Incorporation of Fluorene-containing Pyridyl Ligand into Discrete Supramolecules via Coordination-Driven Self-Assembly

Abstract

During the past decade, coordination-driven self-assembly with Pt or Pd acceptors has drawn much attention and been successfully applied for the production of various discrete 2D or 3-D supramolecules. Well-matched pre-designed donors and metal-containing acceptors have provided discrete self-assembles in an almost quantitative yield. Exclusive production of discrete ensembles rather than oligomeric or polymeric structures has been unique and interesting outcome in that similar result can not be expected with other synthetic methodologies. This result could be ascribed to the fact that the coordination between metal and ligand is thermodynamically controlled and the formation of discrete entities favors in enthalpy over that of oligomeric forms. Thus, the right combination of donor and acceptor in the type of their reacting elements and bond angles was crucial for the effective production of discrete selfassemblies. Most of the reactions required specific donor and acceptors with fixed-bond angles, and the shape of supramolecular product was decided by the fixed-bond angles used. Only a few flexible ligands in the presence of template molecules self-assembled into discrete entities. Recently, ligands with variable bond angles were successfully applied without assistance of any template for the synthesis of discrete supramolecules via coordination-driven self-assembly. The important and challenging next question might be their applicability of self-assembled supramolecules. Some of their useful functions were reported as an optical sensor, reaction catalyst or molecular switch. However, their known applicability to date is rather restricted and needs to be searched for further uses. In this sense, we are interested in the synthesis of discrete supramolecules containing a fluorene moiety since fluorene-based compounds are regarded as excellent fluorescent materials and often used as emitting materials in eletroluminescent devices. Herein, we report the preparation of fluorene-substituted pyridyl ligand 1 (Scheme 1) then describe the non-templated self-assembly of this ligand into discrete supramolecules 2-3 upon reaction with platinum or palladium acceptors 4-5 (Scheme 2). The synthesis of bidentate ligand 1 was achieved in a moderate yield (43%) by palladium-mediated coupling of commercially available 2,7-dibromofluorene with an excess of 4-ethynyl pyridine using the reported conditions. The self-assembly of supramolecules 2-3 was performed in NMR solvents for the facile monitor by NMR analysis. An acetone-d6/D2O solution of 1 and organoplatinum acceptor 4 was heated at 60 C for 4 h. Anion exchange with KPF6