Rhenium(I) complexes of readily functionalized bidentate pyridyl-1,2,3-triazole click ligands: A systematic synthetic, spectroscopic and computational study

A family of electronically tuned fac-Re(CO)(3)Cl pyridyl-1,2,3-triazole complexes have been synthesized by refluxing methanol solutions of [Re(CO)(5)Cl] and the substituted 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine ligands (pytri-R). The resulting rhenium(I) complexes were characterised by elemental analysis, HR-ESMS, IR, H-1 and C-13 NMR and in one case the molecular structure was confirmed by X-ray crystallography. The electronic structure of this family of fac-[(pytri-R)Re(CO)(3)Cl] complexes was probed using UV-Vis, Raman and emission spectroscopy and cyclic voltammetry techniques. The complexes show intense absorptions in the visible region, comprising strong pi -> pi* and metal-to-ligand charge-transfer (MLCT) transitions, which were modelled using time-dependent density functional theory (TD-DFT). Interestingly, the MLCT transition energy and the emission maxima are unaffected by the nature of the R substituent on the 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine ligand indicating that the 1,2,3-triazoyl unit is acting as an electronic insulator. The emission lifetimes of the complexes are modestly dependent on the nature of the 1,2,3-triazole substituent, with the conjugated complexes displaying longer lifetimes than the non-conjugated ones. The shorter lifetimes for complexes with non-conjugated ligands are attributed to the free-rotor effect which allows molecules to relax through non-radiative pathways. In this case, the freely rotating CH2 group located between the triazole and the R group causes the decrease in excited lifetime. The electrochemistry of all examples is defined by irreversible Re oxidation and triazole based ligand reduction processes. The nitro substituted complexes show additional nitrobenzene type reduction features. Similarly, the ferrocenyl substituted complex displays the expected reversible one electron oxidation process. Consistent with the spectroscopic data, the position of the oxidation and reduction processes are essentially unaffected by the electronic nature of the 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine substituent. (C) 2012 Elsevier Ltd. All rights reserved.