A Spin-Frustrated Trinuclear Copper Complex Based on Triaminoguanidine with an Energetically Well-Separated Degenerate Ground State

We
present the synthesis and crystal structure of the trinuclear copper
complex [Cu3(saltag)­(bpy)3]­ClO4·3DMF
[H5saltag = tris­(2-hydroxybenzylidene)­triaminoguanidine;
bpy = 2,2′-bipyridine]. The complex crystallizes in the trigonal
space group R3̅, with all copper ions being
crystallographically equivalent. Analysis of the temperature dependence
of the magnetic susceptibility shows that the triaminoguanidine ligand
mediates very strong antiferromagnetic interactions (JCuCu = −324 cm–1). Detailed analysis
of the magnetic susceptibility and magnetization data as well as X-band
electron spin resonance spectra, all recorded on both powdered samples
and single crystals, show indications of neither antisymmetric exchange
nor symmetry lowering, thus indicating only a very small splitting
of the degenerate S = 1/2 ground
state. These findings are corroborated by density functional theory
calculations, which explain both the strong isotropic and negligible
antisymmetric exchange interactions.