Ligands carbon dioxide

A chain mechanism is proposed for this reaction. The first step is oxidation of a carboxylate ion coordinated to Pb(IV), with formation of alkyl radical, carbon dioxide, and Pb(III). The alkyl radical then abstracts halogen from a Pb(IV) complex, generating a Pb(IIl) species that decomposes to Pb(II) and an alkyl radical. This alkyl radical can continue the chain process. The step involving abstraction of halide from a complex with a change in metal-ion oxidation state is a ligand-transfer type reaction. 

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

Note that carbon monoxide inserts into the Zr-H bond of 1 (2 equiv.) to afford an T -formaldehydo-type complex [(Cp2ZrCl)]2(g-CH20) [200-202]. Iminoacyl zir-conocene complexes are formed after addition of 1 to isonitriles [203]. Carbon dioxide [183, 202] is reduced to formaldehyde with 1 (2 equiv.). C02-like molecules such as isocyanates RNCO [204], isothiocyanates RNCS [205], and carbodiimides RNCNR [204] are readily converted to the corresponding bidentate form-amido ligands. 

In 1963 Vaska 164) discovered that the iridium complex Ir(PPh3)2C (CO) takes up molecular oxygen reversibly with 1 1 stoichiometry. This complex has since been shown to reversibly sorb (1 1) ethylene (755), carbon dioxide (765), F2C=CF2 and F3C—C=C—CF3 (767), as well as various other ligands (765). Ibers md La Placa (769)... 

Kostic et al. reported the use of various palladium(II) aqua complexes as catalysts for the hydration and alcoholysis of nitriles,435,456 decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.457 Labile aqua or other solvent ligands can be displaced by a substrate. In many cases, the coordinated substrate thus becomes activated toward nucleophilic addition of water or alcohols. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

The characterization and crystal structure of the dimer [Pt2( -dppm)3] (dppm = bis(diphenyl-phosphino)methane), first reported as a deep red complex in 1978, was described by Manojlovic-Muir et al. in 1986.11 The structure, the first of its type, is made up of two parallel and almost eclipsed trigonal-planar platinum moieties bridged by three diphosphine ligands. The Pf Pt separation is 3.0225(3) A, too long to be considered a bond.11 [Pt2(//-dppm)3] catalyzes the hydrogenation/reduction of carbon dioxide with dimethylamine to give dimethylformamide12 (Equation (1)) and the reverse reaction.13... 

Synthesis of functional models of carbonic anhydrase has been attempted with the isolation of an initial mononuclear zinc hydroxide complex with the ligand hydrotris(3-t-butyl-5-methyl-pyrazolyl)borate. Vahrenkamp and co-workers demonstrate the functional as well as the structural analogy to the enzyme carbonic anhydrase. A reversible uptake of carbon dioxide was observed, although the unstable bicarbonate complex rapidly forms a dinuclear bridged complex. In addition, coordinated carbonate esters have been formed and hydrolyzed, and inhibition by small ions noted.462 A number of related complexes are discussed in the earlier Section 6.8.4. 

Triply bridging carbonates between three zinc centers have been identified in nine different X-ray structures deposited in the CSD 458,461,465-467 For example, a binuclear ft-OH zinc complex with a tetradentate /V-donor ligand absorbs atmospheric carbon dioxide to a triply bridged carbonate.468 Examples are also known where the metal atoms are in varying coordination environments. The complex cation [Zn3(bipyridine)6(/U3-C03)(H20)2]4+ contains one penta- and two hexacoordinate zinc centers.469 A tetrapodal compartmental ligand forms a tetrameric complex with zinc that contains the carbonate bridging between three of the four zinc centers.470... 

Organozinc diiminates of various ligand platforms (particularly /2-diketiminates (BDIs)) have been used extensively in catalysis, especially for the polymerizations of lactides and epoxides and the co-polymerization of epoxides with carbon dioxide. 

Iridium(III) hydride forms complexes with DIOP, BDPP (2,4-bis(diphenyl-phosphino)pentane), NORPHOS, and BINAP ligands to produce amines in 11 -80% ee.679 Similar modest results are obtained in the reduction of N-arylketimines with an iridium(HI) complex with (2S,3 S) -C HIRA PHOS as the chiral ligand.680 The indium complexes with chiral phosphinodihydrooxazoles catalyze the enantioselective hydrogenation of imines in supercritical carbon dioxide with up to 80% ee, but generally lower ee values are observed in... 

Keurentjes et al. performed a continuous hydrogenation of 1-butene in supercritical carbon dioxide.[9,10] A fluorous derivative of Wilkinson s catalyst was prepared in situ by mixing the ligand with [(COD)RhCl]2 under hydrogen / carbon dioxide pressure (Figure 4.37). 

It has already been mentioned that utilization of the permethylated ligand (L19)2 in place of (L23)2 drastically alters the ease of substitution reactions of the [M2(L19)(C1)]+ complexes (Section III.D). Further studies revealed a remarkable influence of the hydrophobic pocket on the rate and course of several substrate transformations, as for instance the fixation of carbon dioxide (239) (Scheme 8), the cis-bromination of a,/)-unsaturated carboxylate ligands (256), and some Diels-Alder reactions (215). Of these the latter two reactions will now be discussed. 

Carbon Dioxide 0=C=0 The reactions of the metallocene sources 1-6 with carbon dioxide depend strongly on the metal and ligands used. Complex 1 gives, by elimination of half of the alkyne, the dimer 93, which forms the titanafuranone 94 after aerial oxidation [49]. 

Y. Kayaki, Y. Noguchi, S. Iwasa, T. Ikariya, R. Noyori, An Effitient Carbonylation of Aryl Halides Catalysed by Palladium Complexes with Phosphite Ligands in Supercritical Carbon Dioxide , Chem Commun. 1999, 1235-1236. 

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