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Introduction to Organometallics
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Samir H. Chikkali, Sandeep Netalkar
The oxidative addition (OA) may be described as the process of addition of a substrate molecule to the transition metal complex accompanied by an increase in the oxidation state of the metal ion by +2 units (Figure 1.17). The exact opposite of oxidative addition is reductive elimination (RE), where the two ligands (Y and Z) are eliminated from the metal center as Y–Z.
Living Polymerizations of π-Conjugated Semiconductors
Published in John R. Reynolds, Barry C. Thompson, Terje A. Skotheim, Conjugated Polymers, 2019
Jeffrey Buenaflor, Christine Luscombe
The electronic character of the monomer also influences KCTP, specifically the stability of the Ni(0) π-aryl complex. When the complex is very stable or unstable, intramolecular transfer for oxidative addition is hindered. This is due to either the dissociation or trapping of the Ni(0) catalyst. Controlled polymerizations in KCTP have typically employed electron-rich monomers, such as thiophenes. On the other hand, examples of controlled polymerizations with n-type monomers via KCTP are limited. It has been reasoned that the weaker π-donation from the n-type polymer backbone hinders intramolecular transfer for oxidative addition due to a less stable Ni(0) π-aryl complex.47–49 Alternatively, it has been suggested that preferential binding of the Ni(0) catalyst on electron-deficient arenes inhibits oxidative addition.50–52 This hypothesis can be attributed to π-backbonding, which is known to be stronger with electron-poor ligands as observed with alkenes.53 Computational studies by Bilbrey showed an elongation of the unsaturated C-C bond on the thiophene, indicating the occurrence of π-backbonding with the Ni(0) π-aryl complex.13 For either scenario, chain termination reactions start to become competitive, reducing control over polymerization. For the synthesis of poly(pyridine-2,5-diyl), Yokozawa et al. observed the dissociation of the Ni catalyst. It was proposed that coordinations between the pyridine nitrogen and the Ni metal center of two monomers resulted in an increased tendency for disproportionation.47 Another example of catalyst dissociation involving the synthesis of a P3HT-pyridine copolymer will be discussed in a later section. Examples of a very stable Ni(0) complex were observed in the attempted polymerization of thiophene-benzothiadiazole-thiophene, thienothiophenes and p-phenylene vinylenes (Scheme 6.9).50,53,54 The very stable Ni(0) complex prevented polymerization from occurring as the Ni catalyst remained trapped and unable to perform the subsequent oxidative addition. Pd metal containing catalysts were able to polymerize the thienothiophenes monomer, due to the weaker association of Pd(0) intermediate to the polymer backbone.53 It is worth noting that thienothiophenes are electron-rich monomers, while benzothiadiazole is electron-deficient. It is not clear as to what is the dominating factor that leads to a stable Ni(0) π-aryl complex, and that it may vary depending on the monomer and ligand structure.
N-Heterocyclic carbene-Pd(II)-PPh3 complexes as a new highly efficient catalyst system for the Sonogashira cross-coupling reaction: Synthesis, characterization and biological activities
Published in Journal of Coordination Chemistry, 2018
L. Boubakri, L. Mansour, A. H. Harrath, I. Özdemir, S. Yaşar, N. Hamdi
Since the discovery of the first N-heterocyclic metal-carbene complexes in 1968 by the Öfele [1] and Wanzlick [2] groups, these compounds have aroused great interest in the field of coordination chemistry, which has been redoubled after the isolation of the first free-NHC in 1991 by Arduengo [3]. Later studies from Herrmann showed that NHC and phosphine have important similarities in properties, which make them particularly useful ligands for applications in catalysis [4]. However, NHCs are in general more σ-donating and have lower electronic parameters than phosphines. This leads to the remarkable stability of the metal-carbene bond. Different transition metal-NHC complexes have been synthesized [5] and used as catalyst on various organic transformations [6]. The prominence of metal–NHC complexes is likely due to their catalytic efficiency and robustness against air, moisture and high temperature. The basis of this interest lies in the facile modulation of electronic and steric parameters reflected by a strong σ-donor and weak π-acceptor ability of NHC ligand. Complexes bearing sterically bulky and electron-rich ligands exhibit enhanced catalytic activity in oxidative addition and reductive elimination reactions that are key elemental steps of many catalytic reactions [7] (Scheme 1).
Synthesis and structural characterization of palladium(II) 2-(arylazo)naphtholate complexes and their catalytic activity in Suzuki and Sonogashira coupling reactions
Published in Journal of Coordination Chemistry, 2019
Sathya Munusamy, Premkumar Muniyappan, Venkatachalam Galmari
Steric bulk of the ligand is believed to increase the rate of reductive-elimination to regenerate the active catalyst. Despite the bulky ligand increasing the reductive elimination rate, the rate of the oxidative addition step can be dramatically affected by the increasing size of the ligand. According to these points a balance between the steric and electronic properties of the ligand is an essential requirement. Though many reports are available on azo ligands with ruthenium and osmium, less research has been reported for palladium(II) arylazo complexes. We are therefore interested in continuing our studies on synthesis of catalysts derived from inexpensive and easily synthesized ligand sets.
Understanding oxidative addition in organometallics: a closer look
Published in Journal of Coordination Chemistry, 2022
Nabakrushna Behera, Sipun Sethi
The concept of oxidative addition is essential in organometallic chemistry as it has a significant role for synthesis and catalysis. Discussion can be initiated from the Vaska’s complex, as early study of oxidative addition was made by Vaska in the 1960s [1]. Vaska in conjunction with DiLuzio reported trans-IrCl(CO)(PPh3)2 (1), a 16-electron species, and studied its reaction with HCl which resulted in the formation of IrHCl2(CO)(PPh3)2 in a process known today as oxidative addition (OA) [1]. A year later, the reactions of 1 with Cl2 as well as H2 yielded IrCl3(CO)(PPh3)2 and IrH2Cl(CO)(PPh3)2 [2]. This reactivity with small molecules provided a benchmark status to Vaska’s complex in the field of transition metal organometallic chemistry [3]. A closer look at the above reaction products reveals the following: (a) facile addition of small molecules occur at the metal center, (b) formal oxidation state of metal increases by two units, (c) coordination number of metal increases by two units, and (d) electron count increased by two units. The type of changes that occur in the above reactions are the basis for oxidative addition. Although these reactions are widely accepted as OA, Vaska did not mention in his original papers that these are oxidative addition reactions. Collman, in 1965, used the term ‘oxidative addition’ while describing the related chemistry of Ru(CO)3(PPh3)2 with the citation of Vaska’s work and others [4]. Although reductive elimination, which is an equally important step in many organometallic catalytic processes, is very closely associated with oxidative addition reaction reversibly, it will not be discussed here as it would far exceed the scope of this account. Thus, the focus will be made on the oxidative addition reactions covering a rather extensive class of reactions.