Metallkomplexe von Heteroarenen, VIF: Bis^^S^-tetramethyl-^-pyrazin^anadium, der erste Bis(aren)metall- Komplex mit vier N-Atomen in der Koordinationssphäre

M a th ia s N ow otny , C h ris to p h E lschenb ro ich* , A n d reas B ehrendt, W erner M assa, S igrid W ocad lo Fachbereich Chemie der Philipps-Universität, Hans-Meerwein-Straße, D-35032 Marburg Herrn Prof. Dr. Dr. h.c. mult. E. O. Fischer zum 75. Geburtstag gewidmet Z. Naturforsch. 48b, 1581-1588 (1993); eingegangen am 1. Juli 1993 Pyrazine Coordination, Vanadium(O), X-Ray, EPR Spectra, Cyclic Voltammetry Cocondensation of 2,3,5,6-tetramethyl-pyrazine with vanadium atoms affords [2,3,5,6(CH3)4->/6-l,4-C4N2]2V (6) which has been studied by X-ray crystallography, EPR-spectroscopy 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 con­ formation adopted by the cyclic ligands in the crystal has a torsional angle of 44.2° which comes close to the angle o f 45° expected for maximum back-bonding, the filled vanadium orbitals 3d x* 2 and 3dxy interacting separately with unoccupied b 3u(7 r*) orbitals of the individ­ ual 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 3dz2—*ag(<7',+) overlap, the latter function representing the in-phase combina­ tion of the nitrogen “lone pair” orbitals. Contrary to bis(^6-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 ^-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 appro­ priate combination of 7r-donor and ^-acceptor properties essential for synergetic bond forma­ tion.