Exploration of the role of carboxylate bridges on the magnetisation dynamics of {LnIII2} (Ln = Tb, Dy, Ho, and Er) paddle-wheel single-molecule magnets: structure-property correlations.

Lanthanide-based complexes are excellent candidates for observing the single-molecule magnet (SMM) behaviour since the Ln(III) ions possess strong magnetic anisotropy, which arises mainly from the spin-orbit coupling of associated with each Ln(III) ion. Among them, the dinuclear Ln(III) complexes are an interesting system for studying the SMM behaviour as they exhibit magnetic exchange in addition to magnetic anisotropy. The exchange interaction between the Ln(III) ions can be influenced by various bridges, such as hydroxides, halides, and alkoxides, which, in general, are smaller and mainly connect the Ln(III) ions through their single donor atoms. The {Ln2} dinuclear complexes with more than two longer bridges, e.g., carboxylate ions (-O-C-O-), are scarce. When {Ln2} complexes have four benzoate bridges, they can adopt a fascinating paddle-wheel structure, providing several superexchange pathways between the Ln(III) ions. Considering these advantages, we synthesised four paddle-wheel {LnIII2} complexes, having the molecular formula of [Ln2(µ-PFBA)4(PFBA)2(solv)2(bpy)2], where Ln = Tb (1-Tb), Dy (2-Dy), Ho (3-Ho), and Er (4-Er), PFBA- = pentafluorobenzoate (C6F5COO-), bpy = 2,2'-bipyridine, and solv = MeOH or H2O. Direct current magnetic susceptibility investigations demonstrated a ferromagnetic exchange interaction in the 1-Tb and 2-Dy complexes and an antiferromagnetic interaction between the LnIII ions in 3-Ho and 4-Er. Complexes 1-Tb, 2-Dy, and 4-Er display out-of-phase susceptibility signals in the presence of an external magnetic field, demonstrating the slow relaxation of magnetization, for which the effective energy barrier (Ueff) is 37.7 ± 1.9 K for 2-Dy. Comprehensive ab initio CASSCF + RASSI-SO + SINGLE_ANISO + POLY_ANISO simulations rationalise the observed SMM behaviour and indicate that the intensity of magnetic exchange changes from weak ferromagnetic to weak antiferromagnetic in the subsequent lanthanide sequence: 1-Tb (+0.2 cm-1) < 2-Dy (+0.04 cm-1) < 3-Ho (-0.47 cm-1) < 4-Er (-0.55 cm-1). These findings underscore the significance of magnetic superexchange between LnIII⋯LnIII ions in augmenting the SMM properties of the {Ln2} complexes in addition to each Ln(III) ion's anisotropy. Further, a few models were analysed, and results implied that the assembly of different carboxylate bridges in an ideal paddle-wheel topology can effectively tune the Ueff of these {Dy2} SMMs.