Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China.
Chemistry. 2012 Aug 6;18(32):9870-6. doi: 10.1002/chem.201201042. Epub 2012 Jul 4.
Co(II)-substituted α-Keggin-type 12-tungstenphosphate (n-C(4)H(9))(4)NH[PW(11)Co(H(2)O)O(39)]-(PW(11)Co) is synthesized and used as a single-component, solvent-free catalyst in the cycloaddition reaction of CO(2) and epoxides to form cyclic carbonates. The mechanism of the cycloaddition reaction is investigated using DFT calculations, which provides the first computational study of the catalytic cycle of polyoxometalate-catalyzed CO(2) coupling reactions. The reaction occurs through a single-electron transfer from the doublet Co(II) catalyst to the epoxide and forms a doublet Co(III)-carbon radical intermediate. Subsequent CO(2) addition forms the cyclic carbonate product. The existence of radical intermediates is supported by free-radical termination experiments. Finally, it is exhilarating to observe that the calculated overall reaction barrier (30.5 kcal mol(-1)) is in good agreement with the real reaction rate (83 h(-1)) determined in the present experiments (at 15 °C).
钴（II）取代的α-Keggin 型 12-钨磷酸[(n-C（4）H（9））（4）N]（4）H[PW（11）Co（H（2）O）O（39）]-（PW（11）Co）被合成并用作 CO（2）和环氧化物加成反应形成环状碳酸酯的单组分、无溶剂催化剂。使用 DFT 计算研究了加成反应的机理，这为多金属氧酸盐催化 CO（2）偶联反应的催化循环提供了首次计算研究。反应通过来自双重态 Co（II）催化剂到环氧化物的单电子转移发生，并形成双重态 Co（III）-碳自由基中间体。随后的 CO（2）加成形成环状碳酸酯产物。自由基中间体的存在得到了自由基终止实验的支持。最后，令人兴奋的是，计算得出的总反应势垒（30.5 kcal mol（-1））与本实验（在 15°C 下）确定的实际反应速率（83 h（-1））非常吻合。
