Using Structures Formed by Dirhodium Tetra(trifluoroacetate) with Polycyclic Aromatic Hydrocarbons to Prospect for Maximum π-Electron Density: Hückel 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 polycyclic aromatic hydrocarbons (PAHs) as donors
has been prepared using a solventless technique. As a result, a number of novel adducts [Rh2(O2CCF3)4]x(L)y
with various stoichiometries, x:y = 1:2, 1:1, 3:2, and 3:1, have been isolated in crystalline form. The following
PAHs have been employed:  acenaphthylene C12H8 (L1); acenaphthene C12H10 (L2); anthracene (L3) and
phenanthrene (L4), C14H10; pyrene (L5) and fluoranthene (L6), C16H10; a series of isomers of the C18H12
composition:  1,2-benzanthracene (L7), triphenylene (L8), and chrysene (L9). Single-crystal X-ray diffraction
studies have revealed a variety of structural motifs ranging from discrete molecules to extended 1D chains and
2D networks. In the bis-adducts, [Rh2(O2CCF3)4](L)2, an aromatic ligand is axially coordinated to the rhodium
atoms through two long inequivalent Rh−C linkages at each end of the dirhodium complex. In the 1D complexes
{[Rh2(O2CCF3)4](L)}∞ aromatic ligands serve 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 η2 coordination toward a single rhodium center. Simple Hückel calculations
performed on the PAHs were used to calculate π-electron densities for the C−C bonds, and these densities
were compared to the experimental results.