1,2,3-triazolin-5-ylidenes: Synthesis of hetero-bis(carbene) Pd(II) complexes, determination of donor strengths, and catalysis

Abstract

A series of hetero-bis(carbene) complexes trans-[PdBr 2( iPr 2-bimy)(trz)] 1-4 ( iPr 2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; trz = 1,2,3-triazolin-5-ylidene) bearing the constant iPr 2-bimy and varying mesoionic 1,2,3-triazolin-5-ylidenes with different N-substituents has been synthesized as complex probes. Their 13C NMR spectroscopic evaluation shows that mesoionic 1,2,3-triazolin-5-ylidenes are, in general, stronger donors than classical NHCs, while weaker than some nonclassical NHCs, such as pyrazolin-3-ylidenes and mesoionic imidazolin-4-ylidenes. More importantly and for the first time, this methodology proves useful in establishing substituent effects in the donating abilities of 1,2,3-triazolin-5-ylidenes on a finer level. In addition, the trifluoroacetato analogues [Pd(O 2CCF 3) 2( iPr 2-bimy)(trz)] 5-7 have been synthesized through salt metathesis of 1, 2, and 4 with AgO 2CCF 3. The catalytic activities of complexes 1, 2, and 4-7 were examined in the direct arylation of pentafluorobenzene. Complexes bearing less donating trz ligands perform better in this catalysis, and trifluoroacetato complexes outperformed their bromido analogues. © 2011 American Chemical Society.