Synthesis and Stereochemistry of Half-Sandwich Alkynyl Amino Acidate Complexes of Rhodium(III), Iridium(III), and Ruthenium(II)
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Abstract
The synthesis and characterization of optically active amino acidate alkynyl complexes of general formula [(ηn-ring)M(aa)(C⋮CR)] ((ηn-ring)M = (η5-C5Me5)Rh; aa = l-prolinate (Pro); R = CMe3 (1), SiMe3 (2); (ηn-ring) M = (η5-C5Me5)Ir; aa = Pro; R = CMe3 (3), SiMe3 (4); aa = N-methyl-l-prolinate (MePro); R = CMe3 (5), SiMe3 (6); (ηn-ring)M = (η6-p-MeC6H4iPr)Ru; aa = l-alaninate (Ala); R = Ph (7), CO2Me (8); aa = Pro; R = Ph (9), CO2Me (10)) are reported. The crystal structures of (RIr,SC,SN)-3b and (SIr,SC,SN)-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 η5-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 1H NMR spectroscopy. Complex 3b epimerizes at Ir in CDCl3 obeying a first-order rate law with ΔH⧧ = 91.4 ± 3.4 kJ mol-1 and ΔS⧧-= −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.
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