C3 -Symmetric Assemblies from Trigonal Polycarbene Ligands and MI Ions for the Synthesis of Three-Dimensional Polyimidazolium Cations

Abstract

Metallosupramolecular poly-NHC-metal assemblies were prepared from trigonal hexakis (H₆ -1 a(PF₆ )₆ and H₆ -1 b(PF₆ )₆ ) or nonakis (H₉ -3(BF₄ )₉ ) imidazolium salts and Ag₂ O. Complexes [Ag₆ (1 a)₂ ](PF₆ )₆ and [Ag₆ (1 b)₂ ](PF₆ )₆ are built from six Ag⁺ ions sandwiched between two trigonal hexacarbene ligands with an inner and an outer NHC donor in each of the three ligand arms. The metal atoms are arranged in two triangles. The hexakis-NHC ligands bear cinnamic ester groups at the outlying NHC donors, used in postsynthetic [2+2] cycloaddition reactions linking two hexakis-NHC ligands by three cyclobutane units to give complexes [Ag₆ (2 a)](PF₆ )₆ and [Ag₆ (2 b)](PF₆ )₆ bearing a dodecacarbene ligand. From the related nonakisimidazolium salt H₉ -3(BF₄ )₉ , complex [Ag₉ (4)](BF₄ )₉ bearing an octadecacarbene ligand was obtained. Removal of the template metals yielded very large, stable, polyimidazolium cations with 12 or 18 internal imidazolium groups.

Keywords: N-heterocyclic carbenes; organometallic chemistry; photochemical modification; polyimidazolium cages; supramolecular chemistry.