Dimerization of A-alpha-[SiNb(3)W(9)O(40)](7-) by pH-controlled formation of individual Nb-mu-O-Nb linkages.

The reversible, stepwise formation of individual Nb-mu-O-Nb linkages during acid condensation of 2 equiv of A-alpha-SiNb(3)W(9)O(40) (1) to the tri-mu-oxo-bridged structure A-alpha-Si(2)Nb(6)W(18)O(77) (4) is demonstrated by a combination of X-ray crystallography and variable-pD solution (183)W and (29)Si NMR spectroscopy. Addition of DCl to a pD 8.4 solution of 1 (Li(+) salt in D(2)O) results in formation of a mono-Nb-mu-O-Nb-linked dimer, A-alpha-Si(2)Nb(6)W(18)O(79) (2; pD = 3.0-1.3). At pD values between 1.6 and 0.3, two isomers (syn and anti) of the di-mu-oxo-bridged dimer, A-alpha-Si(2)Nb(6)W(18)O(78) (3), are observed by (183)W NMR (C(2v) and C(2h) symmetry for the syn and anti isomers, respectively; 5 (183)W 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-alpha-Cs(8)H(2)[Si(2)Nb(6)W(18)O(78)].18H(2)O, orthorhombic, Cmcm, a = 40.847(2), b = 13.2130(7), and c = 16.8179(9) A at 173K, Z = 4, final R(1) = 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 (29)Si NMR spectroscopy. Reversibility of the above processes was subsequently demonstrated by acquisition of (183)W NMR spectra after incremental additions of LiOH to D(2)O solutions of 4 to effect its stepwise hydrolysis to 2 equiv of 1.