Synthesis, spectroscopic, electrochemical and computational studies of rhenium(i) dicarbonyl complexes based on meridionally-coordinated 2,2′:6′,2′′-terpyridine

Abstract

A homologous series of meridionally-coordinated tridentate 2,2′:6′,2′′-terpyridine Re(I) dicarbonyl complexes have been prepared and investigated. The complexes mer,cis-[Re(tpy-κ³N)(CO)₂(L)]ⁿ (L = Cl⁻ (1), OSO₂CF₃⁻ (2), NCCH₃ (3), CN⁻ (4), NC₅H₅ (5), PMe₃ (6), PEt₃ (7), PPh₃ (8), P(OMe)₃ (9) and P(OEt)₃ (10); n = 0 or +1) have been synthesized and characterized by elemental analysis, ¹H NMR and infrared spectroscopy. The electrochemistry of these compounds has been studied and compared to that of other known rhenium compounds using an electrochemical parameterization model. Cyclic voltammetry measurements have shown that the first oxidation of the complexes varies systematically in potential as the ligand L is altered. Many of these one-electron oxidations occur at lower potentials compared to more familiar bidentate diimine Re(I) tricarbonyl complexes. A correlation exists between CO, a strong π-acidic ligand, and deviations of the observed reduction potentials from the calculated values. Many of the complexes absorb light throughout a significant portion of the visible spectrum. Two of these complexes, mer,cis-Re(tpy-κ³N)(CO)₂Cl (1) and mer,cis-Re(tpy-κ³N)(CO)₂CN (4), absorb light throughout the entire visible spectrum. Low temperature emission spectra were obtained for the compounds mer,cis-Re(tpy-κ³N)(CO)₂Cl (1) and mer,cis-[Re(tpy-κ³N)(CO)₂(P(OEt)₃)]⁺ (10) at 77 K in a 4 : 1 methanol–ethanol glass matrix and give metal-to-ligand charge transfer (³MLCT) luminescence. Density functional theory (DFT) calculations of the electronic structure are in good agreement with the experimental data.