Synthesis and Molecular Structure of a Water-Soluble, Dimeric Tri-Titanium(IV)-Substituted Wells–Dawson Polyoxometalate Containing Two Bridging (C₅Me₅)Rh²⁺ Groups

A novel trititanium­(IV)-substituted Wells–Dawson polyoxometalate (POM)-based organometallic complex, i.e., a dimeric POM containing two bridging Cp*Rh²⁺ groups (Cp* = C₅Me₅) or [{α-P₂W₁₅Ti₃O₆₀(OH)₂}₂(Cp*Rh)₂]^16– (D-1) with C_i symmetry, was synthesized in an analytically pure form by a 1:2 -molar ratio reaction of the organometallic precursor [Cp*RhCl₂]₂ with the separately prepared, monomeric trititanium­(IV)-substituted Wells–Dawson POM, “[P₂W₁₅Ti₃O₅₉(OH)₃]^9–” (M-1). The crystalline sample (NaK-D-1) of the water-soluble, mixed sodium/potassium salt of D-1 was obtained in the 14.7% yield, which has been characterized by complete elemental analysis, TG/DTA, FTIR, single-crystal X-ray structure analysis, and solution (¹⁸³W, ³¹P, ¹H and ¹³C­{¹H}) NMR spectroscopy. Single-crystal X-ray structure analysis revealed that the two species of the protonated Wells–Dawson subunits, “[P₂W₁₅Ti₃O₆₀(OH)₂]^10‑” were bridged by the two Cp*Rh²⁺ groups, resulting in the an overall C_i symmetry. The Cp*Rh²⁺ groups were linked to the two terminal oxygen atoms of the titanium­(IV) sites and one edge-sharing oxygen atom of the surface Ti–O–Ti bond. The ¹⁸³W NMR of D-1 dissolved in D₂O showed that its solution structure was represented as a dimeric POM with a formula of [{α-P₂W₁₅Ti₃O₆₀(OH)₃}₂{Cp*Rh­(OH)}₂]^16– (D-2) with Ci (or S₂) symmetry. A trititanium­(IV)-substituted Wells–Dawson POM-supported organometallic complex has never been reported so far, and thus D-1 in the solid state and D-2 in solution are the first example of this type of complex.