Multiple\nCoordination Exchanges for Room-Temperature Activation of Open-Metal\nSites in Metal–Organic Frameworks

Abstract

The activation of\nopen coordination sites (OCSs) in metal–organic frameworks\n(MOFs), i.e., the removal of solvent molecules coordinated at the\nOCSs, is an essential step that is required prior to the use of MOFs\nin potential applications such as gas chemisorption, separation, and\ncatalysis because OCSs often serve as key sites in these applications.\nRecently, we developed a “chemical activation” method\ninvolving dichloromethane (DCM) treatment at room temperature, which\nis considered to be a promising alternative to conventional thermal\nactivation (TA), because it does not require the application of external\nthermal energy, thereby preserving the structural integrity of the\nMOFs. However, strongly coordinating solvents such as <i>N</i>,<i>N</i>-di­methyl­form­amide (DMF), <i>N</i>,<i>N</i>-di­ethyl­form­amide\n(DEF), and dimethyl sulfoxide (DMSO) are difficult to remove solely\nwith the DCM treatment. In this report, we demonstrate a multiple\ncoordination exchange (CE) process executed initially with acetonitrile\n(MeCN), methanol (MeOH), or ethanol (EtOH) and subsequently with DCM\nto achieve the complete activation of OCSs that possess strong extracoordination.\nThus, this process can serve as an effective “chemical route”\nto activation at room temperature that does not require applying heat.\nTo the best of our knowledge, no previous study has demonstrated the\nactivation of OCSs using this multiple CE process, although MeOH and/or\nDCM has been popularly used in pretreatment steps prior to the TA\nprocess. Using MOF-74­(Ni), we demonstrate that this multiple CE process\ncan safely activate a thermally unstable MOF without inflicting structural\ndamage. Furthermore, on the basis of in situ ¹H nuclear\nmagnetic resonance (¹H NMR) and Raman studies, we propose\na plausible mechanism for the activation behavior of multiple CE.