[W(CO)5(pyrazine)]:: evidence for a stereochemically-rigid complex up to 373 K

[W(CO)(5)(pz)], pz = pyrazine, has been prepared by 365 nm photolysis of W(CO)(6) in C6H6 Or Me2CO at 22 degrees C. The isolated product exhibits the typical signature i.r. and electronic spectral features of [W(CO)(5)L] (L = N-heterocycle) complexes, coordinated at the N-heterocyclic position, in agreement with prior studies of other workers. The room temperature H-1-n.m.r. spectrum in [H-2(6)]acetone shows two singlets at 9.09 and 8.75 p.p.m. (J(2,3) = J(5,6) = 5 MHz), downfield of the free ligand resonance (8.61 p.p.m.), as anticipated for a stereochemically rigid pyrazine. Temperature dependence studies were carried out in diglyme solvent from 298 to 393 K in search of evidence for a 1,4-metallotropic shift of the coordinated ligand. The singlets of the separate adjacent (H-2 and H-6) protons and removed (H-3, H-5) protons remained sharp even up to 373 K where ligand dissociation destroyed the complex. Free pyrazine is not in rapid exchange with the coordinated pyrazine over the entire temperature range. These results stand in contrast to the 1,2-shift observed in a previous study for [W(CO)(5)(pyd)], pyd = pyridazine. Pyridazine has greater sigma-donation and pi-acceptor power compared with pyrazine, which should significantly raise the kinetic barrier to bond-breaking of the W-pyd bond versus W-pz bond. This apparently leads to facile dissociation rather than a 1,4-shift with [W(CO)(5)(pz)].