Half-sandwich (η⁶-arene)- ruthenium(II) chiral Schiff base complexes: analysis of the diastereomeric mixtures in solution by 2D-NMR spectroscopy

Abstract

2D NMR spectroscopy has been used to determine the metal configuration in solution of three complexes, viz. [(η⁶-p-cymene)Ru(L^∗)Cl] (1) and [(η⁶-p-cymene)Ru(L^∗)(L')] (ClO₄) (L'=H₂O, 2; PPh₃, 3), where L^∗ is the anion of (S)-(1-phenylethyl)salicylaldimine. The complexes exist in two diastereomeric forms in solution. Both the (R_Ru,S_C)- and (S_Ru,S_C)-diastereomers display the presence of attractive CH/π interaction involving the phenyl group attached to the chiral carbon and the cymene ring hydrogens. This interaction restricts the rotation of the C^∗-N single bond and, as a result, two structural types with either the hydrogen atom attached to the chiral carbon (C^∗) or the methyl group attached to C^∗ in close proximity of the cymene ring protons get stabilized. Using 2D NMR spectroscopy as a tool, the spatial interaction involving these protons are studied in order to obtain the metal configuration(s) of the diastereomeric complexes in solution. This technique has enabled us to determine the metal configuration as (R_Ru,S_C) for the major isomers of 1-3 in solution.