Journal of the American Chemical Society, Vol.133, No.13, 4955-4964, 2011
Kubas-Type Hydrogen Storage in V(III) Polymers Using Tri- and Tetradentate Bridging Ligands
Oxalic acid, oxamide, glycolic acid, and glycolamide were employed as 2-carbon linkers to synthesize a series of one-dimensional V(III) polymers from trismesityl vanadium(III).THF containing a high concentration of low-valent metal sites that can be exploited for Kubas binding in hydrogen storage. Synthesized materials were characterized by powder X-ray diffraction (PXRD), nitrogen adsorption (BET), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), Raman spectroscopy, thermogravimetric analysis, and elemental analysis. Because each of these organic linkers possesses a different number of protons and coordinating atoms, the products in each case were expected to have different stoichiometries with respect to the number of mesityl groups eliminated and also a different geometry about the V(III) centers. For example, the oxalate and glycolate polymers contained residual mesityl groups; however, these could be exchanged with hydride via hydrogenolysis. The highest adsorption capacity was recorded on the product of trismesityl vanadium(III).THF with oxamide (3.49 wt % at 77 K and 85 bar). As suggested by the high enthalpy of adsorption (17.9 kJ/mol H-2), a substantial degree of performance of the vanadium metal centers was retained at room temperature (25%), corresponding to a gravimetric adsorption of 0.87 wt % at 85 bar, close to the performance of MOF-177 at this temperature and pressure. This is remarkable given the BET surface area of this material is only 9 m(2)/g. A calculation on the basis of thermogravimetric results provides 0.88 hydrogen molecule per vanadium center under these conditions. Raman studies with H-2 and D-2 showed the first unequivocal evidence for Kubas binding on a framework metal in an extended solid, and IR studies demonstrated H(D) exchange of the vanadium hydride with coordinated D-2. These spectroscopic observations are sufficient to assign the rising trends in isosteric heats of hydrogen adsorption observed previously by our group in several classes of materials containing low-valent transition metals to the Kubas interaction.