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 isomeri sm. The novel [Ni(2-MePy)(2)(DBM)(2)] complex host exists as the trans isom er in five structurally distinct phases comprising two guest-free polymorph s as well as inclusion compounds with benzene, carbon tetrachloride, chloro benzene, and toluene. All architectures arise from van der Waals packing, t he host materials having guest species located in channels or cages. The st ability of inclusions correlates with the shape of the cavity space. For zi nc dibenzoylmethanate only one 2-MePy unit coordinates to give the [Zn(2-Me Py)(DBM)(2)] complex that does not exhibit inclusion properties. With cadmi um 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 ear lier, the ability to form clathrates was attributed only to trans-configure d molecules of this type; the presence of a more stable cis form, with a tr ans-to-cis phase conversion enthalpy gain of as much as 17.2(6) kJ/mol, exp lains 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 nove l materials where the potential for inclusion may be switched through molec ular-level control of the isomeric state.