2-Pyridine cyclic triimidazole as a chelating and bridging ligand in mono- and hexa-nuclear Re(i) complexes with emissive properties in solution and in the solid state

Abstract

Two luminescent Re(I) complexes, namely mononuclear fac-[ReCl(CO)₃(TT-Py)] (1) and hexanuclear [{Re(μ-Cl)(CO)₃(μ-TT-Py)}{Re₂(μ-Cl)₂(CO)₆}]₂ (2), were isolated in high yields by a one-pot reaction between [ReCl(CO)₅] and different amounts of nitrogen-rich 2-pyridine cyclic triimidazole (TT-Py), which acts as a chelating ligand in both derivatives. The metallo-supramolecular cyclic structure of 2, which can be described as a metallacycle in which two mononuclear units of 1 act as bridging ligands towards two “Re₂(μ-Cl)₂(CO)₆” fragments, was confirmed by single-crystal X-ray diffraction analysis. Both complexes were fully characterized in solution by 1D and 2D multinuclear NMR spectroscopy. 1 and 2 exhibit photoluminescence at room temperature in solution, with a broad unstructured emission in the 530–570 nm range. The nature of the excited states involved in the electronic absorption spectrum of 1 was ascertained by a combined density functional theory and time-dependent density functional theory (TD-DFT) study. Both compounds exhibit strong aggregation induced emission enhancement (AIEE), which is more pronounced for 2 leading to a solid state photoluminescence quantum yield of 2.5% that is one of the highest values ever reported for Re(I) polynuclear metallacycles so far. The versatility of TT-Py to adopt variegated coordination modes, previously observed for Cu(I) and Cu(II) derivatives, is confirmed supporting further use of this ligand to obtain new and peculiar emissive mono-, polynuclear and extended coordination systems.