Copper can exist in various oxidation states, and the solubility of copper compounds in solvents such as trimethyl citrate can depend on the specific form of the copper used. Generally, soluble copper compounds include copper(II) salts like copper(II) acetate or copper(II) citrate.
Trimethyl citrate, being a citrate ester, may be able to solubilize copper(II) complexes due to its chelating properties and ability to form coordination complexes.