Search

This work describes the preparation of a new thiosemicarbazone derivative, (Z)-N-ethyl-2-(6-oxo-1,10-phenanthrolin-5(6H)-ylidene)hydrazinecarbothioamide (phet) and its respective Re(i) tricarbonyl chloro complex, fac-[ReCl(CO)3(phet)]. The spectroscopic, photophysical and electrochemical properties of the new complex were fully investigated through steady state and time-resolved techniques along with computational calculations. In fac-[ReCl(CO)3(phet)], the new ligand is coordinated to the metal center through the pyridyl rings of the phenanthroline moiety. The unbound electron pairs in the S atom of the bending thiosemicarbazone group induce new low energy lying electronic transitions. Consequently, enhanced visible light absorption up to 550 nm is observed in acetonitrile due to the overlap between MLCTRe→phet and ILphet(n→π*) transitions. The absorption bands and emission quantum yields of fac-[ReCl(CO)3(phet)] are sensitive to proton concentration due to an acid-basic equilibrium in the N atoms of the thiosemicarbazone. Proton dissociation constants of 10.0 ± 0.1 and 11.4 ± 0.2 were determined respectively for the ground and excited states of the new complex. Spectral changes could also be observed in the presence of Zn2+ cations which can be further explored for sensing applications. The electrochemical behavior of the new complex was studied in detail, revealing up to four one electron reduction processes in the range from 0 to -2.4 V vs. Fc+/Fc. With support of DFT calculations, the first three processes are ascribed to the reduction of the coordinated phet ligand followed by the ReI/0 reduction and consequent Cl- release. The new complex was able to act as an electrocatalyst for CO2 reduction into CO (Eonset = -1.92 V vs. Fc+/Fc), with a turnover frequency of 2.81 s-1 and turnover number of 24 ± 1 in anhydrous acetonitrile, being the first Re(i) tricarbonyl complex with a thiosemicarbazone derivative described for this goal. The detailed characterization carried out here can drive the development of new Re(i)-thiosemicarbazone derivatives for different applications.

Reactions of thiosemicarbazide ligands (LR: R = cyclohexyl (Cy) or phenyl (Ph)) with [CuCl(PPh3)3] led to the formation of colorless Cu(I) complexes of composition [CuCl(PPh3)2(LR)]. Both complexes were characterized by spectroscopic methods and further studied by single crystal X-ray diffractometry. The crystal structures confirmed the tetrahedral geometry for the Cu(I) metal center, which is coordinated by a chlorido, two triphenylphosphane and by the thiosemicarbazide as a neutral S-donor monodentate ligand. Photophysical studies revealed that the complexes exhibit emission at room temperature with maxima around 480 nm. At 77 K, the emission is shifted to higher energy, a characteristic behavior of MLCT emitters. The main low lying molecular orbitals were calculated by TD-DFT and confirmed that the S0 → S1 transitions have mainly MLCT character as well as that the peripheral groups in the thiosemicarbazide ligand play an important role on this transition. Moreover, the emission bands are considerably suppressed in the presence of O2, indicating the triplet character of the emitter excited states. The intersystem crossing process is also suggested by the energy diagrams built through TD-DFT calculations. Overall, both experimental and theoretical analysis suggest 3MLCT radiative decays occurrence for the complexes. Furthermore, biological assays showed that the complexes are active on the intracellular amastigote form of Trypanosoma cruzi (Tulahuen Lac-Z strain), and present a significant cytotoxic effect against metastatic melanoma cells. Finally, no significant toxicity to LLC-MK2 cells was observed, resulting in a therapeutic index of about 30 for one of the complexes, in the same order of magnitude as that of the standard drug benznidazole.

In the present work the influence of the distance of the protonatable site of different ancillary ligands to the metal center on the luminescence quenching of ReI polypyridyl complexes by hydroquinone are evaluated by means of experimental and theoretical studies. In these systems, it is expected the occurrence of proton-coupled electron transfer (PCET) reactions upon excitation, which is a key process in solar-to-fuels energy conversion. The series fac-[Re(CO)3(2,2-bpy)(L)]PF6, L = pyridine, 1,4-pyrazine, 4,4'-bipyridyl, 1,2-bis-(4-pyridyl)ethane were synthesized and the luminescence quenching rate constant (kq) by hydroquinone in CH3CN and 1:1 CH3CN/H2O were determined by steady-state and lifetime measurements. In bare acetonitrile, the 1,4-pyrazine exhibits the higher kq (3.49 ± 0.02) × 109 L mol-1 s-1 among the species investigated, followed by 4,4'-bipyridyl (kq = 2.50 ± 0.02) × 109 L mol-1 s-1. In 1:1 CH3CN/H2O, the kq values for all complexes are very similar evidencing the role of water molecules as proton acceptor following the reductive quenching of the complexes by hydroquinone. In CH3CN, the proton release for the solvent is not spontaneous and the higher basicity of the coordinated 1,4-pyrazine and 4,4'-bipyridyl in relation to 1,2-bis-(4-pyridyl)ethane after metal-to-ligand charge transfer (MLCT) excitation contributes to the proton transfer step. These results are corroborated by time-dependent density functional theory (TD-DFT) calculations. Moreover, the low H/D kinetic isotope effect (KIE) in 3:1 CH3CN/X2O (X = H or D) confirms that the major PCET pathway is the electron transfer followed by proton transfer, but for 1,4-pyrazine and 4,4'-bipyridyl the concerted proton-electron transfer seems to play a role at high hydroquinone concentrations.