Ruthenium-Aminoallenylidene Complexes from Butatrienylidene Intermediates via an Aza-Cope Rearrangement: Synthetic, Spectroscopic, Electrochemical, Spectroelectrochemical, and Computational Studies

Ruthenium-aminoallenylidene complexes trans-[Cl(L2)2RuCCC(NR2)CH2R′]EF6 (4af; E ) P, Sb, L2 ) 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(L2)2RudCdCdCdCH2] and the initial addition products trans-[Cl(L2)2Ru-CtCC(NR2R′)dCH2] via an Aza-Cope type rearrangement. Amine adducts have been isolated for (dimethylamino)-2-pentyne (3f) and 1-methyl-1,2,5,6-tetrahydropyridine (3g). The former cleanly converts to its aminoallenylidene isomer upon warming. All products have been characterized by various spectroscopic techniques, including NMR, IR, and UV/vis spectroscopy and cyclic voltammetry; complex 4b was also characterized by X-ray crystallography. Most notable are the considerable bond length alternations along the unsaturated C3 ligand and the trigonalplanar nitrogen, indicative of its sp2 character. Aminoallenylidene complexes of this type are best described as a hybrid between true cumulenic and iminium alkynyl resonance forms, with major contributions of the latter, as is also evident from the high energy barriers for rotation around the iminium type CdN 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(L2)2RuCCC(NMe2)C4H7]EF6 (4a-c), which only differ in the nature of the chelating diphosphine ligand. Aminoallenylidene complexes 4 undergo reversible one-electron oxidation. In contrast, their reduction is irreversible at room temperature but partially reversible at temperatures between 233 and 195 K. The spectroscopic changes accompanying oxidation were monitored by in situ UV/ vis, IR, and EPR techniques. DFT calculations have been performed on the model complexes trans-[Cl(L2)2RudCdCdCdCH2] and trans-[Cl(L2)2RuC3{N(CH3)2}CH3]. Our results explain the regioselectivity of nucleophilic addition to the proposed butatrienylidene intermediate and the spectroscopic and electrochemical properties of aminoallenylidene complexes 4. Both orbital and steric effects are equally important in the regioselective addition to C3. The calculations further indicate primarily metal-based oxidation and ligand-based reduction of complexes 4, in accordance with experimental observations. They also let us assign the experimental UV/vis bands and the two main IR absorptions in the 2000-1500 cm-1 region.