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

• Published in: Journal of the American Chemical Society Vol. 141; no. 19; pp. 7853 - 7864
• Main Authors: Song, Dahae, Bae, Jinhee, Ji, Hoon, Kim, Min-Bum, Bae, Youn-Sang, Park, Kyo Sung, Moon, Dohyun, Jeong, Nak Cheon
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15.05.2019
• Subjects: acetonitrile; air; coordination polymers; copper; dissociation; electron transfer; geometry; hydroquinone; ions; oxidation; quinones
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.9b02114

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 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. 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.