High-order Cu(II) chloro-complexes in LiCl brines: Insights from density function theory and molecular dynamics

Cu(II) complexation by chloride is relevant to the transport of copper in near-surface geologic environments, yet the existence of high-order Cu(II) chloro-complexes still remains in dispute. In this study, the structure characteristics and stabilities of [CuClx]2−xaq (x = 3, 4, 5) complexes have been investigated using density functional theory (DFT) methods and molecular dynamics (MD) simulations. [CuCl3]− and [CuCl4]2− species can both be tracked, while the [CuCl5]3−aq complex cannot be recorded during MD simulations of trace Cu2+ in chloride-rich brines. DFT calculations indicate that contact ion pair (CIP) conformers of [CuCl3]− species are less stable than its solvent separated ion pair (SSIP) conformers, in which one Cl− stays in the second coordination sphere of the centered Cu2+. MD simulations also reveal that the SSIP conformer is apt to appear in the aqueous solution than its CIP conformer. It seems that the third Cl− is more likely in the second coordination shell of center Cu2+ in [CuCl3]−. Meanwhile, the characteristic peak around 385 nm resolved in UV-Vis spectra experiments, which was attributed to the [CuCl3]− complex, could also be resulted from some SSIP structures of the [CuCl3]− complex. In MD simulations, the complex [CuCl4]2−aq is found more stable than [CuCl3]−aq. The surrounding water molecules around [CuClx]2−xaq (x = 3, 4) enhance their stabilities in Cl− brines, especially for [CuCl4]2−aq. The hydration shell of [CuCl4]2−aq species is more intact than that of [CuCl3]2−aq, and the residence time of a water molecule in the second coordination sphere of Cu ion in the [CuCl4]2−aq complex is also obviously longer than that of [CuCl3]−aq. The [CuCl4]2−aq complex can even be recorded in the less concentrated (6.33 m) Cl− solution, while the [CuCl3]−aq complex is tracked only in the 16.32 m Cl− brine. Meanwhile, the possibilities of [CuCl3]−aq and [CuCl4]2−aq complexes found in 16.32 m Cl− solutions both decrease with increasing temperature, [CuCl3]−aq cannot be recorded during our simulations at 373 K and 423 K, and only few [CuCl4]2−aq at 423 K.