Chapter 6 - Homogeneous catalysis: Characterization and spectroscopy

The metal‐catalyzed oligomerization and polymerization of alkenes by homogeneous metal complexes has not only provided extraordinary insights into the chain‐growth process, it has also become of increasing commercial importance in delivering products of greater purity or improved properties. Novel materials not accessible using conventional heterogeneous catalysts can now be synthesized. Metals are no longer constrained to certain functions (titanium for polymerization, nickel for oligomerization) but span the range of molecular weights and polymer structures. This review discusses the fundamentals of chain‐growth polymerization catalyzed by homogeneous complexes and new developments in the field.

General rate equations and their applications for cyclic reaction networks: multi-pathway systems

Chiral gold complexes have been applied in homogeneous catalytic reactions since 1986, in some cases with high enantioselectivity. Acyclic diaminocarbene (ADC) ligands are acyclic analogues of N-heterocyclic carbenes (NHCs) that have larger N-CCarbene-N angles and stronger donating ability. ADCs have been developed as alternatives to phosphine and NHC ligands in homogeneous gold catalysis. In 2012, a new series of chiral gold(I) ADCs were first developed by Slaughter's group and were shown to give remarkable enantioselectivities in some reactions. Because of the hindered rotation of the N-CCarbene bonds of ADC, chiral ADC substituents can easily get close to the metal center in some conformations, although two rotameric structures are formed if the chiral amine is nonsymmetric. The selective of specific ADC conformations was the initial focus of this study. Formational selectivity of one diastereomer of an ADC ligand during synthesis was examines by measuring the relative rates of diastereomer formation in a 1H NMR kinetic study. The potential for converting multiple conformational isomers of ADCs into a single conformation, or at least a simpler mixture, was examined. This study used the analogy that anti- isomer has electronic and structural similarity with urea/thiourea, raising the possibility that 1,8-naphthyridine can be used to favor certain conformations through a self-assembled hydrogen-bonding complex. Gold(I) is a soft carbophilic Lewis acid able to active C-C π bonds to nucleophilic attack, and ADC-gold complexes are potentially useful in this regard. Therefore, biaryl gold(I) ADC complexes were examine with silver salt additives in catalytic 1,6-enyne cyclization reaction. A detailed study found that the counteranion affects the regioselectivities of these reactions more than substituents on the ancillary ADC ligands.

Published data on homogeneous catalytic redox reactions involving U, Np, Pu, and Am ions in acidic and alkaline media are summarized. The kinetic data and possible reaction mechanisms are discussed.

Bio-based process for the catalytic production of ethyl levulinate from cellulose

Dendrimers in Catalysis

Homogeneous transition metal catalysis: from the water gas shift reaction to nuclear waste vitrification

Synthesis of a water-soluble Pd(II) complex of N′-(pyridin-2-yl)propane-1,3-diamine modified β-cyclodextrin (Pd(II)@PyPDA:β-CD) is reported. The synthesized Pd(II)@PyPDA:β-CD, characterized by FT–IR, NMR, mass, UV–vis spectroscopies and molecular modeling studies, is used as a homogeneous, ecofriendly and reusable catalyst under mild reaction conditions for transfer hydrogenation of aromatic ketones to secondary alcohols using isopropyl alcohol as a green reducing agent. This homogeneous catalyst is reused five times with only a small decrease in its catalytic activity. The presence of a chiral cavity in cyclodextrin, is found to facilitate significant chiral induction, evident from preliminary chiral HPLC studies. A plausible mechanism for hydrogenation is proposed. Other significant advantages of the present catalyst, namely, simple preparation, stability, substrate selectivity, easy recovery, and reusability highlight its potential utility in other reactions and also in industrial applications.

Recent advancement in deoxygenation of fatty acids via homogeneous catalysis for biofuel production

Microwave heating was applied in homogeneous and in heterogeneous reactions and the results were compared from the point of view of activation of chemical reactions. Reactions including the addition of halo compounds to alkenes catalyzed by copper and ruthenium complexes in different solvents and NaY zeolite catalyzed alkylation of secondary amine in the absence of solvent were studied as model reactions to compare possibilities of microwave activation of reactants and catalysts. Rate enhancement of over one order of magnitude in homogeneous reactions was caused mainly by thermal dielectric heating effect which resulted from the effective coupling of microwaves to polar solvents. Activation of reactants and catalysts was very low if any. In heterogeneously catalyzed alkylation reactions highly efficient activation of zeolite catalyst was recorded. The results indicated that the best reaction conditions were in experiments when both activation of catalyst and performance of reaction were carried out under microwave conditions. Rate enhancement was most probably caused by "hot spots" or by "selective heating" of active sites. In both homogeneous and heterogeneous reactions non-thermal activation (specific effect) was excluded.

In recent years, the concept of Frustrated Lewis Pairs (FLPs), which consist of a combination of Lewis acid (LA) and Lewis base (LB) active sites arranged in a suitable geometric configuration, has been widely utilized in homogeneous catalytic reactions. This concept has also been extended to solid supports such as zeolites, metal oxide surfaces, and metal/covalent organic frameworks, resulting in a diverse range of heterogeneous FLP catalysts that have demonstrated notable efficiency and recyclability in activating small molecules. This study presents the successful immobilization of FLP active sites onto the surface of ligand-stabilized copper nanoclusters with atomic precision, leading to the development of copper nanocluster FLP catalysts characterized by high reactivity, stability, and selectivity. Specifically, thiol ligands containing 2-methoxyl groups were strategically designed to stabilize the surface of [Cu34S7(RS)18(PPh3)4]2+ (where RSH = 2-methoxybenzenethiol), facilitating the formation of FLPs between the surface copper atoms (LA) and ligand oxygen atoms (LB). Experimental and theoretical investigations have demonstrated that these FLPs on the cluster surface can efficiently activate H2 through a heterolytic pathway, resulting in superior catalytic performance in the hydrogenation of alkenes under mild conditions. Notably, the intricate yet precise surface coordination structures of the cluster, reminiscent of enzyme catalysts, enable the hydrogenation process to proceed with nearly 100% selectivity. This research offers valuable insights into the design of FLP catalysts with enhanced activity and selectivity by leveraging surface/interface coordination chemistry of ligand-stabilized atomically precise metal nanoclusters.

This review provides a broad overview of the literature related to the importance of pyridine and related ligands in homogeneous catalysis. In particular, it describes the various ways by which this ligand can stabilised the metal within a complex for homogeneous catalysis. We surveyed the important transition metal homogenous catalysts containing pyridine and related ligand acting as backbone for other ligands in homogeneous catalytic reactions explicitly from 2011 up to early 2014 and summarized their comparative catalytic activities.

Recent developments in the one‐step thermal and photochemical catalytic hydroxylation of benzene to phenol using various oxidants are described, focusing on the catalytic mechanisms. Hydroxylation of benzene with hydrogen peroxide is catalyzed by homogeneous metal complexes and heterogeneous metal oxides to produce phenol when high‐valent metal‐oxo complexes are reactive species The selective hydroxylation of benzene to phenol is achieved by using mesoporous silica‐alumina as the catalyst support, which prohibits further oxidation of phenol due to the strongly acidic sites. Photocatalytic oxidation of benzene with dioxygen and water is made possible by using organic photocatalysts via photoinduced electron transfer from benzene to the excited states of the photocatalysts, followed by the hydroxylation of the benzene radical cation with water to yield phenol selectively. The further oxidation of phenol is prohibited because of the fast back electron transfer from the one‐electron reduced species of the photocatalyst to the phenol radical cation, whereas back electron transfer from the one‐electron reduced species to the benzene radical cation is slowed down because of the large driving force of the back electron transfer, which is in the Marcus inverted region.

In this study, we report for the first time rhodium porphyrins as single-site catalysts for the hydrolytic dehydrogenation of ammonia borane (AB), overcoming the traditional role of porphyrin-based materials as stabilizing supports for metal nanoparticles. Rhodium complexes were synthesized by reaction of RhCl3·3H2O with four free-base porphyrins. To explore homogenous conditions, we chose two cationic (TMPyP_H2 and TMAP_H2) and one anionic (TSP_H2) water-soluble core, whereas water-insoluble neutral TAP_H2 was used as a heterogeneous catalysis. Rhodium porphyrins were fully characterized by UV-vis, ATR-FTIR and NMR spectroscopy, and the content of rhodium was estimated by ICP-MS. Considerably, all the investigated rhodium porphyrins catalyze effectively the hydrolytic dehydrogenation of ammonia borane at 303 K and 1 atm, with varying efficiency. Anionic TSP_Rh showed the best performance (70.7% H2 yield in 9 min, TOF = 15.72 × 106 h-1), whereas the two cationic catalysts differed in activity and yield (TMPyP_Rh: 68.7% H2 yield in 7 min, TOF = 3.41 × 106 h-1; TMAP_Rh: 57.5% H2 yield in 21 min, TOF = 11.50 × 106 h-1). The stability tests pointed out a progressive deactivation of ionic rhodium porphyrins, mainly due to the reaction environment. Indeed, its impact is likely greater on water-soluble complexes, as suggested by a detailed spectroscopic investigation performed on the homogeneous catalysts after use. Conversely, the performances of heterogeneous TAP_Rh remained stable over seven successive runs, although the catalyst exhibited lower activity (65.4% H2 yield in 28 min, TOF = 3.65 × 106 h-1). These results strongly demonstrate the efficiency of Rh-based porphyrins as homogeneous and heterogeneous catalysts for the ultra-pure hydrogen production from ammonia borane.

An artificial enzyme-based assay: DNA detection using a peroxidase-like copper–creatinine complex