Synthesis and structural characterization of well-defined bis(oxamato)palladate(II) precatalysts for Suzuki and Heck reactions

Abstract

Abstract A family of tetra-n-butylammonium salts of bis(oxamato)palladate(II) complexes of formula (n-Bu4N)2[Pd(Lm)2]·pH2O [m = 1–9; L1 = N-phenyloxamate (pma) and p = 2 (1), L2 = N-2-methylphenyloxamate (2-Mepma) and p = 4 (2), L3 = N-4-methylphenyloxamate (4-Mepma) and p = 2 (3), L4 = N-2,3-dimethylphenyloxamate (2,3-Me2pma) and p = 3 (4), L5 = N-2,4-dimethylphenyloxamate (Me2pma) and p = 4 (5), L6 = N-2,5-dimethylphenyloxamate (2,5-Me2pma) and p = 7 (6), L7 = N-3,4-dimethylphenyloxamate (3,4-Me2pma) and p = 6 (7), L8 = N-3,5-dimethylphenyloxamate (3,5-Me2pma) and p = 5 (8)] have been prepared and characterized by spectroscopic methods. The crystal structure of 1 that has been solved by single crystal X-ray diffraction shares main structural features with that reported for (n-Bu4N)2[Pd(L9)2]·2CH3CONH2 [L9 = N-2,4,6-dimethylphenyloxamate (2,4,6-Me3pma) and CH3CONH2 = acetamide] (see F.R. Fortea-Perez et al. (2016)). Each palladium atom in 1 is four-coordinate with two oxygen and two nitrogen atoms from two fully deprotonated pma ligands building a centrosymmetric square-planar surrounding, the Pd N/Pd O bond lengths being 2.020(3)/2.003(2) A. The bis(oxamato)palladate(II) units in 1 are well separated from each other by the bulky organic cations, the resulting value of the shortest metal–metal separation being 9.56 A. This series of very stable, phosphine-free, complexes with well-defined structures were shown to efficiently catalyze the Heck reaction of aryl halides with olefins in DMF at 80 °C in 3 h, most likely involving Pd(II)/Pd(IV) species in the vinylation reaction of aryl halides, as well as the Suzuki coupling of aryl halides with arylboronic acids in the same solvent under similar conditions. The reaction conditions for both Heck and Suzuki couplings were optimized by varying the amount of the catalyst, and their scope was systematically studied by using different olefins and aryl halides. The influence of the number and positions of the methyl substituents from the phenyloxamate ligand on the catalytic activity is analyzed and discussed in the light of the results obtained.