Zusammenfassung
The optically active bidentate Schiff base ligands (+)-2-N-[(S)-1-phenylethyl]pyrrolcarbaldimine NN*H and (+)-N-[(S)-1-phenylethyl]salicylaldimine ON*H react with [Cp*MCl2](2) (M = Rh, Ir) to give the half-sandwich complexes Cp*Rh(NN*)Cl (1), Cp*Ir(NN*)Cl (2), Cp*Rh(ON*)Cl (3) and Cp*Ir(ON*)Cl (4). In all four cases two diastereomers (R-M,S-C) and (S-M,S-C) arise, differing only in the metal ...
Zusammenfassung
The optically active bidentate Schiff base ligands (+)-2-N-[(S)-1-phenylethyl]pyrrolcarbaldimine NN*H and (+)-N-[(S)-1-phenylethyl]salicylaldimine ON*H react with [Cp*MCl2](2) (M = Rh, Ir) to give the half-sandwich complexes Cp*Rh(NN*)Cl (1), Cp*Ir(NN*)Cl (2), Cp*Rh(ON*)Cl (3) and Cp*Ir(ON*)Cl (4). In all four cases two diastereomers (R-M,S-C) and (S-M,S-C) arise, differing only in the metal configuration. However, in each of the systems 1-4, only the (R-M,S-C) diastereomer crystallized. X-ray structure analyses established the absolute configuration in the crystals of all four complexes. In solution the compounds epimerize via a change of the metal configuration and approach equilibria (R-M,S-C) (S-M,S-C) in which for 1 and 2 the (S-M,S-C) diastereomers dominate. Time dependent integration of H-1 NMR signals of the complexes I and 2 revealed that the epimerization is a clean first-order reaction. The half-lives in CD2Cl2 solution at -50 degrees C were 19.4 min for 1 and 30.6 min for 2. From the temperature dependence Delta H-not equal was determined as 62.4 kJ mol(-1) for 1 and 44.3 kJ mol(-1) for 2 and Delta S-not equal as 148 J K-1 mol(-1) for 1 and 65 J K-1 mol(-1) for 2. For the compounds 3 and 4, the kinetic data for the epimerization with respect to the metal center were derived From H-1 NMR coalescence experiments: half lives 35 ms for 3 at 21 +/- 3 degrees (CH2Cl2 solution) and 58 ms for 4 at -1.8 +/- 3 degrees C (CHCl3 solution). These epimerization studies show that the metal configuration is labile in complexes 1 and 2 and extremely labile in complexes 3 and 4. (C) 2000 Elsevier Science Ltd. All rights reserved.
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