Capturing HBCy2: Using N,O-Chelated Complexes of Rhodium(I) and Iridium(I) for Chemoselective Hydroboration

Coinage metal amido and thiolate SNS complexes: consequences of catalyst speciation in Cu(I)-catalysed carbonyl hydroboration

Three new IPr-Ag- and -Au-SNS amido and thiolate complexes were synthesized and compared to their previously reported Cu analogues as carbonyl hydroboration catalysts (IPr = bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Although these complexes showed no catalytic activity, treatment of the IPr-Ag-SNS amido complex with pinacolborane released the N-borylated ligand, S^MeN^BpinS^Me, (L1-Bpin). This finding led us to reinvestigate the IPr-Cu-SNS amido precatalyst, revealing that immediate loss of L1-Bpin converts our catalyst system to [CuH(IPr)]₂.

Solvent-free hydroboration of alkenes and alkynes catalyzed by rhodium-ruthenium nanoparticles on carbon nanotubes

A heterogeneous catalyst consisting of bimetallic rhodium-ruthenium particles immobilized on carbon nanotubes was used in the hydroboration reaction and proved highly effective for a variety of alkenes and alkynes. The reactions were carried out with low catalytic loadings (0.04 mol%), under solvent-free conditions, and at room temperature. In addition, to demonstrate its recyclability, the catalyst was recovered by a simple centrifugation process and reused over 5 consecutive cycles without losing any activity.

Synthesis and Chemistry of Dihydridoborate Group 7 Metal Complexes with Varied N,E-Chelated Ligands (E = O, NH, or S)

Several dihydridoborate group 7 metal complexes have been synthesized and their structural aspects have been described from various N,S-, N,N-, and N,O-chelated borate species, such as Na[(H₃B)mp] (mp = 2-mercaptopyridyl), Na[(H₃B)amt] (amt = 2-amino-5-mercapto-1,3,4-thiadiazolyl), Na[(H₃B)hp] (hp = 2-hydroxypyridyl), Na[(H₂B)bap] (bap = bis(2-aminopyridyl)), and Na[(H₂B)bdap] (bdap = bis(2,6-diaminopyridyl)). Room temperature photolysis of [M₂(CO)₁₀] (M = Mn or Re) with these borate species afforded dihydridoborate complexes [(CO)₃M(μ-H)₂BHL] 1-6 (1, M = Mn, L = mp; 2, M = Re, L = mp; 3, M = Mn, L = amt; 4, M = Mn, L = hp; 5, M = Mn, L = ap; 6, M = Mn, L = dap, ap = 2-aminopyridyl, dap = 2,6-diaminopyridyl). In complexes 1-3, the corresponding (H₂BHL) units are coordinated to the metal centers through the (κ³-H,H,S) mode. However, in complexes 4 and 5 (or 6), the connection is via (κ³-H,H,O) and (κ³-H,H,N) modes of coordination, respectively. Complexes 1 and 5 underwent hydroboration reactions with terminal alkynes that yielded trans-hydroborated species [Mn(CO)₃(μ-H)₂(NC₅H₄E)B(PhC═CH₂)] (7, E = S; 8, E = NH). Density functional theory (DFT) calculations have been carried out to investigate the electronic structures of these dihydridoborate species as well as the nature of bonding in them.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Cooperative B-H activation by Cp* based κ2-N,S-chelated Ru(II) and Mo(II) complexes (Cp* = η5-C5Me5)

The chemistry of the Cp* based κ²-N,S-chelated ruthenium complex, [Cp*RuPPh₃(κ²-N,S-(NC₇H₄S₂)], 1 with different boranes has been explored. The room temperature reaction of 1 with BH₃·THF and bulky boranes, such as MesBH₂ and H₂BAr^F, led to the formation of different dihydridoborate complexes, [{κ³-S,H,H-(NBH₂R)(S₂H₄C₇)}RuCp*], 2-4 (2: R = H, 3: R = Mes, and 4: R = Ar^F; Mes = 2,4,6-trimethylphenyl, and Ar^F = 3,5-bistrifluoromethyl-benzene). In contrast, the Cp* based κ²-N,S-chelated molybdenum complex, [Cp*Mo(CO)₂{κ²-N,S-(NC₇H₄S₂)}], 5, yielded the agostic borate species, [Cp*Mo(CO)₂{κ²-S,H-(NBH₂R) (NC₇H₄S₂)}], 6 and 7 (6: R = Mes and 7: R = Ar^F) at elevated temperatures.

Cooperative B-H and Si-H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes

Cooperative E-H (E = B, Si) bond activations employing κ²-N,S-chelated ruthenium borate species, [PPh₃{κ²-N,S-(NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(NC₇H₄S₂)₂}], (1) are established. Treatment of 1 with BH₃·SMe₂ yielded the six-membered ruthenaheterocycle [PPh₃{κ²-S,H-(BH₃NS₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (2) formed by a hemilabile ring opening of a Ru-N bond and capturing of a BH₃ unit coordinated in an "end-on" fashion. On the other hand, the bulky borane H₂BMes shows different reactivity with 1 that led to the formation of the two dihydroborate complexes [{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (3) and [PPh₃{κ³-S,H,H-(NBH₂Mes)(S₂C₇H₄)}Ru(κ²-N,S-C₇H₄NS₂)] (4), in which H₂BMes has been inserted into the Ru-N bond of the initial κ²-N,S-chelated ligand. In an attempt to directly activate hydrosilanes by 1, reactions were carried out with H₂SiPh₂ that yielded two isomeric five-membered ruthenium silyl complexes, namely [PPh₃{κ²-S,Si-(NSiPh₂)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a,b), and the hydridotrisilyl complex [Ru(H){κ²-S,Si-(SiPh₂NC₇H₄S₂}₃] (6). These complexes were generated by Si-H bond activation with the release of H₂ and the formation of N-Si and Ru-Si bonds. When the reaction of 1 was carried out in the presence of PhSiH_3, the reaction only produced the analogous complexes [PPh₃{κ²-S,Si-(NSiPhH)(S₂C₇H₄)}Ru{κ³-H,S,S'-H₂B(C₇H₄NS₂)₂}] (5a',b'). Density functional theory (DFT) calculations have been used to probe the bonding modes of boranes/silane with the ruthenium center.

Controlled partial transfer hydrogenation of quinolines by cobalt-amido cooperative catalysis

Catalytic hydrogenation or transfer hydrogenation of quinolines was thought to be a direct strategy to access dihydroquinolines. However, the challenge is to control the chemoselectivity and regioselectivity. Here we report an efficient partial transfer hydrogenation system operated by a cobalt-amido cooperative catalyst, which converts quinolines to 1,2-dihydroquinolines by the reaction with H₃N·BH₃ at room temperature. This methodology enables the large scale synthesis of many 1,2-dihydroquinolines with a broad range of functional groups. Mechanistic studies demonstrate that the reduction of quinoline is controlled precisely by cobalt-amido cooperation to operate dihydrogen transfer from H₃N·BH₃ to the N=C bond of the substrates.

Controlling the partial reduction of quinolines is challenging, given the competing overreduction to tetrahydroquinolines. Here, the authors report a cobalt-amido cooperative catalyst for the selective, partial transfer hydrogenation of quinolines to 1,2-dihydroquinolines with H₃N·BH₃ as reductant.

Catalytic Hydroboration of Aldehydes, Ketones, and Alkenes Using Potassium Carbonate: A Small Key to Big Transformation

An efficient transition-metal-free protocol for the hydroboration of aldehydes and ketones (reduction) was developed. The hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) under the K₂CO₃ catalyst has been studied. The reaction system is practical and reliable and proceeds under extremely mild and operationally simple conditions. No prior preparation of the complex metal catalyst was required, and hydroboration occurred stoichiometrically. Further, the chemoselective reduction of aldehydes over ketones was carried out. Moreover, we demonstrated the use of K₂CO₃ as an efficient catalyst for the hydroboration of alkenes. The operational simplicity, inexpensive and transition-metal-free catalyst, and the applicability to gram-scale synthesis strengthen its potential applications for hydroboration (reduction) at an industrial scale.

Dihydridoboranes: Selective Reagents for Hydroboration and Hydrodefluorination

The preparation of a new series of dihydridoboranes supported by N,N-chelating ligands, [R₂NCH₂CH₂NAr]^- (R = alkyl, Ar = aryl), is reported. These new boranes react selectively with carbonyls, imines, and a series of electron-deficient fluoroarenes. The reactivity is complementary to recognized reagents such as pinacolborane, catecholborane, NHC-BH₃, and borane (BH₃) itself. Selectivities are rationalized by invoking both open- and closed-chain forms of the reagents as part of equilibrium mixtures.

Cleavage of Si-H, B-H, and C-H Bonds by Metal-Ligand Cooperation

Metal-ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal-ligand cooperation has enabled unprecedented reactivities. Especially, metal-ligand cooperative H-H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This review summarizes the recent progress in the metal-ligand cooperative cleavages of Si-H, B-H, and C-H bonds and their catalytic applications.

Mercapto-benzothiazolyl based ruthenium(ii) borate complexes: synthesis and reactivity towards various phosphines

Ruthenium complexes featuring phosphinate and dual Ru⋯H–B interactions between Ru and B–H bonds of borate ligands supported by mercapto-benzothiazolyl heterocycles have been synthesized.

Silylene induced cooperative B–H bond activation and unprecedented aldehyde C–H bond splitting with amidinate ring expansion

Silylene mediated B–H and aldehyde C–H bond splitting were realized under ambient conditions.

Catalyst-free and solvent-free hydroboration of ketones

The hydroboration of a wide range of ketones with HBpin under catalyst-free and solvent-free conditions was reported in high yields.

Recent advances in the catalytic hydroboration of carbonyl compounds

The latest development in the catalytic hydroboration of CO groups is summarized in this review. Access to borate ester intermediates provides a pathway to convert them into the corresponding valuable functionalized alcohols.

M–H–BR3 and M–Br–BR3 interactions in rhodium and nickel complexes of an ambiphilic phosphine–thioether–borane ligand

Reaction of [Rh(μ-Cl)(CO)(TXPB)] (1; TXPB = 2,7-di-tert-butyl-5-diphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene) with NaBH4 yielded square planar [Rh(μ-H)(CO)(TXPB)] (2) in which the hydride ligand bridges between rhodium and the borane unit of TXPB. The Rh–H, Rh–B, and Rh–Cipso distances are short at 1.84(5), 2.456(6), and 2.568(5) Å, respectively, whereas the B–H bond, 1.59(6) Å, falls at the longer end of the usual range. Compound 2 is compared with the previously reported series of rhodium TXPB complexes: [RhX(CO)(TXPB)] {X = F (3), Cl (1), Br (4), I (5)}. Compound 4 in this series features the only crystallographically characterized example of an M–Br–BR3 interaction, and to expand this area, [NiBr(μ-Br)(TXPB)] (6) was prepared via the reaction of [NiBr2(dme)2] (dme = 1,2-dimethoxyethane) with TXPB. An X-ray crystal structure of light purple 6 revealed a square-planar geometry with a strong B–Br interaction {B–Br = 2.311(6) Å; ∑(C–B–C) = 344.5(7)°}. An 11B NMR chemical shift of 23 ppm was observed for 6, indicating that an appreciable B–Br interaction is maintained in solution. No signals were observed in the 31P{1H} NMR spectrum at room temperature, whereas a broadened 31P signal was observed at −20 °C, evolving into a sharp singlet at −67 °C. This behaviour suggests that at room temperature, square planar 6 exists in equilibrium with a paramagnetic tetrahedral isomer, present at a level below that detectable through Evans magnetic measurements.

Benz-amidinato calcium iodide catalyzed aldehyde and ketone hydroboration with unprecedented functional group tolerance

A benz-amidinato calcium compound, [PhC(NiPr)₂CaI] (1), catalyzed hydroboration of a wide range of aldehydes and ketones using pinacolborane (HBpin) at room temperature is reported. The catalyst shows functional group tolerance even towards OH and NH groups. The strategy was further extended to imines.

Theoretical investigations of the Ir-catalyzed direct borylation of B(3,6)–H of o-carborane: the actual catalyst, mechanism, and origin of regioselectivity

The Ir^III species is proved to be the actual catalyst. The electron-delocalized structure and the inductive effects of the carbon centers is account for the regioselectivity.

Highly efficient hydroboration of carbonyl compounds catalyzed by tris(methylcyclopentadienyl)lanthanide complexes

(MeCp)₃Ln complexes are reported as highly efficient catalysts in promoting hydroboration and a plausible stepwise mechanism is proposed.

Small bite-angle 2-phosphinophosphinine ligands enable rhodium-catalysed hydroboration of carbonyls

2-Phosphinophosphinine ligands generate Rh catalysts for the hydroboration of ketones and imines in contrast to standard phosphine ligands.

Direct Determination of Electron-Transfer Properties of Dicopper-Bound Reduced Dioxygen Species by a Cryo-Spectroelectrochemical Approach

Direct experimental determination of redox properties of superoxo (O2.- ) and peroxo (O22- ) embedded in dicopper complexes bearing an unsymmetrical binucleating ligand was achieved using cryo-electrochemistry and cryo-spectroelectrochemistry in dichloromethane. Cyclic voltammetry for dicopper(I) (1+ ) oxidation to a CuI CuII mixed-valent species (12+ ) under inert atmosphere at 193 K reveals slow heterogeneous electron-transfer kinetics, indicative of a large reorganization energy. Oxygenation of the dicuprous complex 1+ gives the bridged peroxo dicopper(II) species 3+ , which is reversibly oxidized to the superoxo complex 22+ at E0 =0.11 V (vs. SCE) with a small inner sphere electron-transfer reorganization energy, λi =0.54 eV, determined from variable temperature electrochemical impedance spectroscopy. The data suggest that the O2.- /O22- redox process occurs directly on the O2 -derived core.

Ytterbium-Catalyzed Hydroboration of Aldehydes and Ketones

The well-defined heavy rare-earth ytterbium iodide complex 1 (L₂YbI) has been successfully employed as an efficient catalyst for the hydroboration of a wide range of aldehydes and ketones with pinacolborane (HBpin) at room temperature. The protocol requires low catalyst loadings (0.1-0.5 mol %) and proceeds rapidly (>99% conversion in <10 min). Additionally, catalyst 1 shows a good functional group tolerance even toward the hydroxyl and amino moieties and displays chemoselective hydroboration of aldehydes over ketones under mild conditions.

Tris(cyclopentadienyl)lanthanide Complexes as Catalysts for Hydroboration Reaction toward Aldehydes and Ketones

It was found that homoleptic cyclopentadienyl lanthanide complexes Cp₃Ln (Ln = Y (1), Yb (2), Sm (3), Nd (4), La (5), Cp = cyclopentadienyl) can be employed as excellent catalysts for the hydroboration of various aldehydes and ketones toward pinacolborane. These robust lanthanide catalysts exhibited high reactivity with low catalyst loadings (0.01-1 mol %) under mild conditions and good functional group tolerability. These complexes also demonstrated uniquely carbonyl-selective hydroboration in the presence of alkenes and alkynes.

Rhenium-catalysed hydroboration of aldehydes and aldimines

The first examples of the catalysed hydroboration of aldehydes and aldimines using low- and high-valent rhenium complexes are reported.