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.