Modified Metal Dibenzoylmethanates for Soft Supramolecular Materials:  Extension to Oligomeric and Polymeric Host Receptors with Nanosized Void Spaces^†

Recently reported molecular host complexes, modified metal dibenzoylmethanates (metal DBMs), are extended to polynuclear host receptors by connecting metal DBM units with bridging ligand 4,4‘-dipyridyl (bipy). Series of new solvent-free as well as inclusion materials were prepared and studied with XRD, NMR, and other methods. Template-directed synthesis gave inclusion compounds with three distinct types of host receptors:  dimeric [Zn₂(bipy)(DBM)₄], trimeric [Zn₃(bipy)₂(DBM)₆], and polymeric [Zn(bipy)(DBM)₂]_n complexes. The dimeric complex was isolated as a solvent-free compound and as inclusion compounds with tert-butylbenzene and fluorobenzene (host per guest molar ratios of 1:1 and 1:²/₃, respectively). The trimeric complex was prepared as an inclusion compound with DMSO (1:5 molar ratio). The polymeric complex was observed in inclusion compounds with fluorobenzene (host unit per guest molar ratio 1:2), tetrahydrofuran (1:2), 2-pentanone (1:2), bipy/DMSO (1:¹/₂:¹/₂), and bipy/tert-butylbenzene (1:1:2); a solvent-free material was obtained upon desolvation of tetrahydrofuran and 2-pentanone inclusions. The isolated host species have five-coordinated terminal zinc centers (two chelating DBMs and a singly coordinating bipy) and octahedrally coordinated intrinsic zinc centers (two chelating equatorial DBMs and two singly coordinating bipys). The bipy ligands bridge the zinc centers to give the di-, tri-, and polynuclear complexes. Nanosized voids between the DBM units coordinated to adjacent zinc centers form the basis for cages and channels in the inclusion materials. These voids have, in many cases, a typical size of between 1 and 2 nm and accommodate up to five molecules per single cavity or two rows of guest molecules in a single channel. When compared to previously reported metal DBMs, the multinuclear metal DBM receptors reported here exhibit stronger inclusion affinity and enhanced inclusion capacity.