Supramolecular Triruthenium Cluster-Based Benzene Hydrogenation Catalysis: Fact or Fiction?

Nitrogen–nitrogen-functionalized N-heterocyclic carbene ruthenium(ii) complexes realized efficient CO2 hydrogenation to formate

Three new Ru–CNN complexes are synthesized for the hydrogenation of CO2 to formate. The Ru–CNN complex exhibits a long lifetime of over 400 h at 170 °C with a high TON of 6.5 × 105.

MOF-Mediated Synthesis of Supported Fe-Doped Pd Nanoparticles under Mild Conditions for Magnetically Recoverable Catalysis*

Metal-organic framework (MOF)-driven synthesis is considered as a promising alternative for the development of new catalytic materials with well-designed active sites. This synthetic approach is used here to gradually transform a new bimetallic MOF, with Pd and Fe as the metal components, by the in situ generation of aniline under mild conditions. This methodology results in a compositionally homogeneous nanocomposite formed by Fe-doped Pd nanoparticles that, in turn, are supported on iron oxide-doped carbon. The nanocomposite has been fully characterized by several techniques such as IR and Raman spectroscopy, TEM, XPS, and XAS. The performance of this nanocomposite as an heterogeneous catalyst for hydrogenation of nitroarenes and nitrobenzene coupling with benzaldehyde has been evaluated, proving it to be an efficient and reusable catalyst.

Copper Metal-Organic Framework Surface Catalysis: Catalyst Poisoning, IR Spectroscopic, and Kinetic Evidence Addressing the Nature and Number of the Catalytically Active Sites En Route to Improved Applications

The metal-organic framework (MOF) H₃[(Cu₄Cl)₃-(BTTri)₈, H₃BTTri = 1,3,5-tris(¹H-1,2,3-triazol-5-yl)benzene] (CuBTTri) is a precatalyst for biomedically relevant nitric oxide (NO) release from S-nitrosoglutathione (GSNO). The questions of the number and nature of the catalytically most active, kinetically dominant sites are addressed. Also addressed is whether or not the well-defined structural geometry of MOFs (as solid-state analogues of molecular compounds) can be used to generate specific, testable hypotheses about, for example, if intrapore vs exterior surface metal sites are more catalytically active. Studies of the initial catalytic rate vs CuBTTri particle external surface area to interior volume ratio show that intrapore copper sites are inactive within the experimental error (≤1.7 × 10^-5% of the observed catalytic activity)-restated, the traditional MOF intrapore metal site catalysis hypothesis is disproven for the current system. All observed catalysis occurs at exterior surface Cu sites, within the experimental error. Fourier transform infrared (FT-IR) analysis of CN^--poisoned CuBTTri reveals just two detectable Cu sites at a ca. ≥0.5% detection limit, those that bind three or one CN^- ("Cu(CN)₃" and "CuCN"), corresponding to the CN^- binding expected for exterior surface, 3-coordinate (Cu_surface) and intrapore, 5-coordinate (Cu_pore) sites predicted by the idealized, metal-terminated crystal structure. Two-coordinate Cu defect sites are ruled out at the ≥0.5% FT-IR detection limit as such defect sites would have been detectable by the FT-IR studies of the CN^--poisoned catalyst. Size-selective poisoning studies of CuBTTri exterior surface sites reveal that 1.3 (±0.4)% of total copper in 0.6 ± 0.4 μm particles is active. That counting of active sites yields a normalized turnover frequency (TOF), TOF_norm = (4.9 ± 1.2) × 10^-2 mol NO (mol Cu_surface)^-1 s^-1 (in water, at 20 min, 25 °C, 1 mM GSNO, 30% loss of GSNO, and 1.3 ± 0.4 mol % Cu_surface)-a value ∼100× higher than the TOF calculated without active site counting. Overall, Ockham's razor interpretation of the data is that exterior surface, Cu_surface sites are the catalytically most active sites present at a 1.3 (±0.4)% level of total Cu.

A New Mechanism of Metal-Ligand Cooperative Catalysis in Transfer Hydrogenation of Ketones

We report iridium catalysts IrCl(η⁵-Cp*)(κ²-(2-pyridyl)CH₂NSO₂C₆H₄X) (1-Me, X = CH₃ and 1-F, X = F) for transfer hydrogenation of ketones with 2-propanol that operate by a previously unseen metal-ligand cooperative mechanism. Under the reaction conditions, complexes 1 (1-Me and 1-F) derivatize to a series of catalytic intermediates: Ir(η⁵-Cp*)(κ²-(C₅H₄N)CHNSO₂Ar) (2), IrH(η⁵Cp*)(κ²-(2-pyridyl)CH₂NSO₂Ar) (3), and Ir(η⁵-Cp*)(κ³-(2-pyridyl)CH₂NSO₂Ar) (4). The structures of 1-Me and 4-Me were established by single-crystal X-ray diffraction. A rate-determining, concerted hydrogen transfer step (2 + R₂CHOH ⇄ 3 + R₂CO) is suggested by kinetic isotope effects, Eyring parameters (ΔH ^≠ = 29.1(8) kcal mol^-1 and ΔS ^≠ = -17(19) eu), proton-hydride fidelity, and DFT calculations. According to DFT, a nine-membered cyclic transition state is stabilized by an alcohol molecule that serves as a proton shuttle.

One-pot dual catalysis for the hydrogenation of heteroarenes and arenes

A catalytic system resulting from a monohydrido bridged ruthenium complex hydrogenated both heteroarenes and arenes, exhibited dual catalysis and provided access to valuable saturated heterocycles and cycloalkanes.

Asymmetric hydrogenation of an α-unsaturated carboxylic acid catalyzed by intact chiral transition metal carbonyl clusters – diastereomeric control of enantioselectivity

Asymmetric hydrogenation catalysis by [(μ-H)₂Ru₃(μ₃-S)(CO)₇(μ-P–P*)] (P–P* = chiral diphosphine) indicates intact chiral clusters as active catalysts.

A prolific catalyst for dehydrogenation of neat formic acid

Formic acid is a promising energy carrier for on-demand hydrogen generation. Because the reverse reaction is also feasible, formic acid is a form of stored hydrogen. Here we present a robust, reusable iridium catalyst that enables hydrogen gas release from neat formic acid. This catalysis works under mild conditions in the presence of air, is highly selective and affords millions of turnovers. While many catalysts exist for both formic acid dehydrogenation and carbon dioxide reduction, solutions to date on hydrogen gas release rely on volatile components that reduce the weight content of stored hydrogen and/or introduce fuel cell poisons. These are avoided here. The catalyst utilizes an interesting chemical mechanism, which is described on the basis of kinetic and synthetic experiments.

Ionic liquids in catalysis: molecular and nanometric metal systems

The catalyst immobilization in a liquid phase represents an attractive means to preserve high activities and selectivities, also permitting an easy recycling. To attain this goal, organic products should be extracted in a simple way from the catalytic phase leading to metal-free target compounds; for this reason, ionic liquids exhibiting high affinity for metallic species and low affinity for low polar compounds, turn into a promising medium, in particular for the synthesis of fine chemicals. In the present Accounts, we illustrate this approach through our research involving both molecular organometallic compounds and metallic nanoparticles dispersed in an ionic liquid phase.

A new mechanism for allylic alcohol isomerization involving ruthenium nanoparticles as a ‘true catalyst’ generated through the self-assembly of supramolecular triruthenium clusters

A new five step mechanism has been established for allylic alcohol isomerization involving ruthenium nanoparticles as the ‘true catalyst’.

Oxidative decomposition of Au25(SR)18 clusters in a catalytic context

Gold nanoparticle catalysis of chemical transformations has emerged as a subject of intense interest over the past decade. In particular, Au25(SR)18 has emerged as a model catalyst. In an effort to investigate their potential as intact, homogeneous, unsupported catalysts, we have discovered that Au25(SR)18 clusters are not stable in oxidizing conditions reported for catalytic styrene oxidation. Further investigation suggests that the active catalytic species is an Au(I) species resulting from oxidative decomposition of the starting gold cluster. This conclusion appears independent of R-group on thiolate-ligated Au25(SR)18 clusters.

The free-energy barrier to hydride transfer across a dipalladium complex

We use density-functional theory molecular dynamics (DFT-MD) simulations to determine the hydride transfer coordinate between palladium centres of the crystallographically observed terminal hydride locations, Pd–Pd–H, originally postulated for the solution dynamics of the complex bis-NHC dipalladium hydride [{(MesIm)₂CH₂}₂Pd₂H][PF₆], and then calculate the free-energy along this coordinate. We estimate the transfer barrier-height to be about 20 kcal mol⁻¹ with a hydride transfer rate in the order of seconds at room temperature. We validate our DFT-MD modelling using inelastic neutron scattering which reveals anharmonicity of the hydride environment that is so pronounced that there is complete failure of the harmonic model for the hydride ligand. The simulations are extended to high temperature to bring the H-transfer to a rate that is accessible to the simulation technique.

Diastereomeric control of enantioselectivity: evidence for metal cluster catalysis

Enantioselective hydrogenation of tiglic acid effected by diastereomers of the general formula [(μ-H)2Ru3(μ3-S)(CO)7(μ-P-P*)] (P-P* = chiral Walphos diphosphine ligand) strongly supports catalysis by intact Ru3 clusters. A catalytic mechanism involving Ru3 clusters has been established by DFT calculations.

Chromium(0) nanoparticles as effective catalyst for the conversion of glucose into 5-hydroxymethylfurfural

It's nano: Small and uniform chromium nanoparticles, either preformed or generated in situ, effectively catalyze the conversion of glucose into 5-hydroxymethyl furfural. The results compare favorably with those achieved by using a catalyst system based on divalent CrCl(2) in ionic liquids (ILs). In addition, the chromium nanoparticles are found in the CrCl(2)/IL system, and the implications of their presence in that system is investigated.

Efficient cluster-based catalysts for asymmetric hydrogenation of α-unsaturated carboxylic acids

The new clusters [H(4)Ru(4)(CO)(10)(μ-1,2-P-P)], [H(4)Ru(4)(CO)(10) (1,1-P-P)] and [H(4)Ru(4)(CO)(11)(P-P)] (P-P=chiral diphosphine of the ferrocene-based Josiphos or Walphos ligand families) have been synthesised and characterised. The crystal and molecular structures of eleven clusters reveal that the coordination modes of the diphosphine in the [H(4)Ru(4)(CO)(10)(μ-1,2-P-P)] clusters are different for the Josiphos and the Walphos ligands. The Josiphos ligands bridge a metal-metal bond of the ruthenium tetrahedron in the "conventional" manner, that is, with both phosphine moieties coordinated in equatorial positions relative to a triangular face of the tetrahedron, whereas the phosphine moieties of the Walphos ligands coordinate in one axial and one equatorial position. The differences in the ligand size and the coordination mode between the two types of ligands appear to be reflected in a relative propensity for isomerisation; in solution, the [H(4)Ru(4)(CO)(10)(1,1-Walphos)] clusters isomerise to the corresponding [H(4)Ru(4)(CO)(10)(μ-1,2-Walphos)] clusters, whereas the Josiphos-containing clusters show no tendency to isomerisation in solution. The clusters have been tested as catalysts for asymmetric hydrogenation of four prochiral α-unsaturated carboxylic acids and the prochiral methyl ester (E)-methyl 2-methylbut-2-enoate. High conversion rates (>94%) and selectivities of product formation were observed for almost all catalysts/catalyst precursors. The observed enantioselectivities were low or nonexistent for the Josiphos-containing clusters and catalyst (cluster) recovery was low, suggesting that cluster fragmentation takes place. On the other hand, excellent conversion rates (99-100%), product selectivities (99-100% in most cases) and good enantioselectivities, reaching 90% enantiomeric excess (ee) in certain cases, were observed for the Walphos-containing clusters, and the clusters could be recovered in good yield after completed catalysis. Results from high-pressure NMR and IR studies, catalyst poisoning tests and comparison of catalytic properties of two [H(4)Ru(4)(CO)(10)(μ-1,2-P-P)] clusters (P-P=Walphos ligands) with the analogous mononuclear catalysts [Ru(P-P)(carboxylato)(2)] suggest that these clusters may be the active catalytic species, or direct precursors of an active catalytic cluster species.

A Three-Stage Mechanistic Model for Ammonia Borane Dehydrogenation by Shvo's Catalyst

We propose a mechanistic model for three-stage dehydrogenation of ammonia borane (AB) catalyzed by Shvo's cyclopentadienone-ligated ruthenium complex. We provide evidence for a plausible mechanism for catalyst deactivation, the transition from fast catalysis to slow catalysis, and relate those findings to the invention of a second-generation catalyst that does not suffer from the same deactivation chemistry.The primary mechanism of catalyst deactivation is borazine-mediated hydroboration of the ruthenium species that is the active oxidant in the fast catalysis case. This transition is characterized by a change in the rate law for the reaction and changes in the apparent resting state of the catalyst. Also, in this slow catalysis situation, we see an additional intermediate in the sequence of boron, nitrogen species, aminodiborane. This occurs with concurrent generation of NH(3), which itself does not strongly affect the rate of AB dehydrogenation.

The mechanism of efficient asymmetric transfer hydrogenation of acetophenone using an iron(II) complex containing an (S,S)-Ph2PCH2CH═NCHPhCHPhN═CHCH2PPh2 ligand: partial ligand reduction is the key

On the basis of a kinetic study and other evidence, we propose a mechanism of activation and operation of a highly active system generated from the precatalyst trans-[Fe(CO)(Br)(Ph(2)PCH(2)CH═N-((S,S)-C(Ph)H-C(Ph)H)-N═CHCH(2)PPh(2))][BPh(4)] (2) for the asymmetric transfer hydrogenation of acetophenone in basic isopropanol. An induction period for catalyst activation is observed before the catalytic production of 1-phenethanol. The activation step is proposed to involve a rapid reaction of 2 with excess base to give an ene-amido complex [Fe(CO)(Ph(2)PCH(2)CH═N-((S,S)-C(Ph)H-C(Ph)H)-NCH═CHPPh(2))](+) (Fe(p)) and a bis(enamido) complex Fe(CO)(Ph(2)PCH═CH-N-(S,S-CH(Ph)CH(Ph))-N-CH═CHPPh(2)) (5); 5 was partially characterized. The slow step in the catalyst activation is thought to be the reaction of Fe(p) with isopropoxide to give the catalytically active amido-(ene-amido) complex Fe(a) with a half-reduced, deprotonated PNNP ligand. This can be trapped by reaction with HCl in ether to give, after isolation with NaBPh(4), [Fe(CO)(Cl)(Ph(2)PCH(2)CH(2)N(H)-((S,S)-CH(Ph)CH(Ph))-N═CHCH(2)PPh(2))][BPh(4)] (7) which was characterized using multinuclear NMR and high-resolution mass spectrometry. When compound 7 is treated with base, it directly enters the catalytic cycle with no induction period. A precatalyst with the fully reduced P-NH-NH-P ligand was prepared and characterized by single crystal X-ray diffraction. It was found to be much less active than 2 or 7. Reaction profiles obtained by varying the initial concentrations of acetophenone, precatalyst, base, and acetone and by varying the temperature were fit to the kinetic model corresponding to the proposed mechanism by numerical simulation to obtain a unique set of rate constants and thermodynamic parameters.

Chiral ammonium-capped rhodium(0) nanocatalysts: synthesis, characterization, and advances in asymmetric hydrogenation in neat water

Optically active amphiphilic compounds derived from N-methylephedrine, N-methylprolinol, or cinchona derivatives possessing bromide or chiral lactate counterions were efficiently used as protective agents for rhodium(0) nanoparticles. The full characterization of these surfactants and the obtained nanocatalysts was performed by means of different techniques. These spherical nanoparticles, with sizes between 0.8-2.5 nm depending on the stabilizer, were evaluated in the hydrogenation of model substrates in neat water as a green solvent. The rhodium catalysts showed relevant kinetic properties, but modest enantiomeric excess values of up to 13 % in the hydrogenation of ethyl pyruvate. They were also investigated in the hydrogenation of disubstituted arenes, such as m-methylanisole, providing interesting catalytic activities and a preferential cis selectivity of around 80 %; however, no asymmetric induction was observed.

Is it homogeneous or heterogeneous catalysis derived from [RhCp*Cl2]2? In operando XAFS, kinetic, and crucial kinetic poisoning evidence for subnanometer Rh4 cluster-based benzene hydrogenation catalysis

Determining the true, kinetically dominant catalytically active species, in the classic benzene hydrogenation system pioneered by Maitlis and co-workers 34 years ago starting with [RhCp*Cl(2)](2) (Cp* = [η(5)-C(5)(CH(3))(5)]), has proven to be one of the most challenging case studies in the quest to distinguish single-metal-based "homogeneous" from polymetallic, "heterogeneous" catalysis. The reason, this study will show, is the previous failure to use the proper combination of: (i) in operando spectroscopy to determine the dominant form(s) of the precatalyst's mass under catalysis (i.e., operating) conditions, and then crucially also (ii) the previous lack of the necessary kinetic studies, catalysis being a "wholly kinetic phenomenon" as J. Halpern long ago noted. An important contribution from this study will be to reveal the power of quantitiative kinetic poisoning experiments for distinguishing single-metal, or in the present case subnanometer Rh(4) cluster-based catalysis, from larger, polymetallic Rh(0)(n) nanoparticle catalysis, at least under favorable conditions. The combined in operando X-ray absorption fine structure (XAFS) spectroscopy and kinetic evidence provide a compelling case for Rh(4)-based, with average stoichiometry "Rh(4)Cp*(2.4)Cl(4)H(c)", benzene hydrogenation catalysis in 2-propanol with added Et(3)N and at 100 °C and 50 atm initial H(2) pressure. The results also reveal, however, that if even ca. 1.4% of the total soluble Rh(0)(n) had formed nanoparticles, then those Rh(0)(n) nanoparticles would have been able to account for all the observed benzene hydrogenation catalytic rate (using commercial, ca. 2 nm, polyethyleneglycol-dodecylether hydrosol stabilized Rh(0)(n) nanoparticles as a model system). The results--especially the poisoning methodology developed and employed--are of significant, broader interest since determining the nature of the true catalyst continues to be a central, often vexing issue in any and all catalytic reactions. The results are also of fundamental interest in that they add to a growing body of evidence indicating that certain, appropriately ligated, coordinatively unsaturated, subnanometer M(4) transition-metal clusters can be relatively robust catalysts. Also demonstrated herein is that Rh(4) clusters are poisoned by Hg(0), demonstrating for the first time that the classic Hg(0) poisoning test of "homogeneous" vs "heterogeneous" catalysts cannot distinguish Rh(4)-based subnanometer catalysts from Rh(0)(n) nanoparticle catalysts, at least for the present examples of these two specific, Rh-based catalysts.

Industrial Ziegler-type hydrogenation catalysts made from Co(neodecanoate)2 or Ni(2-ethylhexanoate)2 and AlEt3: evidence for nanoclusters and sub-nanocluster or larger Ziegler-nanocluster based catalysis

Ziegler-type hydrogenation catalysts are important for industrial processes, namely, the large-scale selective hydrogenation of styrenic block copolymers. Ziegler-type hydrogenation catalysts are composed of a group 8-10 transition metal precatalyst plus an alkylaluminum cocatalyst (and they are not the same as Ziegler-Natta polymerization catalysts). However, for ∼50 years two unsettled issues central to Ziegler-type hydrogenation catalysis are the nature of the metal species present after catalyst synthesis, and whether the species primarily responsible for catalytic hydrogenation activity are homogeneous (e.g., monometallic complexes) or heterogeneous (e.g., Ziegler nanoclusters defined as metal nanoclusters made from combination of Ziegler-type hydrogenation catalyst precursors). A critical review of the existing literature (Alley et al. J. Mol. Catal. A: Chem. 2010, 315, 1-27) and a recently published study using an Ir model system (Alley et al. Inorg. Chem. 2010, 49, 8131-8147) help to guide the present investigation of Ziegler-type hydrogenation catalysts made from the industrially favored precursors Co(neodecanoate)(2) or Ni(2-ethylhexanoate)(2), plus AlEt(3). The approach and methods used herein parallel those used in the study of the Ir model system. Specifically, a combination of Z-contrast scanning transmission electron microscopy (STEM), matrix assisted laser desorption ionization mass spectrometry (MALDI MS), and X-ray absorption fine structure (XAFS) spectroscopy are used to characterize the transition metal species both before and after hydrogenation. Kinetic studies including Hg(0) poisoning experiments are utilized to test which species are the most active catalysts. The main findings are that, both before and after catalytic cyclohexene hydrogenation, the species present comprise a broad distribution of metal cluster sizes from subnanometer to nanometer scale particles, with estimated mean cluster diameters of about 1 nm for both Co and Ni. The XAFS results also imply that the catalyst solutions are a mixture of the metal clusters described above, plus unreduced metal ions. The kinetics-based Hg(0) poisoning evidence suggests that Co and Ni Ziegler nanoclusters (i.e., M(≥4)) are the most active Ziegler-type hydrogenation catalysts in these industrial systems. Overall, the novelty and primary conclusions of this study are as follows: (i) this study examines Co- and Ni-based catalysts made from the actual industrial precursor materials, catalysts that are notoriously problematic regarding their characterization; (ii) the Z-contrast STEM results reported herein represent, to our knowledge, the best microscopic analysis of the industrial Co and Ni Ziegler-type hydrogenation catalysts; (iii) this study is the first explicit application of an established method, using multiple analytical methods and kinetics-based studies, for distinguishing homogeneous from heterogeneous catalysis in these Ziegler-type systems; and (iv) this study parallels the successful study of an Ir model Ziegler catalyst system, thereby benefiting from a comparison to those previously unavailable findings, although the greater M-M bond energy, and tendency to agglomerate, of Ir versus Ni or Co are important differences to be noted. Overall, the main result of this work is that it provides the leading hypothesis going forward to try to refute in future work, namely, that sub, M(≥4) to larger, M(n) Ziegler nanoclusters are the dominant, industrial, Co- and Ni- plus AlR(3) catalysts in Ziegler-type hydrogenation systems.