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(O 3 PC 6 H 4 −X)· n H 2 O (X = p -NO 2, m -F, p -Cl, p -F, H, p -CH 3 ) have been investigated. In the isostructural n = 1 series (X = p -NO 2, m -F, p -F, H), the paramagnetic d 1 vanadyl centers are coupled via the O−P−O pathways of V(OPO) 2 V 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- NO 2 gives way to increasing antiferromagnetic coupling between the V IV centers as the electron-donating ability of X is increased. Consistent with a chair-like V(OPO) 2 V 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 -CH 3 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 μ 2 -bridging oxygens of a V(μ 2 -O) 2 V 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.