Synthesis of Heterobimetallic RuMn Complexes and the Coupling Reactions of Epoxides with Carbon Dioxide Catalyzed by these Complexes

• Published in: Chemistry : a European journal 2006-01, Vol.12 (4), p.1004-1015
• Main Authors: Man, Man Lok, Lam, King Chung, Sit, Wing Nga, Ng, Siu Man, Zhou, Zhongyuan, Lin, Zhenyang, Lau, Chak Po
• Format: Article
• Language: English
• Subjects: coupling reactions; cyclic carbonates; density functional calculations; heterobimetallic complexes; reaction mechanisms
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500780

The heterobimetallic complexes [(η5‐C5H5)Ru(CO)(μ‐dppm)Mn(CO)4] and [(η5‐C5Me5)Ru(μ‐dppm)(μ‐CO)2Mn(CO)3] (dppm= bis‐diphenylphosphinomethane) have been prepared by reacting the hydridic complexes [(η5‐C5H5)Ru(dppm)H] and [(η5‐C5Me5)Ru(dppm)H], respectively, with the protonic [HMn(CO)5] complex. The bimetallic complexes can also be synthesized through metathetical reactions between [(η5‐C5R5)Ru(dppm)Cl] (R=H or Me) and Li+[Mn(CO)5]−. Although the complexes fail to catalyze the hydrogenation of CO2 to formic acid, they catalyze the coupling reactions of epoxides with carbon dioxide to yield cyclic carbonates. Two possible reaction pathways for the coupling reactions have been proposed. Both routes begin with heterolytic cleavage of the RuMn bond and coordination of an epoxide molecule to the Lewis acidic ruthenium center. In Route I, the Lewis basic manganese center activates the CO2 by forming the metallocarboxylate anion which then ring‐opens the epoxide; subsequent ring‐closure gives the cyclic carbonate. In Route II, the nucleophilic manganese center ring‐opens the ruthenium‐attached epoxide to afford an alkoxide intermediate; CO2 insertion into the RuO bond followed by ring‐closure yields the product. Density functional calculations at the B3LYP level of theory were carried out to understand the structural and energetic aspects of the two possible reaction pathways. The results of the calculations indicate that Route II is favored over Route I. Cyclic carbonates can be obtained in CO2/epoxide coupling reactions by using heterobimetallic RuMn complexes. Density functional calculations were carried out at the B3LYP level of theory in order to understand and compare the structural and energetic aspects of the two possible reaction pathways involved in the catalytic reaction (see scheme), both of which begin with heterolytic metalmetal bond cleavage.