Head-on versus side-on [5-5]bitrovacenes featuring benzene and naphthalene units as spacers: How π-stacking affects exchange coupling and redox splitting

The biradicals 1,8-di([5]trovacenyl)naphthalene (6), 1,5-di([5]trovacenyl)naphthalene (7), and 1,3-di([5]trovacenyl)benzene (8) and the monoradical 1-[5]trovacenylnaphthalene (9) have been prepared and studied by means of single-crystal X-ray diffraction (6, 8, 9), cyclic voltammetry, EPR spectroscopy, and magnetic susceptometry. Comparison of the magnetic properties of 6-9 reveals that π-stacking, as encountered in 6, largely enhances exchange interaction between the two singly occupied V(3d_z2) orbitals. The effect of π-stacking on the electrochemical properties, as manifested in the redox splitting between consecutive electron transfer steps, is less pronounced. Redox splittings δE_1/2(2+/+, +/0) for consecutive reductions exceed those for oxidations of binuclear trovacenes, δE_1/2(2+/+, +/0) being apparent for π-stacked 6 only. Because of the orthogonality of the metal-centered redox orbitals V1(d_z2) and V2(d_z2) and the η⁵-cyclopentadienyl π-orbitals, electro- and magnetocommunication are indirect processes. Electrocommunication rests on changes of the donor/acceptor properties of V1,V2 caused by oxidation or reduction which govern metal-ligand charge distribution; the latter changes are transmitted via the spacer. Magnetocommunication takes the form of antiferromagnetic coupling, which can be traced to spin polarization of filled π-orbitals of the bridge by orthogonal singly occupied vanadium 3d_z2 orbitals.