Conformational Isomerism in (p-RC6H4NC)2W(dppe)2:  Substantial Structural Changes Resulting from Subtle Differences in the π-Acidity of p-RC6H4NC

• Published in: Journal of the American Chemical Society 2000-11, Vol.122 (44), p.10856-10867
• Main Authors: Wagner, Nicole L, Laib, Frank E, Bennett, Dennis W
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0018466

Bis(arylisocyanide) complexes of tungsten containing bulky bidentate aryl phosphine ligands adopt either cis or trans conformations, depending on the nature of the para-substituent group on the aryl isocyanide ligand. Ab initio calculations on a series of ligands, p-RC6H4NC, indicate that the energy of the LUMO is determined by the electron-withdrawing/donating capabilities of the substituent group, which determine the relative π-acidity of the ligand. Strong π acids, which contain electron-withdrawing groups, tend to polarize sufficient charge density away from the metal center to effect the formation of the sterically less favorable, but electronically stabilized, cis conformer. Weaker π acids, containing donor substituents, do not provide enough electronic stabilization to overcome steric repulsion, resulting in the adoption of trans geometry. (CNC6H4NC)W(dppe)2 exists as the cis conformer, as does (CNC6H2(CH3)2NC)W(dppe)2, while (CNC6(CH3)4NC)W(dppe)2, with increased steric bulk coupled with decreased π-acidity, exists as the trans conformer. Theoretical and experimental studies indicate that the energy match between isocyanide ligands and the metal center can be modified significantly by the selection of various substituents and that relatively subtle variations in the electronic structure of the ligands can have dramatic effects on the structures of the resulting complexes.