The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species.

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the -selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(μ-Cl)(DPEPhos)] and [Rh(DPEPhos)(MeOH)] with benzotriazole and allene, giving coordination, or coupling of the substrates.

How solvents affect the stability of cationic Rh(I) diphosphine complexes: a case study of MeCN coordination.

Cationic rhodium(I) diphosphine complexes, referred to as Schrock-Osborn catalysts, are privileged homogeneous catalysts with a wide range of catalytic applications. The coordination of solvent molecules can have a significant influence on reaction mechanisms and kinetic scenarios. Although solvent binding is well documented for these rhodium species, comparative quantifications for structurally related systems are not available to date.

Synthesis, Coordination Chemistry, and Mechanistic Studies of P,N-Type Phosphaalkene-Based Rh(I) Complexes.

The synthesis of P,N-phosphaalkene ligands, py-CH═PMes* (, py = 2-pyridyl, Mes* = 2,4,6-Bu-CH) and the novel quin-CH═PMes* (, quin = 2-quinolinyl) is described. The reaction with [Rh(μ-Cl)cod] produces Rh(I) bis(phosphaalkene) chlorido complexes and with distorted trigonal bipyramidal coordination environments. Complexes and show a pronounced metal-to-ligand charge transfer (MLCT) from Rh into the ligand P═C π* orbitals.

Iridium(III) bis(thiophosphinite) pincer complexes: synthesis, ligand activation and applications in catalysis.

Iridium(III) bis(thiophosphinite) complexes of the type [(PSCSP)Ir(H)(Cl)(py)] (PSCSP = κ-(2,6-SPR)CH) (R = Bu, iPr, Ph) can be prepared from the ligand precursors 1,3-(SPR)CH by C-H activation at Ir using [Ir(COE)Cl] or [Ir(COD)Cl]. Optimisation of the protocol for complexation showed that direct cyclometallation in the absence or presence of pyridine, as well as C-H activation in the presence of H are viable options that, depending on the phosphine substituent furnish the five-coordinate Ir(III) hydride chloride complexes 2-R or the base stabilised species 3-R in good yields. In case of the PSCSP ligand, P-S activation results in the formation of a thiophosphine stabilised Ir(III) hydride complex [(PSCSP)Ir(H)(Cl)(PPhSH)] (4).

Catalyst Deactivation During Rhodium Complex-Catalyzed Propargylic C-H Activation.

Detailed mechanistic investigations on our previously reported synthesis of branched allylic esters by the rhodium complex-catalyzed propargylic C-H activation have been carried out. Based on initial mechanistic studies, we present herein more detailed investigations of the reaction mechanism. For this, various analytical (NMR, X-ray crystal structure analysis, Raman) and kinetic methods were used to characterize the formation of intermediates under the reaction conditions.

Two precatalysts for application in propargylic CH activation.

The complexes {bis[(2-diphenylphosphanyl)phenyl] ether-κP,P'}(η-norbornadiene)rhodium(I) tetrafluoridoborate, [Rh(CH)(CHOP)]BF, and {bis[(2-diphenylphosphanyl)phenyl] ether-κP,P'}[η-(Z,Z)-cycloocta-1,5-diene]rhodium(I) tetrafluoridoborate dichloromethane monosolvate, [Rh(CH)(CHOP)]BF·CHCl, are applied as precatalysts in redox-neutral atomic-economic propargylic CH activation [Lumbroso et al. (2013). Angew.

Insight into the Activation of In Situ Generated Chiral Rh Catalysts and Their Application in Cyclotrimerizations.

We report a detailed study concerning the efficient generation of highly active chiral rhodium complexes of the general structure [Rh(diphosphine)(solvent) ] as well as their exemplary successful utilization as catalysts for cyclotrimerizations. Such solvent complexes could likewise be prepared from novel ammonia complexes of the type [Rh(diphosphine)(NH ) ] . A valuable, feasible approach to generate novel chiral Rh complexes was found by in situ generation from Wilkinson's catalyst [RhCl(PPh ) ] with chiral P,N ligands.

Unravelling the Mechanism of Basic Aqueous Methanol Dehydrogenation Catalyzed by Ru-PNP Pincer Complexes.

Ruthenium PNP complex 1a (RuH(CO)Cl(HN(CHPi-Pr))) represents a state-of-the-art catalyst for low-temperature (<100 °C) aqueous methanol dehydrogenation to H and CO. Herein, we describe an investigation that combines experiment, spectroscopy, and theory to provide a mechanistic rationale for this process. During catalysis, the presence of two anionic resting states was revealed, Ru-dihydride (3) and Ru-monohydride (4) that are deprotonated at nitrogen in the pincer ligand backbone.

NNP-Type Pincer Imidazolylphosphine Ruthenium Complexes: Efficient Base-Free Hydrogenation of Aromatic and Aliphatic Nitriles under Mild Conditions.

A series of seven novel N(Im)N(H)P-type pincer imidazolylphosphine ruthenium complexes has been synthesized and fully characterized. The use of hydrogenation of benzonitrile as a benchmark test identified [RuHCl(CO)(N(Im)N(H) P(tBu))] as the most active catalyst. With its stable Ru-BH4 analogue, in which chloride is replaced by BH4, a broad range of (hetero)aromatic and aliphatic nitriles, including industrially interesting adiponitrile, has been hydrogenated under mild and base-free conditions.

Two precatalysts for application in asymmetric homogeneous hydrogenation.

The title compounds, [(1R,1'R,2R,2'R)-2,2'-bis(diphenylphosphanyl)-1,1'-dicyclopentane](η(4)-norbornadiene)rhodium(I) tetrafluoridoborate, [Rh(C34H36P2)(C7H8)]BF4, (I), and [(1R,1'R,2R,2'R)-2,2'-bis(diphenylphosphanyl)-1,1'-dicyclopentane][η(4)-(Z,Z)-cycloocta-1,5-diene]rhodium(I) tetrafluoridoborate dichloromethane monosolvate, [Rh(C34H36P2)(C8H12)]BF4·CH2Cl2, (II), are applied as precatalysts in asymmetric homogeneous hydrogenation, e.g. in the reduction of dehydroamino acids, affording excellent enantiomeric excesses [Zhu, Cao, Jiang & Zhang (1997).

New pentacoordinated rhodium species as unexpected products during the in situ generation of dimeric diphosphine-rhodium neutral catalysts.

Dimeric rhodium complexes of the type [Rh(PP)(μ2 -Cl)]2 (PP=diphosphine) are often used as precatalysts and are generated "in situ" from the corresponding diolefin complexes by exchange of the diene with the desired diphosphine. Herein, we report that the "in situ" procedure also leads to unexpected monomeric pentacoordinated neutral complexes of the type [RhCl(PP)(diolefin)], for the first time herein characterized by NMR spectroscopy and X-ray crystallography for the ligands 1,4-bis(diphenylphosphino)propane (DPPP), 1,4-bis(diphenylphosphino)butane (DPPB), and 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP). The pentacoordinated complexes are in equilibrium with the dimeric target compound [Rh(PP)(μ2 -Cl)]2 .

Mechanistic investigations of the rhodium catalyzed propargylic CH activation.

Previously we reported the redox-neutral atom economic rhodium catalyzed coupling of terminal alkynes with carboxylic acids using the DPEphos ligand. We herein present a thorough mechanistic investigation applying various spectroscopic and spectrometric methods (NMR, in situ-IR, ESI-MS) in combination with DFT calculations. Our findings show that in contrast to the originally proposed mechanism, the catalytic cycle involves an intramolecular protonation and not an oxidative insertion of rhodium in the OH bond of the carboxylic acid.

Selective hydrogen production from methanol with a defined iron pincer catalyst under mild conditions.

Molecularly well-defined iron pincer complexes promote the aqueous-phase reforming of methanol to carbon dioxide and hydrogen, which is of interest in the context of a methanol and hydrogen economy. For the first time, the use of earth-abundant iron complexes under mild conditions for efficient hydrogen generation from alcohols is demonstrated.

Development of an improved rhodium catalyst for z-selective anti-markovnikov addition of carboxylic acids to terminal alkynes.

To develop more active catalysts for the rhodium-catalyzed addition of carboxylic acids to terminal alkynes furnishing anti-Markovnikov Z enol esters, a thorough study of the rhodium complexes involved was performed. A number of rhodium complexes were characterized by NMR, ESI-MS, and X-ray analysis and applied as catalysts for the title reaction. The systematic investigations revealed that the presence of chloride ions decreased the catalyst activity.

Low-temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide.

Hydrogen produced from renewable resources is a promising potential source of clean energy. With the help of low-temperature proton-exchange membrane fuel cells, molecular hydrogen can be converted efficiently to produce electricity. The implementation of sustainable hydrogen production and subsequent hydrogen conversion to energy is called "hydrogen economy".

Investigations into the formation and stability of cationic rhodium diphosphane η6-arene complexes.

Rhodium-η(6)-arene complexes can be generated in the presence of arenes following the hydrogenation of the diolefin in rhodium catalyst precursors of the type [Rh(PP*)(diolefin)]X (PP* = chelating diphosphane, X = noncoordinating anion). In this paper we report the characterization of such arene complexes with the ligands DuPhos, dipamp, dppe, Tangphos, dppf, and diop by means of NMR spectroscopy ((31)P, (103)Rh) and X-ray analysis. A procedure that follows the approach to equilibrium as a function of time monitored by using an UV/Vis diode array was used to determine 20 stability constants.

Molecular vibration spectroscopy studies on novel trinuclear rhodium-7-hydride complexes of the general type {[Rh(PP*)X]3(μ(2-X)3(μ3-X)}(BF4)2 (X = H, D).

Novel trinuclear rhodium-hydride complexes with diphosphine ligands Tangphos, t-Bu-BisP*, and Me-DuPHOS which contain bridging μ(2)- and μ(3)-hydrides as well as terminal hydrides in one molecule have been reported recently. In this work, these different rhodium-hydride bonds are characterized by Raman spectroscopy and the results are compared with those obtained by means of the more commonly applied IR spectroscopy. Density functional theory (DFT) calculations have been carried out to support the experimental findings.

(+)-Chlorido[(1,2,3,4-η;κP)-2'-diphenyl-phosphanyl-2-diphenyl-phosphoryl-1,1'-binaphth-yl]rhodium(I) methanol monosolvate.

In the title complex, [RhCl(C(44)H(32)OP(2))]·CH(3)OH, the Rh(I) ion is coordinated by a naphthyl group of a partially oxidized 2,2'-bis-(diphenyl-phosphan-yl)-1,1'-binaphthyl (BINAP) ligand in a η(4) mode, one P atom of the diphenyl-phosphanyl group and one Cl atom. The P=O group does not inter-act with the Rh(I) ion but accepts an O-H⋯O hydrogen bond from the methanol solvent mol-ecule.

Unravelling the reaction path of rhodium-MonoPhos-catalysed olefin hydrogenation.

The mechanism of the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate (mac) catalysed by [Rh(MonoPhos)(2)(nbd)]SbF(6) (MonoPhos: 3,5-dioxa-4-phosphacyclohepta[2,1-a:3,4-a']dinaphthalen-4-yl)dimethylamine) was elucidated by using (1)H, (31)P and (103)Rh NMR spectroscopy and ESI-MS. The use of nbd allows one to obtain in pure form the rhodium complex that contains two units of the ligand. In contrast to the analogous complexes that contain cis,cis-1,5-cyclooctadiene (cod), this complex shows well-resolved NMR spectroscopic signals.

Trinuclear rhodium hydride complexes.

Three novel trinuclear rhodium hydride complexes of the type {[Rh(PP*)H](3)(μ(2)-H)(3)(μ(3)-H)}[BF(4)](2) containing diphosphines Tangphos, t-Bu-BisP* and Me-DuPHOS have been synthesised. The new compounds are very stable. Their structures have been characterized by X-ray analysis in the solid state and by NMR-spectroscopic investigations in solution.

Coordination chemistry of new selective ethylene trimerisation ligand Ph2PN(iPr)P(Ph)NH(R) (R = iPr, Et) and tests in catalysis.

The synthesis of [Ph(2)PN((i)Pr)P(Ph)NH(R)] (R = (i)Pr, Et) (1, 2) is described and the structure of 2 has been determined by single-crystal X-ray analysis. Compound 1 readily reacts with chromium(0), nickel(0), nickel(II), palladium(II), platinum(II) and iron(II) complexes to give four-membered rings (3-10) via P,P' coordination. The molecular structures of [Cr(CO)(4){Ph(2)PN((i)Pr)P(Ph)NH(R)-P,P'}] (R = (i)Pr, Et) (3, 4), [Cr(CO)(3)(NCCH(3)){Ph(2)PN((i)Pr)P(Ph)NH((i)Pr)-P,P'}] (5), [Ni{Ph(2)PN((i)Pr)P(Ph)NH((i)Pr)-P,P'}(2)] (6), cis-[MX(2){Ph(2)PN((i)Pr)P(Ph)NH((i)Pr)-P,P'}] (M = Ni, Pd, Pt; X = Cl or Br) (7, 8, 9) and trans-[Fe(NCCH(3))(2){Ph(2)PN((i)Pr)P(Ph)NH((i)Pr)-P,P'}(2)](BF(4))(2) (10) have been determined by X-ray diffraction.

(+)-{1,2-Bis[(2R,5R)-2,5-diethyl-phospho-lan-1-yl]ethane-κP,P'}(η-cyclo-octa-1,5-diene)rhodium(I) tetra-fluoridoborate.

The title compound, [Rh(C(8)H(12))(C(18)H(36)P(2))]BF(4), exhibits a rhodium(I) complex cation with a bidentate bis-phosphine ligand and a bidentate η(2),η(2)-coordinated cyclo-octa-1,5-diene ligand. The ligands form a slightly distorted square-planar coordination environment for the Rh(I) atom. An intra-molecular P-Rh-P bite angle of 83.

(+)-{1,2-Bis[(2R,5R)-2,5-dimethyl-phospho-lan-1-yl]ethane-κP,P'}(η-cyclo-octa-1,5-diene)rhodium(I) tetra-fluorido-borate.

The title compound, [Rh(C(8)H(12))(C(14)H(28)P(2))]BF(4), exhibits a rhodium(I) complex cation with a bidentate bis-phosphine ligand and a bidentate η(2),η(2)-coordinated cyclo-octa-1,5-diene. Together the ligands create a slightly distorted square-planar cordination environment for the Rh(I) atom. There are three mol-ecules in the asymmetric unit and intra-molecular P-Rh-P bite angles of 82.

Trinuclear rhodium complexes and their relevance for asymmetric hydrogenation.

Various trinuclear rhodium complexes of the type [Rh(3)(PP)(3)(mu(3)-OH)(x)(mu(3)-OMe)(2-x)]BF(4) (where PP = Me-DuPhos, dipamp, dppp, dppe; different ligands and mu-bridging anions) are presented, which are formed upon addition of bases such as NEt(3) to solvate complexes [Rh(PP)(solvent)(2)]BF(4). They were extensively characterized by X-ray diffraction and NMR spectroscopy ((103)Rh, (31)P, (13)C, (1)H). Their in situ formation resulting from basic additives (NEt(3)) or basic prochiral olefins (without addition of another base) can cause deactivation of the asymmetric hydrogenation.

The major/minor concept: dependence of the selectivity of homogeneously catalyzed reactions on reactivity ratio and concentration ratio of the intermediates.

The homogeneously catalyzed asymmetric hydrogenation of prochiral olefins with cationic Rh(I) complexes is one of the best-understood selection processes. For some of the catalyst/substrate complexes, experimental proof points out the validation of the major/minor principle; the concentration-deficient minor substrate complex, which has very high reactivity, yields the excess enantiomer. As exemplified by the reaction system of [Rh(dipamp)(MeOH)2]+/methyl (Z)-alpha-acetamidocinnamate (dipamp=1,2-bis((o-methoxyphenyl)phenylphosphino)ethane), all six of the characteristic reaction rate constants have been previously identified.