Octanuclear Zinc Phosphates with Hitherto Unknown Cluster Architectures: Ancillary Ligand and Solvent Assisted Structural Transformations Thereof

Structural variations in zinc phosphate cluster chemistry have been achieved through a careful selection of phosphate ligand, ancillary ligand, and solvent medium. The use of 4-haloaryl phosphates (X-dippH₂) as phosphate source in conjunction with 2-hydroxypyridine (hpy) ancillary ligand in acetonitrile solvent resulted in the isolation of the first examples of octameric zinc phosphates [Zn₈(X-dipp)₈(hpy)₄­(CH₃CN)₂(H₂O)₂]·4H₂O (X = Cl 2, Br 3) and not the expected tetranuclear D4R cubane clusters. Use of 2,3-dihydroxypyridine (dhpy) as ancillary ligand, under otherwise similar reaction conditions with the same set of phosphate ligands and solvent, resulted in isolation of another type of octanuclear zinc phosphate clusters {[(Zn₈(X-dipp)₄(X-dippH)₄­(dhpyH)₄­(dhpyH₂)₂(H₂O)₂]·2solvent} (X = Cl, solvent = MeCN 4; Br, solvent = H₂O 5), as the only isolated products. X-ray crystal diffraction studies reveal that 2 and 3 are octanuclear clusters that are essentially formed by edge fusion of two D4R zinc phosphates. Although 4 and 5 are also octanuclear clusters, they exhibit a completely different cluster architecture and have been presumably formed by the ability of 2,3-dihydroxypyridine to bridge zinc centers in addition to the X-dipp ligands. Dissolution of both types of octanuclear clusters in DMSO followed by crystallization yields D4R cubanes [Zn­(X-dipp)­(DMSO)]₄ (X = Cl 6, Br 7), in which the ancillary ligands such as hpy, H₂O, and CH₃CN originally present on the zinc centers of 2–5 have been replaced by DMSO. DFT calculations carried out to understand the preference of Zn₈ versus Zn₄ clusters in different solvent media reveal that use of CH₃CN as solvent favors the formation of fused cubanes of the type 2 and 3, whereas use of DMSO as the solvent medium promotes the formation of D4R structures of the type 6 and 7. The calculations also reveal that the vacant exocluster coordination sites on the zinc centers at the bridgehead positions prefer coordination by water to hpy or CH₃CN. Interestingly, the initially inaccessible D4R cubanes [Zn­(X-dipp)­(hpy)]₄·2MeCN (X = Cl 8, Br 9) could be isolated as the sole products from the corresponding DMSO-decorated cubanes 6 and 7 by combining them with hpy in CH₃CN.