6533b7cefe1ef96bd12578a9

RESEARCH PRODUCT

Preparation and reactions of palladium(II) complexes with C2-bonded heteroaromatic ligands trans[PdCl(RN)(PPh3)2] (RN = 2-pyridyl, 2-pirazyl, 2-pyrimidyl group). A new reaction pathway in the insertion of isocyanides into the Pdc bond of trans-[PdXR(L)2] compounds

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Abstract The complexes trans -[PdCl(R N )(PPh 3 ) 2 ] (I) [R N = 2-pyridyl (2-Py), 2-pyrazyl (2-pyz), 2-pyrimidyl (2-pym) group] have been prepared in high yield by deprotonation with NEt 3 of the corresponding cationic compounds trans [PdCl(R N H) (PPh 3 ) 2 ] + (R N H = N -protonated C 2 -heteroaromatic ligand) in the presence of an excess of PPh 3 . In chlorinated solvents, complexes I undergo a slow reversible dimerization into the binuclear derivatives [PdCl(μ-R N )(PPh 3 )] 2 (II) (μ-R N = C 2 , N 1 -bridging ligand). From the 31 P NMR spectra in 1,2-dichloroethane the following dissociation constants were obtained: 1.9 mol 1 −1 (R N = 2-py), 5.1 × 10 −2 (2-pym), 6.6 × 10 −3 (2-pyz). The dimerization becomes fast and quantitative if the PPh 3 , involved in the equilibrium is removed by oxidation or by reaction with [PdCl(η 3 -2-MeC 3 H 4 )] 2 . Only the 2-pyridyl complex Ia reacts (slowly) with CO yielding the migratory insertion product trans -[PdCl{C(2-py)O}(PPh 3 ) 2 ], together with the dimer IIa. All the complexes I undergo migratory insertion of t-butylisocyanide with formation of trans -[PdCl{C(R N ) = NCMe 3 }(PPh 3 ) 2 ]] at rates which depend on the heterocyclic group (R N = 2-py > 2-pyz ⪢ 2-pym). The reaction of the 2-pyrazyl complex Ib with CNCMe 3 has been studied in detail by conductivity measurements and by IR and 31 P NMR spectroscopy. The data suggest a complex mechanism in which insertion occurs through rearrangement of a four-coordinate intermediate [PdCl(2-pyz)(CNCMe 3 )(PPh 3 )], and through interaction of a cationic intermediate trans -[Pd(2-pyz)(CNCMe 3 )(PPh 3 ) 2 ] + (Vb) with Cl − and with the free isocyanide of the initial equilibria. The occurrence of the latter reactions is confirmed by independent experiments in which the cationic complex Vb (isolated as perchlorate salt) is treated with an equimolar amount of [AsPh 4 ]Cl or CNCMe 3 . The isocyanide-promoted insertion step represents a new mechanistic pathway for isocyanide insertion into the PdC bond of trans -[PdXR(L) 2 ] complexes.