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Spin Frustration Determines the Stability and Reactivity of Metal-Organic Frameworks with Triangular Iron(III)-Oxo
Patrick Lechner1,2, Gaurab Ganguly1, Michael J Sahre3
1Institute of Theoretical Chemistry, Faculty of Chemistry, University of Vienna, Währinger Str. 17, Vienna, 1090, Austria.
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Density functional theory (DFT) is the standard approach for modeling MIL-101(Fe) and related Fe-based metal-organic frameworks, typically assuming a ferromagnetic high-spin configuration. However, this widely adopted approach overlooks a key electronic feature: Spin frustration in the triangular -O) nodes. Using flip-spin, broken-symmetry DFT, we identify the true ground state as an antiferromagnetic state that standard DFT fails to capture. We demonstrate that neglecting spin frustration in MIL-101(Fe) leads to structural distortions, incorrect energetics, and misleading predictions of stability and reactivity. By explicitly accounting for spin frustration, we recover the correct structure and rationalize the temperature-dependent and CO binding. Spin frustration enhances fixation at room temperature, while its loss upon partial reduction suppresses this activity but promotes CO adsorption via -backbonding. These findings challenge current computational conventions and highlight spin frustration as a critical electronic feature in these frameworks.
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