In this work, (1,5-hexadiene)Ni(SIPr) (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) is used in place of Ni(COD)2/SIPr·HBF4/KOtBu (COD = 1,5-cyclooctadiene) as a more robust catalyst for regioselective reductive coupling of ynoates and aldehydes with triethylsilane. The catalytic reaction of ethyl 3-(trimethylsilyl)propiolate and methyl 4-formylbenzoate shows first-order dependence on aldehyde and catalyst concentrations, inverse first-order dependence on [ynoate], and no dependence on [silane]. The kinetics data, coupled with deuterium-labeling experiments, support a mechanism involving dissociation of the ynoate from a catalytically dormant nickelacyclopentadiene intermediate prior to turnover-limiting formation of a catalytically active nickeladihydrofuran.

•Synthesis of four POCOP-pincer ligated palladium hydrocarbyl complexes.•Crystal structures of two of the palladium hydrocarbyl complexes.•Reactions of these hydrocarbyl complexes with phenylacetylene.•A systematic comparison of the palladium-carbon bond lengths.•A systematic comparison of the reactivity of the hydrocarbyl complexes.POCOP-pincer ligated palladium hydrocarbyl complexes [2,6-(R2PO)2C6H3]PdR′ have been prepared from the reaction between [2,6-(R2PO)2C6H3]PdCl and R'Li. X-ray crystallographic studies of these complexes show that the Pd–R′ bond becomes shorter when the metal-bound carbon has more s character. Comparing the relative reactivity of these palladium hydrocarbyl (including hydride) complexes toward phenylacetylene suggests that the rate of the C–H exchange process follows a decreasing order of Pd–H > Pd–Csp3 > Pd–Csp2 > Pd–Csp. The palladium phenyl complex [2,6-(R2PO)2C6H3]PdPh is exceptionally slow for the reaction with phenylacetylene, possibly due to a high-energy conformer required for the palladium-carbon bond cleavage.The reaction of POCOP-pincer ligated palladium hydrocarbyl complexes with phenylacetylene shows a general reactivity order of Pd–Csp3 > Pd–Csp2 > Pd–Csp with the phenyl complex being an exception. The related palladium hydride complex is much more reactive than these hydrocarbyl complexes. Download high-res image (113KB)Download full-size image

Mechanochemistry, more specifically high-speed ball milling, has garnered significant attention in several areas of chemistry, particularly for the synthesis of inorganic materials, cocrystals, organic compounds, discrete metal complexes and metal organic frameworks. This methodology is creating exciting research opportunities because, unlike traditional synthesis, a reaction carried out in a high-speed ball mill does not necessarily need a solvent, thus representing an environmentally friendly solution to the issue of solvent waste. This Chemistry Frontiers article delves into a unique area of ball milling that capitalizes on the solventless nature of the synthesis by using reaction vials, balls, foils, and pellets as both reaction medium and the catalyst. Several examples are highlighted, from nanoparticle synthesis and nanocatalysis to using transition metals in their metallic forms as catalysts. This article is aimed to show both the advantages and challenges present in the field, and to spark interest in further development of this research area.

Nickel hydride complexes, defined herein as any molecules bearing a nickel hydrogen bond, are crucial intermediates in numerous nickel-catalyzed reactions. Some of them are also synthetic models of nickel-containing enzymes such as [NiFe]-hydrogenase. The overall objective of this review is to provide a comprehensive overview of this specific type of hydride complexes, which has been studied extensively in recent years. This review begins with the significance and a very brief history of nickel hydride complexes, followed by various methods and spectroscopic or crystallographic tools used to synthesize and characterize these complexes. Also discussed are stoichiometric reactions involving nickel hydride complexes and how some of these reactions are developed into catalytic processes.

Two Fe–Cu heterobimetallic complexes have been synthesized from the reactions of the (cyclopentadienone)iron complexes {2,5-(EMe3)2-3,4-(CH2)4(η4-C4C═O)}Fe(CO)3 (E = Si, 1a; E = C, 1b) with (IPr)CuOH (IPr = 1,3-bis(diisopropylphenyl)imidazol-2-ylidene). X-ray crystallographic studies show that these complexes adopt a structure featuring a bridging hydride which can be described by the formula {2,5-(EMe3)2-3,4-(CH2)4(η4-C4C═O)}(CO)2Fe(μ-H)Cu(IPr) (E = Si, 4a′; E = C, 4b′). When they are dissolved in toluene, THF, or cyclohexane, these complexes form a rapidly equilibrated isomeric mixture of 4a′ or 4b′ and the terminal iron hydride {2,5-(EMe3)2-3,4-(CH2)4(η5-C4COCuIPr)}Fe(CO)2H (E = Si, 4a; E = C, 4b). The solution structure for the Me3Si derivative is dominated by 4a. Both 4a and 4b/4b′ react with HCO2H to form a monometallic iron hydride and (IPr)CuOCHO. They also undergo displacement of (IPr)CuH by CO. The heterobimetallic complexes are effective catalysts for the reduction of PhCHO under water-gas shift conditions; 4a exhibits higher activity than 4b/4b′. Control experiments with monometallic species establish the cooperativity between a bifunctional iron fragment and a copper fragment during the catalytic reaction. A mechanistic investigation including stoichiometric reactions, various control experiments, and labeling studies has led to the proposal of two different catalytic cycles.Keywords: cooperativity; copper; heterobimetallic complexes; iron; Knölker complex; reduction; water-gas shift reaction

A solvent-free, nickel-catalyzed [2 + 2+2 + 2] cycloaddition of alkynes to synthesize substituted cyclooctatetraene (COT) derivatives has been developed. This mechanochemical approach takes advantage of the frictional energy created by reusable nickel pellets, which also act as the catalyst. In contrast to solution chemistry, the major products are cyclooctatetraene isomers rather than substituted benzenes.Keywords: Cyclooctatetraene; Cyclotetramerization; Cyclotrimerization; Green chemistry; High speed ball mill; Mechanochemistry; Nickel catalysis;

•A dozen new POCOP pincer nickel isothiocyanate and azide complexes were synthesized.•Metathesis reactivity and ligand exchange reactions of these complexes were studied.•The metathesis reaction is faster with a less electron rich nickel center.•In competing with NCS−, N3−N3− prefers a less electron rich nickel center.•The experimental results were supported by DFT calculations and the HSAB theory.A series of nickel pincer complexes of the type [4-Z-2,6-(R2PO)2C6H2]NiX (R = tBu, iPr, Ph; Z = H, CO2Me; X = NCS, N3) have been synthesized from the reactions of the corresponding nickel chloride complexes [4-Z-2,6-(R2PO)2C6H2]NiCl and potassium thiocyanate or sodium azide. X-ray structure determinations of these complexes have shown that the thiocyanate ion binds to the nickel center through the nitrogen. A comparable Ni–N bond length (approx. 1.87 Å for the isothiocyanate complexes and 1.91 Å for the azide complexes) and an almost identical Ni–Cipso bond length (approx. 1.89 Å) have been observed for these complexes. Metathesis reactivity of [4-Z-2,6-(R2PO)2C6H2]NiCl and ligand exchange reactions between the nickel isothiocyanate and nickel azide complexes have been investigated. The metathesis reactions with thiocyanate/azide complexes are faster with a less electron rich and more sterically accessible nickel center. The thermodynamic stability of these nickel complexes has been rationalized using hard-soft acid-base theory (HSAB theory); a harder ligand prefers a less electron rich nickel center. These experimental results have been supported by quantum chemical analysis of the coordinating nitrogen atoms in SCN– and N3−N3−.

The reaction of the formate complex {2,6-(R2PO)2C6H3}Ni(OCHO) (R = tBu, 5; R = iPr, 6) with CS2 shows first-order kinetics in nickel concentration and zero-order in [CS2] when CS2 is used in large excess. Rate measurement at different temperatures gives activation parameters ΔH⧧ = 22.6 ± 0.9 kcal/mol and ΔS⧧ = −5.2 ± 3.0 eu for the decarboxylation of 5 and ΔH⧧ = 22.6 ± 1.0 kcal/mol and ΔS⧧ = −4.3 ± 3.2 eu for the decarboxylation of 6. Comparing the decarboxylation rate constants for 6 and {2,6-(iPr2PO)2C6H3}Ni(OCDO) (6-d) yields KIE values of 1.67–1.90 within the temperature range 30–45 °C. On the basis of these experimental results and DFT calculations, an ion pair mechanism has been proposed for the decarboxylation process. The CS2 insertion products {2,6-(R2PO)2C6H3}Ni(SCHS) (R = tBu, 3; R = iPr, 4) have been characterized by X-ray crystallography.

An efficient synthesis of a green dye from oxidative coupling of p-phenylenediamine (PPD) and resorcinol (in a 2:1 ratio) has been developed. Reactivity studies of this dye molecule with a variety of reagents (PPD, resorcinol, the oxidized form of the green dye itself, and a dinuclear indo dye) demonstrate that it cannot be the key reactive intermediate in reported oxidative oligomerization of PPD and resorcinol. However, the trinuclear species does form large aggregates. At least one viable pathway of oligomerization has been demonstrated with the dinuclear indo dye.

The reductions of aldehydes, ketones, and esters to alcohols are important processes for the synthesis of chemicals that are vital to our daily life, and the reduction of CO2 to methanol is expected to provide key technology for carbon management and energy storage in our future. Catalysts that affect the reduction of carbonyl compounds often contain ruthenium, osmium, or other precious metals. The high and fluctuating price, and the limited availability of these metals, calls for efforts to develop catalysts based on more abundant and less expensive first-row transition metals, such as nickel and iron. The challenge, however, is to identify ligand systems that can increase the thermal stability of the catalysts, enhance their reactivity, and bypass the one-electron pathways that are commonly observed for first-row transition metal complexes. Although many other strategies exist, this Account describes how we have utilized pincer ligands along with other ancillary ligands to accomplish these goals. The bis(phosphinite)-based pincer ligands (also known as POCOP-pincer ligands) create well-defined nickel hydride complexes as efficient catalysts for the hydrosilylation of aldehydes and ketones and the hydroboration of CO2 to methanol derivatives. The hydride ligands in these complexes are substantially nucleophilic, largely due to the enhancement by the strongly trans-influencing aryl groups. Under the same principle, the pincer-ligated nickel cyanomethyl complexes exhibit remarkably high activity (turnover numbers up to 82,000) for catalytically activating acetonitrile and the addition of H–CH2CN across the C═O bonds of aldehydes without requiring a base additive. Cyclometalation of bis(phosphinite)-based pincer ligands with low-valent iron species “Fe(PR3)4” results in diamagnetic Fe(II) hydride complexes, which are active catalysts for the hydrosilylation of aldehydes and ketones. Mechanistic investigation suggests that the hydride ligand is not delivered to the carbonyl substrates but is important to facilitate ligand dissociation prior to substrate activation. In the presence of CO, the amine-bis(phosphine)-based pincer ligands are also able to stabilize low-spin Fe(II) species. Iron dihydride complexes supported by these ligands are bifunctional as both the FeH and NH moieties participate in the reduction of C═O bonds. These iron pincer complexes are among the first iron-based catalysts for the hydrogenation of esters, including fatty acid methyl esters, which find broad applications in industry. Our studies demonstrate that pincer ligands are promising candidates for promoting the first-row transition metal-catalyzed reduction of carbonyl compounds with high efficiency. Further efforts in this research area are likely to lead to more efficient and practical catalysts.

Phosphinite-based metallacycles are important intermediates, catalyst precursors, or catalysts for a variety of chemical transformations. Facile, reversible formation of P–O bonds allows phosphinites to act as catalytic directing groups that lead to more efficient and selective metal-catalyzed processes. Relatively low costs and convenient syntheses make metallacyclic phosphinite complexes attractive compounds for catalytic studies. Given the high interest in developing the related phosphine- and phosphite-based catalysts, this Perspective focuses specifically on the comparisons between these different metallacyclic complexes in catalytic reactions. In a number of examples, phosphinite-based metallacycles are more efficient catalysts due to better-matched ligand properties for rate-limiting steps or faster conversion of the complexes to catalytically active species.Keywords: cyclometalation; directing groups; metallacycles; phosphines; phosphinites; phosphites; pincer complexes

A new bidentate secondary phosphine oxide (SPO) was synthesized from diphenyl ether via ortho-lithiation, phosphorylation with PhP(Cl)NEt2, and hydrolysis in an acidic medium. Nickel(0) species ligated with this new SPO was established as a more effective catalyst than Ni(0)–Ph2P(O)H for the cross-coupling of aryl iodides with aryl thiols.

Nickel POCOP-pincer hydride complexes [2,6-(R2PO)2C6H3]NiH (R = iPr, 4a; R = cPe = cyclopentyl, 4b) react with phenylacetylene to generate [2,6-(R2PO)2C6H3]NiC(Ph)CH2 (5a–b) as the major product and (E)-[2,6-(R2PO)2C6H3]NiCHCHPh (6a–b) as the minor product. The 2,1-insertion is more favorable than the 1,2-insertion and both pathways involve cis addition of Ni–H across the CC bond. Unlike the palladium case, alkynyl complexes [2,6-(R2PO)2C6H3]NiCCPh (7a–b) and H2 are not produced in the nickel system. The more bulky hydride complex [2,6-(tBu2PO)2C6H3]NiH (4c) shows no reactivity towards phenylacetylene. Catalytic hydrogenation of phenylacetylene with 4a–b takes place at an elevated temperature (70–100 °C) and proves to be heterogeneous. The structures of 5b, 6a, 7a and 7b have been studied by X-ray crystallography.

A series of ruthenium- and iron-based pincer catalysts have been tested for the homogeneous hydrogenation of fatty acid methyl esters to fatty alcohols with turnover numbers (TONs) of up to 1860. These catalysts operate under neat conditions (no solvent) from the gram to the kilogram scale. The required temperature (135 °C) and H2 pressure (300–1000 psig) are much lower than those needed for current industrial processes, which rely on heterogeneous copper chromite catalysts. The fatty alcohol products can be purified through distillation, resulting in low levels of Ru (1 ppm) and P (<25 ppm). The five-coordinate Milstein catalyst and Takasago’s borane-stabilized catalyst are also effective for the direct hydrogenation of coconut oil to fatty alcohols. These results hold the promise of using homogeneous catalysts for ester hydrogenation in an industrial setting.

The P-stereogenic nickel complex {2,6-[(t-Bu)(Ph)PO]2C6H3}NiCl (2) has been synthesized via cyclometalation of the POCOP-pincer ligand 1,3-[(t-Bu)(Ph)PO]2C6H4 (1) with NiCl2. The initially isolated 2 consists of a 1:1 mixture of racemic and meso isomers that are separable through repeated crystallization and is configurationally stable even at 110 °C. Upon mixing with t-BuOK, the meso isomer (2-meso) displays a higher ligand substitution rate than the racemic isomer (2-rac), likely because its nickel center is sterically more accessible. Complex 2, as either pure 2-rac or a 2-rac/2-meso mixture, can be converted to the nickel triflate complex {2,6-[(t-Bu)(Ph)PO]2C6H3}NiOTf (3) or the nickel formate complex {2,6-[(t-Bu)(Ph)PO]2C6H3}NiOCHO (7) without epimerization at the phosphorus centers. Under a dynamic vacuum at 90 °C, decarboxylation of 7-meso is faster than that of 7-rac, suggesting that in the transition state the formato hydrogen approaches the nickel center from the axial site rather than the equatorial site. The structure of 2-rac has been studied by X-ray crystallography.

Hydrogenation of esters is vital to the chemical industry for the production of alcohols, especially fatty alcohols that find broad applications in consumer products. Current technologies for ester hydrogenation rely on either heterogeneous catalysts operating under extreme temperatures and pressures or homogeneous catalysts containing precious metals such as ruthenium and osmium. Here, we report the hydrogenation of esters under relatively mild conditions by employing an iron-based catalyst bearing a PNP-pincer ligand. This catalytic system is also effective for the conversion of coconut oil derived fatty acid methyl esters to detergent alcohols without adding any solvent.

A series of iron bis(phosphinite) pincer complexes with the formula of [2,6-(iPr2PO)2C6H3]Fe(PMe2R)2H (R = Me, 1; R = Ph, 2) or [2,6-(iPr2PO)2-4-(MeO)C6H2]Fe(PMe2Ph)2H (3) have been tested for catalytic dehydrogenation of ammonia borane (AB). At 60 °C, complexes 1–3 release 2.3–2.5 equiv of H2 per AB in 24 h. Among the three iron catalysts, 3 exhibits the highest activity in terms of both the rate and the extent of H2 release. The initial rate for the dehydrogenation of AB catalyzed by 3 is first order in 3 and zero order in AB. The kinetic isotope effect (KIE) observed for doubly labeled AB (kNH3BH3/kND3BD3 = 3.7) is the product of individual KIEs (kNH3BH3/kND3BH3 = 2.0 and kNH3BH3/kNH3BD3 = 1.7), suggesting that B–H and N–H bonds are simultaneously broken during the rate-determining step. NMR studies support that the catalytically active species is an AB-bound iron complex formed by displacing trans PMe3 or PMe2Ph (relative to the hydride) by AB. Loss of NH3 from the AB-bound iron species as well as catalyst degradation contributes to the decreased rate of H2 release at the late stage of the dehydrogenation reaction.

DFT calculations have been performed to gain mechanistic insight into ester hydrogenation to alcohols (exemplified by PhCO2CH3 +2H2 → PhCH2OH + CH3OH), catalyzed by a well-defined Fe-PNP pincer hydridoborohydride complex (1). The entire catalytic process includes precatalyst activation to an active species trans-dihydride complex 2, 2-catalyzed transformation of PhCO2CH3 + H2 → PhCHO + CH3OH, hydrogenation of PhCHO to PhCH2OH, and catalyst regeneration via H2 addition to the dehydrogenated 2 (i.e., complex 5). The transformation, PhCO2CH3 + H2 → PhCHO + CH3OH, proceeds via hydrogenation of PhCO2CH3 to a hemiacetal PhCH(OH)(OCH3), followed by decomposition of the hemiacetal to methanol and benzaldehyde. The Fe-complex 5 was found to be capable of facilitating the decomposition of the hemiacetal. The ineffectiveness of the catalytic system in hydrogenating methyl salicylate is attributed to the intrinsically lower reactivity of the ester toward C═O reduction and a very facile side-reaction, which is adding the phenol OH group of the hemiacetal intermediate stemmed from methyl salicylate to the Fe–N active site of 5. Computations of various catalyst initiation pathways show that the initiation without the aid of an additive is very unfavorable, thus we suggest the use of a Lewis base such as NR3 (R = Me and Et) and PR3 (R = nBu and tBu) to accelerate precatalyst (1) activation, because a Lewis base could form a stable adduct (BH3–NR3/PR3) with the BH3 moiety of 1. In agreement with greatly enhanced kinetics and thermodynamics of the initiation process suggested by the DFT calculations, experimental study shows that the addition of a catalytic amount of NEt3 doubled the yield of benzyl alcohol from the hydrogenation of methyl benzoate, when compared to the case without using any additive. The trans effect of the hydride on the reactivity of 2 and steric effect of the pincer substituents on the stability of 2 are also discussed.Keywords: catalyst initiation; DFT calculations; hydrogenation of esters; iron catalysis; Lewis base; mechanism

Treatment of iron POCOP-pincer hydride complexes cis-[2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)2 (1-H), [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)(CO) (2-H, trans H/CO; 2′-H, cis H/CO), and cis-[2,6-(iPr2PO)2C6H3]Fe(H)(CO)2 (3-H) with HBF4·Et2O in CD3CN/THF-d8 results in a rapid evolution of H2. Except for the reaction of 1-H, which leads to decomposition of the pincer structure, all other hydrides are converted cleanly to acetonitrile-trapped cationic complexes. Protonation of these hydrides with the weaker acids CF3CO2H and HCO2H establishes the basicity order of 1-H > 2-H > 2′-H > 3-H, with 3-H bearing the least basic hydride ligand. An alternative method of abstracting hydride by [Ph3C]+[BF4]− gives complicated products; the reaction of 2-H generates two pincer products, [HPMe3]+[BF4]− and Gomberg’s dimer, which supports a single electron transfer pathway. Cationic complexes {[2,6-(iPr2PO)2C6H3]Fe(CO)(PMe3)(CH3CN)}+[BF4]− (2+-BF4, trans CO/CH3CN) and cis-{[2,6-(iPr2PO)2C6H3]Fe(CO)2(CH3CN)}+[BF4]− (3+-BF4) are prepared from protonation of 2-H (or 2′-H) and 3-H with HBF4·Et2O, respectively. Both compounds react with H2 with the aid of iPr2NEt to yield neutral hydride complexes and [iPr2N(H)Et]+[BF4]−. In addition, they catalyze the hydrosilylation of benzaldehyde and acetophenone with (EtO)3SiH and show higher catalytic activity than the neutral hydrides 2-H/2′-H and 3-H. The mechanism for the formation of 2+-BF4 and the X-ray structure of 2+-BF4 are also described.

Nickel pincer complexes of the type [2,6-(R2PO)2C6H3]NiH (R = tBu, 1a; R = iPr, 1b; R = cPe, 1c) react with BH3·THF to produce borohydride complexes [2,6-(R2PO)2C6H3]Ni(η2-BH4) (2a–c), as confirmed by NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. The reactions are irreversible at room temperature but reversible at 60 °C. Compound 1a exchanges its hydrogen on the nickel with the borane hydrogen of 9-BBN or HBcat, but does not form any observable adduct. The less bulky hydride complexes 1b and 1c, however, yield nickel dihydridoborate complexes reversibly at room temperature when mixed with 9-BBN and HBcat. The dihydridoborate ligand in these complexes adopts an η2-coordination mode, as suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of 1a–c, reduction of CO2 leads to the methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, 1a, involves bulky substituents on the phosphorus donor atoms. Catalytic reactions involving 1b and 1c are less efficient because of the formation of dihydridoborate complexes as the dormant species as well as partial decomposition of the catalysts by the boranes.

A nickel-based catalytic system has been developed for [2 + 2 + 2] cyclotrimerization of various alkynes, especially ynoates. This catalytic system enables facile construction of substituted aromatic compounds in excellent yields with high regioselectivity.

Pincer complexes of the type [2,6-(R2PO)2C6H3]NiSC6H4Z (R = Ph and i-Pr; Z = p-OCH3, p-CH3, H, p-Cl, and p-CF3) have been synthesized from [2,6-(R2PO)2C6H3]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni–S bond length (approx. 2.20 Å) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with “[2,6-(Ph2PO)2C6H3]Ni” rather than “[2,6-(i-Pr2PO)2C6H3]Ni”, and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH2Br, CH3I, CCl4, and Ph3CCl have been examined and several mechanistic pathways have been explored.

The development of efficient methods for the synthesis of molecules with 1,4-difunctionalities has been a dire need of the synthetic community. In this work, intermolecular reductive coupling of ynoates and aldehydes (in the presence of a silane) has been accomplished for the first time using catalytic amounts of Ni(COD)2, an N-heterocyclic carbene ligand, and PPh3. High regioselectivity has been demonstrated for the multicomponent coupling reactions, and more than a dozen invaluable silyl-protected γ-hydroxy-α,β-enoates have been synthesized. This methodology provides a quick entry to many other 1,4-difunctional compounds and oxygen-containing five-membered rings. The intermediacy of metallacycles in the catalytic process has been established by deuterium-labeling experiments.

The mechanistic details of nickel-catalyzed reduction of CO2 with catecholborane (HBcat) have been studied by DFT calculations. The nickel pincer hydride complex ({2,6-C6H3(OPtBu2)2}NiH = [Ni]H) has been shown to catalyze the sequential reduction from CO2 to HCOOBcat, then to CH2O, and finally to CH3OBcat. Each process is accomplished by a two-step sequence at the nickel center: the insertion of a C═O bond into [Ni]H, followed by the reaction of the insertion product with HBcat. Calculations have predicted the difficulties of observing the possible intermediates such as [Ni]OCH2OBcat, [Ni]OBcat, and [Ni]OCH3, based on the low kinetic barriers and favorable thermodynamics for the decomposition of [Ni]OCH2OBcat, as well as the reactions of [Ni]OBcat and [Ni]OCH3 with HBcat. Compared to the uncatalyzed reactions of HBcat with CO2, HCOOBcat, and CH2O, the nickel hydride catalyst accelerates the Hδ− transfer by lowering the barriers by 30.1, 12.4, and 19.6 kcal/mol, respectively. In general, the catalytic role of the nickel hydride is similar to that of N-heterocyclic carbene (NHC) catalyst in the hydrosilylation of CO2. However, the Hδ− transfer mechanisms used by the two catalysts are completely different. The Hδ− transfer catalyzed by [Ni]H can be described as hydrogen being shuttled from HBcat to nickel center and then to the C═O bond, and the catalyst changes its integrity during catalysis. In contrast, the NHC catalyst simply exerts an electronic influence to activate either the silane or CO2, and the integrity of the catalyst remains intact throughout the catalytic cycle. The comparison between [Ni]H and Cp2Zr(H)Cl in the stoichiometric reduction of CO2 has suggested that ligand sterics and metal electronic properties play critical roles in controlling the outcome of the reaction. A bridging methylene diolate complex has been previously observed in the zirconium system, whereas the analogous [Ni]OCH2O[Ni] is not a viable intermediate, both kinetically and thermodynamically. Replacing HBcat with PhSiH3 in the nickel-catalyzed reduction of CO2 results in a high kinetic barrier for the reaction of [Ni]OOCH with PhSiH3. Switching silanes to HBcat in NHC-catalyzed reduction of CO2 generates a very stable NHC adduct of HCOOBcat, which makes the release of NHC less favorable.

Treatment of resorcinol-derived bis(phosphinite) ligands 1,3-(R2PO)2C6H4 (R = iPr and Ph) with Fe(PMe3)4 furnishes iron POCOP-pincer hydride complexes [2,6-(R2PO)2C6H3]Fe(H)(PMe3)2 (R = iPr, 1a; R = Ph, 1b) with two PMe3cis to each other. The isopropyl complex 1a undergoes ligand substitution upon mixing with CO to give [2,6-(iPr2PO)2C6H3]Fe(H)(PMe3)(CO). The kinetic product (2a) of this process contains a CO ligand trans to the hydride, whereas the thermodynamic product (2a′) has a CO ligand cis to the hydride. The displacement of PMe3 in 2a by CO takes place at an elevated temperature, resulting in the formation of [2,6-(iPr2PO)2C6H3]Fe(H)(CO)2 (3a). These new iron POCOP-pincer hydride complexes catalyze the hydrosilylation of aldehydes and ketones with different functional groups, and 1a is the most efficient catalyst for this process. Isotopic labeling experiments rule out the hydride ligand being directly involved in the reduction. The hydrosilylation reactions are more likely to proceed via the activation of silanes or carbonyl substrates after ligand (PMe3, or CO in the case of 3a) dissociation from the iron center.

Nickel hydride with a diphosphinite-based ligand catalyzes the highly efficient reduction of CO2 with catecholborane, and the hydrolysis of the resulting methoxyboryl species produces CH3OH in good yield. The mechanism involves a nickel formate, formaldehyde, and a nickel methoxide as different reduced stages for CO2. The reaction may also be catalyzed by an air-stable nickel formate.

The use of first-row transition metals for the catalytic reduction of carbonyl functionalities has become increasingly important in homogeneous catalysis. This Perspective examines the mechanistic aspects of these reduction reactions, with a focus on various interactions between metal complexes and substrates. Four different types of catalytic pathways, namely catalysis with dihydride (or dihydrogen) complexes, catalysis with monohydride complexes, metal–ligand bifunctional catalysis, and catalysis involving ionic mechanisms, are discussed with recent examples highlighted.

Nickel bis(phosphinite) pincer complexes [2,6-(R2PO)2C6H3]NiCl (R = Ph, 1a; R = Me, 1b; R = iPr, 1c; R = tBu, 1d) show catalytic activity in cross-coupling of aryl iodides and aryl thiols. The optimal catalytic conditions involve 1 mol % of 1a and 2 equiv of KOH (with respect to aryl thiols) in DMF at 80 °C and tolerate a variety of functional groups in the substrates. The potential intermediates in these catalytic reactions, such as nickel thiolate complexes [2,6-(Ph2PO)2C6H3]NiSAr (Ar = Ph, 2a; Ar = p-MeC6H4, 3a; Ar = p-MeOC6H4, 4a), have been synthesized and spectroscopically characterized. The reaction between 2a and PhI in DMF-d7 is slow enough to argue against 2a being directly involved in the C−S bond-forming step. NMR studies suggest that the pincer ligand framework in complexes 1a, 3a, and 4a is destroyed by KOH via the cleavage of P−O bonds to release Ph2POK, and further decomposition leads to Ph3P and other phosphorus-containing products. The cross-coupling reactions are more effectively catalyzed by Ni(COD)2/Ph2P(O)H. The structures of 1b, 2a, 4a, and [2,6-(Ph2PO)2C6H3]NiP(O)Ph2 (5a), which is relevant to the decomposition process, have been studied by X-ray crystallography.

Nickel PCP-pincer hydride complexes catalyze chemoselective hydrosilylation of C═O bonds of aldehydes and ketones in the presence of other functional groups. The mechanism involves C═O insertion into a nickel−hydrogen bond, followed by cleavage of the newly formed Ni−O bond with a silane.

Zirconocene−1-aza-1,3-diene complexes, [Me2Si(C5H4)2]Zr[N(Ar)CH═CHCH(Ph)] (Ar = Ph, 2a; Ar = p-MeOC6H4, 2b) and Cp2Zr[N(Ar)CH═CHCH(Ph)] (Ar = Ph, 3a; Ar = p-MeOC6H4, 3b), have been synthesized and characterized by NMR spectroscopy. X-ray crystal structure determinations of compounds 2a and 3a,b reveal folded five-membered-ring moieties for the zirconacycles. DFT calculations and variable-temperature NMR experiments for complex 2a establish a rapid ring-flipping process at room temperature, with the conformation bearing a pseudoequatorial Ph group more stable by 5.6 kcal/mol. Kinetic studies on ketone insertion into these zirconocene complexes show second-order reactions, and the insertion is more favorable in the presence of a [Me2Si] ansa bridge, a less electron-rich substituent on the nitrogen, and a more basic ketone. One of the insertion products, namely Cp2Zr[N(Ph)CH═CHCH(Ph)CPh2O] (5a), has also been characterized by X-ray crystallography.

Nickel POCOP-pincer hydride complexes [2,6-(R2PO)2C6H3]NiH (R = iPr, 4a; R = cPe = cyclopentyl, 4b) react with phenylacetylene to generate [2,6-(R2PO)2C6H3]NiC(Ph)CH2 (5a–b) as the major product and (E)-[2,6-(R2PO)2C6H3]NiCHCHPh (6a–b) as the minor product. The 2,1-insertion is more favorable than the 1,2-insertion and both pathways involve cis addition of Ni–H across the CC bond. Unlike the palladium case, alkynyl complexes [2,6-(R2PO)2C6H3]NiCCPh (7a–b) and H2 are not produced in the nickel system. The more bulky hydride complex [2,6-(tBu2PO)2C6H3]NiH (4c) shows no reactivity towards phenylacetylene. Catalytic hydrogenation of phenylacetylene with 4a–b takes place at an elevated temperature (70–100 °C) and proves to be heterogeneous. The structures of 5b, 6a, 7a and 7b have been studied by X-ray crystallography.

The use of first-row transition metals for the catalytic reduction of carbonyl functionalities has become increasingly important in homogeneous catalysis. This Perspective examines the mechanistic aspects of these reduction reactions, with a focus on various interactions between metal complexes and substrates. Four different types of catalytic pathways, namely catalysis with dihydride (or dihydrogen) complexes, catalysis with monohydride complexes, metal–ligand bifunctional catalysis, and catalysis involving ionic mechanisms, are discussed with recent examples highlighted.

Highly efficient catalytic reduction of CO2 with catecholborane has been developed by using bis(phosphinite) pincer ligated palladium thiolate complexes. Turnover frequencies up to 1780 h−1 have been achieved at room temperature under an atmospheric pressure of CO2. These thiolate complexes represent the most efficient homogeneous catalysts known to date for the reduction of CO2 to methanol under mild conditions.

A nickel-based catalytic system has been developed for [2 + 2 + 2] cyclotrimerization of various alkynes, especially ynoates. This catalytic system enables facile construction of substituted aromatic compounds in excellent yields with high regioselectivity.

A new bidentate secondary phosphine oxide (SPO) was synthesized from diphenyl ether via ortho-lithiation, phosphorylation with PhP(Cl)NEt2, and hydrolysis in an acidic medium. Nickel(0) species ligated with this new SPO was established as a more effective catalyst than Ni(0)–Ph2P(O)H for the cross-coupling of aryl iodides with aryl thiols.

Pincer complexes of the type [2,6-(R2PO)2C6H3]NiSC6H4Z (R = Ph and i-Pr; Z = p-OCH3, p-CH3, H, p-Cl, and p-CF3) have been synthesized from [2,6-(R2PO)2C6H3]NiCl and sodium arylthiolate. X-ray structure determinations of these thiolate complexes have shown a somewhat constant Ni–S bond length (approx. 2.20 Å) but an almost unpredictable orientation of the thiolate ligand. Equilibrium constants for various thiolate exchange (between a nickel thiolate complex and a free thiol, or between two different nickel thiolate complexes) reactions have been measured. Evidently, the thiolate ligand with an electron-withdrawing substituent prefers to bond with “[2,6-(Ph2PO)2C6H3]Ni” rather than “[2,6-(i-Pr2PO)2C6H3]Ni”, and bonds least favourably with hydrogen. The reactions of the thiolate complexes with halogenated compounds such as PhCH2Br, CH3I, CCl4, and Ph3CCl have been examined and several mechanistic pathways have been explored.

Palladium POCOP-pincer hydride complexes [2,6-(R2PO)2C6H3]PdH (R = tBu, 2a; R = iPr, 2b; R = cPe, 2c, cPe = cyclopentyl) have been synthesized from [2,6-(R2PO)2C6H3]PdCl (1a–c) and LiAlH4 or LiBEt3H. These hydride complexes react with phenylacetylene to afford H2, [2,6-(R2PO)2C6H3]PdCCPh (3a–c) and a small amount of styrene. When the R groups are isopropyl groups, a second palladium species is generated, and it has been identified as an alkenyl complex (E)-[2,6-(iPr2PO)2C6H3]PdCHCHPh (4b). Mechanistic studies have shown that decomposition of these palladium pincer complexes and related palladium methyl complexes [2,6-(R2PO)2C6H3]PdCH3 (5a–c) occurs at room temperature in the presence of H2 (1 atm or lower), resulting in the leaching of palladium particles. These particles have been shown to catalyze the hydrogenation of phenylacetylene and diphenylacetylene to their alkene and alkane products. A mechanism for the formation of palladium particles has been proposed. The structures of 1a, 1c, 2a, 2c, 3a, 4b and 5b have been studied by X-ray crystallography.

Mechanochemistry, more specifically high-speed ball milling, has garnered significant attention in several areas of chemistry, particularly for the synthesis of inorganic materials, cocrystals, organic compounds, discrete metal complexes and metal organic frameworks. This methodology is creating exciting research opportunities because, unlike traditional synthesis, a reaction carried out in a high-speed ball mill does not necessarily need a solvent, thus representing an environmentally friendly solution to the issue of solvent waste. This Chemistry Frontiers article delves into a unique area of ball milling that capitalizes on the solventless nature of the synthesis by using reaction vials, balls, foils, and pellets as both reaction medium and the catalyst. Several examples are highlighted, from nanoparticle synthesis and nanocatalysis to using transition metals in their metallic forms as catalysts. This article is aimed to show both the advantages and challenges present in the field, and to spark interest in further development of this research area.