Catalytic Cyclic Carbonate Synthesis Using Epoxide and Carbon Dioxide: Combined Catalytic Effect of Both Cation and Anion of an Ionic Crv(O) Amido Macrocyclic Complex

We have studied an ionic Cr-v(O) complex of a cyclic amido ligand (1) for cyclic carbonate synthesis using epoxide and carbon dioxide (CO2) as starting materials. Lithium salt of 1 (Li1) showed a high turnover number (TON) of 2950 at 180 degrees C and 450 psig CO2 pressure. Reactions were run under solvent-free conditions and in the presence of a Lewis base, which acts as a cocatalyst. Important reaction parameters such as pressure, temperature, and variation of cocatalyst to catalyst ratio were studied in order to obtain optimum reaction conditions. Besides the obvious effect of the anionic Cr-v on the catalytic activity, the cation of the complex has a profound effect on the overall catalytic activity of the complex. The effect is most significant when the cation is Li+. In order to probe the effect of cation and particularly of Li+ on the catalytic activity, several Cr-v(O) complexes were synthesized with various cations. Both inorganic (Na+, K+) and organic cations ((Ph)(4)P+, (Bu)(4)N+) were used to synthesize the complexes and eventually to understand the effect of the cations. The activity dropped to almost half (TON 1500) for the complexes with cations of Na+ (Na1), K+ (K1), (Ph)(4)P+ (PPh(4)1), and (Bu)(4)N+ (Bu(4)N1), indicating the significant role of Li+ on the overall catalytic activity of Li1. Addition of a calculated amount of Li salt to the complexes with larger cations restores the catalytic activity to the same level as that of Li1, thus reinforcing the role of Li+. We have attempted to correlate the high activity of Li+ compared to larger cations based on its relatively higher Lewis acidity. Charge densities (Z(eff)/volume) of all inorganic cations indicate that Li+ has the highest charge density and thus the highest Lewis acidity, which in turn helps in producing the high catalytic activity of the complex. A probable mechanism signifying the role of Li+ and ably supported by ab initio theoretical calculations is presented.