The photo-isomerization of the cyclononatetraenyl ligand and related rare earth complexes.

The cyclononatetraenyl (Cnt) ligand is a large monoanionic ligand. It is easily synthesized by ring expansion after cyclopropanation of the cyclooctatetraenyl (Cot) ligand. The Cnt ligand can be reported as the --- () isomer, where the aromatic ring is flat, and all carbon atoms form a homogenous ring, and as the --- () isomer, where one carbon places itself inside the ring. The isomerization from the to the form has been reported numerous times in previous articles, but no quantitative analysis has been proposed due to contradictory data. This article proposes a detailed analysis involving light to rationalize this intrigue concerning isomerization. A careful synthesis at low temperatures and with light protection yields the ligand in its form (Cnt-). The controlled photo-isomerization of the Cnt- ligand is reported herein. A series of divalent or trivalent rare earth complexes, (Cnt)Sm, and (Cot)(Cnt)Ln (Ln = Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Er, Ho), which synthesis, solid-state X-ray diffraction and solution H NMR and UV-visible measurements, have been revised according to the synthesis using the Cnt- ligand. The photo-isomerization of the (Cnt-)Sm evolves to the intermediate (Cnt-)(Cnt-)Sm and the (Cnt-)Sm complex as the thermodynamical product. The photoisomerization of the trivalent (Cot)(Cnt)Ln complexes highlights the formation of a photostationary state (PSS) after several minutes of irradiation, in which both Cnt- and Cnt- ligands are present. The ratio of these two forms varies according to metal and irradiation wavelength: low-energy wavelengths favor the isomer, and high-energy wavelengths favor the isomer. DFT and TD-DFT were performed to provide a tentative orbital explanation.