Cyclic(Alkyl)(Amino)Carbene (CAAC)‐Supported Zn Alkyls: Synthesis, Structure and Reactivity in Hydrosilylation Catalysis

Divergent Synthesis of Chelating Aziridines and Cyclic(Alkyl)(Amino)Carbenes (CAACs) from Pyridyl-Tethered Robust Azomethine Ylides

Azomethine ylides are typically in situ generated synthons for making N-heterocycles through cycloaddition reactions. But an offbeat aspect about them is the isomeric nature of aldiminium-based azomethine ylides and (alkyl/aryl)(amino)carbenes, interconvertible by a formal 1,3-H+ transfer. Herein, two thermally robust azomethine ylides with a N-appended picolyl sidearm are isolated, which cyclize to py aziridines at 80 °C but unprecedentedly result N-pico CAAC-CuCl (CAAC=cyclic(alkyl)(amino)carbene) complexes when heated with CuCl at merely 60 °C. The pendant Npy , as revealed by computational analysis, plays a crucial role in this unusual 1,3-H+ shift using a deprotonation-protonation sequence, as well as in placing the CuCl at the carbenic site in tandem. The softer nature of Cu(I) is also critical. Chelating CAACs are rare and one with a N-tethered additional donor is priorly unknown. Both N-pico CAAC and py aziridine are bidentate chelators giving highly active cationic Rh(I) catalysts for hydrosilylating unactivated olefins by Et3 SiH. Notably, the py aziridine-Rh(I) is superior than the N-pico CAAC-Rh(I) catalyst.

Thermally stable zinc hydride catalyst for hydrosilylation of CO2 to silyl formate at atmospheric pressure

Neutral zinc complexes supported by H[PNNO], a diaminophenolate ligand bearing a pendant phosphine group, were synthesized and characterized. The phosphine arm adopts two different configurations in solution and prevents aggregation. The monomeric zinc hydride complex is stable at elevated temperatures up to 125 °C and reacts readily with CO₂ to afford a zinc formate complex. The zinc hydride is active for CO₂ hydrosilylation at atmospheric CO₂ pressure and is selective for CO₂ reduction to the silyl-formate product.

Bicyclic (alkyl)(amino)carbene stabilized zinc(0) complex with singlet biradicaloid ground state

A storable bicyclic (alkyl)(amino)carbene (BICAAC) stabilized two coordinate zinc(0) complex [(BICAAC)2Zn] (2) was synthesized. DFT calculations reveal that BICAAC plays a decisive role in imparting the stability to 2. This complex activates the C(sp3)-Cl bond of trityl chloride generating the Gomberg's free radical with greater efficiency than metallic Zn powder.

Cationic Zinc Hydride Catalyzed Carbon Dioxide Reduction to Formate: Deciphering Elementary Reactions, Isolation of Intermediates, and Computational Investigations

Zinc has been an element of choice for carbon dioxide reduction in recent years. Zinc compounds have been showcased as catalysts for carbon dioxide hydrosilylation and hydroboration. The extent of carbon dioxide reduction can depend on various factors, including electrophilicity at the zinc center and the denticity of the ancillary ligands. In a few cases, the addition of Lewis acids to zinc hydride catalysts markedly influences carbon dioxide reduction. These factors have been investigated by exploring elementary reactions of carbon dioxide hydrosilylation and hydroboration by using cationic zinc hydrides bearing tetradentate tris[2-(dimethylamino)ethyl]amine and tridentate N,N,N',N'',N''-pentamethyldiethylenetriamine in the presence of triphenylborane and tris(pentafluorophenyl)borane.

Recent Representative Advances on the Synthesis and Reactivity of N-Heterocyclic-Carbene-Supported Zinc Complexes

The present account reviews the most recent noteworthy developments on the synthesis, structure and catalytic applications of Zn-NHC species, a class of complexes that have attracted attention over the past five to ten years due to their enhanced robustness and hydrolytic stability versus classical Zn organometallics. In particular, thanks to NHC stabilization, access to unprecedented Zn species were recently achieved, including two-coordinate Zn(II) organocations and thermally stable molecularly well-defined Zn hydride species, opening the way to effective Zn-mediated hydro-silylation/-boration catalysis of various unsaturated substrates under mild conditions. The potential of NHC-Zn species for the stabilization of unprecedented Zn species and use in various catalytic applications is only emerging and the vast array of readily available NHC structures should promote future developments of the field.

Heterobimetallic rare earth metal-zinc catalysts for reactions of epoxides and CO2 under ambient conditions

Four homodinuclear rare earth metal (RE) complexes 1-4 bearing a multidentate diglycolamine-bridged bis(phenolate) ligand were synthesized. In addition, seven heterobimetallic RE-Zn complexes 5-11 were prepared through a one-pot strategy. In these heterobimetallic complexes, two RE centers are bridged by either Zn(OAc)₂ or Zn(OBn)₂ moieties. All complexes were characterized by single crystal X-ray diffraction, elemental analysis, IR spectroscopy, and multinuclear NMR spectroscopy (in the case of diamagnetic complexes 1, 4, 7 and 11). Moreover, the multi-nuclear structures of complexes 4 and 11 in solution were also studied by ¹H DOSY spectroscopy. These complexes were applied in catalyzing the coupling reaction of carbon dioxide (CO₂) with epoxides. Zn(OAc)₂- and Zn(OBn)₂-bridged heterobimetallic complexes showed comparable catalytic activities under ambient conditions and were more active than monometallic RE complexes. Significant synergistic effect in heterobimetallic complexes is observed. Mono-substituted epoxides were converted into cyclic carbonates under 1 atm CO₂ at 25 °C in 88-96% yields, whereas di-substituted epoxides reacted under 1 atm CO₂ at higher temperatures in 40-80% yields.

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).

C-Methylenation of anilines and indoles with CO2 and hydrosilane using a pentanuclear zinc complex catalyst

The one-step C-methylenation of anilines and indoles with CO2 and phenylsilane was catalyzed by a pentanuclear ZnII complex to give diarylmethanes via geminal C-H and C-C bond formation. It is proposed that the zinc-hydride complex generated in situ is a catalytically active species and that bis(silyl)acetal is a key intermediate. When aniline was used as a substrate, both the C-methylenation and N-methylation proceeded.

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.

Hydroboration of carbon dioxide enabled by molecular zinc dihydrides

Molecular zinc dihydrides were found to be active catalysts for hydroboration of carbon dioxide, selectively giving boryl formate, bis(boryl)acetal, or methoxy-borane compounds by varying the borane reductant.

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.