Pincer Catalyst Research Articles

A High-Valent Ru-PCP Pincer Catalyst for Hydrogenation of Carbonyl and Carboxyl Compounds under Molecular Hydrogen.

Low-valent metals traditionally dominate the domain of catalytic hydrogenation. However, metal-ligand cooperating (MLC) catalytic systems, operating through heterolytic H-H bond splitting by a Lewis acidic metal and a basic ligand site, do not require an electron-rich metal. On the contrary, high-valent metals that induce weaker back donation facilitate heterolytic bond activation. Here we report, for the first time, the efficient hydrogenation of carbonyl and carboxyl compounds under molecular hydrogen catalyzed by a structurally well-defined RuIV catalyst bearing a bifunctional PCP pincer ligand. The catalyst exhibits reactivity toward molecular hydrogen superior to that of the low-valent analog and allows hydrogen activation even at room temperature.

Basic Promotors Impact Thermodynamics and Catalyst Speciation in Homogeneous Carbonyl Hydrogenation.

Homogeneously catalyzed reactions often make use of additives and promotors that affect reactivity patterns and improve catalytic performance. While the role of reaction promotors is often discussed in view of their chemical reactivity, we demonstrate that they can be involved in catalysis indirectly. In particular, we demonstrate that promotors can adjust the thermodynamics of key transformations in homogeneous hydrogenation catalysis and enable reactions that would be unfavorable otherwise. We identified this phenomenon in a set of well-established and new Mn pincer catalysts that suffer from persistent product inhibition in ester hydrogenation. Although alkoxide base additives do not directly participate in inhibitory transformations, they can affect the equilibrium constants of these processes. Experimentally, we confirm that by varying the base promotor concentration one can control catalyst speciation and inflict substantial changes to the standard free energies of the key steps in the catalytic cycle. Despite the fact that the latter are universally assumed to be constant, we demonstrate that reaction thermodynamics and catalyst state are subject to external control. These results suggest that reaction promotors can be viewed as an integral component of the reaction medium, on its own capable of improving the catalytic performance and reshaping the seemingly rigid thermodynamic landscape of the catalytic transformation.

Iridium‐Catalyzed C(sp3)−H Borylation Using Silyl‐Bipyridine Pincer Ligands

AbstractNew ligands for the iridium‐catalyzed C(sp3)−H borylation of aliphatic compounds were established. In sharp contrast to 6‐methyl‐2,2′‐bipyridine and 6‐isobutyl‐2,2′‐bipyridine, 2,2′‐bipyridine and 1,10‐phenanthroline derivatives bearing a hydrosilylmethyl group (which would give a thermally stable NNSi pincer complex) served as suitable ligands for the reaction. Among them, a phenanthroline‐based NNSi pincer ligand was shown to be an excellent ligand, and various aliphatic compounds were efficiently converted to the corresponding borylated products using the Ir/NNSi pincer catalyst system. The NNSi pincer ligand showed unique selectivity and enabled the iridium‐catalyzed C(sp3)−H borylation using pinacolborane [H−B(pin)] instead of B2(pin)2. The formation of an iridium complex bearing a quinoline‐based NNSi pincer ligand from [IrCl(cod)]2 was observed, and the catalytic activity of the complex was demonstrated.

Iridium-Catalyzed C(sp3 )-H Borylation Using Silyl-Bipyridine Pincer Ligands.

New ligands for the iridium-catalyzed C(sp3 )-H borylation of aliphatic compounds were established. In sharp contrast to 6-methyl-2,2'-bipyridine and 6-isobutyl-2,2'-bipyridine, 2,2'-bipyridine and 1,10-phenanthroline derivatives bearing a hydrosilylmethyl group (which would give a thermally stable NNSi pincer complex) served as suitable ligands for the reaction. Among them, a phenanthroline-based NNSi pincer ligand was shown to be an excellent ligand, and various aliphatic compounds were efficiently converted to the corresponding borylated products using the Ir/NNSi pincer catalyst system. The NNSi pincer ligand showed unique selectivity and enabled the iridium-catalyzed C(sp3 )-H borylation using pinacolborane [H-B(pin)] instead of B2 (pin)2 . The formation of an iridium complex bearing a quinoline-based NNSi pincer ligand from [IrCl(cod)]2 was observed, and the catalytic activity of the complex was demonstrated.

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.

Hydration of Nitriles Enabled by PNP−manganese Pincer Catalyst

AbstractA general and practical methodology for the hydration of nitriles to primary amides enabled by manganese catalyst is presented. The described protocol shows broad substrate scope with good functional group tolerance, including a wide range of (hetero)aromatic and aliphatic nitriles, thus afforded the corresponding amides in good to excellent isolated yields under mild conditions. Preliminary mechanistic studies indicated that metal‐ligand cooperation (MLC) mode was involved in this catalytic process.

Palladium-Catalyzed Domino Cycloisomerization/Double Condensation of Acetylenic Acids with Dinucleophiles

Metal-catalyzed cascade processes based on hydrofunctionalization of alkynes are receiving much more attention because of their potential to provide advantageous approaches to otherwise synthetically challenging compounds. An alternative catalyst system has been found for the domino cycloisomerization/cyclocondensation reaction involving acetylenic acids and heterodinucleophiles. A CNN pincer palladium(II) complex, acting as a homogeneous catalyst, provides the corresponding polyheterocycles with a higher substrate/catalyst ratio. Other palladium sources were also tested and discarded, and a number of mechanistic studies including poisoning assays, kinetic plots, TEM images, XRD spectra and UPLC-MS analysis of reaction intermediates were conducted in order to shed light on the role of this pincer catalyst and the catalytic cycle involved in the cascade reaction. As a result, a more nuanced mechanism is tentatively proposed.

Catalytic Furfural/5-Hydroxymethyl Furfural Oxidation to Furoic Acid/Furan-2,5-dicarboxylic Acid with H2 Production Using Alkaline Water as the Formal Oxidant.

Furfural and 5-hydroxymethyl furfural (HMF) are abundantly available biomass-derived renewable chemical feedstocks, and their oxidation to furoic acid and furan-2,5-dicarboxylic acid (FDCA), respectively, is a research area with huge prospective applications in food, cosmetics, optics, and renewable polymer industries. Water-based oxidation of furfural/HMF is a lucrative approach for simultaneous generation of H2 and furoic acid/FDCA. However, this process is currently limited to (photo)electrochemical methods that can be challenging to control, improve, and scale up. Herein, we report well-defined ruthenium pincer catalysts for direct homogeneous oxidation of furfural/HMF to furoic acid/FDCA, using alkaline water as the formal oxidant while producing pure H2 as the reaction byproduct. Mechanistic studies indicate that the ruthenium complex not only catalyzes the aqueous oxidation but also actively suppresses background decomposition by facilitating initial Tishchenko coupling of substrates, which is crucial for reaction selectivity. With further improvement, this process can be used in scaled-up facilities for a simultaneous renewable building block and fuel production.

Two step activation of Ru-PN3P pincer catalysts for CO2 hydrogenation

Activation of homogeneous catalysts is crucial for their operation. We describe the consecutive double activation of Ru pincer catalyst required to trigger its catalytic activity.

Room temperature chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds by using a well-defined mixed donor Mn(i) pincer catalyst.

Chemoselective hydrogenation of C[double bond, length as m-dash]C, C[double bond, length as m-dash]O and C[double bond, length as m-dash]N bonds in α,β-unsaturated ketones, aldehydes and imines is accomplished at room temperature (27 °C) using a well-defined Mn(i) catalyst and 5.0 bar H2. Amongst the three mixed-donor Mn(i) complexes developed, κ3-(R2PN3NPyz)Mn(CO)2Br (R = Ph, iPr, t Bu); the t Bu-substituted complex ( tBu2PN3NPyz)Mn(CO)2Br shows exceptional chemoselective catalytic reduction of unsaturated bonds. This hydrogenation protocol tolerates a range of highly susceptible functionalities, such as halides (-F, -Cl, -Br, and -I), alkoxy and hydroxy, including hydrogen-sensitive moieties like acetyl, nitrile, nitro, epoxide, and unconjugated alkenyl and alkynyl groups. Additionally, the disclosed method applies to indole, pyrrole, furan, thiophene, and pyridine-containing unsaturated ketones leading to the corresponding saturated ketones. The C[double bond, length as m-dash]C bond is chemoselectively hydrogenated in α,β-unsaturated ketones, while the aldehyde's C[double bond, length as m-dash]O bond and imine's C[double bond, length as m-dash]N bond are preferentially reduced over the C[double bond, length as m-dash]C bond. A detailed mechanistic study highlighted the non-innocent behavior of the ligand in the ( tBu2PN3NPyz)Mn(i) complex and indicated a metal-ligand cooperative catalytic pathway. The molecular hydrogen (H2) acts as a hydride source, whereas MeOH provides a proton for hydrogenation. DFT energy calculations supported the facile progress of most catalytic steps, involving a crucial turnover-limiting H2 activation.

Correction: Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Correction for ‘Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes’ by David Decker et al., Inorg. Chem. Front., 2022, DOI: 10.1039/d1qi00998b.

Catalytic and mechanistic studies of a highly active and E-selective Co(ii) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

A Co(II) PNNH pincer catalyst system for the highly E-selective transfer-semihydrogenation of internal alkynes using ammonia borane and MeOH as the hydrogen source is described. The reaction is proposed to occur via the Co(ii) oxidation state.

Manganese N,N,N-pincer complex-catalyzed epoxidation of unactivated aliphatic olefins

Combination of quinoline with an in situ generated manganese pincer catalyst enables selective epoxidation of aliphatic olefins in good yields.

Isocyanate‐free synthesis of ureas and polyureas via ruthenium catalyzed dehydrogenation of amines and formamides

AbstractEthyl formate is very effective for the formylation of amines and diamines to yield formamides and diformamides, respectively. Subsequent dehydrogenation of 1:1 mixtures of formamides and amines or diformamides and diamines with ruthenium pincer catalysts furnishes ureas and polyureas, respectively. This approach provides access to a large structural variety of ureas and polyureas because a wide range of primary amines and diamines are readily converted to the corresponding ureas and polyureas. Small molecule ureas were isolated in yields of up to 95%, whereas polyureas were isolated in lower yields (up to 68%) due to strong adhesion to the reaction flask and poor solubility impeding product removal. Furthermore, the ease with which hydrogen can be removed from the reaction medium enables the synthesis of polyureas with molecular weights that are among the highest reported to date (Mn up to 31,100 g mol−1). Notably, this synthetic approach does not necessitate high pressure conditions, simplifying equipment needed for their preparation, and it avoids the use of isocyanates, which have significant health concerns.

Manganese(I) Catalyzed Alkenylation of Phosphine Oxides Using Alcohols with Liberation of Hydrogen and Water.

Herein, a catalytic cross-coupling of methyldiphenylphosphine oxide with arylmethyl alcohols leading to the alkenylphosphine oxides is reported. A manganese pincer catalyst catalyzes the reactions, which provides exclusive formation of trans-alkenylphosphine oxides. Mechanistic studies indicate that reactions proceed via aldehyde intermediacy and the catalyst promotes the C═C bond formation. Reactions are facilitated by dearomatization, and aromatization metal-ligand cooperation operates in catalyst. Use of abundant base metal catalyst and formation of water and H2 as the only byproducts make this catalytic protocol sustainable and environmentally benign.

Ascendancy of Nitrogen Heterocycles in the Computationally Designed Mn(I)PNN Pincer Catalysts on the Hydrogenation of Carbon Dioxide to Methanol.

The development of sustainable catalysts to get methanol from CO2 under milder conditions and without any additives is still considered an arduous task. In many instances, transition-metal-catalyzed carbon dioxide to formic acid formation is more facile than methanol formation. This article provides comprehensive density functional theoretic investigations of six new Mn(I)PNN complexes, which are designed to perform CO2 to methanol conversion under milder reaction conditions. All these six catalysts have similar structural features except at terminal nitrogen, -N (1), where adenine-inspired nitrogen heterocycles containing pyridine and pyrimidine moieties are attached to instill an electron withdrawing effect on the central metal and thus to facilitate dihydrogen polarization during the catalyst regeneration. All these computationally modeled Mn(I)PNN complexes demonstrate the promising catalytic activity to get methanol through cascade catalytic cycles at 298.15 K. The metal-ligand cooperative (MLC) as well as noncooperative (NC) pathways are investigated for each catalytic cycle. The NC pathway is the preferred pathway for formic acid and formaldehyde formation, whereas methanol formation proceeds through only the MLC pathway. Different nitrogen heterocycles attached to the -N (1) terminal manifested a considerable amount of impact on the Gibbs free energies, overall activation energies, and computed turnover frequencies (TOFs). Among all the catalysts, SPCAT02 provides excellent TOFs for HCO2H (500 151 h-1), HCHO (11 912 h-1), and CH3OH (2 372 400 h-1) formation at 50 °C. SPCAT04 is found to be a better catalyst for the selective formation of formic acid formation at room temperature than the rest of the catalysts. The computed TOF results are found reliable upon comparison with experimentally established catalysts. To establish the structure-activity relationship, the activation strain model and Fukui function calculations are performed on all the catalysts. Both these studies provide complementary results. The present study revealed a very important finding that a more electrophilic metal center could facilitate the CO2 hydrogenation reaction robustly. All computationally designed catalysts could be cheaper and better alternatives to convert CO2 to methanol under mild reaction conditions in an aqueous medium.

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.

Selective α‐Deuteration of Cinnamonitriles using D2O as Deuterium Source

AbstractThe selective α‐deuteration of α,β‐unsaturated nitriles using the strong base tBuOK or a metal‐ligand cooperative Ru pincer catalyst is described. With D2O as deuterium source and glyme as solvent at 70 °C, tBuOK is an efficient catalyst for deuteration at the α‐C(sp2) position of cinnamonitriles, providing access to a broad range of deuterated derivatives in good to excellent yields and with very high levels of deuterium incorporation. While the tBuOK‐catalysed protocol does not tolerate base‐sensitive functional groups, cinnamonitrile derivatives containing a benzylic bromide or ester moiety were deuterated in excellent yields using Milstein's ruthenium PNN pincer catalyst. Moreover, the activity for H/D exchange of the metal‐ligand cooperative Ru catalyst is found to be significantly higher than that of tBuOK, allowing reactions to proceed well even at room temperature. A mechanistic proposal is put forward that involves deprotonation of the cinnamonitrile α‐CH position when using tBuOK as catalyst, whereas H/D exchange catalysis with the Ru PNN pincer likely proceeds via (reversible) oxa‐Michael addition of D2O.magnified image

Manganese(I) catalyzed cross-coupling of secondary allylic alcohols and primary alcohols

Cross-coupling of alcohols to value-added products by using sustainable catalytic reactions has gained attention in recent years. Isomerization of secondary allylic alcohol to the corresponding enolizable ketone is an atom economical and known transformation. Herein, a selective cross-coupling of secondary allylic alcohol and primary alcohol is reported to afford the corresponding α-alkenyl or alkylation products. These catalytic protocols proceed via acceptorless dehydrogenative coupling (ADC) or borrowing hydrogen (BH) strategies, which liberates water and/or hydrogen as the only byproducts. Highly abundant manganese-based pincer catalysts catalyze the reactions.

Unlocking the Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with Superbases

The efficient synthesis of hydrosilanes by catalytic ydrogenolysis of chlorosilanes is described, using an Iridum (III) pincer catalyst. A careful selection of a nitrogen base (incl. sterically hindered guanidines and phosphazenes) can unlock the preparation of Me3SiH, Et3SiH and Me2SiHCl in high yield (up to 98%), directly from their corresponding chlorosilanes.