Where Are the Protons in α-[HxW12O40](8-x)- (x = 2−4)?
Where Are the Protons in α-[HxW12O40](8-x)- (x = 2−4)?
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DOI：
10.1021/ic061370c
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发表时间：
2006-10
影响因子：
4.6
通讯作者：
Calvin R. Sprangers;J. Marmon;D. C. Duncan
中科院分区：
文献类型：
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作者：
Calvin R. Sprangers;J. Marmon;D. C. Duncan
用核磁共振方法研究了多金属氧酸盐α-(H2)W(12)O(40)]6-(1)与Q+Br-/CH(2)Cl(2)的相转移阳离子歧化反应，分离出(n-Bu)4N+(Q+)盐Q(6)1和α-Q5[(H3)W(12)O(40)](Q(5)2)。几个小组报告了质子化阴离子H(X)1(x=1和2)的盐，但对质子数量或位置没有共识。本文报道，结合1H和183W核磁共振证据、元素分析、酸滴定测量和H/D同位素指定，证实在非水介质中，1的内部密码和空腔可逆地只容纳一个质子形成2。由于外部质子必须通过紧密堆积的氧化钨表面转移，这应该构成一个重要的活化势垒，重要的是，1H核磁共振转换是瞬时的(1当量HBr在CH(3)CN中)，而反向过程是缓慢的(T1/2约17.4h；1当量Q+OH-)。
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-).