Metal sandwich molecules: Planar metal atom arrays between aromatic hydrocarbons

Sandwich molecules M_nS₂ 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 C _2v 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 n²- and n³-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 (C ₂₄H₁₂), ovalene (C₃₂H₁₄), circumanthracene (C₄₀H₁₆) and circumcoronene (C ₅₄H₁₈).