Symmetrical and Non-symmetrical Pd (II) Pincer Complexes Bearing Mesoionic N-heterocyclic Thiones: Synthesis, Characterizations and Catalytic Properties

In contrast to well-established symmetrical pincer complexes, non-symmetrical metal pincers with the loss of C(2v)symmetry are much less studied. In this work, mesoionic NHTs (NHTs = N-heterocyclic thiones), which can be viewed as the sulfur adducts of mesoionic carbenes, are incorporated into pincer complexes for the first time. Two symmetrical and non-symmetrical phenylene-bridged bis (NHT) compounds3a/3bwere synthesized as proligands via a cycloaddition-alkylation-thionation reaction sequence. In a case to access bis (NHT) compound3c, N-dealkylation reactions occurred. The carbene NMR signals of NHTs are only partially correlated to the pi-accepting abilities of carbenes, which is different from Bertrand's carbene-phosphinidene system. The structural analysis of3a/3bindicates that they possess C-S partial double bonds.3aand3bserved as the precursors to access two aryl anion-linked [SCS/S '] pincer complexes6a/6b. An external base proved to be essential for this cyclopalladation process. The catalytic properties of6a/6bin the cycloisomerizations of alkynoic acids have been examined. Finally, non-symmetrical complex6ashows superior catalytic performance in such transformations contrasting to its symmetrical counterpart6b.