Coordinative Reduction: A Chemical Strategy to Enhance the Hydrolytic Stability of a Paddle-Wheel MOF

Abstract

Enhancement of hydrolytic stability of metal−organic frameworks (MOFs) is a challenging issue in MOF chemistry due to the limitation of their applications under a humid environment. Meanwhile, inner sphere electron transfer has constituted one of the most extensively studied subjects in contemporary chemistry. In this presentation, we show a new conceptual “coordinative reduction” of Cu2+ ion, which is realized in a paddlewheel MOF, HKUST-1, with a postsynthetic manner via inner sphere “single” electron transfer from hydroquinone (H2Q) to Cu2+ through its coordination bond. H2Q treatment of HKUST-1 under anhydrous conditions leads to the single charge (1+) reduction of approximately 30% of Cu2+ ions. Thus, this coordinative reduction is an excellent reduction process to be self-controlled in both oxidation state and quantity. More concretely, once Cu2+ ions are reduced to Cu+, the reduction reaction does not proceed further, in terms of their oxidation state as well as their amount. Also, we show that a half of the Cu+ ions (about 15%) remains in paddlewheel framework with pseudo square planar geometry and the other half of the Cu+ ions (about 15%) forms [Cu(MeCN)4]+ complex in a small cage in the fashion of a ship-in-a-bottle after dissociation from the framework. Furthermore, we show that the coordinative reduction results in substantial enhancement of the hydrolytic stability of HKUST-1 to the extent that its structure remains intact even after exposure to humid air for two years.