METAL-COMPLEXES OF HETEROARENES .7. BIS(2 ,3,5,6-TETRAMETHYL-ETA(6)-PYRAZINE)VANADIUM, THE 1ST BIS(ARENE)METAL COMPLEX WITH 4 N-ATOMS IN THE COORDINATION SPHERE

Cocondensation of 2,3,5,6-tetramethyl-pyrazine with vanadium atoms aff ords [2,3,5,6-(CH3)4-eta6-1,4-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) th e ligand in 6 is folded along the N-N axis (dihedral angle 13.6-degree s). The conformation adopted by the cyclic ligands in the crystal has a torsional angle of 44.2-degrees which comes close to the angle of 45 -degrees expected for maximum back-bonding, the filled vanadium orbita ls 3d(x2-y2) and 3d(xy) interacting separately with unoccupied b3u(pi ) orbitals of the individual pyrazine rings. The hyperfine coupling co nstant a(V-51) = 4.45 mT attests to comparatively small spin density a t the metal nucleus. It is proposed, that substantial transfer of spin density is effected by V 3d(z2)-->a(g)(sigma(n+)) overlap, the latter function representing the in-phase combination of the nitrogen ''lone pair'' orbitals. Contrary to bis(eta6-phosphinine)vanadium 8, for the pyrazine complex 6 only the anion 6- is formed reversibly in cyclovol tammetry, oxidation to 6+ being irreversible. However, in common with 8, the redox potentials for 6 show pronounced anodic shifts. Attempts towards eta6-coordination of 2,4,6-trimethyl-1,3,5-triazine failed. Ap parently, the 1:1 ratio of CH3/N is insufficient to furnish the triazi ne ring with the appropriate combination of pi-donor and delta-accepto r properties essential for synergetic bond formation.