Bridging Azines in the Coordination Sphere of Low-Valent Vanadocene Derivatives

Complexes of vanadocene(II) and -(III) derivatives with aromatic N-heterocyclic ligands are scarcely known. Here we present the first syntheses of several mono-, di-, and trinuclear complexes of this type and their detailed characterization. Reactions of vanadocene(II) derivatives are limited to N-heterocycles with distinct π-acceptor ability such as 4,4′-azobis(pyridine), which reacts as a bridging ligand to form [(Cp2V)2(μ2-Py2N2)] (13). In contrast Cp2VII reacts with, for example, 1,2-bis(4-pyridiyl)acetylene (C2Py2) solely to give the corresponding metallacyclopropene [Cp2V(η2-C2Py2)] (12) without affecting the pyridyl moieties. On the other hand, the reactivity of vanadocene(III) cations with this class of ligands is unrestricted and dominated by the acid−base properties of the reactants. The application of permethylated cyclopentadienyl ligands (Cp*) significantly enhances the solubility of those complexes, which leads to a better crystallizability. Due to the cationic properties of vanadocenium(III) derivatives, reactions with the smallest aromatic bridging N-heterocycle, pyrazine, afforded the rarely known mononuclear pyrazine complexes [Cp′2V(Pz)][BPh4] (Cp′ = Cp (17), Cp* (20); Pz = C4H4N2), whereas other N-heterocycles with expanded π-systems such as 4,4′-bipyridine (4-Bipy), C2Py2, and 1,3,5-tris(4-pyridyl)-2,4,6-triazine (4-TPT) grant access to multinuclear vanadocenium(III) derivatives ([(Cp*2V)2(μ2-4-Bipy)][BPh4]2 (21), [(Cp*2V)2(μ2-Py2C2)][BPh4]2 (24), [(Cp*2V)3(μ3-4-TPT)][BPh4]3 (26)). Compositions of all these complexes were confirmed by single-crystal X-ray diffraction.