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 alkynyl complexes of general formula [(η n -ring)M(aa)(C⋮CR)] ((η n -ring)M = (η 5 -C 5 Me 5 )Rh; aa = l -prolinate (Pro); R = CMe 3 ( 1 ), SiMe 3 ( 2 ); (η n -ring) M = (η 5 -C 5 Me 5 )Ir; aa = Pro; R = CMe 3 ( 3 ), SiMe 3 ( 4 ); aa = N -methyl- l -prolinate (MePro); R = CMe 3 ( 5 ), SiMe 3 ( 6 ); (η n -ring)M = (η 6 - p -MeC 6 H 4 i Pr)Ru; aa = l -alaninate (Ala); R = Ph ( 7 ), CO 2 Me ( 8 ); aa = Pro; R = Ph ( 9 ), CO 2 Me ( 10 )) are reported. The crystal structures of ( R Ir, 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 ligands around the chiral iridium center. The metals are η 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, and bonded to a terminal almost linear tert -butyl ethynyl ligand. The configurational stability of the complexes was studied by circular dichroism and 1 H NMR spectroscopy. Complex 3b epimerizes at Ir in CDCl 3 obeying a first-order rate law with Δ H ⧧ = 91.4 ± 3.4 kJ mol - 1 and Δ S ⧧ - = −20.8 ± 10.8 J K - 1 mol - 1 . The equilibrium constant for 3b ⇌ 3a is 2.70 ± 0.41 in methanol at 20 °C. A mechanistic interpretation of the epimerization process is proposed.