Pd2L2 metallacycles as molecular containers for small molecules

Abstract

M₂L₂ type metallacyclic complexes, [Pd₂(L1)₂Cl₄]·1.5CH₂Cl₂ (1), [Pd₂(L1)₂Cl₄]·2CHCl₃ (2), [Pd₂(L2)₂Cl₄]·2CH₂Cl₂·2CH₃CN (3), [Pd₂(L2)₂Cl₄]·2CHCl₃·2CH₃CN (4) and [Pd₂(L3)₂Cl₄]·CH₂Cl₂·2CH₃CN (5), have been prepared from three semi-rigid benzimidazol or benzotriazol ligands, 1,4-bis(benzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L1), 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L2) and 1,4-bis(benzotriazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L3). All the complexes were structurally characterized by single-crystal X-ray diffraction and the phase purity was confirmed by powder X-ray diffraction (PXRD) measurements. The solution structure of representative complex 1 was studied by ¹H NMR titration and ESI mass spectroscopy. The thermal stability and guest-exchange properties of 1, 3 and 4 were investigated, revealing that the Pd₂L₂ metallacycles can act as a selective receptor for CH₂Cl₂ or CHCl₃ guest molecules. The catalytic activity of 1 in Suzuki–Miyaura coupling reaction was also studied and 1 could be recycled at least 5 times under heterogeneous conditions, indicative of a potential self-supported catalyst.