Shape-selective separation of molecular isomers with tunable hydrogen-bonded host frameworks

The propensity of hydrogen-bonded guanidinium (G) organodisulfonates (S) to form crystalline inclusion compounds has been investigated in the context of separating isomeric mixtures of xylenes and dimethylnaphthalenes via sel ective inclusion. Pairwise competition experiments, in which inclusion comp ounds are grown from solutions containing an isomeric mixture of guests, ma p the inclusion selectivity of a particular host as a function of guest con tent in solution. Whereas the G(2)[4,4 ' -biphenyldisulfonate] host is mini mally selective with respect to inclusion of o-, m-, or p-xylene, the homol ogous G(2)[2,6-naphthalenedisulfonatel is highly selective toward the inclu sion of p-xylene, by a factor of 36 and 160 versus o-xylene and m-xylene, r espectively. Similarly, the hosts of the homologous series G2[2,6-naphthale nedisulfonatel, G(2)[4,4 ' -biphenyldisulfonate], G(2)[2,6-anthracenedisulf onatel, and G(2)[4,4 ' -azobenzenedisulfonatel display different selectivit y for the 10 isomers of dimethylnaphthalene. The details of the selectivity behavior are highly dependent on the molecular structure of the GS host an d the solid-state structures of the corresponding inclusion compounds. Sing le crystal structure determinations reveal that isomer selectivity is most pronounced when the structures of corresponding inclusion compounds are sig nificantly different, i.e., when the isomeric guests template different arc hitectural isomers of the host. Furthermore, selectivity appears to be a co nsequence of size and shape compatibility between the host and guest. The o bservation of selective inclusion demonstrates the feasibility of a crystal lization-based separation process based on these host compounds.