Rational Design of Cobalt Complexes Based on the trans Effect of Hybrid Ligands and Evaluation of their Catalytic Activity in the Cycloaddition of Carbon Dioxide with Epoxide

Abstract

A series of cobalt complexes are presented as effective catalysts for the synthesis of cyclic carbonates from epoxides and CO2. The catalytic potentials of the cobalt complexes, in combination with tetrabutylammonium bromide, have been demonstrated to solve some challenges in the synthesis of cyclic carbonates, including the room-temperature conversion of terminal epoxides and activation-challenging substrates such as internal epoxides and fatty acid derived epoxides. A key factor in the success of the strategy is the use of cobalt complexes that are prepared on the basis of the trans effect of hybrid ligands. The trans effect between N-heterocyclic carbenes and acetylacetone has been proved by a number of spectroscopic measurements, including UV–vis, ESI-MS, EPR, and in situ FT-IR and by DFT calculations; these support the notion that acetylacetone prefers to dissociate from the cobalt center, which will result in one coordination site for the activation of a substrate molecule at the cobalt atom and thus give rise to high reactivity.

Tolerant Bimetallic Macrocyclic [OSSO]-Type Zinc Complexes for Efficient CO2 Fixation into Cyclic Carbonates

“…It was found that the coupling reaction hardly proceeded with the complex 3 or TBAB as the sole catalyst (entries 10 and 11, Table ), with only 4.3% and 5.5% PO conversion into PC observed at 130 °C for 1.0 h, which is much lower than the yield of 69.8% for the 3 /TBAB binary system. This confirms that the presence of both a Lewis acidic bimetallic zinc complex and a Lewis base as a nucleophile is necessary for an ideal catalyst system to achieve high efficiency. , Furthermore, the outcome of this coupling reaction dramatically depended on the nature of the nucleophile employed as shown in entries 3 and 6–9 in Table . The binary system zinc complex 3 /TBAB afforded the best result.…”

“…This confirms that the presence of both a Lewis acidic bimetallic zinc complex and a Lewis base as a nucleophile is necessary for an ideal catalyst system to achieve high efficiency. 40,41 Furthermore, the outcome of this coupling reaction dramatically depended on the nature of the nucleophile employed as shown in entries 3 and 6−9 in Table 1. The binary system zinc complex 3/TBAB afforded the best result.…”

“…Generally, internal epoxides display a low reactivity for transformation into cyclic carbonate due to the high steric hindrance. 40,41,67 Of interest, such catalytic system could catalyze the coupling reaction of internal epoxides, CHO and VCHO, with CO 2 to produce corresponding carbonates with TOF of 158 h −1 per Zn center (63.2% yield) and 127 h −1 per Zn center (50.8%), respectively. Furthermore, the catalyst shows the high stereoselectivity for the coupling reaction of CHO and CO 2 to generate cis-CHC as evidenced by the presence of proton signal at 4.42 ppm in 1 H NMR (Figure S34) and a peak at 74.8 ppm in the 13 C NMR spectrum (Figure S35).…”

“…Another focus was put on the cycloaddition of bicyclic epoxide with CO 2 because the resultant bicarbonate could serve as feedstock for the preparation of polyurethanes without using toxic isocyanate. , One commercial diepoxide, ethylene glycol diglycidyl ether (EGDE), was evaluated and could be transformed into corresponding bis-carbonate with a yield of 80.6% under identical 3 and TBAB loadings (entry 6, Table ). Generally, internal epoxides display a low reactivity for transformation into cyclic carbonate due to the high steric hindrance. ,, Of interest, such catalytic system could catalyze the coupling reaction of internal epoxides, CHO and VCHO, with CO 2 to produce corresponding carbonates with TOF of 158 h –1 per Zn center (63.2% yield) and 127 h –1 per Zn center (50.8%), respectively. Furthermore, the catalyst shows the high stereoselectivity for the coupling reaction of CHO and CO 2 to generate cis -CHC as evidenced by the presence of proton signal at 4.42 ppm in 1 H NMR (Figure S34) and a peak at 74.8 ppm in the 13 C NMR spectrum (Figure S35).…”

Hemilabile N‐heterocyclic carbene and nitrogen ligands on Fe (II) catalyst for utilization of CO2 into cyclic carbonate

“…The results are consistent with the experiments of UV–vis spectra, suggesting that the interaction between the catalyst 1d and the epichlorohydrin was occurred. Our recent published work 28 reported the trans effect of cobalt complexes. In the report, we observed a characteristic absorption peak of IR for the acetylacetone in situ FT‐IR experiments for cobalt complexes, which supported the acetylacetone preferring to dissociate from the cobalt center.…”