Counteranion-Dependent Reaction Pathways in the Protonation of Cationic Ruthenium-Vinylidene Complexes

The tetraphenylborate salts of the cationic vinylidene complexes (Cp*Ru=C=CHR((Pr2PNHPy)-Pr-i)](+) (R = p-C6H4CF3 (1a-BPh4), Ph (1b-BPh4), p-C6H4CH3 (1c-BPh4), p-C6H4Br (1d-BPh4), Bu-t (1e-BPh4), H (1f-BPh4)) have been protonated using an excess of HBF4 center dot OEt2 in CD2Cl2, furnishing the dicationic carbyne complexes [Cp*Ru CCH2R((Pr2PNHPy)-Pr-i)](2+) (R = p-C6H4CF3 (2a), Ph (2b), p-C6H4CH3 (2c), p-C6H4Br (2d), Bu-t (2e), H (21)), which were characterized in solution at low temperature by NMR spectroscopy. The corresponding reaction of the chloride salts 1a-Cl, 1b-Cl, 1c-Cl, and 1d-Cl followed a different pathway, instead affording the novel alkene complexes [Cp*RuCl(kappa(1)(N),eta(2)(C,C)-C5H4N-(NHPPr2CH)-Pr-i=CHR)][BF4] (3a-d). In these species, the entering proton is located at the alpha-carbon atom of the former vinylidene ligand, which also forms a P C bond with the phosphorus atom of the (Pr2PNHPy)-Pr-i ligand. To shed light on the reaction mechanism, DFT calculations have been performed by considering several protonation sites. The computational results suggest metal protonation followed by insertion. The coordination of chloride to ruthenium leads to alkenyl species which can undergo a P C coupling to yield the corresponding alkene complexes. The noncoordinating nature of [BPh4](-) does not allow the stabilization of the unsaturated species coming from the insertion step, thus preventing this alternative pathway.