The interaction model between metal cation and tropylium: a quantum chemistry predication
Introduction
Recent experimental and theoretical investigations have revealed that the interaction between π systems and cations play significant roles in many aspects, such as molecular recognition, drug action and protein folding [1], [2], [3], [4], [5], [6]. The cations could be either simple metal ions, ammonium or tetramethylammonium, and the π systems could be the typical aromatic compounds, such as benzene, heterocyclic or nucleobase [7], [8], [9], [10], [11], [12]. The binding strength could be as strong as common chemical bonding. For example, the MP2/6-31G** calculated binding enthalpy for the complexes formed by benzene with Be2+, Mg2+ or Ca2+ are −215.3, −106.1 or −71.9 kcal/mol, respectively [9]. However, it was noticed that all the studied π systems are neutral or negatively charged molecules [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. To the best of our knowledge, no π system was published based on cations. Therefore, a question remains whether it is possible that the π system could be a cation in cation–π complexes. To clarify this doubt, we carried out theoretical calculations on several unusual cation–π systems formed by metal ions and the aromatic cation tropylium using the ab initio Hartree–Fock (HF) and second order Møller–Plesset perturbation theory (MP2) methods [14], [15].
Section snippets
Computational details
The positively charged π system (called π+ hereinafter) used in this paper is C7H7+ (tropylium ion), which is a delocalized carbenium ion formed by detachment of one hydride ion from the CH2 group of cyclohepta-1,3,5-triene; and the metal cations are Li+, Na+, K+, Be2+, Mg2+ and Ca2+. The initial structures with different interaction distances, ranging from 0.0 to 10.0 Å, were designed with the symmetry of C7v, in which metal cations are at the normal axis of the C7H7+ ring (Fig. 1). The
The potential energy curve
The potential energies of the system formed by C7H7+ with alkaline metal ion or calcium ion were found to monotonously increase as the two separated cations move towards each other, suggesting no cation–π+ complex formed. However the potential curves of the complexes formed by C7H7+ with Be2+ or Mg2+ have local minima (Fig. 2). Therefore, Be2+ and Mg2+ are capable of forming M2+–π+ complex with C7H7+. As the separated π+ and M2+ approach each other, the systems proceed energetically uphill to
Conclusions
In conclusion, it is possible that a metal cation binds the aromatic cation, tropylium ion or C7H7+, to form a cation–π+ complex, but it is a metastable structure as the complexation is endothermic. The ES, PL, CT and MIX play important roles in the process of metal cation approaching aromatic cation. While the ES repulsion is the dominant interaction before the system reaches TS1, the PL and CT increase fast versus interaction distance after TS1, resulting in the binding of a metal cation to C7
Acknowledgements
We acknowledge the financial supports from the National Natural Science Foundation of China (Grant 29725203), the State Key Program of Basic Research of China (Grant 1998051115). We thank Profs. Zhenyang Lin and Shihe Yang in Hong Kong University of Science and Technology and Mrs. Tan–Lee Kwee Choo in Singapore Polytechnic for their fruitful discussions. The quantum chemistry calculations were performed on Power Challenge R10000 at the Network Information Center, CAS, Beijing, China and on P4
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