Bimetallic η6,η1 NCN-pincer ruthenium palladium complexes with η6-RuCp coordination: synthesis, X-ray structures, and catalytic properties

The synthesis, structure, and catalytic activity of a series of bimetallic η6,η1-NCN-pincer ruthenium palladium complexes, [3]+−[5]+, have been studied, in which η6-coordination of [Ru(C5R5)]+ (R = H or Me) is realized either directly to the central arene ring of the NCN-pincer palladium complex ([3]+ and [4]+) or to the para-phenyl substituent ([5]+). The X-ray structures of [4]+ and [5]+, as well as the regioselectivity observed in the synthesis of [5]+, provide clear evidence of intramolecular steric interactions in [4]+ between the amine arms of the pincer moiety and the methyl groups of the cyclopentadienyl ligand. Cyclovoltammetry data point to strong electronic interactions between the two metal fragments in [3]+ and [4]+ and only limited interactions in [5]+. Catalytic studies in the cross-coupling reaction between trans-phenylvinylboronic acid and vinyl epoxide show that the catalytic activity of the palladium center is enhanced by η6-coordination of the ruthenium fragment. These modifications of the catalytic activity of palladium are not correlated to the decreased electron density on palladium, as confirmed by DFT calculations. We hence propose a mechanism in which transmetalation is the rate-determining step