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

Two novel Re(I) complexes with the general formula fac-[Re(CO)(3)(L)(nHo)]CF3SO3, where L = 2,2'-bipyridine (bpy) or 1,10 phenanthroline (phen) and nHo (9H-pyrido[3,4-b] indole; norharmane) have been synthesized. The Re(I)-nHo complexes were characterized by structural X-ray diffraction, H-1 and C-13 NMR, UV-vis absorption and FT-IR spectroscopy, and by a combination of two mass spectrometry techniques, namely ESI-MS and UV-MALDI-MS. All characterizations showed that nHo is coordinated to the metal atom by the pyridine nitrogen of the molecule. X-ray structural analysis revealed that the crystal lattices for both complexes are further stabilized by a strong >N-H center dot center dot center dot O bond between the pyrrole NH group of the pyridoindole ligand and one oxygen atom of the trifluoromethanesulfonate counter-ion. Ground state geometry optimization by DFT calculations showed that in fluid solution the nHo ligand may rotate freely. The nature of the electronic transitions of Re(CO)(3)(bpy)(nHo)(+) were established by TD-DFT calculations. The set of the most important electronic transitions present in this complex are comprised of pi ->pi* electronic transitions centered on bpy and nHo moieties, LLCTnHo -> COs, MLLCTRe(CO)3 -> bpy and LLCTnHo -> bpy transitions. Additionally, TD-DFT calculations predict the existence of another two intense MLLCTRe(CO)(3 -> nHo) electronic transitions. Calculated UV-vis absorption spectra are in good agreement with the corresponding experimental data for the bpy-containing complex.