σ-Aromatic cyclic M3+ (M = Cu, Ag, Au) clusters and their complexation with dimethyl imidazol-2-ylidene, pyridine, isoxazole, furan, noble gases and carbon monoxide

The sigma-aromaticity of M-3(+) (M = Cu, Ag, Au) is analyzed and compared with that of Li-3(+) and a prototype sigma-aromatic system, H-3(+). Ligands (L) like dimethyl imidazol-2-ylidene, pyridine, isoxazole and furan are employed to stabilize these monocationic M-3(+) clusters. They all bind M-3(+) with favorable interaction energy. Dimethyl imidazol-2-ylidene forms the strongest bond with M-3(+) followed by pyridine, isoxazole and furan. Electrostatic contribution is considerably more than that of orbital contribution in these M-L bonds. The orbital interaction arises from both L -> M sigma donation and L <- M back donation. M-3(+) clusters also bind noble gas atoms and carbon monoxide effectively. In general, among the studied systems Au-3(+) binds a given L most strongly followed by Cu-3(+) and Ag-3(+). Computation of the nucleus-independent chemical shift (NICS) and its different extensions like the NICS-rate and NICS in-plane component vs. NICS out-of-plane component shows that the s-aromaticity in L bound M-3(+) increases compared to that of bare clusters. The aromaticity in pyridine, isoxazole and furan bound Au-3(+) complexes is quite comparable with that in the recently synthesized Zn-3(C-5(CH3)(5))(3)(+). The energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital also increases upon binding with L. The blue-shift and red-shift in the C-O stretching frequency of M-3(CO)(3)(+) and M-3(OC)(3)(+), respectively, are analyzed through reverse polarization of the sigma- and pi-orbitals of CO as well as the relative amount of OC -> M s donation and M -> CO pi back donation. The electron density analysis is also performed to gain further insight into the nature of interaction.