Solid-state coexistence of {Zr 12 } and {Zr 6 } zirconium oxocarboxylate clusters †

Ligand metathesis, Co( II ) coordination, and partial condensation reactions of an archetypal {Zr 6 } zirconium oxocarboxylate cluster result in the first example of the coexistence of the distinct zirconium oxide frameworks {Zr 6 O 8 } and {Zr 12 O 22 }. Even minor modifications to the reaction conditions push this apparent equilibrium towards the {Zr 6 O 8 }-based product

Molecular high-nuclearity zirconium(IV) and cerium(III/IV) oxide structures have emerged as an extension to classical polyoxometalate chemistry with numerous structural analogies and similar formation paradigms. 1 In this context, we have explored the magnetic functionalization of several zirconium(IV) oxide clusters based on highly condensed {Zr 6 O 8 } 2 and {Zr 12 O 22 } 3 frameworks.Whereas little is known about the equilibria of different structural zirconium oxide archetypes in their reaction solutions, their similarity to group-5/6 polyoxometalates, where numerous species or isomers commonly coexist, suggests that, under certain boundary reaction conditions, different zirconium oxide structures could coexist as well.We now scanned the reaction parameter space for Co(II)-substituted zirconium oxocarboxylate clusters, starting from a member of the well-established {Zr 6 O 4 (OH) 4 (carboxylate) 12 } 2type cluster family of clusters, namely [Zr 6 O 4 (OH) 4 (pr) 12 ] (pr: propionate).Reactions were performed in the presence of potential bridging ligands in acetonitrile and small, welldefined amounts of water that facilitate further hydrolysis or condensation steps.‡ This approach yielded a crystalline phase of composition {H[Zr 12 Co 2 O 8 (OH) 14 (pr) 22 (MeCN) 2 (μ-pz)] [Zr 6 Co 6 O 8 (pr) 12 (Hbda) 6 ](NO 3 ) 3 •8MeCN} n (1), in which clusters of both the {Co 6 Zr 6 } and the {Co 2 Zr 12 } types co-crystallize (H 2 bda: N-butyldiethanolamine).Here, pyrazine (pz) selectively coordinates to the Co centers of the {Co 2 Zr 12 } moieties and interlinks the latter into a one-dimensional coordination polymer; we note that while a few zirconium oxide-based MOFs characterized by single-crystal X-ray diffraction exist, 4 1 represents the first structurally characterized heterometallic zirconium oxide-based coordination polymer.Importantly, replacing the pz linker with an alternative potential linker reagent, 4,4′-biphenol (H 2 bph), under otherwise comparable reaction conditions results in the formation of crystals of only the discrete {Co 6 Zr 6 }-type cluster, isolated as [Co 6 Zr 6 O 8 (pr) 12 (Hbda) 6 ] (Hbph) 2 •(H 2 bph) (2).
The two (crystallographically equivalent) Co(II) sites each coordinate to two oxygen atoms of two propionate ligands, and a pyrazine ligand coordinates to two adjacent Co sites of neighboring {Co 2 Zr 12 } groups, resulting in an infinite helical chain (Fig. S1 †

Conclusions
The close similarity between the reaction conditions yielding compounds 1 and 2 highlights the sensitive nature of the equilibrium conditions under which different {Zr 6 } and {Zr 12 } zirconium oxide cluster archetypes can co-crystallize.We note that we, however, cannot rule out the co-existence of these cluster types in the reaction solution that eventually produces crystals of 2: whereas the {Zr 12 } units in 1 are interlinked by pyrazine into a 1D coordination polymer, effectively reducing their solubility and facilitating precipitation, the 4,4′-biphenol linker employed in the reaction solution for compound 2 might simply be less affine to Co(II) coordination so as to adopt the same role.More systematic in situ studies of the reaction solutions are currently underway to determine how far the coexistence relations seen in the solid state relate to the conditions in the solution.Full-sphere data collection with exposures of 30 s per frame was made with ω scans in the range of 0-180°at φ = 0, 120, and 240°.A semiempirical absorption correction was based on the fit of a spherical harmonic function to the empirical transmission surface as sampled by multiple equivalent measurements 6 using SADABS software. 7Measurements were optimized to collect data to a resolution of 0.71 Å; however, the datasets have been truncated to obtain the statistically relevant resolution.The positions of metal atoms were found by direct methods.The remaining atoms were located in an alternating series of least-squares cycles and difference Fourier maps.All non-hydrogen atoms were refined in full-matrix anisotropic approximation.All hydrogen atoms were placed in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients.

Notes and references
Fig. 1 A segment of the solid-state structure of 1 illustrating the coexistence of one helical polymer strand A, consisting of three pyrazine-linked {Co 2 Zr 12 } groups, and two adjacent {Co 6 Zr 6 } (B) clusters.The zirconium oxide substructures are shown in an octahedral representation.Co: purple spheres, C: grey, N: blue, O: red.Nitrate groups, acetonitrile solvate molecules, and hydrogen positions are omitted for clarity.

Fig. 3
Fig. 3 Molecular structure of B. Color codes as in Fig. 2. The {Co 6 Zr 6 } cluster in is virtually isostructural to B. Hydrogen and nBu groups belonging to bda ligands are omitted for clarity.

Fig. 4
Fig. 4 Arrangement of the three nitrate groups (shown in their van der Waals spheres) around the {Zr 12 } core (in a polyhedral representation) in 1.