title: Where are the protons in alpha-[H(x)W(12)O(40)](8-x)- (x = 2-4)?
abstract: The equilibria and speciation of the proton cryptate polyoxometalate alpha-(H2)W(12)O(40)]6- (1) were examined by NMR following the phase-transfer cation metathesis of aqueous Na(6)1 with Q+Br-/CH(2)Cl(2), leading to the isolation of the (n-Bu)4N+ (Q+) salts Q(6)1 and alpha-Q5[(H3)W(12)O(40)](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 1H and 183W 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 1H NMR (1 equiv of HBr in CH(3)CN), whereas the reverse process is slow (t1/2 approximately 17.4 h; 1 equiv of Q+OH-).
authors.affiliation: Department of Chemistry and Biochemistry, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, USA.
authors.forename: Calvin R
authors.initials: CR
authors.lastname: Sprangers
authors.forename: Jason K
authors.initials: JK
authors.lastname: Marmon
authors.forename: Dean C
authors.initials: DC
authors.lastname: Duncan