Iridium and Palladium Complexes with N-Heterocyclic Carbene Ligands and Their Catalytic Applications

• Main Authors: Ching-Feng Fu, 傅勁逢
• Other Authors: Shiuh-Tzung Liu
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/65698823858939744954

博士 === 臺灣大學 === 化學研究所 === 98 === Recently, the use of N-heterocyclic carbenes (denoted as NHC) as ligands for transition metal ions in catalysis has been increased significantly due to their strong sigma-donor nature. In this work, we have synthesized a series of iridium and palladium complexes with the saturated NHCs bearing different side chains by transmetalation from (NHC)W(CO)n to the metal precursors. It was demonstrated that NHCs were better sigma-donors than phosphines via comparison of IR stretching of the carbonyl groups in (NHC)Ir(CO)2Cl versus the phosphine analogs. This trend was also consistent with the rate determination of the CO exchanging in (NHC)Ir(CO)2Cl and (Cy3P)Ir(CO)2Cl. However, the 13CO exchanging was slower in the case of N-pyridinyl substituted iridium complex due to the block of pyridinyl group at the axial position. Chelation of bis-pyridinyl NHC ligand toward the Pd center resulted in the formation of a pincer complex, which was confirmed by single crystal X-ray diffraction. Treatment of trans-(NHC)2PdCl2 with silver salt in CD3CN scavenges chlorides could generate stable cationic species. Due to the trans-influence, the cationic species transformed to the thermodynamically stable cis-biscarbene complex. Upon coordination of chloride, cis-(NHC)2PdCl2 could be isolated. For the same reason, cis-(NHC)(PPh3)PdCl2 would be formed from the reaction between bridge-Pd carbene complexes and triphenylphosphine. Reactions between bridge-Pd carbene complexes and other carbene precursors could form unsymmetry bis-NHC Pd complexes, which contained saturated and unsrturated-NHCs on the same Pd center. Treatment of those unsymmetry carbene complexes with I2 , CF3COOH, or AgBF4 to investigate the stability of carbene-Pd bonds, it was found that unsaturated-NHC Pd bonds were more easily to break. Iridium complexes were able to catalyze intramolecular hydroamination of alkynes, cyclization of acetylenic carboxylic acid, and oxidative cyclization of 2-(o-aminophenyl)ethanol with excellent activities. In addition, (NHC)Ir(CO)2Cl could act as catalysts for the hydrogen-transfer reduction of ketones, especially for acetophenones with electron-withdrawing groups. Due to the higher electron density on the Pd center, the saturated bis-carbene palladium complexes exhibited good activities on the catalysis of the Suzuki-Miyaura coupling of chloroarenes with phenylbonoric acid. In contrast, the carbene-phosphine complex with lower electron density performed better activity for C-S coupling. The reason was that transmetalation of thiolates were slower than oxidative addition of aryl bromides during the catalytic cycle. The stable cationic species generated from the abstraction of NHC-Pd-Cl with silver salt actively catalyzed the tandem reaction of 2-(phenylethynyl)anilines with benzaldehydes to give bisindoles and the conjugate addition between indoles and nitrostyrenes.