Impact of Spin-Phonon Coupling on Magnetic Relaxation of a Co(II) Single-Molecule Magnet

Single-molecule magnets (SMMs) based on transition metals have appeared as enticing targets exploiting the magnetic anisotropy in 3d elements. Among transition elements, Co based SMMs are very prominent as they often exhibit a high spin-reversal barrier (Ueff ), owing to large unquenched orbital angular momentum. Employing the wave function-based multireference CASSCF/NEVPT2 calculations, herein we substantiate the zero-field splitting parameters of four mononuclear Co complexes and one of them has been realized as a prospective SMM. The mechanism of magnetic relaxation has been studied to underpin the molecular origin of the slow relaxation of magnetization. The combination of suppressed quantum tunnelling of magnetization (QTM) at the ground state and the high negative D value usually manifests SMM behavior at zero-applied magnetic field. However, mere fulfillment of these conditions ensure little about their SMM behavior, as spin-phonon couplings often play the role of spoilsports by lowering the spin-relaxations channels. A detailed study accounting all the 46 vibrational modes below the first-excited state, for the prospective Co(II) complex, reveals one of the vibrational modes, providing lower spin-relaxation pathway. This results an SMM with Ueff value of 239.30 cm−1, decreased by ~81 cm-1 from the value without spin-vibrational coupling.