(Me2NH2)10[H2-Dodecatungstate] polymorphs: dodecatungstate cages embedded in a variable dimethylammonium cation plus water of crystallization matrix

Two polymorphs and a solvatomorph of a new dimethylammonium polytungstate-decakis(dimethylammonium) dihydrogendodecatungstate, (Me2NH2)(10)(W12O42)center dot nH(2)O (n = 10 or 11)-have been synthesized. Their structures were characterized by single-crystal X-ray diffraction and solid-phase NMR methods. The shape of the dodecatungstate anions is essentially the same in all three structures, their interaction with the cations and water of crystallization, however, is remarkably variable, because the latter forms different hydrogen-bonded networks, and provides a highly versatile matrix. Accordingly, the N-HMIDLINE HORIZONTAL ELLIPSISO and C-HMIDLINE HORIZONTAL ELLIPSISO hydrogen bonds are positioned in each crystal lattice in a variety of environments, characteristic to the structure, which can be distinguished by solid-state H-1-CRAMPS, C-13, N-15 CP MAS and H-1-C-13 heteronuclear correlation NMR. Thermogravimetry of the solvatomorphs also reflect the difference and multiformity of the environment of the water molecules in the different crystal lattices. The major factors behind the variability of the matrix are the ability of ammonium cations to form two hydrogen bonds and the rigidity of the polyoxometalate anion cage. The positions of the oxygen atoms in the latter are favourable for the formation of bifurcated and trifurcated cation-anion hydrogen bonds, some which are so durable that they persist after the crystals are dissolved in water, forming ion associates even in dilute solutions. The H atom involved in furcated hydrogen bonds cannot be exchanged by deuterium when the compound is dissolved in D2O. An obvious consequence of the versatility of the matrix is the propensity of these compounds to form multiple polymorphs.

Automated summary

A new dimethylammonium polytungstate has been synthesized, showing two polymorphs and a solvatomorph with highly versatile matrix properties. The structures were characterized using single-crystal X-ray diffraction and solid-phase NMR methods, revealing different hydrogen-bonded networks formed by the cations and water of crystallization. The versatility of the matrix is attributed to the ability of ammonium cations to form multiple hydrogen bonds and the rigidity of the polyoxometalate anion cage, leading to the formation of multiple polymorphs in these compounds.