Design and modification of rhodium and iridium N-heterocyclic carbene complexes for asymmetric transfer hydrogenation and antimicrobial activity
The two projects described in this dissertation demonstrate the wide utility of noble metal N-heterocyclic carbene (NHC) complexes. The first project details the design of iridium NHC amino acid complexes for asymmetric transfer hydrogenation (ATH) of prochiral ketones. Iridium(I) bis-NHC complexes were found to undergo oxidative addition with a variety of alpha-amino acids, generating chiral iridium(III) complexes of the form Ir(NHC)2(aa)(H)(X) (aa = amino acid, X = halide). The complexes were screened for ATH of aryl and alkyl ketones, and optimization studies found enantioselectivity in this system was highly sensitive to the reaction temperature, NHC ligand, and amino acid. Incorporation of secondary amino acids was essential to enantioselectivity. Aryl ketones were reduced in high conversion and enantioselectivity when employing the Ir(IMe)2(L-Pro)(H)(I) catalyst in isopropyl alcohol, in some cases giving over 90% ee of the alcohol products. Density functional theory calculations were conducted in order to gain insight into the active catalytic species, and the results suggest that the high enantioselectivity of this system primarily arises from steric effects.
The second project details the design of rhodium and iridium NHC piano-stool complexes featuring derivatized tetramethylcyclopentadienyl ligands (CpR, R = alkyl or aryl substituent) for antimicrobial applications. Complexes of the form (CpR)M(NHC)Cl2 (M = Rh or Ir) were synthesized by transmetallation of the NHC ligand using silver(I) oxide in the presence of the desired noble metal Cp*R dimer. The complexes were screened for biological activity against various bacteria, yeast, and fungi. Many of these compounds were highly active against Mycobacterium smegmatis, displaying minimum inhibitory concentrations (MICs) as low at 0.25 microgram per mL. Analysis of structure-activity relationships found that incorporation of the NHC ligand greatly enhances the antimicrobial properties of rhodium and iridium piano-stool complexes, more so than previously investigated diamine, amino acid, or beta-diketonato ligands. Cytotoxicity studies on one of the rhodium NHC complexes showed this compound was nontoxic towards mammalian cells at low concentrations, which strengthens the potential of these types of compounds as viable drug candidates.