Selective C[sbnd]H bond activation of ethane by free gold clusters

Abstract

The activation and potential dissociation of ethane mediated by small cationic gold clusters Aux+ (x = 2–4) has been explored by infrared multiple photon dissociation (IR-MPD) spectroscopy and density functional theory (DFT) calculations. The calculations show that the interaction between the gold clusters and ethane is mainly governed by the mixing of the ethane CH3 bond-forming orbitals π−(CH3) with gold d-orbitals. While the CC single bond appears to be unaffected, this mixing leads to the selective activation of up to two ethane C–H bonds and a reduction of the activation barrier for C–H bond dissociation to up to 0.82 eV, making the reaction kinetically feasible at room temperature. In agreement with this, experimental IR-MPD spectra of the complexes Au2(C2H6)2+ and Au2(C2D6)2+ show strong indications for the co-existence of two isomeric structures, one representing the encounter complex and one where a single ethane C–H bond is dissociated resulting in a complex which contains an ethyl group and a bridge-bonded H atom along with a second, adsorbed C2H6 molecule. A similar C–H bond dissociation mechanism is theoretically predicted for the Au3(C2H6)y+ (y = 2,3) and Au4(C2H6)y+ (y = 2,3) complexes, albeit thermodynamically less favorable. IR-MPD spectra of Au3(C2H6)y+ (y = 2,3) and Au4(C2H6)y+ (y = 2,3) confirm the encounter product to be the dominant one, although the co-existence of isomers containing ethyl groups cannot be excluded. Various pathways for C–H bond activation are theoretically explored and the ethane activation mechanism is compared to the gold mediated activation of methane and ethylene.