1D magnetic interactions in Cuⁱⁱ oxovanadium phosphates (VPO), magnetic susceptibility, DFT, and single-crystal EPR

We report the crystal face indexing and molecular spatial orientation, magnetic properties, electron paramagnetic resonance (EPR) spectra, and density functional theory (DFT) calculations of two previously reported oxovanadium phosphates functionalized with Cu^II complexes, namely, [Cu(bipy)(VO₂)(PO₄)]_n (1) and [{Cu(phen)}₂(VO₂(H₂O)₂)(H₂PO4)₂ (PO₄)]_n (2), where bipy = 2,2′-bipyridine and phen = 1,10-phenanthroline, obtained by a new synthetic route allowing the growth of single crystals appropriate for the EPR measurements. Compounds 1 and 2 crystallize in the triclinic group P1¯and in the orthorhombic Pccn group, respectively, containing dinuclear copper units connected by two -O-P-O- bridges in 1 and by a single -O-P-O- bridge in 2, further connected through -O-P-O-V-O- bridges. We emphasize in our work the structural aspects related to the chemical paths that determine the magnetic properties. Magnetic susceptibility data indicate bulk antiferromagnetism for both compounds, allowing to calculate J = -43.0 cm^-1 (d_Cu-Cu = 5.07 Å; J defined as H_ex(i,j) = -J S_i·S_j), considering dinuclear units for 1, and J = -1.44 cm^-1 (d_Cu-Cu = 3.47 Å) using the molecular field approximation for 2. The single-crystal EPR study allows evaluation of the g matrices, which provide a better understanding of the electronic structure. The absence of structure of the EPR spectra arising from the dinuclear character of the compounds allows estimation of weak additional exchange couplings |J′| > 0.3 cm^-1 for 1 (d_Cu-Cu = 5.54 Å) and a smaller value of |J′| 0.15 cm^-1 for 2 (d_Cu-Cu = 6.59 Å). DFT calculations allow evaluating two different exchange couplings for each compound, specifically, J = -36.60 cm^-1 (d_Cu-Cu = 5.07 Å) and J′ = 0.20 cm^-1 (d_Cu-Cu =5.54 Å) for 1 and J = -1.10 cm^-1 (d_Cu-Cu =3.47 Å) and J′ = 0.01 cm^-1 (d_Cu-Cu = 6.59 Å) for 2, this last value being in the range of the uncertainties of the calculations. Thus, these values are in good agreement with those provided by magnetic and single-crystal EPR measurements.