Metal-ligand Cooperation Research Articles

Manganese(I)‐Catalyzed β‐Methylation of Alcohols Using Methanol as C1 Source

AbstractHighly selective β‐methylation of alcohols was achieved using an earth‐abundant first row transition metal in the air stable molecular manganese complex [Mn(CO)2Br[HN(C2H4PiPr2)2]] 1 ([HN(C2H4PiPr2)2]=MACHO‐iPr). The reaction requires only low loadings of 1 (0.5 mol %), methanolate as base and MeOH as methylation reagent as well as solvent. Various alcohols were β‐methylated with very good selectivity (>99 %) and excellent yield (up to 94 %). Biomass derived aliphatic alcohols and diols were also selectively methylated on the β‐position, opening a pathway to “biohybrid” molecules constructed entirely from non‐fossil carbon. Mechanistic studies indicate that the reaction proceeds through a borrowing hydrogen pathway involving metal–ligand cooperation at the Mn‐pincer complex. This transformation provides a convenient, economical, and environmentally benign pathway for the selective C−C bond formation with potential applications for the preparation of advanced biofuels, fine chemicals, and biologically active molecules

Manganese(I)-Catalyzed β-Methylation of Alcohols Using Methanol as C1 Source.

Highly selective β‐methylation of alcohols was achieved using an earth‐abundant first row transition metal in the air stable molecular manganese complex [Mn(CO)2Br[HN(C2H4PiPr2)2]] 1 ([HN(C2H4PiPr2)2]=MACHO‐iPr). The reaction requires only low loadings of 1 (0.5 mol %), methanolate as base and MeOH as methylation reagent as well as solvent. Various alcohols were β‐methylated with very good selectivity (>99 %) and excellent yield (up to 94 %). Biomass derived aliphatic alcohols and diols were also selectively methylated on the β‐position, opening a pathway to “biohybrid” molecules constructed entirely from non‐fossil carbon. Mechanistic studies indicate that the reaction proceeds through a borrowing hydrogen pathway involving metal–ligand cooperation at the Mn‐pincer complex. This transformation provides a convenient, economical, and environmentally benign pathway for the selective C−C bond formation with potential applications for the preparation of advanced biofuels, fine chemicals, and biologically active molecules

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal-Ligand Cooperation.

The reversible activation of H2 via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H2 via an MLC pathway to the widely used iron catalyst [(iPrPNP)FeH(CO)] (1) (iPrPNP = N(CH2CH2PiPr2)2-). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H2 using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H2 activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H2 activation. The results of this study provide guidance about the design of catalysts and additives to promote H2 activation via an MLC pathway.

Formamides as Isocyanate Surrogates: A Mechanistically Driven Approach to the Development of Atom-Efficient, Selective Catalytic Syntheses of Ureas, Carbamates, and Heterocycles.

Despite the hazardous nature of isocyanates, they remain key building blocks in bulk and fine chemical synthesis. By surrogating them with less potent and readily available formamide precursors, we herein demonstrate an alternative, mechanistic approach to selectively access a broad range of ureas, carbamates, and heterocycles via ruthenium-based pincer complex catalyzed acceptorless dehydrogenative coupling reactions. The design of these highly atom-efficient procedures was driven by the identification and characterization of the relevant organometallic complexes, uniquely exhibiting the trapping of an isocyanate intermediate. Density functional theory (DFT) calculations further contributed to shed light on the remarkably orchestrated chain of catalytic events, involving metal-ligand cooperation.

Facile C=O Bond Splitting of Carbon Dioxide Induced by Metal-Ligand Cooperativity in a Phosphinine Iron(0) Complex.

New iron complexes [Cp*FeL]− (1‐σ and 1‐π, Cp*=C5Me5) containing the chelating phosphinine ligand 2‐(2′‐pyridyl)‐4,6‐diphenylphosphinine (L) have been prepared, and found to undergo facile reaction with CO2 under ambient conditions. The outcome of this reaction depends on the coordination mode of the versatile ligand L. Interaction of CO2 with the isomer 1‐π, in which L binds to Fe through the phosphinine moiety in an η5 fashion, leads to the formation of 3‐π, in which CO2 has undergone electrophilic addition to the phosphinine group. In contrast, interaction with 1‐σ—in which L acts as a σ‐chelating [P,N] ligand—leads to product 3‐σ in which one C=O bond has been completely broken. Such CO2 cleavage reactions are extremely rare for late 3d metals, and this represents the first such example mediated by a single Fe centre.

Rhodapentalenes: Pincer Complexes with Internal Aromaticity.

SummaryPincer complexes are a remarkably versatile family benefited from their stability, diversity, and tunability. Many of them contain aromatic organic rings at the periphery, and aromaticity plays an important role in their stability and properties, whereas their metallacyclic cores are not aromatic. Herein, we report rhodapentalenes, which can be viewed as pincer complexes in which the metallacyclic cores exhibit considerable aromatic character. Rhodapentalenes show good thermal stability, although the rhodium-carbon bonds in such compounds are fragile. Experimental and computational studies suggest that the stabilization of rigid CCC pincer architectures together with an intrinsic aromaticity is vital for these metallacyclic rhodium species. Dearomatization-aromatization reactions, corresponding to metal-ligand cooperation of classical aromatic pincer complexes, were observed in this system. These findings suggest a new concept for pincer chemistry, the internal aromaticity involving metal d-orbitals, which would be useful for exploiting the nature of construction motif and inspire further applications.

A Silyl-Nickel Moiety as a Metal-Ligand Cooperative Site.

Nickel(II) complexes supported by a diphosphinosilyl ligand, [PhSi(2-PiPr2C6H4)2]- (PhSiP2), reveal unusual metal-ligand cooperativity (MLC). While (PhSiP2)Ni(NHMes) (2a) was cleanly isolated at room temperature, a nickel triisopropylphenylamido species, (PhSiP2)Ni(NHTrip) (2b), slowly transformed into a nickel(II) phenyl species, [(ArNH)SiP2]Ni(Ph) (3b), where (ArNH)SiP2- = [(NHTrip)Si(2-PiPr2C6H4)2]-. The X-ray crystallographic data of 3b exhibit a Si-N bond generated from Si-N coupling between the silyl moiety and amino group, along with cleavage of a Si-C bond. Because substoichiometric amounts of π-acidic ligands, such as isocyanide, enhance the conversion rate of 2 to 3 (kobs = 0.28 vs 0.44 h-1), this reaction may involve reductive elimination (RE) and oxidative addition (OA) operating at the silyl-nickel moiety. This is further supported by the fact that the presence of excess π-acidic ligands results in the generation of demetalated ligands (ArNH)PhSiP2 (4) having both Si-N and Si-Ph bonds and the nickel(0) species. Theoretical evaluations also agree on such a pathway. Interestingly, the reaction of a nickel phenyl species (3) with gaseous carbon dioxide (CO2(g)) produces a nickel(II) carbamato complex, (PhSiP2)Ni(OC(O)NHAr) (6), which may involve a RE-OA process occurring at a nickel center. Although 2 and 3 might be in equilibrium, the reaction of 3 with CO2 does not follow this pathway. Instead, a CO2 interaction induces RE at the silyl-nickel moiety, followed by amide group transfer, to give a carbamato product, 6, based on our experimental and theoretical evaluations. These results highly support that group transfer involving MLC can be managed via a RE-OA pathway at the silyl-nickel(II) moiety.

Cleavage of Si-H, B-H, and C-H Bonds by Metal-Ligand Cooperation.

Metal-ligand cooperation, in which metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. In contrast to the classical bond cleavage by active metal centers with spectator ligands, metal-ligand cooperation has enabled unprecedented reactivities. Especially, metal-ligand cooperative H-H bond cleavage has been extensively studied and applied to various catalysts. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This review summarizes the recent progress in the metal-ligand cooperative cleavages of Si-H, B-H, and C-H bonds and their catalytic applications.

Ruthenium-Catalyzed Direct Cross-Coupling of Secondary Alcohols to β-Disubstituted Ketones

The β-disubstituted ketone functionality is prevalent in biologically active compounds and in pharmaceuticals. A ruthenium-catalyzed direct synthesis of β-disubstituted ketones by cross-coupling of two different secondary alcohols is reported. This new protocol was applied to the synthesis of variety of β-disubstituted ketones from various cyclic, acyclic, symmetrical, and unsymmetrical secondary alcohols. An amine–amide metal–ligand cooperation in a Ru catalyst facilitates the activation and formation of covalent bonds in selective sequences to provide the products. Kinetic and deuterium-labeling experiments suggested that aliphatic alcohols oxidize faster than benzylic secondary alcohols. A plausible mechanism is proposed on the basis of mechanistic and kinetic studies. Water and H2 are the only byproducts from this selective cross-coupling of secondary alcohols.1 Introduction2 Catalytic Self- or Cross-Coupling of Alcohols and Selectivity Challenges3 Recent Developments in the Synthesis of β-Disubstituted Ketones4 Scope of Ruthenium-Catalyzed Cross-Couplings of Secondary Alcohols5 Mechanistic Studies and Proposed Mechanism6 Conclusion

Cooperative Bond Activation Reactions with Nickel and Palladium Carbene Complexes with a PCcarbeneS Pincer Ligand

A new PCcarbeneS ligand has been designed to stabilize late transition metal carbene complexes with Schrock-type reactivity for bond activations via metal–ligand cooperation. This ligand combines previous approaches to such complexes by stabilizing the carbene moiety through either charge delocalization into adjacent aryl groups or the use of an anion-stabilizing substituent. Nickel and palladium complexes of the PCcarbeneS pincer ligand could be prepared by dehydrohalogenation of the precursors (PCsp3S)NiCl and (PCsp3S)PdCl and were characterized in solution and solid state. X-ray diffraction (XRD) analyses as well as density functional theory (DFT) studies demonstrate that the electronic structure of these complexes can be described by a carbene as well as a zwitterionic complex with a M–C single bond. Due to the strong nucleophilic character at the carbon atom, both complexes are highly reactive and undergo sulfur transfer to form thioketone complexes. The nickel carbene complex is capable of cooperati...

Thermodynamic Analysis of Metal-Ligand Cooperativity of PNP Ru Complexes: Implications for CO2 Hydrogenation to Methanol and Catalyst Inhibition.

The hydrogenation of CO2 in the presence of amines to formate, formamides, and methanol (MeOH) is a promising approach to streamlining carbon capture and recycling. To achieve this, understanding how catalyst design impacts selectivity and performance is critical. Herein we describe a thorough thermochemical analysis of the (de)hydrogenation catalyst, (PNP)Ru-Cl (PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine; Ru = Ru(CO)(H)) and correlate our findings to catalyst performance. Although this catalyst is known to hydrogenate CO2 to formate with a mild base, we show that MeOH is produced when using a strong base. Consistent with pKa measurements, the requirement for a strong base suggests that the deprotonation of a six-coordinate Ru species is integral to the catalytic cycle that produces MeOH. Our studies also indicate that the concentration of MeOH produced is independent of catalyst concentration, consistent with a deactivation pathway that is dependent on methanol concentration, not equivalency. Our temperature-dependent equilibrium studies of the dearomatized congener, (*PNP)Ru, with various H-X species (to give (PNP)Ru-X; X = H, OH, OMe, OCHO, OC(O)NMe2) reveal that formic acid equilibrium is approximately temperature-independent; relative to H2, it is more favored at elevated temperatures. We also measure the hydricity of (PNP)Ru-H in THF and show how subsequent coordination of the substrate can impact the apparent hydricity. The implications of this work are broadly applicable to hydrogenation and dehydrogenation catalysis and, in particular, to those that can undergo metal-ligand cooperativity (MLC) at the catalyst. These results serve to benchmark future studies by allowing comparisons to be made among catalysts and will positively impact rational catalyst design.

Metal-Ligand Cooperative Approach To Achieve Dehydrogenative Functionalization of Alcohols to Quinolines and Quinazolin-4(3H)-ones under Mild Aerobic Conditions.

A simple metal-ligand cooperative approach for the dehydrogenative functionalization of alcohols to various substituted quinolines and quinazolin-4(3H)-ones under relatively mild reaction conditions (≤90 °C) is reported. Simple and easy-to-prepare air-stable Cu(II) complexes featuring redox-active azo-aromatic scaffolds, 2-arylazo-(1,10-phenanthroline) (L1,2), are used as catalyst. A wide variety of substituted quinolines and quinazolin-4(3H)-ones were synthesized in moderate to good isolated yields via dehydrogenative coupling reactions of various inexpensive and easily available starting materials under aerobic conditions. A few control experiments and deuterium labeling studies were carried out to understand the mechanism of the dehydrogenative coupling reactions, which indicate that both copper and the coordinated azo-aromatic ligand participate in a cooperative manner during the catalytic cycle.

Manganese(I)-Catalyzed Cross-Coupling of Ketones and Secondary Alcohols with Primary Alcohols.

Catalytic cross-coupling of ketones and secondary alcohols with primary alcohols is reported. An abundant manganese-based pincer catalyst catalyzes the reactions. Low loading of catalyst (2 mol %) and catalytic use of a mild base (5–10 mol %) are sufficient for efficient cross-coupling. Various aryl and heteroaryl ketones are catalytically cross-coupled with primary alcohols to provide the selective α-alkylated products. Challenging α-ethylation of ketones is also attained using ethanol as an alkylating reagent. Further, direct use of secondary alcohols in the reaction results in in situ oxidation to provide the ketone intermediates, which undergo selective α-alkylation. The reaction proceeds via the borrowing hydrogen pathway. The catalyst oxidizes the primary alcohols to aldehydes, which undergo subsequent aldol condensation with ketones, promoted by catalytic amount of Cs2CO3, to provide the α,β-unsaturated ketone intermediates. The hydrogen liberated from oxidation of alcohols is used for hydrogenation of α,β-unsaturated ketone intermediates. Notably either water or water and dihydrogen are the only byproducts in these environmentally benign catalytic processes. Mechanistic studies allowed inferring all of the intermediates involved. Dearomatization–aromatization metal–ligand cooperation in the catalyst facilitates the facile O–H bond activation of both primary and secondary alcohols, and the resultant manganese alkoxide complexes produce corresponding carbonyl compounds, perhaps via β-hydride elimination. The manganese(I) hydride intermediate plays dual role as it hydrogenates α,β-unsaturated ketones and liberates molecular hydrogen to regenerate the catalytically active dearomatized intermediate. Metal–ligand cooperation allows all of the manganese intermediates to exist in same oxidation state (+1) and plays an important role in these catalytic cross-coupling reactions.

Parallels between Metal-Ligand Cooperativity and Frustrated Lewis Pairs.

Metal ligand cooperativity (MLC) and frustrated Lewis pair (FLP) chemistry both feature the cooperative action of a Lewis acidic and a Lewis basic site on a substrate. A lot of work has been carried out in the field of FLPs to prevent Lewis adduct formation, which often reduces the FLP reactivity. Parallels are drawn between the two systems by looking at their reactivity with CO2, and we explore the role of steric bulk in preventing dimer formation in MLC systems.

Manganese(I)-Catalyzed α-Alkenylation of Ketones Using Primary Alcohols.

A simple protocol of manganese catalyzed selective α-alkenylation of ketones using primary alcohols is reported. The reactions proceeded well with a low loading of catalyst (0.3 mol %). The overall transformation operates through O-H bond activation of primary alcohols via dearomatization-aromatization metal ligand cooperation in the catalyst to provide the corresponding aldehydes, which further undergo condensation with methylene ketones to deliver α,β-unsaturated ketones. This selective α-alkenylation proceeds with the release of water and liberation of molecular hydrogen.

Optimizing reaction paths for methanol synthesis from CO2 hydrogenation via metal-ligand cooperativity

As diversified reaction paths exist over practical catalysts towards CO2 hydrogenation, it is highly desiderated to precisely control the reaction path for developing efficient catalysts. Herein, we report that the ensemble of Pt single atoms coordinated with oxygen atoms in MIL-101 (Pt1@MIL) induces distinct reaction path to improve selective hydrogenation of CO2 into methanol. Pt1@MIL achieves the turnover frequency number of 117 h−1 in DMF under 32 bar at 150 °C, which is 5.6 times that of Ptn@MIL. Moreover, the selectivity for methanol is 90.3% over Pt1@MIL, much higher than that (13.3%) over Ptn@MIL with CO as the major product. According to mechanistic studies, CO2 is hydrogenated into HCOO* as the intermediate for Pt1@MIL, whereas COOH* serves as the intermediate for Ptn@MIL. The unique reaction path over Pt1@MIL not only lowers the activation energy for the enhanced catalytic activity, but also contributes to the high selectivity for methanol.

Pyridine-Based PCP-Ruthenium Complexes: Unusual Structures and Metal-Ligand Cooperation.

Metal-ligand cooperation (MLC) by dearomatization/aromatization provides a unique way for bond activation, which has led to the discovery of various acceptorless dehydrogenative coupling reactions. However, most of the studies are based on pincer complexes with a central nitrogen donor. Aiming at exploration of the possibility of MLC by PCP-type pincer complexes, we report herein the synthesis, characterization, structure, and reactivity of pyridine-based PCP-Ru complexes. X-ray structures and DFT calculations indicate a carbenoid character of quaternized pyridine-based PCP-Ru complexes. These complexes undergo dearomatization by direct deprotonation, and the dearomatized complex can react with hydrogen, alcohols, or nitriles to regain aromatization via MLC.

Isolation of a Diylide-Stabilized Stannylene and Germylene: Enhanced Donor Strength through Coplanar Lone Pair Alignment.

The preparation of the first stable diylide‐substituted stannylene and germylene (Y2E, with E=Ge, Sn and Y=[PPh3‐C‐SO2Tol]−) is reported. The synthesis is easily accomplished in one step from the sulfonyl‐substituted metalated ylide YNa and the corresponding ECl2 precursors. Y2Ge and Y2Sn exhibit unusual structures in the solid state and in solution, in which the three adjacent lone pairs in the C‐E‐C linkage are arranged coplanar to each other. As shown by DFT studies, this bonding situation is preferred over the typical π‐donation from the ligands into the empty p‐orbital at the metal due to the strong anion‐stabilizing ability of the sulfonyl groups in the ylide backbone and their additional coordination to the metal. The alignment of the three lone pairs leads to a remarkable boost of the HOMO energy and thus of the donor strengths of the tetrylenes. Hence, Y2Ge and Y2Sn become stronger donors than their diamino or diaryl congeners and comparable to cyclic alkyl(amino)carbenes. First reactivity studies confirm the high reactivity of Y2Ge and Y2Sn, which for example undergo an intramolecular C−H activation reaction via metal–ligand cooperation.

Rh(I) Complex with a Tridentate Pyridine–Amino–Olefin Actor Ligand–Metal–Ligand Cooperative Activation of CO2 and Phenylisocyanate under C–C and Rh–E (E = O, N) Bond Formation

We synthesized and characterized the novel olefinic multidentate actor ligand dbap-py. The ligand consists of a 2-methyl pyridine unit and an azepine component fused by an N-alkylation. The coordin...

Phosphine-NHC Manganese Hydrogenation Catalyst Exhibiting a Non-Classical Metal-Ligand Cooperative H2 Activation Mode.

Deprotonation of the MnI NHC-phosphine complex fac-[MnBr(CO)3 (κ2 P,C-Ph2 PCH2 NHC)] (2) under a H2 atmosphere readily gives the hydride fac-[MnH(CO)3 (κ2 P,C-Ph2 PCH2 NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn(CO)3 (κ3 P,C,C-Ph2 PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO)3 (κ2 P,C-Ph2 P=CHNHC)] (6 b) as key intermediate able to activate H2 via a non-classical mode of metal-ligand cooperation implying a formal λ5 -P-λ3 -P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre-catalysts for ketone hydrogenation in the MnI series reported to date (TON up to 6200).