Two Distinct Cu(II)-V(IV) Superexchange Interactions with Similar Bond Angles in a Triangular ?CuV2? Fragment

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(H2O)dmbpy](2)[V2O2F8] (dmbpy = 4,4 '-dimethyl-2,2 '-bipyridyl) that is composed of triangular CuV2 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 CuV2 fragment. Counterintuitively, their strong disparity is found to originate from the restricted symmetry of the half-filled Cu d(x2-y2) 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(H2O)(2)[V2O2F8] (enH(2) = ethylene diammonium) containing similar triangular CuV2 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.

Automated summary

This study investigates a novel molecular tetramer compound composed of triangular CuV2 fragments and reveals drastically different coupling strengths of superexchange Cu-V interactions in the presence of similar bond angles. By analyzing the magnetic orbitals and crystal field, the origin of the disparity is identified and compared with a similar chain compound. The findings highlight the possibility of observing fundamentally different magnetic couplings between magnetic ions in a single spin motif.