CO₂ Fixation by Dimeric Tb(III) Complexes: Synthesis, Structure, and Magnetism

Two dinuclear complexes, [Tb2(L1)2(piv)2(NO3)2]·H2O (1) and [Tb2(L2)2(CF3CO2)2(H2O)4].2NO3 (2), have been prepared and characterized by single-crystal X-ray diffraction, where each metal ion is doubly phenoxido-bridged by the two phenolato oxygen atoms of the tetradentate Schiff-base ligand. Previous magnetic studies of 1 show that it is not a single-molecule magnet (SMM), while AC magnetic measurements of 2 show that it relaxes quite fast with μSQUID measurements revealing the presence of an interaction operating between the Tb ions. Through DC, μSQUID, and CASSCF calculations, the strength of the interaction in 2 can be quantified, which is of dipolar origin. Both complexes showed efficient catalytic activity toward the carbon dioxide insertion reaction into epoxides for the formation of organic cyclic carbonates. Catalytic synthesis of organic cyclic carbonates smoothly occurred at 60 °C under1 bar carbon dioxide pressure and neat conditions. Exocyclic as well as endocyclic epoxides produced a respective cyclic carbonate product with moderate to high yield (43–100%). Moreover, a high turnover number (7300–10000) along with a high turnover frequency (537.5–5000 h–1) are found in this catalytic reaction.