Dual spherical-spherical aromaticity in [M(η2-P4)2]+ (M = Cu, Ag, Au) and [P(P4)2]+. Evaluation of bonding nature and spherical aromatic character of the P4 aggregates from DFT calculations

The favorable coordination of two P-4 units to a coinage metal center, given by [M(eta(2)-P-4)(2)](+) and [P(P-4)(2)](+) clusters, offers an interesting template for evaluating how two spherical aromatic units interact when bridged by either a metal or a main-group element center. Our results indicate a M+-(P-4)(2) stabilization trend in the order Au > Cu > Ag, involving mainly electrostatic character, followed by a sizable contribution from covalent (orbital) character, with a slight London dispersion type stabilization. The coordination scheme is based on a sigma- M+<-(eta(2)-P-4)(2) charge transfer, followed by sigma-metal-to-ligand backbonding, in addition to a set of two sigma-ligand-to-metal and pi-ligand-to-metal charge transfer bonds. Such structural features bring together two spherical aromatic P-4 units, leading to the formation of a dual aromatic cluster mediated by a coinage metal center, i.e. two independent aromatic units within the same cluster. However, in [P(P-4)(2)](+) involving stronger covalent P-4-P bonds, the spherical aromatic character decreases, owing to the larger distortion of the P-4 units, as result of the larger bonding interaction towards the central P atom. Hence, the coordination towards a coinage metal center of the P-4 unit incorporates multiple aromatic units into a single molecular entity. This provides a model to envisage larger connected spherical-aromatic species serving as building blocks while retaining their inherent characteristics.