From tetranuclear to pentanuclear [Co–Ln] (Ln = Gd, Tb, Dy, Ho) complexes across the lanthanide series: effect of varying sequence of ligand addition

Abstract

Two new families of cobalt(II/III)–lanthanide(III) coordination aggregates have been reported: tetranuclear [Ln^III₂Co^III₂L₂(N-BuDEA)₂(O₂CCMe₃)₄(H₂O)₂]·(MeOH)_n·(H₂O)_m (Ln = Gd, 1; Tb, 2; Dy, 3; n = 2, m = 10 for 1 and 2; n = 6, m = 2 for 3) and pentanuclear Ln^III₂Co^IICo^III₂L₂(N-BuDEA)₂(O₂CCMe₃)₆(MeOH)₂ (Ln = Dy, 4; Ho, 5) formed from the reaction of two aggregation assisting ligands H₂L (o-vanillin oxime) and N-BuDEAH₂ (N-butyldiethanolamine). A change in preference from a lower to higher nuclearity structure was observed on going across the lanthanide series brought about by the variation in the size of the Ln^III ions. An interesting observation was made for the varying sequence of addition of the ligands into the reaction medium paving the way to access both structural types for Ln = Dy. HRMS (+ve) of solutions gave further insight into the formation of the aggregates via different pathways. The tetranuclear complexes adopt a modified butterfly structure with a more complex bridging network while trapping of an extra Co^II ion in the pentanuclear complexes destroys this arrangement putting the Co–Co–Co axis above the Ln–Ln axis. Direct current (dc) magnetic susceptibility measurements reveal weak antiferromagnetic coupling in 1. Complexes 2 and 5 display no slow magnetic relaxation, whereas complexes 3 and 4 display out-of-phase signals at low temperature in ac susceptibility measurements. All compounds were analyzed with DFT and CASSCF calculations and informations about the single-ion anisotropies and mutual 4f–4f/4f–3d magnetic interactions were derived.