Bojana G. Jeliazkova

Charge-transfer photochemistry of copper(II) dithiocarbamate mixed-ligand complexes

Abstract The photoredox behaviour of copper(II) diethyldithiocarbamate mixed-ligand complexes, Cu(Et 2 dtc)X (X = Cl, ClO 4 , NO 3 ), is characterized by strongly allowed ligand-to-metal charge transfer (LMCT) transition in the visible spectra. The reaction from the lowest LMCT state is reduction of copper(II) to copper(I) proceeding via an intramolecular electron transfer from an equatorially bound sulfur atom of the ligand to copper(II). Irradiation of Cu II (Et 2 dtc) + …NO 3 − and Cu II (Et 2 dtc) + …ClO 4 − in the CT band in CHCl 3 -iso-pro (1:1) solutions gave rise to Cu II (Et 2 dtc)Cl as an intermediate. Final reaction products in all systems were CuCl 2 and tetraethylthiuramdisulfide. The mechanism of the primary photochemical reaction is discussed as well as the effect of the anion and the solvent on the transformation of the primary photoproduct. The assignments are supported by EPR data and UV-vis absorption spectra of the complexes.

A calorimetric study ofN,N-dimethylformamide complexes of copper(II) in acetonitrile

Complex formation of copper(II) with N,N-dimethylformamide(DMF) has been investigated calorimetrically in acetonitrile at 25°C. Calorimetric titration curves obtained are explained in terms of formation of [Cu(dmf)n]2+ (n=1–4, 6) and their formation constants, enthalpies and entropies were determined. Formation of [Cu(dmf)5]2+ is uncertain. The stepwise enthalpies ΔS30 and entropies ΔSn0 at each consecutive step are all negative except for ΔS30. The overall enthalpies of formation of [Cu(dmf)6]2+ is −(77.8±5.4) kJ-mol−1, which is compared with the enthalpy of transfer of copper(II) ion, ΔHto=−79.7 kJ-mol−1, from acetonitrile to DMF.

Effect of alcohols on the photochemical reaction of bis(N,N′-diethyldithiocarbamato)copper(II) in CHCl3

Abstract Bis( N , N ′-diethyldithiocarbamato)copper( II ) (Cu(Et 2 dte) 2 is initially photoreduced to Cu 1 within its donor-acceptor (DA) complex with CHCl 3 and the quantum yields obtained are independent of the composition of the CHCl 3 EtOH solvent mixture. Two intermediate copper(II) complexes are subsequently formed in equilibrium, in a dark reaction of the primary photoproduct with CHCl 3 , when Cu(ET 2 dtc) 2 photolysis is carried out in CHCl 3 EtOH (150:1 to 50:1). These are the monomeric species Cu II (Et 2 dtc)Cl and its chloride-bridged dimeric form Cu 2 (Et 2 dtc) 2 Cl 2 . The latter is the only intermediate Cu II complex obtained in pure CHCl 3 . A reaction mechanism is proposed and the role of alcohol is discussed from a combined analysis of spectrophotometric and electron paramagnetic resonance (EPR) data.

Effect of solvent and remote ligand substituents on the photochemical behaviour of copper(II) dithiocarbamates and dithiophosphates

The photochemical properties of bis(dithiocarbamato)CuII, Cu(R1dtc)2, and bis(dithiophosphato)CuII, Cu(R22dtp)2, complexes with different remote ligand substituents (R1 = piperidine, morpholine, pyrrolidine and 4-phenylpiperazine; R2 = Me, Et and i-Pr) have been studied in chloromethanes (CCl4, CHCl3, CH2Cl2), chloromethanes/EtOH and PhMe. The monomeric species CuII(R1dtc)Cl and its chloride-bridged dimeric form Cu2(R1dtc)2Cl2 were subsequently obtained during Cu(R1dtc)2 photolysis in chloromethane/EtOH mixtures and the steady-state concentration of Cu2(R1dtc)2Cl2 was found to depend on the EtOH content in the mixed solvents as well as on the nature of R1 and the oxidising ability of the chloromethane. The appearance of the mixed-ligand complex CuII(R22dtp)Cl has been observed as an intermediate photoproduct after longer u.v.-irradiation of Cu(R22dtp)2 in chloromethanes/EtOH.

Charge-transfer photochemistry of the ternary complex (dithiocarbamato, dithiophosphato)copper(II)

Abstract Photolysis of the ternary system consisting of diethyldiselenocarbamate (Et 2 dsc), diisopropyldithiophosphate ( i -Pr 2 dtp) and copper(II) (1:1:1) has been studied in toluene, CH 2 Cl 2 , CHCl 3 , CCl 4 and chloroalkane–EtOH solutions. The results obtained by EPR techniques and UV–Vis data indicate that a homolytic CuS bond cleavage involving dtp ligand appears as the primary photoprocess in Cu(dsc)(dtp) photolysis. Further conversion of the primary photoproduct Cu I (Et 2 dsc) is discussed in terms of specific interaction with the solvent. According to quantum yield data solvent efficiencies in photolysis could be related to their oxidizing power. A reaction mechanism is proposed well fitting the experimental results.

Thermodynamics of ligand exchange reactions between bis(dithiocarbamato)copper(II) and copper(II) salts. An EPR study.

EPR spectroscopy was chosen to investigate the ligand exchange reactions between copper(II) bis(dithiocarbamate), Cu(dtc)2, and copper(II) salts which proceeds with the formation of mixed-ligand complexes of the type Cu(dtc)X, where X = Cl, NO3, ClO4. Large concentrations of 1:1 mixed-ligand complexes of this type are obtained as indicated by the EPR spectra of acetone, CHCl3/EtOH, CHCl3/i-PrOH, CCl4/EtOH and CCl4/i-PrOH, solutions of Cu(dtc)2 and the appropriate copper(II) salt CuCl2, Cu(NO3)2 or Cu(ClO4)2. Double integration of Cu(dtc)2 EPR signals obtained at temperatures between 240 and 310 K affords the calculation of the equilibrium constant (K) of the reaction: Cu(dtc)2 + CuX2 <==> 2 Cu(dtc)X in all solvents as a function of T. From the values of K the stability constant beta of the mixed-ligand complexes has been derived. The error associated with the calculated stability constant is +/- 10%. Thermodynamic parameters (deltaH0, deltaG0 and deltaS0) are determined from the temperature dependence of K as measured by EPR spectroscopy.

Thermal and photochemical reactions of copper(II) dithiocarbamate mixed-ligand complexes with chloroalkanes

Abstract Thermal and/or photochemical conversion of a series of copper(II) complexes containing mixed dithiocarbamato–alcohol ligands, Cu II (Et 2 dtc) + . . . Y − (Y=ClO 4 − , NO 3 − ), into Cu II (Et 2 dtc)Cl in chloroalkane/alcohol solutions, where chloroalkane=CCl 4 , CHCl 3 or CH 2 Cl 2 and alcohol=MeOH, EtOH or i -BuOH, proceeds in chloroalkanes. Both reactions follow a similar pathway which is more effective the stronger the acceptor properties of chloroalkanes and the weaker the coordinating abilities of alcohols are. A detailed reaction mechanism is proposed which well fits the experimental results obtained by EPR and UV–VIS spectra and quantum yields.

Charge-transfer photochemistry of the ternary complex (dithio-diseleno-carbamato)copper(II)

Photolysis of the ternary system consisting of diethyldithiocarbamate (Et2dtc), diethyldiselenocarbamate (Et2dsc) and copper(II) (1:1:1) has been studied in isobutylmethylketone (IBMK), toluene, chloromethane and chloromethane/ROH solutions (chloromethane = CCl4, CHCl3 or CH2Cl2 and ROH = EtOH or i-PrOH). The results obtained by EPR techniques and UV-Vis data indicate that a homolytic Cu-S bond cleavage involving the dithiocarbamate (dtc) ligand appears as the primary photo-process in Cu(Et2dtc)(Et2dsc) photolysis. Further conversion of the primary photoproduct Cu(I)(Et2dsc) is discussed in terms of a specific interaction with the solvent. In chloromethanes and chloromethane/ROH Cu(I)(Et2dsc) is oxidised by the solvent to give the corresponding paramagnetic mixed-ligand Cu(II)(Et2dsc)Cl complex and/or its chloride-bridged and EPR silent dimer Cu2(Et2dsc)2Cl2. The formation of the monomeric species occurs through a co-ordination of the alcohol molecule in the xy plane of the complex. Because of its co-ordination inertness, toluene poorly stabilises the primary photoproduct Cu(I)(Et2dsc), thus providing an effective primary recombination process and lower efficiency of Cu(Et2dtc)(Et2dsc) photolysis. The formation of the bis-solvated mixed-ligand complex Cu(II)(Et2dsc)+ in IBMK is also discussed.

Charge-transfer photochemistry of bis(diethyl-diselenocarbamato)copper(II).

The photochemical reactions of bis(diethyl-diselenocarbamato)copper(II), Cu(Et2dsc)2, complex have been studied in toluene, CH2Cl2, CHCl3 and chloroalkane/EtOH mixed solvents. Charge-transfer irradiation induces intramolecular oxidation of the ligand and reduction of copper(II) to copper(I) as evidenced by EPR and UV-Vis spectra of the complex as well as quantum yield results. When photolysis is carried out in CHCl3 or CH2Cl2 or in the solvent mixture CHCl3/EtOH resp. CH2Cl2/EtOH of lower than 1:1 EtOH content, the primary photoproduct CuI(Et2dsc) is further oxidised in a dark reaction with the chloroalkane producing the corresponding paramagnetic mixed-ligand CuII(Et2dsc)Cl complex in equilibrium with its chloride-bridged and EPR silent, dimeric form Cu2(Et2dsc)2Cl2. At low concentration of EtOH the equilibrium is shifted to the dimeric form whereas at higher than 1:1 EtOH content in the mixed solvent CHCl3/EtOH it is shifted to CuII(Et2dsc)Cl. A reaction mechanism is proposed and the role of ethanol is discussed.

EPR STUDY ON THE CHARGE-TRANSFER PHOTOCHEMISTRY OF THE BROMIDE MIXED-LIGAND DITHIOCARBAMATE COMPLEX OF COPPER(II)

Abstract The EPR technique has been used to study the photolysis of the mixed-ligand complex Cu II (Et 2 dtc)Br in a 1:1 solvent mixture of chloroalkane and alcohol, where the chloroalkane is CCl 4 , CHCl 3 or CH 2 Cl 2 and the alcohol is MeOH, EtOH, i -PrOH or i -BuOH, in comparison with Cu II (Et 2 dtc)Cl photolysis in CHBr 3 :ROH. It was found that while Cu II (Et 2 dtc)Br photolysis in chloroalkane:ROH yielded Cu II (Et 2 dtc)Cl as an intermediate, the opposite conversion of Cu II (Et 2 dtc)Cl to Cu II (Et 2 dtc)Br proceeded via Cu II (Et 2 dtc)Cl photolysis in CHBr 3 :ROH. The final photolytic products in both cases were tetraethylthiuramdisulphide and the corresponding copper(II) salt (CuCl 2 or CuBr 2 , respectively). The results obtained by EPR allowed to get some insight into the behaviour of the primary photolytic products towards both components of the mixed solvent.