Cyclopalladated o-carboranes. Palladium transfer from cyclopalladated N,N-dimethylbenzylamines, 1C-diphenylphosphino- o-carborane and η 3-allyls in carboxylic acid solvents

Abstract

1-Diphenylphosphinomethyl- o-carborane, o-HCB 10H 1OCCH 2PPh 2 (DPMC), reacts with Na 2PdCl 4 (methanol) or PdCl 2(PhCN) 2 (benzene) to give the “bis-adduct” trans- [PdCl 2(DPMC) 2], which on thermolysis in n-propanol transforms into the five-membered PdPCCB palladacycle, di-μ-chlorobis[(1-diphenylphosphinomethyl- o-carboranyl-B,P)palladium(II)] ( IIa), as a mixture of B(3) and B(4) metallated isomers according to 1H and 31P NMR. If the internal palladation is performed in the presence of LiBr or NaI, the corresponding bromo- or iodo-bridged dimers are obtained. Complex IIa can also be prepared via ligand exchange in carboxylic acid solvents. In particular, palladium(II) migrates from cyclopalladated chloro-bridged N,N-dimethylbenzylamine or 1C-diphenylphosphino- o-carborane, i.e. from complexes with C,N-five- and B,P-four-membered palladacycles, respectively, to 1-diphenylphosphinomethyl- o -carborane yielding IIa. Migration of palladium(II) is also observed from the compounds with great stability towards acetic acid, namely the [PdCl(η 3-allyl)] 2 dimers. No such processes except with this particular ligand were found in the case of palladium(II) allyls and other entering ligands (azobenzene, 2-phenylpyridine, α-methylstyrene). The exchange processes described clearly show that complex IIa is characterized by a remarkable thermodynamic stability in acetic acid which accounts for the course of many exchange reactions.