Coordination chemistry and catalytic activity of N-heterocyclic carbene iridium(i) complexes

Abstract

Iridium complexes [(CO)₂Ir(NHC-R)Cl] (R = Et–, 3a; PhCH₂–, 3b; CH₃OCH₂CH₂–, 3c; o-CH₃OC₆H₄CH₂–, 3d; NHC: N-heterocyclic carbene) are prepared via the carbene transfer from [(NHC-R)W(CO)₅] to [Ir(COD)Cl]₂. By using substitution with ¹³CO, we are able to estimate the activation energy (ΔG^‡) of the CO-exchange in 3a–d, which are in the range of 12–13 kcal mol⁻¹, significantly higher than those for the phosphine analog [(CO)₂Ir(PCy₃)Cl]. Reactions of 3b and 3d with an equimolar amount of PPh₃ result in the formation of the corresponding [(NHC-R)Ir(CO)(PPh₃)Cl] with the phosphine and NHC in trans arrangement. In contrast, the analogous reaction of 3a or 3c with phosphine undergoes substitution followed by the anion metathesis to yield the corresponding di-substituted [(NHC-R)Ir(CO)(PPh₃)₂]BF₄ (5) directly. Treatment of 3b or 3d with excess of PPh₃ leads to the similar product of disubstitution 5b and 5d. The analysis for the IR data of carbonyliridium complexes provides the estimation of electron-donating power of NHCs versus phosphines. The NHC moiety on the iridium center cannot be replaced by phosphines, even 1,2-bis(diphenylphohino)ethane (dppe). All the carbene moieties on the iridium complexes are inert toward sulfur treatment, indicating a strong interaction between NHC and the iridium centers. Complexes 3a–c are active on the catalysis of the oxidative cyclization of 2-(o-aminophenyl)ethanol to yield the indole compound. The phosphine substituted complexes or analogs are less active.