Polyoxometalate Catalyst Precursors. Improved Synthesis, H⁺-Titration Procedure, and Evidence for ³¹P NMR as a Highly Sensitive Support-Site Indicator for the Prototype Polyoxoanion−Organometallic-Support System [(<i>n</i>-C₄H₉)₄N]₉P₂W₁₅Nb₃O₆₂

Abstract

An improved synthesis of the prototype polyoxoanion−organometallic-support system, P2W15Nb3O629-, as its organic solvent-soluble all-[(n-C4H9)4N]+ salt is described. The improvements result in a 30% faster procedure which also provides a 60% increase in yield of [(n-C4H9)4N]9P2W15Nb3O62, 2. One key improvement in this synthesis is a pH titration of protonated [(n-C4H9)4N]5H4P2W15Nb3O62, 1, using [(n-C4H9)4N]OH and phenolpthalein indicator, which leads to fully deprotonated 2the key synthon required for clean support of organometallic cations onto the P2W15Nb3O629- polyoxoanion's “Nb3O93-” cap. The material obtained from this procedure is of the highest purity (≥95%) seen to date as judged by 31P NMR spectroscopy. [(n-C4H9)4N]9P2W15Nb3O62 has been further characterized by 1H, 13C, 17O, and 183W NMR spectroscopy as well as IR spectroscopy and complete elemental analysis. Also reported are control experiments that (i) confirm the identical reactivity of 2, obtained by either the procedure reported herein or by our previously published accounts, with organometallic cations such as [IrI(1,5-COD)]+, [RhI(1,5-COD)]+ and [RhIII(C5Me5)]2+, (ii) show that the pH titration employed herein may be used to cleanly deprotonate polyoxoanions containing varying degrees of protonation, (iii) show that trace amounts of excess OH- present in P2W15Nb3O629- prevent clean organometallic cation-support reactions, and (iv) show that 31P NMR titration of P2W15Nb3O629- using [(1,5-COD)Ir]+ is an effective and simple method of determining the purity of P2W15Nb3O629-.