Controlling the symmetry of hexamonodentate 3d-transition metal complexes through symmetry propagation from high-symmetry Ti–Mo and Zr–Mo clusters via hydrogen-bonding interactions

Abstract

The symmetry of one of the simplest hexamonodentate complexes, [M(H₂O)₆]²⁺ (M = Fe, Co, Ni, and Zn), was controlled by tuning interactions in the second coordination sphere. Highly symmetric Ti–Mo or Zr–Mo cluster cations acted as symmetry templates, imposing a crystallographic trigonal coordination geometry in the hexamonodentate complexes through intermolecular hydrogen-bonding interactions. Magnetic measurements revealed that the ideal trigonal symmetry results in weak spin–orbit coupling for high-spin Fe^II complexes, despite the T-term ground state in the octahedral geometry. In contrast, high-spin Co^II analogues with the T-term ground state exhibited strong spin–orbit coupling. DFT studies supported that a d⁶ Fe^II ion in the D₃ symmetry has a ⁵A₁ ground state while a d⁷ Co^II in the same symmetry has a ⁴E ground state. Single-ion magnet behavior was observed in the Co^II complexes. These results demonstrate that incorporating a diamagnetic, highly symmetric cluster enables precise symmetry control in single-ion magnets containing only monodentate ligands.