Pentacoordinate complexes are often proposed to be intermediates in palladium-catalyzed reactions, but the true nature of such intermediates usually remains elusive. Our interest in the field originated by a mechanistic study on the palladium-phenanthroline catalyzed carbonylation of nitroarenes to afford isocyanates (ArNCO) or carbamates (ArNHCOOR). Mechanistic studies point to Pd(Phen)(COOMe)2 (1a) as a key species. A kinetic study indicates that reversible coordination of a CO ligand to 1a occurs. By using Phen as ligand, no intermediate is spectroscopically detectable. By using 2,9-Me2Phen, the intermediate CO adduct is observed, but it is too unstable to be isolated. Several analogous derivatives having carboxylato groups in place of the two –COOMe moieties were also detected but not isolated and, by working at -50 °C, crystals of Pd(2,9-Me2Phen)Cl2(CO) could be grown and the structure solved by X-ray diffraction. The complex is better described as square planar, with a weak additional Pd-N interaction, rather than as a true pentacoordinate adduct. This has important consequences for catalysis since it implies that the ligand becomes monodentate during the catalytic cycle
Tetra- versus Penta-Coordination in Palladium Carbonyl Complexes. X-ray Structure of Pd(2,9-Me2Phen)Cl2(CO) and Relevance to the Reductive Carbonylation of Nitroarenes and Oxidative Carbonylation of Amines / F. Ragaini, M. Gasperini, S. Cenini, L. Arnera, A. Caselli, P. Macchi, N. Casati. ((Intervento presentato al 1.. convegno European Chemistry Congress tenutosi a Budapest (H) nel 2006.
Tetra- versus Penta-Coordination in Palladium Carbonyl Complexes. X-ray Structure of Pd(2,9-Me2Phen)Cl2(CO) and Relevance to the Reductive Carbonylation of Nitroarenes and Oxidative Carbonylation of Amines
F. RagainiPrimo
;M. GasperiniSecondo
;S. Cenini;A. Caselli;P. MacchiPenultimo
;N. CasatiUltimo
2007
Abstract
Pentacoordinate complexes are often proposed to be intermediates in palladium-catalyzed reactions, but the true nature of such intermediates usually remains elusive. Our interest in the field originated by a mechanistic study on the palladium-phenanthroline catalyzed carbonylation of nitroarenes to afford isocyanates (ArNCO) or carbamates (ArNHCOOR). Mechanistic studies point to Pd(Phen)(COOMe)2 (1a) as a key species. A kinetic study indicates that reversible coordination of a CO ligand to 1a occurs. By using Phen as ligand, no intermediate is spectroscopically detectable. By using 2,9-Me2Phen, the intermediate CO adduct is observed, but it is too unstable to be isolated. Several analogous derivatives having carboxylato groups in place of the two –COOMe moieties were also detected but not isolated and, by working at -50 °C, crystals of Pd(2,9-Me2Phen)Cl2(CO) could be grown and the structure solved by X-ray diffraction. The complex is better described as square planar, with a weak additional Pd-N interaction, rather than as a true pentacoordinate adduct. This has important consequences for catalysis since it implies that the ligand becomes monodentate during the catalytic cyclePubblicazioni consigliate
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