Tungsten(II)-catalyzed rearrangements of norbornadiene: Effects of alternative complexation stages

The spontaneous transformation of norbornadiene (nbd, bicyclo[2.2.1]hepta-2,5-diene) to an alkylidene, within a selection of Group 6 organometallic complexes, was studied using density-functional theory computations. This as-yet-unknown mechanism is postulated to be required in the spontaneous ring-opening metathesis polymerization of nbd via Group 6 complexes. In a previous study, an extensive computational search for intermediates and transition states was made for the rearrangement within the originally 7-coordinate WI2(CO)3(η2,η2-nbd) complex, which we call the Category I searches. In the current study, we perform similar searches but for two other stages of the complex, which we designate Category II and Category III. The Category II searches assume one carbonyl has been dissociated, thus beginning with a 16-electron (16e) complex. The Category III searches assume that the dissociated carbonyl has been replaced by an η2-nbd, which allows possible coupling of two nbd molecules to form the initial alkylidene needed for polymerization. Category II resulted in activation barriers equally as high as for Category I, if not higher, due to the increased reluctance of the electrophilic metal centre to allow ligands to loosen their metal–ligand bonds for rearrangement. Category III, however, resulted in activation energies half as large as the single-nbd rearrangements of Categories I and II.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (105 K)Download as PowerPoint slideHighlights► This is a DFT study of the transformation of norbornadiene to an alkylidene. ► Mechanisms from different complexation stages were studied. ► Over 60 transition states found. ► WI2(CO)3(η2,η2-nbd) was the starting model complex. ► Oxidative coupling of two nbd units lowers barrier by half.