Investigation of the electronic and structural properties of bidentate phosphine-tungsten complexes containing aromatic isocyanide ligands

Zero-valent, electron-rich tungsten phosphine complexes containing aromatic isocyanide ligands of the type (p-CN-C6H 4NC)2W(dppe)2 (dppe = 1,2-bis(diphenylphosphino)ethane) have the potential to be used as building blocks for molecular-level wires of the form -n[-CNC6H4NC-W(dppe)2-CNC 6H4NC-W(dppe)2-CNC6H4NC-]- n. The versatile p-CN-C6H4NC ligand would not only bridge the metal fragments together, but would also allow those bridged metal fragments to "electronically" communicate with one another by virtue of the pi system of its aromatic ring.;In initial studies toward this end, a cis-( p-CN-C6H4NC)2W(dppe)2 building block was synthesized [1]. In order to gain insight into the electronic and structural properties of cis-(p-CN-C 6H4NC)2W(dppe)2, a systematic study was undertaken in which the para group on the aromatic isocyanide ligand was replaced with several different substituents and the properties of the resultant complexes (p-R-C6H4NC) 2W(dppe)2 examined.;When R was an electron-withdrawing group (R = O2N, CN, F 3C, FO2S), the (p-R-C6H 4NC)2W(dppe)2 complexes adopted a cis geometry. When R was less electron-withdrawing (R = F, H3C, H3CO), the (p-R-C6H4NC) 2W(dppe)2 complexes adopted a trans geometry. It was determined that the pi-acidity of the p-R-C 6H4NC ligands was responsible for the geometry that the ( p-R-C6H4NC)2W(dppe)2 complexes adopted. The ability to produce the trans isomer of the complexes was significant because in molecular wires utilizing these complexes, the tungsten-isocyanide-tungsten pi-system overlap would be maximized in the trans conformation enhancing the conduction capabilities of the molecular-level wire.;Additionally, the reactions between trans-(N2) 2W(dppe)2 and p-R-C6H4NC were studied. If R = CN, and the reaction was allowed to proceed 7--10 days in toluene, a trans dimer, trimer and tetramer were formed: [CNC6H4NC-W(dppe)2-]n-CNC 6H4NC (n = 2, 3, 4). This discovery was important because it demonstrated that p-CN-C6H4NC would bridge the electron-rich W(dppe)2 fragments and that it did so in the preferred trans geometry, crucial milestones on the path toward the synthesis of molecular-level wires.;It was also determined that the mechanism of the reactions between trans-(N2)2W(dppe)2 and the p-R-C6H4NC ligands involved a dppe ring-opening on trans-(N2)2W(dppe)2 (rather than the dissociation of the N2 ligands from trans-(N 2)2W(dppe)2) to generate open coordination sites for the p-R-C6H4NC ligands.