Experimental and theoretical studies of ancillary ligand in rhenium (I) core

What is it about?

The fac-[Re(CO)3(deeb)L]+ complex (C2) where L is (E)-2-((3-amino-pyridin-4-ylimino)-methyl)-4,6-diterbutylphenol ancillary ligand, which presents an intramolecular hydrogen bond, has been synthesized and characterized using UV-vis, 1H-NMR, FT-IR, cyclic voltammetry and DFT calculations. The UV-vis absorption and emission properties have been studied at room temperature and the results were compared with TDDFT calculations including spin-orbit effects. We report an alternative synthesis route for the fac-Re(CO)3(deeb)Br (C1) complex where deeb=(4,4'-diethanoate)-2,2'-bpy. Besides, we have found that the C1 shows a red shift in the emission spectrum due to the nature of the ancillary electron donating ligand, while the C2 complex shows a blue shift in the emission spectrum suggesting that the ancillary ligand L has electron withdrawing ability and the important role of the intramolecular hydrogen bond. The calculations suggest that an experimental mixed absorption band at 361 nm could be assigned to MLCT and LLCT transitions. The electron withdrawing nature of the ancillary ligand in C2 explain the electrochemical behavior, which shows the oxidation of ReI at 1.83 V and the reduction of deeb at -0.77 V.

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Why is it important?

The research of rhenium(I) tricarbonyl diimine complexes of the general form fac- Re(CO)3(N,N)(L), where, N,N are substituted bipyridine and L the ancillary ligand might be halogen or pyridine, dates back from the 1970s. Since then, it has attracted much attention due to their appealing photophysical properties, due to their luminescent properties based on the ReI core which are adjustable, and might be optimized by the variation of the (N,N) ligand structure, or more subtly, by tuning the electronic nature of the ancillary ligand L. Several conjugated chelating ligands, such as, diimine, indole and benzimidazoles, have been attached to the fac-Re(CO)3+ core showing therapeutic properties.