Optically active transition metal complexes .107. Synthesis, properties, stereochemistry and crystal structures of diastereomeric benzene-ruthenium(II) complexes with a chiral salicylideneaminato ligand

The reaction of [{Ru(eta(6)-C6H6)Cl-2}(2)] with the sodium salt of (S)-N-(1-phenylethyl)salicylideneamine (HL-L) in CH2Cl2 led to a diastereomer mixture of (R(Ru),S-C)- and (S-Ru,S-C)-[Ru(eta(6)-C6H6)(L-L)Cl] 1a and 1b, in a ratio of 86:14. Mediated by AgPF6 in acetone at -30 to -35 degrees C, the chloride ligand in 1a/1b was substituted by 4-methylpyridine (4Me-py), 2-methylpyridine (2Me-py) or triphenylphosphane (PPh(3)) to give the two diastereomers 2a/2b of [Ru(eta(6)-C6H6)(L-L)(4Me-py)]PF6, the pure diastereomer 3 of [Ru(eta(6)-C6H6)-(L-L)(2Me-py)]PF6 and the two diastereomers 4a/4b of [Ru(eta(6)-C6H6)(L-L)(PPh(3))]PF6. At room temperature in [H-2(6)]acetone, under equilibrium conditions, the diastereomer ratio 2a:2b was 67:33, 3 was diastereomerically pure and the ratio 4a:4b was 93.4:6.6. Variable-temperature H-1 NMR spectroscopy of complexes 2a/2b and 4a/4b from -80 degrees C to room temperature demonstrated configurational lability of the ruthenium configuration. Since equilibration occurred during reaction and work-up, the ruthenium configuration was not retained in the substitution reactions. Diastereomer 2a was obtained diastereomerically pure by crystallisation. The diastereomers 4a and 4b were separated and examined by variable-temperature NMR spectroscopy. The crystal structures of the (R(Ru),S-C) diastereomer of complex 1 and of the thermodynamically more stable (R(Ru),S-C) diastereomers 2a and 4a'' were determined by X-ray analysis. A conformational analysis based on the NMR spectroscopic results showed that two main factors govern the orientation of the I-phenylethyl group relative to the [Ru(eta(6)-C6H6)(L-L)L'] moiety (L' = Cl, 4Me-py, 2Me-py or PPh(3)): (i) the face-on orientation of the phenyl substituent with respect to the pi-bonded aromatic benzene ligand and (ii) the steric demand of the unidentate ligands with respect to the 1-phenylethyl group.