Synthesis, structure, and magnetism of heterobimetallic trinuclear complexes {[L₂Co₂Ln][X]} [Ln = Eu, X = Cl; Ln = Tb, Dy, Ho, X = NO₃; LH₃ = (S)P[N(Me)NCH-C₆H₃-2-OH-3-OMe]₃]: a 3d-4f family of single-molecule magnets

Abstract

Sequential reaction of LH₃ (LH₃ = (S)P[N(Me)N=CH-C₆H₃-2-OH-3-OMe]₃) with Co(OAc)₂·4H₂O followed by reaction with lanthanide salts afforded trinuclear heterobimetalllic compounds {[L₂Co₂Ln][X]} [Ln = Eu (1), X = Cl; Ln = Tb (2), Dy (3), Ho (4), X = NO₃] in excellent yields. These compounds retain their integrity in solution as determined by electrospray ionization mass spectrometry studies. The molecular structures of 1-4 were confirmed by a single-crystal X-ray structural study and reveal that these are isostructural. In all of the compounds, the three metal ions are arranged in a perfectly linear manner and are held together by two trianionic ligands, L^3-. The two terminal Co^II ions contain a facial coordination environment (3N, 3O) comprising three imino nitrogen atoms and three phenolate oxygen atoms. The coordination geometry about the cobalt atom is severely distorted. An all-oxygen coordination environment (12O) is present around the central lanthanide ion, which is present in a distorted icosahedral geometry. The coordination sphere around the lanthanide ion is achieved by utilizing three phenolate oxygen atoms and three methoxy oxygen atoms of each ligand. In all of these trinuclear complexes (1-4), the Co-Ln distances are around 3.3 Å, while the Co-Co distances range from 6.54 to 6.60 Å. The screw-type coordination mode imposed by the ligand induces chirality in the molecular structure, although all of the complexes crystallize as racemates. Magnetic properties of 1-4 have been studied in detail using dc and ac susceptibility measurements. Dynamic measurements reveal that 2-4 display a single-molecule magnet behavior, while the Co₂Eu (1) analogue does not show any out-of-phase ac susceptibility.