Acid/base responsive assembly/dis-assembly of a family of zirconium(IV) clusters with a cyclic imide-dioxime ligand

The hydrolytically stable dioxime ligand (2Z-6Z)-piperidine-2,6-dione (H(3)pidiox) acts as a strong chelator mainly with hard metals in high oxidation states, a pre-requisite for potential applications in metal sequestering processes from aqueous solutions. Reaction of ZrCl4 with H(3)pidiox in methanol gives the mononuclear compound [Zr-IV(eta(1),eta(1),eta(2)-H(2)pidiox-O,N,O')(2)(OH2)(2)]Cl-2 center dot H2O center dot CH3OH (1), while the same reaction mixture in the presence of KOH gave the pentanuclear ZrOC [Zr-5(IV)(mu 2-OH)4(OH2)4(mu(2)-eta(1),eta(1),eta(2)-Hpidiox-O,N,O')4(eta(1),eta(1),eta(1)-HpidioxO,N,O')(4)]center dot 5KCl center dot 3CH(3)OH center dot 8H(2)O (2). Compound 1 is formed at very acidic pH = 0, and the pentanuclear ZrOC 2 at higher pH values (pH = 2). Compounds 1 and 2 were characterized by single crystal X-ray structure analysis, multi-nuclear NMR spectroscopy and ESI-MS spectrometry. The single crystal X-ray structure analysis of 1 revealed a mononuclear zirconium(IV) compound containing an eight-coordinate zirconium atom bound to two singly deprotonated H(2)pidiox(-) ligands and two water molecules in a severely distorted bicapped octahedral geometry. The pentanuclear ZrOC 2 constitutes the second example of a Zr-5 cluster to be reported and the first one in which the four zirconium atoms are arranged in a tetrahedral arrangement with the fifth occupying the center of the tetrahedron. 1D and 2D NMR spectroscopies of the acidic CD3OD solutions of complex 1 reveal a fast equilibrium between 1 and 2. Addition of KOH into a CH3OH solution of 2 results in the controlled fast transformation of 2 to an asymmetric hexanuclear ZrOC 3 as evidenced by the NMR and real-time ESI-MS solution studies. Further addition of KOH to the solution of 3 leads to the ZrOC 4, and on the basis of NMR and ESI-MS data and in comparison with the known hexanuclear titanium(IV)/H(3)pidiox cluster, it is concluded that the cluster 4 should have a hexanuclear structure. Electrospray ionization mass spectrometry (ESI-MS) demonstrated not only the structural stability 1 and 2 in solution, but also revealed the reversible pH driven dis-assembly/re-assembly process between the monomeric 1 and the pentanuclear ZrOC 2.

Automated summary

The hydrolytically stable dioxime ligand, H(3)pidiox, exhibits strong chelation with hard metals in high oxidation states, potentially useful in metal sequestering processes from aqueous solutions. Experimental data analysis indicates that zirconium compounds with different structural features can be synthesized under different pH conditions.