How the Aromatic 4-Membered Hydrido-Bridged Copper Rings Respond to Successive Nucleophilic Attack?

Electronic structure calculations (B3LYP/6-311+G**) predict that nucleophilic attack of the aromatic cycloCu4(μ)4 ring yields ligand-stabilized tetranuclear Cu4 clusters formulated as cyclo-Cu4(μ)4Nucn (n = 1-4; Nuc = N2, CO, H2O, NH3 and PH3). Depending on the number of added nucleophiles, the tetranuclear Cu4 clusters adopt planar, bent or 3D tetrahedral geometries. These molecules exhibit aromatic character, which is primarily due to 4s and 3d cyclic electron delocalization over the Cu4 framework (s and d-orbital aromaticity). The aromaticity of the novel ligand stabilized tetranuclear Cu4 clusters was verified by a number of established criteria of aromaticity. In particular, the nucleusindependent chemical shift, NICS(0) and NICS(1) and their out-of-plane components NICSzz(0), NICSzz(1) and the NICS scan pictures are indicative for the aromaticity of the planar, bent and tetrahedral Cu4 clusters. The effect of the substituents on the aromatic character of the Cu4 clusters is also a main concern of the present work. It was found that increasing the number of the attacking nucleophiles increases the diatropic response of the aromatic tetranuclear Cu4 clusters. Moreover, the aromaticity of the 3D structures (spherical aromaticity) is higher than the aromaticity of the planar and bent structures. The effect of the coordinated nucleophiles, Nuc, on the stability, geometry, electronic structure and bonding mode of the cyclo-Cu4(μ)4Nucn molecules is also thoroughly discussed.