Insertion Reactions Involving Palladium Complexes with Nitrogen Ligands. 2. Carbon monoxide and Alkene Insertion Reactions with Novel Palladium Compounds Containing Terdentate Ligands

Neutral and ionic methylpalladium compounds containing trinitrogen ligands (N-N-N)Pd(Me)(Y) (N-N-N = trinitrogen ligand; Y = Cl -, CF 3 SO 3 -, 4-Me-C 6 H 4 SO 3 - ) have been synthesized and characterized by spectroscopic methods. The coordination mode of the trinitrogen ligand depends not only on the rigidity of the ligand but also on the solvent and on the anion Y. With the weakly coordinating ligands CF 3 SO 3 - and BF 4 - the trinitrogen ligands readily adopt a terdentate coordination mode, resulting in complexes of the type [(σ 3 -N-N-N)Pd(Me)]Y. In methylchloropalladium compounds, the flexible trinitrogen ligands adopt a bidentate coordination mode, resulting in the complexes (σ 2 -N-N-N)Pd(Me)(Cl), whereas the terdentate coordination mode has been found for rigid ligands, resulting in the ionic complexes [(σ 3 -N-N-N)Pd(Me]Cl. Polar solvents, such as acetonitrile, stabilize the formation of ionic complexes. The methylpalladium compounds readily insert carbon monoxide, resulting in the facile formation of the acetylpalladium compounds (σ 2 -N-N-N)Pd(C(O)Me)(Cl) and [(σ 3 -N-N-N)Pd(C(O)Me)]Y (Y = Cl -, OTf - ), in which the coordination mode of the ligands remains unchanged. CO insertion half-lives of the methylpalladium compounds show that substituents adjacent to one nitrogen donor atom accelerate the insertion for both the neutral and the ionic complexes. Interestingly, the CO insertion half-life is not correlated to the rigidity of the trinitrogen ligand, since the compound [(terpy)Pd(Me)]Cl, which contains the rigid ligand terpy, undergoes a faster carbon monoxide insertion than analogous complexes with the flexible ligands C 5 H 4 N-2-C(H) N(CH 2 ) 2 C 5 H 4 N. Alkene insertion has been tested by reacting the acetylpalladium compounds with norbornadiene (NBD). The NBD-inserted compounds contain the C 7 H 8 C(O)CH 3 moiety coordinating in an unprecedented monodentate σ-C fashion. Peculiarly, [(terpy)Pd(C(O)Me)]Cl not only undergoes NBD insertion but does so at a very high rate. On the basis of these findings a tentative mechanism is suggested.