Chromium aromatic hydrocarbon sandwich molecules and the eighteen-electron rule

Ab initio density functional theory (DFT) calculations are reported for the chromium sandwich structure Cr_nR₂, where n = 7 and R is the aromatic hydrocarbon hexabenzocoronene (C₄₂H₁₈). This system is remarkable in that the structure of the chromium sites strongly resemble those in chromium bis-benzene C_r1(C₆H ₆)₂, as judged by geometry and charge density properties. The electron localization function of the sandwich shows a hexagonally arrayed set of V(C, Cr, C) valence basins about each chromium atom with modification due to local site symmetry. This system satisfies an extension of the 18-electron rule to components of a conjugated molecular system. This idea is explored further by examining the electronic and geometric properties of the series Cr_n,R₂, where n and R are given by n = 1, benzene C ₆H₆ as reference; n = 2, biphenyl (C₆H ₅)₂; n = 3, triphenylene C₁₈H₁₂; n = 3, coronene C₂₄H₁₈; and n = 4, dibenzopyrene C ₂₄H₁₄. On the basis of electron counting and ring isolation, all the sandwich structures in this series could satisfy the extension of the 18-electron rule, with the exception of coronene, which was deliberately included. The DFT calculations predict spin-paired ground states for some but not all of the sandwich structures, implying that the Cr-ring interactions at work require understanding at a deeper level. Thus, while sandwiches with n = 1, n = 2, n = 4 and n = 7 have spin paired singlet ground states and appear to satisfy the rule, those with n = 3 (triphenylene, coronene) have antiferromagnetic singlet ground states and do not. This is attributed to nonuniformity in the electronic charge density of the rings of the isolated hydrocarbons and to a reduction of symmetry from D_3h to C _2v with a concomitant spin-charge density change in the sandwiches.