Vanadium 7,7,8,8-Tetracyano-p-quinodimethane (V[TCNQ]₂)-Based Magnets

A new family of molecule-based magnets of general formula V[TCNQR₂]₂·zCH₂Cl₂ has been synthesized and characterized (TCNQ = 7,7,8,8-tetracyano-p-quinodimethane; R = H, Br, Me, Et, i-Pr, OMe, OEt, and OPh). In addition, solid solutions of V[TCNQ]_x[TCNQ(OEt)₂]_2-x·zCH₂Cl₂ composition have been prepared. Except R = Br, magnetic ordering was observed for all materials, with T_c values between 7.5 K (R = Me) and 106 K (R = OEt), with R = H at 52 K. The substitution of electron-donating OMe and OEt groups for H in TCNQ increased T_c, whereas the substitution of less electron-donating alkyl groups (with respect to alkoxy groups) decreased T_c. The results of MO calculations indicate that neither the spin nor charge densities of the disubstituted TCNQs are sufficiently different to explain the wide range of critical temperatures. Although the structures of the amorphous materials are not known, it is proposed that the oxygen atom of the [TCNQR₂]^•- acceptor (R = OMe and OEt) and the V(II) interact to form a seven-membered ring. This interaction could stabilize the structure and enhance the magnetic coupling, leading to an increased T_c. The magnetic properties of V[TCNQ]_x[TCNQ(OEt)₂]_2-x·zCH₂Cl₂ deviated from the expected linear relationship with respect to x, exhibiting magnetic behavior more characteristic of a step function in a plot of T_c versus x.