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

Abstract

Protonolysis of [ZnH 2 ] 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 [(L 2)ZnH] + ,i solable either as the mononuclear THF adduct [(L 2)ZnH(thf)] + [BAr F 4 ] À (L 2 = TMEDA; BAr F 4 À = [B-(3,5-(CF 3) 2-C 6 H 3) 4 ] À)o ra st he dimer [{(L 2)Zn)} 2 (m-H) 2 ] 2+-[BAr F 4 ] À 2 (L 2 = TEEDA). In contrast to [ZnH 2 ] n ,the cationic zinc hydrides are thermally stable and soluble in THF. [(L 2)ZnH] + was also shown to form di-and trinuclear adducts of the elusive neutral [(L 2)ZnH 2 ]. All hydride-containing cations readily inserted CO 2 to give the corresponding formate complexes.[ (TMEDA)ZnH] + [BAr F 4 ] À catalyzedt he hydrosilylation of CO 2 with tertiary hydrosilanes to give stepwise formoxys ilane,m ethyl formate,a nd methoxy silane.T he unexpected formation of methyl formate was shown to result from the zinc-catalyzed transesterification of methoxy silane with formoxys ilane,w hichw as eventually converted into methoxy silane as well.

Zinc catalysed electrophilic C–H borylation of heteroarenes

“…The three functionals all gave the correct relative Lewis acidity towards hydride ( Scheme 9 ). Based on these observations and its recent utility in other zinc-cation catalysed processes, 32 B3PW91 was used for further calculations, and only results with this functional are discussed.…”

“… 31 However, another C–H borylation mechanism may be more relevant for substrates less acidic than alkynes, with no (hetero)arene deprotonation by a zinc-hydride reported to date to the best of our knowledge. 32 One alternative mechanism involves activation of a hydroborane by a zinc Lewis acid. This would enhance electrophilicity at boron and form a functional equivalent of a borenium cation, species well documented to borylate π systems.…”

“…23,26 Therefore, identifying a Brønsted base that when combined with a zinc Lewis acid functions as a FLP maybe essential to facilitate C–H deprotonation and ultimately borylation (particularly given the lack of precedence for Zn–H units acting as effective Brønsted bases towards heteroarenes). 32 While some notable work on zinc based FLPs has been reported, their use to date has been limited ( Fig. 2B ), 37 and they have not been applied in arene C–H functionalisation to our knowledge.…”

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

“…They are employed in the reduction of olefins, [5–8] imines, [9–11] aldehydes, ketones, [12–18] and carbon dioxide [19–30] . Catalytic reduction of carbon dioxide through catalytic hydrosilylation [31–34] and, to some extent, hydroboration [35, 36] with zinc catalysts has been investigated [37–44] . Zinc acetate is known to hydrosilylate CO 2 to a mixture of reduced products: silyl formates, bis(silyl) acetals, methoxysilanes, and methane [45] .…”

“…Due to inherent Lewis acidity, alkyl zinc cations promote CO 2 hydrosilylation through activation of the C=O bond. Recent investigations have shown that zinc hydrides catalyze CO 2 hydrosilylation, resulting in selective reduction to silyl formates or methoxysilanes [43, 46] . Mechanistic studies have revealed that molecular zinc hydrides insert CO 2 across Zn−H bond.…”