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

Tsun Hung Hsiao, Tzu Liang Wu, Sandipan Chatterjee, Chin Yi Chiu, Hon Man Lee, Lorenzo Bettucci, Claudio Bianchini, Werner Oberhauser

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Abstract

The imidazolium salts 1,1′-dibenzyl-3,3′-propylenediimidazolium dichloride and 1,1′-bis(1-naphthalenemethyl)-3,3′-propylenediimidazolium dichloride have been synthesized and transformed into the corresponding bis(NHC) ligands 1,1′-dibenzyl-3,3′-propylenediimidazol-2-ylidene (L1) and 1,1′-bis(1-naphthalenemethyl)-3,3′-propylenediimidazol-2-ylidene (L2) that have been employed to stabilize the PdII complexes PdCl22-C,C-L1) (2a) and PdCl22-C,C-L2) (2b). Both latter complexes together with their known homologous counterparts PdCl22-C,C-L3) (1a) (L3 = 1,1′-dibenzyl-3,3′-ethylenediimidazol-2-ylidene) and PdCl22-C,C-L4) (1b) (L4 = 1,1′-bis(1-naphthalenemethyl)-3,3′-ethylenediimidazol-2-ylidene) have been straightforwardly converted into the corresponding palladium acetate compounds Pd(κ1-O-OAc)22-C,C-L3) (3a) (OAc = acetate), Pd(κ1-O-OAc)22-C,C-L4) (3b), Pd(κ1-O-OAc)22-C,C-L1) (4a), and Pd(κ1-O-OAc)22-C,C-L2) (4b). In addition, the phosphanyl-NHC-modified palladium acetate complex Pd(κ1-O-OAc)22-P,C-L5) (6) (L5 = 1-((2-diphenylphosphanyl)methylphenyl)-3-methyl-imidazol-2-ylidene) has been synthesized from corresponding palladium iodide complex PdI22-P,C-L5) (5). The reaction of the former complex with p-toluenesulfonic acid (p-TsOH) gave the corresponding bis-tosylate complex Pd(OTs)22-P,C-L5) (7). All new complexes have been characterized by multinuclear NMR spectroscopy and elemental analyses. In addition the solid-state structures of 1b·DMF, 2b·2DMF, 3a, 3b·DMF, 4a, 4b, and 6·CHCl3·2H2O 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 7′ under catalytic reaction conditions.

Original languageEnglish
Pages (from-to)4014-4024
Number of pages11
JournalJournal of Organometallic Chemistry
Volume694
Issue number25
DOIs
Publication statusPublished - 2009 Dec 1

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All Science Journal Classification (ASJC) codes

  • Biochemistry
  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry
  • Materials Chemistry

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