Ruthenium-aminoallenylidene complexes from butatrienylidene intermediates via an aza-cope rearrangement: Synthetic, spectroscopic, electrochemical, spectroelectrochemical, and computational studies

Ruthenium-aminoallenylidene complexes trans- [Cl(L-2)(2)RuCCC(NR2)CH2R']+EF 6- (4a- f; E = P, Sb, L-2 = chelating diphosphine) are accessible from the respective dichloro precursors, NaEF6, butadiyne, and an allylic amine in a one-pot procedure. The reactions proceed via the primary butatrienylidene intermediate trans- [Cl(L-2)(2)Ru=C=C=C=CH2](+) and the initial addition pr oducts trans- [Cl(L-2)(2)Ru-C drop CC(NR2R')=CH2](+) via an Aza-Cope type r earrangement. Amine adducts have been isolated for (dimethylamino)-2-pentyn e (3f) and 1-methyl-1,2,5,6-tetrahydropyridine (3g). The former cleanly con verts to its aminoallenylidene isomer upon warming. All products have been characterized by various spectroscopic techniques, including NMR, IR, and U V/vis spectroscopy and cyclic voltammetry; complex 4b was also characterize d by X-ray crystallography. Most notable are the considerable bond length a lternations along the unsaturated CQ ligand and the trigonal-planar nitroge n, indicative of its sp(2) character. Aminoallenylidene complexes of this t ype are best described as a hybrid between true cumulenic and iminium alkyn yl resonance forms, with major contributions of the latter, as is also evid ent from the high energy barriers for rotation around the iminium type C=N bond. The effect of the electron density on the metal on the spectroscopic and electrochemical properties of the cations in 4 has been probed for the dimethylallylamine-derived complexes trans- [Cl(L-2)(2)RuCCC(NMe2)C4H7]+EF6 - (4a-c), which only differ in the nature of the chelating diphosphine liga nd. Aminoallenylidene complexes 4 undergo reversible one-electron oxidation . In contrast, their reduction is irreversible at room temperature but part ially reversible at temperatures between 233 and 195 K. The spectroscopic c hanges accompanying oxidation were monitored by in situ UV/ vis, IR, and EP R techniques. DFT calculations have been performed on the model complexes t rans-[Cl(L-2)(2)Ru=C=C=C=CH2](+) and trans- [Cl(L-2)(2)RuC3{N(CH3)(2)}CH3]( +). Our results explain the regioselectivity of nucleophilic addition to th e proposed butatrienylidene intermediate and the spectroscopic and electroc hemical properties of aminoallenylidene complexes 4. Both orbital and steri c effects are equally important in the regioselective addition to Ca The ca lculations further indicate primarily metal-based oxidation and ligand-base d reduction of complexes 4, in accordance with experimental observations. T hey also let us assign the experimental UV/vis bands and the two main IR ab sorptions in the 2000-1500 cm(-1) region.