Toward Prediction of Magnetic Properties in Layered Vanadyl Phosphonates:  Correlation of Magnetic Exchange with the Hammett σ Parameter

The magnetic properties of the layered vanadyl phosphonates (LVPs) VO(O3PC6H4−X)·nH2O (X = p-NO2, m-F, p-Cl, p-F, H, p-CH3) have been investigated. In the isostructural n = 1 series (X = p-NO2, m-F, p-F, H), the paramagnetic d1 vanadyl centers are coupled via the O−P−O pathways of V(OPO)2V chair-like subunits. The magnetic properties of these LVPs can be systematically controlled by modification of the P atom's electronic environment via variations of the X group in the aryl pendant. The simple paramagnetism of the LVP featuring the strong electron withdrawing substituent X = p-NO2 gives way to increasing antiferromagnetic coupling between the VIV centers as the electron-donating ability of X is increased. Consistent with a chair-like V(OPO)2V exchange pathway between pairs of vanadyl centers, the temperature-dependent magnetic susceptibility data fit a Bleaney−Bowers dimer model. When the substituent is large, as is the case for the p-Cl and p-CH3 species, a structurally different class of LVPs is obtained, in which n = 1.5. In this case, the contribution of the O−P−O linkage is overwhelmed by the presence of the more direct exchange pathway of vanadyl centers through μ2-bridging oxygens of a V(μ2-O)2V dimer, obviating the effects of electronic variations in the phosphonate bridge. Our results show that magnetic coupling correlates with a simple measure of electronic perturbation of the exchange pathway in LVPs, implying that such interactions can be tuned using the traditional tools of physical organic chemistry.