The impact of anion shielding on the catalytic activity of CO2 fixation into cyclic carbonates†

A multi-active site 1,1′,1′′-(2-hydroxybenzene-1,3,5-triyl)tris(N-benzyl-N,N-dimethylmethanammonium)bromide (3) and hierarchical ionic polymers 9, 11, 13, and 16 have been synthesized. Compound 3 showed the best catalytic activity under co-catalyst free and mild reaction conditions, namely, 1 mL of substrate (epoxide), 2 mol% catalyst loading at 90 °C for 24 h under atmospheric pressure, as compared to its polymeric equivalents (9, 11, 13, and 16). Interestingly, an increase in the number of active sites within the backbone of 3 showed an inverse relationship with the halide nucleophilicity associated with it, viz., Cl⁻ > Br⁻. This phenomenon arises from the large size of the bromide anion, leading to a ‘congested active site’ effect, as demonstrated by both experimental and theoretical studies. This was evident as the benzylic ammonium catalyst with bromide ions achieved higher epoxide conversion than its chloride counterpart, whereas the opposite was observed for catalyst 3, likely due to a less congested catalyst structure with chlorides. Unexpectedly, during a five-run recyclability experiment, a modified and less active form of the organocatalyst 3′ was isolated. The overall reaction mechanism was elucidated and further supported by DFT calculations.