Protonated diolefin complexes: Model systems for C?;h activation via metal complexation

On protonation of the diolefin complexes M(C₅R₅)(diene)] (R = H, CH₃; M = Co, Rh, Ir; diene = 2,3-dimethylbutadiene, 1,3-cyclohexadiene) with HBF₄, cationic species are isolated which, at room temperature, show fluxional behaviour on the NMR time scale. Depending on R and M, three different ground states are observed for these cationic complexes in the NMR spectra at low temperatures. While for M = Ir a classical metal-hydride structure M–H is observed, the Co and Rh complexes show ground states with ‘agostic’ H-bridges M‥H‥C. The protonated species are characterized by ¹H-, ¹³C- and ¹⁰³Rh-NMR spectra. Total line-shape analysis of the ¹H and ¹³C spectra in the 298–154 K range gave the free enthalpies of activation ΔG ^≠ for methyl rotation and 1, 4-H shift in the agostic structures 2b , 2b′ , 2c , and 2c′ . The Rh complexes show the lowest ΔG^± values for the 1,4-H shift, and the strength of the agostic bond appears to decrease in the order CoC₅H₅ > CoC₅Me₅ > RhC₅H₅ > RhC₅Me₅. Only for R = H and M = Rh and in the presence of traces of Lewis bases (H₂O, Pyridine, or acetone), variable amounts of coordinatively saturated allyl complexes competing with the agostic species are observable. Morethan equimolar amounts of basic solvents lead to irreversible deprotonation and recovery of the starting complexes. Stable allyl-halide complexes are formed on reaction with HCI, while protonation with HBF₄, in the presence of CO, gives high yields of complexes M(CO)(allyl)(C₅R₅)] BF₄]. The different ground states observed for the protonated complexes and the dynamic behaviour in solution are compared with other hydride-transfer reactions observed in organometallic chemistry, specifically with the β-hydride elimination and the catalytic hydrogenation of olefins.