Stepwise C-H Bond Activations and Assembly of Binuclear Ln3+ Species Coordinated by a Hexaanionic Anti-Η5:Η5-dibenzopentalene-bridged Bis(carbazolyl) Framework.

New binuclear rare-Earth complexes [(μ2-η5:η5;κ2:κ2-N,C-L)Ln2(THF)4] (Ln = Y3+ (1Y), Tb3+ (1Tb), Dy3+ (1Dy), and Er3+ (1Er)) containing a hexaanionic dibenzopentalene-bridged bis(carbazolyl) framework were obtained via spontaneous self-assembly in the reactions of Ln(CH2SiMe3)3(THF)2 with an equimolar amount of 3,6-di-tert-butyl-1,8-bis(phenylethynyl)-9H-carbazole. Two Ln3+ ions are μ-anti-η5:η5-coordinated with the dianionic dibenzopentalene fragment resulting from a series of successive intramolecular ortho-CH activations in the phenyl rings of the ethynyl substituents followed by intermolecular dimerization involving Ln-CAr bond addition to a CC triple bond. Magnetic investigations show that 1Dy behaves as a genuine single-molecule magnet (SMM) with relaxation dynamics described by a multicomponent phenomenological relaxation model yielding an effective energy barrier of 689 cm-1. In contrast, 1Tb and 1Er exhibit only field-induced slow magnetic relaxation at low temperature. Ab initio calculations reveal a substantially larger crystal-field splitting and stronger axial anisotropy for Dy3+ than for the Er3+ analogue. Weak antiferromagnetic interactions between the lanthanide centers were identified with J = -0.23 cm-1 for 1Dy, -0.06 cm-1 for 1Tb, and -0.28 cm-1 for 1Er. This indicates that the slow magnetic relaxation is governed primarily by single-ion crystal-field effects rather than exchange coupling.