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

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.