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

Abstract

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.

Spin Interactions and Magnetic Anisotropy in a Triangular Nickel(II) Complex with Triaminoguanidine Ligand Framework

“…Upon lowering the temperature, the χ M T value decreases to nearly zero at very low temperatures indicative for a diamagnetic ground state ( S = 0) for the trinuclear complex. This is consistent with the typically observed antiferromagnetic exchange interaction between paramagnetic centers mediated by N–N diazine bridges provided by the triaminoguanidine backbone of the ligand system , , . In fact, antiferromagnetically exchange coupled trinuclear systems with equilateral triangular spin topology and individual centers of even spin such as nickel(II) ions ( S = 1), are well‐known to possess a diamagnetic ground state…”

“…The magnetic exchange interactions present in the complex cations of compounds 1 and 2 were studied by broken‐symmetry DFT (BS‐DFT) calculations utilizing the B3‐LYP hybrid functional, which was previously found to give reasonable results for magnetic exchange coupling constants in 3d transition metal complexes and in particular for cases of triangular systems , , . For these calculations, two different approaches have been used: (i) Evaluation of the exchange coupling constant from a dinuclear model structure generated by diamagnetic substitution of one paramagnetic nickel(II) center with a zink(II) ion, namely [Ni 2 Zn(saltag t Bu )(bpy) 3 (H 2 O) 3 ] + , further denoted as 1‐Ni 2 Zn .…”

“…However, tritopic ligands that induce strict C 3 symmetry in the resulting complex molecules are still rather scarce and to the best of our knowledge limited to ligands constituted from backbones based on triaminoguanidine, and phloroglucinol . Interestingly, the triaminoguanidine core unit generally leads to antiferromagnetic exchange, whereas for copper(II) complexes with phloroglucinol‐based ligands ferromagnetic exchange interactions via a spin‐polarization mechanism are observed.…”

[CrF(O2CtBu)2]9: Synthesis and Characterization of a Regular Homometallic Ring with an Odd Number of Metal Centers and Electrons

“…TheINS data for the compound reveal nearly frustrated spin dynamics,with aslight lifting of the degeneracyofthe lowestenergy states resulting in as plitting of the two S = 1/2 multiplets of only 0.1 meV.C ompared to systems such as triangles where highly rigid ligand systems are required to ensure spin frustration, [22] obtaining anear perfect frustrated ground state from such as terically undemanding set of starting materials is remarkable.G iven the strong evidence for spin frustration in 1,further studies of the spin properties, such as spin density or correlation dynamics,w ill provide fascinating information on the physics of such large frustrated spin systems.…”

Crystal Structure and Spin Frustration Behavior of an Oxo‐centered Trinuclear Chromium(III) Complex

“…The spins on M(1) and M(2) are paired, which is supposed to frustrate the spins on the M(3), as the intrinsic nature of the interaction is antiferromagnetic and M(3) cannot pair with M(1) and M(2) at the same time . It has been reported that spin frustration usually occurs in the isolated [M 3 O(O 2 CR) 6 L 3 ] and similar structures, when M = Cr, Mn, Fe, Co, Cu, or Ru . Recently, it has been demonstrated that frustration gives rise to complex magnetic properties such as chiral spin structures, orbitally driven magnetism, spin‐ice behavior exhibiting Dirac strings with magnetic monopoles, valence‐bond solids, and spin liquids …”