Spin-frustration determines the stability and reactivity of MOFs with triangular Fe(III) centers

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. This widely adopted approach overlooks a key electronic feature: spin-frustration in the triangular Fe₃O nodes. Using flip-spin DFT, we identify the true ground state as an antiferromagnetic 𝑀 = 6 state that standard DFT fails to capture. We show that using standard DFT for 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 N₂ and CO binding: spin-frustration enhances N₂ fixation at room temperature, while its loss upon partial Fe(III) reduction suppresses this activity but promotes CO adsorption via 𝜋-back bonding. These findings challenge the current conventions and highlight spin-frustration as a critical electronic feature.