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

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.