Haptotropic Phenomena in Digold(I) Triple-Bonded Complexes

Addition of either IPrAuOTf or IPrCuOTf (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; OTf = trifluoromethanesulfonate anion) to the digold acetylide IPrAuC≡CAuIPr results in the selective formation of the corresponding trimetallic cationic species [IPrAuC≡C(π-MIPr)AuIPr][OTf] (M = Au or Cu). Variable-temperature ¹H NMR experiments (VT-NMR) reveal that while the homotrimetallic gold complex exhibits dynamic σ,π-exchange in solution at temperatures even as low as -130 °C, the heterometallic analogue presents a static scenario. On the other hand, extension of the acetylide bridge by one additional acetylide unit using IPrAuC≡C-C≡CAuIPr introduces a new fluxional process in the corresponding analogous trimetallic compounds [IPrAuC≡C(π-MIPr)-C≡CAuIPr][OTf] (M = Au or Cu), namely π,π-exchange. In the case of the copper-containing complex, this exchange occurs even at low temperatures, whereas exchange can be thermally arrested in the trigold system at temperatures below -10 °C. Computational studies indicate that the divergent behavior between gold and copper regarding π,π-exchange does not appear to stem from their interaction with the alkyne fragment but rather in how this interaction changes along the reaction coordinate toward the transition state geometry.