Chapter 2 - Homogeneous catalysts preparation methods

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTMetal-assisted CO insertion reaction on a new surface rhodium dimer catalyst observed by an in situ EXAFS techniqueK. Asakura, K. Kitamura-Bando, K. Isobe, H. Arakawa, and Y. IwasawaCite this: J. Am. Chem. Soc. 1990, 112, 8, 3242–3244Publication Date (Print):April 1, 1990Publication History Published online1 May 2002Published inissue 1 April 1990https://doi.org/10.1021/ja00164a072RIGHTS & PERMISSIONSArticle Views140Altmetric-Citations15LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (409 KB) Get e-Alerts Get e-Alerts

Limited natural resources, high energy consumption, economic considerations, and environmental concerns demand that we develop new technologies for the sustainable production of chemicals and fuels. New methods that combine the selective activation of C-H bonds of hydrocarbons with oxidation by a green oxidant such as molecular oxygen would represent huge advances toward this goal. The spectacular selectivity of transition metals in cleaving C-H bonds offers the potential for the direct use of hydrocarbons in the production of value-added organics such as alcohols. However, the use of oxygen, which is abundant, environmentally benign, and inexpensive (particularly from air), has proven challenging, and more expensive and less green oxidants are often employed in transition-metal-catalyzed reactions. Advances in the use of oxygen as an oxidant in transition-metal-catalyzed transformations of hydrocarbons will require a better understanding of how oxygen reacts with transition metal alkyl and hydride complexes. For alkane oxidations, researchers will need to comprehend and predict how metals that have shown particularly high activity and selectivity in C-H bond activation (e.g. Pt, Pd, Rh, Ir) will react with oxygen. In this Account, we present our studies of reactions of late metal alkyls and hydrides with molecular oxygen, emphasizing the mechanistic insights that have emerged from this work. Our studies have unraveled some of the general mechanistic features of how molecular oxygen inserts into late metal hydride and alkyl bonds along with a nascent understanding of the scope and limitations of these reactions. We present examples of the formation of metal hydroperoxide species M-OOH by insertion of dioxygen into Pt(IV)-H and Pd(II)-H bonds and show evidence that these reactions proceed by radical chain and hydrogen abstraction pathways, respectively. Comparisons with recent reports of insertion of oxygen into other Pd(II)-H complexes, and also into Ir(III)-H and Rh(III)-H complexes, point to potentially general mechanisms for this type of reaction. Additionally, we observed oxygen-promoted C-H and H-H reductive elimination reactions from five-coordinate Ir(III) alkyl hydride and dihydride complexes, respectively. Further, when Pd(II)Me(2) and Pt(II)Me(2) complexes were exposed to oxygen, insertion processes generated M-OOMe complexes. Mechanistic studies for these reactions are consistent with radical chain homolytic substitution pathways involving five-coordinate M(III) intermediates. Due to the remarkable ability of Pt(II) and Pd(II) to activate the C-H bonds of hydrocarbons (RH) and form M-R species, this reactivity is especially exciting for the development of partial alkane-oxidation processes that utilize molecular oxygen. Our understanding of how late transition metal alkyls and hydrides react with molecular oxygen is growing rapidly and will soon approach our knowledge of how other small molecules such as olefins and carbon monoxide react with these species. Just as advances in understanding olefin and CO insertion reactions have shaped important industrial processes, key insight into oxygen insertion should lead to significant gains in sustainable commercial selective oxidation catalysis.

A realistic picture of the complete insertion reaction for a representative example of the formation of carbamates from lithium amides is given by combining experimental information with detailed DFT investigations. The chiral lithium (S)‐2‐(pyrrolidin‐1‐ylmethyl)pyrrolidide (2) reacts with CO2 to form its corresponding carbamate. In this reaction the second pyrrolidine ring functions as an intramolecular ligand, and therefore, “symmetrical” as well as “unsymmetrical” dimers of 2 have to be taken into account. On the energetically preferred reaction pathway this amide exists predominantly as a (solvated) dimer 4. Attack by CO2 occurs in an “end‐on” manner to give several heterodimers 5 which are composed of a lithium amide and a lithium carbamate. This exergonic step is rate determining, and the activation barrier was estimated to be about 10 kcal mol−1. A second lithium‐mediated CO2 insertion into a nitrogen−lithium bond yields the encounter complex 5d−(CO2), which then goes on to form several almost isoenergetic homodimeric carbamates 6. This strongly exergonic insertion reaction has a barrier of 9.7 kcal mol−1. We also report on novel lithium‐mediated CO2 insertion reactions into oxygen−lithium bonds in lithium carbamates. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The carbon-carbon (C-C) bond activation of [n]cycloparaphenylenes ([n]CPPs) by a transition-metal complex is herein reported. The Pt0 complex Pt(PPh3 )4 regioselectively cleaves two C-C σ bonds of [5] CPP and [6]CPP to give cyclic dinuclear platinum complexes in high yields. Theoretical calculations reveal that the relief of ring strain drives the reaction. The cyclic complex was further transformed into a cyclic diketone by using a CO insertion reaction.

Allyl groups of (η3-allyl)Fe(CO)2(NO) complexes undergo regioselective cabonylation at the less substituted carbon of the η3-allyl ligands in the presence of 1,2-bis(diphenylphosphino)ethane to give (β,γ-unsaturated acyl)Fe(CO)(NO)(dppe) complexes in good yields. X-ray analysis of the acyl complex thus obtained revealed that the complex has a trigonal bipyramid geometry. A characteristic substituent effect of the η3-allyl ligands on the CO insertion reaction was observed.

This article presents a review of the methods of synthesis of α-methylene lactones, esters, lactams, ketones, and aldehydes.The methods are classified into the following categories.1. Routes via compounds bearing a leaving group at the β-carbon.1-1. Oxygen functional groups.1-2. Nitrogen functional groups.1-3. Sulfur, Selenium functional groups and Halogens.2. Routes via compounds bearing a leaving group at the α-carbon.3. Routes via compounds bearing leaving groups at α- and β-carbons.4. Synthesis utilizing methylene compounds such as4-1. Acrylic acid derivatives.4-2. Allyl alcohol derivatives or Acrolein derivatives.5. Routes via cyclopropane derivatives.6. Rearrangement.7. CO insertion reaction.8. Miscellaneous.

A mechanism, which is distinct from the traditional one when sodium alkoxide was used instead of tertiary amines, was proposed for the alkoxycarbonylation of aryl iodides. The catalytic cycle was composed of oxidative addition, subsequent ArPdOR formation, CO insertion to Pd-OR, and final reductive elimination of ArPdCOOR. The kinetic simultaneity of the formation of deiodinated side product from the aryl iodide and aldehyde from corresponding alcohol provided strong evidence for the existence of ArPdOR species. The observation of thioether, as the other competitive product in palladium catalyzed thiocarbonylation of aryl iodides and sodium alkylthiolate, also indicate the possibility of metathesis between ArPdI and sodium alkylthiolate. Preliminary kinetic studies revealed that neither oxidative addition nor reductive elimination was rate limiting. DFT calculation displayed preference for CO insertion into Pd-OR bond. The advantage of this novel mechanism had been demonstrated in the facile alkoxycarbonylation and thiocarbonylation. The ethoxycarbonylation of aryl iodides under room temperature and balloon pressure of CO in the presence of EtONa were examined, and good to high yields were obtained; the t-butoxycarbonylation reactions in the presence of t-BuONa were achieved, and the alkylthiocarbonylation (including the t-butylthiocarbonylation) of aryl iodides in the presence of sodium alkylthiolate were also investigated.

Transition metal-catalyzed alkane dehydrogenation

Oxidative addition and reductive elimination are key steps in a wide variety of catalytic reactions mediated by transition-metal complexes. Historically, this reactivity has been considered to be the exclusive domain of d-block elements. However, this paradigm has changed in recent years with the demonstration of transition-metal-like reactivity by main-group compounds. This Review highlights the substantial progress achieved in the past decade for the activation of robust single bonds by main-group compounds and the more recently realized activation of multiple bonds by these elements. We also discuss the significant discovery of reversible activation of single bonds and distinct examples of reductive elimination at main-group element centers. The review consists of three major parts, starting with oxidative addition of single bonds, proceeding to cleavage of multiple bonds, and culminated by the discussion of reversible bond activation and reductive elimination. Within each subsection, the discussion is arranged according to the type of bond being cleaved or formed and considers elements from the left to the right of each period and down each group of the periodic table. The majority of results discussed in this Review come from the past decade; however, earlier reports are also included to ensure completeness.

The individual steps of a model Pd-catalyzed Negishi coupling between vinyl bromide and MeZnClS2 (S = THF), using Pd(PMe3)2 as catalyst, have been examined with the aid of DFT computations in THF solution with the B97D functional to gain further insight into the role of Pd–Zn interactions on the oxidative addition and reductive elimination steps. Comparisons between such processes taking place on monometallic Pd complexes and on bimetallic Pd–Zn complexes reveal a detrimental effect of Pd–Zn interactions in oxidative addition, whereas reductive elimination is favored by Pd–Zn cooperativity. Additionally, a cis transmetalation pathway has been characterized that complements previous studies involving trans complexes.

Neutral ((Ni-1)Pd(CH3)(Cl)) and cationic ([(Ni-1)Pd(CH3)(OEt2)][BAr‘4]) complexes containing the strongly electron-donating metallodithiolate ligand Ni-1 ([(N,N‘-bis(2-mercaptoethyl)-N,N‘-diazacycl...

Transition metal complexes play a vital role in modern catalysis due to their unique ability to facilitate a wide range of chemical transformations with high efficiency, selectivity, and sustainability. This study explores the fundamental mechanisms by which transition metal complexes function as catalysts and highlights their extensive industrial applications. The catalytic activity of these complexes arises from the variable oxidation states, coordination geometries, and electronic properties of transition metals, which enable them to activate substrates and stabilize reactive intermediates during chemical reactions. Mechanistic pathways such as oxidative addition, reductive elimination, insertion, and ligand exchange are central to the catalytic cycles of many metal-based systems. Well-known examples include palladium-catalyzed cross-coupling reactions, rhodium- and ruthenium-based hydrogenation and hydroformylation, and vanadium or molybdenum complexes used in oxidation reactions. These reactions are foundational in the synthesis of pharmaceuticals, polymers, agrochemicals, and fine chemicals. From an industrial perspective, transition metal catalysts contribute significantly to green chemistry by reducing energy consumption, minimizing waste, and improving atom economy. Homogeneous and heterogeneous catalytic systems utilizing metals such as nickel, cobalt, platinum, and copper have revolutionized large-scale processes like petroleum refining, ammonia synthesis, and polymer production. This paper emphasizes the importance of understanding catalytic mechanisms at the molecular level to design more efficient and environmentally friendly catalysts. Continued research in this area holds promise for the development of novel catalytic systems tailored to meet the demands of sustainable chemical manufacturing and energy transformation.

This thesis describes the synthesis of a broad range of heterocyclic carbene complexes of transition metals, their fundamental reaction chemistry and their use as catalysts in a number of reactions. Mechanistic studies have been performed in order to gain insight into likely catalytic cycles and the decomposition processes of the complexes. Pd complexes of donor-functionalised heterocyclic carbene ligands have been synthesised through a route involving carbene transfer from Aglcarbene precursors. A variety of complexes containing 0 and N donor-functionalised carbenes have been prepared and tested as catalysts for Heck-type and polymerisation reactions. The complexes are highly active for Heck and Suzuki couplings, but do not catalyse chain growth reactions such as ethylene polymerisation or CO-ethylene copolymerisation. Zerovalent carbene complexes of Pd of formula Pd(tmiy)2(alkene) (tmiy = 1,3,4,5- tetramethylimidazolin-2-ylidene, alkene = maleic anhydride, tetracyanoethylene) have been synthesised. Spectroscopic studies on the complexes provide strong evidence of the almost purely donor nature of the carbene ligand. Reaction with a number of substrates, including aryl halides, leads to Pd11 complexes through oxidative addition. The zerovalent Ni complex Ni(tmiy)2 has also been produced in situ and found to undergo oxidative addition with organohalides and dihalides. Halide abstraction from Pd(tmiy)2(4-nitrophenyl)I in the presence of n-butylacrylate leads to migratory insertion of the olefin and elimination of the Heck coupling product. Under stoichiometric reaction conditions hydrocarbyl-imidazolium ions are also produced as major byproducts, indicating that carbene elimination as the hydrocarbyl-imidazolium salt is a probable route to catalyst deactivation. Additionally, the reaction of CO with [PdMe(tmiy)(µ-C1)]2 has been studied, and the migratory insertion of CO into the PdMe bond, followed by rapid decomposition by elimination of an acyl-imidazolium cation is reported. This observation provides an explanation for the lack of activity of the Pd carbene complexes in chain growth reactions. Elimination of the imidazolium ion from hydrocarbyl Pd and Ni carbene complexes was considered a potentially serious route to catalyst deactivation. In order to elucidate the mechanism of the reaction a number of new methyl Pdri complexes of the form [PdMe(tmiy)L2]BF4 (L = cyclooctadiene, methyldiphenylphosphine, triphenylphosphite, triphenylphosphine) were prepared and their decomposition kinetics studied. The complexes decompose via elimination of pentamethylimidazolium and generation of Pd° following first order kinetics. The kinetic investigations combined with density functional studies show that the complexes decompose via a mechanism of concerted reductive elimination. An understanding of the mechanism of the reaction led to the prediction of methods by which it can be impeded. Density functional studies of Ni, Pd and Pt complexes show that the energetics of the reaction can be changed such that oxidative addition of imidazolium to M ° is energetically favourable. This is demonstrated experimentally by the synthesis of hydrido Pt carbene complexes, via oxidative addition of azolium salts to Pt °. The results suggest that conducting reactions in ionic liquids of imidazolium salts will be a promising method of stabilising M-carbene complexes against decomposition. Consistent with this proposal, Ni complexes of the form NiI2(carbene)2 have been synthesised (and crystal structures obtained) and shown to be very active and stable catalysts for olefin dimerisation when immobilised in ionic liquids. A variety of carbenes and imidazolium salts with two flanking donor groups were prepared and their coordination behaviour investigated. Coordination of a dipicolyl carbene ligand to Pd was found to increase the stability of the complex (to reductive elimination) relative to other cationic methyl Pd carbene complexes. The ligands were tested in conjunction with the metals Ti, V, Cr and Pd as catalysts for ethylene polymerisation, and several combinations were found to give moderately active catalysts.

The rates of CO2 insertion into trans‐Ru(dmpe)2(Me)H [1, dmpe = 1,2‐bis(dimethylphosphino)ethane] and trans‐Ru(dmpe)2(Me)2 (2) derivatives were monitored by in situ infrared and 1H NMR spectroscopy. The reactions are first order in both CO2 and metal complex concentrations, and CO2 insertion into the Ru–H bond of 1 occurs instantaneously at 0 °C. The reverse process, decarboxylation, was observed to occur readily at ambient temperature as revealed by 13CO2 exchange with subsequent CO2 insertion into the Ru–Me bond at higher temperatures. No further CO2 insertion into the Ru–H bond of the resulting acetate complex was observed. The activation barrier for CO2 insertion into the first Ru–Me bond of 2 was determined to have ΔH‡ and ΔS‡ values of 12.7 ± 0.6 kcal mol–1 and –31.9 ± 2.0 e.u., respectively, which are indicative of a highly ordered transition state. The rate of CO2 insertion into the second Ru–Me bond was two orders of magnitude slower at ambient temperature and resulted in the formation of trans‐Ru(dmpe)2(O2CMe)2. In general, the insertion of CO2 into the Ru–H or Ru–Me bonds of trans‐Ru(dmpe)2(X)R (R = H or Me) was disvavored in the presence of poorly electron‐donating X ligands. For example, the insertion of CO2 into the Ru–H bond of trans‐Ru(dmpe)2(Cl)H was not observed even under forcing conditions. Computational results were in excellent agreement with these observations and predict a significant enhancement in CO2 activity and resultant complex stability if dmpe is replaced with tetramethylethylenediamine (tmeda).

### Background For the past 37 years the Mineralogical Society of America, and in conjunction with the Geochemical Society (since 2000), have sponsored and published 72 review volumes that communicate the results of significant advances in research in the Earth sciences. Several of these have either directly or indirectly addressed the fundamental importance, role, and behavior of volatile components on processes influencing magma rheology, crystallization, evolution, eruption, and related metasomatism and mineralization. Volume 30—which was published in 1994—focused on this topic broadly, and this volume has provided a lasting summary on the geochemical and physical behaviors of a wide variety of magmatic volatiles (Carroll and Holloway 1994). Since that year, continued research has brought important and new knowledge about the role of the volatile component sulfur in natural magmas, and significant progress was made simultaneously in understanding the role of sulfur in industrial or technical processes such as glass or steel production. Here, in volume 73, we have assembled in 15 chapters the current state of research concerning sulfur in melts based on the extensive experience of various authors practically working on these topics. The behavior of sulfur in melts and its implications for natural and industrial processes are still insufficiently understood, and hence, are difficult to apply as a tool for interpreting problems of geological or industrial interest. In recent decades, various new investigations in the geosciences as well as in the engineering and material sciences have employed modern spectroscopic, analytical, theoretical, and experimental techniques to improve our understanding of the complex and volatile behavior of sulfur in a wide variety of molten systems. However, these different research initiatives (e.g., empirical vs. applied research and natural vs. technical applications) were rarely well integrated, and the scientific goals were usually approached with specific and relatively focused points of view. Consequently, bridging this …