Trapping of a ruthenium-butatrienylidene intermediate by tertiary amines. 2-ammoniobutenynyl complexes

2-Ammoniobutenynyl Ru-complexes trans-[Cl(dppm)(2)Ru-C=C-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)p yridine binds to the unsaturated carbon ligand via the pyridine nitrogen to give 3h. These results may be rationalized by envoking the butatrienyliden e complex trans-[Cl(dppm)(2)Ru=C=C=C=CH2](+) as the reactive intermediate. Amine complexes cis-[Ru(dppm)(2)Cl(NR2R')](+) are frequently formed as side products and the 4-dimethylaminopyridine derivative cis-[Ru(dppm)(2)Cl(4-D TMAP)](+) was obtained in high yield in the absence of butadiyne but under otherwise identical reaction conditions. Complexes 3 have been characterize d by various spectroscopic and electrochemical techniques including cyclic and square wave voltammetry and, in the case of the NEt3 derivative 3a, als o by X-ray crystallography. Most complexes 3 undergo a fully reversible one -electron oxidation at half-wave potentials that depend on the amine substi tuent. This provides evidence for delocalization over the conjugated Cs bri dge. EPR spectroscopic investigations of the oxidized forms of the NEt3 and PhCH2NMe2 derived complexes 3a and 3f point to Ru-centered radical dicatio ns. The irreversible reduction of complexes 3 occurs in two closely spaced, poorly defined waves and ultimately releases the respective free amine, wh ich 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 C=C stre tch.