Transition metal sandwich molecules with large (Cn, n ⩾ 24) zigzag poly aromatic hydrocarbons

Ab initio plane wave based density functional theory was used to study the electronic structure and geometry of sandwich structures MnR2 consisting of a layer of palladium metal atoms between large eclipsed pericondensed aromatic hydrocarbon molecules: ovalene C32H14, circumanthracene C40H16, circumpyrene C42H16 and circumcoronene C54H18. The analysis was guided by the results from the smaller sandwiches: Pd(C6H6)2, symmetric isomers of Pd2(C16H10)2, Pd4(C16H10)2, Pd6(C24H12)2 and Pd7(C24H12)2 comprised of benzene, pyrene and coronene, which represented the different types of metal binding sites and their combinations found in the larger systems. Isometric surfaces of the total electronic charge density were used to interpret the bonding and coordination of metal atoms and were especially useful when their sites lacked symmetry. Starting from an initial geometry where each metal was assigned to each eclipsed pair of rings, geometry optimization showed that there was a preference for the metal atoms to coordinate to carbon atoms on the circumference of the sandwich where the C-C bond charge density in the isolated hydrocarbon was highest. This resulted in many nearest metal-metal atom distances greater than that in the bulk metal and a frustration of the formation of metal clusters with direct metal-metal bonds. In the two largest sandwiches with circumpyrene and circumcoronene, the perimeter-to-area ratio was reduced enough so that the total frustration was lifted and small core metal arrays were established in the central region. In the circumpyrene sandwich, the four metal core atoms had M-M bonds and some M-C bonds which held the sandwich flat. In contrast, the circumcoronene sandwich displayed a distinctive central bulge around a seven atom M-M bonded core. This study showed that interior metal bonding can have considerable variability over that previously reported and it provided confirmation for M-C edge bond sites with η2- and η3-coordination that were previously identified in small sandwich systems.