Photoredox behaviour of chlorocopper(II) complexes in acetonitrile: mechanism and quantum yields

Direct Evidence of Visible Light-Induced Homolysis in Chlorobis(2,9-dimethyl-1,10-phenanthroline)copper(II)

Developments in the field of photoredox catalysis that leveraged the long-lived excited states of Ir(III) and Ru(II) photosensitizers to enable radical coupling processes paved the way for explorations of synthetic transformations that would otherwise remain unrealized. While first row transition metal photocatalysts have not been as extensively investigated, valuable synthetic transformations covering broad scopes of olefin functionalization have been recently reported featuring photoactivated chlorobis(phenanthroline) Cu(II) complexes. In this study, the photochemical processes underpinning the catalytic activity of [Cu(dmp)₂Cl]Cl (dmp = 2,9-dimethyl-1,10-phenanthroline) were studied. The combined results from static spectroscopic measurements and conventional photochemistry, ultrafast transient absorption, and electron paramagnetic resonance spin trapping experiments strongly support blue light (λ_ex = 427 or 470 nm)-induced Cu-Cl homolytic bond cleavage in [Cu(dmp)₂Cl]⁺ occurring in <100 fs. On the basis of electronic structure calculations, this bond-breaking photochemistry corresponds to the Cl → Cu(II) ligand-to-metal charge transfer transition, unmasking a Cu(I) species [Cu(dmp)₂]⁺ and a Cl atom, thereby serving as a departure point for both Cu(I)- or Cu(II)-based photoredox transformations. No net photochemistry was observed through direct excitation of the ligand-field transitions in the red (λ_ex = 785 or 800 nm), and all combined experiments indicated no evidence of Cu-Cl bond cleavage under these conditions. The underlying visible light-induced homolysis of a metal-ligand bond yielding a one-electron-reduced photosensitizer and a radical species may form the basis for novel transformations initiated by photoinduced homolysis featuring in situ-formed metal-substrate adducts utilizing first row transition metal complexes.

Removal of Chloride Ions from Strongly Acidic Wastewater Using Cu(0)/Cu(II): Efficiency Enhancement by UV Irradiation and the Mechanism for Chloride Ions Removal

Strongly acidic wastewater, which is usually generated from nonferrous metal smelting industries, has the ability to be recycled as sulfuric acid. Before this wastewater is recycled, the removal of chloride ions is necessary to improve the quality of the recycled sulfuric acid. At present, the widely used method to remove chloride ions from acidic wastewater in the form of CuCl precipitate has several disadvantages, including low removal efficiency, high temperature, long treatment time, and high dosage of Cu(II). This study proposed an improved new method of removing Cl(-I) using Cu(0)/Cu(II) under UV irradiation, and the mechanism was investigated. The Cl(-I) concentration was lowered to below 50 mg/L at a Cu(II) dosage of 1200 mg/L. Under UV irradiation, ligand-to-metal charge transfer takes place, thereby resulting in the formation of Cl^•. Next, CuCl precipitates form through the reaction between Cu(0) and Cl^• and produce h⁺/^•OH under UV irradiation, which can oxidize Cl(-I) to Cl^•. Simultaneously, Cl₂ gas also forms directly from Cl^•. This study offered a theoretical foundation for the application of UV irradiation for the enhanced removal of chloride ions from strongly acidic wastewater.

Solvent Effects on Nonradiative Relaxation Dynamics of Low-Energy Ligand-Field Excited States: A CuCl42- Complex

Nonradiative relaxation dynamics of CuCl₄^2- complexes photoexcited into the highest-energy ligand-field electronic state (²A₁) is studied in acetonitrile, dichloromethane, and chloroform solvents, as well as in acetonitrile-water and in acetonitrile-deuterated water mixtures. Due to ultrafast internal conversion, this excited state directly converts to the electronic ground state in dichloromethane and chloroform. The nonradiative relaxation constant is similar in anhydrous acetonitrile. Addition of water to acetonitrile solutions efficiently quenches the excited ligand-field ²A₁ state. The quenching is proposed to be due to the diffusion-controlled formation of an electronically excited pentacoordinated [CuCl₄H₂O]^2- encounter complex or a short-lived exciplex of similar structure, in which the electronic excitation energy transfers into the O-H stretch of the coordinated H₂O molecule. This is followed by the dissociation of the pentacoordinated species, resulting in the reformation of the ground-state CuCl₄^2- and free H₂O molecules.

Ultrafast Photochemistry of Copper(II) Monochlorocomplexes in Methanol and Acetonitrile by Broadband Deep-UV-to-Near-IR Femtosecond Transient Absorption Spectroscopy

Photochemistry of copper(II) monochlorocomplexes in methanol and acetonitrile solutions is studied by UV-pump/broadband deep-UV-to-near-IR probe femtosecond transient absorption spectroscopy. Upon 255 and 266 nm excitation, the complexes in acetonitrile and methanol, respectively, are promoted to the excited ligand-to-metal charge transfer (LMCT) state, which has a short (sub-250 fs) lifetime. From the LMCT state, the complexes decay via internal conversion to lower-lying ligand field (LF) d-d excited states or the vibrationally hot ground electronic state. A minor fraction of the excited complexes relaxes to the LF electronic excited states, which are relatively long-lived with lifetimes >1 ns. Also, in methanol solutions, about 3% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming copper(I) solvatocomplexes and chlorine atoms, which then further react forming long-lived photoproducts. In acetonitrile, about 50% of the LMCT-excited copper(II) monochlorocomplexes dissociate forming radical and ionic products in a ratio of 3:2. Another minor process observed following excitation only in methanol solutions is the re-equilibration between several forms of the copper(II) ground-state complexes present in solutions. This re-equilibration occurs on a time scale from sub-nanoseconds to nanoseconds.

Mechanism of Formation of Copper(II) Chloro Complexes Revealed by Transient Absorption Spectroscopy and DFT/TDDFT Calculations

Copper(II) complexes are extremely labile with typical ligand exchange rate constants on the order of 10(6)-10(9) M(-1) s(-1). As a result, it is often difficult to identify the actual formation mechanism of these complexes. In this work, using UV-vis transient absorption when probing in a broad time range (20 ps to 8 μs) in conjunction with DFT/TDDFT calculations, we studied the dynamics and underlying reaction mechanisms of the formation of extremely labile copper(II) CuCl4(2-) chloro complexes from copper(II) CuCl3(-) trichloro complexes and chloride ions. These two species, produced via photochemical dissociation of CuCl4(2-) upon 420 nm excitation into the ligand-to-metal-charge-transfer electronic state, are found to recombine into parent complexes with bimolecular rate constants of (9.0 ± 0.1) × 10(7) and (5.3 ± 0.4) × 10(8) M(-1) s(-1) in acetonitrile and dichloromethane, respectively. In dichloromethane, recombination occurs via a simple one-step addition. In acetonitrile, where [CuCl3](-) reacts with the solvent to form a [CuCl3CH3CN](-) complex in less than 20 ps, recombination takes place via ligand exchange described by the associative interchange mechanism that involves a [CuCl4CH3CN](2-) intermediate. In both solvents, the recombination reaction is potential energy controlled.