Dinuclear zinc hydride supported by an anionic bis(N-heterocyclic carbene) ligand

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.

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.

Hydrosilane σ-Adduct Intermediates in an Adaptive Zinc-Catalyzed Cross-dehydrocoupling of Si-H and O-H Bonds

Three-coordinate Ph BOXMe 2 ZnR (Ph BOXMe 2 =phenyl-(4,4-dimethyl-oxazolinato; R=Me: 2 a, Et: 2 b) catalyzes the dehydrocoupling of primary or secondary silanes and alcohols to give silyl ethers and hydrogen, with high turnover numbers (TON; up to 107 ) under solvent-free conditions. Primary and secondary silanes react with small, medium, and large alcohols to give various degrees of substitution, from mono- to tri-alkoxylation, whereas tri-substituted silanes do not react with MeOH under these conditions. The effect of coordinative unsaturation on the behavior of the Zn catalyst is revealed through a dramatic variation of both rate law and experimental rate constants, which depend on the concentrations of both the alcohol and hydrosilane reactants. That is, the catalyst adapts its mechanism to access the most facile and efficient conversion. In particular, either alcohol or hydrosilane binds to the open coordination site on the Ph BOXMe 2 ZnOR catalyst to form a Ph BOXMe 2 ZnOR(HOR) complex under one set of conditions or an unprecedented σ-adduct Ph BOXMe 2 ZnOR(H-SiR'3 ) under other conditions. Saturation kinetics provide evidence for the latter species, in support of the hypothesis that σ-bond metathesis reactions involving four-centered electrocyclic 2σ-2σ transition states are preceded by σ-adducts.

Synthetic and Structural Studies of Ethyl Zinc β-Amidoenoates and β-Ketoiminates

A set of heteroleptic ethyl zinc β-amidoenoates (1, 2) and β-ketoiminates (3) of the form [LZnEt]2 with varying steric bulk have been synthesised via the reaction of diethylzinc with β-aminoenoate ligands HL1 and HL2 and β-ketoimine HL3. These complexes have been characterised via 1H and 13C NMR, mass spectrometry and single-crystal X-ray diffraction, which unambiguously determined all three structures as dimeric species in the solid state. We observe the unusual dimerisation of 1 and 2 through coordination of the central zinc atom to the methine carbon of the second monomer, which gives these complexes high reactivity. The thermal properties of complex 3 are explored via thermal gravimetric analysis (TGA), to investigate their potential as single-source precursors to zinc oxide, which shows that 3 has a significantly lower decomposition temperature as compared to its bis-ligated counterpart [Zn(L3)2], which gives 3 promise as a single-source precursor to zinc oxide.

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.

An air-stable, Zn2+-based catalyst for hydrosilylation of alkenes and alkynes

Hydrosilylation of C[double bond, length as m-dash]C double and C[triple bond, length as m-dash]C triple bonds is one of the most widely used processes in organosilicon chemistry, mostly catalyzed by Pt-based complexes. We report here the synthesis of an air-stable dicationic Zn2+-based complex in a hemilabile tris(2-methyl-6-pyridylmethyl) phosphine (TmPPh) ligand, 12+[B(C6F5)4]2. When heated, 12+[B(C6F5)4]2 activates Si-H bonds reversibly via ligand/metal cooperation between Lewis acidic Zn2+ and Lewis basic N centers in a frustrated Lewis pair (FLP) type fashion. Consequently, 12+[B(C6F5)4]2 was found to be an effective catalyst for hydrosilylation reactions of C[double bond, length as m-dash]C double and C[triple bond, length as m-dash]C triple bonds. Remarkably, these hydrosilylation reactions can be loaded under aerobic conditions, as well as, in some cases, work under neat conditions. The mechanism of the activation of the Si-H bond and the hydrosilylation reaction is proposed based on experiments and density functional theory (DFT) calculations.

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.

Reactivity patterns for the activation of CO2 and CS2 with alumoxane and aluminum hydrides

Carbon dioxide is readily fixed when reacting with either alumoxane dihydride [{MeLAl(H)}2(μ-O)] (1) or aluminum dihydride [MeLAlH2] (2) (MeL = HC[(CMe)N(2,4,6-Me3C6H2)]2-) to produce bimetallic aluminum formates [(MeLAl)2(μ-OCHO)2(μ-O)] (3) and [(MeLAl)2(μ-OCHO)2(μ-H)2] (5), respectively. Furthermore, [(MeLAl)2(μ-OCHO)2(μ-OH)2] (4) is easily obtained upon the reaction of 3 or 5 with H2O. The stability of the unusual dialuminum diformate dihydride core observed in 5 stems from the proximity of the Al centers allowing the formation of two Al-HAl bridges and precluding further hydride transfer to the HCO2 moieties. Contrary to this behavior, 1 and 2 react with CS2 giving cyclic alumoxane and aluminum sulfides [(MeLAl)2(μ-S)(μ-O)] (6) and [{MeLAl(μ-S)}2] (7), respectively. The molecular structures of 3-7 were characterized by IR, Raman, solution or solid-state (MAS) NMR spectroscopy and mass spectrometry and for 4-7 were characterized by X-ray diffraction studies. NMR kinetic studies and DFT calculations suggest that the mechanisms for the formation of 6 and 7 involve the transfer of a hydride group forming transient aluminum thioformate intermediates which proceed to form Al-S-Al moieties through the cleavage of C-S bonds and insertion of a sulfur atom, followed by the elimination of thioformaldehyde.

Terminal hydridozinc cation

Thermally stable hydridozinc cation catalytically converts CO₂ to PhSi(OCHO)₃ using PhSiH₃ in the presence of a mild Lewis acid BPh₃.

Calcium formamidinate derivatives by hydride insertion of carbodiimides

Insertion reactions of N,N′-dialkyl and N,N′-diaryl carbodiimides with a dimeric calcium hydride provide formamidinate derivatives whose structures on the steric demands of the carbodiimide reagent.

Constructing reactive Fe and Co complexes from isolated picolyl-functionalized N-heterocyclic carbenes

The Fe(ii) and Co(ii) complexes prepared from the isolated free carbene form of the picolyl-NHC ligands display excellent catalytic activity towards the hydrosilylation of ketones.

Chemoselective Polymerizations from Mixtures of Epoxide, Lactone, Anhydride, and Carbon Dioxide

Controlling polymer composition starting from mixtures of monomers is an important, but rarely achieved, target. Here a single switchable catalyst for both ring-opening polymerization (ROP) of lactones and ring-opening copolymerization (ROCOP) of epoxides, anhydrides, and CO2 is investigated, using both experimental and theoretical methods. Different combinations of four model monomers-ε-caprolactone, cyclohexene oxide, phthalic anhydride, and carbon dioxide-are investigated using a single dizinc catalyst. The catalyst switches between the distinct polymerization cycles and shows high monomer selectivity, resulting in block sequence control and predictable compositions (esters and carbonates) in the polymer chain. The understanding gained of the orthogonal reactivity of monomers, specifically controlled by the nature of the metal-chain end group, opens the way to engineer polymer block sequences.

Two alternative approaches to access mixed hydride-amido zinc complexes: synthetic, structural and solution implications

In the presence of a bulky N-heterocyclic carbene, Zn(HMDS)₂can be converted in to either a mononuclear amido/hydride complex or a tetranuclear hydride rich cluster depending on the hydride source and reaction conditions employed.