Spin-polarization in 1,3,5-trihydroxybenzene-bridged first-row transition metal complexes

The late Olivier Kahn formulated a research objective for molecular magnetism: ‘The normal trend for the molecular state is the pairing of electrons […] with a cancellation of the electron spins. The design of a molecule-based magnet requires that this trend be successfully opposed.’ One strategy for enforcing ferromagnetic interactions is the spin-polarization mechanism. While this mechanism is almost always working in organic chemistry, the application to transition metal complexes is not straight-forward. We have established a structurally related series of trinuclear complexes bridged by modified 1,3,5-trihydroxybenzene (phloroglucinol) ligands. The trinuclear CuII complexes all exhibit weak, but ferromagnetic interactions. The trinuclear VIV complex exhibits even smaller ferromagnetic interactions, while a trinuclear Mn complex exhibits antiferromagnetic interactions. The correlation between structural and magnetic parameters in the series of Cu complexes gives experimental insight into the spin-polarization mechanism. The crucial parameter for an efficient spin-polarization mechanism through the bridging benzene unit seems to be the amount of spin density in the pzπ orbitals of the phenolic oxygen atoms. This spin density crucially depends on the remaining coordination sites and on the ligand folding at the central Cu–phenolate bond. The spin transfer from the metal to the phenolate oxygen atom occurs by two different mechanisms, namely spin-polarization and spin-delocalization, which can provide opposing contributions. The main conclusion of this study is that for a more efficient spin-polarization through the central benzene ring the spin density in the phenolate O pzπ orbital must be maximized, which can mainly be achieved by increasing the covalency of the metal–phenolate bond.