Spin Frustration Determines the Stability and Reactivity of Metal-Organic Frameworks with Triangular Iron(III)-Oxo Clusters.

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 ${\mathrm{Fe}}_3({\mu }_3$ -O) nodes. Using flip-spin, broken-symmetry DFT, we identify the true ground state as an antiferromagnetic $2S+1=6$ 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 $N_2$ and CO binding. Spin frustration enhances $N_2$ fixation at room temperature, while its loss upon partial ${\mathrm{Fe}}^{\mathrm{III}}$ reduction suppresses this activity but promotes CO adsorption via $\pi$ -backbonding. These findings challenge current computational conventions and highlight spin frustration as a critical electronic feature in these frameworks.