Catalytic Conversion of Carbon Dioxide Using Binuclear Double-Stranded Helicates: Cyclic Carbonate from Epoxides and Diol.

The construction of sophisticated molecular architectures from chemical subunits requires careful selection of the spacers, precise synthetic strategies, and substantial efforts. Here, we report a series of binuclear double-stranded helicates synthesized from different combinations of pyridyl hydrazone-based multidentate ligands (H(2) 1, H(2) 2, H(2) 3) by increasing the methylene spacer and transition metals (Co, Ni, and Zn). The ligands H(2) 1 (N'1,N'3-bis((E)-pyridin-2-ylmethylene)malonohydrazide), H(2) 2 (N'1,N'4-bis((E)-pyridin-2-ylmethylene)succinohydrazide), and H(2) 3 (N'1,N'5-bis((E)-pyridin-2-ylmethylene)glutarohydrazide) and their respective complexes with Co, Ni, and Zn were obtained. Single-crystal X-ray diffraction studies of these binuclear metallohelicates confirm the double-stranded helical structure of the complexes derived from H(2) 2. The set of helicates Co-1, Co-2, and Co-3; Ni-1, Ni-2, and Ni-3; and Zn-1, Zn-2, and Zn-3 were investigated for its catalytic activity in the cyclic carbonate formation reaction. Intriguingly, among the synthesized catalyst, Co-1 was found to be better in terms of conversions with the calculated TOF (turnover frequency) of 128/h. The catalytic performance was significantly improved by adding 0.2 mmol of tetrabutylammonium bromide by achieving 76% conversion in 30 min, with the observed TOF of 15,934 h(-1)/molecule and 7967 h(-1)/Co center. The results obtained herein show that the double-stranded helicates are effective catalysts for converting both terminal and non-terminal epoxides into their corresponding cyclic carbonates. The striking feature of this catalytic protocol lies in demonstrating the catalytic activity for the conversion of diol to cyclic carbonate, and the detailed kinetic experiments tempted us to propose a tentative reaction mechanism for this conversion.