Two Distinct Cu(II)–V(IV) Superexchange Interactions with Similar Bond Angles in a Triangular “CuV₂” Fragment

Abstract

The strength and sign of superexchange interactions are often predicted on the basis of the bond angles between magnetic ions, but complications may arise in situations with a nontrivial arrangement of the magnetic orbitals. We report on a novel molecular tetramer compound [Cu(H 2 O)dmbpy] 2 [V 2 O 2 F 8 ] (dmbpy = 4,4′-dimethyl-2,2′bipyridyl) that is composed of triangular "CuV 2 " fragments and displays a spin gap behavior. By combining first-principles calculations and electronic models, we reveal that superexchange Cu−V interactions carry drastically different coupling strengths along two Cu−F−V pathways with comparable bond angles in the triangular "CuV 2 " fragment. Counterintuitively, their strong disparity is found to originate from the restricted symmetry of the half-filled Cu d x 2 −y 2 orbital stabilized by the crystal field, leading to one dominating antiferromagnetic Cu−V coupling in each fragment. We revisit the magnetic properties of the reported spin-gapped chain compound [enH 2 ]Cu-(H 2 O) 2 [V 2 O 2 F 8 ] (enH 2 = ethylene diammonium) containing similar triangular "CuV 2 " fragments, and the magnetic behavior of the molecular tetramer and the chain compounds is rationalized as that of weakly coupled spin dimers and spin trimers, respectively. This work demonstrates that fundamentally different magnetic couplings can be observed between magnetic ions with similar bond angles in a single spin motif, thus providing a strategy to introduce various exchange interactions combined with low dimensionality in heterometallic Cu(II)−V(IV) compounds.

Beyond π–π Stacking: Understanding Inversion Symmetry Breaking in Crystalline Racemates

“…19 Similar parallel heterochiral π−π packings were observed in racemic compounds with other methyl-substituted ligands and the aromatic ligand 1,10-phenanthroline, and an inversion center is always found between the racemates. 40,42,43 As a result, although the asymmetric unit of the discussed compounds contains equivalent BBUs, the BBUs are related by different symmetry elements, resulting in different extended structures. The racemic chains display a similar pattern but have different compositions between the NCS and CS structures.…”

Metamagnetism in a coordination polymer built of trimeric cobalt units and melamine

“…In general, field-induced metamagnetic transitions were reported in a wide range of materials from alloys to molecular magnets. Various origins of anomalous staircase magnetism includes zero-field splitting [7], ligand-field splitting [19], spin waves [20], spin gaps or spin dimers [21,22], magnetic superlattices [23,24], structural phase transitions [25], and exchange bias. The magnetic superlattices and structural phase transitions reported in magnetic alloys and arrays of magnetic particles, and exchange bias that occurs at boundaries between soft and hard magnets, are not of molecular origin expected for CPs.…”