Synthesis and stereochemistry of half-sandwich alkynyl amino acidate complexes of rhodium(III), iridium(III), and ruthenium(II)

The synthesis and characterization of optically active amino acidate alkyny l complexes of general formula [(eta(n)-ring)M(aa)(C drop CR)] ((eta(n)-rin g)M = (eta(5)-C5Me5)Rh; aa = L-prolinate (Pro); R = CMe3 (1), SiMe3 (2); (e ta(n)-ring)M = (eta(5)-C5Me5)Ir; aa = Pro; R = CMe3 (3), SiMe3 (4); aa = N- methyl-L-prolinate (MePro); R = CMe3 (5), SiMe3 (6); (eta(n)-ring)M = (eta( 6)-p-(MeC6H4Pr)-Pr-i)-Ru; aa = L-alaninate (Ala); R = Ph (7), CO2Me (8); aa = Pro; R = Ph (9), CO2Me (10)) are reported. The crystal structures of(R-I r,S-C,S-N)-3b and (S-Ir,S-C,S-N)-5 were determined by X-ray analysis. Both molecular structures exhibit analogous pseudo-octahedral arrangements of li gands around the chiral iridium center. The metals are eta(5)-bonded to the pentamethylcyclopentadienyl group, linked in a chelate fashion through the aminic nitrogen and one of the carboxylate oxygens to the amino acidate, a nd bonded to a terminal almost linear tert-butyl ethynyl ligand. The config urational stability of the complexes was studied by circular dichroism and H-1 NMR spectroscopy. Complex 3b epimerizes at Ir in CDCl3 obeying a first- order rate law with Delta H-double dagger = 91.4 +/- 3.4 kJ mol(-1) and Del ta Sdouble dagger- = -20.8 +/- 10.8 J K-1 mol(-1). The equilibrium constant for 3b reversible arrow 3a is 2.70 +/- 0.41 in methanol at 20 degrees C. A mechanistic interpretation of the epimerization process is proposed.