Optically Active Transition Metal Complexes. 109.¹ Novel (η⁶-Benzene)ruthenium Complexes with a Chiral Pyrrolecarbaldiminato Chelate Ligand:  Synthesis, Crystal Structures, Properties, and Stereochemistry at the Ruthenium Atom

The reaction of [(η 6 -C 6 H 6 )RuCl 2 ] 2 with the sodium salt of (+)-( S )- N -(1-phenylethyl)pyrrolecarbaldimine (HLL*) in CH 2 Cl 2 yielded a mixture of the two diastereomers ( S Ru, S C )- and ( R Ru, S C )-[(η 6 -C 6 H 6 )Ru(LL*)Cl] ( 1a, b ) in a ratio of 68:32. The chloride ligand in 1a, b was replaced in methanol by triphenylphosphane to give the two diastereomers ( S Ru, S C )- and ( R Ru, S C )-[(η 6 -C 6 H 6 )Ru(LL*)(PPh 3 )]PF 6 ( 2a,b ). According to variable-temperature 1 H NMR studies the formation of configurationally labile solvate intermediates has to be assumed in the reaction of the chloro complexes 1a, b with triphenylphosphane in the solvent methanol. In contrast to the diastereomers 1a, b, the ruthenium configuration in the phosphane complexes 2a, b is configurationally stable at room temperature. The diastereomers 2a, b were separated by crystallization. The crystal structures of ( S Ru, S C )- 1a, ( S Ru, S C )- 2a, and ( R Ru, S C )- 2b were determined by X-ray analysis. The epimerization of 2b at 85 °C in nitromethane- d 3 gave a 93.5:6.5 equilibrium mixture of 2a and 2b (τ 1/2 (min) = 58.2 ± 0.4). Conformational analyses showed that two main factors govern the orientation of the 1-phenylethyl group relative to the [(η 6 -C 6 H 6 )Ru(LL*)X] moiety (X = Cl, PPh 3 ): (i) the face-on orientation of the phenyl substituent with respect to the π-bonded benzene ligand and (ii) the orientation of the hydrogen substituent toward the unidentate ligand L.