Influence of Radical Bridges on Electron Spin Coupling

Increasing interactions between spin centers in molecules and molecular materials is a desirable goal for applications such as single-molecule magnets for information storage or magnetic metal–organic frameworks for adsorptive separation and targeted drug delivery and release. To maximize these interactions, introducing unpaired spins on bridging ligands is a concept used in several areas where such interactions are otherwise quite weak, in particular, lanthanide-based molecular magnets and magnetic metal–organic frameworks. Here, we use Kohn–Sham density functional theory to study how much the ground spin state is stabilized relative to other low-lying spin states by creating an additional spin center on the bridge for a series of simple model compounds. The di- and triradical structures consist of nitronyl nitroxide (NNO) and semiquinone (SQ) radicals attached to a meta-phenylene­(R) bridge (where R = −NH•/–NH2, −O•/OH, −CH2 •/CH2). These model compounds are based on a fully characterized SQ–meta-phenylene–NNO diradical with moderately strong antiferromagnetic coupling. Replacing closed-shell substituents CH3 and NH2 with their radical counterparts CH2 • and NH• leads to an increase in stabilization of the ground state with respect to other low-lying spin states by a factor of 3–6, depending on the exchange–correlation functional. For OH compared with O• substituents, no conclusions can be drawn as the spin state energetics depend strongly on the functional. This could provide a basis for constructing sensitive test systems for benchmarking theoretical methods for spin state energy splittings. Reassuringly, the stabilization found for a potentially synthesizable complex (up to a factor of 3.5) is in line with the simple model systems (where a stabilization of up to a factor of 6.2 was found). Absolute spin state energy splittings are considerably smaller for the potentially stable system than those for the model complexes, which points to a dependence on the spin delocalization from the radical substituent on the bridge.