Transition Metal Cation-π Interactions: Complexes Formed by Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ Binding with Benzene...

A computational investigation of the structures and interaction energies of complexes formed by Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ binding with benzene (Bz) molecules is performed employing high level ab initio quantum chemical methods, such as the second-order perturbation theory (MP2), coupled-cluster singles and doubles (CCSD), and coupled-cluster singles and doubles with perturbative triples [CCSD(T)] methods, along with the 6-311++G(2d,2p) and 6-311++G(d,p) basis sets. As far as we know, the present work is the first to study the structures and energetics of Bz-M²⁺ and Bz-M²⁺-Bz type complexes (M = Co, Ni, Cu, and Zn). Relativistic effects are also investigated via Douglas-Kroll-Hess second-order scalar relativistic computations for the complexes considered. Our results demonstrate that there are strong bindings between transition metal cations and benzene molecules. The computed interaction energies, including relativistic energy corrections, for the Bz-M²⁺ type complexes at the CCSD(T)/6-311++G(2d,2p) level are -131.9 (Bz-Fe²⁺), -172.6 (Bz-Co²⁺), -189.8 (Bz-Ni²⁺), -181.1 (Bz-Cu²⁺), and -158.2 (Bz-Zn²⁺) kcal mol^-1. Similarly, interaction energies for the Bz-M²⁺-Bz type complexes at the CCSD(T)/6-311++G(d,p) level are -206.4 (Bz-Fe²⁺-Bz), -213.4 (Bz-Co²⁺-Bz), -249.7 (Bz-Ni²⁺-Bz), -258.6 (Bz-Cu²⁺-Bz), and -235.2 (Bz-Zn²⁺-Bz) kcal mol^-1. Further, our results also demonstrate that the relativistic effects are very important in accurate computations of interaction energies. The predicted relativistic energy corrections to interaction energies, using the ωB97X-D functional, are between -1.9 and -7.7 kcal mol^-1. The transition metal cation-π interactions investigated in this study prove significantly larger binding energies compared to arbitrary π-π interactions and main group cation-π interactions. We believe that the present study may open new avenues in cation-π interactions.