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

Abstract

In contrast to well‐established symmetrical pincer complexes, non‐symmetrical metal pincers with the loss of C2v 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) compounds 3a/3b were synthesized as proligands via a “cycloaddition‐alkylation‐thionation” reaction sequence. In a case to access bis (NHT) compound 3c, N‐dealkylation reactions occurred. The carbene NMR signals of NHTs are only partially correlated to the π‐accepting abilities of carbenes, which is different from Bertrand's carbene‐phosphinidene system. The structural analysis of 3a/3b indicates that they possess C‐S partial double bonds. 3a and 3b served as the precursors to access two aryl anion‐linked [SCS/S′] pincer complexes 6a/6b. An external base proved to be essential for this cyclopalladation process. The catalytic properties of 6a/6b in the cycloisomerizations of alkynoic acids have been examined. Finally, non‐symmetrical complex 6a shows superior catalytic performance in such transformations contrasting to its symmetrical counterpart 6b.