A series of cobalt complexes are presented as effective catalysts for the synthesis of cyclic carbonates from epoxides and CO2. The catalytic potentials of the cobalt complexes, in combination with tetrabutylammonium bromide, have been demonstrated to solve some challenges in the synthesis of cyclic carbonates, including the room-temperature conversion of terminal epoxides and activation-challenging substrates such as internal epoxides and fatty acid derived epoxides. A key factor in the success of the strategy is the use of cobalt complexes that are prepared on the basis of the trans effect of hybrid ligands. The trans effect between N-heterocyclic carbenes and acetylacetone has been proved by a number of spectroscopic measurements, including UV–vis, ESI-MS, EPR, and in situ FT-IR and by DFT calculations; these support the notion that acetylacetone prefers to dissociate from the cobalt center, which will result in one coordination site for the activation of a substrate molecule at the cobalt atom and thus give rise to high reactivity.

A new type of pincer palladium complexes C1～C6 based on the strong donor strength of carbazoles skeleton were synthesized. The air-and moisture-stable complexes C1～C6 act as efficient catalysts for the direct arylation of azoles with (hetero)aryl bromides in good to excellent yields with broad substrate scope used KOAc as sole base under aerobic conditions. It was demonstrated that this developed protocol was the most catalytic system for the direct C-H bond arylation for thiazoles under relatively mild reaction conditions at a low catalyst loading of 0.5 mol%. Keywords carbazole ligand; palladium complex; direct arylation; aryl bromides; coupling reaction Recently, N-heterocyclic carbene ligand [12] and diimine ligand [13] palladium complexes catalyzed C-H arylation reactions have been reported, in which two catalytic systems made a significant progress in this area with K 2 CO 3 as base and pivalate acid (PivOH) as additive. Research in the structure-activity relationship of previously reported palladium catalysts found that the strong donor strength and high steric hindrance demand of ligands are directly related to the catalytic activities of complexes. [12,13] Carbazole is a strong electron-donating group and commonly used as ligand for metal complexes synthesis. A series of metal complexes based on carbazole skeleton are synthesized, such as ruthenium, [14] copper, [15] iridium, [16] platinum, [17] nickel, [18] and found widely applications in the advanced material synthesis.