Electron-rich, bulky PNN-type ruthenium complexes: synthesis, characterization and catalysis of alcohol dehydrogenation

Production of carbon monoxide and hydrogen from methanol using a ruthenium pincer complex: a DFT study

To understand the mechanism of the dehydrogenation of methanol to CO and H2 catalyzed by a ruthenium pincer complex, a density functional theory (DFT) study has been conducted on two different cycles which differ in the substances entering the cycle (methanol (cycle 1) versus methoxymethanol (cycle 2)). Our calculated results show that both cycles consist of three stages: dehydrogenation of alcohol to aldehyde (stage I); hydrogen formation (stage II); and decarbonylation with the regeneration of the catalyst (stage III). The energy barriers of the rate-determining steps for cycles 1 and 2 are 49.6 and 28.5 kcal mol-1, respectively. Thus cycle 2 is more energetically feasible. For stage III of cycle 2, our results did not support the mechanism proposed in the experiment (CO release occurs prior to decarbonylation). Instead, we suggested and examined an alternative pathway, that is, decarbonylation occurs prior to CO release. The mechanistic insights gained in the present paper could be beneficial for further designing of these kinds of reactions.

Chemically robust and readily available quinoline-based PNN iron complexes: application in C-H borylation of arenes

Iron catalysts have been used for over a century to produce ammonia industrially. However, the use of iron catalysts generally remained quite limited until relatively recently, when the abundance and low toxicity of iron spurred the development of a variety of iron catalysts. Despite the fact that iron catalysts are being developed as alternatives to precious metal catalysts, their reactivities and stabilities are quite different because of their unique electronic structures. In this context, our group previously developed a new family of quinoline-based PNN pincer-type ligands for low- to mid-valent iron catalysts. These chemically robust PNN ligands provide air- and moisture-tolerant iron complexes, which exhibit excellent catalytic performances in the C-H borylation of arenes. This feature article summarises our recent work on PNN iron complexes, including their conception and design, as well as related reports on iron pincer complexes and iron-catalysed C-H borylation reactions.

NHC ligand-based half-sandwich iridium complexes: synthesis, structure and catalytic activity in acceptorless dehydrogenation and transfer hydrogenation

Half-sandwich iridium complexes exhibited high catalytic activity for acceptorless dehydrogenation of alcohols and transfer hydrogenation of ketones.

Silylamido supported dinitrogen heterobimetallic complexes: syntheses and their catalytic ability

Molybdenum dinitrogen complexes supported by monodentate arylsilylamido ligand, [Ar(Me₃Si)N]₃MoN₂Mg(THF)₂[N(SiMe₃)Ar] (5) and [Ar(Me₃Si)N]₃MoN₂SiMe₃ (6) (Ar = 3,5-Me₂C₆H₃) were synthesized and structurally characterized, and proved to be effective catalysts for the disproportionation of cyclohexadienes and isomerization of terminal alkenes. The ¹H NMR spectrum suggested that the bridging nitrogen ligand remains intact during the catalytic reaction, indicating possible catalytic ability of the Mo-N=N motif.

Co(I) complexes with a tetradentate phenanthroline-based PNNP ligand as a potent new metal-ligand cooperation platform

A series of low spin cobalt(i) complexes bearing a tetradentate phenanthroline-based PNNP ligand (2,9-bis((diphenylphosphanyl)methyl)-1,10-phenanthroline), [CoCl(PNNP)] (1), [CoMe(PNNP)] (2) and [Co(CH2SiMe3)(PNNP)] (3), were synthesized and structurally identified. Complex 3 underwent a structural rearrangement of the PNNP skeleton upon heating to form [Co(PNNP')] (4), which is supported by an asymmetrical PNNP' ligand with a dearomatized phenanthroline backbone. Mechanistic studies supported that the transformation from 3 to 4 was initiated by the homolysis of either a Co-CH2SiMe3 bond or a benzylic C-H bond. Complex 4 achieved H-H bond cleavage of H2 (1 atm) at ambient temperature, to form [Co(PNNP'')] (6), in which two H atoms were incorporated into the endocyclic double bond of the PNNP'' ligand backbone.

Proton-responsive naphthyridinone-based RuII complexes and their reactivity with water and alcohols

We report the synthesis and reactivity of RuII complexes with a new naphthyridinone-substituted phosphine ligand, 7-(diisopropylphosphinomethyl)-1,8-naphthyridin-2(1H)-one (L-H), which contains two reactive sites that can potentially be deprotonated by a strong base: an NH proton of naphthyridinone and a methylene arm attached to the phosphine. In the absence of a base, the stable bis-ligated complex Ru(L-H)2Cl2 (1) containing two NH groups in the secondary coordination sphere is formed. Upon further reaction with a base, a doubly deprotonated, dimeric complex is obtained, [Ru2(L*-H)2(L)2] (2), in which two of the four ligands undergo deprotonation at the NH (L), while the other two ligands are deprotonated at the methylene groups (L*-H) as confirmed by an X-ray diffraction study; intramolecular hydrogen bonding is present between the NH group of one ligand and an O-atom of another ligand in the dimeric structure, which stabilizes the observed geometry of the complex. Complex 2 reacts with protic solvents such as water or methanol generating aqua Ru(L)2(OH2)2 (3) or methanol complexes Ru(L)2(MeOH)2 (4), respectively, both exhibiting intramolecular H-bonded patterns with surrounding ligands at least in the solid state. These complexes react with benzyl alcohols to give aldehydes via base-free acceptorless dehydrogenation.

Activation of Dinitrogen by Polynuclear Metal Complexes

Activation of dinitrogen plays an important role in daily anthropogenic life, and the processes by which this fixation occurs have been a longstanding and significant research focus within the community. One of the major fields of dinitrogen activation research is the use of multimetallic compounds to reduce and/or activate N₂ into a more useful nitrogen-atom source, such as ammonia. Here we report a comprehensive review of multimetallic-dinitrogen complexes and their utility toward N₂ activation, beginning with the d-block metals from Group 4 to Group 11, then extending to Group 13 (which is exclusively populated by B complexes), and finally the rare-earth and actinide species. The review considers all polynuclear metal aggregates containing two or more metal centers in which dinitrogen is coordinated or activated (i.e., partial or complete cleavage of the N₂ triple bond in the observed product). Our survey includes complexes in which mononuclear N₂ complexes are used as building blocks to generate homo- or heteromultimetallic dinitrogen species, which allow one to evaluate the potential of heterometallic species for dinitrogen activation. We highlight some of the common trends throughout the periodic table, such as the differences between coordination modes as it relates to N₂ activation and potential functionalization and the effect of polarizing the bridging N₂ ligand by employing different metal ions of differing Lewis acidities. By providing this comprehensive treatment of polynuclear metal dinitrogen species, this Review aims to outline the past and provide potential future directions for continued research in this area.

Preparation of monocarbonyl ruthenium complexes bearing bidentate nitrogen and phosphine ligands and their catalytic activity in carbonyl compound reduction

A series of novel monocarbonyl ruthenium catalysts containing bidentate dinitrogen or/and diphosphine ligands are easily obtained through a general and straightforward approach.

A Ruthenium Hydrido Dinitrogen Core Conserved across Multielectron/Multiproton Changes to the Pincer Ligand Backbone

A series of ruthenium(II) hydrido dinitrogen complexes supported by pincer ligands in different formal oxidation states have been prepared and characterized. Treating a ruthenium dichloride complex supported by the pincer ligand bis(di-tert-butylphosphinoethyl)amine (H-PNP) with reductant or base generates new five-coordinate cis-hydridodinitrogen ruthenium complexes each containing different forms of the pincer ligand. Further ligand transformations provide access to the first isostructural set of complexes featuring all six different forms of the pincer ligand. The conserved cis-hydridodinitrogen structure facilitates characterization of the π-donor, π-acceptor, and/or σ-donor properties of the ligands and assessment of the impact of ligand-centered multielectron/multiproton changes on N₂ activation. Crystallographic studies, infrared spectroscopy, and ¹⁵N NMR spectroscopy indicate that N₂ remains weakly activated in all cases, providing insight into the donor properties of the different pincer ligand states. Ramifications on applications of (pincer)Ru species in catalysis are considered.

Efficient acceptorless dehydrogenation of secondary alcohols to ketones mediated by a PNN-Ru(ii) catalyst

The ruthenium(ii) complex, [fac-PN_HN]RuCl₂(PPh₃) (C), in combination with t-BuOK proved an effective and versatile catalyst allowing aromatic-, aliphatic- and cycloalkyl-containing alcohols to be efficiently converted to their corresponding ketones with high values of TON achievable.

Dehydrogenation of benzyl alcohol with N2O as the hydrogen acceptor catalyzed by the rhodium(i) carbene complex: insights from quantum chemistry calculations

The detailed mechanisms of the dehydrogenation of benzyl alcohol with N₂O as the hydrogen acceptor catalyzed by the rhodium(i) carbene complex for the formation of the corresponding carboxylic acid or ester have been investigated via density functional theory (DFT) calculations at the M06 level of theory. Three cycles were considered for the formation of benzaldehyde, benzyl benzoate and benzoic acid. On the basis of the calculations, the rate-determining step for these three cycles is involved in N₂O activation by the rhodium ammine hydride complex with an activation barrier of only 22.6 kcal mol^-1, which is different from the previous mechanism proposed by Gianetti and co-workers, where the hydride is transferred from the Rh atom to the oxygen atom of N₂O with a barrier of 30.5 kcal mol^-1. In addition, the calculations also demonstrated that one more N₂O is necessary to give benzoic acid, and the reaction can only take place under anhydrous conditions. Present calculations are in good agreement with the experimental observations and provide new insights into the dehydrogenation of benzyl alcohol with N₂O as the hydrogen acceptor.

Synthesis and coordination chemistry of tridentate (PNN) amine enamido phosphine ligands with ruthenium

Tridentate amine-imine-phosphine ligands, R2PC5H7NC2H4NEt2 [(R)PNN(H)], where R = Pr(i) or Bu(t) are synthesized using a straightforward protocol of condensation, deprotonation, and addition of a chlorodialkylphosphine. Multinuclear NMR spectroscopy shows the ligands exist exclusively in the enamine tautomeric form in solution. Treating these ligands with RuHCl(PPr(i)3)2(CO) forms the desired coordination compounds, RuHCl[(R)PNN(H)](CO), where the imine tautomeric form of the ligands coordinates to ruthenium. Deuterium labelling experiments show Ru-H/N-D scrambling occurs during ligand coordination. Treating the RuHCl[(R)PNN(H)](CO) precursors with potassium tert-butoxide allows for the synthesis of two new ruthenium enamido-phosphine complexes, RuH[(R)PNN](CO), which were fully characterized. The structure of one of the derivatives was confirmed by X-ray crystallography (R = Pr(i)). The reactivity of the enamido-phosphine complexes with H2 and benzyl alcohol is also reported. For the enamido phosphine complex where R = Pr(i), the reaction with H2 is reversible and forms (RuH(CO)[(Pri)PNN(H)])2(μ-H)2, a hydride-bridged dimer that results from cooperative activation of H2. The reactivity of both amine-enamido-phosphine ruthenium compounds with benzyl alcohol establishes that the complexes are catalyst precursors for acceptorless dehydrogenation (AD), although the turnover frequencies measured using both catalyst precursors are modest.

Non-symmetric pincer ligands: complexes and applications in catalysis

Pincer ligands have become ubiquitous in organometallic chemistry and homogeneous catalysis. Recently, new varieties of pincer ligands with non-symmetrical backbones and/or ligating groups have been reported and their application in transition metal complexes has been exploited in a variety of catalytic transformations. This non-symmetric approach vastly increases the structural and electronic diversity of this class of ligand. This approach has proven beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, which can dissociate and thereby create a vacant coordination site to increase the catalyst activity. Additionally, this provides further access to chiral ligands and complexes for asymmetric induction. This perspective highlights recent, important examples of non-symmetric pincer ligands, which feature aryl or pyridine backbones, and the synthesis and use of subsequent complexes in catalytic transformations, and discusses the future potential of this type of ligand system.

Ruthenium(II) Complexes Containing Lutidine-Derived Pincer CNC Ligands: Synthesis, Structure, and Catalytic Hydrogenation of C-N bonds

A series of Ru complexes containing lutidine-derived pincer CNC ligands have been prepared by transmetalation with the corresponding silver-carbene derivatives. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N-heterocyclic carbene fragments. In the presence of tBuOK, the Ru-CNC complexes are active in the hydrogenation of a series of imines. In addition, these complexes catalyze the reversible hydrogenation of phenantridine. Detailed NMR spectroscopic studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand-assisted activation of dihydrogen. More interestingly, upon deprotonation, the Ru-CNC complex 5 e(BF4 ) is able to add aldimines to the metal-ligand framework to yield an amido complex. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer-sphere stepwise hydrogen transfer to the C-N bond assisted either by the pincer ligand or a second coordinated H2 molecule.

An efficient N-heterocyclic carbene-ruthenium complex: application towards the synthesis of polyesters and polyamides

The ruthenium benzimidazolylidene-based N-heterocyclic carbene (NHC) complex 4 catalyzes the direct dehydrogenative condensation of primary alcohols into esters and primary alcohols in the presence of amines to the corresponding amides in high yields. This efficient new catalytic system shows a high selectivity towards the conversion of diols to polyesters and of a mixture of diols and diamines to polyamides. The only side product formed in this reaction is molecular hydrogen. Remarkable is the conversion of hydroxytelechelic polytetrahydrofuran (Mn = 1000 g mol(-1))--a polydispers starting material--into a hydrolytically degradable polyether with ester linkages (Mn = 32 600 g mol(-1)) and, in the presence of aliphatic diamines, into a polyether with amide linkages in the back bone (Mn = 16 000 g mol(-1) ).

Nitrogen enriched mesoporous organic polymer anchored copper(ii) material: an efficient and reusable catalyst for the synthesis of esters and amides from aromatic systems

New mesoporous polymer anchored copper acetate (Cu-mPMF) has been synthesized and well characterized. The catalytic performance of this complex has been tested for the synthesis of esters and amides.

Oxidant free dehydrogenation of alcohols using chitosan/polyacrylamide entrapped Ag nanoparticles

Silver nanoparticles encaged in nanoporous chitosan/polyacrylamide interpenetrating polymer network (Ag@IPN) were synthesized, characterized and used for oxidant free dehydrogenation of alcohols.

Direct oxidative esterification of alcohols and hydration of nitriles catalyzed by a reusable silver nanoparticle grafted onto mesoporous polymelamine formaldehyde (AgNPs@mPMF)

Oxidative esterification and hydration of nitriles are catalyzed by a newly synthesized AgNPs@mPMF. The catalyst is well characterized and reusable.