Stability and dynamic processes in 16VE iridium(III) ethyl hydride and rhodium(I) σ-ethane complexes: experimental...

Iridium(I) and rhodium(I) ethyl complexes, (PONOP)M(C2H5) (M = Ir (1-Et), Rh (2-Et)) and the iridium(I) propyl complex (PONOP)Ir(C3H7) (1-Pr), where PONOP is 2,6-(tBu2PO)2C5H3N, have been prepared. Low-temperature protonation of the Ir complexes yields the alkyl hydrides, (PONOP)Ir(H)(R) (1-(H)(Et)(+) and 1-(H)(Pr)(+)), respectively. Dynamic (1)H NMR characterization of 1-(H)(Et)(+) establishes site exchange between the Ir-H and Ir-CH2 protons (ΔG(exH)(‡)(-110 °C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate. By dynamic (13)C NMR spectroscopy, the exchange barrier between the α and β carbons ("chain-walking") was measured (ΔG(exC)(‡)(-110 °C) = 8.1(1) kcal/mol). The barrier for ethane loss is 17.4(1) kcal/mol (-40 °C), to be compared with the reported barrier to methane loss in 1-(H)(Me)(+) of 22.4 kcal/mol (22 °C). A rhodium σ-ethane complex, (PONOP)Rh(EtH) (2-(EtH)(+)), was prepared by protonation of 2-Et at -150 °C. The barrier for ethane loss (ΔG(dec)(‡)(-132 °C) = 10.9(2) kcal/mol) is lower than for the methane complex, 2-(MeH)(+), (ΔG(dec)(‡)(-87 °C) = 14.5(4) kcal/mol). Full spectroscopic characterization of 2-(EtH)(+) is reported, a key feature of which is the upfield signal at -31.2 ppm for the coordinated CH3 group in the (13)C NMR spectrum. The exchange barrier of the hydrogens of the coordinated methyl group is too low to be measured, but the chain-walking barrier of 7.2(1) kcal/mol (-132 °C) is observable by (13)C NMR. The coordination mode of the alkane ligand and the exchange pathways for the Rh and Ir complexes are evaluated by DFT studies. On the basis of the computational studies, it is proposed that chain-walking occurs by different mechanisms: for Rh, the lowest energy path involves a η(2)-ethane transition state, while for Ir, the lowest energy exchange pathway proceeds through the symmetrical ethylene dihydride complex.