Dimerization of A-α-[SiNb₃W₉O₄₀]^7- by pH-Controlled Formation of Individual Nb−μ-O−Nb Linkages

The reversible, stepwise formation of individual Nb−μ-O−Nb linkages during acid condensation of 2 equiv of A-α-[SiNb3W9O40]7- (1) to the tri-μ-oxo-bridged structure A-α-[Si2Nb6W18O77]8- (4) is demonstrated by a combination of X-ray crystallography and variable-pD solution 183W and 29Si NMR spectroscopy. Addition of DCl to a pD 8.4 solution of 1 (Li+ salt in D2O) results in formation of a mono-Nb−μ-O−Nb-linked dimer, A-α-[Si2Nb6W18O79]12- (2; pD = 3.0−1.3). At pD values between 1.6 and 0.3, two isomers (syn and anti) of the di-μ-oxo-bridged dimer, A-α-[Si2Nb6W18O78]10- (3), are observed by 183W NMR (C2v and C2h symmetry for the syn and anti isomers, respectively; 5 183W NMR signals for each isomer in the ratio 2:2:2:2:1). X-ray-quality crystals of syn-3 were isolated in 53% yield (syn-A-α-Cs8H2[Si2Nb6W18O78]·18H2O, orthorhombic, Cmcm, a = 40.847(2), b = 13.2130(7), and c = 16.8179(9) Å at 173K, Z = 4, final R1 = 0.0685). At the low-pD limit of −0.08 (1.2 M DCl), 4 alone is observed. Additional supporting data are provided by variable-pD 29Si NMR spectroscopy. Reversibility of the above processes was subsequently demonstrated by acquisition of 183W NMR spectra after incremental additions of LiOH to D2O solutions of 4 to effect its stepwise hydrolysis to 2 equiv of 1.