Pincer Complexes Research Articles

Reactivity of a Strictly T-Shaped Phosphine Ligated by an Acridane Derived NNN Pincer Ligand.

The steric tuning of a tridentate acridane-derived NNN pincer ligand allows for the isolation of a strictly T-shaped phosphine that exhibits ambiphilic reactivity. Well-defined phosphorus-centered reactivity towards nucleophiles and electrophiles is reported, contrasting with prior reports on this class of compounds. Reactions towards oxidants are also described. The latter result in the two-electron oxidation of the phosphorus atom from +III to +V and are accompanied by a strong geometric distortion of the NNN pincer ligand. By contrast, cooperative activation of E-H (HCl, HBcat, HOMe) bonds proceeds with retention of the phosphorus redox state. When using H2 O as a substrate, the reaction results in the full disassembly of H2 O to its constituent atoms, highlighting the potential of this platform for small molecule activation reactions.

Synthesis, Structure, and Photophysical Properties of Platinum(II) (N,C,N') Pincer Complexes Derived from Purine Nucleobases†.

The synthesis of a series of Pt{κ3-N,C,N'-[L]}X (X = Cl, RC≡C) pincer complexes derived from purine and purine nucleosides is reported. In these complexes, the 6-phenylpurine skeleton provides the N,C-cyclometalated fragment, whereas an amine, imine, or pyridine substituent of the phenyl ring supplies the additional N'-coordination point to the pincer complex. The purine N,C-fragment has two coordination positions with the metal (N1 and N7), but the formation of the platinum complexes is totally regioselective. Coordination through the N7 position leads to the thermodynamically favored [6.5]-Pt{κ3-N7,C,N'-[L]}X complexes. However, the coordination through the N1 position is preferred by the amino derivatives, leading to the isomeric kinetic [5.5]-Pt{κ3-N1,C,N'-[L]}X complexes. Extension of the reported methodology to complexes having both pincer and acetylide ligands derived from nucleosides allows the preparation of novel heteroleptic bis-nucleoside compounds that could be regarded as organometallic models of Pt-induced interstrand cross-link. Complexes having amine or pyridine arms are green phosphorescence emitters upon photoexcitation at low concentrations in CH2Cl2 solution and in poly(methyl methacrylate) (PMMA) films. They undergo self-quenching at high concentrations due to molecular aggregation. The presence of intermolecular π-π stacking and weak Pt···Pt interactions was also observed in the solid state by X-ray diffraction analysis.

Pitfalls for POCOP-Type Palladium Pincer Complexes in Catalytic Reduction of CO2 with Catecholborane

Reduction of CO2 with catecholborane (HBcat) is known to be catalyzed by nickel hydride complexes supported by a bis(phosphinite)-based or POCOP-type pincer ligand. The objective of this research is to examine how changing the metal center from nickel to palladium would impact the outcome of CO2 reduction. Stoichiometric studies show that the palladium hydride complexes react rapidly with CO2, although the resulting palladium formate complexes can lose CO2 under reduced pressure or in chloroform. In the presence of HBcat, the formate complexes are converted back to the palladium hydride species while the formate group is reduced to CH3OBcat. This process is also accompanied by the formation of palladium bis(catecholato)borate complexes, which diverts some of the HBcat to yield B2(cat)3 and B2H6. For palladium hydrides stabilized by a POCOP-pincer ligand with relatively small phosphorus substituents (e.g., isopropyl or cyclopentyl groups), HBcat cleaves the P–O bonds in the ligand backbone to degrade the catalysts to secondary phosphine complexes. Without CO2, HBcat also undergoes H2 elimination with the Pd–H moiety to form palladium boryl complexes. These side reactions with HBcat and the palladium pincer complexes play profound roles in the catalytic reduction of CO2 with the borane.

The Role of Non-Covalent Interactions in the Reactions between Palladium Hydrido Complex with Amidoarylphosphine Pincer Ligand and Brønsted Acids

The interaction between (PNP)PdH (1); PNP = bis(2-diisopropylphosphino-4-methylphenyl)amide and different acids (CF3SO3H, HBF4∙Et2O, fluorinated alcohols and formic acid) was studied in benzene or toluene as well as in neat alcohols by IR and NMR spectroscopies. The structures of hydrogen-bonded complexes were also optimized at the DFT/ωB97-XD/def2-TZVP level. The nitrogen atom of the amidophosphine pincer ligand readily accepts proton not only from strong Brønsted acids but from relatively weak fluorinated alcohols. That suggests that binding to palladium(II) increases the diarylamine basicity, making it a strong base. Nevertheless, H+ can be taken from [(PN(H)P)PdH]+ (2) by pyridine or hexamethylphosphoramide (HMPA). These observations confirm the need for a shuttle base to form [(PN(H)P)PdH]+ (2) as the result of the heterolytic splitting of H2 by [(PNP)Pd]+. At that, a stoichiometric amount of formic acid protonates a hydride ligand yielding an unstable η2-H2 complex that rapidly converts into formate (PNP)Pd(OCHO), which loses CO2 to restore (PNP)PdH, whereas the relatively high acid excess hampers this reaction through competitive protonation at nitrogen atom.

Metal Identity Effects in the Pincer Complex-Catalyzed Dehydrogenative Coupling of Formamides with Alcohols to Form Carbamates

This manuscript describes the impact of metal (ruthenium versus iron versus manganese) on the PNP pincer complex-catalyzed dehydrogenative coupling of formamides with alcohols to yield carbamates. Studies are conducted to compare the structure and reactivity of key organometallic intermediates as a function of metal. These reveal that all three pincer catalysts undergo fast reaction with isocyanates (the initial organic products of formamide dehydrogenation) to form off-cycle metallacyclic species. However, the structures and reactivities of these intermediates vary as a function of metal. The N-bound isomer of the metallacycle is formed with Fe (literature) and Ru (this work), while both O-bound (kinetic) and N-bound (thermodynamic) metallacycles are observed with Mn (this work). At 100 °C, the O-bound Mn cycloadduct reacts rapidly with cyclohexanol to release the pincer catalyst along with the corresponding carbamate. In contrast, all three of the N-bound metallacycles show dramatically lower reactivity toward catalyst regeneration under analogous conditions. The reactivity of the pincer complexes toward formamide dehydrogenation also varies as a function of metal, with Ru and Fe ≫ Mn at 100 °C. Overall, the Mn pincer complex is an effective catalyst for the conversion of a variety of formamides and alcohols to carbamates at 150 °C with catalyst loadings as low as 1 mol %. The catalyst performance and substrate scope are comparable to those of the Fe analogue.

Pincer Ligand Supported Fe Complex Catalysed Dinitrogen Reduction to NH3 Using FLP‐H2 as the Hydrogen Source: DFT Studies

AbstractSynthesising ammonia at low temperature and catalyst with high stability, selectivity and activity other than Haber Bosch process and enzymatic process is still a challenging task for chemists. Computational studies serve as a valuable tool for testing novel ideas in order to either realise a theoretically viable and practicably synthesizable catalyst or to identify more of the recognised catalysts both of which are important for the growth of this field. In this report, DFT calculations have been performed on a hypothetical Fe‐ imidazolyl‐pyridine complex to explore the potential of dinitrogen reduction to ammonia at optimum temperature and pressure using Fe bound Nitrogen as a catalyst and FLP‐H2 as a co‐catalyst. FLP acts as a hydrogen source which prevents the binding of H2 with metal centre. The computed Gibbs free energies of the elementary reactions revealed that the Fe bound nitrogen can act as a catalyst for activating and functionalising dinitrogen to yield NH3 and N2H2.

DFT Mechanistic Investigation on Manganese Pincer Complex Catalysed Cross-Coupling of Methanol with Benzyl Alcohol to Afford Methyl Benzoate.

In this paper the density functional theory (DFT) method was employed to investigate the cross-coupling of methanol with benzyl alcohol to afford methyl benzoate, catalysed by Mn-PNN pincer complex. The whole reaction process mainly includes three stages: the dehydrogenation of benzyl alcohol to benzaldehyde, the coupling of benzaldehyde with methanol to hemiacetal and the dehydrogenation of hemiacetal to methyl benzoate. The calculated results indicated that two dehydrogenation processes are influenced by two competitive mechanisms of inner and outer spheres. Dehydrogenation of benzyl alcohol to benzaldehyde is the rate-determining step of the whole reaction, with the energy barrier of 22.1 kcal/mol. In addition, the regeneration of catalyst is also extremely important. Compared with direct dehydrogenation, the dehydrogenation mode assisted by formic acid is more advantageous. This work might provide theoretical insights and shed light on the design of cheap transition-metal catalysts for the dehydrogenation reaction.

Mechanistic Study and Conceptual Chemical Reactivity Analysis of Hydroboration of Carbon Dioxide Catalyzed by a Manganese(I)-PNP-Pincer Complex.

Designing efficient and selective catalysts for carbon dioxide reduction is an intensive research area in the recent literature on homogeneous catalysis. In this work, we study the catalytic activity of a newly reported Mn(I)-PNP-pincer catalyst with an embedded aromatic ring. First, we systematically examine its capability to yield different products and highlight the importance of ligand aromaticity and steric effects on metal-ligand cooperativity. We then further conceptually probe its reactivity with descriptors from both conceptual density functional theory and an information-theoretic approach, thereby proposing a novel partitioning of the reaction coordinate into three relevant regions. Our results show that the reactivity in these different regions is governed by different properties such as steric effects, electrophilicity/nucleophilicity, or aromaticity. We anticipate that this methodology, with the analytical tools employed in this study, can be generalized and extended to other catalytic systems and find applications in designing better catalysts.

Oxidative cross dehydrogenative coupling directed carbamates synthesis using Cu(II) pincer complex as active catalyst under mild reaction condition

AbstractDevelopment of an air‐stable and cheap transition metal‐based catalyst for the formation of carbamates via the cross‐coupling reaction of various 2‐carbonyl substituted phenolic derivatives or 1,3‐diketo derivatives with N,N‐disubstituted formamides (C(sp2)O bond formation) under mild conditions is the key focus of synthetic chemists. In this context, we have developed a novel benzothiazole appended 2,6‐di picolinamide‐based Cu(II) complex for the facile generation of carbamates at a slightly elevated temperature (50°C) with a short reaction time and low catalyst loading (0.5 mol%) for the first time. The synthesized complex was well characterized by ultraviolet–visible, infrared, electron paramagnetic resonance spectroscopy, cyclic voltammetry, high‐resolution mass spectrometry (HRMS), as well as by single crystal X‐ray diffraction analysis. A wide range of aromatic carbamates was synthesized using complex 1 in good/excellent yield with decent functional group tolerance under mild conditions. Control experiments suggest that the reaction proceeds via the radical mechanism, and the formation of radical‐mediated intermediates was confirmed by the gas chromatography (GC), GCMS and HRMS. The use of low cost and environmentally benign catalysts in mild conditions along with the tolerance of a wide range of functional groups makes this an easy synthetic approach for the development of carbamates.

Ruthenium (II) Complexes of CNC Pincers and Bipyridine in the Photocatalytic CO2 Reduction Reaction to CO Using Visible Light: Catalysis, Kinetics, and Computational Insights

A series of five ruthenium (II) complexes containing a tridentate CNC pincer ligand, a bidentate 2,2′-bipyridine (bpy) ligand, and a monodentate ligand (chloride, bromide, or acetonitrile) were synthesized. The CNC pincer ligands used imidazole or benzimidazole-derived N-heterocyclic carbenes (NHCs) as the C donors and a 4-methoxy-substituted central pyridyl ring as the N donor. All of the complexes were characterized by analytical, spectroscopic, and single-crystal X-ray diffraction methods. These complexes were used as catalysts for visible-light-driven CO2 reduction in the presence and absence of an external photosensitizer (PS). Notably, complex 4C with a benzimidazole-derived CNC pincer ligand and bromide as the monodentate ligand was the most active catalyst tested for both sensitized and self-sensitized CO2 reduction. Thus, this catalyst was the subject of further mechanistic studies using transient absorption spectroscopy (TAS), absorption spectroelectrochemistry (SEC), and computational studies. A mechanism has been proposed for self-sensitized CO2 reduction involving (1) light excitation of the catalyst, (2) reduction by sacrificial donors, (3) halide loss, and (4) CO2 binding to form [RuCO2]+ as the catalyst resting state. The timeline for these steps and the structures of key intermediates are all supported by experimental observations (including TAS and SEC) and supporting computational studies. Subsequent steps in the cycle past [RuCO2]+ were not experimentally observable, but they are supported by computations. Experiments were also used to explain the differences observed for sensitized catalysis. Catalyst 4C is an unusually active catalyst for both sensitized and self-sensitized CO2 reduction, and thus being able to understand how it functions and which steps are turnover-limiting is an important development facilitating the design of commercially viable catalysts for solar fuel formation.

Designing New Magnesium Pincer Complexes for Catalytic Hydrogenation of Imines and N-Heteroarenes: H2 and N-H Activation by Metal-Ligand Cooperation as Key Steps.

Utilization of main-group metals as alternatives to transition metals in homogeneous catalysis has become a hot research area in recent years. However, their application in catalytic hydrogenation is less common due to the difficulty in heterolytic cleavage of the H-H bond. Employing aromatization/de-aromatization metal-ligand cooperation (MLC) highly enhances the H2 activation process, offering an efficient approach for the hydrogenation of unsaturated molecules catalyzed by main-group metals. Herein, we report a series of new magnesium pincer complexes prepared using PNNH-type pincer ligands. The complexes were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H2 and N-H bonds by MLC employing these pincer complexes was developed. Using the new magnesium complexes, homogeneously catalyzed hydrogenation of aldimines and ketimines was achieved, affording secondary amines in excellent yields. Control experiments and DFT studies reveal that a pathway involving MLC is favorable for the hydrogenation reactions. Moreover, the efficient catalysis was extended to the selective hydrogenation of quinolines and other N-heteroarenes, presenting the first example of hydrogenation of N-heteroarenes homogeneously catalyzed by early main-group metal complexes. This study provides a new strategy for hydrogenation of C═N bonds catalyzed by magnesium compounds and enriches the research of main-group metal catalysis.

Catalytic production of ammonia from dinitrogen employing molybdenum complexes bearing N-heterocyclic carbene-based PCP-type pincer ligands

Mechanistic insight into the catalytic production of ammonia from dinitrogen is needed to improve the synthesis of this vital molecule. Here we study the use of samarium diiodide (SmI2) and water in the presence of molybdenum complexes that bear PCP-type pincer ligands to synthesize ammonia. The proton-coupled electron transfer during the formation of a N–H bond on the molybdenum imide complex was found to be the rate-determining step at high catalyst concentrations. Additionally, the dimerization step of the catalyst became the rate-determining step at low catalyst concentrations. We designed PCP-type pincer ligands with various substituents at the 5- and 6-positions and observed that electron-withdrawing groups promoted the reaction rate, as predicted by density functional theory calculations. A molybdenum trichloride complex that bears a trifluoromethyl group functioned as the most effective catalyst and produced up to 60,000 equiv. ammonia based on the molybdenum atom of the catalyst, with a molybdenum turnover frequency of up to 800 equiv. min−1. The findings reported here can contribute to the development of an environmentally friendly next-generation nitrogen-fixation system.

Manganese(I)-Pincer Catalyzed α-Alkylation of Sulfones by Alcohols

Sulfones are building blocks in organic synthesis and pharmaceutical chemistry. Direct α-alkylation of sulfones using alcohols with water as the sole byproduct is an efficient and promising method for their structural modifications. However, competitive reactions always exist in this transformation that influences the reaction selectivity. For example, in addition to the desired α-alkylation, Julia-type olefination and α-alkenylation are frequently observed, especially when benzyl alcohol derivatives were used as alkylation reagents. Herein, we report α-alkylation of sulfones using alcohols catalyzed by a pincer complex of earth-abundant manganese. The Mn-PNN-bipyridyl complex exhibited good performance in reaction selectivity. The amount of base also had an important influence on the smooth transformation. Aromatic and aliphatic alcohols are suitable alkylation reagents for sulfones at low metal catalyst loading (0.5 mol %), highlighting the practicality of the developed system.

C-H versus C-O Addition: A DFT Study of the Catalytic Cleavage of the β-O-4 Ether Linkage in Lignin by Iridium and Cobalt Pincer Complexes

The potential energy surfaces of the reactions involved in the catalytic cleavage of 2-phenoxy-1-phenylethanol, a model of the β-O-4 linkage in lignin, by (iprPCP)-Ir, (iprPCOP)-Ir, (iprPCP)-Co and (iprPCOP)-Co complexes have been studied using the M06/6-311G**/LANL2TZ level of theory. Both iridium and cobalt are found to be active towards the cleavage of the β-O-4 linkage, with rate constants of 44.7 s−1 and 5.1 × 106 s−1, respectively. The iridium catalysts prefer the ‘initial C-H addition’ pathway, showing a kinetic preference of 16.8 kcal mol−1 over the ‘direct C-O insertion’ pathway, while the cobalt catalysts prefer the ‘direct C-O insertion’ route which is kinetically favored by 15.7 kcal mol−1 over the ‘initial C-H addition’ pathway. A two-state reactivity occurs along the preferred pathway for the cobalt-catalyzed reaction.

Unexplored Facet of Pincer Ligands: Super-Reductant Behavior Applied to Transition-Metal-Free Catalysis.

Pincer ligands are well-established supporting ancillaries to afford robust coordination to metals across the periodic table. Despite their widespread use in developing homogeneous catalysts, the redox noninnocence of the ligand backbone is less utilized in steering catalytic transformations. This report showcases a trianionic, symmetric NNN-pincer to drive C-C cross-coupling reactions and heterocycle formation via C-H functionalization, without any coordination to transition metals. The starting substrates are aryl chlorides that can tease the limit of a catalyst's ability to promote a reductive cleavage at a much demanding potential of -2.90 V vs SCE. The reducing power of the simple trianionic ligand backbone has been tremendously amplified by shining visible light on it. The catalyst's success relies on its easy access to the one-electron oxidized iminosemiquinonate form that has been thoroughly characterized by X-band electron paramagnetic resonance spectroscopy through spectroelectrochemical experiments. The moderately long-lived excited-state lifetime (10.2 ns) and such a super-reductive ability dependent on the one-electron redox shuttle between the bisamido and iminosemiquinonato forms make this catalysis effective.

Aromatic and olefinic C-H alkenylation by catalysis with spirocyclic NHC Ru(IV) pincer complex

Aromatic and olefinic C-H alkenylation by catalysis with spirocyclic NHC Ru(IV) pincer complex

Synthesis and characterization of cobalt (II) pincer complexes and their application as dyes in dye-sensitized solar cells

A series of eight cobalt(II) pincer complexes were prepared by using NNN tridentate and NN bidentate pincer ligand systems. All the ligands and complexes were completely characterized using various spectroscopic techniques and analytical methods including single crystal X-ray analysis. Optimization of the molecular structures of all the compounds was done by B3LYP computational method and the HOMO-LUMO energy gaps of the compounds were calculated. The optical band-gaps of the cobalt complexes were determined by using electronic spectral data and the corresponding electrochemical gaps were obtained from cyclic voltammetry studies. All the cobalt (II) complexes were used as dyes in the dye-sensitized solar cell and the photovoltaic parameters of the constructed solar cells were investigated. Complex C7, comprising of bis(benzimidazole) system with an ethylene backbone exhibited highest power conversion efficiency among the tested compounds.

Exploring the Effect of Pincer Rigidity on Oxidative Addition Reactions with Cobalt(I) Complexes.

Cobalt complexes containing the 2,6-diaminopyridine-substituted PNP pincer (iPrPNMeNP = 2,6-(iPr2PNMe)2(C5H3N)) were synthesized. A combination of solid-state structures and investigation of the cobalt(I)/(II) redox potential established a relatively rigid and electron-donating chelating ligand as compared to iPrPNP (iPrPNP = 2,6-(iPr2PCH2)2(C5H3N)). Based on a buried volume analysis, the two pincer ligands are sterically indistinguishable. Nearly planar, diamagnetic, four-coordinate complexes were observed independent of the field strength (chloride, alkyl, aryl) of the fourth ligand completing the coordination sphere of the metal. Computational studies supported a higher barrier for C-H oxidative addition, largely a result of the increased rigidity of the pincer. The increased oxidative addition barrier resulted in stabilization of (iPrPNMeNP)Co(I) complexes, enabling the characterization of the cobalt boryl and the cobalt hydride dimer by X-ray crystallography. Moreover, (iPrPNMeNP)CoMe served as an efficient precatalyst for alkene hydroboration likely because of the reduced propensity to undergo oxidative addition, demonstrating that reactivity and catalytic performance can be tuned by rigidity of pincer ligands.

Kinetic of Synthesis Pincer Ligand Based on Thiol and Phosphorus: Thermodynamic Their Complexes

Kinetic of Synthesis Pincer Ligand Based on Thiol and Phosphorus: Thermodynamic Their Complexes

Improved Trianionic Pincer Ligand Synthesis for Cyclic Polymer Catalysts

Improved Trianionic Pincer Ligand Synthesis for Cyclic Polymer Catalysts