# Complexes of Zinc-Coordinated Heteroaromatic N-Oxides with Pyrene: Lewis Acid Effects on the Multicenter Donor–Acceptor Bonding

## TL;DR

This study explores how zinc coordination affects the bonding between N-oxides and pyrene, revealing stronger interactions through electrostatic and orbital effects.

## Contribution

The paper introduces new insights into how Lewis acids like ZnCl2 enhance multicenter donor–acceptor bonding in N-oxide–pyrene complexes.

## Key findings

- Coordination with ZnCl2 increases the positive electrostatic potential on π-acceptor surfaces.
- ZnCl2 coordination strengthens multicenter bonding through electrostatic and molecular-orbital interactions.
- Dark co-crystals form from Zn-coordinated N-oxides and pyrene, similar to protonated or BF3-bonded systems.

## Abstract

4-Nitroquinoline-N-oxide (NQO) and 4-nitropyridine-N-oxide (NPO) are important precursors for the synthesis of substituted heterocycles while NQO is a popular model mutagen and carcinogen broadly used in cancer research; intermolecular interactions are critical for their reactions or functioning in vivo. Herein, the effects of the coordination of N-oxide’s oxygen atom to Lewis acids on multicenter donor–acceptor bonding were explored via a combination of experimental and computational studies of the complexes of NQO and NPO with a typical π-electron donor, pyrene. Coordination with ZnCl2 increased the positive electrostatic potentials on the surfaces of these π-acceptors and lowered the energy of their LUMO. Analogous effects were observed upon the protonation of the N-oxides’ oxygen or bonding with boron trifluoride. The interaction of ZnCl2, NPO, or NQO and pyrene resulted in the formation of dark co-crystals comprising π-stacked Zn-coordinated N-oxides and pyrene similar to that found with protonated or (reported earlier) BF3-bonded N-oxides. Computational studies indicated that the coordination of N-oxides to zinc(II), BF3, or protonation led to the strengthening of the multicenter bonding of the nitro-heterocycle with pyrene, and this effect was related both to the increased electrostatic attraction and molecular–orbital interactions in their complexes.