Strong direct exchange coupling and single-molecule magnetism in indigo-bridged lanthanide dimers

The synthesis, structure and magnetic properties of the indigo-bridged dilanthanide complexes [{(η⁵-Cp*)₂Ln}₂(μ-ind)]ⁿ⁻ with Ln = Gd or Dy and n = 0, 1 or 2 are described. The gadolinium complexes with n = 0 and 2 show typically weak exchange coupling, whereas the complex bridged by the radical [ind]³⁻ ligand shows an unusually large coupling constant of J = −11 cm⁻¹ (−2J formalism). The dysprosium complexes with n = 0 and 1 are single-molecule magnets in zero applied field, whereas the complex with n = 2 does not show slow magnetic relaxation.