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

摘要

Aminoallenylidene complexes trans-[Cl(dppm)_2RuC_3(NRR')(CH_3)]~+ are obtained from the regioselective addition of secondary amines to trans-[Cl(dppm)_2Ru=C=C=C=CH_2]~+. Unsymmetrically substituted amines give rise to Z/E isomeric mixtures. Dynamic ~(31)P 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 ident to C-C(=NRR')(CH_3)]~+. This is also indicated by the pronounced bond length alternation within the RuC)_3N-chain as is revealed by X-ray structure analysis of the Z isomer of the (benzylmethyl) methylamine derivative 2d. The issue of NR_2 rotation was also addressed by DFT calculations on the trans-[Cl(dhpm)_2RuC_3{N(CH_3)_2}(CH_3)]~+ model complex (dhpm = H_2PCH_2PH_2). 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.