Synthesis, conformational isomerism, and fluxional behavior of octahedral bis(alkyne)tungsten(0) complexes

Article abstract of DOI:10.1021/om00028a026

trans-Bis(alkyne)tungsten complexes W(CO)₂(NN)(RC≡CR′)₂ (1-4) were synthesized via the exchange reaction of W(CH₃CN)₃(CO)₃ with 1 equiv of chelate nitrogen ligand NN and 2 equiv of alkyne, where NN = bpy (a), phen (b), 4-Mephen (b′), en (c), 2-(NH₂CH₂)py (d) and RC≡CR′ = MeO₂CCCCO₂Me (1), PhCCCO₂Et (2), HCCCO₂Me (3), HCCCO₂Et (4), at ambient temperature. The solid-state structure of a representative of the series, 2a (NN = bpy, RC≡CR′ = PhCCCO₂Et) was determined by the X-ray diffraction method. The results indicate that the geometry of the compound is octahedral, with the two CO groups cis to each other but each trans to a pyridyl group, while the two PhCCCO₂Et ligands are trans to each other and are cis to the CO and the bpy ligands. The two alkynes are mutually orthogonal, and each eclipses a N-M-C vector. In addition, the X-ray study shows that the phenyl end of the alkyne ligand is close to the CO group and the ester group is near the bpy ligand. This compound crystallizes in the monoclinic space group Cc with unit cell dimensions α = 15.523(5) A?, b = 9.676(2) A?, c = 21.471(6) A?, and β = 108.17(2)° for Z = 4. According to variable-temperature NMR spectra, complexes 2-4 exist in solution as a mixture of conformational isomers arising from different orientations of the unsymmetric alkyne ligands relative to the N-W-CO vectors. For 2a and 2b, the three conformational isomers A-C were detected at low temperatures, although only conformation A was observed in the solid state. For complexes 3a,b and 4a,b, only conformations A and B were found, but for the en complex 2c, only A was observed in the low-temperature NMR spectra of these complexes. Several mechanisms have been considered for the observed dynamic NMR behavior of these complexes. A dissociative process involving the exchange of free and coordinated alkynes has been eliminated. Nondissociative mechanisms involving a concerted disrotatory motion and an independent motion of the alkyne ligands also cannot account for all observations. Only the mechanism of conrotatory motion of the two alkyne ligands is in agreement with all experimental facts.