Understanding Ligand Effects on Bielectronic Transitions: Chemo- and Electroreduction of Rhodium Bis(Diphosphine) Complexes to Low Oxidation States.

Rhodium complexes in the -I and 0 oxidation states are of great potential interest in catalytic applications. In contrast to their rhodium +I congeners, however, the structural and electronic parameters governing their access and stability are far less understood. Herein, we investigate the two-electron reduction of a parameterized series of bis(diphosphine) Rh complexes [Rh(dxpy)2]NTf2 (x = P-substituent, y = alkanediyl bridging P atoms). Through (electro)reductions from the RhI parents, Rh-I d10-complexes were obtained and characterized spectroscopically, including 103Rh NMR data. The reductive steps convolute with structural rearrangements from square planar to tetrahedral coordination. We found that the extent of these reorganisations defines whether the first E0(RhI/0) and second E0(Rh0/-I) reduction potentials are normally ordered, leading to monoelectronic stepwise events, or inverted, giving bielectronic transitions. Reductionist approaches based on Hammett parameters or the P-Rh-P bite angles provide only partial correlations with the redox potentials. However, we identified the C-O stretch of analogue diphosphine complexes as an expedient computational parameter that enables these correlations through both electronic and geometric features, even in a predictive manner. Gaining control over two-electron reduction behaviors through rationalized ligand effects has potential impact beyond Rh complexes, for molecular and enzymatic metal sites commonly exhibiting bielectronic transitions.

Computational design of cooperatively acting molecular catalyst systems: carbene based tungsten- or molybdenum-catalysts with rhodium- or iridium-complexes for the ionic hydrogenation of N2 to NH3.

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol-1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol-1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected.

Pd(II)-catalyzed carboxylation of aromatic C─H bonds with CO2.

The carboxylation of nonactivated C─H bonds provides an attractive yet hitherto largely elusive chemical process to synthesize carboxylic acids by incorporation of CO2 into the chemical value chain. Here, we report on the realization of such a reaction using simple and nonactivated arenes as starting materials. A computationally designed Pd(II) complex acts as organometallic single-component catalyst, and apart from a base, necessary for thermodynamic stabilization of the intermediates, no other additives or coreagents are required. Turnover numbers up to 102 and high regioselectivities are achieved. The potential of this catalytic reaction for "green chemistry" is demonstrated by the synthesis of veratric acid, an intermediate for pharmaceutical production, from CO2 and veratrol.

Factors Governing the Catalytic Insertion of CO2 into Arenes - A DFT Case Study for Pd and Pt Phosphane Sulfonamido Complexes.

The potential of Pd/Pt complexes for catalytic carboxylation of arenes with CO2 is investigated by means of computational chemistry. Recently we reported that the bis[(2-methoxyphenyl)phosphino]-benzenesulfonamido palladium complex 1 inserts CO2 reversibly in its Pd-C(aryl) bond generating carboxylato complex 2. In the present work we study how geometric and electronic factors of various ligands and substrates influence the overall activation barrier (energy span, ES) of a potential catalytic cycle for arene carboxylation comprising this elementary step. The tendency of the key intermediates to dimerize and thus deactivating the potential catalysts is examined as well as the role of the base, which inevitably is needed to stabilize the reaction product. We show that Pd and Pt complexes I(Pd)-L16-S1 and I(Pt)-L16-S1 do not dimerize, enable the computation of complete catalytic cycles, and show interestingly low ES values of 26.8 and 24.5 kcal/mol, respectively.

Hydroamination of Aromatic Alkynes to Imines Catalyzed by Pd(II)-Anthraphos Complexes.

Herein, we report the synthesis, characterization, and catalytic performance of cationic Pd(II)-Anthraphos complexes in the intermolecular hydroamination of aromatic alkynes with aromatic amines. The reaction proceeds with 0.18 mol % of catalyst loading, at 90 °C for 4 h under neat conditions. Good to excellent yields could be obtained for a broad range of amines and alkynes.

Carbon monoxide and hydrogen (syngas) as a C1-building block for selective catalytic methylation.

A catalytic reaction using syngas (CO/H2) as feedstock for the selective ß-methylation of alcohols was developed whereby carbon monoxide acts as a C1 source and hydrogen gas as a reducing agent. The overall transformation occurs through an intricate network of metal-catalyzed and base-mediated reactions. The molecular complex [Mn(CO)2Br[HN(C2H4P i Pr2)2]] 1 comprising earth-abundant manganese acts as the metal component in the catalytic system enabling the generation of formaldehyde from syngas in a synthetically useful reaction. This new syngas conversion opens pathways to install methyl branches at sp3 carbon centers utilizing renewable feedstocks and energy for the synthesis of biologically active compounds, fine chemicals, and advanced biofuels.

Manganese(I)-Catalyzed ß-Methylation of Alcohols Using Methanol as C1 Source.

Highly selective ß-methylation of alcohols was achieved using an earth-abundant first row transition metal in the air stable molecular manganese complex [Mn(CO)2 Br[HN(C2 H4 Pi Pr2 )2 ]] 1 ([HN(C2 H4 Pi Pr2 )2 ]=MACHO-i Pr). The reaction requires only low loadings of 1 (0.5 mol %), methanolate as base and MeOH as methylation reagent as well as solvent. Various alcohols were ß-methylated with very good selectivity (>99 %) and excellent yield (up to 94 %). Biomass derived aliphatic alcohols and diols were also selectively methylated on the ß-position, opening a pathway to "biohybrid" molecules constructed entirely from non-fossil carbon. Mechanistic studies indicate that the reaction proceeds through a borrowing hydrogen pathway involving metal-ligand cooperation at the Mn-pincer complex. This transformation provides a convenient, economical, and environmentally benign pathway for the selective C-C bond formation with potential applications for the preparation of advanced biofuels, fine chemicals, and biologically active molecules.

Manganese-catalyzed hydroboration of carbon dioxide and other challenging carbonyl groups.

Reductive functionalization of the C=O unit in carboxylic acids, carbonic acid derivatives, and ultimately in carbon dioxide itself is a challenging task of key importance for the synthesis of value-added chemicals. In particular, it can open novel pathways for the valorization of non-fossil feedstocks. Catalysts based on earth-abundant, cheap, and benign metals would greatly contribute to the development of sustainable synthetic processes derived from this concept. Herein, a manganese pincer complex [Mn(Ph2PCH2SiMe2)2NH(CO)2Br] (1) is reported to enable the reduction of a broad range of carboxylic acids, carbonates, and even CO2 using pinacolborane as reducing agent. The complex is shown to operate under mild reaction conditions (80-120 °C), low catalyst loadings (0.1-0.2 mol%) and runs under solvent-less conditions. Mechanistic studies including crystallographic characterisation of a borane adduct of the pincer complex (1) imply that metal-ligand cooperation facilitates substrate activation.

Catalytic Hydrogenation of Cyclic Carbonates using Manganese Complexes.

Catalytic hydrogenation of cyclic carbonates to diols and methanol was achieved using a molecular catalyst based on earth-abundant manganese. The complex [Mn(CO)2 (Br)[HN(C2 H4 Pi Pr2 )2 ] 1 comprising commercially available MACHO ligand is an effective pre-catalyst operating under relatively mild conditions (T=120 °C, p(H2 )=30-60 bar). Upon activation with NaOt Bu, the formation of coordinatively unsaturated complex [Mn(CO)2 [N(C2 H4 Pi Pr2 )2 )] 5 was spectroscopically verified, which confirmed a kinetically competent intermediate. With the pre-activated complex, turnover numbers up to 620 and 400 were achieved for the formation of the diol and methanol, respectively. Stoichiometric reactions under catalytically relevant conditions provide insight into the stepwise reduction form the CO2 level in carbonates to methanol as final product.

Double Dehydrogenation of Primary Amines to Nitriles by a Ruthenium Complex Featuring Pyrazole Functionality.

A ruthenium(II) complex bearing a naphthyridine-functionalized pyrazole ligand catalyzes oxidant-free and acceptorless selective double dehydrogenation of primary amines to nitriles at moderate temperature. The role of the proton-responsive entity on the ligand scaffold is demonstrated by control experiments, including the use of a N-methylated pyrazole analogue. DFT calculations reveal intricate hydride and proton transfers to achieve the overall elimination of 2 equiv of H2.

On the applicability of density functional theory to manganese-based complexes with catalytic activity toward water oxidation.

The present contribution assesses the performance of several popular and accurate density functionals, namely B3LYP, BP86, M06, MN12L, mPWPW91, PBE0, and TPSSh toward manganese-based coordination complexes. These compounds show promising properties toward application to catalytic water oxidation. Although manganese with N- and O-biding ligands tends to give rise to high spin complexes, the results show that BP86, mPWPW91, and specially MN12L, tend to yield low-spin complexes. The usage of these functionals for such compounds is, thus, discouraged. All the functionals considered deliver accurate geometries. The present results show, however, that B3LYP delivers geometries deviating from experimental values when compared to the other functionals of the set. M06, PBE0, and TPSSh deliver geometries of similar accuracy, PBE0 outstanding slightly with respect to the other two. © 2017 Wiley Periodicals, Inc.

Interaction of formaldehyde with a water-tolerant frustrated Lewis pair.

A facile complexation of formaldehyde with the water-tolerant frustrated Lewis pair (FLP) B(C6F5)3/PtBu3 and its Al-analog under ambient conditions is reported. Unprecedented formaldehyde adducts 1, 2 and 4 have been identified and crystallographically characterized.

Catalytic NH3 Synthesis using N2 /H2 at Molecular Transition Metal Complexes: Concepts for Lead Structure Determination using Computational Chemistry.

While industrial NH3 synthesis based on the Haber-Bosch-process was invented more than a century ago, there is still no molecular catalyst available which reduces N2 in the reaction system N2 /H2 to NH3 . As the many efforts of experimentally working research groups to develop a molecular catalyst for NH3 synthesis from N2 /H2 have led to a variety of stoichiometric reductions it seems justified to undertake the attempt of systematizing the various approaches of how the N2 molecule might be reduced to NH3 with H2 at a transition metal complex. In this contribution therefore a variety of intuition-based concepts are presented with the intention to show how the problem can be approached. While no claim for completeness is made, these concepts intend to generate a working plan for future research. Beyond this, it is suggested that these concepts should be evaluated with regard to experimental feasibility by checking barrier heights of single reaction steps and also by computation of whole catalytic cycles employing density functional theory (DFT) calculations. This serves as a tool which extends the empirically driven search process and expands it by computed insights which can be used to rationalize the various challenges which must be met.

Hydrogenation of CO2 to Formic Acid with a Highly Active Ruthenium Acriphos Complex in DMSO and DMSO/Water.

The novel [Ru(Acriphos)(PPh3 )(Cl)(PhCO2 )] [1; Acriphos=4,5-bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO2 to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H2 O without the need for amine bases as co-reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h(-1) were achieved, thus rendering 1 one of the most active catalysts for CO2 hydrogenations under additive-free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L(-1) ) was shown to be limited by product-dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L(-1) , which corresponds to optimized values of TON=16×10(3) and TOFavg ≈10(3)  h(-1) .

A DFT Study on the Co-polymerization of CO2 and Ethylene: Feasibility Analysis for the Direct Synthesis of Polyethylene Esters.

The co-polymerization of CO2 with the non-polar monomer ethylene, though highly desirable, still presents a challenge whereas the palladium-catalyzed CO/C2 H4 co-polymerization is well understood. Building on this analogy, the goal of this study was to elucidate the feasibility of developing suitable catalysts for co-polymerizing CO2 with ethylene to polyethylene esters. Computational methods based on density functional theory were hereby employed. In the search for new catalyst lead structures, a closed catalytic cycle was identified for the palladium-catalyzed CO2 /C2 H4 co-polymerization reaction. The computational study on palladium complexes with a substituted anionic 2-[bis(2,4-dimethoxyphenyl)-phosphine]-benzene-2-hydroxo ligand revealed key aspects that need to be considered when designing ligand sets for potential catalysts for the non-alternating co-polymerization of CO2 and ethylene.

Can Contemporary Density Functional Theory Predict Energy Spans in Molecular Catalysis Accurately Enough To Be Applicable for in Silico Catalyst Design? A Computational/Experimental Case Study for the Ruthenium-Catalyzed Hydrogenation of Olefins.

The catalytic hydrogenation of cyclohexene and 1-methylcyclohexene is investigated experimentally and by means of density functional theory (DFT) computations using novel ruthenium Xantphos(Ph) (4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) and Xantphos(Cy) (4,5-bis(dicyclohexylphosphino)-9,9-dimethylxanthene) precatalysts [Ru(Xantphos(Ph))(PhCO2)(Cl)] (1) and [Ru(Xantphos(Cy))(PhCO2)(Cl)] (2), the synthesis, characterization, and crystal structures of which are reported. The intention of this work is to (i) understand the reaction mechanisms on the microscopic level and (ii) compare experimentally observed activation barriers with computed barriers. The Gibbs free activation energy ΔG(⧧) was obtained experimentally with precatalyst 1 from Eyring plots for the hydrogenation of cyclohexene (ΔG(⧧) = 17.2 ± 1.0 kcal/mol) and 1-methylcyclohexene (ΔG(⧧) = 18.8 ± 2.4 kcal/mol), while the Gibbs free activation energy ΔG(⧧) for the hydrogenation of cyclohexene with precatalyst 2 was determined to be 21.1 ± 2.3 kcal/mol. Plausible activation pathways and catalytic cycles were computed in the gas phase (M06-L/def2-SVP). A variety of popular density functionals (ωB97X-D, LC-ωPBE, CAM-B3LYP, B3LYP, B97-D3BJ, B3LYP-D3, BP86-D3, PBE0-D3, M06-L, MN12-L) were used to reoptimize the turnover determining states in the solvent phase (DF/def2-TZVP; IEF-PCM and/or SMD) to investigate how well the experimentally obtained activation barriers can be reproduced by the calculations. The density functionals B97-D3BJ, MN12-L, M06-L, B3LYP-D3, and CAM-B3LYP reproduce the experimentally observed activation barriers for both olefins very well with very small (0.1 kcal/mol) to moderate (3.0 kcal/mol) mean deviations from the experimental values indicating for the field of hydrogenation catalysis most of these functionals to be useful for in silico catalyst design prior to experimental work.

NH3 Synthesis in the N2/H2 Reaction System using Cooperative Molecular Tungsten/Rhodium Catalysis in Ionic Hydrogenation: A†DFT Study.

The ionic hydrogenation of N2 with H2 to give NH3 is investigated by means of density functional theory (DFT) computations using a cooperatively acting catalyst system. In this system, N2 binds to a neutral tungsten pincer complex of the type [(PNP)W(N2)3] (PNP=pincer ligand) and is reduced to NH3. The protons and hydride centers necessary for the reduction are delivered by heterolytic cleavage of H2 between the N2-tungsten complex and the cationic rhodium complex [Cp*Rh{2-(2-pyridyl)phenyl}(CH3 CN)](+). Successive transfer of protons and hydrides to the bound N2, as well as all Nx Hy units that occur during the reaction, enable the computation of closed catalytic cycles in the gas and in the solvent phase. By optimizing the pincer ligands of the tungsten complex, energy spans as low as 39.3 kcal mol(-1) could be obtained, which is unprecedented in molecular catalysis for the N2/H2 reaction system.

Hydrogenation of carbon dioxide to methanol using a homogeneous ruthenium-Triphos catalyst: from mechanistic investigations to multiphase catalysis.

The hydrogenation of CO2 to methanol can be achieved using a single molecular organometallic catalyst. Whereas homogeneous catalysts were previously believed to allow the hydrogenation only via formate esters as stable intermediates, the present mechanistic study demonstrates that the multistep transformation can occur directly on the Ru-Triphos (Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane) centre. The cationic formate complex [(Triphos)Ru(η2-O2CH)(S)]+ (S = solvent) was identified as the key intermediate, leading to the synthesis of the analogous acetate complex as a robust and stable precursor for the catalytic transformation. A detailed mechanistic study using DFT calculations shows that a sequential series of hydride transfer and protonolysis steps can account for the transformation of CO2via formate/formic acid to hydroxymethanolate/formaldehyde and finally methanolate/methanol within the coordination sphere of a single Ru-Triphos-fragment. All experimental results of the systematic parameter optimisation are fully consistent with this mechanistic picture. Based on these findings, a biphasic system consisting of H2O and 2-MTHF was developed, in which the active cationic Ru-complex resides in the organic phase for recycling and methanol is extracted with the aqueous phase.

Highly versatile catalytic hydrogenation of carboxylic and carbonic acid derivatives using a Ru-triphos complex: molecular control over selectivity and substrate scope.

The complex [Ru(Triphos)(TMM)] (Triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane, TMM = trimethylene methane) provides an efficient catalytic system for the hydrogenation of a broad range of challenging functionalities encompassing carboxylic esters, amides, carboxylic acids, carbonates, and urea derivatives. The key control factor for this unique substrate scope results from selective activation to generate either the neutral species [Ru(Triphos)(Solvent)H2] or the cationic intermediate [Ru(Triphos)(Solvent)(H)(H2)](+) in the presence of an acid additive. Multinuclear NMR spectroscopic studies demonstrated together with DFT investigations that the neutral species generally provides lower energy pathways for the multistep reduction cascades comprising hydrogen transfer to C═O groups and C-O bond cleavage. Carboxylic esters, lactones, anhydrides, secondary amides, and carboxylic acids were hydrogenated in good to excellent yields under these conditions. The formation of the catalytically inactive complexes [Ru(Triphos)(CO)H2] and [Ru(Triphos)(µ-H)]2 was identified as major deactivation pathways. The former complex results from substrate-dependent decarbonylation and constitutes a major limitation for the substrate scope under the neutral conditions. The deactivation via the carbonyl complex can be suppressed by addition of catalytic amounts of acids comprising non-coordinating anions such as HNTf2 (bis(trifluoromethane)sulfonimide). Although the corresponding cationic cycle shows higher overall barriers of activation, it provides a powerful hydrogenation pathway at elevated temperatures, enabling the selective reduction of primary amides, carbonates, and ureas in high yields. Thus, the complex [Ru(Triphos)(TMM)] provides a unique platform for the rational selection of reaction conditions for the selective hydrogenation of challenging functional groups and opens novel synthetic pathways for the utilization of renewable carbon sources.

Bifunctional ruthenium(II) hydride complexes with pendant strong Lewis acid moieties: structure, dynamics, and cooperativity.

The synthesis of a novel class of bifunctional ruthenium hydride complexes incorporating Lewis acidic BR(2) moieties is reported. Determination of the molecular structures in the solid state and in solution provided evidence for tunable interaction between the two functionalities. Cooperative effects on the reactivity of the complexes were demonstrated including the activation of small Lewis basic molecules by reversible anchoring at the boron center.