Trapping of a Ruthenium−Butatrienylidene Intermediate by Tertiary Amines. 2-Ammoniobutenynyl Complexes<sup>†</sup>

1999-09-01T00:00:00Z (GMT) by Rainer F. Winter F. M. Hornung
2-Ammoniobutenynyl Ru-complexes <i>trans-</i>[Cl(dppm)<sub>2</sub>Ru−C<b>⋮</b>C−C(NR<sub>2</sub>R‘)CH<sub>2</sub>]<sup>+</sup>PF<sub>6</sub><sup>-</sup> (<b>3a</b>−<b>g</b>) have been prepared in a one-pot procedure from <i>cis-</i>[RuCl<sub>2</sub>(dppm)<sub>2</sub>], excess butadiyne, and various tertairy amines, whereas 4-(dimethylamino)pyridine binds to the unsaturated carbon ligand via the pyridine nitrogen to give <b>3h</b>. These results may be rationalized by envoking the butatrienylidene complex <i>trans-</i>[Cl(dppm)<sub>2</sub>RuCCCCH<sub>2</sub>]<sup>+</sup> as the reactive intermediate. Amine complexes <i>cis</i>-[Ru(dppm)<sub>2</sub>Cl(NR<sub>2</sub>R‘)]<sup>+</sup> are frequently formed as side products and the 4-dimethylaminopyridine derivative <i>cis-</i>[Ru(dppm)<sub>2</sub>Cl(4-DMAP)]<sup>+</sup> was obtained in high yield in the absence of butadiyne but under otherwise identical reaction conditions. Complexes <b>3</b> have been characterized by various spectroscopic and electrochemical techniques including cyclic and square wave voltammetry and, in the case of the NEt<sub>3</sub> derivative <b>3a</b>, also by X-ray crystallography. Most complexes <b>3</b> undergo a fully reversible one-electron oxidation at half-wave potentials that depend on the amine substituent. This provides evidence for delocalization over the conjugated C<sub>3</sub> bridge. EPR spectroscopic investigations of the oxidized forms of the NEt<sub>3</sub> and PhCH<sub>2</sub>NMe<sub>2</sub> derived complexes <b>3a</b> and <b>3f</b> point to Ru-centered radical dications. The irreversible reduction of complexes <b>3</b> occurs in two closely spaced, poorly defined waves and ultimately releases the respective free amine, which itself is reactive toward the Ru(III) oxidation product. Following the oxidation of <b>3a </b>and <b>3f</b> by UV/vis and IR spectroelectrochemistry revealed a blue shift of the prominent absorption band and a bleaching of the C<b>⋮</b>C stretch.