Metal ion exchange in Prussian blue analogues: Cu(ii)-exchanged Zn–Co PBAs as highly selective catalysts for A3 coupling

The occurrence of metal ion exchange in Zn3[Co(CN)6]2 and Cu3[Co(CN)6]2 Prussian blue analogues (Zn–Co and Cu–Co PBAs) was demonstrated for the first time. While Cu(ii) ion exchange easily occurs in Zn–Co PBA, the exchange of Cu(ii) atoms in Cu–Co PBA by Zn(ii) proved to be more difficult. At low to medium Cu(ii) loadings, the catalytic activity of the exchanged PBAs for the A3 coupling reaction of benzaldehyde, piperidine and phenylacetylene was higher than that of the bimetallic PBAs and that of multi metal PBAs of similar composition prepared by co–precipitation. This result showcases the benefits of the ion exchange process as a preparation method of PBA catalysts, since it is believed to lead to the incorporation of the desired metal in a more accessible position for reactant molecules. At higher Cu(ii) loadings, ion exchange with Cu(CH3COO)2·H2O also resulted in co-incorporation of CH3COO−. This incorporation considerably boosted the catalytic activity of the PBAs by providing a basic function that facilitates the C–H activation of phenylacetylene. The most active of the studied PBAs, catallytically outperforms other Cu(ii) based A3 coupling catalysts and completely suppresses the activity for the homocoupling of phenylacetylene, even under oxidative conditions. Furthermore, the basicity of the PBAs was investigated in the nitroaldol (Henry) reaction, where a clear effect of the presence of CH3COO− was observed. The CH3COO− containing PBAs exhibited an activity three times higher than the rest of the PBAs. The presence of the basic CH3COO− groups represents the first case of basic functionalization of PBAs.