Abstract
The cover picture shows the concept of selective anion separation using metal–organic frameworks. Functionalization of the coordination polymers with hydrogen‐bonding groups for specific anion binding and recognition results in highly selective separations by direct anion exchange or competitive crystallization processes. The diagram in the lower‐left corner depicts a qualitative model for anion separation selectivity, captured in three orthogonal concepts: complementarity (strength and number of anion‐binding interactions), organizational rigidity of the framework, and softness (or degree of covalency for anion binding). The solvation‐controlled (Hofmeister) selectivity that is normally observed in anion separations can be reversed (anti‐Hofmeister) or completely replaced by peak selectivity (non‐Hofmeister) when rigid, structurally constrained frameworks having a sufficient number of complementary anion‐binding groups are employed, or when the anion is bound with a strong degree of covalency (soft–soft interactions). The background shows a structurally rigid metal–organic framework functionalized with –COOH anion‐binding groups that exhibit peak selectivity for the Cl(H~2~O)~4~^−^ anionic cluster. Details are presented in the Microreview by R. Custelcean and B. A. Moyer on p. 1321 ff.