Iridium PCP pincer complexes: highly active and robust catalysts for novel homogeneous aliphatic dehydrogenations

Abstract

The PCP pincer complexes, IrH₂{C₆H₃(CH₂PR₂)₂-2,6} (R = Bu^t, Prⁱ) are extraordinarily active and robust catalysts for aliphatic dehydrogenation reactions. Their application to alkane dehydrogenation has resulted in the first efficient catalytic systems for homogeneous thermochemical alkane dehydrogenation without the use of a sacrificial hydrogen acceptor and for dehydrogenation of n-alkanes to α-olefins. The pincer catalysts also effect other aliphatic dehydrogenations which had not previously been accomplished via homogeneous catalysis including the conversion of cycloalkanes to arenes, ethylbenzene to styrene and tetrahydrofuran to furan. All of these catalytic reactions are inhibited by even traces of nitrogen. This is apparently due to the formation of the remarkably stable dinitrogen complexes: [Ir{C₆H₃(CH₂PR₂)₂-2,6}]₂(µ-N₂). Mechanistic studies have indicated that the high activity of the pincer catalysts may be in part due to ‘agostic promotion’ by phosphino aliphatic groups.