The electronic structures and aromaticities for zinc sandwich, half-sandwich and zinc–zinc [formula omitted] sandwich complexes within density functional theory

Abstract

The equilibrium geometries, energies, harmonic frequencies, and nucleus-independent chemical shifts of the zinc sandwich, the half-sandwich, and the zinc–zinc ( Zn 2 2 + ) sandwich complexes are computed by B3LYP/6-311++G(d,p) within the density functional theory. The staggered [(η 5-C 5H 5) 2Zn 2] ( 1 A 1, D 5 d ) is the most stable minimum with higher binding energy and slightly stronger aromaticity than those of the η 5- C 5 H 5 − ( 1 A 1, D 5 h ). The eclipsed [(η 5-C 5H 5) 2Zn 2] ( 1 A 1, D 5 h ) is a transition state with a small ring rotation barrier. The Zn-containing half-sandwich complexes are minima while with different stabilities and aromaticity. Particularly, the [(η 5-C 5H 5)Zn] ( 2 A 1, C 5 v ) has larger binding energy with aromaticity slightly weaker than η 5 − C 5 H 5 − , this compound features a simple while bona fide monovalent zinc molecular compound. The Zn 2+ sandwich complex, [Zn(η 5-C 5H 5) 2] ( 1 A 1, D 5 h and D 5 d ), with aromaticity weaker than that of the slip–sandwich complex, is a saddle point on potential energy surface. The slip–sandwich complex, (η 1-C 5H 5)Zn(η 5-C 5H 5) ( 1 A 1, C 1), with aromaticity close to that of η 5 − C 5 H 5 − ( 1 A 1, D 5 h ), is a local minimum. Both the eclipsed and the staggered [(C 5(CH 3) 5) 2Zn 2]( C 1) are aromatic with aromaticity close to that of [(η 5-C 5H 5) 2Zn 2] ( D 5 d ). According to the analysis of molecular orbitals, the Wiberg bond indices, the magnitude of charge transfer, the total nucleus-independent chemical shifts distributions, and the nucleus-independent chemical shifts contribution distributions of various bonds manifests, the stabilities of all the Zn-containing sandwich, the half-sandwich, and the Zn 2 2 + sandwich complexes are accredited to both ionic electrostatic interactions and covalent binding, especially the ionic electrostatic interactions, between the metal center and the η 5 − C 5 H 5 − building blocks.