Manganese 2-phosphinophosphinine precatalysts for methanol/ethanol upgrading to isobutanol

Two Mn-phosphinophosphinine complexes were synthesised from reaction of the proligand with [MnBr(CO)5] at 80 °C for 2 h; 2-diphenylphosphino-3-methyl-6-trimethylsilylphosphinine manganese tricarbonyl bromide (2TMS) and 2-diphenylphosphino-3-methyl-phosphinine manganese tricarbonyl bromide (2H). 31P{1H} NMR spectroscopy revealed characteristic chemical shifts for the phosphinine and phosphine donors bound to Mn (255.4 and 23.7 ppm for 2TMS; 234.2 and 24.8 ppm for 2H), and single crystal X-ray diffraction established the structure of the chelating complex 2TMS. Rapid reaction of both complexes with water was observed with 2TMS reacting to eventually yield a single product, syn-3TMS, from the syn-1,2-addition of water across the PC multiple bond on the bromide face, confirmed by X-ray diffraction for both an unsolvated and solvated structure, where MeOH was found to be H-bonding to the P-OH functionality. The reaction of 2R with dry methanol gave multiple products that were not in equilibrium with each other, and the molecular structure of one isomer was definitively established by X-ray diffraction as an unusual 1,4-addition product (1,4-4TMS). However, reaction of 2R with methanol in the presence of trace water showed that hydrolysis products 3R were formed preferentially. Both phosphinine complexes acted as pre-catalysts for the Guerbet upgrading of methanol/ethanol to isobutanol at 180 °C over 90 h, giving yields of isobutanol (based on moles of ethanol) of 22% for 2TMS and 27% for 2H. This is superior to known Mn dppm complexes [dppm = bis(diphenylphosphino)methane], including the 21% yield recorded for the best derivative [MnBr(κ2-PPh2C(H)PhPPh2)(CO)3] shown to date.

The Mechanism of Guerbet Reaction by Metal Ligand Cooperation Catalyst Mn-PCP

The Guerbet reaction is important for the synthesis of longer-chain monoalcohols like isobutanol through catalytic transfer hydrogenation from short-chain methanol and ethanol. However, the mechanism becomes complicated, especially considering the variations in the different metal-ligand cooperation (MLC) catalysts used. In order to further understand the Guerbet reaction, DFT studies were performed to figure out the detailed mechanism initiated by the unique Mn-PCP MLC Catalyst. Our results suggest that even with the assistance of the carbanion site of the PCP ligand, the direct substitution mechanism is less favored than the condensation-reduction mechanism. The key step of the reaction is the final reduction of the carbonyl, in which the 1,4-reduction of the unsaturated aldehyde is prior to the 3,4-reduction or 1,2-reduction due to the stronger interaction between the catalyst and the substrate. It is found that the production of isobutanol is preferred over n-butanol because of the lower total free energy barrier and lower relative free energy of the product. Finally, by changing the electronic effect of the carbanion site of the catalyst, we found that the relation between the electronic effect and the highest free energy span was not monotonous and a point with optimal electronic effect exists numerically.

DPPM-Bridged Binuclear Pt(II) Pincer Complexes: Chemistry, Structure, and Photophysics in Solution Revisited

A series of luminescent binuclear ([dppm{Pt(NNC)}2]2+) and mononuclear ([PPh3Pt(NNC)]+) complexes containing pincer ligands were synthesized and characterized. Photophysical characteristics of both types of complexes were studied in dichloromethane solution. In the solid phase, the binuclear compounds adopt a syn configuration where the {Pt(NNC)} fragments are held together due to intramolecular Pt-Pt bonding and π-stacking of the pincer ligand aromatic systems. Analysis of the complexes' molecular structure in solution by multinuclear NMR spectroscopy showed that the stacked intramolecular configuration is retained in fluid media, which is in complete agreement with a considerable red shift of the emission wavelength due to formation of the intramolecular Pt-Pt bond, leading to the transformation of an emissive excited state to 3MMLCT. It was also found that triethylamine quenches the emission of both types of complexes; the mechanism of quenching is a combination of dynamic and static channels of excited-state deactivation. In the case of binuclear complexes, deprotonation of the dppm methylene bridge by triethylamine also contributes to the chromophore quenching. To explain the observed chemistry of binuclear complex interactions with Et3N, a chemical equilibrium scheme was suggested, which was confirmed by quantitative monitoring of the 31P signal variations as a function of triethylamine concentration.

Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(Ph2PCH2NHC)Mn(CO)3Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000. Mechanistic investigation combining experimental and computational studies allowed to rationalize this beneficial effect as an enhanced stabilization of reaction intermediates including anionic hydride species fac-[(Ph2PC(Ph)NHC)Mn(CO)3H]- playing a crucial role in the hydrogenation process. These results highlight the interest of such carbon bridge substitution strategy being rarely employed in the design of chemically non-innocent ligands.

Experimental and theoretical insights for designing Zn2+ complexes to trigger chemo-selective hetero-coupling of alcohols

Designing well-defined Zn-complexes for sustainable dehydrogenative catalysis overcoming the difficulties associated with activating Zn2+(d10)-metal species is considered paramount goal in catalysis. Herein, we explore the plausibility of β-alkylation of secondary alcohols with primary alcohols by well-defined 3d10 Zn-complexes. Detailed organometallic and catalytic investigations, in conjunction with computational analyses, were conducted to ascertain the potential involvement of the catalyst at various stages of the catalytic process.

Two active species from a single metal halide precursor: a case study of highly productive Mn-catalyzed dehydrogenation of amine-boranes via intermolecular bimetallic cooperation

Metal-metal cooperation for inert bond activation is a ubiquitous concept in coordination chemistry and catalysis. While the great majority of such transformations proceed via intramolecular mode in binuclear complexes, to date only a few examples of intermolecular small molecule activation using usually bimetallic frustrated Lewis pairs (Mδ+⋯M'δ-) have been reported. We introduce herein an alternative approach for the intermolecular bimetallic cooperativity observed in the catalytic dehydrogenation of amine-boranes, in which the concomitant activation of N-H and B-H bonds of the substrate via the synergetic action of Lewis acidic (M+) and basic hydride (M-H) metal species derived from the same mononuclear complex (M-Br). It was also demonstrated that this system generated in situ from the air-stable Mn(i) complex fac-[(CO)3(bis(NHC))MnBr] and NaBPh4 shows high activity for H2 production from several substrates (Me2NHBH3, tBuNH2BH3, MeNH2BH3, NH3BH3) at low catalyst loading (0.1% to 50 ppm), providing outstanding efficiency for Me2NHBH3 (TON up to 18 200) that is largely superior to all known 3d-, s-, p-, f-block metal derivatives and frustrated Lewis pairs (FLPs). These results represent a step forward towards more extensive use of intermolecular bimetallic cooperation concepts in modern homogeneous catalysis.

The Dichotomy of Mn-H Bond Cleavage and Kinetic Hydricity of Tricarbonyl Manganese Hydride Complexes

Acid-base characteristics (acidity, pKa, and hydricity, ΔG°_H- or k_H-) of metal hydride complexes could be a helpful value for forecasting their activity in various catalytic reactions. Polarity of the M-H bond may change radically at the stage of formation of a non-covalent adduct with an acidic/basic partner. This stage is responsible for subsequent hydrogen ion (hydride or proton) transfer. Here, the reaction of tricarbonyl manganese hydrides mer,trans-[L₂Mn(CO)₃H] (1; L = P(OPh)₃, 2; L = PPh₃) and fac-[(L-L')Mn(CO)₃H] (3, L-L' = Ph₂PCH₂PPh₂ (dppm); 4, L-L' = Ph₂PCH₂-NHC) with organic bases and Lewis acid (B(C₆F₅)₃) was explored by spectroscopic (IR, NMR) methods to find the conditions for the Mn-H bond repolarization. Complex 1, bearing phosphite ligands, features acidic properties (pKa 21.3) but can serve also as a hydride donor (ΔG^≠_298K = 19.8 kcal/mol). Complex 3 with pronounced hydride character can be deprotonated with KHMDS at the CH₂-bridge position in THF and at the Mn-H position in MeCN. The kinetic hydricity of manganese complexes 1-4 increases in the order mer,trans-[(P(OPh)₃)₂Mn(CO)₃H] (1) < mer,trans-[(PPh₃)₂Mn(CO)₃H] (2) ≈ fac-[(dppm)Mn(CO)₃H] (3) < fac-[(Ph₂PCH₂NHC)Mn(CO)₃H] (4), corresponding to the gain of the phosphorus ligand electron-donor properties.

Fac-to-mer isomerization triggers hydride transfer from Mn(I) complex fac-[(dppm)Mn(CO)3H]

Low-temperature IR and NMR studies combined with DFT calculations revealed the mechanistic complexity of apparently simple reactions between Mn(I) complex fac-[(dppm)Mn(CO)₃H] and Lewis acids (LA = Ph₃C⁺, B(C₆F₅)₃) involving the formation of so-far elusive meridional hydride species mer-[(dppm)Mn(CO)₃H⋯LA] and unusual dearomatization of the Ph₃C⁺ cation upon hydride transfer.

Cage-like manganesesilsesquioxanes: features of their synthesis, unique structure, and catalytic activity in oxidative amidations

A family of Mn-based cage-like silsesquioxanes (and complexes with 1,10-phenanthroline) exhibits unique types of molecular architectures and catalytic activity in oxidative amidation reactions.

Carbenes and phosphonium ylides: a fruitful association in coordination chemistry

Among a plethora of σ-donor ligands available, carbon-centered ones have become essential, in particular with the emergence of N-heterocyclic carbenes (NHCs), positioning themselves as credible alternatives to traditional nitrogen- and phosphorus-based systems. Phosphonium ylides representing another class of neutral η¹-bonded carbon ligands have also been shown to act as effective Lewis bases. Considering the intrinsic features of the carbene and phosphonium ylide ligands, similar in terms of electronic properties, but different in terms of bonding mode, the design of hybrid systems combining these two types of carbon functionalities appeared to be a natural and exciting challenge. This Perspective comprehensively covers the chemistry of such ligand architectures from synthesis and fundamental aspects to catalytic applications.

Detection and Structural Investigation of Elusive Palladium Hydride Intermediates Formed from Simple Metal Salts

The Mizoroki-Heck reaction is one of the most known and best studied catalytic transformations and has provided an outstanding driving force for the development of catalysis and synthetic applications. Three out of four classical Mizoroki-Heck catalytic cycle intermediates contain Pd-C bonds and are well known and studied in detail. However, a simple palladium hydride (which is formed after the product-releasing β-H-elimination step) is a kind of elusive intermediate in the Mizoroki-Heck reaction. In the present study, we performed a combined theoretical and mass spectrometry (MS) study of palladium hydride complexes [PdX₂H]^- (X = Cl, Br, and I), which are reactive intermediates in the Mizoroki-Heck reaction. Static and molecular dynamic calculations revealed that these species have a T-shaped structure with a trans-arrangement of halogen atoms. Other isomers of [PdX₂H]^- are unstable and easily rearrange into the T-shaped form or decompose. These palladium hydride intermediates were detected by MS in precatalyst activation using NaBH₄, Et₃N, and a solvent molecule as reducing agents. Online MS monitoring allowed the detection of [PdX₂H]^- species in the course of the Mizoroki-Heck reaction.

High spin polarized Fe2 cluster combined with vicinal nonmetallic sites for catalytic ammonia synthesis from a theoretical perspective

Compared to other Fen (n > 2) clusters, Fe2 cluster catalysts combined with vicinal nonmetallic sites are expected to be an ideal catalyst for ammonia synthesis with a lower N–H formation (0.47 eV) and N–N dissociation (0.50 eV) energy barrier at the same time.

Manganese Diphosphine and Phosphinoamine Complexes Are Effective Catalysts for the Production of Biofuel Alcohols via the Guerbet Reaction

We report a variety of manganese-based catalysts containing both chelating diphosphine (bis(diphenylphosphino)methane (dppm: 1, 2, and 7) or 1,2-bis(diphenylphosphino)ethane (dppe: 3)), and mixed-donor phosphinoamine (2-(diphenylphosphino)ethylamine (dppea: 4–6)) ligands for the upgrading of ethanol and methanol to the advanced biofuel isobutanol. These catalysts show moderate selectivity up to 74% along with turnover numbers greater than 100 over 90 h, with catalyst 2 supported by dppm demonstrating superior performance. The positive effect of substituting the ligand backbone was also displayed with a catalyst supported by C-phenyl-substituted dppm (8) having markedly improved performance compared to the parent dppm catalysts. Catalysts supported by the phosphinoamine ligand dppea are also active for the upgrading of ethanol to n-butanol. These results show that so-called PNP-pincer ligands are not a prerequisite for the use of manganese catalysts in Guerbet chemistry and that simple chelates can be used effectively.