A sustainable strategic approach for N-alkylation of amines with activation of alcohols triggered via a hydrogen auto-transfer reaction using a Pd(II) complex: evidence for metal-ligand cooperativity

This work describes a new well-defined, air-stable, phosphine free palladium(II) [Pd(L)Cl] (1) catalyst. This catalyst was utilized for N-alkylation of amines and indole synthesis where H2O was found to be the by-product. A broad range of aromatic amines were alkylated using this homogeneous catalyst with a catalyst loading of 0.1 mol%. Greener aromatic and aliphatic primary alcohols were utilized and a hydrogen auto-transfer strategy via a metal-ligand cooperative approach was investigated. The precursor of the antihistamine-containing drug molecule tripelennamine was synthesized on a gram scale for large-scale applicability of the current synthetic methodology. A number of control experiments were performed to investigate the possible reaction pathway and the outcomes of these experiments indicated the azo-chromophore as a hydrogen reservoir during the catalytic cycle.

Readily available Ti-based in situ catalytic system for oxo/imido heterometathesis

We investigate Ti(NEt2)4 supported on silica dehydroxylated at 700 °C as an easily accessible pre-catalyst for oxo/imido heterometathesis reactions. Being activated with TolNH2, the supported Ti amide (SiO)Ti(NEt2)3 (1) demonstrates catalytic activity in the imidation of ketones with N-sulfinylamines comparable with the most active previously described well-defined imido catalyst (SiO)Ti(NtBu)(Me2Pyr)(py)2 (2) (Me2Pyr = 2,5-dimethylpyrrolyl), which implies the in situ formation of surface imido species in this system. The materials obtained via treatment of 1 with anilines (TolNH2 (1a) and p-MeOC6H415NH2 (1b)) were studied with IR, EA and 1H, 13C, 15N and 2D solid-state NMR, although the proposed imido intermediate has not been detected, pointing towards tris-amides (SiO)Ti(NHC6H4X)3 (X = Me, OMe) being the major surface species in the isolated materials 1a and 1b. The system 1/TolNH2 was tested in a range of imidation reactions and demonstrated excellent performance for express high-yielding preparation of ketimines, formamidines, lactone imidates and sulfurdiimines, making it a convenient alternative to the well-defined supported Ti imido catalysts.

Hydroaminoalkylation of alkenes using transition metals complexes grafted on silica SBA15 as catalysts

The gas-phase hydroaminoalkylation reaction of propene catalyzed by group 4 (M = Ti, Zr and Hf) metal amido complexes [(≡Si-O-)(M(-NMe₂)₃] was investigated by using PBE0-D3/SVP//TZVP level of theory. The geometrical analysis traced the formation of the metallaaziridines and the azametallacyclopentanes as key intermediates in these reactions. The metallaaziridines were simulated through the activation of α-C-H bonds of the amido groups; while the azametallacyclopentanes were configured by slotting the propene double bond onto the M - C bonds of the metallaaziridines. The latter reaction was considered the rate-determining step. Thermochemical calculations showed that the order of catalytic activity is: Ti ≥ Zr > Hf; while the preference of the azametallacyclopentanes is: Hf > Zr ≥ Ti.

N═N Bond Cleavage by Tantalum Hydride Complexes: Mechanistic Insights and Reactivity

The reaction of [TaCp^RX₄] (Cp^R = η⁵-C₅Me₅, η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) with SiH₃Ph resulted in the formation of the dinuclear hydride tantalum(IV) compounds [(TaCp^RX₂)₂(μ-H)₂], structurally identified by single-crystal X-ray analyses. These species react with azobenzene to give the mononuclear imide complex [TaCp^RX₂(NPh)] along with the release of molecular hydrogen. Analogous reactions between the [{Ta(η⁵-C₅Me₅)X₂}₂(μ-H)₂] derivatives and the cyclic diazo reagent benzo[c]cinnoline afford the biphenyl-bridged (phenylimido)tantalum complexes [{Ta(η⁵-C₅Me₅)X₂}₂(μ-NC₆H₄C₆H₄N)] along with the release of molecular hydrogen. When the compounds [(TaCp^RX₂)₂(μ-H)₂] (Cp^R = η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) were employed, we were able to trap the side-on-bound diazo derivatives [(TaCp^RX)₂{μ-(η²,η²-NC₆H₄C₆H₄N)}] (Cp^R = η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) as intermediates in the N=N bond cleavage process. DFT calculations provide insights into the N=N cleavage mechanism, in which the ditantalum(IV) fragment can promote two-electron reductions of the N=N bond at two different metal–metal bond splitting stages.

The series of dinuclear tantalum(IV) hydrides [{TaCp^RX₂}₂(μ-H)₂] (Cp^R = η⁵-C₅Me₅, η⁵-C₅H₄SiMe₃, η⁵-C₅HMe₄; X = Cl, Br) show the ability to promote N=N bond cleavage in their reactions with azobenzene and benzo[c]cinnoline in absence of reducing reagents. Both the characterization of intermediate species and DFT studies point to a mechanism in two stages, in which the Ta−Ta bond splitting is key for the reduction of the N=N bond and its complete scission.

Olefin metathesis: what have we learned about homogeneous and heterogeneous catalysts from surface organometallic chemistry?

Since its early days, olefin metathesis has been in the focus of scientific discussions and technology development. While heterogeneous olefin metathesis catalysts based on supported group 6 metal oxides have been used for decades in the petrochemical industry, detailed mechanistic studies and the development of molecular organometallic chemistry have led to the development of robust and widely used homogeneous catalysts based on well-defined alkylidenes that have found applications for the synthesis of fine and bulk chemicals and are also used in the polymer industry. The development of the chemistry of high-oxidation group 5-7 alkylidenes and the use of surface organometallic chemistry (SOMC) principles unlocked the preparation of so-called well-defined supported olefin metathesis catalysts. The high activity and stability (often superior to their molecular analogues) and molecular-level characterisation of these systems, that were first reported in 2001, opened the possibility for the first direct structure-activity relationships for supported metathesis catalysts. This review describes first the history of SOMC in the field of olefin metathesis, and then focuses on what has happened since 2007, the date of our last comprehensive reviews in this field.

Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N═N Bond of 2,2'-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N═N bond cleavage of 2,2'-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2'-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(═Npy)Cl3]n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N═N bond of 2,2'-azopyridine. The initial products of the above reactions were determined to be 2,2'-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2'-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N═N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [nBu4N][Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of nBu4NCl to complex 1a, while addition of nBu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [nBu4N]n/2[{Nb(═Npy)Cl3}2(μ-Cl)]n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2'-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(═Npy)Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2'-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]2 to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl3(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).

The Comparison between Single Atom Catalysis and Surface Organometallic Catalysis

Single atom catalysis (SAC) is a recent discipline of heterogeneous catalysis for which a single atom on a surface is able to carry out various catalytic reactions. A kind of revolution in heterogeneous catalysis by metals for which it was assumed that specific sites or defects of a nanoparticle were necessary to activate substrates in catalytic reactions. In another extreme of the spectrum, surface organometallic chemistry (SOMC), and, by extension, surface organometallic catalysis (SOMCat), have demonstrated that single atoms on a surface, but this time with specific ligands, could lead to a more predictive approach in heterogeneous catalysis. The predictive character of SOMCat was just the result of intuitive mechanisms derived from the elementary steps of molecular chemistry. This review article will compare the aspects of single atom catalysis and surface organometallic catalysis by considering several specific catalytic reactions, some of which exist for both fields, whereas others might see mutual overlap in the future. After a definition of both domains, a detailed approach of the methods, mostly modeling and spectroscopy, will be followed by a detailed analysis of catalytic reactions: hydrogenation, dehydrogenation, hydrogenolysis, oxidative dehydrogenation, alkane and cycloalkane metathesis, methane activation, metathetic oxidation, CO₂ activation to cyclic carbonates, imine metathesis, and selective catalytic reduction (SCR) reactions. A prospective resulting from present knowledge is showing the emergence of a new discipline from the overlap between the two areas.

Catalytic Oxo/Imido Heterometathesis by a Well-Defined Silica-Supported Titanium Imido Complex

Grafting Ti(=NtBu)(Me₂ Pyr)₂ (py)₂ (Me₂ Pyr= 2,5-dimethylpyrrolyl, py=pyridine) onto the surface of silica partially dehydroxylated at 700 °C gives the well-defined silica-supported Ti imido complex (≡SiO)Ti(=NtBu)(Me₂ Pyr)(py)₂ , which is fully characterized by IR and solid-state NMR spectroscopy as well as elemental and mass balance analyses. While stoichiometric imido-transfer reactivity is typical for Ti imides, the obtained surface complex is unique in that it enables catalytic transformations involving Ti imido and oxo intermediates. In particular, it efficiently catalyzes imidation of carbonyl compounds with N-sulfinylamines by oxo/imido heterometathesis.

Catalysis by Design: Well-Defined Single-Site Heterogeneous Catalysts

Heterogeneous catalysis, a field important industrially and scientifically, is increasingly seeking and refining strategies to render itself more predictable. The main issue is due to the nature and the population of catalytically active sites. Their number is generally low to very low, their "acid strengths" or " redox properties" are not homogeneous, and the material may display related yet inactive sites on the same material. In many heterogeneous catalysts, the discovery of a structure-activity reationship is at best challenging. One possible solution is to generate single-site catalysts in which most, if not all, of the sites are structurally identical. Within this context and using the right tools, the catalyst structure can be designed and well-defined, to reach a molecular understanding. It is then feasible to understand the structure-activity relationship and to develop predictable heterogeneous catalysis. Single-site well-defined heterogeneous catalysts can be prepared using concepts and tools of surface organometallic chemistry (SOMC). This approach operates by reacting organometallic compounds with surfaces of highly divided oxides (or of metal nanoparticles). This strategy has a solid track record to reveal structure-activity relationship to the extent that it is becoming now quite predictable. Almost all elements of the periodical table have been grafted on surfaces of oxides (from simple oxides such as silica or alumina to more sophisticated materials regarding composition or porosity). Considering catalytic hydrocarbon transformations, heterogeneous catalysis outcome may now be predicted based on existing mechanistic proposals and the rules of molecular chemistry (organometallic, organic) associated with some concepts of surface sciences. A thorough characterization of the grafted metal centers must be carried out using tools spanning from molecular organometallic or surface chemistry. By selection of the metal, its ligand set, and the support taken as a X, L ligands in the Green formalism, the catalyst can be designed and generated by grafting the organometallic precursor containing the functional group(s) suitable to target a given transformation (surface organometallic fragments (SOMF)). The choice of these SOMF is based on the elementary steps known in molecular chemistry applied to the desired reaction. The coordination sphere necessary for any catalytic reaction involving paraffins, olefins, and alkynes also can thus be predicted. Only their most complete understanding can allow development of catalytic reactions with the highest possible selectivity, activity, and lifetime. This Account will examine the results of SOMC for hydrocarbon transformations on oxide surfaces bearing metals of group 4-6. The silica-supported catalysts are exhibiting remarkable performances for Ziegler-Natta polymerization and depolymerization, low temperature hydrogenolysis of alkanes and waxes, metathesis of alkanes and cycloalkanes, olefins metathesis, and related reactions. In the case of reactions involving molecules that do not contain carbon (water-gas shift, NH3 synthesis, etc.) this single site approach is also valid but will be considered in a later review.