Trovacenyl-based organometallic triradicals: Spin frustration, competing interactions and redox splitting

Two new triradicals based on trovacene [(η⁷-tropylium) vanadium(η⁵-cyclopentadienyl)], 1,3,5-tri([5]trovacenyl)benzene 4 and 1,3,5-tri([5]trovacenyl)-6-methoxybenzene 5 were prepared and their magnetic properties were studied by continuous-wave X-band electron paramagnetic resonance (EPR) spectroscopy and by temperature-dependent magnetic susceptometry. The EPR spectra of 4 and 5 in liquid toluene solution demonstrate that the three unpaired electrons localized on the vanadium atoms interact with each other in both complexes. The data from magnetic susceptometry revealed that the electron spins in both triradicals are antiferromagnetically coupled despite the meta-phenylene bridge. The exchange coupling constants are equal in the C₃-symmetrical triradical 4 (J = J′ = -0.68 cm ^-1), which leads to a twofold degenerate spin ground state (spin frustration). The symmetry lowering by methoxy substitution of the benzene spacer in 5 results in the effect of competing interactions (J = -1.83 cm ^-1 and J′ = -2.38 cm^-1). In addition to magnetocommunication, the effect of ring substitution on electrocommunication is also discernable. It manifests itself in disparate redox splittings δE_1/2 (0/-, -/2-) and δE_1/2 (-/2-, 2-/3-) for 5, while these parameters are equal for the C₃-symmetrical trinuclear complex 4.