Slow‐Proton Dynamics within a Zirconium‐Containing Sandwich‐Like Complex Based on the Trivacant Anion α‐[SiW9O34]10– – Synthesis, Structure and NMR Spectroscopy

Abstract

AbstractReaction of ZrOCl2 with the potassium tungstosilicate A‐α‐[SiW9O34]10– in water at pH = 4 leads to the formation of the sandwich‐type complex [Zr3O(OH)2(SiW9O34)2]12– (H2‐1) which has been structurally characterised by single‐crystal X‐ray diffraction. It consists of a [Zr3O(OH)2] triangular central cluster closely embedded between two A‐α‐[SiW9O34]10– subunits. Inspection of the geometric parameters within the {Zr3O3} core unambiguously reveals the presence of two attached protons giving two hydroxo Zr–OH–Zr bridges and a single oxo Zr–O–Zr junction. The anions Hx‐1 with x = 3, 2 or 1 were obtained in solution at pH = 2, 4 and 7.5, respectively, and were subsequently characterised by 183W and 29Si NMR spectroscopy in aqueous solution. 183W NMR spectra appear strongly dependent upon the protonation state of the trizirconium central core, {HxZr3O3}. 183W NMR spectroscopy of H2‐1 gives a two‐line spectrum, consistent only with the hopping of the two H+ ions over the three bridging Zr–O–Zr oxygen atoms. Interestingly, the resonance attributed to the six tungsten atoms of the belt connected to the zirconium atoms through W–O–Zr junctions exhibits substantial line‐broadening (Δν1/2 = 21 Hz) interpreted as a slow dynamic process involving the H+ ions. Such behaviour contrasts with the fast H+ mobility generally observed in other POM systems and this will be discussed in relation to the peculiar environment of the {Zr3O3} core which hinders structural relaxation consequent with the H+ hopping. Both 183W resonances of the H3‐1 anion remain sharp as usual (Δν1/2 = 1.6 Hz) consistent with the presence of the triprotonated species [Zr3(OH)3(SiW9O34)2]11– whereas for the monoprotanated anion (H‐1), the resonance of the belt becomes broad enough to be indistinguishable. Infrared spectroscopy, elemental analysis, pH titration and polarographic measurements of Hx‐1 were also carried out.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)