# Trickier than It Looks: Isomerization between Five- and Six-Coordinated Zinc in Heterometallic Li2Zn2 Molecule

## TL;DR

This paper reports the synthesis and characterization of two isomers of a heterometallic Li2Zn2 molecule, showing how solvent choice affects their coordination and solubility.

## Contribution

The study reveals a reversible isomerization between five- and six-coordinated zinc in a heterometallic molecule depending on the solvent.

## Key findings

- 6-coordinated zinc isomer forms in acetone with high yield.
- 5-coordinated zinc isomer is obtained in ethanol and has low solubility in alkanes and haloalkanes.
- Isomerization between the two is reversible and solvent-dependent.

## Abstract

This report describes the synthesis and characterization
of two
heterobimetallic Li–Zn coordination isomers
[Li2Zn2(tbaoac)6] (tbaoac = tert-butyl acetoacetato) that have been isolated separately
by the same stoichiometric reaction run in different organic solvents.
The 6-coordinated zinc isomer (6-Zn) was synthesized
in acetone with high yield, while the 5-coordinated one (5-Zn) was readily obtained from ethanol. The 5-Zn isomer
has a low solubility in organic solvents such as alkanes and haloalkanes,
while its 6-Zn counterpart exhibits a good solubility
in almost all common solvents. Two isomeric molecules feature similar
centrosymmetric tetranuclear cyclic assemblies, which are different
in their arrangement of tbaoac ligands. While all ligands act as μ2-type in the structure of 5-Zn, the two tbaoac
groups chelating Li appear as μ3-type in 6-Zn, thus providing an additional coordination for Zn ions. However,
the real structural transformation between these isomers was shown
to be more complex than simply making or breaking a couple of Zn–O
bonds. X-ray single-crystal structure analysis, powder X-ray diffraction,
multinuclear NMR, DART mass spectrometry, ICP-OES analysis, and TGA
have been employed for the characterization of the isomers. The combination
of powder X-ray diffraction and 1H NMR investigation revealed
that 6-Zn isomer can be quantitatively transformed to 5-Zn in ethanol, while the reverse conversion instantly takes
place in acetone.

Two coordination
isomers of the heterometallic tetranuclear
cyclic assembly [Li2Zn2(tbaoac)6]
(tbaoac = tert-butyl acetoacetato) have been isolated.
It has been found that 6-coordinated Zn can quantitatively isomerize
to 5-coordinated Zn in ethanol, while the latter rearranges back in
acetone.