Photoluminescence and structural evaluation of O,O’ and N,N’ bidentate ligands and its Re(I) tricarbonyl complexes
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Date
2021-03
Authors
Nkoe, Pheello Isaac
Journal Title
Journal ISSN
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Publisher
University of the Free State
Abstract
Despite the increase in research efforts, cancer remains amongst the leading causes of death worldwide. The survival rates vary with the type and stage of the tumour, including the sex, age and socioeconomic situation of each patient. Cancer treatment depends on the location, type and stage of the tumour, but the majority of patients undergo chemotherapy either as the main treatment or in combination with surgery or radiotherapy. Over 50 % of tumours are treated with platinum-based drugs alone or co-administered with other chemotherapeutic agents. In the past decades, many organometallic complexes have been confirmed as extremely promising anti-cancer drug candidates. Organometallic complexes provide structural and stereochemical variety, the possibility of rational ligand design and diverse mechanisms of action. The cytotoxicity of organometallic complexes and their potential are also reported in literature. The potential cytotoxicity of rhenium(I) tricarbonyl complexes are evident from literature. The Rhenium(I) tricarbonyl core possesses intrinsic properties advantageous for the development of novel anti-cancer candidates. Rhenium(I) tricarbonyl complexes are good luminescence probes with long-lived emission states, high quantum yields, large Stokes shifts and emissions that can be tuned by varying the ligands. One of the aims of this study was to investigate the ability of the synthesized O,O’- bidentate ligands and the chosen N,N’- and O,O’- bidentate ligands to coordinate to the fac-[Re(CO)3]+ core. The chosen ligands include: 2,2,2-trifluoro-1-(3-hydroxy-1H-inden-2-yl)ethan-1-one (TIFH), 2,2,2-trifluoro-1-(3-hydroxy-1-methyl-1H-inden-2-yl)ethan-1-one (MeTIFH), 2,2,2-trifluoro-1-(3-hydroxy-1-phenyl-1H-inden-2-yl)ethan-1-one (PhTIFH), 1,3-bis(3,5-dimethylphenyl)propane-1,3-dione (3,5-DiMePh), 2-methyl-1,3-diphenylpropane-1,3-dione (MeDiPhPr), 2-bromo-1,3-diphenylpropane-1,3-dione (BrDiPhPr), benzoic anhydride (BAHY), 1,3-bis(4-methoxyphenyl)propane-1,3-dione (4-MeTPh), 4,4'-dimethyl-2,2'-bipyridine (DiMePy) and 4,4'-dimethoxy-2,2'-bipyridine (DiMeOPy). The ligands and complexes were characterized by IR, UV/Vis and NMR (1H and 13C). Crystal structures of the following were obtained: 4-MeTPh, fac [Re(CO)3(Br)(DiMePy)], fac-[Re(CO)3(Act)(DiMePy)][NO3], fac- [Re(CO)3(NO3)(DiMePy)], fac-[Re(CO)3(Br)(DiMeOPy)], and fac- [Re(CO)3(NO3)(DiMeOPy)]. The lability and mechanism of substitution of the methanol ligands by simple monodentate nucleophiles were investigated by the methanol substitution reaction of fac-[Re(CO)3(MeOH)(DiMePy)]+, fac [Re(CO)3(MeOH)(DiMeOPy)]+, fac- [Re(CO)3(MeOH)(4-MeTPh)], fac-[Re(CO)3(MeOH)(BrDiPhPr)], fac- [Re(CO)3(MeOH)(TIF)] and fac-[Re(CO)3(MeOH)(MeTIF)] with a variety of monodentate ligands (thiourea, sodium thiocyanate, triphenylphosphine or tricyclohexylphosphine and sodium bromide). A variation is seen in the ΔS‡ values, ranging from small negative (-77 ± 4 JK-1mol-1) to small positive (94 ± 8 JK-1mol-1), pointing towards an interchange associative to an interchange dissociative type of activation. A large difference is mostly seen in the two ΔS‡ values, calculated from the Eyring equation and the global fit, emphasizing the large errors in the ΔS‡ values, highlighting the need for high pressure studies to unambiguously determine the mechanism of the reaction. The photoluminescent properties of three ligand systems (TIFH, MeTIFH and PhTIFH) and twenty two Re(I) complexes were determined. The excitation wavelength of the synthesized bidentate ligands range from 534 nm to 650 nm and the emission wavelength range from 595 nm to 740 nm. The excitation wavelength of the Re(I) tricarbonyl complexes range from 285 nm to 705 nm and the emission wavelengths range from 450 nm to 740 nm. The Re(I) tricarbonyl complexes with bomido ligands in the sixth position yield higher quantum yields compared to the other Re(I) tricarbonyl complexes. The photoluminescent behaviour of fac- [Re(CO)3(H2O)(DiMePy)]+ and fac-[Re(CO)3(H2O)(DiMeOPy)]+ were further investigated in solution state. Both complexes were excited at 285 nm and gave rise to a maximum emission peak at 610 nm for fac-[Re(CO)3(H2O)(DiMePy)]+ and 625 nm for fac-[Re(CO)3(H2O)(DiMeOPy)]+. The solution state results (when compared to the solid state results), is red-shifted for both the aqua species; and the longer wavelength is potentially more appropriate for model PDT agents than the solid state results. The ligands and complexes investigated in this study gave good results with maximum excitation and emission wavelengths that are in the recommended range of 534 nm to 705 nm and 595 nm to 740 nm respectively and can potentially be used as PDT agents, which should definitely be investigated further.
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Keywords
Thesis (Ph.D. (Chemistry))--University of the Free State, 2021, Rhenium(I), Cancer treatment, Organometallic complexes, Rhenium(I) tricarbonyl complexes