Computational Predictions and Experimental Validation of Alkane Oxidative Dehydrogenation by Fe₂M MOF Nodes

Abstract

The modular structure of metal−organic frameworks (MOFs) makes them promising platforms for catalyst design and for elucidating structure/performance relationships in catalysis. In this work, we systematically varied the composition of the metal nodes (Fe 2 M) of the MOF PCN-250 and used density functional theory (DFT) to identify promising catalysts for light alkane C−H bond activation. Oxidative dehydrogenation (ODH) of alkanes was studied using N 2 O as the oxidant to understand the reactivity of the oxocentered Fe 2 M nodes found in PCN-250, where the Fe ions are in the +3 oxidation state and M is a metal with the oxidation state of +2. We show that the N 2 O activation barrier is positively correlated with the oxygen-binding energy at the metal center, and the C−H activation barrier is negatively correlated with this same quantity. For clusters containing early transition metals, oxygen binds strongly, facilitating N 2 O activation but hindering C−H activation. To validate the DFT predictions, we synthesized and tested PCN-250(Fe 2 M) with M = Mn, Fe, Co, and Ni and found that PCN-250(Fe 2 Mn) and PCN-250(Fe 3 ) are more active than PCN-250(Fe 2 Co) and PCN-250(Fe 2 Ni) in agreement with the DFT predictions, demonstrating the power of DFT calculations to predict and identify promising MOF catalysts for alkane C−H bond activation in advance of experiments.

Why Conventional Design Rules for C–H Activation Fail for Open-Shell Transition-Metal Catalysts

“…Starting from a resting state structure (1), a high-valent terminal metal-oxo (2) is formed via two-electron oxidation ( Figure 1). We calculate the thermodynamic reaction energy, ΔE(oxo), with the common oxidant 33,98 N 2 O as the oxygen atom source:…”

High‐Valent Metal–Oxo Species at the Nodes of Metal–Triazolate Frameworks: The Effects of Ligand Exchange and Two‐State Reactivity for C−H Bond Activation

“…Beyond the choice of anionic bridging ligands,t he donor strength of the organic linker is also likely to influence the redox activity of the framework in cases where an [MO] 2+ site can form. To test this,w ec ompared the MÀOf ormation enthalpies of the Fe-BBTA-X cluster models with those of Fe 0.1 Mg 1.9 (DOBDC) (H 4 DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid) and Fe 3 (m 3 -O)(HCOO) 6 ( Figure S11), the latter of which is representative of the node in the Fe-MIL-100/101 (and Fe-PCN-250) series.T hese MOFs were chosen as points of comparison, as they have been shown to activate strong CÀHb onds via ap resumed Fe IV -oxo intermediate [18,25,26] and have carboxylate linkers,w hich are weaker donors than the triazolate linkers of the M 2 X 2 (BBTA) series. [44] At the B3LYP-D3(BJ)/def2-TZVP level of theory, [59][60][61][62][63][64] DH O is predicted to be + 24 kJ mol À1 for Fe 0.1 Mg 1.9 (DOBDC) and + 35 kJ mol À1 for Fe 3 (m 3 -O)(HCOO) 6 .W hile Fe-BBTA-Br has a DH O value nearly equivalent to that of Fe 0.1 Mg 1.9 -(DOBDC), all other members of the Fe-BBTA-X series have more favorable Fe IV -oxo formation enthalpies,w ith Fe-BBTA-X (X = OH, SH, SeH) having DH O values that are % 34 kJ mol À1 more exothermic than Fe 0.1 Mg 1.9 (DOBDC) and % 45 kJ mol À1 more exothermic than Fe-MIL-100/Fe-MIL-101/Fe-PCN-250 (Table S9).…”

“…As aresult, relatively little is known about how to enhance the reactive properties of these materials despite the modular nature of the underlying secondary building units. [17] To date, nearly all MOFs studied for the activation of light alkanes via terminal metal-oxo species contain metals connected to one another with carboxylate linkers that yield an all-oxido coordination environment, as is the case for the metal sites of Fe-MOF-74 (and its structural analogues), [18][19][20][21][22] Mn-exchanged MOF-5, [23] M 3 (BTC) 2 (M = Cr, Fe,C o, Ni, Cu, Zn; H 3 BTC = 1,3,5-benzenetricarboxylic acid), [24] FeM 2 -PCN-250 (M = Fe,M n, Co,N i; PCN = Porous Coordination Network), [25] and Fe-MIL-100 (MIL = Materials Institut Lavoisier). [26,27] Fe-BTT (H 3 BTT = 1,3,5-benzenetristetrazolate), which has tetrazolate linkers,i so ne exception;h owever,i t is believed that framework defects (rather than the computationally investigated, crystallographic framework sites) are responsible for the materialsreactivity toward ethane in the presence of N 2 O.…”

“…[44] The greater redox activity of the framework is particularly relevant, as mechanistic studies based on density functional theory (DFT) calculations frequently indicate that the reactivity of MOFs with open metal sites are limited by their ability to form high-valent metal-oxo active species. [25,26,45] In the present study,w ef ocus on the family of metaltriazolate frameworks with the general formula M 2 X 2 (BBTA) (M = divalent metal, X = monovalent bridging anion). M 2 X 2 -(BBTA) is topologically analogous to the MOF-74 family, containing ahigh density of coordinatively unsaturated metal sites with square pyramidal coordination geometries and ahoneycomb-like crystal structure ( Figure 1).…”

Synthetic and computational assessment of a chiral metal–organic framework catalyst for predictive asymmetric transformation

“…[5][6][7][8] In this context, crystalline porous materials like Metal-Organic Frameworks (MOF) have been explored as ideal model crystalline structures for in silico investigations of catalytic transformations, however challenged by the increasing sophistication of their hybrid organic-inorganic structures. [9][10][11][12][13] Nevertheless, turning to enantioselective reactions, the computationally-driven rationalization of their catalytic activity in heterogeneous asymmetric transformations is challenging. To the best of our knowledge, such rationalization has never been achieved so far notwithstanding the major analogous achievements in computational molecular catalysis, 14,15 at least partly because of the complexity of the chemical reactions at the solid's interface when chiral supramolecular interactions come into play.…”