# Mechanochemical Cis/Trans Isomerization of a Metal Centre Involving a Metal‐Organic Halogen‐Bonded (MOXB) Cocrystal

## TL;DR

This paper shows how halogen bonding in a metal-organic cocrystal enables mechanochemical isomerization of a metal complex from cis to trans geometry.

## Contribution

The study introduces a mechanochemical method for cis→trans isomerization enabled by metal-organic halogen-bonded (MOXB) cocrystals.

## Key findings

- Ball-milling with halogen bonding leads to transient MOXB cocrystal formation and cis→trans isomerization.
- Periodic DFT calculations confirm that halogen bonding makes the isomerization more enthalpically favorable.
- MOXB cocrystals enable new responsive behaviors in metal-based systems.

## Abstract

Halogen bonding enables the mechanochemical ball‐milling isomerization of an otherwise persistent cis‐coordinated metal complex into the corresponding trans‐isomer. The importance of halogen bonding for enabling the cis→trans isomerization of the metal centre is evidenced by real‐time in situ synchrotron powder X‐ray diffraction monitoring of the ball‐milling experiments that showed the transient appearance of a cis‐geometry metal‐organic halogen‐bonded (MOXB) cocrystal, which is rapidly replaced by the corresponding trans‐geometry one, with any excess, non‐halogen‐bonded cis‐geometry complex being retained throughout the milling experiment. The importance of cocrystallization for cis
→
trans isomerization is supported by periodic density‐functional theory calculations which show that the process becomes notably more enthalpically favourable in the presence of the halogen bond donor. The presented work indicates that the formation of MOXB cocrystals can open the door to new, metal‐based responsive behaviours, different from those of parent solid‐state coordination complexes.

The presented work demonstrates a cocrystallization‐enabled mechanochemical interconversion of geometrical isomers of a coordination complex, illustrating the use of halogen‐bond‐driven cocrystal formation for generating new metal‐based responsive behaviours.