Norharmane rhenium(I) polypyridyl complexes: synthesis, structural and spectroscopic characterization

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Photochemistry

Background:

• Rhenium(I) complexes are known for their luminescent properties and potential applications in catalysis and sensing.
• The incorporation of heterocyclic ligands like norharmane (nHo) can tune the photophysical and electrochemical properties of metal complexes.
• Understanding ligand coordination and electronic transitions is crucial for designing functional rhenium complexes.

Purpose of the Study:

• To synthesize and characterize novel rhenium(I) complexes incorporating the 9H-pyrido[3,4-b]indole (norharmane, nHo) ligand.
• To elucidate the coordination mode of the nHo ligand and the structural features of the resulting complexes.
• To investigate the electronic transitions and photophysical properties of these rhenium(I) complexes using computational methods.

Main Methods:

• Synthesis of fac-[Re(CO)3(L)(nHo)]CF3SO3 complexes, where L = 2,2'-bipyridine (bpy) or 1,10 phenanthroline (phen).
• Characterization using X-ray diffraction, NMR spectroscopy (1H, 13C), UV-vis absorption, FT-IR spectroscopy, ESI-MS, and UV-MALDI-MS.
• Density Functional Theory (DFT) calculations for ground state geometry optimization and Time-Dependent DFT (TD-DFT) for electronic transition analysis.

Main Results:

• Successful synthesis and full characterization of two novel rhenium(I) complexes with nHo and bpy or phen ligands.
• X-ray diffraction confirmed nHo coordination through its pyridine nitrogen and revealed hydrogen bonding between the nHo pyrrole NH and the triflate counter-ion.
• DFT and TD-DFT calculations provided insights into ligand rotation in solution and identified key electronic transitions, including π→π*, LLCT, and MLLCT, matching experimental UV-vis data for the bpy complex.

Conclusions:

• The synthesized rhenium(I) complexes exhibit unique structural features due to nHo coordination and hydrogen bonding.
• The electronic transitions are a complex interplay of ligand-centered, metal-to-ligand, and ligand-to-ligand charge transfers.
• Computational studies successfully predict and explain the experimental spectroscopic properties, aiding in the design of future rhenium-based functional materials.