Aminoallenylidene complexes of ruthenium(II) from the regioselective addition of secondary amines to butatrienylidene intermediates: a combined experimental and theoretical study of the hindered rotation around the CN-bond

Aminoallenylidene complexes trans-[Cl(dppm)2RuC3(NRR)(CH3)]+ are obtained from the regioselective addition of secondary amines to trans-[Cl(dppm)2Ru=C=C=C=CH2]+. Unsymmetrically substituted amines give rise to Z/E isomeric mixtures. Dynamic 31P NMR spectroscopy gave an energy barrier of about 85 kJ mol^-1 for rotation around the CN-bond pointing to a large contribution of the iminium alkynyl resonance form trans-[Cl(dppm)2Ru-C (identical)C-C(=NRRʹ)(CH3)]+. This is also indicated by the pronounced bond length alternation within the RuC3N-chain as is revealed by X-ray structure analysis of the Z isomer of the (benzylmethyl)methylamine derivative 2d. The issue of NR2 rotation was also addressed by DFT calculations on the trans-[Cl(dhpm)2RuC3{N(CH3)2}(CH3)]+ model complex (dhpm = H2PCH2PH2). Upon rotation around the iminium type CN bond, the nitrogen lone pair and the (pi)-system of the allenylidene ligand are decoupled, resulting in a significantly longer CN bond and a tetrahedrally coordinated nitrogen atom.