Metal Sandwich Molecules: Planar Metal Atom Arrays between Aromatic Hydrocarbons

Abstract

Sandwich molecules MnS2 consisting of a layer of transition metal atoms M (palladium) between planar polyacene aromatic hydrocarbon molecules (S) are studied using ab initio density functional theory. The polyacenes range from tetracene C18H12 (four rings) to circumcoronene C54H18 (nineteen rings). Geometry optimization shows that in simple arrangements, where one metal is assigned to each ring, there is a preference for metal atoms to coordinate to carbon atoms on the circumference of the sandwich. This can result in metal-metal distances greater than in bulk metal and so the establishment of planar metal clusters with metal-metal bonds in small systems is frustrated. This effect is studied by changing the number of metal atoms and relaxing symmetry constraints. A neutral molecule Pd5(C18H12)2 with C2v symmetry and a lopsided arrangement of metal atoms is shown to be consistent with experimental work on dications by Murahashi et al. [Science 313 (2006) 1104–1107]. In tetracene sandwiches with n=5 and 9 the palladium atoms adopt η2- and η3-coordination to the edge carbon atoms. In addition to edge bonding, motifs for interior metal atoms are identified from a series of sandwiches with increasing size containing: coronene (C24H12), ovalene (C32H14), circumanthracene (C40H16) and circumcoronene (C54H18).