and fully characterized to mimic the tungstoenzyme acetylene hydratase (AH). The complexes [W(CO)(C2 H2 )(CHCH-SPy)(SPy)] (4) and [W(CO)(C2 H2 )(CHCH-SPn)(SPn)] (5) were formed by intramolecular nucleophilic attack of the nitrogen donors of the ligand on the coordinated C2 H2 molecule. Labelling experiments using C2 D2 with the SPy system revealed the insertion reaction proceeding via a bis-acetylene intermediate. The starting complex [W(CO)(C2 H2 )(SPy)2 ] (6) for these studies was accessed by the new acetylene precursor mixture [W(CO)(C2 H2 )n (MeCN)3-n Br2 ] (n=1 and 2; 7). All complexes represent rare examples in the field of W-C2 H2 chemistry with 4 and 5 being the first of their kind. In the ongoing debate on the enzymatic mechanism, the findings support activation of acetylene by the tungsten center.

Towards Extended Zinc Ethylsulfinate Networks by Stepwise Insertion of Sulfur Dioxide into Zn-C Bonds

The ability to utilize polluting gases in efficient metal-mediated transformations is one of the most pressing challenges of modern chemistry. Despite numerous studies on the insertion of SO₂ into M-C bonds, the chemical reaction of SO₂ with organozinc compounds remains little explored. To fill this gap, we report here the systematic study of the reaction of Et₂ Zn towards SO₂ as well as the influence of Lewis bases on the reaction course. Whereas the equimolar reaction provided a novel example of a structurally characterized organozinc ethylsulfinate compound of general formula [(EtSO₂ )ZnEt]_n , the utilization of an excess of SO₂ led to the formation of the zinc(II) bis(ethylsulfinate) compound [(EtSO₂ )₂ Zn]_n . Moreover, we have discovered that the presence of N-donor Lewis bases represents an efficient tool for the preparation of extended zinc ethylsulfinates, which in turn led to the formation of 1D [(EtSO₂ ZnEt)₂ (hmta)]_n and 2D [((EtSO₂ )₂ Zn)₂ (DABCO)]_n ⋅solv (in which solv=THF or toluene, hmta= hexamethylenetetramine, and DABCO=1,4-diazabicyclo[2.2.2]octane) coordination polymers, respectively. The results of DFT calculations on the reactivity of SO₂ towards selected Zn-C reactive species as well as the role of an N-donor Lewis base on the stabilization of the transition states complement the discussion.

Diverse Catalytic Systems and Mechanistic Pathways for Hydrosilylative Reduction of CO2

Catalytic hydrosilylation of carbon dioxide has emerged as a promising approach for carbon dioxide utilization. It allows the reductive transformation of carbon dioxide into value-added products at the levels of formate, formaldehyde, methanol, and methane. Tremendous progress has been made in the area of carbon dioxide hydrosilylation since the first reports in 1981. This focus review describes recent advances in the design and catalytic performance of leading catalyst systems, including transition-metal, main-group, and transition-metal/main-group and main-group/main-group tandem catalysts. Emphasis is placed on discussions of key mechanistic features of these systems and efforts towards the development of more selective, efficient, and sustainable carbon dioxide hydrosilylation processes.