Selective C-H bond activation of ethane by free gold clusters

The activation and potential dissociation of ethane mediated by small cationic gold clusters Au-x(+)(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 pi-(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 Au-2(C2H6)(2)(+) and Au-2(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 Au-3(C2H6)(y)(+) (y = 2,3) and Au-4(C2H6)(y)(+) (y = 2,3) complexes, albeit thermodynamically less favorable. IR-MPD spectra of Au-3(C2H6)(y)(+) (y = 2,3) and Au-4(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. (C) 2018 Elsevier B.V. All rights reserved.