# Steric Restraints in Redox‐Active Guanidine Ligands and Their Impact on Coordination Chemistry

## TL;DR

Researchers synthesized new benzene ligands with guanidino groups and found that slight structural changes affect their ability to coordinate with metals.

## Contribution

The first synthesis of cyclic diguanidino-benzene ligands and the discovery of structural parameters affecting coordination ability.

## Key findings

- An ethylene bridge prevents coordination while maintaining basicity, similar to Huenig's base.
- A propylene bridge restores coordination ability without losing nucleophilic properties.
- Macrocyclic tetraguanidines were formed and analyzed for structural restraints.

## Abstract

The introduction of redox‐active ligands into coordination compounds is attractive for a number of applications; intramolecular electron transfer between a redox‐active ligand and a metal is the basis for applications in switchable devices and advanced redox catalysis for multielectron substrate activations. A fine‐tuning of the properties of redox‐active ligands focusses on the redox potential and frontier orbital energies, as well as the steric demand and coordination mode. In this work, we report the first synthesis of new o‐diguanidino‐benzene ligands in which the two guanidino groups are connected through an alkyl chain of different length. The introduction of an ethylene bridge between the two guanidino groups turns a strongly coordinating ligand into a noncoordinating molecule, while keeping its strong Brønsted basicity. These features qualify the ethylene‐bridged compound as a powerful alternative to Huenig's base, diisopropyl‐ethyl‐amine. The change from an ethylene to a propylene bridge switches back on the coordination ability. It is also possible to introduce a methyl group to the central C atom of the propylene bridge without loss of nucleophilic properties. The analysis, based on a variety of metal complexes, identifies three structural parameters to assay the structural restraint. Macrocyclic tetraguanidines are also formed.

One carbon makes the difference; a diguandine with an ethylene bridge does not coordinate metals, while a diguanidine with a propylene bridge coordinates to a variety of metals. In this work, we report the synthesis and coordination chemistry of the first cyclic diguanidino‐benzene ligands and compare their coordination chemistry with that of open diguanidino‐benzene ligands.