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 σ-aromaticity of M3+ (M = Cu, Ag, Au) is analyzed and compared with that of Li3+ and a prototype σ-aromatic system, H3+. Ligands (L) like dimethyl imidazol-2-ylidene, pyridine, isoxazole and furan are employed to stabilize these monocationic M3+ clusters. They all bind M3+ with favorable interaction energy. Dimethyl imidazol-2-ylidene forms the strongest bond with M3+ 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 σ donation and L ← M back donation. M3+ clusters also bind noble gas atoms and carbon monoxide effectively. In general, among the studied systems Au3+ binds a given L most strongly followed by Cu3+ and Ag3+. 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 σ-aromaticity in L bound M3+ increases compared to that of bare clusters. The aromaticity in pyridine, isoxazole and furan bound Au3+ complexes is quite comparable with that in the recently synthesized Zn3(C5(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 M3(CO)3+ and M3(OC)3+, respectively, are analyzed through reverse polarization of the σ- and π-orbitals of CO as well as the relative amount of OC → M σ donation and M → CO π back donation. The electron density analysis is also performed to gain further insight into the nature of interaction.