Stability and Dynamic Processes in 16VE Iridium(III) Ethyl Hydride and Rhodium(I) σ‑Ethane Complexes: Experimental and Computational Studies

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 1H NMR characterization of 1-(H)­(Et)+ establishes site exchange between the Ir–H and Ir–CH2 protons
(ΔGexH‡(−110
°C) = 7.2(1) kcal/mol), pointing to a σ-ethane intermediate.
By dynamic 13C NMR spectroscopy, the exchange barrier between
the α and β carbons (“chain-walking”) was
measured (ΔGexC‡(−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 (ΔGdec‡(−132 °C) = 10.9(2) kcal/mol)
is lower than for the methane complex, 2-(MeH)+, (ΔGdec‡(−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 13C 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 13C 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
η2-ethane transition state, while for Ir, the lowest
energy exchange pathway proceeds through the symmetrical ethylene
dihydride complex.