Using structures formed by dirhodium tetra(trifluoroacetate) with polycyclic aromatic hydrocarbons to prospect for maximum pi-electron density: Huckel calculations get it right

A new class of supramolecular assemblies derived from a powerful Lewis acid in the form of dirhodium(II) tetra(trifluoroacetate) and various planar po lycyclic aromatic hydrocarbons (PAHs) as donors has been prepared using a s olventless technique. As a result, a number of novel adducts [Rh-2(O2CCF3)( 4)](x)(L)(y) with various stoichiometries, x:y = 1:2, 1:1, 3:2, and 3: 1, h ave been isolated in crystalline form. The following PAHs have been employe d: acenaphthylene C12H8 (L1); acenaphthene C12H10 (L2); anthracene (L3) and phenanthrene (L4), C14H10; pyrene (L5) and fluoranthene (L6), C16H10; a se ries of isomers of the C18H12 composition: 1,2-benzanthracene (L7), triphen ylene (L8), and chrysene (L9). Single-crystal X-ray diffraction studies hav e revealed a variety of structural motifs ranging from discrete molecules t o extended ID chains and 2D networks. In the bis-adducts, [Rh-2(O2CCF3)(4)] (L)(2), an aromatic ligand is axially coordinated to the rhodium atoms thro ugh two long inequivalent Rh-C linkages at each end of the dirhodium comple x. In the 1D complexes {[Rh-2(O2CCF3)(4)](L)}(infinity) aromatic ligands se rve as bidentate links between two dirhodium units, while in 2D structures PAHs act as polydentate linkers, each coordinated to several rhodium atoms. Each linkage of a PAH consisted of an off-centered eta (2) coordination to ward a single rhodium center. Simple Huckel calculations performed on the P AHs were used to calculate pi -electron densities for the C-C bonds, and th ese densities were compared to the experimental results.