Theoretical Determination of the Structural, Bonding, and Magnetoresponsive Properties of Square-Planar Ligand-Protected Noble Metal (Cu, Ag, Au) Clusters

Electronic structure calculations (DFT) suggest that “ligand-protected” four-membered rings of noble metals formulated as [c-M4(μ2-L)4] (M = Cu, Ag, Au; L = H, CH3, SiH3, GeH3, NH2, PH2, OH, F, Cl, Br, I) are thermodynamically stable molecules with respect to their dissociation either to ML monomers or to M2L2 dimers. The M2L2 dimers are the products of the thermodynamically favored oxidative addition reactions of the L2 to M2 diatomic species. The [c-M4(μ2-L)4] (M = Cu, Ag, Au; L = H, CH3, SiH3, GeH3, NH2, PH2, OH, F, Cl, Br, I) molecules adopt square-planar structures involving bridging stabilizing ligands L, except [c-Au4(μ2-CH3)4]. All [c-M4(μ2-L)4] clusters are characterized by perfect planarity and equalization of all metal−metal bonds in the metallic rings, with the only exception being the [c-Cu4(μ2-Br)4] and [c-Cu4(μ2-I)4] clusters, which adopt a diamond-like core structure. In all four-membered metallic rings the intermetallic interactions are relatively weak, described as cuprophilic, argentophilic, and aurophilic interactions. The four-membered metallic rings are further stabilized by the bonding of the μ2-L bridges, which corresponds to a three-center two-electron (3c-2e) bond. Analysis of the magnetotropicity (diatropicity/paratropicity) of the [c-M4(μ2-L)4] clusters based on the NICS zz -scan curves in conjunction with symmetry-based selection rules for the most significant translationally and rotationally allowed transitions revealed that all rings exhibit long-range diatropicity (aromaticity) with minima in the region of 1.8 to 4.0 Å above and below the ring and paratropicity (antiaromaticity) in the ring plane up to 0.7 to 2.5 Å above and below the ring where a nodal plane resides and having maxima at the ring center. It should be noted that the estimated NICS zz min values are indicative of remarkable aromaticity only for the [c-M4(μ2-L)4] (M = Cu, Ag, Au; L = H, CH3) clusters; all other species should be nonaromatic or even antiaromatic.