Synthesis and structure of binuclear O/S-bridged organobismuth complexes and their cooperative catalytic effect on CO ₂ fixation

In the synthesis of binuclear organobismuth complexes (1-6) through treatment of organobismuth chlorides with NaOH or Na ₂S·9H ₂O, the two 5,6,7,12-tetrahydrodibenz [c,f]-[1,5]azabismocine frameworks are cross-linked by either a sulfur or an oxygen atom. The complexes (1-6) show high catalytic efficiency in the synthesis of cyclic carbonates from 2-(chloromethyl)oxirane and CO ₂. Compared with their precursor chlorides (7-9), methoxide 10 and methanethioate 11 which are mononuclear organobismuth complexes, the binuclear organobismuth complexes show higher cooperative catalytic effect. However, the complexes with an oxygen bridge (1-3) are not stable in air and lose their catalytic efficiency because of hydrolysis or CO ₂ adsorption (forming organobismuth carbonates in the latter case). Nonetheless, the binuclear organobismuth complexes (4-6) with a sulfur bridge are highly stable in air and can be applied in the synthesis of cyclic carbonates (with the co-presence of Bu ₄NI) across various kinds of epoxides, thus exhibiting satisfactory efficiency and selectivity.