A Ligand Field Molecular Mechanics Study of CO₂‐Induced Breathing in the Metal–Organic Framework DUT‐8(Ni)

Abstract

Flexible metal-organic Frameworks (MOFs) are an interesting class of materials due to their diverse properties. One representative of this class is the layered-pillar MOF DUT-8(Ni). This MOF consists of Ni 2 paddle wheels interconnected by naphthalene dicarboxylate linkers and dabco pillars (Ni 2 (ndc) 2 (dabco), ndc = 2,6-naphthalene-dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane). DUT-8(Ni) undergoes a volume change of over 140% upon adsorption of guest molecules. Herein, a ligand field molecular mechanics (LFMM) study of the CO 2 -induced flexibility of DUT-8(Ni) is presented. LFMM is able to reproduce experimental and DFT structural features as well as properties that require large simulation cells. It is shown that the transformation energy from a closed to open state of the MOF is overcompensated fivefold by the host-guest interactions. Structural characteristics of the MOF explain the shape of the energy profile at different loading states and provide useful insights to the interpretation of previous experimental results.the possibilities of employing ligand field molecular mechanics (LFMM) simulations to study flexible MOFs and their processes. The method employed is not limited to adsorption processes but as has been shown previously [8][9][10][11] to be applicable to other properties, for example, spin states or magnetic properties. Our goal is to present simulations using the example of CO 2induced breathing in the pillared layer MOF DUT-8(Ni) [12] (Ni 2 (ndc) 2 (dabco), ndc = 2,6naphthalenedicarboxylate, dabco = 1,4diazabicyclo-[2.2.2]-octane, see Figure 1) using an LFMM approach. Motivated by its outstanding properties, DUT-8(Ni) is being investigated extensively recently. [12][13][14][15][16][17][18][19][20][21][22][23] Some examples of the properties of interest are the enormous volume change upon opening (more than 140 %), [14] its isomorphism, [22] the property dependence on the metal center, [14] and crystallite size effects. [19] Alzahrani and Deeth [24] already employed LFMM to investigate clusters of Zn 2 (bdc) 2 (dabco) and showed the applicability to this structurally related flexible MOF. We present here the first application of LFMM to simulate the breathing of an openshell transition-metal paddle wheel pillared layer MOF in periodic boundary conditions.

Molecular Dynamics Simulations of the Breathing Phase Transition of MOF Nanocrystallites II: Explicitly Modeling the Pressure Medium

“…Out of 28 simulations, 16 end up in a diamond-shaped and 12 in a disturbed diamond-shaped closed pore form, independent of the conformation and the system size. The observation of differently shaped closed pore forms agrees with already reported deviations from the perfect periodic image [31]. The DUT-8 NCs show an equivalent number of different closed pore forms as the DMOF-1 model system.…”

London Dispersion Governs the Interaction Mechanism of Small Polar and Non-Polar Molecules in Metal-Organic Frameworks

“…We recently presented the possibility to perform molecular dynamics (MD) simulations of DUT8-(Ni) by employing Ligand Field Molecular Mechanics (LFMM) for the description of the metal nodes. 25 Alzahrani and Deeth lately also confirmed the applicability of LFMM for the description of flexibility in MOFs. 29 Experimental adsorption energies of chlorinated methanes (CH x Cl 4-x , x = 0 to 3) in DUT-8(Ni) (−25 to −40 , −35 to −50 , −30 to −40 and −20 to −40 kJ mol −1 for CCl 4 to CH 3 Cl respectively, see Figure 2b) 26,30 suggest, in agreement with chemical intuition, a significant impact of the guest's dipole moment on the adsorption energy.…”

“…A thoroughly investigated representative of breathing flexible Metal-Organic Frameworks (MOFs), [1][2][3][4][5][6] sometimes also referred to as thirdgeneration MOFs, 7-9 is the layered pillar MOF Ni 2 (ndc) 2 (dabco) (ndc = 2,6-naphthalene-dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]octane), also known as DUT-8(Ni). [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] It consists of layers of Ni 2 (COO) 4 paddle wheel nodes interconnected by ndc linkers (Figure 1). The so formed layers are interconnected in the third dimension by dabco pillars (right side of Figure 1).…”

Breathing Metal–Organic Polyhedra Controlled by Light for Carbon Dioxide Capture and Liberation

“…The studies of gas adsorption in breathing materials remain dominated by molecular simulation at present. 60 Hence, DFT calculations were performed to support the unusual adsorption behavior (Figures 6e and 6f). Upon visible light irradiation, the binding energy of NUT-101 adsorbing CO 2 is negative, suggesting that the CO 2 molecule can be captured within the cage (Supporting Information Table S4).…”