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 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.