Metal-ligand Cooperative Mechanism Research Articles

Z-selective dimerization of terminal alkynes by a (PNNP)FeII complex.

A tetradentate bis(amido)bis(phosphine) FeII complex, (PNNP)Fe, is shown to activate the terminal C-H bond of aryl alkynes across its Fe-Namide bonds. (PNNP)Fe is also shown to catalytically dimerize terminal aryl alkynes to produce 1,3-enynes with Z : E ratios as high as 96 : 4 with yields up to 95% and loadings as low as 1 mol% at 30 °C in 2 h. A plausible metal-ligand cooperative mechanism invoking a vinylidene intermediate is proposed.

Aminophosphine PNH complexes of Mn(I), Fe(II), and Co(II) and evaluation of their activities in the transfer hydrogenation of nitriles.

A series of Mn(I), Fe(II), and Co(II) complexes with PNH ligands bearing secondary amine functionality were prepared and tested in the catalytic transfer hydrogenation of nitriles using ammonia borane as a hydrogen source. Among all tested complexes, a tetracoordinate Fe(II) bromide, (PNH)FeBr2, proved the most effective, representing a rare example of a highly active iron-based catalytic system for transfer hydrogenation reactions beyond carbonyl compounds and the first example of the iron catalyst for the transfer hydrogenation of nitriles to the corresponding primary amines. Mechanistic studies point out a metal-ligand cooperative mechanism enabled by the secondary amine moiety of the PNH ligand.

Cyclopentadienone Vanadium(I) Complex: Synthesis, Red/Ox Activity, and Application to Catalytic Transfer Hydrogenation

Cyclopentadienone late transition metal complexes have exhibited various catalytic transformation and elementary reactions by transferring electrons from the metal to ligand, the so-called metal–ligand cooperation mechanism. Herein, we report a cyclopentadienone low-valent early transition metal, a vanadium(I), complex. The obtained complex furnished the most extended C═O bond length among all of the analogous CpO metal complexes, showing a substantial degree of metal-to-ligand electron transfer. The red/ox activity of the vanadium center in the cyclopentadienone V(I) complex was confirmed upon the addition of the proton source to yield the hydroxycyclopentadienyl V(III) complex. The system was applied to catalytic transfer hydrogenation of carbonyl compounds, indicating the possible application of the red/ox of the early transition metals in metal–ligand cooperative organic transformations.

Molecular hydrogen and water activation by transition metal frustrated Lewis pairs containing ruthenium or osmium components: catalytic hydrogenation assays.

The transition metal frustrated Lewis pair compounds [(Cym)M(κ3S,P,N-HL1)][SbF6] (Cym = η6-p-MeC6H4iPr; H2L1 = N-(p-tolyl)-N'-(2-diphenylphosphanoethyl)thiourea; M = Ru (5), Os (6)) have been prepared from the corresponding dimer [{(Cym)MCl}2(μ-Cl)2] and H2L1 by successive chloride abstraction with NaSbF6 and AgSbF6 and NH deprotonation with NaHCO3. Complexes 5 and 6 and the previously reported phosphano-guanidino compounds [(Cym)M(κ3P,N,N'-HL2)][SbF6] [H2L2 = N,N'-bis(p-tolyl)-N''-(2-diphenylphosphanoethyl) guanidine; M = Ru (7), Os (8)] and pyridinyl-guanidino compounds [(Cym)M(κ3N,N',N''-HL3)][SbF6] [H2L3 = N,N'-bis(p-tolyl)-N''-(2-pyridinylmethyl) guanidine; M = Ru (9), Os (10)] heterolytically activate H2 in a reversible manner affording the hydrido complexes [(Cym)MH(H2L)][SbF6] (H2L = H2L1; M = Ru (11), Os (12); H2L = H2L2; M = Ru (13), Os (14); H2L = H2L3; M = Ru (15), Os (16)). DFT calculations carried out on the hydrogenation of complex 7 support an FLP mechanism for the process. Heating 9 and 10 in methanol yields the orthometalated complexes [(Cym)M(κ3N,N',C-H2L3-H)][SbF6] (M = Ru (17), Os (18)). The phosphano-guanidino complex 7 activates deuterated water in a reversible fashion, resulting in the gradual deuteration of the three cymene methyl protons through sequential C(sp3)-H bond activation. From DFT calculations, a metal-ligand cooperative reversible mechanism that involves the O-H bond activation and the formation of an intermediate methylene cyclohexenyl complex has been proposed. Complexes 5-10 catalyse the hydrogenation of the CC double bond of styrene and a range of acrylates, the CO bond of acetophenone and the CN bond of N-benzylideneaniline and quinoline. The CC double bond of methyl acrylate adds to catalyst 9, affording complex 19 in which a new ligand exhibiting a fac κ3N,N',C coordination mode has been incorporated.

Catalytic Mechanisms of Transfer Hydrogenation of Azobenzene with Ammonia Borane by Pincer Bismuth Complex: Crucial Role of C=N Functional Group on the Pincer Ligand.

Transfer hydrogenation of azobenzene with ammonia borane mediated by pincer bismuth complex 1 was systematically investigated through density functional theory calculations. An unusual metal-ligand cooperation mechanism was disclosed, in which the saturation/regeneration of the C=N functional group on the pincer ligand plays an essential role. The reaction is initiated by the hydrogenation of the C=N bond (saturation) with ammonia borane to afford 3CN , which is the rate-determining step with Gibbs energy barrier (ΔG≠ ) and Gibbs reaction energy (ΔG) of 25.6 and -7.3 kcal/mol, respectively. 3CN is then converted to a Bi-H intermediate through a water-bridged pathway, which is followed up with the transfer hydrogenation of azobenzene to produce the final product N,N'-diphenylhydrazine and regenerate the catalyst. Finally, the catalyst could be improved by substituting the phenyl group for the tert-butyl group on the pincer ligand, where the ΔG≠ value (rate-determining step) decreases to 24.0 kcal/mol.

Exploring the conversion mechanism of formaldehyde to CO2 and H2 catalyzed by bifunctional ruthenium catalysts: A DFT study

Bifunctional catalysts have a wide range of applications in chemical hydrogen storage. However, the versatile structure and complex influencing factors of catalysts still limit the current mechanistic understanding. Herein, a theoretical study based on density functional theory calculation is performed to illuminate the mechanistic preference of the conversion from formaldehyde to CO2 and H2 catalyzed by bifunctional ruthenium catalysts. The computational results indicate: (1) In contrast to the previously proposed mechanism, the catalyst is involved in the formaldehyde hydrolysis reaction and effectively reduces the activation free energy barrier. (2) Metal ligand cooperation mechanism is preferred instead of the metal-centred mechanism due to the stronger interaction between substrate and dual active sites on the bifunctional catalyst. (3) The catalyst with the OH group exhibits better catalytic activity compared with the NH group due to appropriate catalytic driving force, which is governed by the intrinsic electronic effects of the Lewis basic functional ligand. The pKa value can be used as a reliable descriptor to evaluate the catalytic activity of the functional ligand. Our study highlights the advantages of bifunctional catalysts in dehydrogenation-related reactions and proposes feasible strategies for the regulation of bifunctional catalyst activity, which is expected to provide new inspiration for future bifunctional catalyst design.

Unexpected role of two ortho-OH groups for the hydrogenation of CO2 to methanol catalyzed by Fe bipyridinol complexes

A density functional theory study of Fe-catalyzed hydrogenation of CO2 to methanol reveals an unexpected metal–ligand cooperation mechanism with two ortho-OH groups play key roles for the breaking of C–O and O–H bonds and formation of O–H…O hydrogen bonds. The formation of methanediol via hydride transfer from Fe to formic acid (4′ → TS6,7) is the rate-determining step with a total barrier of 30.8 kcal mol−1 in free energy. Furthermore, by replacing the two meta (R1) and two para (R2) hydrogen atoms in the bipyridinol ligand and the coordinated CN and CO with different functional groups, we built a series of stable iron complexes and computationally predicted their catalytic activities for hydrogenation of CO2 to methanol. Among all newly proposed iron complexes, 1t and 1u are promising catalyst candidates for hydrogenation of CO2 to methanol with rather low total free energy barriers of 26.6 and 28.8 kcal mol−1, respectively. The electrostatic potential map analysis indicates that stronger electron donating ligands on iron may lead to higher catalytic activities.

Well-Defined Phosphine-Free Manganese(II)-Complex-Catalyzed Synthesis of Quinolines, Pyrroles, and Pyridines.

Herein, we report a simple, phosphine-free, and inexpensive catalytic system based on a manganese(II) complex for synthesizing different important N-heterocycles such as quinolines, pyrroles, and pyridines from amino alcohols and ketones. Several control experiments, kinetic studies, and DFT calculations were carried out to support the plausible reaction mechanism. We also detected two potential intermediates in the catalytic cycle using ESI-MS analysis. Based on these studies, a metal-ligand cooperative mechanism was proposed.

Hydrogenative Catalysis with Three-Coordinate Zinc Complexes Supported with PN Ligands is Enhanced Compared to PNP Analogs.

This work details the synthesis, characterization, and catalytic activity of reactive low-coordinate organozinc complexes. The complexes activate hydrogen and they appear to be more active in hydrogenation of ketones and imines than their tridentate pincer analogs. This is thought, in part, to be due to the lack of trailing third phosphorus arm present in previous work. DFT computations reveal a sigma-bond metathesis mechanism is comparable to an alternative aromatization/dearomatization metal-ligand cooperative mechanism.

Nucleophilic Reactivity at a ═CH Arm of a Lutidine-Based CNC/Rh System: Unusual Alkyne and CO2 Activation.

Reaction of an amido pincer complex [(CNC)*Rh(CO)] (1) (CNC* is the deprotonated form of CNC) with carbon dioxide gave a neutral complex [(CNC-CO2)Mes*Rh(CO)] (2), which is the result of a C-C bond-forming reaction between the deprotonated arm of the CNC* ligand and CO2. The molecular structure of 2 showed a zwitterionic complex, where the CO2 moiety is covalently connected to the former ═CH arm of the CNC* pincer ligand. The unusual structure of 1 allowed us to explore the reactivity of the CO2 moiety with selected primary amines RNH2 (benzylamine and ammonia), which afforded cationic complexes [(CNC)MesRh(CO)][HRNC(O)O] (R = Bz (3), H (4)). Compounds 3 and 4 are the result of a C-N coupling between the incoming amine and the CO2 fragment covalently connected to the pincer ligand in 2, a process that involves protonation of the "CH-CO2" fragment in 2 from the respective amines. Once revealed the nucleophilic character of the ═CH fragment in 1, we explored its reactivity with alkynes, a study that enlightened a novel reactivity trend in alkyne activation. Reaction of 1 with terminal alkynes RC≡CH (R = Ph, 2-py, 4-C6H4-CF3) yielded neutral complexes [(CNC-CH═CHR)Mes*Rh(CO)] (R = Ph (5), 2-py (6), 4-C6H4-CF3 (7)) in good yields. Deuterium labeling experiments with PhC≡CD confirmed that complex 5 is the product of a formal insertion of the alkyne into the C(sp2)-H bond of the deprotonated arm in 1. This structural proposal was further confirmed by the X-ray molecular structure of phenyl complex 5, which showed the alkyne covalently linked to the pincer ligand. Besides, this novel transformation was analyzed by DFT methods and showed a metal-ligand cooperative mechanism, based on the initial electrophilic attack of the alkyne to the ═CH arm of the CNCMes* ligand (making a new C-C bond) followed by the action of a protic base (HN(SiMe3)2), which is able to perform a proton rearrangement that leads to the final product 5.

From hydrogen autotransfer process to deuterium autotransfer process: The N-trideuteromethylation of amines with deuterated methanol to trideuteromethylated amines catalyzed by a Cp*Ir complex bearing a flexible bridging and functional ligand

A Cp*Ir complex bearing a flexible and functional 6,6′-(OH)2-2,2′-dipyridylamine ligand was designed, synthesized and found to be a general and efficient catalyst for the synthesis of trideuteromethylated amines via the N-trideuteromethylation of amines with deuterated methanol based on the proposed deuterium autotransfer process. The synthetic strategy is attractive due to easily available deuterium source, high atom efficiency and environmental friendliness. It was confirmed that OH units in the ligand are crucial for the catalytic activity. Furthermore, a metal-ligand cooperative mechanism was also proposed based on experimental results and DFT calculations.

Acceleration Effect of Bases on Mn Pincer Complex-Catalyzed CO2 Hydroboration

Herein, we report a comprehensive study of CO2 hydroboration catalyzed by Mn pincer complexes. The traditional metal-ligand cooperation (MLC) mechanism based on the H-Mn-N-Bpin pincer complex is not viable due to the competing abstraction of the Bpin group from the H-Mn-N-Bpin complex by NaOtBu. Instead, we propose an ionic mechanism based on the H-Mn-N-Na species with a low energy span (22.5 kcal/mol) and unveil the acceleration effect of bases. The X groups in the H-Mn-N-X catalyst models are further modulated, and the steric hindrance and H→B donor-acceptor interactions of the X group increase the energy barrier of the hydride transfer. The hydrogen bond and electrostatic interactions of the X group can accelerate the hydride transfer to HCOOBpin and HCHO molecules except for the nonpolar CO2 molecule. Based on these discoveries, we designed a pyridine-based Mn pincer catalyst system, which could achieve CO2 hydroboration in low-temperature and base-free conditions through a metal-ligand cooperation mechanism.

Reductive Alkylation of Azides and Nitroarenes with Alcohols: A Selective Route to Mono- and Dialkylated Amines

Herein, we demonstrated an efficient protocol for reductive alkylation of azides/nitro compounds via a borrowing hydrogen (BH) method. By following this protocol, selective mono- and dialkylated amines were obtained under mild and solvent-free conditions. A series of control experiments and deuterium-labeling experiments were performed to understand this catalytic process. Mechanistic studies suggested that the Ir(III)-H was the active intermediate in this reaction. KIE study revealed that the breaking of the C-H bond of alcohol might be the rate-limiting step. Notably, this solvent-free strategy disclosed a high TON of around 5600. Based on kinetic studies and control experiments, a metal-ligand cooperative mechanism was proposed.

Mechanistic insight into the isomerization of allyl alcohol catalyzed by the Co(II)-PNP catalyst: Crucial role of spectator ligand

Pincer complexes based on first-row transition metals have a wide range of applications in the field of homogeneous catalysis. However, the variable coordination geometry and electronic structure bring about complicated influencing factors, which restricts the deep understanding of the catalytic mechanisms. Herein, a detailed theoretical study based on the density functional theory (DFT) was conducted to illuminate the mechanistic preference of the isomerization of allyl alcohol catalyzed by the Co(II) pincer catalyst developed by de Vries et al. (Chem. Eur. J., 25 (2019) 7820–7825). Three different coordination geometries, two spin states (doublet and quartet) and corresponding pathways were taken into consideration. The calculation results suggest that the Co(II)-PNP catalyst prefers the inner-sphere migratory insertion mechanism at doublet state instead of metal-ligand cooperation (MLC) mechanism, which is consistent with the experimental results. Although the catalyst bears the NH functional ligands, the π−basic spectator ligands lead to the changeable coordination geometries, resulting in the emergence of spin crossover process with high energy barrier. Based on these findings, we designed a series of model catalysts and evaluated their catalytic activity on a theoretical level. The results indicate that π−acidic ligands and rigid tetradentate ligands can effectively avoid the occurrence of spin crossover processes and ensure that the Co(II) catalyst follows the MLC mechanism at low spin state. In addition, the activity of the catalyst can be precisely adjusted by changing the ligand substituents electronic effect. These findings and predictions are expected to provide guidance for future catalyst design.

Carbonylic-Carbon-Centered Mechanism for Catalytic α-Methylation

The multicarbonyl manganese pincer complex-catalyzed α-methylation reaction is studied. In this theoretical work, an unprecedented carbonylic-carbon-centered mechanism is discovered. That is, the catalytic center is on the carbonyl ligand instead of the metal atom, such as the common aromatization/dearomatization metal–ligand cooperation mechanism. Further, the origins of chemoselectivity of enone hydrogenation are addressed with the $DEL-type NBO analyses. With a manganese pincer catalyst, the donor–acceptor electron transfer is found from the C═C bond to the C═O antibond, which can be regarded as the original driving force for the enone C═C hydrogenation. In contrast, the Noyori-type RuH2(diphosphine)(diamine) catalysts favor the C═O hydrogenation, where the driving force is the donor–acceptor interaction associated with the O···H–N hydrogen bond.

Reversible PtII-CH3 deuteration without methane loss: metal-ligand cooperation vs. ligand-assisted PtII-protonation.

Di(2-pyridyl)ketone dimethylplatinum(ii), (dpk)PtII(CH3)2, reacts with CD3OD at 25 °C to undergo complete deuteration of Pt–CH3 fragments in ∼5 h without loss of methane to form (dpk)PtII(CD3)2 in virtually quantitative yield. The deuteration can be reversed by dissolution in CH3OH or CD3OH. Kinetic analysis and isotope effects, together with support from density functional theory calculations indicate a metal–ligand cooperative mechanism wherein DPK enables Pt–CH3 deuteration by allowing non-rate-limiting protonation of PtII by CD3OD. In contrast, other model di(2-pyridyl) ligands enable rate-limiting protonation of PtII, resulting in non-rate-limiting C–H(D) reductive coupling. Owing to its electron-poor nature, following complete deuteration, DPK can be dissociated from the PtII-centre, furnishing [(CD3)2PtII(μ-SMe2)]2 as the perdeutero analogue of [(CH3)2PtII(μ-SMe2)]2, a commonly used PtII-precursor.

A Proton-Responsive Annulated Mesoionic Carbene (MIC) Scaffold on Ir Complex for Proton/Hydride Shuttle: An Experimental and Computational Investigation on Reductive Amination of Aldehyde

A Cp*Ir(III) complex (1) bearing a proton-responsive hydroxy unit on an annulated imidazo[1,2-a][1,8]naphthyridine based mesoionic carbene scaffold was synthesized by two different synthetic routes. The molecular structure of 1 revealed an anionic lactam form of the ligand. The acid–base equilibrium between the lactam-lactim tautomers on the ligand scaffold was examined by 1H NMR and UV–vis spectra. The pKa of the appendage −OH group in the lactim form of 1 was estimated to assess the proton transfer property of the catalyst. The catalytic efficacy of 1 for reductive amination of aldehyde was evaluated by utilizing three different hydrogen sources: molecular H2, iPrOH/KOtBu combination, and HCOOH/Et3N (5:2) azeotropic mixture. The HCOOH/Et3N (5:2) azeotropic mixture protocol was found to be the best among the three different hydrogenation methods. Catalyst 1 hydrogenates imines chemoselectively over carbonyls under the reaction conditions. A range of aldehydes was reductively aminated to the corresponding secondary amines using the HCOOH/Et3N (5:2) azeotropic mixture. Further, catalyst 1 showed high efficiency for the reduction of a wide variety of N-heterocyclic imine derivatives. The lactam-lactim tautomerization of the ligand system is proposed for direct hydrogenation, whereas only the lactam form operates in the strongly basic medium (iPrOH/KOtBu). Under HCOOH/Et3N (5:2) conditions, the lactam scaffold is not protonated; rather, an outer-sphere hydride transfer from formate to the Ir is proposed, which is supported by 1H NMR and DFT calculations. Finally, ligand-promoted hydride transfer from metal-hydride to the protonated imine affords the corresponding amine. A close agreement between the experimentally estimated and computed thermodynamic/kinetic parameters gives credence to the metal-ligand cooperative mechanism for the imine hydrogenation reaction using the HCOOH/Et3N (5:2) azeotropic mixture.

Hydrogenation or Dehydrogenation of N-Containing Heterocycles Catalyzed by a Single Manganese Complex.

A highly chemoselective base-metal catalyzed hydrogenation and acceptorless dehydrogenation of N-heterocycles is presented. A well-defined Mn complex operates at low catalyst loading (as low as 2 mol %) and under mild reaction conditions. The described catalytic system tolerates various functional groups, and the corresponding reduced heterocycles can be obtained in high yields. Experimental studies indicate a metal-ligand cooperative catalysis mechanism.

Ru0 or RuII: A Study on Stabilizing the "Activated" Form of Ru-PNP Complexes with Additional Phosphine Ligands in Alcohol Dehydrogenation and Ester Hydrogenation.

The complex Ru-MACHO has been previously shown to undergo uncontrolled degradation subsequent to base-induced dehydrochlorination in the absence of a substrate. In this study, we report that stabilization of the dehydrochlorinated Ru-MACHO with phosphines furnishes complexes whose structures depend on the phosphines employed: while PMe3 led to the expected octahedral RuII complex, PPh3 provided access to a trigonal-bipyramidal Ru0 complex. Because both complexes proved to be active in base-free (de)hydrogenation reactions, thorough quantum-chemical calculations were employed to understand the reaction mechanism. The calculations show that both complexes lead to the same mechanistic scenario after phosphine dissociation and, therefore, only differ energetically in this step. According to the calculations, the typically proposed metal-ligand cooperation mechanism is not the most viable pathway. Instead, a metal-ligand-assisted pathway is preferred. Finally, experiments show that phosphine addition enhances the catalyst's performance in comparison to the PR3-free "activated" Ru-MACHO.

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(η5-Cp⁎)(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.