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

Abstract

The hydrolytically stable dioxime ligand (2Z-6Z)-piperidine-2,6-dione (H₃pidiox) 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 H₃pidiox in methanol gives the mononuclear compound [Zr^IV(η¹,η¹,η²-H₂pidiox-O,N,O′)₂(OH₂)₂]Cl₂·H₂O·CH₃OH (1), while the same reaction mixture in the presence of KOH gave the pentanuclear ZrOC [Zr^IV₅(μ₂-OH)₄(OH₂)₄(μ₂–η¹,η¹,η²-Hpidiox-O,N,O′)₄(η¹,η¹,η¹-HpidioxO,N,O′)₄]·5KCl·3CH₃OH·8H₂O (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 H₂pidiox⁻ ligands and two water molecules in a severely distorted bicapped octahedral geometry. The pentanuclear ZrOC 2 constitutes the second example of a Zr₅ 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)/H₃pidiox 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.