Where are the protons in α-[HxW12O40](8-x)-(x=2-4)?

The equilibria and speciation of the proton cryptate polyoxometalate alpha-[(H-2)W12O40](6-) (1) were examined by NMR following the phase-transfer cation metathesis of aqueous Na(6)1 with Q(+)Br(-)/CH2Cl2, leading to the isolation of the (n-Bu)(4)N+ (Q(+)) salts Q(6)1 and alpha-Q(5)[( H-3)W12O40] (Q(5)2). Several groups report salts of the protonated anions H(x)1 (x = 1 and 2) with no consensus on proton numbers or locations. Reported herein, a combination of H-1 and W-183 NMR evidence, elemental analysis, acid titration measurements, and H/D isotopomer assignments establishes that in nonaqueous media the internal cryptand cavity of 1 reversibly accommodates only one more proton to form 2. Because an external proton must transfer across the close- packed tungsten oxide surface of 1, which should constitute a substantial activation barrier, it is significant that the transformation is instantaneous by H-1 NMR (1 equiv of HBr in CH3CN), whereas the reverse process is slow (t(1/2) similar to 17.4 h; 1 equiv of Q(+)OH(-)).