Head-on versus Side-on [5-5] Bitrovacenes Featuring Benzene and Naphthalene Units as Spacers: How p-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-[51trovacenylnaphthalene (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 pi-stacking, as encountered in 6, largely enhances exchange interaction between the two singly occupied V(3d(z)(2)) orbitals. The effect of pi-stacking on the electrochemical properties, as manifested in the redox splitting between consecutive electron transfer steps, is less pronounced. Redox splittings deltaE(1/2) for consecutive reductions exceed those for oxidations of binuclear trovacenes, deltaE(1/2)(2+/+, +/0) being apparent for pi-stacked 6 only, Because of the orthogonality of the metal-centered redox orbitals V1(d(z)(2)) and V2(d(z)(2)) and the eta(5)-cyclopentadienyl pi-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 pi-orbitals of the bridge by orthogonal singly occupied vanadium 3d(z)(2)orbitals.