Metallkomplexe von Heteroarenen, VII: Bis(2,3,5,6-tetramethyl-η6-pyrazin)vanadium, der erste Bis(aren)metall-Komplex mit vier N-Atomen in der Koordinationssphäre / Metal Complexes of Heteroarenes, VII: Bis(2,3,5,6-tetramethyl-η6-pyrazine)vanadium, the First Bis(arene)metal Complex with Four N Atoms in the Coordination Sphere

Abstract

Cocondensation of 2,3,5,6-tetramethyl-pyrazine with vanadium atoms affords [2,3,5,6-(CH3)4-η6-1,4-C4N2]2V (6) which has been studied by X-ray crystallography, EPR-spectro-scopy and cyclic voltammetry. As shown by unequal metal ring-atom distances (V—C = 219.6, V—N = 210.9 pm) the ligand in 6 is folded along the N—N axis (dihedral angle 13.6°). The conformation adopted by the cyclic ligands in the crystal has a torsional angle of 44.2° which comes close to the angle of 45° expected for maximum back-bonding, the filled vanadium orbitals 3 dx2-γ2 and 3 dxy interacting separately with unoccupied b3u(π*) orbitals of the individual pyrazine rings. The hyperfine coupling constant a(51V) = 4.45 mT attests to comparatively small spin density at the metal nucleus. It is proposed, that substantial transfer of spin density is effected by V 3dz 2→ag(σn+) overlap, the latter function representing the in-phase combination of the nitrogen “lone pair” orbitals. Contrary to bis(η6-phosphinine)vanadium 8, for the pyrazine complex 6 only the anion 6⁻ is formed reversibly in cyclovoltammetry, oxidation to 6+ being irreversible. However, in common with 8, the redox potentials for 6 show pronounced anodic shifts. Attempts towards η6-coordination of 2,4,6-trimethyl-1,3,5-triazine failed. Apparently, the 1:1 ratio of CH3/N is insufficient to furnish the triazine ring with the appropriate combination of π-donor and δ-acceptor properties essential for synergetic bond formation.