Role of the Ligand and of the Size and Flexibility of the Palladium−Ancillary Ligand Cycle on the Reactivity of Substituted Alkynes toward Palladium(0) Complexes Bearing Potentially Terdentate Nitrogen−Sulfur−Nitrogen or Nitrogen−Nitrogen−Nitrogen Ligands:  Kinetic and Structural Study

Abstract

The reaction between palladium(0) complexes bearing potentially terdentate ligands and dimethyl acetylenedicarboxylate (DMA) to give the corresponding palladacyclopentadiene complexes was studied under kinetic conditions. The reactivity of the complexes was markedly influenced by the nature of the ancillary ligand. Thus, when pyridyldithioether (SNS) and dipyridylthioether (NSN) ligands are used, the reactivity and the rate law of the corresponding derivatives are similar to those of the unsubstituted bidentate pyridylthioether substrates and, therefore, a marked rate increase can be obtained only by reduction of the olefin steric requirement. When terdentate NNN ligands are used, an apparent difference in reactivity between the derivatives bearing the pyridine−amine−pyridine and pyridine−amine−quinoline ligands is observed. On the basis of a detailed structural study (NMR, X-ray) and on kinetic investigations, an interpretation which takes into account the flexibility of the cycle formed between the ligand and palladium is proposed. Thus, irrespective of the size of the cycle, the complexes in which the ligand forms flexible cycles undergo ring opening less easily, with a consequent reduction of reactivity. Conversely, rigid rings cannot undergo associative attack without companion ring opening, this phenomenon being crucial in favoring the alkyne attack.

Synthesis, stability and reactivity of palladium(0) olefin complexes bearing labile or hemi-labile ancillary ligands and electron-poor olefins

“…Interestingly, when potentially ter-dentate nitrogen ligands are employed the usual trigonal arrangement around palladium is observed since one coordinating atom of the ligand itself remains uncoordinated. [75] coordination occurs via the aromatic pyridine (or quinoline) nitrogen with the consequent formation of an eight-membered ring, the central amino group of the ancillary ligand being uncoordinated in keeping with the general observation that electron delocalization from the electron rich metal centre to an electron accepting moiety (pyridine or quinoline) would better stabilize the ensuing molecule. For the sake of completeness it may be interesting to cite the works of Gade, Spek, and Privalov in which ligands or olefins not described in Schemes 1 and 2 were used.…”

“…The mechanism of attack of substituted alkynes on palladium(0) olefin derivatives bearing bi-and ter-dentate ligands giving palladacyclopentadiene complexes was studied in detail for the first time [44,75]. The reactivity of the complexes was found to be strongly dependent on the bulkiness of the entering alkyne Table 3 Kinetic constants for the oxidative addition of ArI to [Pd(g 2 -dmfu)(P-NR)] in different solvents at 25°C in the presence of dmfu (30-fold in excess).…”

“…X-ray diffractometric studies 10.1. Complexes bearing mono-bi-and ter-dentate nitrogen ligands A significant number of palladium(0) olefin compounds bearing the title ligands was synthesized by several authors [30,31,35,37,75,[78][79][80][81][82][83][84][85][86]. All the structures display the typical trigonal arrangement around the palladium centre with the Pd-N bond lengths being scarcely influenced by the olefin nature, spanning within 2.10 and 2.18 Å.…”

Addition of halogens and interhalogens on palladacyclopentadienyl complexes stabilized by pyridyl−thioether N−S spectator ligands

“…The ligands NS-R (R ¼ Me, t-Bu, Ph) [7] and the complexes 1aec [8], were obtained according to published protocols by adding the appropriate ligand to the polymer [PdC 4 (COOMe) 4 ] n [9]. The complexes 2aec, 3aec, 4aec and 5a are newly synthesized compounds.…”

“…As already stated the ligands [7] and the complexes 1 [8] were synthesized according to published procedures.…”