Tetranuclear {LnIII}4 complexes possessing homometallic O-capped structural subunits: study of magnetic and photoluminescent properties

Abstract

A series of homometallic tetranuclear Ln₄ complexes, [Ln₄(μ₃-OH)₂{py₂C(OH)O}₂(O₂CCMe₃)₈] [{py₂C(OH)O}⁻ = monoanionic gem-diol form of di-2-pyridyl ketone; Ln = Nd (1), Eu (2), Tb (3), Dy (4), Er (5) and Yb (6)], have been synthesized and characterized. The asymmetric unit of each of the tetranuclear derivatives comprises the dinuclear motif, [Ln₂(μ₃-OH){py₂C(OH)O}(O₂CCMe₃)₄]. The core structure of this Ln₄ family possesses two homometallic O-capped structural subunits, Ln^III₃O, which are further connected through the bridging μ₃-OH ligands. Mean plane analysis indicates that all the lanthanide centers lie in the same plane. There are two types of eight-coordinated lanthanide centers present. The shape analysis revealed that the Ln1 center possesses a distorted elongated trigonal bipyramidal geometry while the Ln2 center has a distorted Johnson elongated triangular bipyramidal geometry. Solid-state room temperature photoluminescence studies of 1–5 show the ligand center emission band at 306 nm when excited at 270 nm. Compound 1, when excited at 380 nm emits at 517, 624, and 724 nm. On the other hand, excitation at 360 nm of compound 3, leads to emission peaks at 489, 542, 583, 590, and 624 nm. These bands are due to the f–f transitions of the Tb(III) centers. For all complexes, the dependence of direct current (DC) and alternating current (AC) magnetization on temperature and magnetic field was measured. The magnetic anisotropy and magnetic relaxation mechanism were also studied using the complete active space self-consistent field (CASSCF) methodology for complexes 1–6. The SINGLE_ANISO module is used to extract the relevant spin-Hamiltonian parameters. We have calculated the magnetic exchange couplings (J₁–J₃) for all the complexes. For complexes 3 and 4, we observed a sizable tunnel splitting of 1.5 × 10⁻³ cm⁻¹ and 1.9 × 10⁻³ cm⁻¹, indicating that QTM is still a dominant mechanism for magnetic relaxation in the exchange-coupled systems, resulting in diminished SMM behaviour.