Inclusion Potential, Polymorphism, and Molecular Isomerism of Metal Dibenzoylmethanates Coordinated with 2-Methylpyridine^†

In this study, new complexes of metal(II) (Ni, Zn, Cd) dibenzoylmethanates (DBMs) coordinated with 2-methylpyridine (2-MePy) were prepared and studied for host properties as a function of their structure and molecular isomerism. The novel [Ni(2-MePy)₂(DBM)₂] complex host exists as the trans isomer in five structurally distinct phases comprising two guest-free polymorphs as well as inclusion compounds with benzene, carbon tetrachloride, chlorobenzene, and toluene. All architectures arise from van der Waals packing, the host materials having guest species located in channels or cages. The stability of inclusions correlates with the shape of the cavity space. For zinc dibenzoylmethanate only one 2-MePy unit coordinates to give the [Zn(2-MePy)(DBM)₂] complex that does not exhibit inclusion properties. With cadmium dibenzoylmethanate, two polymorphic forms of the composition [Cd(2-MePy)₂(DBM)₂] were isolated. A metastable form contains complex molecules in the trans isomeric state, while a stable form contains cis isomers. As earlier, the ability to form clathrates was attributed only to trans-configured molecules of this type; the presence of a more stable cis form, with a trans-to-cis phase conversion enthalpy gain of as much as 17.2(6) kJ/mol, explains the failure to isolate any inclusion compounds of the Cd complex. The precedent of isolating both cis and trans isomers for the same complex of metal dibenzoylmethanate host is discussed in the context of designing novel materials where the potential for inclusion may be switched through molecular-level control of the isomeric state.