Intermediates in the Synthesis of Zr-Based Metal-Organic Framework UiO-66

We recently discovered that aging a solution of zirconium(IV) tetrachloride (ZrCl4) in N,N-dimethylformamide (DMF) in the presence of water, followed by addition of terephthalic acid linker, reduces the crystallite size of the metal-organic framework UiO-66 [Chem. Commun. 2016, 52, 6411-6414]. In an effort to shed light into the nature of the aging effect and on its relationship with the crystallite size of UiO-66, we report here the isolation and structural characterization of a microcrystalline zirconium-based compound of formula [ZrCl(OH)2(DMF)2]Cl, which is formed during the aging process. The Zr(IV) ions are coordinated by hydroxide, DMF and chloride to produce a one-dimensional polymer. Thanks to the presence of two –OH groups per zirconium atom, [ZrCl(OH)2(DMF)2]Cl is a suitable precursor for the synthesis of UiO-66 in dry DMF, affording a product having smaller crystallite size than that obtained from a reaction mixture having the same chemical composition, but using ZrCl4 as the Zr(IV) source. By starting from ZrCl4 and generating [ZrCl(OH)2(DMF)2]Cl in situ in solution through aging, we obtained smaller crystallites as the aging time increased, proving that [ZrCl(OH)2(DMF)2]Cl plays a role in the aging process. The possible role of [ZrCl(OH)2(DMF)2]Cl in the crystallization mechanism of UiO-66 is also discussed.