Determination of the surface temperature of magnetically heated nanoparticles using a catalytic approach

Herein we describe a new method for the determination of the surface temperature of magnetically heated nanoparticles in solution using the temperature dependency of the catalytic performances of iron carbide nanoparticles coated with ruthenium (Fe_2.2C@Ru) for acetophenone hydrodeoxygenation. A correlation between nanoparticle surface temperature and magnetic field could be established. Very high surface temperatures could be estimated in different solvents, which were also found similar at a given magnetic field and well above some solvent boiling points.

Heterolytic cleavage of dihydrogen (HCD) in metal nanoparticle catalysis

Supports, ligands and additives can promote heterolytic H₂ splitting by a cooperative mechanism with metal nanoparticles.

In memoriam Paul Kamer (March 13 1960 – November 19 2020)

Celebrating the life of Paul Kamer.

Novel nickel nanoparticles stabilized by imidazolium-amidinate ligands for selective hydrogenation of alkynes

Novel magnetically recoverable nickel nanoparticles (Ni NPs) stabilized by imidazolium-amidinate ligands selectively hydrogenate alkynes into (Z)-alkenes under mild conditions.

σ-H-H, σ-C-H, and σ-Si-H Bond Activation Catalyzed by Metal Nanoparticles

Activation of H-H, Si-H, and C-H bonds through σ-bond coordination has grown in the past 30 years from a scientific curiosity to an important tool in the functionalization of hydrocarbons. Several mechanisms were discovered via which the initially σ-bonded substrate could be converted: oxidative addition, heterolytic cleavage, σ-bond metathesis, electrophilic attack, etc. The use of metal nanoparticles (NPs) in this area is a more recent development, but obviously nanoparticles offer a much richer basis than classical homogeneous and heterogeneous catalysts for tuning reactivity for such a demanding process as C-H functionalization. Here, we will review the surface chemistry of nanoparticles and catalytic reactions occurring in the liquid phase, catalyzed by either colloidal or supported metal NPs. We consider nanoparticles prepared in solution, which are stabilized and tuned by polymers, ligands, and supports. The question we have addressed concerns the differences and similarities between molecular complexes and metal NPs in their reactivity toward σ-bond activation and functionalization.

Halogen bonding effects on the outcome of reactions at metal centres

Key findings regarding the effects of ligand preorganisation via halogen bonding on the outcome of reactions at rhodium are reported.

Monitoring of nanoparticle reactivity in solution: interaction of l-lysine and Ru nanoparticles probed by chemical shift perturbation parallels regioselective H/D exchange

Thanks to new water-soluble Ru nanoparticles (NPs) stabilized by sulfonated NHC ligands, we demonstrate that it is possible to monitor the catalyst/substrate interaction using NMR chemical shift perturbations (CSPs), under conditions that closely resemble those applied during the enantiospecific C-H deuteration of l-lysine. Correlating the pH dependence of the interaction of l-lysine with the surface of the RuNPs and its subsequent deuteration, our study underscores the importance of oriented binding to the surface as a critical factor for H/D exchange.

: a halogen-bond assembled supramolecular catalyst

XBphos-Rh constitutes the first example of halogen bonding as the driving force behind the assembly of a transition-metal catalyst for hydroborations.

The use of halogen bonding as a tool to construct a catalyst backbone is reported. Specifically, pyridyl- and iodotetrafluoroaryl-substituted phosphines were assembled in the presence of a rhodium(i) precursor to form the corresponding halogen-bonded complex XBphos-Rh. The presence of fluorine substituents at the iodo-containing supramolecular motif was not necessary for halogen bonding to occur due to the template effect exerted by the rhodium center during formation of the halogen-bonded complex. The halogen-bonded supramolecular complexes were successfully tested in the catalytic hydroboration of terminal alkynes.

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Amide versus amine ligand paradigm in the direct amination of alcohols with Ru-PNP complexes

Ruthenium complexes of PNP-ligands bearing secondary amines were found to be inactive in the alcohol amination with NH₃, while all complexes of homologous ligands bearing tertiary amines gave active catalysts!

A protic ionic liquid as an atom economical solution for palladium catalyzed asymmetric allylic alkylation

A recyclable palladium catalyst for asymmetric allylic alkylation that generates its own ionic liquid solvent.

Ruthenium nanoparticles ligated by cholesterol-derived NHCs and their application in the hydrogenation of arenes

Herein we present ruthenium nanoparticles (Ru-NPs) stabilized with two rigid NHC ligands derived from cholesterol.

An iridium–SPO complex as bifunctional catalyst for the highly selective hydrogenation of aldehydes

An Ir(iii) hydride catalytic system shows very high activities and selectivities in the chemoselective hydrogenation of various substituted aldehydes.

Characterization of secondary phosphine oxide ligands on the surface of iridium nanoparticles

The coordination mode of secondary phosphine oxide ligands on the surface of iridium nanoparticle catalysts was elucidated by solid-state NMR.

Ligand effects in rhodium-catalyzed hydroformylation with bisphosphines: steric or electronic?

Do wide bite angles lead to high linear regioselectivity in hydroformylation, or is an electronic effect operative?

Correlation between the Selectivity and the Structure of an Asymmetric Catalyst Built on a Chirally Amplified Supramolecular Helical Scaffold

For the first time, supramolecular helical rods composed of an achiral metal complex and a complementary enantiopure monomer provided a good level of enantioinduction in asymmetric catalysis. Mixtures containing an achiral ligand monomer (BTA(PPh2), 2 mol %) and an enantiopure ligand-free comonomer (ester BTA, 2.5 mol %), both possessing a complementary benzene-1,3,5-tricarboxamide (BTA) central unit, were investigated in combination with [Rh(cod)2]BArF (1 mol %) in the asymmetric hydrogenation of dimethyl itaconate. Notably, efficient chirality transfer occurs within the hydrogen-bonded coassemblies formed by BTA Ile and the intrinsically achiral catalytic rhodium catalyst, providing the hydrogenation product with up to 85% ee. The effect of the relative content of BTA Ile as compared to the ligand was investigated. The amount of chiral comonomer can be decreased down to one-fourth of that of the ligand without deteriorating the enantioselectivity of the reaction, while the enantioselectivity decreases for mixtures containing high amounts of BTA Ile. The nonlinear relationship between the amount of chiral comonomer and the enantioselectivity indicates that chirality amplification effects are at work in this catalytic system. Also, right-handed helical rods are formed upon co-assembly of the achiral rhodium complex of BTA(PPh2) and the enantiopure comonomer BTA Ile as confirmed by various spectroscopic and scattering techniques. Remarkably, the major enantiomer and the selectivity of the catalytic reaction are related to the handedness and the net helicity of the coassemblies, respectively. Further development of this class of catalysts built on chirally amplified helical scaffolds should contribute to the design of asymmetric catalysts operating with low amounts of chiral entities.

Novel iminopyridine derivatives: ligands for preparation of Fe(ii) and Cu(ii) dinuclear complexes

A series of imino- and amino-pyridine ligands based on dihydrobenzofurobenzofuran (BFBF) and methanodibenzodioxocine (DBDOC) backbones have been synthesized. These ligands form exclusively dinuclear complexes with metals such as iron(II) and copper(II). The structures for complexes 15, 16, 18, 19, 20, 21, 23, and 24 were determined by X-ray crystallography. The complexes show large distances for the metal nuclei and different geometries depending on the nature of the metal. An octahedral geometry was observed for the iron(II) complexes, while copper(II) complex 24 showed a distorted trigonal bipyramidal geometry. The iron(II) complexes showed activity as catalysts in the cycloaddition of CO2 to epoxides, obtaining moderate yields of cyclic carbonates.

Enantioselective hydrogenation of ketones by iridium nanoparticles ligated with chiral secondary phosphine oxides

Chiral iridium nanoparticles (IrNPs) were synthesized by H₂reduction of (1,5-cyclooctadiene)(methoxy)iridium(i) dimer ([Ir(OMe)(COD)]₂) in the presence of an asymmetric secondary phosphine oxide.

Palladium catalyzed oxidative carbonylation of alcohols: effects of diphosphine ligands

The best catalytic performance was exhibited by a Lewis base P∩P ligand with a relatively wide bite angle capable of maintaining a cis geometry.

Highly active, chemo- and enantioselective Pt-SPO catalytic systems for the synthesis of aromatic carboxamides

Platinum complexes of the chiral non-racemizing SPO ligand 1 have been discovered to be the first artificial transition metal complexes providing kinetic resolution in the hydration of a racemic chiral nitrile.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.

Tunable asymmetric catalysis through ligand stacking in chiral rigid rods

Chiral benzene-1,3,5-tricarboxamide (BTA) ligands, comprising one diphenylphosphino group and one or two remote chiral 1-methylheptyl side chains, were evaluated in the rhodium-catalyzed asymmetric hydrogenation of dimethyl itaconate. Despite the fact that the rhodium atom and the chiral center(s) are separated by more than 12 covalent bonds, up to 82% ee was observed. A series of control and spectroscopic experiments confirmed that the selectivity arises from the formation of chiral helical polymers by self-association of the BTA monomers through noncovalent interactions. The addition of a phosphine-free chiral BTA, acting as a comonomer for the chiral BTA ligands, increases the level of enantioselectivity (up to 88% ee). It illustrates how the selectivity of the reaction can be increased in a simple fashion by mixing two different BTA monomers. The concept was further probed by performing the same experiment with an achiral BTA ligand, i.e. a phosphine-functionalized BTA that contains two remote octyl side chains. It afforded an encouraging 31% ee, thus demonstrating the catalytically relevant transfer of chirality between the self-assembled units. It constitutes a unique example of the sergeants-and-soldiers principle applied to catalysis.

Ortho-phosphinobenzenesulfonate: a superb ligand for palladium-catalyzed coordination-insertion copolymerization of polar vinyl monomers

Ligands, Lewis bases that coordinate to the metal center in a complex, can completely change the catalytic behavior of the metal center. In this Account, we summarize new reactions enabled by a single class of ligands, phosphine-sulfonates (ortho-phosphinobenzenesulfonates). Using their palladium complexes, we have developed four unusual reactions, and three of these have produced novel types of polymers. In one case, we have produced linear high-molecular weight polyethylene, a type of polymer that group 10 metal catalysts do not typically produce. Secondly, complexes using these ligands catalyzed the formation of linear poly(ethylene-co-polar vinyl monomers). Before the use of phosphine-sulfonate catalysts, researchers could only produce ethylene/polar monomer copolymers that have different branched structures rather than linear ones, depending on whether the polymers were produced by a radical polymerization or a group 10 metal catalyzed coordination polymerization. Thirdly, these phosphine-sulfonate catalysts produced nonalternating linear poly(ethylene-co-carbon monoxide). Radical polymerization gives ethylene-rich branched ethylene/CO copolymers copolymers. Prior to the use of phosphine-sulfonates, all of the metal catalyzed processes gave completely alternating ethylene/carbon monoxide copolymers. Finally, we produced poly(polar vinyl monomer-alt-carbon monoxide), a copolymerization of common polar monomers with carbon monoxide that had not been previously reported. Although researchers have often used symmetrical bidentate ligands such as diimines for the polymerization catalysis, phosphine-sulfonates are unsymmetrical, containing two nonequivalent donor units, a neutral phosphine, and an anionic sulfonate. We discuss the features that make this ligand unique. In order to understand all of the new reactions facilitated by this special ligand, we discuss both the steric effect of the bulky phosphines and electronic effects. We provide a unified interpretation of the unique reactivity by considering of the net charge and the enhanced back donation in the phosphine-sulfonate complexes.

Diphosphine capsules for transition-metal encapsulation

Self-assembly and characterization of novel heterodimeric diphosphine capsules formed by multiple ionic interactions and composed of one tetracationic diphosphine ligand and one complementary tetraanionic calix[4]arene are described. Encapsulation of a palladium atom within a diphosphine capsule is achieved successfully by using the metal complex of the tetracationic diphosphine ligand for the assembly process. In this templated approach to metal encapsulation, the transition-metal complex is an integrated part of the capsule with the transition metal located inside the capsule and is not involved in the assembly process. We present two approaches for capsule assembly by mixing solutions of the precharged building blocks in methanol and mixing solutions of the neutral building blocks in methanol. The scope of the diphosphine capsules and the metallodiphosphine capsules is easily extended by applying tetracationic diphosphine ligands with different backbones (ethylene, diphenyl ether, and xanthene) and cationic binding motifs (p-C(6)H(4)-CH(2)-ammonium, m-C(6)H(4)-ammonium, and m-C(6)H(4)-guanidinium). These tetracationic building blocks with different flexibilities and shapes readily associate into capsules with the proper capsular structure, as is indicated by (1)H NMR spectroscopy, 1D NOESY, ESI-MS, and modeling studies.

Bis(metallo) capsules based on two ionic diphosphines

Self-assembly and characterization of heterodimeric diphosphine capsules formed by multiple ionic interactions are described. The first type of capsules is composed of one novel tetrasulfonato-xantphos ligand and one complementary tetraammonium calix[4]arene. Encapsulation of a transition metal is achieved by self-assembly of a rhodium complex containing the tetraanionic diphosphine ligand and a tetracationic calix[4]arene. The second type of capsules is composed of two oppositely charged diphosphine ligands: one tetrasulfonato-xantphos and one tetraammonium-diphosphine (of the xantphos-, DPEphos-, and 1,2-bis(diphenylphosphino)ethane (dppe)-type). Bis(metallo) capsules, that is, simultaneous encapsulation of two different transition metals, are created by self-assembly of a palladium or platinum complex containing a tetracationic ligand and a rhodium complex containing a tetraanionic ligand. Diphosphine ligands with different flexibilities and shapes assemble into metallocapsules with a proper capsular structure, as is indicated by (1)H NMR and 1D-NOESY spectroscopy, ESIMS, and modeling studies.

Relationship between conformational flexibility and chelate cooperativity

A family of four biscarbamates (AA) and four bisphenols (DD) were synthesized, and H-bonding interactions between all AA•DD combinations were characterized using (1)H NMR titrations in carbon tetrachloride. A chemical double mutant cycle analysis shows that there are no secondary electrostatic interactions or allosteric cooperativity in these systems, and the system therefore provides an ideal platform for investigating the relationship between chemical structure and chelate cooperativity. Effective molarities (EMs) were measured for 12 different systems, where the number of rotors in the chains connecting the two H-bond sites was varied from 5 to 20. The association constants vary by less than an order of magnitude for all 12 complexes, and the variation in EM is remarkably small (0.1-0.9 M). The results provide a relationship between EM and the number of rotors in the connecting chains (r): EM ≈ 10r(-3/2). The value of 10 M is the upper limit for the value of EM for a noncovalent intramolecular interaction. Introduction of rotors reduces the value of EM from this maximum in accord with a random walk analysis of the encounter probability of the chain ends (r(-3/2)). Noncovalent EMs never reach the very high values observed for covalent processes, which places limitations on the magnitudes of the effects that one is likely to achieve through the use of chelate cooperativity in supramolecular assembly and catalysis. On the other hand, the decrease in EM due to the introduction of conformational flexibility is less dramatic than one might expect based on the behavior of covalent systems, which limits the losses in binding affinity caused by poor preorganization of the interaction sites.