Assembly of titanium embedded polyoxometalates with unprecedented structural features

Abstract

Two titanium embedded polyoxometalates with unprecedented structural features are presented: a monotitanium containing tungstoantimonate Na₁₃H₃[TiO(SbW₉O₃₃)₂]·33 H₂O featuring a {TiO}²⁺ moiety (1) and a hexatitanium containing tungstoarsenate K₆[Ti₄(H₂O)₁₀(AsTiW₈O₃₃)₂]·30 H₂O containing a {Ti₄(H₂O)₁₀}¹⁶⁺ moiety (2). Both compounds have been fully characterised by single crystal X-ray diffraction, elemental analysis, IR and TGA. 1 is constructed from two α-B-{Sb^IIIW₉O₃₃} fragments linked by five sodium cations and an unprecedented square pyramidal Ti(O)O₄ group with a terminal TiO bond, and 2 exhibits a Krebs-type structure composed of two {AsTiW₈O₃₃} fragments, where one W(VI) centre has been substituted for a Ti(IV) centre in each, fused together via a belt of four additional Ti(IV) centres. This system represents the tungsten Ti-incorporated polyoxoanion with one of the highest Ti : W ratios so far reported. Additionally, 2 could also be isolated as an n-tetrabutylammonium salt and has been further characterised by electrochemistry and electrospray ionisation (ESI) MS studies. Due to the unique nature of these systems, both have been fully investigated using DFT calculations yielding highly interesting results. Structure 1 has been optimised with five sodium atoms in the belt position, which in addition to reducing the high charge of the cluster influence a stabilisation of the antimony lone pairs. Electrostatic potential calculations highlight the high electronegativity of the terminal oxygen on the titanium centre, enhancing real potentiality as a reactive site for catalysis.