Trapping of a Ruthenium−Butatrienylidene Intermediate by Tertiary Amines. 2-Ammoniobutenynyl Complexes

Abstract

2-Ammoniobutenynyl Ru-complexes trans-[Cl(dppm)2Ru−CC−C(NR2R‘)CH2]+PF6- (3a−g) have been prepared in a one-pot procedure from cis-[RuCl2(dppm)2], excess butadiyne, and various tertairy amines, whereas 4-(dimethylamino)pyridine binds to the unsaturated carbon ligand via the pyridine nitrogen to give 3h. These results may be rationalized by envoking the butatrienylidene complex trans-[Cl(dppm)2RuCCCCH2]+ as the reactive intermediate. Amine complexes cis-[Ru(dppm)2Cl(NR2R‘)]+ are frequently formed as side products and the 4-dimethylaminopyridine derivative cis-[Ru(dppm)2Cl(4-DMAP)]+ was obtained in high yield in the absence of butadiyne but under otherwise identical reaction conditions. Complexes 3 have been characterized by various spectroscopic and electrochemical techniques including cyclic and square wave voltammetry and, in the case of the NEt3 derivative 3a, also by X-ray crystallography. Most complexes 3 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 C3 bridge. EPR spectroscopic investigations of the oxidized forms of the NEt3 and PhCH2NMe2 derived complexes 3a and 3f point to Ru-centered radical dications. The irreversible reduction of complexes 3 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 3a and 3f by UV/vis and IR spectroelectrochemistry revealed a blue shift of the prominent absorption band and a bleaching of the CC stretch.