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.