Imidazole and NHC-Carbene based Coordination Architectures and Their Applications

Abstract

Modern-day chemistry demands novel molecular materials having aesthetically pleasing structures along with fascinating chemical and physical properties. New synthetic strategies have been applied for structural modulation of such structures to incorporate and improve different functional properties. Covalent attachment approach to incorporate different functions into new systems has been marred with tedious synthesis and complex purification techniques. Coordination-driven self-assembly offers limitless possibilities for designing elegant structures which can be efficiently preprogramed by the selection of suitable building blocks encrypted with unique properties. These complementary building blocks acknowledge each other through non-covalent interactions and self-assemble to form sophisticated functional architectures. The relatively simple design principle, high bond enthalpy, predictable directionality and easy to achieve pre-designed supramolecules in high yields under mild reaction condition have made coordination-driven self-assembly a great technique.1 In this regard, Ag-carbene bond directed metallacycle and metallacage were designed using two new 1,4-dihydropyrrolo[3,2-b]pyrrole based aggregation induced emission (AIE)-inactive di- and tetra-imidazolium salts. Transmetallation of these complexes allowed their facile conversion to their respective Au(I)-analogues. Without using any AIEgen, final assemblies exhibited linkage induced enhancement of fluorescence (LIEF) in dilute solution.2 In another report, propeller shaped fluorophore triphenylamine unit was self-coupled to develop a new imidazole donor. Self-assembly of imidazole donor and Ag2O gave rise to a metallacage, which exhibited metal-coordination assisted enhanced emission in solution state. Finally, the enhanced emission was utilized in picric acid sensing.3 AIE-active TPE unit was incorporated in a flexible tetra-imidazole ligand to synthesize an Ag-carbene complex which showed rigidification induced enhancement in fluorescence in solution state as compared to its aggregated state. Moreover, this Ag-carbene complex was explored as artificial light-harvesting system with good energy transfer efficiency.4 Further, a new Pd6 molecular barrel was synthesized employing three 1,4-dihydropyrrolo[3,2-b]pyrrole panels that were clipped through six cis-PdII acceptors. Large open windows, cavity, and favourable interactions between the ligand panels and fullerene (C60 and C70) allowed the barrel to encapsulate fullerenes efficiently. Detailed investigation revealed that barrel has a stronger binding affinity towards C70 over C60, resulting in the predominant extraction of C70 from a mixture of the two.5