HIGH-PRESSURE NMR AND AB-INITIO COMPUTATIONAL STUDIES ON THE INSERTION MECHANISM OF CARBON-MONOXIDE INTO CATIONIC MONOORGANOPALLADIUM(II) COMPLEXES BEARING TRIDENTATE NITROGEN DONOR LIGANDS

A number of methyl and arylpalladium(II) cations bearing tridentate ni trogen-donor ligands has been prepared, and their reactivity in the in sertion of carbon monoxide has been studied. The resulting acetyl and aroylpalladium complexes of the ligands N-(2-picolyl)N,N',N'-trimethyl ethylenediamine (pico) and N,N,N',N '' N ''-pentamethyldiethylenetriam ine (pmdeta) were isolated and characterized. In the case of the 4-nit rophenylpalladium and 2,4,6-trimethylphenylpalladium pico- and pmdeta complexes, no carbonylation products could be isolated. In these cases , either the equilibrium of the carbonylation reaction did not lie ful ly to the side of the insertion product even at 10 atm of CO or the in sertion product decarbonylated upon attempted isolation. The methylpal ladium complex of the 2,6-bis[(dimethylamino)methyl]pyridine (NN'N) li gand reacted quantitatively with CO in CD3-COCD3 to give acetic anhydr ide, palladium metal, and the protonated ligand; a mechanism for the a nhydride formation is proposed. With the exception of the 1-naphthoyl derivative, the aroyl complexes of the NN'N ligand could not be isolat ed due to decarbonylation upon attempted isolation. Most of the unstab le insertion products could, however, be characterized by IR and high- pressure NMR. Crystals of the 1-naphthoylpalladium complex with the NN 'N ligand were obtained from acetone/pentane under a CO atmosphere. Th is complex is the first example of an aroylpalladium(II) cation. Two r eaction pathways for the carbonyl insertion reaction, i.e., dissociati ve and associative, have been evaluated using H-1 NMR studies and ab i nitio calculations. The insertion reaction at 10 atm of CO pressure in CD3-COCD3 is complete within 2.5 min for most complexes, with the exc eption of those bearing strongly electron-withdrawing para substituent s (e.g., NO2) or sterically demanding ortho substituents (e.g., 2,4,6- trimethyl) on the aryl ring. Ab initio calculations at the RHF, MP2//S CF, and CAS-SCF/CI levels on the cationic model system [Pd(CH3)(NH3)(3 )](+) + CO and the neutral system [Pd(CH3)(2)(NH3)(2)] + CO show that the carbonylation reaction follows a hybrid pathway, i.e., a concerted replacement of NH3 by CO followed by migratory insertion of CO into t he Pd-C bond instead of a purely dissociative or associative mechanism . For both the neutral and the cationic systems the rate-determining s tep is the migratory insertion. The insertion process is enhanced by c oordination of the dissociated amine and is slightly more favorable in the neutral system. Together with the low-energy replacement of NH3 b y CO, this implies that in both systems the rate of carbonyl insertion should be independent of the applied CO pressure.