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

Cleavage of the [Ir(η4-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(η4-COD)(NHCR)(η1-Cp)] and [Ir(η2-COD)(NHCR)(η5-Cp)] is obtained. The latter is favored for R = Cy, while the former predominates for R = Me. Conversely, [Ir(η4-COD)(NHCR)(η1-Ind)] is the only product of the reaction with NaInd, despite the R substituent. DFT/B3LYP calculations confirmed that the η1 coordination mode of the ring gives rise to the most stable structures, namely square planar complexes of 5d8 Ir(I). The energy of the 18 electron species containing η2-COD and η5-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(η4-COD)(NHCR)(η1-Ind)], is associated to the low energy of the η3-Ind species required in the conversion process, and is not easily observed in the cyclopentadienyl derivatives, where a similar intermediate is disfavored.