N‐Heterocyclic Carbene Based Tri‐organyl‐Zn–Alkyl Cations: Synthesis, Structures, and Use in CO₂ Functionalization

Abstract

Tri-organyl and tricoordinate N-heterocyclic carbene (NHC) Zn-NHC alkyl cations [(nNHC) Zn-Me] (nNHC=C2-bonded-IMes/-IDipp; 3 and 4 ; IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were first synthesized and structurally characterized by ionization of the corresponding neutral precursors [(nNHC)ZnMe ] with [Ph C][B(C F ) ] in the presence of one equivalent of free NHC. Whereas cation [(nIMes) Zn-Me] (3 ) is stable, its sterically congested analogue [(nIDipp) Zn-Me] (4 ) readily undergoes an nNHC-to-aNHC isomerization in the presence of THF or IDipp to afford the more thermodynamically stable [(aIDipp)(nIDipp)Zn-Me] (aIDipp=C4-bonded IDipp, 5 ), reflecting the adaptable-to-sterics coordination chemistry of these cations for improved stability. Cations 3 -5 are the first Zn cations of the type Zn(C)(C')(C'') (C, C', C''=σ-donor carbyl ligand). Kinetic studies combined with DFT calculations agree with an nNHC-to-aNHC process proceeding through the initial deprotonation of 4 (at a Zn-bonded C4-IDipp moiety) by IDipp. Unlike 3 and 4 , the rearranged cation 5 reacts with CO through insertion into the Zn-Me bond yielding the corresponding Zn(κ -OAc) cation 6 . Both cations 5 and 6 were successfully used in CO hydrosilylation catalysis for silylformate formation.

Homogeneous Catalytic Reduction of CO2 with Silicon‐Hydrides, State of the Art

“…Examples of zinc‐catalyzed CO 2 hydrosilylation processes have also been reported ,. Thus, the hydride complex [Zn(H)(κ 3 ‐Tptm)] ( 16 ) (Tptm=tris(2‐pyridylthio)methyl) (0.1 mol %) catalyzes the reaction of CO 2 (7.0 bar) with HSi(EtO) 3 at 373 K, to selectively afford HCO 2 Si(OEt) 3 on a 20 g scale (TOF=2.9 h −1 ) (Scheme ) …”

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

“…[3] Thecatalytic reduction of CO 2 with silanes is well-known, mainly using ruthenium complexes with ac ombination of high CO 2 pressures (20-80 bar) and moderate to high temperatures (45-100 8 8C), [4] but also Cu, [5] Ir, [6] and Pd [7] complexes were found to catalyze this reaction under mild conditions.In terms of price,a bundance,a nd toxicity of the metal center, the use of zinc-based systems is of significant interest. [8][9][10][11][12] However,these systems require either high temperatures and high pressures or only show low to moderate turnover numbers and turnover frequencies.O nly the recently introduced [Tism PriBenz ]b ased system was found to catalyze the hydrosilylation of CO 2 under ambient conditions,albeit with the use of ab orane additive and with only low turnover frequencies. [8][9][10][11][12] However,these systems require either high temperatures and high pressures or only show low to moderate turnover numbers and turnover frequencies.O nly the recently introduced [Tism PriBenz ]b ased system was found to catalyze the hydrosilylation of CO 2 under ambient conditions,albeit with the use of ab orane additive and with only low turnover frequencies.…”

“…Indeed, we found [Tntm]ZnH to catalytically reduce CO 2 with trimethoxysilane at room temperature and under mild conditions.T his is the first zinc-based system capable of catalytically reducing CO 2 at room temperature without the need of any additive and is also the most active system up to date.T he intermediate formate complex [Tntm]Zn(O 2 CH) was isolated and fully characterized. [8][9][10][11][12] [Tntm]ZnH (2)was prepared by reaction of the respective zinc trimethylsilanolate complex [Tntm]Zn(OSiMe 3 )(1)with aslight excess of (MeO) 3 SiH in THF in 85 %yield, similar to apreviously described method. Reported zinc-based catalysts for the hydrosilylation of CO 2 .…”

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

“…Thus,it is surprising that only very few examples of zinc-catalyzed hydrosilylation of CO 2 have been reported:t wo cationic,Nheterocyclic carbene (NHC) stabilized zinc complexes,ad icationic NHC-stabilized zinc hydride cluster,a swell as the tris(thiopyridyl)methane based [Tptm]ZnH and the tris[(1isopropylbenzimidazol-2-yl)dimethylsilyl]methane based [Tism PriBenz ]ZnH complexes were shown to catalytically react with CO 2 in presence of silanes ( Figure 1). [8][9][10][11][12] However,these systems require either high temperatures and high pressures or only show low to moderate turnover numbers and turnover frequencies.O nly the recently introduced [Tism PriBenz ]b ased system was found to catalyze the hydrosilylation of CO 2 under ambient conditions,albeit with the use of ab orane additive and with only low turnover frequencies. [11] Herein we show that the recently introduced [Tntm] system [13] allows the isolation of the terminal zinc hydride complex [Tntm]ZnH.…”

“…Reported zinc-based catalysts for the hydrosilylation of CO 2 . [8][9][10][11][12] [Tntm]ZnH (2)was prepared by reaction of the respective zinc trimethylsilanolate complex [Tntm]Zn(OSiMe 3 )(1)with aslight excess of (MeO) 3 SiH in THF in 85 %yield, similar to apreviously described method. [14] In the solid state,t he hydrido compound is moderately stable at ambient atmosphere whereas in solution it is sensitive towards moisture and decomposes to yet unidentified products.H owever,i nd ichloromethane,a lso in absence of moisture, 2 is only stable below À30 8 8C, while at room temperature it decomposes in the course of 4-6 h, predominantly forming [k 4 -C,N,S,S-Tntm]Zn(Pn) (4,P n = 6-tertbutyl-3-thiopyridazine;S upporting Information, Figures S8-S10, S19).…”