Hydrosilylation of Aldehydes and Ketones Catalyzed by a Terminal Zinc Hydride Complex, [κ3-Tptm]ZnH

Hydrosilylation of nitriles and tertiary amides using a zinc precursor

We report a competent and selective hydrosilylation of nitriles and tertiary amides catalyzed by the readily available zinc bis(hexamethyldisilazide) under solvent-free and mild conditions, making it a sustainable and desirable alternative to existing methods. Both protocols afforded high conversion, superior selectivity, and a broad substrate scope, from electron-withdrawing to electron-donating and heterocyclic substitutions.

2-Anilidomethylpyridine-Derived Three-Coordinate Zinc Hydride: The Journey Unveils Anilide Backbone's Reactive Nature

Low-coordinate heteroleptic zinc hydrides are catalytically important but rare and synthetically challenging. We herein report three-coordinate monomeric zinc hydride on a 2-anilidomethylpyridine framework (NNL). The synthetic success comes through systematically screening a few different routes from different precursors. During the process, the ligand's anilide backbone interestingly appears to be more reactive than Zn's terminal site to electrophilic Lewis and Brønsted acids. The proligand NNLH reacts with [Zn{N(SiMe3)2}2] and ZnEt2 to give [(NNL)ZnA] (A = N(SiMe3)2 (1), Et(2)). Both are inert to PhSiH3 and H2 but react with HBpin only through the internal Zn-Nanilide bond to give the borylated ligand NNLBpin (3). The reactions of 1 and 2 with Ph3EOH (E = C, Si) afford a series of divergent compounds like [(NNLH)Zn(OSiPh3)2] (4), [Zn3(OSiPh3)4Et2] (5), and [EtZn(OCPh3)] (6). But in all cases, it is invariably the Zn-Nanilide bond protonated by the -OH with equal or higher preference than the terminal Zn-N or Zn-C bonds. A DFT analysis rationalizes the origin of such a reactivity pattern. Realizing that an acid-free route might be the key, reacting [(NNL)Li] with ZnBr2 gives [(NNL)Zn(μ-Br)]2 (7), which on successively treating with KOSiPh3 and PhSiH3 gives the desired [(NNL)ZnH] (8) as a three-coordinate monomer with a terminal Zn-H bond. Estimating the ligand steric in 8 shows the openness in Zn's coordination sphere, a desired criterion for efficient catalysis. This and a positive influence of the pyridyl sidearm is reflected in 8's superior activity in hydroborating PhC(O)Me by HBpin in comparison to Jones' two-coordinate anilido zinc hydride.

Catalytic reduction of carbon dioxide by a zinc hydride compound, [Tptm]ZnH, and conversion to the methanol level

The zinc hydride compound, [Tptm]ZnH, may achieve the reduction of CO₂ by (RO)₃SiH (R = Me, Et) to the methanol oxidation level, (MeO)_xSi(OR)_4-x, via the formate species, HCO₂Si(OR)₃. However, because insertion of CO₂ into the Zn-H bond is more facile than insertion of HCO₂Si(OR)₃, conversion of HCO₂Si(OR)₃ to the methanol level only occurs to a significant extent in the absence of CO₂.

Synthesis of bis(2-pyridylthio)methyl zinc hydride and catalytic hydrosilylation and hydroboration of CO2

The reactions of bis(2-pyridylthio)methane with Me₂Zn and Zn[N(SiMe₃)₂]₂ afford [Bptm]ZnMe and [Bptm]ZnN(SiMe₃)₂, thereby providing access to a variety of other [Bptm]ZnX derivatives, including the zinc hydride complex [Bptm]ZnH, which serves as a catalyst for the reduction of CO₂ and other carbonyl compounds via hydrosilylation and hydroboration.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Synthesis, Structure, and Reactivity of a Terminal Cadmium Hydride Compound, [κ3-TismPriBenz]CdH

The terminal cadmium hydride compound, [κ³-Tism^PriBenz]CdH, which features the tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, may be obtained via the reactions of either [κ³-Tism^PriBenz]CdN(SiMe₃)₂ or [Tism^PriBenz]CdOSiPh₃ with PhSiH₃. The Cd-H bond of [κ³-Tism^PriBenz]CdH undergoes (a) metathesis reactions with MeI, Me₃SiX (X = Cl, Br, I, NCO), and Me₃SnX (X = Cl, Br, I) to afford the corresponding [Tism^PriBenz]CdX derivative, (b) insertion with CO₂ and CS₂ to afford respectively [Tism^PriBenz]Cd(κ¹-O₂CH) and [Tism^PriBenz]Cd(κ¹-S₂CH), and (c) hydride abstraction with B(C₆F₅)₃ to afford {[Tism^PriBenz]Cd}[HB(C₆F₅)₃] that possesses a rare trigonal monopyramidal geometry for cadmium.

Zinc Hydride Catalyzed Chemoselective Hydroboration of Isocyanates: Amide Bond Formation and C=O Bond Cleavage

Herein, a remarkable conjugated bis-guanidinate (CBG) supported zinc hydride, [{LZnH}₂ ; L={(ArHN)(ArN)-C=N-C=(NAr)(NHAr); Ar=2,6-Et₂ -C₆ H₃ }] (I) catalyzed partial reduction of heteroallenes via hydroboration is reported. A large number of aryl and alkyl isocyanates, including electron-donating and withdrawing groups, undergo reduction to obtain selectively N-boryl formamide, bis(boryl) hemiaminal and N-boryl methyl amine products. The compound I effectively catalyzes the chemoselective reduction of various isocyanates, in which the construction of the amide bond occurs. Isocyanates undergo a deoxygenation hydroboration reaction, in which the C=O bond cleaves, leading to N-boryl methyl amines. Several functionalities such as nitro, cyano, halide, and alkene groups are well-tolerated. Furthermore, a series of kinetic, control experiments and structurally characterized intermediates suggest that the zinc hydride species are responsible for all reduction steps and breaking the C=O bond.

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.

Rhenium versus cadmium: an alternative structure for a thermally stable cadmium carbonyl compound

An alternative description is provided for the previously reported novel tetranuclear cadmium carbonyl compound, [Cd(CO)3(C6H3Cl)]4. Specifically, consideration of single crystal X-ray diffraction data indicates that the compound is better formulated as the rhenium compound, [Re(CO)3(C4N2H3S)]4. Furthermore, density functional theory calculations predict that, if it were to exist, [Cd(CO)3(C6H3Cl)]4 would have a very different structure to that reported. While it is well known that X-ray diffraction may not reliably distinguish between atoms of similar atomic number (e.g. N/C and Cl/S), it is not generally recognized that two atoms with very different atomic numbers could be misassigned. The misidentification of two elements as diverse as Re and Cd (ΔZ = 27) is unexpected and serves as an important caveat for structure determinations.

Zinc Hydride-Catalyzed Hydrofuntionalization of Ketones

Three new dimeric bis-guanidinate zinc(II) alkyl, halide, and hydride complexes [LZnEt]₂ (1), [LZnI]₂ (2) and [LZnH]₂ (3) were prepared. Compound 3 was successfully employed for the hydrosilylation and hydroboration of a vast number of ketones. The catalytic performance of 3 in the hydroboration of acetophenone exhibits a turnover frequency, reaching up to 5800 h^-1, outperforming that of reported zinc hydride catalysts. Notably, both intra- and intermolecular chemoselective hydrosilylation and hydroboration reactions have been investigated.

Metal-Ligand Cooperation Facilitates Bond Activation and Catalytic Hydrogenation with Zinc Pincer Complexes

A series of PNP zinc pincer complexes capable of bond activation via aromatization/dearomatization metal–ligand cooperation (MLC) were prepared and characterized. Reversible heterolytic N–H and H–H bond activation by MLC is shown, in which hemilability of the phosphorus linkers plays a key role. Utilizing this zinc pincer system, base-free catalytic hydrogenation of imines and ketones is demonstrated. A detailed mechanistic study supported by computation implicates the key role of MLC in facilitating effective catalysis. This approach offers a new strategy for (de)hydrogenation and other catalytic transformations mediated by zinc and other main group metals.

Heavier Alkaline-Earth Catalyzed Dehydrocoupling of Silanes and Alcohols for the Synthesis of Metallo-Polysilylethers

The dehydrocoupling of silanes and alcohols mediated by heavier alkaline-earth catalysts, [Ae{N(SiMe₃ )₂ }₂ ⋅(THF)₂ ] (I-III) and [Ae{CH(SiMe₃ )₂ }₂ ⋅(THF)₂ ], (IV-VI) (Ae=Ca, Sr, Ba) is described. Primary, secondary, and tertiary alcohols were coupled to phenylsilane or diphenylsilane, whereas tertiary silanes are less tolerant towards bulky substrates. Some control over reaction selectivity towards mono-, di-, or tri-substituted silylether products was achieved through alteration of reaction stoichiometry, conditions, and catalyst. The ferrocenyl silylether, FeCp(C₅ H₄ SiPh(OBn)₂ ) (2), was prepared and fully characterized from the ferrocenylsilane, FeCp(C₅ H₄ SiPhH₂ ) (1), and benzyl alcohol using barium catalysis. Stoichiometric experiments suggested a reaction manifold involving the formation of Ae-alkoxide and hydride species, and a series of dimeric Ae-alkoxides [(Ph₃ CO)Ae(μ₂ -OCPh₃ )Ae(THF)] (3 a-c, Ae=Ca, Sr, Ba) were isolated and fully characterized. Mechanistic experiments suggested a complex reaction mechanism involving dimeric or polynuclear active species, whose kinetics are highly dependent on variables such as the identity and concentration of the precatalyst, silane, and alcohol. Turnover frequencies increase on descending Group 2 of the periodic table, with the barium precatalyst III displaying an apparent first-order dependence in both silane and alcohol, and an optimum catalyst loading of 3 mol % Ba, above which activity decreases. With precatalyst III in THF, ferrocene-containing poly- and oligosilylethers with ferrocene pendent to- (P1-P4) or as a constituent (P5, P6) of the main polymer chain were prepared from 1 or Fe(C₅ H₄ SiPhH₂ )₂ (4) with diols 1,4-(HOCH₂ )₂ -(C₆ H₄ ) and 1,4-(CH(CH₃ )OH)₂ -(C₆ H₄ ), respectively. The resultant materials were characterized by NMR spectroscopy, gel permeation chromatography (GPC) and DOSY NMR spectroscopy, with estimated molecular weights in excess of 20,000 Da for P1 and P4. The iron centers display reversible redox behavior and thermal analysis showed P1 and P5 to be promising precursors to magnetic ceramic materials.

Photoactivated silicon–oxygen and silicon–nitrogen heterodehydrocoupling with a commercially available iron compound

A commercially available iron readily engages in catalytic Si–O and Si–N bond formation under visible light irradiation.

New anionic rhodium complexes as catalysts for the reduction of acetophenone and its derivatives

New anionic rhodium(iii) complexes, obtained by a simple reaction of RhCl3 with organic chlorides (derivatives of imidazole and pyridine), have been employed as catalysts for hydrosilylation (reduction) of acetophenone derivatives. The reactions, in which 1,1,1,3,5,5,5-heptamethyltrisiloxane was a reducing agent, proceeded in a biphasic system because the above complexes are insoluble in the reaction medium. Thereby easy isolation of the complexes from post-reaction mixtures was possible after reaction completion. This is the first example of the application of rhodium complexes of this type as catalysts for ketone reduction. The complexes have shown high activity and enabled obtaining the hydrosilylation product in a very short time and in the range of low concentrations (0.1 mol%). By using FT-IR in situ analysis that enables measuring product concentrations in real time, a comparison has been made of the catalytic activity for hydrosilylation of acetophenone and methoxyacetophenone isomers shown by four rhodium complexes ([C+][RhCl4 -]) differing in cations and the most effective catalyst for this process has been distinguished.

Zinc Oxide-Catalyzed Dehydrogenation of Primary Alcohols into Carboxylic Acids

Zinc oxide has been developed as a catalyst for the dehydrogenation of primary alcohols into carboxylic acids and hydrogen gas. The reaction is performed in mesitylene solution in the presence of potassium hydroxide, followed by workup with hydrochloric acid. The transformation can be applied to both benzylic and aliphatic primary alcohols and the catalytically active species was shown to be a homogeneous compound by a hot filtration test. Dialkylzinc and strongly basic zinc salts also catalyze the dehydrogenation with similar results. The mechanism is believed to involve the formation of a zinc alkoxide which degrades into the aldehyde and a zinc hydride. The latter reacts with the alcohol to form hydrogen gas and regenerate the zinc alkoxide. The degradation of a zinc alkoxide into the aldehyde upon heating was confirmed experimentally. The aldehyde can then undergo a Cannizzaro reaction or a Tishchenko reaction, which in the presence of hydroxide leads to the carboxylic acid.

Carbonyl and ester C–O bond hydrosilylation using κ4-diimine nickel catalysts

(^Ph2PPrDI)Ni chemoselectively catalyzes α-allyl ester C–O bond hydrosilylation to prepare silyl esters with turnover frequencies of up to 990 h⁻¹.

A porous metal–organic aerogel based on dirhodium paddle-wheels as an efficient and stable heterogeneous catalyst towards the reduction reaction of aldehydes and ketones

A new mesoporous metal–organic aerogel based on dirhodium paddle-wheels has been successfully synthesized and applied in the hydrosilylation reaction of aldehydes and ketones.

Surprisingly high sensitivity of copper nanoparticles toward coordinating ligands: consequences for the hydride reduction of benzaldehyde

Depending on the ligand, ligand-induced leaching of copper nanoparticles may produce catalytically active species for the reduction of benzaldehyde.

Tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl metal complexes, [TismPriBenz]M: a new class of metallacarbatranes, isomerization to a tris(N-heterocyclic carbene) derivative, and evidence for an inverted ligand field

The tris[(1-isopropylbenzimidazol-2-yl)dimethylsilyl]methyl ligand, [TismPriBenz], has been employed to form carbatrane compounds of both the main group metals and transition metals, namely [TismPriBenz]Li, [TismPriBenz]MgMe, [TismPriBenz]Cu and [TismPriBenz]NiBr. In addition to the formation of atranes, a zinc compound that exhibits κ3-coordination, namely [κ3-TismPriBenz]ZnMe, has also been obtained. Furthermore, the [TismPriBenz] ligand may undergo a thermally induced rearrangement to afford a novel tripodal tris(N-heterocyclic carbene) variant, as shown by the conversion of [TismPriBenz]Cu to [κ4-C4-TismPriBenz*]Cu. The transannular M-C bond lengths in the atrane compounds are 0.19-0.32 Å longer than the sum of the respective covalent radii, which is consistent with a bonding description that features a formally zwitterionic component. Interestingly, computational studies demonstrate that the Cu-Catrane interactions in [TismPriBenz]Cu and [κ4-C4-TismPriBenz*]Cu are characterized by an "inverted ligand field", in which the occupied antibonding orbitals are localized more on carbon than on copper.

Selective hydrosilylation of alkynes and ketones: contrasting reactivity between cationic 3-iminophosphine palladium and nickel complexes

New palladium(allyl) complexes proved effective for the hydrosilylation of electron-deficient alkynes, while nickel analogues excelled with ketones and internal alkynes.

Pure phosphotriesters as versatile ligands in transition metal catalysis: efficient hydrosilylation of ketones and diethylzinc addition to aldehydes

The phosphate fate: will proving the versatility of phosphate ligands in catalysis make them finally attractive?

Cadmium Compounds with an [N3C] Atrane Motif: Evidence for the Generation of a Cadmium Hydride Species

Tris(2-pyridylthio)methane ([Tptm]H) has been employed to synthesize a series of cadmium carbatrane compounds that feature an [N3C] coordination environment. Specifically, [Tptm]H reacts with Cd[N(SiMe3)2]2 to afford [Tptm]CdN(SiMe3)2, which thereby provides access to other derivatives. For example, [Tptm]CdN(SiMe3)2 reacts with (i) CO2 to form {[Tptm]Cd(μ-NCO)}2 and (ii) Me3SiOH and Ph3SiOH to form {[κ3-Tptm]Cd(μ-OSiMe3)}2 and [Tptm]CdOSiPh3, respectively. The siloxide compound {[κ3-Tptm]Cd(μ-OSiMe3)}2 reacts with Me3SiX (X = Cl, Br, O2CMe) to give [Tptm]CdX, while the reaction with PhSiH3 in the presence of CO2 generates the formate complex, [Tptm]CdO2CH, thereby providing evidence for the generation of a proposed cadmium hydride intermediate, {[Tptm]CdH}.

Investigation of N-Heterocyclic Carbene-Supported Group 12 Triflates as Pre-catalysts for Hydrosilylation/Borylation

N-Heterocyclic carbene (NHC) complexes of Cd and Hg triflates (OTf) were prepared and their attempted conversion into rare cadmium and mercury hydrides was explored. In contrast to zinc, which forms stable [ZnH]⁺ complexes with NHCs, the heavier Cd and Hg congeners could not be formed; the increased instability of Cd-H and Hg-H units was rationalized with the aid of computations. It was also discovered that the dimeric adduct [IPr⋅Cd(μ-OTf)₂ ]₂ (IPr=[(HCNDipp)₂ C:]; Dipp=2,6-iPr₂ C₆ H₃ ) is an active precatalyst for the hydrosilylation and hydroborylation of hindered aldehydes and ketones. The related zinc congener was inactive as a catalyst highlighting a distinct advantage of using heavy Group 12 metals to promote catalytic hydrosilylation/borylation.

Ring-Shaped Phosphinoamido-Magnesium-Hydride Complexes: Syntheses, Structures, Reactivity, and Catalysis

A series of magnesium(II) complexes bearing the sterically demanding phosphinoamide ligand, L(-) =Ph2 PNDip(-) , Dip=2,6-diisopropylphenyl, including heteroleptic magnesium alkyl and hydride complexes are described. The ligand geometry enforces various novel ring and cluster geometries for the heteroleptic compounds. We have studied the stoichiometric reactivity of [(LMgH)4 ] towards unsaturated substrates, and investigated catalytic hydroborations and hydrosilylations of ketones and pyridines. We found that hydroborations of two ketones with pinacolborane using various Mg precatalysts is very rapid at room temperature with very low catalyst loadings, and ketone hydrosilylation using phenylsilane is rapid at 70 °C. Our studies point to an insertion/σ-bond metathesis catalytic cycle of an in situ formed "MgH2 " active species.

Synthesis, structure and reactivity of [TmBut]ZnH, a monomeric terminal zinc hydride compound in a sulfur-rich coordination environment: access to a heterobimetallic compound

The zinc hydride complex, [Tm^But]ZnH, undergoes insertion of CO₂ and facile protolytic cleavage, of which the latter provides access to heterobimetallic [Tm^But]ZnMo(CO)₃Cp.

Two-coordinate terminal zinc hydride complexes: synthesis, structure and preliminary reactivity studies

The first examples of essentially two-coordinate, monomeric zinc hydride complexes have been stabilised by incorporation of “super bulky” amide ligands. Crystallographic studies show them to possess near linear N–Zn–H fragments (see picture).