Dimensionality Alteration and Intra- versus Inter-SBU Void Encapsulation in Zinc Phosphate Frameworks

4,4′-Bipyridine-N-oxide (BIPYMO, 1), a less commonly employed coordination polymer linker, has been used as a ditopic spacer to bridge double-four-ring (D4R) zinc phosphate clusters to form novel framework coordination polymers. Zinc phosphate framework compounds [Zn₄(X-dipp)₄(BIPYMO)₂]_n·2MeOH [X = H (2), Cl (3), Br (4), I (5); dipp = 2,6-diisopropylphenyl phosphate] have been obtained by treating a methanol solution of zinc acetate with X-dippH₂ and BIPYMO (in a 1:1:1 molar ratio) at ambient conditions. Framework phosphates 2–5 can also be obtained by treating the preformed D4R cubanes [Zn­(X-dipp)­(DMSO)]₄ with required quantities of BIPYMO in methanol. Single-crystal X-ray diffraction studies reveal that these framework solids are two-dimensional (2D) networks as opposed to the diamondoid networks obtained when the parent unoxidized 4,4′-bipyridine is used as the linker (Inorg. Chem. 2014, 53, 8959). The two types of voids (viz., smaller intra-D4R and larger inter-D4R) present in these framework solids can be utilized for different types of encapsulation processes. For example, the in situ generated 2D framework 2 encapsulates fluoride ions accompanied by a change in the dimensionality of the framework to yield {[(nC₄H₉)₄N]­[F@(Zn₄(dipp)₄(BIPYMO)₂)]}_n (6). The three-dimensional framework 6 represents the first structurally characterized example of a fluoride-ion-encapsulated polymeric coordination compound or a metal–organic framework. The possibility of utilizing inter-D4R voids as hosts for small organic molecules has been explored by treating in situ generated 2 with a series of organic molecules of appropriate size. Framework 2 has been found to be a selective host for benzil and not for other structurally similar molecules such as benzoquinone, benzidine, anthracene, naphthalene, α-pyridoin, etc. The benzil-occluded isolated framework [benzil@{Zn₄(dipp)₄(BIPYMO)₂}]_n (7) has been isolated as single crystals, and its crystal structure determination revealed the binding of benzil molecules to the framework through strong π–π interactions.