Polymorph and isomer conversion of complexes based on CuI and PPh3 easily observed via luminescence

Abstract

Reactions between copper(I) iodide and triphenylphosphine have been explored in solution and in the solid state and six luminescent coordination complexes have been obtained and characterized by X-ray diffraction and UV-vis spectroscopy and photophysics. Solid-state reactions of CuI with PPh₃ in different conditions (kneading, vapour digestion) and stoichiometries resulted in the formation of high ratio ligand : metal compounds while tetrameric structures could be obtained only by solution reactions. Crystal structures were determined by single crystal X-ray diffraction while purity of the bulk product was checked by powder diffraction (XRPD). Three different tetrameric structures with 1 : 1 stoichiometry have been synthesized: two closed cubane-type polymorphs [CuI(PPh₃)]₄ (form 1a) and [CuI(PPh₃)]₄ (form 1b) and an open step-like isomer [CuI(PPh₃)]₄ (form 2). The conversions between the polymorphs and isomers have been studied and characterized by XRPD. The most stable form [CuI(PPh₃)]₄ (form 1b) can convert into the open step-like isomer [CuI(PPh₃)]₄ (form 2) in a slurry experiment with EtOH or CH₂Cl₂ or AcCN and converts back into [CuI(PPh₃)]₄1b when exposed to vapors of toluene. At room temperature all the tetrameric compounds exhibit luminescence in the solid state and, notably, the two polymorphs show a dissimilar dual emission at low temperature. The luminescence features in the solid state seem to be peculiarly related to the presence of the aromatic phosphine ligand and depend on the Cu–Cu distance in the cluster.