# How Thermodynamic, Electronic, and Steric Factors Influence Mesitylcopper Oligomers

## TL;DR

This paper explains how mesitylcopper forms stable clusters by studying the balance of electronic, thermodynamic, and steric effects using computer simulations.

## Contribution

The study reveals the electronic driving force and optimal balance of factors leading to stable mesitylcopper oligomers.

## Key findings

- Strong electronic interactions drive aggregation in mesitylcopper oligomers with n ≥ 3.
- Midsized oligomers (n = 4–5) balance electronic, steric, and entropic factors optimally.
- Mesityl groups act as bridging ligands in oligomers larger than dimers.

## Abstract

Mesitylcopper (CuMes) is a highly versatile organocopper
reagent
used in both organic and inorganic syntheses. It has previously been
shown that CuMes exists as a tetrameric or pentameric cyclic oligomer
[CuMes]
n
(n = 4, 5),
both in solution and in the solid state. The bonding arrangement between
the [CuMes] units has qualitatively been described as localized three-center
two-electron (3c-2e) bonds. However, the electronic, structural, and
thermodynamic forces driving this aggregation are still not well understood.
For this reason, we employed density functional theory (DFT) calculations
to study mesitylcopper as a monomeric [CuMes] unit and [CuMes]
n
oligomers with n = 2 to n = 7. We found that there is a strong electronic driving
force for aggregation caused by strong mixing between the Cu’s d orbitals and Mes’s π orbitals in oligomers
larger than the dimer. This mixing is only optimized in oligomers
with n ≥ 3, where the mesityl group is no
longer bonded to a single copper center but instead becomes a bridging
ligand. Beyond the trimer, steric and entropic factors become relevant
for determining the relative stabilities of the different aggregates,
with midsized oligomers (n = 4–5) having the
optimal balance between the electronic Cu–C bonding character,
Cu···Cu attractive forces, entropy, reduced internal
ring strain, and reduced steric interactions between the mesityl groups.