Electronic Properties of Rhenium(I) Carbonyl Complexes Bearing Strongly Donating Hexahydro-Pyrimidopyrimidine Based Ligands

Re(I) tricarbonyl complexes have been synthesized using bi- and tridentate ligands equipped with one or two hexahydro-pyrimidopyrimidine (hpp) units attached to either a pyridine or a pyrazine ring. These complexes have been characterized by NMR, ESI-MS, vibrational and optical spectroscopies as well as electrochemistry. Their structures were determined via single-crystal X-ray crystallography and modelled using both DFT and TD-DFT methods. The complexes are non-emissive in solution at room temperature but display emission with mixed intra ligand (major) and metal-ligand (minor) charge transfer characters at 77 K. Additionally, both pyrazine-based complexes appear to be emissive in the solid state, presumably due to the presence of intermolecular interactions, as observed in the crystal structure.