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

Iridium complexes [(CO)(2)Ir(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 (CO)-C-13, we are able to estimate the activation energy (Delta G double dagger) 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)(2)Ir(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

Biscarbene palladium(II) complexes. Reactivity of saturated versus unsaturated N-heterocyclic carbenes

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by 1H and 13C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd−C(saturated NHC) is distinct from that of Pd−C(unsaturated NHC). The Pd−C(saturated NHC) bonds are fairly stable toward reagents such as CF3COOH, AgBF4 and I2, whereas Pd−C(unsaturated NHC) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki−Miyaura coupling follows the order: (sat-NHC)2PdCl2 > (sat-NHC)(unsat-NHC)PdCl2 > (unsat-NHC)2PdCl2