1,2,3-Triazolin-5-ylidenes: Synthesis of Hetero-bis(carbene) Pd(II) Complexes, Determination of Donor Strengths, and Catalysis

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Abstract

A series of hetero-bis(carbene) complexes trans-[PdBr2(iPr2-bimy)(trz)] 1–4 (iPr2-bimy = 1,3-diisopropylbenzimidazolin-2-ylidene; trz = 1,2,3-triazolin-5-ylidene) bearing the constant iPr2-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(O2CCF3)2(iPr2-bimy)(trz)] 5–7 have been synthesized through salt metathesis of 1, 2, and 4 with AgO2CCF3. 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.

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