A Phosphine-Oxide Cobalt(II) Complex and Its Catalytic Activity Studies toward Alcohol Dehydrogenation Triggered Direct Synthesis of Imines and Quinolines

Herein, a new pincer-like amino phosphine donor ligand, H2L1, and its phosphine-oxide analog, H2L2, were synthesized. Subsequently, cobalt(II) complexes 1 and 2 were synthesized by the reaction of anhydrous Co(II)Cl2 with ligands H2L1 and H2L2, respectively. The ligands and complexes were fully characterized by various physicochemical and spectroscopic characterization techniques. Finally, the identity of the complexes 1 and 2 was confirmed by single crystal X-ray structure determination. The phosphine ligand containing complex 1 was converted to the phosphine oxide ligand containing complex 2 in air in acetonitrile solution. Both complexes 1 and 2 were investigated as precatalysts for alcohol dehydrogenation-triggered synthesis of imines in air. The phosphine-oxide complex 2 was more efficient than the phosphine complex 1. A wide array of alcohols and amines were successfully reacted in a mild condition to result in imines in good to excellent yields. Precatalyst 2 was also highly efficient for the synthesis of varieties of quinolines in air. As H2L2 in 2 has side arms that can be deprotonated, we investigated complex 2 for its base (KOtBu) promoted deprotonation events by various spectroscopic studies and DFT calculations. These studies have shown that mono deprotonation of the amine side arm attached to the pyridine is quite feasible, and deprotonation of complex 2 leads to a dearomatized pyridyl ring containing complex 2a. The mechanistic investigations of the catalytic reaction, by a combination of experimental and computational studies, have suggested that the dearomatized complex, 2a acted as an active catalyst. The reaction proceeded through the hydride transfer pathway. The activation barrier of this step was calculated to be 26.5 kcal/mol, which is quite consistent with the experimental reaction temperature under aerobic conditions. Although various pincer-like complexes are explored for such reactions, phosphine oxide ligand-containing complexes are still unexplored.

Robust and efficient transfer hydrogenation of carbonyl compounds catalyzed by NN-Mn(I) complexes

A series of manganese(I) carbonyl complexes bearing structurally related NN- and NNN-chelating ligands have been synthesized and assessed as catalysts for transfer hydrogenation (TH). Notably, the NN-systems based on N-R functionalized 5,6,7,8-tetrahydroquinoline-8-amines, proved the most effective in the manganese-promoted conversion of acetophenone to 1-phenylethanol. In particular, the N-isopropyl derivative, Mn1, when conducted in combination with t-BuONa, was the standout performer mediating not only the reduction of acetophenone but also a range of carbonyl substrates including (hetero)aromatic-, aliphatic- and cycloalkyl-containing ketones and aldehydes with especially high values of TON (up to 17 200; TOF of 3550 h-1). These findings, obtained through a systematic variation of the N-R group of the NN ligand, are consistent with an outer-sphere mechanism for the hydrogen transfer. As a more general point, this Mn-based catalytic TH protocol offers an attractive and sustainable alternative for producing alcoholic products from carbonyl substrates.

Dual electronic effects achieving a high-performance Ni(II) pincer catalyst for CO2 photoreduction in a noble-metal-free system

A carbazolide-bis(NHC) Ni^II catalyst (1; NHC, N-heterocyclic carbene) for selective CO₂ photoreduction was designed herein by a one-stone-two-birds strategy. The extended π-conjugation and the strong σ/π electron-donation characteristics (two birds) of the carbazolide fragment (one stone) lead to significantly enhanced activity for photoreduction of CO₂ to CO. The turnover number (TON) and turnover frequency (TOF) of 1 were ninefold and eightfold higher than those of the reported pyridinol-bis(NHC) Ni^II complex at the same catalyst concentration using an identical Ir photosensitizer, respectively, with a selectivity of ∼100%. More importantly, an organic dye was applied to displace the Ir photosensitizer to develop a noble-metal-free photocatalytic system, which maintained excellent performance and obtained an outstanding quantum yield of 11.2%. Detailed investigations combining experimental and computational studies revealed the catalytic mechanism, which highlights the potential of the one-stone-two-birds effect.

Efficient transfer hydrogenation of ketones by molybdenum complexes through comprehensively verifying auxiliary ligands

Molybdenum complexes, ligated with N1,N1-dialkyl-N2-(5,6,7,8-tetrahydroquinolin-8-yl)ethane-1,2-diamines along with auxiliary ligands, provide various structural features as [NNH/NNHN]Mo(CO)4/3 (Mo1 – Mo3), [NNHN]Mo(CO)2Br] (Mo4 – Mo5), [NNH]Mo(CO)(η3-C3H5)Br](Mo6) and [NNHN/S] Mo(CO)(PPh3)2] (Mo7 – Mo8). All...

Unusual mechanism of paramagnetic nickel-catalysed α-alkylation of amides

Nickel transition-metal catalysts are important materials which are widely used in (de)hydrogenation reactions. Typical NiII catalysts adopt a square planar geometry and a low-spin state owing to their d8 electronic configuration. Here, we describe a mechanistic investigation of a novel octahedral NiII catalyst with a paramagnetic nature catalysing the α-alkylation of amides. Both non-bifunctional and bifunctional pathways were considered. In addition, we clarified the superiority of the high-spin state by comparing the geometries, valence electronic configurations, and rate-limiting energy barriers of the high- and low-spin states. Our results indicate that the novel octahedral nickel catalyst favours the bifunctional pathway and tends to maintain a high-spin state throughout the reaction due to the N-arm ligand. This computational study suggests that the spin state has the potential to influence the catalyst structure and reaction mechanism. Furthermore, these findings present novel insights for the design of NiII catalysts with high-spin states.

Dual roles of the electronic effect on selectivity: pincer nickel-electrocatalyzed CO2 reduction

The unconventional dual roles of the electronic effect on the selectivity are unfolded, i.e., (1) the electronic effect on redox originating from σ-donation and (2) the electronic effect on π-back-donation.

The origin of different driving forces between O–H/N–H functional groups in metal ligand cooperation: mechanistic insight into Mn(i) catalysed transfer hydrogenation

The origin of different catalytic activity between two structurally similar Lewis basic bifunctional catalysts.

Mechanistic insights into the non-bifunctional hydrogenation of esters by Co(ii) pincer complexes: a DFT study

A recent experiment has revealed that additive free ester hydrogenation by Co-pincer complexes might follow an unusual non-bifunctional mechanism, however, the detailed mechanistic pathway is missing. It has been predicted that several intermediates and transition states are involved, having their essential role in the catalytic performances. Detailed theoretical studies are therefore essential in this regard for achieving more efficient ester hydrogenation catalysts. On the basis of first principles calculations, performed over Co(PNP)/(PNN) complexes, we present here the energetics and mechanistic details, showing the distinct orientations of different possible intermediates and transition states, and find the minimum energy pathway for the conversion of esters to alcohols. In the way, we find that some intermediates must undergo structural distortion for achieving the lowest potential energy barrier which must have a severe impact on the catalytic turnover frequency.

Lewis Acid Transition-Metal-Catalyzed Hydrogen Activation: Structures, Mechanisms, and Reactivities

As a new type of bifunctional catalyst, the Lewis acid transition-metal (LA-TM) catalysts have been widely applied for hydrogen activation. This study presents a mechanistic framework to understand the LA-TM-catalyzed H₂ activation through DFT studies. The mer(trans)-homolytic cleavage, the fac(cis)-homolytic cleavage, the synergetic heterolytic cleavage, and the dissociative heterolytic cleavage should be taken as general mechanisms for the field of LA-TM catalysis. Four typical LA-TM catalysts, the Z-type κ⁴ -L₃ B-Rh complex tri(azaindolyl)borane-Rh, the X-type κ³ -L₂ B-Co complex bis-phosphino-boryl (PBP)-Co, the η² -BC-type κ³ -L₂ B-Pd complex diphosphine-borane (DPB)-Pd, and the Z-type κ² -LB-Pt complex (boryl)iminomethane (BIM)-Pt are selected as representative models to systematically illustrate their mechanistic features and explore the influencing factors on mechanistic variations. Our results indicate that the tri(azaindolyl)borane-Rh catalyst favors the synergetic heterolytic mechanism; the PBP-Co catalyst prefers the mer(trans)-homolytic mechanism; the DPB-Pd catalyst operates through the fac(cis)-homolytic mechanism, whereas the BIM-Pt catalyst tends to undergo the dissociative heterolytic mechanism. The mechanistic variations are determined by the coordination geometry, the LA-TM bonding nature, the electronic structure of the TM center, and the flexibility or steric effect of the LA ligands. The presented mechanistic framework should provide helpful guidelines for LA-TM catalyst design and reaction developments.

Cobalt-Catalyzed Aqueous Dehydrogenation of Formic Acid

Among the known liquid organic hydrogen carriers, formic acid attracts increasing interest in the context of safe and reversible storage of hydrogen. Here, the first molecularly defined cobalt pincer complex is disclosed for the dehydrogenation of formic acid in aqueous medium under mild conditions. Crucial for catalytic activity is the use of the specific complex 3. Compared to related ruthenium and manganese complexes 7 and 8, this optimal cobalt complex showed improved performance. DFT computations support an innocent non-classical bifunctional outer-sphere mechanism on the triplet state potential energy surface.

A DFT study of Co(i) and Ni(ii) pincer complex-catalyzed hydrogenation of ketones: intriguing mechanism dichotomy by ligand field variation

Ligand field variation governs the mechanism dichotomy for isoelectronic catalysts.

Towards practical earth abundant reduction catalysis: design of improved catalysts for manganese catalysed hydrogenation

Rational design using kinetic studies has led to a 3-fold-increase in the reaction-rates compared to an already-promising lead catalyst for the reduction of ketones and esters.

Catalyzed or non-catalyzed: chemoselectivity of Ru-catalyzed acceptorless dehydrogenative coupling of alcohols and amines via metal–ligand bond cooperation and (de)aromatization

The mechanistic origin of the chemoselectivity for Ru-catalyzed acceptorless coupling of amines and alcohols.

Homogeneously catalyzed hydrogenation and dehydrogenation reactions – From a mechanistic point of view

Homogeneously catalyzed hydrogenation/dehydrogenation reactions represent not only one of the most synthetically important chemical transformations, but also a promising way to renewably utilize the hydrogen energy. In order to rationally design efficient homogeneous catalysts for hydrogenations/dehydrogenations, it is of fundamental importance to understand their reaction mechanisms in detail. With this aim in mind, we herein provide a brief overview of the mechanistic understanding and related catalyst design strategies. Hydrogenations and dehydrogenations represent the reverse process of each other, and involve the activation/release of H2 and the insertion/elimination of hydride as major steps. The mechanisms discussed in this chapter include the cooperation (bifunctional) mechanism and the non-cooperation mechanisms. Non-cooperation mechanisms usually involve single-site transition metal (TM) catalysts or transition metal hydride (TM-H) catalysts. Cooperation mechanisms usually operate in the state-of-the-art bifunctional catalysts, including Lewis-base/transition-metal (LB-TM) catalysts, Lewis-acid/transition-metal (LA-TM) catalysts, Lewis-acid/Lewis-base (LA-LB; the so-called frustrated Lewis pairs - FLPs) catalysts, newly developed ambiphilic catalysts, and bimetallic transition-metal/transition-metal (TM-TM) catalysts. The influence of the ligands, the electronic structure of the metal, and proton shuttle on the reaction mechanism are also discussed to improve the understanding of the factors that can govern mechanistic preferences. The content presented in this chapter should both inspire experimental and theoretical chemists concerned with homogeneously catalyzed hydrogenation and dehydrogenation reactions, and provide valuable information for future catalyst design.

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

This review is aimed at introducing the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

Unexpected concerted two-proton transfer for amination of formic acid to formamide catalysed by Mn bipyridinol complexes

DFT calculations reveal a concerted two-proton transfer mechanism for the activation of diethylamine and formic acid, as well as the formation of formamide and water catalysed by Mn bipyridinol complexes.

DFT study of CO2 hydrogenation catalyzed by a cobalt-based system: an unexpected formate anion-assisted deprotonation mechanism

An unexpected formate anion-assisted deprotonation mechanism is unfolded by a DFT study of CO₂ hydrogenation catalyzed by a cobalt-based system.

Elucidating metal hydride reactivity using late transition metal boryl and borane hydrides: 2c–2e terminal hydride, 3c–2e bridging hydride, and 3c–4e bridging hydride

A general trend for the hydrogenation reactivity of metal hydride(s): 3c–4e bridging hydride > 2c–2e terminal hydride > 3c–2e bridging hydride.

Hydrogenation of phenyl-substituted CN, CN,CC, CC and CO functional groups by Cr, Mo and W PNP pincer complexes – a DFT study

The hydrogenation of phenyl-substituted CN, CN, CC, CC and CO functional groups catalyzed by PNP pincer amido M(NO)(CO)(PNP) and amino ^HM(NO)(CO)(PN^HP) complexes [M = Cr, Mo and W; PNP = N(CH₂CH₂P(isopropyl)₂)₂] has been computed.

Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes

Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient and clean functional group transformations. In fact, the selective reduction of nitriles, ketones, and aldehydes with molecular hydrogen permits access to a green synthesis of valuable amines and alcohols. Despite more than a century of developments in homogeneous and heterogeneous catalysis, efforts toward the creation of new useful and broadly applicable catalyst systems are ongoing. Recently, Earth-abundant metals have attracted significant interest in this area. In the present study, we describe for the first time specific molecular-defined manganese complexes that allow for the hydrogenation of various polar functional groups. Under optimal conditions, we achieve good functional group tolerance, and industrially important substrates, e.g., for the flavor and fragrance industry, are selectively reduced.