Abstract
A salen based molecular cage, salen@cage, was synthesized and complexed with Co and Al to yield metal-salen molecular cages, Co(ii)@cage, Co(iii)@cage and Al(iii)@cage. These cages were demonstrated to be efficient heterogeneous catalysts for the cycloaddition of CO2 with styrene oxide, achieving full conversion at 25 °C and 1 atm CO2. Good to excellent yields of various cyclic carbonates were also achieved under mild conditions. Al(iii)@cage can be reused up to five times without any significant loss of its high catalytic activity. The capability to access a variety of heterogeneous organometallic catalysts with salen@cage offers new prospects for practical CO2 utilization and chemical manufacturing.
Highlights
Molecular cages have been of much interest over the past decade as these materials have a wide range of applications from molecular recognition[1] to chemical sensors.[2]
The imine-based organic salen@cage was synthesized by the Schiff base reaction as shown in Scheme 1.20 In the Electronic supplementary information (ESI)-MS spectrum, the salen@cage compound showed only one sharp signal at m/z 1447.83 with the expected isotopic pattern which corresponded to the cation of [2 + 3] salen@cage (Fig. S1 in the ESI†)
According to dynamic covalent chemistry, the reversible nature of the imine bonds, high reaction temperature and long reaction duration allowed for the most thermodynamically stable product to be selectively formed in equilibrium.19c,21 The [2 + 3] molecular prism[4] was the most enthalpically favoured and entropically favoured.3d The use of other lower boiling point solvents (e.g. CHCl3 and THF) and shorter reaction durations resulted in the formation of smaller [2 + 2], [2 + 1] and [1 + 2] cages detected by ESI-MS
Summary
Molecular cages have been of much interest over the past decade as these materials have a wide range of applications from molecular recognition[1] to chemical sensors.[2]. The increasing anthropogenic emissions of CO2 have resulted in excessive global warming, and the efficient utilization of CO2 as a C1 source is an appealing subject of investigation.[9] Co(III) and Al(III) salen complexes were effective catalysts for the cycloaddition of CO2 to epoxides and the reaction usually proceeds under mild conditions.[10] The cyclic carbonate products obtained have a range of different applications, including being used as green solvents,[11] electrolytes in lithium-ion batteries[12] and precursors in organic synthesis as intermediates to important chemicals like glycols, polyurethanes, dialkyl carbonates, carbamates, purines and pyrimidines.
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