Binding of transition-metal ions to curved π surfaces:: Corannulene and coronene

Calculations and comparisons were made for a set of seven metal cations binding to corannulene and coronene, namely, the three alkalis Li+, Na+, and K+ and the four transition-metal ions Ti+, Cr+, Ni+, and Cu+ (combined with literature results for Li+/corannulene). In the case of corannulene, the most favorable binding site. for Ti+ and Ni+ is eta(6) over the six-membered ring on the convex face, whereas for Li+, Na+, K+, and Cr+, the five-membered eta(5) ring site on the convex face is about equally good. Cu+ slightly prefers binding at eta(2) edge sites rather than ring-centered sites on corannulene, but edge locations for several other ions were not found to be favored, in contrast to results reported for C-60. For the alkalis, binding to the convex (outside) face is slightly favored relative to binding to the concave face, whereas for the transition metals, a much larger preference for outside binding is found, particularly for the eta(5) sites. An approximate point-charge model calculation is used to separate the electrostatic-plus-polarization contributions to the binding to the eta(5) sites from the electronic orbital contributions: Judging from this analysis, electronic orbital interactions favor outside binding of transition-metal ions by amounts, ranging from about 5 to 11 kcal mol(-1). A molecular orbital picture is proposed that invokes perturbations of pi-d donation and d-pi* back-donation to explain the particularly unfavorable electronic binding interaction on the concave pi face. Binding to the flat coronene pi surface is found to be roughly equal to the outside eta(5) binding sites of the curved corannulene pi surface.