Head-on versus Side-on [5−5]Bitrovacenes Featuring Benzene and Naphthalene Units as Spacers:  How π-Stacking Affects Exchange Coupling and Redox Splitting

Abstract

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 z 2 ) 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 δ E 1/2 for consecutive reductions exceed those for oxidations of binuclear trovacenes, δ E 1/2 (2+/+, +/0) being apparent for π-stacked 6 only. Because of the orthogonality of the metal-centered redox orbitals V1(d z 2 ) and V2(d z 2 ) and the η 5 -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 z 2 orbitals.