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.

Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H2 Production

In this study, we reported the synthesis and structural characterization of a triphos-CoII complex [(κ3-triphos)CoII(CH3CN)2]2+ (1) and a triphos-CoI-H complex [(κ2-triphos)HCoI(CO)2] (4). The facile synthetic pathways from 1 to [(κ3-triphos)CoII(κ2-O2CH)]+ (1') and [(κ3-triphos)CoI(CH3CN)]+ (2), respectively, as well as the interconversion between [(κ3-triphos)CoI(CO)2]+ (3) and 4 have been established. The activation energy barrier, associated with the dehydrogenation of a coordinated formate fragment in 1' yielding the corresponding 2 accompanied by the formation of H2 and CO2, was experimentally determined as 23.9 kcal/mol. With 0.01 mol % loading of 1, a maximum TON ∼ 1735 within 18 h and TOF ∼ 483 h-1 for the first 3 h could be achieved. Kinetic isotope effect (KIE) values of 2.25 (kHCOOH/kDCOOH) and 1.36 (kHCOOH/kHCOOD) for the dehydrogenation of formic acid and its deuterated derivatives, respectively, implicate that the H-COOH bond cleavage is likely the rate-determining step. The catalytic mechanism proposed by density functional theory (DFT) calculations coupled with experimental 1H NMR and gas chromatography-mass spectrometry (GC-MS) analysis unveils two competing pathways for H2 production; specifically, deprotonating a HCOO-H bond by a proposed Co-H intermediate C and homolytic cleavage of the CoII-H moiety of C, presumably via a dimeric Co intermediate D containing a [Co2(μ-H)2]2+ core, to yield the corresponding 2 and H2.

Synthesis and catalytic activity of well-defined Co(I) complexes based on NHC-phosphane pincer ligands

A new methodology for the preparation of Co(I)-NHC (NHC = N-heterocyclic carbene) complexes, namely, [Co(PCNHCP)(CO)2][Co(CO)4] (1) and [Co(PCNHCP)(CO)2]BF4 (2), has been developed (PCNHCP = 1,3-bis(2-(diphenylphosphanyl)ethyl)-imidazol-2-ylidene). Both complexes can be straightforwardly prepared by direct reaction of their parent imidazolium salts with the Co(0) complex Co2(CO)8. Complex 1 efficiently catalyses the reductive amination of furfural and levulinic acid employing silanes as reducing agents under mild conditions. Furfural has been converted into a variety of secondary and tertiary amines employing dimethyl carbonate as the solvent, while levulinic acid has been converted into pyrrolidines under solventless conditions. Dehydrocoupling of the silane to give polysilanes has been observed to occur as a side reaction of the hydrosilylation process.

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.

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

The use of supporting ligands that can store either protons or electrons has emerged as a powerful strategy in catalysis. While these strategies are potent individually, natural systems mediate remarkable transformations by combining the storage of both protons and electrons in the secondary coordination sphere. As such, there has been recent interest in using this strategy to enable fundamentally different transformations. Furthermore, outsourcing H-atom or hydrogen storage to ancillary ligands can also enable alternative mechanistic pathways and thereby selectivity. Here, we describe the application of this strategy to facilitate radical reactivity in Co-based hydrogenation catalysis. Metalation of previously reported dihydrazonopyrrole ligands with Co results in paramagnetic complexes, which are best described as having Co(II) oxidation states. These complexes catalytically hydrogenate olefins with low catalyst loadings under mild conditions (1 atm H₂, 23 °C). Mechanistic, spectroscopic, and computational investigations indicate that this system goes through a radical hydrogen-atom transfer (HAT) type pathway that is distinct from classic organometallic mechanisms and is supported by the ability of the ligand to store H₂. These results show how ancillary ligands can facilitate efficient catalysis, and furthermore how classic organometallic mechanisms for catalysis can be altered by the secondary coordination sphere.

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...

Base-free transfer hydrogenation of aldehydes and ketones catalyzed by imidazolin-2-iminato actinide complexes

A series of thorium and uranium complexes decorated with unsymmetrical imidazolin-2-iminato ligands were found to be effective as catalysts in the transfer hydrogenation of aldehydes and ketones with 2-propanol to form the corresponding alcohols.

Control of Catalyst Isomers Using an N-Phenyl-Substituted RN(CH2CH2PiPr2)2 Pincer Ligand in CO2 Hydrogenation and Formic Acid Dehydrogenation

A novel pincer ligand, ^iPrPN^PhP [PhN(CH₂CH₂PⁱPr₂)₂], which is an analogue of the versatile MACHO ligand, ^iPrPN^HP [HN(CH₂CH₂PⁱPr₂)₂], was synthesized and characterized. The ligand was coordinated to ruthenium, and a series of hydride-containing complexes were isolated and characterized by NMR and IR spectroscopies, as well as X-ray diffraction. Comparisons to previously published analogues ligated by ^iPrPN^HP and ^iPrPN^MeP [CH₃N(CH₂CH₂PⁱPr₂)₂] illustrate that there are large changes in the coordination chemistry that occur when the nitrogen substituent of the pincer ligand is altered. For example, ruthenium hydrides supported by the ^iPrPN^PhP ligand always form the syn isomer (where syn/anti refer to the relative orientation of the group on nitrogen and the hydride ligand on ruthenium), whereas complexes supported by ^iPrPN^HP form the anti isomer and complexes supported by ^iPrPN^MeP form a mixture of syn and anti isomers. We evaluated the impact of the nitrogen substituent of the pincer ligand in catalysis by comparing a series of ^iPrPN^RP (R = H, Me, Ph)-ligated ruthenium hydride complexes as catalysts for formic acid dehydrogenation and carbon dioxide (CO₂) hydrogenation to formate. The ^iPrPN^PhP-ligated species is the most active for formic acid dehydrogenation, and mechanistic studies suggest that this is likely because there are kinetic advantages for catalysts that operate via the syn isomer. In CO₂ hydrogenation, the ^iPrPN^PhP-ligated species is again the most active under our optimal conditions, and we report some of the highest turnover frequencies for homogeneous catalysts. Experimental and theoretical insights into the turnover-limiting step of catalysis provide a basis for the observed trends in catalytic activity. Additionally, the stability of our complexes enabled us to detect a previously unobserved autocatalytic effect involving the base that is added to drive the reaction. Overall, by modifying the nitrogen substituent on the MACHO ligand, we have developed highly active catalysts for formic acid dehydrogenation and CO₂ hydrogenation and also provided a framework for future catalyst development.

Indirect reduction of CO2 and recycling of polymers by manganese-catalyzed transfer hydrogenation of amides, carbamates, urea derivatives, and polyurethanes

The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the corresponding alcohols, amines, and methanol as products. Since these compound classes can be prepared using CO2 as a C1 building block the reported reaction represents an approach to the indirect reduction of CO2. Notably, these are the first examples on the reduction of carbamates and urea derivatives as well as on the C-N bond cleavage in amides by transfer hydrogenation. The general applicability of this methodology is highlighted by the successful reduction of 12 urea derivatives, 26 carbamates and 11 amides. The corresponding amines, alcohols and methanol were obtained in good to excellent yields up to 97%. Furthermore, polyurethanes were successfully converted which represents a viable strategy towards a circular economy. Based on control experiments and the observed intermediates a feasible mechanism is proposed.

Cobalt-Catalyzed Chemoselective Transfer Hydrogenative Cyclization Cascade of Enone-Tethered Aldehydes

The ligand-free Co-catalyzed chemoselective reductive cyclization cascade of enone-tethered aldehydes with i-PrOH as the environmentally benign hydrogen surrogate is developed by this study. Mechanistic studies disclosed that such a protocol is initiated by an ortho-enone-assisted Co(I)-catalyzed reduction of the aldehyde functionality with i-PrOH. Meanwhile, the selectivity from the Michael-Aldol cycloreduction cascade to the oxa-Michael cascade is feasible and readily adjusted by the addition of steric Lewis bases, such as TEMPO and DABCO, delivering substituted 1H-indenes and dihydroisobenzofurans, respectively.

Asymmetric hydrogenation catalyzed by first-row transition metal complexes

This review focuses on asymmetric direct and transfer hydrogenation with first-row transition metal complexes. The reaction mechanisms and the models of enantiomeric induction were summarized and emphasized.

Ruthenium(ii)-catalyzed reductive N–O bond cleavage of N-OR (R = H, alkyl, or acyl) substituted amides and sulfonamides

A convenient method for the reductive cleavage of the N–O bonds of amide derivatives was developed using ruthenium(ii)-catalyzed transfer hydrogenation reaction.

Rational selection of co-catalysts for the deaminative hydrogenation of amides

Theory and experiments were used to rationally design co-catalysts for the deaminative hydrogenation of amides. TBD was found to be the optimal catalyst, assisting the C–N bond cleavage while preventing the formation of adducts with the catalyst.

The catalytic hydrogenation of amides is an atom economical method to synthesize amines. Previously, it was serendipitously discovered that the combination of a secondary amide co-catalyst with (^iPrPNP)Fe(H)(CO) (^iPrPNP = N[CH₂CH₂(PⁱPr₂)]₂^–), results in a highly active base metal system for deaminative amide hydrogenation. Here, we use DFT to develop an improved co-catalyst for amide hydrogenation. Initially, we computationally evaluated the ability of a series of co-catalysts to accelerate the turnover-limiting proton transfer during C–N bond cleavage and poison the (^iPrPNP)Fe(H)(CO) catalyst through a side reaction. TBD (triazabicyclodecene) was identified as the leading co-catalyst. It was experimentally confirmed that when TBD is combined with (^iPrPNP)Fe(H)(CO) a remarkably active system for amide hydrogenation is generated. TBD also enhances the activity of other catalysts for amide hydrogenation and our results provide guidelines for the rational design of future co-catalysts.

Transfer Hydrogenation of Ketones Catalyzed by Symmetric Imino-N-heterocyclic Carbene Co(III) Complexes

The synthesis of new moisture-sensitive imine-functionalized N-heterocyclic carbene (NHC) precursor salts [1-(2-[(hydroxyl-benzylidene)-amino]-ethyl)-3-R-3H-imidazole-1-ium bromide; R = methyl (1a), ethyl (1b), and benzyl (1c)] is reported. Subsequent deprotonation of 1a-c and coordination of the in situ generated NHC ligands to CoBr₂ led to the isolation of air-stable six-coordinate Co(III) complexes 2a-c, respectively. All the salts and complexes were fully characterized. Single-crystal X-ray analysis of 2a and 2c showed octahedral Co centers hexacoordinated to two NHC carbons, two imine nitrogen atoms, and two phenolate oxygens in the form [C^N^O(Co³⁺)C^N^O]. The complexes were used in the catalytic transfer hydrogenation (CTH) of a range of ketones in 2-propanol as the solvent and hydrogen donor. Based on a low catalyst concentration of 0.4 mol %, significant conversions in the range of 70-99% were recorded at high turnover frequencies up to 1635 h^-1. A mechanism to account for the steps involved in the CTH of cyclohexanone by complex 2a is proposed and supported by data from cyclic voltammetry, low-resolution mass spectrometry, UV, and IR spectroscopic techniques.

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.

1-D manganese(ii)-terpyridine coordination polymers as precatalysts for hydrofunctionalisation of carbonyl compounds

Efficient hydroboration and hydrosilylation of ketones and aldehydes has been achieved using a Mn^II-coordination polymer as precatalyst under mild conditions_.

Transfer hydrogenation of aldehydes and ketones catalyzed using an aminophosphinite POCNH pincer complex of Ni(ii)

(POCN^H)NiBr catalyzes the transfer hydrogenation of aldehydes and ketones in iPrOH. The reactions tolerate alkenes, esters, amides, nitriles, and heterocycles and proceed via the metal–ligand cooperative mechanism through (POCN)Ni^II species.

Highly selective hydrogenation of amides catalysed by a molybdenum pincer complex: scope and mechanism

A series of molybdenum pincer complexes has been shown for the first time to be active in the catalytic hydrogenation of amides.

A series of molybdenum pincer complexes has been shown for the first time to be active in the catalytic hydrogenation of amides. Among the tested catalysts, Mo-1a proved to be particularly well suited for the selective C–N hydrogenolysis of N-methylated formanilides. Notably, high chemoselectivity was observed in the presence of certain reducible groups including even other amides. The general catalytic performance as well as selectivity issues could be rationalized taking an anionic Mo(0) as the active species. The interplay between the amide C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O reduction and the catalyst poisoning by primary amides accounts for the selective hydrogenation of N-methylated formanilides. The catalyst resting state was found to be a Mo–alkoxo complex formed by reaction with the alcohol product. This species plays two opposed roles – it facilitates the protolytic cleavage of the C–N bond but it encumbers the activation of hydrogen.

Liquid-Phase Hydrogenation of Nitriles to Amines Facilitated by a Co(II)/Zn(0) Pair: A Ligand-Free Catalytic Protocol

The given report introduces a simple and user-friendly in situ method for the production of catalytically active cobalt particles. The approach circumvents the use of air- and moisture-sensitive reductants as well as the application of anhydrous Co-precursor salts. Accordingly, the described catalytic system is readily assembled under open-flask conditions by simply combining the components in the reaction vessel. Therefore, the arduous charging procedure of the reaction autoclave in a glovebox under an inert gas atmosphere is no longer necessary. In fact, the catalytically active material is obtained upon treatment of readily available Co(OAc)₂·4 H₂O with benign commercial Zn powder. The catalytic performance of the resultant material was tested in the heterogeneous hydrogenation of nitriles to the corresponding primary amines. Both activity and selectivity of the cobalt catalyst are significantly enhanced if a triflate-based Lewis acid and ammonia is added to the reaction mixture.

Iron-PNP-Pincer-Catalyzed Transfer Dehydrogenation of Secondary Alcohols

The well-defined iron PNP pincer complex catalyst [Fe(H)(BH₄ )(CO)(HN{CH₂ CH₂ P(iPr)₂ }₂ ] was used for the catalytic dehydrogenation of secondary alcohols to give the corresponding ketones. Using acetone as inexpensive hydrogen acceptor enables the oxidation with good to excellent yields. DFT computations indicate an outer-sphere mechanism and support the importance of an acceptor to achieve this transformation under milder conditions.

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.

Additive-Free Isomerization of Allylic Alcohols to Ketones with a Cobalt PNP Pincer Catalyst

Catalytic isomerization of allylic alcohols in ethanol as a green solvent was achieved by using air and moisture stable cobalt (II) complexes in the absence of any additives. Under mild conditions, the cobalt PNP pincer complex substituted with phenyl groups on the phosphorus atoms appeared to be the most active. High rates were obtained at 120 °C, even though the addition of one equivalent of base increases the speed of the reaction drastically. Although some evidence was obtained supporting a dehydrogenation-hydrogenation mechanism, it was proven that this is not the major mechanism. Instead, the cobalt hydride complex formed by dehydrogenation of ethanol is capable of double-bond isomerization through alkene insertion-elimination.

Cooperative Mn(i)-complex catalyzed transfer hydrogenation of ketones and imines

A series of Mn(i) complexes bearing amine and benzimidazole fragment containing ligands were investigated in the TH of ketones and imines.