Acceptor Pincer Chemistry of Osmium: Catalytic Alkane Dehydrogenation by (CF3PCP)Os(cod)(H)

Ruthenium-Catalyzed Dehydrogenation Through an Intermolecular Hydrogen Atom Transfer Mechanism

The direct dehydrogenation of alkanes is among the most efficient ways to access valuable alkene products. Although several catalysts have been designed to promote this transformation, they have unfortunately found limited applications in fine chemical synthesis. Here, we report a conceptually novel strategy for the catalytic, intermolecular dehydrogenation of alkanes using a ruthenium catalyst. The combination of a redox-active ligand and a sterically hindered aryl radical intermediate has unleashed this novel strategy. Importantly, mechanistic investigations have been performed to provide a conceptual framework for the further development of this new catalytic dehydrogenation system.

Cyclometalated Osmium Compounds and beyond: Synthesis, Properties, Applications

The synthesis of cyclometalated osmium complexes is usually more complicated than of other transition metals such as Ni, Pd, Pt, Rh, where cyclometalation reactions readily occur via direct activation of C–H bonds. It differs also from their ruthenium analogs. Cyclometalation for osmium usually occurs under more severe conditions, in polar solvents, using specific precursors, stronger acids, or bases. Such requirements expand reaction mechanisms to electrophilic activation, transmetalation, and oxidative addition, often involving C–H bond activations. Osmacycles exhibit specific applications in homogeneous catalysis, photophysics, bioelectrocatalysis and are studied as anticancer agents. This review describes major synthetic pathways to osmacycles and related compounds and discusses their practical applications.

Assembly of a Dihydrideborate and Two Aryl Nitriles to Form a C,N,N′-Pincer Ligand Coordinated to Osmium

The C,N,N′-donor aryl-diimineborate pincer ligand of the complexes OsH₂{κ³-C,N,N-[C₆H₃RCH=NB(cat)N=CHC₆H₄R]}(PⁱPr₃)₂ (R = H, Me) has been generated in a one-pot procedure, by the reaction of the hexahydride OsH₆(PⁱPr₃)₂ with catecholborane (catBH) and two molecules of the corresponding aryl nitrile. The osmium–pincer bonding situation has been analyzed by means of atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analysis coupled with the natural orbitals for chemical valence (EDA-NOCV) methods. According to the results, the complexes exhibit a rather strong electron-sharing Os–C bond, two weaker donor–acceptor N–Os bonds, and two π-back-donations from the transition metal to vacant π* orbitals of the formed metallacycles. In addition, spectroscopic findings and DFT calculations reveal that the donor units of the pincer are incorporated in a sequential manner. First, the central Os–N bond is formed, by the reaction of the dihydrideborate ligand of the intermediate OsH₃{κ²-H,H-(H₂Bcat)}(PⁱPr₃)₂ with one of the aryl nitriles. The subsequent oxidative addition of the o-C–H bond of the aryl substituent of the resulting κ¹-N-(N-boryl-arylaldimine) affords the Os–C bond. Finally, the second Os–N bond is generated from a hydride, an ortho-metalated N-boryl-arylaldimine, and the second aryl nitrile.

Synthesis and coordination chemistry of new asymmetric donor/acceptor pincer ligands, 1,3-C6H4(CH2PtBu(Rf))2 (Rf = CF3, C2F5)

Syntheses of new asymmetric pincer precursors 1,3-C₆H₄{CH₂P(^tBu,X)}₂ (^tBu,XPCPH; X = Cl, SiMe₃, OPh) and a new class of hybrid donor/acceptor pincer ligands 1,3-C₆H₄{CH₂P(^tBu,R_f)}₂ (^tBu,RfPCPH; R_f = CF₃, C₂F₅) are reported.

Challenges and opportunities for alkane functionalisation using molecular catalysts

The conversion of vast low-value saturated hydrocarbons into valuable chemicals is of great interest.

The conversion of vast low-value saturated hydrocarbons into valuable chemicals is of great interest. Thanks to the progression of organometallic and coordination chemistry, transition metal catalysed C sp³–H bond functionalisation has now become a powerful tool for alkane transformations. Specifically, methods for alkane functionalisation include radical initiated C–H functionalisation, carbene/nitrene insertion, and transition metal catalysed C–H bond activation. This perspective provides a systematic and concise overview of each protocol, highlighting the factors that govern regioselectivity in these reactions. The challenges of the existing catalytic tactics and future directions for catalyst development in this field will be presented.

Rapid reversible borane to boryl hydride exchange by metal shuttling on the carborane cluster surface

In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond acting as a supporting hemilabile ligand in exohedrally metalated three-dimensional carborane clusters. The (POBOP)Ru(Cl)(PPh3) pincer complex (POBOP = 1,7-OP(i-Pr)2-m-2-carboranyl) features extreme distortion of the two-center-two-electron Ru-B bond due to the presence of a strong three-center-two-electron B-H···Ru vicinal interaction. Replacement of the chloride ligand with a hydride afforded the (POBOP)Ru(H)(PPh3) pincer complex, which possesses B-Ru, B-H···Ru, and Ru-H bonds. This complex was found to exhibit a rapid exchange between hydrogen atoms of the borane and the terminal hydride through metal center shuttling between two boron atoms of the carborane cage. This exchange process, which involves sequential cleavage and formation of strong covalent metal-boron and metal-hydrogen bonds, is unexpectedly facile at temperatures above -50 °C corresponding to an activation barrier of 12.2 kcal mol-1. Theoretical calculations suggested two equally probable pathways for the exchange process through formally Ru(0) or Ru(iv) intermediates, respectively. The presence of this hemilabile vicinal B-H···Ru interaction in (POBOP)Ru(H)(PPh3) was found to stabilize a latent coordination site at the metal center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and air at 170 °C.

A new and selective cycle for dehydrogenation of linear and cyclic alkanes under mild conditions using a base metal

Selectively converting linear alkanes to α-olefins under mild conditions is a highly desirable transformation given the abundance of alkanes as well as the use of olefins as building blocks in the chemical community. Until now, this reaction has been primarily the remit of noble-metal catalysts, despite extensive work showing that base-metal alkylidenes can mediate the reaction in a stoichiometric fashion. Here, we show how the presence of a hydrogen acceptor, such as the phosphorus ylide, when combined with the alkylidene complex (PNP)Ti=CH^tBu(CH₃) (PNP=N[2-P(CHMe₂)₂-4-methylphenyl]₂^-), catalyses the dehydrogenation of cycloalkanes to cyclic alkenes, and linear alkanes with chain lengths of C₄ to C₈ to terminal olefins under mild conditions. This Article represents the first example of a homogeneous and selective alkane dehydrogenation reaction using a base-metal titanium catalyst. We also propose a unique mechanism for the transfer dehydrogenation of hydrocarbons to olefins and discuss a complete cycle based on a combined experimental and computational study.

Large-Scale Selective Functionalization of Alkanes

Great progress has been made in the past several decades concerning C-H bond functionalization. But despite many significant advances, a commercially viable large-scale process for selective alkane functionalization remains an unreached goal. Such conversions will require highly active, selective, and long-lived catalysts. In addition, essentially complete atom-economy will be required. Thus, any reagents used in transforming the alkanes must be almost free (e.g., O₂, H₂O, N₂), or they should be incorporated into the desired large-scale product. Any side-products should be completely benign or have value as fuels (e.g., H₂ or other alkanes). Progress and promising leads toward the development of such systems involving primarily molecular transition metal catalysts are described.

Ruthenium and osmium complexes of dihydroperimidine-based N-heterocyclic carbene pincer ligands

The reactions of N,N'-bis(phosphinomethyl)dihydroperimidine pro-ligands H2C(NCH2PR2)2C10H6 (R = Cy 1a, R = Ph 1b) with [RuCl2(PPh3)3] give markedly different products. Chelate-assisted double C-H activation in the former affords the perimidinylidene-based N-heterocyclic carbene (per-NHC) pincer complex [RuCl2(OC4H8){κ(3)-P,C,P'-C(NCH2PCy2)2C10H6}] (2), while the latter reaction provides the asymmetric PNP-coordinated complex [RuCl2(PPh3){κ(3)-P,N,P'-CH2(NCH2PPh2)2C10H6}] (3), in which no C-H activation has occurred. Subsequent reactions of the per-NHC complex 2 with carbon monoxide and mesityl isocyanide readily displaced the labile THF ligand to afford the complexes [RuCl2(CA){κ(3)-P,C,P'-C(NCH2PCy2)2C10H6}] (A = O 4, A = NC6H2Me35). Double C-H activation of 1a and 1b was significantly more facile on reaction with [OsCl2(PPh3)3], providing the per-NHC complexes [OsHCl(PPh3){κ(3)-P,C,P'-C(NCH2PR2)2C10H6}] (R = Cy 7a, R = Ph 7b, respectively), each as two isomers. The reactions of 1b with [Ru2(μ-Cl)2Cl2(η-C6H3Me3)2] or [AuCl(THT)] (THT = tetrahydrothiophene) provide the bimetallic complexes [Ru2{μ-H2C(NCH2PPh2)2C10H6}Cl4(η-C6H3Me3)2] (8) and [Au2{μ-H2C(NCH2PPh2)2C10H6}Cl2] (9) without C-H activation occurring.

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product

We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as (iPr4)PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer-Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C-H addition to (pincer)Ir to give an allyl intermediate as was previously reported for ((tBu4)PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.