CO₂/epoxide coupling reactions and thiirane desulphurization with Ru-Mn bimetallic complex, manganese carbonylates, and PPN salts

Abstract

The coupling reactions of carbon dioxide with epoxides to produce five-membered cyclic carbonates were efficiently catalyzed by heterobimetallic complex [(n5-C5H5)Ru(CO)(u-dppm)Mn(CO)4] (1) and complex Li+[Mn(CO)4(PPh3)]- (2). We have studied and compared the reactivity of the two complexes in the coupling reactions. The reactions were carried out under 40 bar of CO2 at 100 C and in the absence of solvents and additives. The higher activity of 1 compared to 2 is attributed to the proximity of the metal centers in the former. Two possible reaction pathways for 1-catalyzed CO2/epoxide coupling reaction have been proposed. Both routes begin with heterolytic cleavage of the Ru-Mn bond and coordination of an epoxide molecule to the Lewis acidic Ru center. In route I, the Lewis basic Mn center activates the CO2 by forming the metallocarboxylate anion, which then ring-opens the epoxide; ring-closure gives the cyclic carbonate. In route II, the nucleophilic Mn center ring-opens the Ru-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 levels 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. We also found that the bimetallic complex [(n5-C5H5)Ru(CO)(u-dppm)Mn(CO)4] (1) is active in catalyzing the desulphurization reaction of thiiranes to produce the corresponding olefins. The reactions were conducted at 100C and dioxane was used as solvent without other additives. The thiiranes were converted to the corresponding olefins with good yields. Comparing the catalytic reactivities of complex 1 with other complexes Li+[Mn(CO)4(PPh3)]- (2) and [(n5-C5H5)Ru(CO)(PPh3)(CH3CN)][BF4]- (3) in the desulphurization reactions found that 1 showed the highest catalytic activity. We believe that some sort of bimetallic co-operativity in 1 facilities the desulphurization reaction. The off-the-shelf reagent [PPN]+[C1]- (4) and the PPN-manganese carbonylate complexes [PPN]+[Mn(CO)5]- (5) and [PPN]+[Mn(CO)4(PPh3)]- (6) were found to be good catalysts for the coupling reactions of CO2 with neat epoxides, under 5 bar CO2, at 100C or lower temperature (80C) without the use of organic solvents, to afford the corresponding cyclic carbonates in good yields. The three catalysts were found to show very similar catalytic activity. However, PPN salts with weak nucleophilic anions such as BF4- and OTf are inactive for the coupling reactions. Two possible mechanisms for the 4-, 5- and 6-catalyzed CO2/epoxide coupling reactions have been proposed.