Coordinative Reduction of Metal Nodes Enhances the Hydrolytic Stability of a Paddlewheel Metal–Organic Framework

Enhancement of hydrolytic stability of metal–organic frameworks (MOFs) is a challenging issue in MOF chemistry because most MOFs have shown limitations in their applications under a humid environment. Meanwhile, inner sphere electron transfer has constituted one of the most intensively studied subjects in contemporary chemistry. In this report, we show, for the first time, a new conceptual <i>coordinative reduction</i> 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. As described below, 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 demonstrate 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.