Structural preferences of cyclopentadienyl and indenyl rings in iridium(I) carbene complexes

Cleavage of the [Ir(η ⁴ -COD)Cl] ₂ dimer in the presence of the corresponding imidazolium salts and the strong base tBuO ^- leads to the formation of Ir(I) derivatives of N-heterocyclic carbenes. When halide is replaced by NaCp, a mixture of [Ir(η ⁴ -COD)(NHC ^R )(η ¹ -Cp)] and [Ir(η ² -COD)(NHC ^R )(η ⁵ -Cp)] is obtained. The latter is favored for R = Cy, while the former predominates for R = Me. Conversely, [Ir(η ⁴ -COD)(NHC ^R )(η ¹ -Ind)] is the only product of the reaction with NaInd, despite the R substituent. DFT/B3LYP calculations confirmed that the η ¹ coordination mode of the ring gives rise to the most stable structures, namely square planar complexes of 5d ⁸ Ir(I). The energy of the 18 electron species containing η ² -COD and η ⁵ -Ind or Cp is higher by 13 and 5 kcal mol ^-1 , respectively. The fluxional behaviour of indenyl, detected by NMR in the solutions of [Ir(η ⁴ -COD)(NHC ^R )(η ¹ -Ind)], is associated to the low energy of the η ³ -Ind species required in the conversion process, and is not easily observed in the cyclopentadienyl derivatives, where a similar intermediate is disfavored.