Reactions of nitrogen donors with cycloheptatrienylidene complexes: Metal coordination versus nucleophilic attack on the carbene ligand

Cycloheptatrienylidene (CHT)-palladium complexes may react with N-donor molecules, showing two different pathways of reaction, either nucleophilic attack on the CHT ligand or coordination to the metal center. The first variant leads to a formation of water-soluble η³-cycloheptatrienyl complexes, as in the case of 3,5-lutidine or 3-chloropyridine. Reaction with 2,6-lutidine, on the other hand, yields monomeric pyridine-substituted CHT-Pd compounds comparable to NHC-PEPPSI complexes. Reaction with 1-methylimidazole yields both a dimeric water-soluble cycloheptatrienyl palladium complex and a monomeric CHT-Pd compound, depending on the conditions of the reaction. Furthermore, a subsequent formation of a Pd₃-sandwich type complex was observed, which has been determined by single-crystal X-ray diffractometry. The nucleophilic attack of morpholine on the CHT ligand reveals another possible reaction path: removal of the CHT ligand from the metal under formation of a tropylidenimmonium cation.