Trinuclear complexes of palladium(ii) with chalcogenated N-heterocyclic carbenes: catalysis of selective nitrile–primary amide interconversion and Sonogashira coupling

Abstract

3-Methyl-1-(2-(phenylthio/seleno)ethyl)-1H-benzo[d]imidazol-3-ium iodide (L1/L2), a precursor of sulfated/selenated N-heterocyclic carbene, was synthesized by the reaction of benzimidazole with 1,2-dichloroethane followed by treatment with PhS/SeNa and MeI. The reaction of L1/L2 with Ag₂O followed by treatment with [Pd(CH₃CN)₂Cl₂] (metal to ligand ratio 3 : 2), i.e. transmetallation, resulted in trinuclear palladium(II) complexes [Pd₃(L1/L2-HI)₂(CH₃CN)Cl₆] (1–2). The complexes were characterized with ¹H, ¹³C{¹H} and ⁷⁷Se{¹H} NMR (2 only), elemental analyses, HR-MS and single-crystal X-ray diffraction. The geometry of three Pd atoms in each complex is nearly square planar. The Pd–S/Se, Pd–C, Pd–N and Pd–Cl bond distances (Å) in 1/2 are 2.3179(19)/2.4312(10), 1.968(7)/1.952(4), 2.073(8)/2.079(4) and 2.2784(19)–2.298(2)/2.292(2)–2.3003(15), respectively. In both the complexes, all Cl are trans to each other. For the central Pd atom, two benzimidazole rings are also trans to each other. The C–H⋯Cl non-covalent interactions result in a three-dimensional network. The moisture and air insensitive trinuclear Pd(II) complexes 1 and 2 are thermally stable and efficient as a catalyst for nitrile–amide interconversion and amine-free Sonogashira C–C coupling (in the presence of CuI). The optimum temperature is 80 °C for the interconversion and 110 °C for the coupling. The catalytic protocols are applicable to both aliphatic and aromatic amides/nitriles. The optimum catalyst loading is 1 mol% for the C–C coupling and 0.5 to 1 mol% for the interconversion. K₂CO₃ as a base gives the best result for Sonogashira C–C coupling. In the conversion of nitriles to amides, the formation of an acid was not detected. After using once, 1/2 can carry out the conversion of ten fresh lots of nitriles to amides with almost the same efficiency. The real catalytic species for the interconversion and coupling appear to be based on Pd(II) and Pd(0), respectively.