Palladium acetate complexes bearing chelating N-heterocyclic carbene (NHC) ligands: Synthesis and catalytic oxidative homocoupling of terminal alkynes

The imidazolium salts 1,10-dibenzyl-3,30-propylenediimidazolium dichloride and 1,10-bis(1-naphthalenemethyl)- 3,30-propylenediimidazolium dichloride have been synthesized and transformed into the corresponding bis(NHC) ligands 1,10-dibenzyl-3,30-propylenediimidazol-2-ylidene (L1) and 1,10-bis(1- naphthalenemethyl)-3,30-propylenediimidazol-2-ylidene (L2) that have been employed to stabilize the PdII complexes PdCl2(j2-C,C-L1) (2a) and PdCl2(j2-C,C-L2) (2b). Both latter complexes together with their known homologous counterparts PdCl2(j2-C,C-L3) (1a) (L3 = 1,10-dibenzyl-3,30-ethylenediimidazol-2-ylidene) and PdCl2(j2-C,C-L4) (1b) (L4 = 1,10-bis(1-naphthalenemethyl)-3,30-ethylenediimidazol-2-ylidene) have been straightforwardly converted into the corresponding palladium acetate compounds Pd(j1-O-OAc)2(j2-C,C-L3) (3a) (OAc = acetate), Pd(j1-O-OAc)2(j2-C,C-L4) (3b), Pd(j1-O-OAc)2(j2-C,C-L1) (4a), and Pd(j1-O-OAc)2(j2-C,C-L2) (4b). In addition, the phosphanyl-NHC-modified palladium acetate complex Pd(j1-O-OAc)2 (j2-P,C-L5) (6) (L5 = 1-((2-diphenylphosphanyl)methylphenyl)-3-methyl-imidazol- 2-ylidene) has been synthesized from corresponding palladium iodide complex PdI2(j2-P,C-L5) (5). The reaction of the former complex with p-toluenesulfonic acid (p-TsOH) gave the corresponding bis-tosylate complex Pd(OTs)2(j2-P,C-L5) (7). All new complexes have been characterized by multinuclear NMR spectroscopy and elemental analyses. In addition the solid-state structures of 1bDMF, 2b2DMF, 3a, 3bDMF, 4a, 4b, and 6CHCl32H2O have been determined by single crystal X-ray structure analyses. The palladium acetate complexes 3a/b, 4a/b, and 6 have been employed to catalyze the oxidative homocoupling reaction of terminal alkynes in acetonitrile chemoselectively yielding the corresponding 1,4-di-substituted 1,3-diyne in the presence of p-benzoquinone (BQ). The highest catalytic activity in the presence of BQ has been obtained with 6, while within the series of palladium-bis(NHC) complexes, 4b, featured with a n-propylene-bridge and the bulky N-1-naphthalenemethyl substituents, revealed as the most active compound. Hence, this latter precursor has been employed for analogous coupling reaction carried out in the presence of air pressure instead of BQ, yielding lower substrate conversion when compared to reaction performed in the presence of BQ. The important role of the ancillary ligand acetate in the course of the catalytic coupling reaction has been proved by variable-temperature NMR studies carried out with 6 and 70 under catalytic reaction conditions.