Metal-Dependent Magnetic Exchange Topology as a Fingerprint of N2 Binding in Isostructural MOF Nodes

Abstract

Triangular metal-oxo nodes are widely employed as catalytic motifs in metal–organic frameworks. Photocatalytic N2 fixation using the all-iron nodes can be switched off by replacing them with all-chromium nodes. However, the two nodes bind N2 in the opposite order to their activity. The present contribution shows that the electronic structure of the metal, which governs the small-molecule binding to the framework, determines the magnetic exchange topology of the [MIII3(μ3-O)] node. Combining broken-symmetry and non-collinear DFT, Hamiltonian-based reconstruction of Mössbauer spectra, and quantitative modeling of temperature-dependent magnetic susceptibility, one finds that FeIII exhibits a genuinely frustrated triangle of comparable antiferromagnetic interactions, whereas CrIII features an asymmetric dimer-plus-monomer governed by unusually strong biquadratic interaction, placing it outside the conventional Heisenberg regime. The distinct electronic structure is reflected in markedly different magnetic topologies and N2 binding strengths, rationalizing the contrasting photocatalytic activities of the two frameworks. These results establish magnetic exchange topology as an experimentally accessible descriptor of chemical function in trinuclear MOF nodes.