Computational Screening of Bimetal-Functionalized Zr 6 O 8 MOF Nodes for Methane C-H Bond Activation Dale R. Pahls, †,§ Manuel A. Ortuñ o, †,§ Peter H. Winegar, †,‡ Christopher J. Cramer,* ,† and Laura Gagliardi* ,† † Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡ Department of Chemistry, Michigan Technical University, Michigan 49331, United States * S Supporting Information ABSTRACT: Zr-based metal -organic frameworks (MOFs) are promising supports for copper-based catalysts able to activate methane. Homo- and heterobimetal- functionalized NU-1000 MOF nodes were selected to computationally screen the effect of ancillary metals for C-H bond activation, allowing us to correlate activation free energies with chemical descriptors. F ossil fuels are depleting, but reserves of light hydrocarbons such as methane are abundant in natural gas reservoirs. 1,2 Despite vast literature on homo- 3 and heterogeneous catalysts, 4 zeolites, 5 and enzymes, 6 the efficient and selective functional- ization of such inert C-H bonds is still a challenge. Here, we focus on metal-organic frameworks (MOFs), 7 a versatile family of mesoporous materials, as platforms to address this challenge. Concerning light hydrocarbons, only a few examples of MOF-mediated C-H bond functionalization are available. Yaghi and co-workers reported the oxidation of methane to acetic acid using the V-based MIL-47 and MOF-48 as catalysts. 8 More recently, Long and co-workers converted ethane to ethanol via Fe-oxo moieties in the magnesium- diluted Fe 0.1 Mg 1.9 (dobdc) MOF-74. 9,10 Zr-based MOFs appear as promising catalyst supports due to their excellent thermal and chemical stability. 11 In particular, we focus on the MOF NU-1000, comprised of [Zr 6 (μ 3 -O) 4 (μ 3 -OH) 4 (OH) 4 (OH 2 ) 4 ] 8+ nodes and tetratopic 1,3,6,8-tetrakis(p-benzoate)pyrene linkers (Figure 1a). 12,13 The presence of terminal hydroxo and aquo ligands at the node (Figure 1b) allows postsynthetic functionalization 14 with a wide variety of metals, such as Fe, 15 Co, 16 Ni, 17 Cu, 18 and Zn, 19 among others, 20 including heterobimetallic Co-Al systems. 21 Metal-functionalized NU- 1000 nodes have been shown to be active catalysts for hydrogenation, 17 oxidation, 16,21 and epoxidation 22 reactions. Indeed, Cu-functionalized NU-1000 nodes exhibit methane to methanol conversion and stand out as proof-of-concept MOF materials for this kind of reactivity. 23 We note that other Zr 6 O 8 MOFs, such as defect sites in the UiO series, 24 PCN-700, and MOF-808, present the same supporting functionality as that found in NU-1000 and such sites have also been employed for the deposition of potentially catalytic metals, 20,25 so results presented here are likely to be relevant to those systems as well. In the present contribution, we employ computational models to explore further the potential activity of metal- functionalized NU-1000 nodes toward C-H bond activation. Inspired by recent studies on Cu-NU-1000 catalysts for methane to methanol reactivity, 23 we designed a series of Cu- based NU-1000 nodes as shown in Figure 1c, bearing Cu and a second metal, namely Fe(II), Co(II), Ni(II), Cu(II), and Zn(II). 26 This approach allows us to screen metals to determine which would be an optimal dopant for methane activation. 27 Since common deposition techniques use water as a coreactant, 12 hydroxo ligands complete the coordination in the precatalytic systems. One of our goals is through systematic study of the effect of the ancillary metal to obtain predictive chemical descriptors of possibly more general utility. 28 All calculations were performed at the density functional theory (DFT) level using the M06-L density functional (see SI for details). From the periodic structure of NU-1000, we extract a neutral cluster model, 13,29 where the pyrene-based linkers are simplified to benzoate and kept fixed to mimic the rigidity of the framework (see SI for details). All Cu-M species were found to have high-spin ground states. We report free energies in kcal mol -1 in the gas phase at 298 K and 1 atm. Received: May 25, 2017 Figure 1. NU-1000 MOF (a), NU-1000 node (b), and bimetallic systems used in this study (c). Communication pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.7b01334 Inorg. Chem. XXXX, XXX, XXX-XXX