A Well-Defined Anionic Dicopper(I) Monohydride Complex that Reacts like a Cluster.

Low-nuclearity copper hydrides are rare and few well-defined dicopper hydrides have been reported. Herein, we describe the first example of a structurally characterized anionic dicopper hydride complex. This complex does not display typical reactivity associated with low-nuclearity copper hydrides, such as alcoholysis or insertion reactions. Instead, its stoichiometric and catalytic reactivity is akin to that of copper hydride clusters. The distinct reactivity is ascribed to the robust dinuclear core that is bound tightly within the dinucleating ligand scaffold.

Metal hydride clusters have historically been studied to unravel their aesthetically pleasing molecular structures and interesting properties, especially toward hydrogen related applications. Central to this work is the hydride ligand, H¯, the smallest closed-shell spherical anion known. Two new developments in polyhydrido nanocluster chemistry include the determination of heretofore unknown hydride coordination modes and novel structural constructs, and conversion from the molecular entities to rhombus-shaped copper nanoparticles (CuNPs). These advances, together with hydrogen evolution and catalysis, have provided both experimentalists and theorists with a rich scientific directive to further explore. The isolation of hexameric [{(Ph3P)CuH}6] (Stryker reagent) could be regarded as the springboard for the recent emergence of polyhydrido copper cluster chemistry due to its utilization in a variety of organic chemical transformations. The stability of clusters of various nuclearity was improved through phosphine, pyridine, and carbene type ligands. Our focus lies with the isolation of novel copper (poly)hydride clusters using mostly the phosphor-1,1-dithiolato type ligands. We found such chalcogen-stabilized clusters to be exceptionally air and moisture stable over a wide range of nuclearities (Cu7 to Cu32). In this Account, we (i) report on state-of-the-art copper hydride cluster chemistry, especially with regards to the diverse and novel structural types generally, and newly discovered hydride coordination modes in particular, (ii) demonstrate the indispensable power of neutron diffraction for the unambiguous assignment and location of hydride ligand(s) within a cluster, and (iii) prove unique transformations that can occur not only between well characterized high nuclearity clusters, but also how such clusters can transform to uniquely shaped nanoparticles of several nanometers in diameter through copper hydride reduction. The increase in the number of low- to high-nuclearity hydride clusters allows for different means by which they can be classified. We chose a classification based on the coordination mode of hydride ligand within the cluster. This includes copper clusters associated with bridging (μ2-H) and capping (μ3-H) hydride modes, followed by an interstitial (μ4-H) hydride mode that was introduced for the first time into octa- and hepta-nuclear copper clusters stabilized by dichalcogen-type ligands. This breakthrough provided a means to explore higher nuclearity polyhydrido nanoclusters, which contain both capping (μ3-H) and interstitial (μ(4-6)-H) hydrides. The presence of bidentate ligands having mixed S/P dative sites led to air- and moisture-stable copper hydride nanoclusters. The formation of rhombus-shaped nanoparticles (CuNPs) from copper polyhydrides in the presence of excess borohydrides suggests the presence of metal hydrides as intermediates during the formation of nanoparticles.

Copper(I) hydride complexes are typically known to react with CO2 to form their corresponding copper formate counterparts. However, recently it has been observed that some multinuclear copper hydrides can feature the opposite reactivity and catalyze the dehydrogenation of formic acid. Here we report the use of a multinuclear PNNP copper hydride complex as an active (pre)catalyst for this reaction. Mechanistic investigations provide insights into the catalyst resting state and the rate-determining step and identify an off-cycle species that is responsible for the unexpected substrate inhibition in this reaction.

Influence of the structure of the diphosphine ligand on the copper fluoride and copper hydride complexes

Mechanistic studies of the copper-catalyzed asymmetric hydroboration of vinylarenes and internal alkenes are reported. Catalytic systems with both DTBM-SEGPHOS and SEGPHOS as the ligands have been investigated. With DTBM-SEGPHOS as the ligand, the resting state of the catalyst, which is also a catalytic intermediate, for hydroboration of 4-fluorostyrene is a phenethylcopper(I) complex ligated by the bisphosphine. This complex was fully characterized by NMR spectroscopy and X-ray crystallography. The turnover-limiting step in the catalytic cycle for the reaction of vinylarenes is the borylation of this phenethylcopper complex with pinacolborane (HBpin) to form the boronate ester product and a copper hydride. Experiments showed that the borylation occurs with retention of configuration at the benzylic position. β-Hydrogen elimination and insertion of the alkene to reform this phenethylcopper complex is reversible in the absence of HBpin but is irreversible during the catalytic process because reaction with HBpin is faster than β-hydrogen elimination of the phenethylcopper complex. Studies on the hydroboration of a representative internal alkene, trans-3-hexenyl 2,4,6-trichlorobenzoate, which undergoes enantio- and regioselective addition of HBpin catalyzed by DTBM-SEGPHOS, KOtBu, and CuCl, also was conducted, and these studies revealed that a DTBM-SEGPHOS-ligated copper(I) dihydridoborate complex is the resting state of the catalyst in this case. The turnover-limiting step in the catalytic cycle for hydroboration of the internal alkene is insertion of the alkene into a copper(I) hydride formed by reversible dissociation of HBpin from the copper dihydridoborate species. With SEGPHOS as the ligand, a dimeric copper hydride was observed as the dominant species during the hydroboration of 4-fluorostyrene, and this complex is not catalytically competent. DFT calculations provide a view into the origins of regio- and enantioselectivity of the catalytic process and indicate that the charge on the copper-bound carbon and delocalization of charge onto the aryl ring control the rate of the alkene insertion and the regioselectivity of the catalytic reactions of vinylarenes.

The reduction of CO2 into formic acid or its conjugate base, using dihydrogen, is an attractive process. While catalysts based on noble metals have shown high turnover numbers, the use of abundant first-row metals is underdeveloped. The key steps of the reaction are CO2 insertion into a metal hydride and regeneration of the metal hydride with H2, along with the concomitant production of formate. For the first step, copper is known as one of the most efficient metals, as shown by the numerous copper-catalysed carboxylation reactions, but this metal has difficulties activating H2 to achieve the second step. Here, we report a catalytic system involving a stable copper hydride that activates CO2, working in tandem with a Lewis pair that heterolytically splits H2. In this system, unprecedented turnover numbers for copper are obtained. Surprisingly, through a combination of stoichiometric and catalytic reactions, we show that classical Lewis pairs outperform frustrated Lewis pairs in this process. Due to its ready availability and low cost, copper is an attractive metal for the homogeneous reduction of CO2 to formate. However, although CO2 can readily insert into copper hydrides to produce metal-bound formate, subsequent regeneration of the catalytic species with H2 is more challenging. Here a dual strategy is used, whereby a copper hydride activates CO2 and a Lewis pair heterolytically splits H2, leading to dramatically improved performance.

Although a large number of copper hydride complexes and clusters have been reported, phosphine-stabilized copper hydrides remain comparatively rare. This is particularly noteworthy given the continuing interest in Stryker’s reagent [HCu(PPh3)]6 due to its use as a hydrogenation reagent. In this work, we report on the synthesis and full characterization of a novel copper hydride cluster, [Cu14H12(PtBu3)6Cl2]. The structure of this copper hydride was determined via SC-XRD. In addition, the reactivity of the hydrides and their position were investigated via a convolutional neural network, quantum chemical calculations, and NMR, and they are compared to the well-known, smaller Stryker’s reagent.

Copper hydrides are very useful in hydrogenation reactions. We report a stable Stryker-type copper hydride reagent protected by hemilabile phosphines: [Cu8H6(dppy)6](OTf)2 (Cu8-H, dppy = diphenylphosphino-2-pyridine). The metal core of this cluster has a bicapped octahedral configuration, and the copper-bound hydrides each triply bridges over a triangular face of the octahedron. This cluster is attractive due to its facile preparation and excellent stability under ambient conditions. The comparable activity and selectivity both in the stoichiometric and catalytic reactions make Cu8-H a promising alternative to Stryker's reagent.

C F bond activation attracts much attention because of the rapid growth of fluorocarbons in pharmaceuticals, agrochemicals, and new materials. Such transformations not only provide new routes to the fluorinated organics, but also offer a fundamental understanding for C F bond cleavage and functionalization. Previous experimental and theoretical studies demonstrated the effectiveness of late transition metals (group 8–10) such as Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, and Pt for their electron-rich nature and the relatively weak M F bonds, which are important for catalytic functionality. Recently, we explored the reactivity of group 11 metal complexes and found that gold(I) complexes have good catalytic reactivity toward hydrodefluorination (HDF) of fluoroarenes. Despite the tremendous progress made, most catalytic systems suffered from either low catalyst turnovers, limited substrate scope, or harsh reaction conditions. For example, gold(I) complexes are reactive only for the substrates with strong electron-withdrawing substituents, and their efficiency was greatly dependent on the electronicdonating additives. To address these issues, we continued to explore the more-active group 11 metals such as copper, as we were inspired by the success of gold. In this context, we report herein the first example of a copper(I)-catalyzed HDF of fluoroarenes, and it features high reactivity, good regioselectivity, and a broad substrate scope. Since Subramanian and Manzer reported the CuF2mediated fluorination of aromatics, copper-catalyzed C F bond formation has been developed by the groups of Hartwig, Lectka, and others. In contrast, for catalytic C F bond activation, copper has been rarely studied. Ribas and coworkers recently showed that a Cu/Cu redox cycle activated C X bonds (X=halogens) using triazamacrocyclic ligand, thus indicating an oxidative addition mechanism. Herein, we found that for HDF reactions, copper hydrides exhibited an unprecedented reactivity toward C F bonds. Moreover, density functional theory (DFT) calculations suggest that C F bond activation by copper hydrides proceeds through a mechanism involving nucleophilic attack, and represents an alternative strategy for copper activating C F bonds. At the outset of our study, we searched for optimal HDF reaction conditions for perfluoronitrobenzene (PFNB). We first applied the our previous catalytic gold system but replaced gold(I) with [Cu(MeCN)4PF6]. Screening silanes (see Table S1 in the Supporting Information) such as HSi(OMe)3, HSiEt3, PMHS, H2SiPh2, and HSiMe2Ph, showed that HSiMe2Ph was the best hydrogen source (conv. 40%). Solvent screening resulted THF providing the highest yield (60%; see Table S2 in the Supporting Information). We then examined ligand effects and the results are listed in Table 1. In

Copper(I) hydride complexes represent a promising entry into formic acid dehydrogenation catalysis. Herein we present the spontaneous decarboxylation of a μ1,3-formate-bridged dicopper(II) complex (1H) to a hexacopper(I) hydride cluster (2H) upon reduction. Isotopic labeling studies revealed that both the H- and CO2 originate from the bound μ1,3-formate in 1H, which represents a key step of the metal-mediated formic acid dehydrogenation. The full reaction equation for the conversion of 1H to 2H is established. The structure of 2H features two Cu3 triangles, each capped by a hydride ligand. Typical hydride reactivity of 2H is demonstrated by the addition of phenylacetylene, leading to the replacement of the hydrides by alkynide ligands -C≡CPh (3) while retaining the hexacopper(I) core. Temperature-dependent dynamic behavior in solution on the NMR time scale was observed for both 2H and 3, reflecting the rich structural landscape of the bis(pyrazolate)-bridged hexacopper(I) core (four isomers each for 2H and 3) predicted by DFT calculations.

Good as gold: gold hydrides were long considered unstable. In the past few years reports have appeared on not only the stable NHC gold(I) monohydride complex 1 (see structure; NHC=N-heterocyclic carbene) but also a dinuclear gold(I) hydride and most recently the gold(III) monohydride complex 2 with a C-N-C pincer ligand. One can expect new and important impulses for inorganic and organometallic chemistry and homogeneous gold catalysis.

Copper hydride compounds have attracted interest in diverse fields as base metallic material in place of rare and noble metals, which have widely been utilized in hydrogenation catalysts, hydrogen storage, and electrochemical materials. Since the first report on the synthesis of copper hydride complex [Cu6H6(PPh3)6] in 1971, copper hydride reagents have been utilized in a variety of organic transformation. While well‐characterized copper hydride complexes have been long limited to a few examples, recently several research groups have reported the synthesis of phosphine‐stabilized copper hydride complexes with various metal‐frameworks and unique reactivity. Here we review recent progress on the synthesis and structures of copper hydride complexes supported by phosphine ligands, including di‐, tri‐, and tetraphosphines, and also describe their reactivity with CO2.

Copper hydrides are highly active catalysts in hydrogenation reactions and reduction processes. Three Stryker-type copper hydride nanoclusters (NCs), [(TPP)CuH]6, [(TCP)CuH]6 and [(TOP)CuH]6 (TPP = triphenylphosphine, TCP = tricyclohexylphosphine and TOP = tri-n-octylphosphine), were synthesized in this study. Due to variations in the electron-donating properties of the phosphine ligands, the UV-visible absorption spectra of the three NCs exhibited notable distinctions. The influence of the phosphine ligands on the effectiveness of the NCs as hydride sources in hydrogenation processes, as well as on the applicability as homogeneous catalysts for reduction reactions, was systematically studied. Due to the highest electron-donating properties of the TOP ligand, [(TOP)CuH]6 was found to exhibit superior performance in both hydrogenation reactions and catalytic reduction reactions. Moreover, these hydrophobic NCs worked well as heterogeneous catalysts in the reduction of 4-nitrophenol.

The reactivity of homoleptic α-metalated dimethylbenzylamine lanthanide complexes (α-Ln(DMBA)3; Ln = La, Y; DMBA = α-deprotonated dimethylbenzylamine) was probed through a series of stoichiometric insertion and catalytic hydrophosphination reactions. Both rare-earth-metal species inserted 3 equiv of various carbodiimides to form the corresponding homoleptic amidinates. α-La(DMBA)3 was also found to be a useful precatalyst for the room-temperature hydrophosphination of heterocumulenes to form phosphaguanidines, phosphaureas, and phosphathioureas in moderate to excellent isolated yields. Furthermore, through a series of stepwise stoichiometric protonation and insertion reactions, a plausible mechanism for the hydrophosphination catalysis was investigated.

Chemoselective hydrogenation of α,β-unsaturated ketones to allylic alcohols, catalyzed by a mononuclear ruthenium complex containing trans P nBu 3 and PPh 3 ligands