Synthesis, properties, stereochemistry and crystal structures of diastereomeric benzene–ruthenium(II) complexes with a chiral salicylideneaminato ligand

Abstract

The reaction of [{Ru(η⁶-C₆H₆)Cl₂}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(η⁶-C₆H₆)(L–L)Cl]1a and 1b, in a ratio of 86 : 14. Mediated by AgPF₆ in acetone at –30 to –35 °C, the chloride ligand in 1a/1b was substituted by 4-methylpyridine (4Me-py), 2-methylpyridine (2Me-py) or triphenylphosphane (PPh₃) to give the two diastereomers 2a/2b of [Ru(η⁶-C₆H₆)(L–L)(4Me-py)]PF₆, the pure diastereomer 3 of [Ru(η⁶-C₆H₆)-(L–L)(2Me-py)]PF₆ and the two diastereomers 4a/4b of [Ru(η⁶-C₆H₆)(L–L)(PPh₃)]PF₆. At room temperature in [²H₆]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 ¹H NMR spectroscopy of complexes 2a/2b and 4a/4b from –80 °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 1-phenylethyl group relative to the [Ru(η⁶-C₆H₆)(L–L)L′] moiety (L′= Cl, 4Me-py, 2Me-py or PPh₃): (i) the faće-on orientation of the phenyl substituent with respect to the π-bonded aromatic benzene ligand and (ii) the steric demand of the unidentate ligands with respect to the 1-phenylethyl group.