Fighting cancer with metals: main focus on Rhemium(I) complexes

Abstract

In recent years, photodynamic therapy (PDT) has aroused significant interest as a potential method of treatment for certain cancers. A non-intrusive treatment, PDT consequently overcomes various obstacles of currently available treatment methods, notably toxicity. However, PDT is not without its limitations. These include, although they are not limited to, low selectivity and an inability to penetrate deep tissues. To overcome such shortcomings, a range of rhenium(I) compounds of the type fac- [Re(CO)3(N,N’-bid(X)]n and cis-trans-[Re(CO)2(N,N’-bid)(X)2]n (n = 0, +1) were synthesised, most of which displayed good yields, ranging from 38.10 to 85.90 % for tricarbonyl and 12.80 to 80.30 % for dicarbonyl complexes, and characterised. Four bimetallic complexes [(Cl2)Pt(O,O’-phenO2-N,N’)-Re(CO)3(Br)], [(Cl2)Pd(O,O’- phenO2-N,N’)-Re(CO)3(Br)], [(Cl2)Pt(O,O’-phenO2-N,N’)-Ru(bpy)2][PF6]2∙2H2O, and [(Cl2)Pd(O,O’-phenO2-N,N’)-Ru(bpy)2][PF6]2 ∙ 2H2O were synthesised and characterised. However, low yields ranging from 10.66 to 45.52 % were obtained. Two 99mTc complexes, cis-trans-[99mTc(CO)2(bpy)(PPh3)2]+ and cis-trans-[99mTc(CO)2(bpy)(PTA)2]+ were synthesised and isolated with retention times of 16.928 and 11.385 minutes respectively. The crystal structure of fac-[Re(CO)3(bpy)(Br)] was obtained and solved and is reported in this study. The rhenium to nitrogen bond distances (Re1-N1, Re1-N2) are reported as 2.171(4) Å and 2.170(4) Å with a bite-angle (N2-Re1-N1) of 74.95(16)°, while the rhenium to bromido bond distance was 2.6126(15) Å. The rhenium to carbonyl carbon bond distances range from 1.912(6) Å to 2.008(7) Å. A distortion of the octahedral sphere is confirmed by the angles N1-Re1-Br1, N2-Re1-Br1, and C1- Re1-Br1 which are 85.66(11)°, 83.50(11)° and 176.16(15)° respectively. A dihedral angle of 7.028(15)° was reported between the equatorial plane and the plane through the 2,2’-bipyridine system. A hydrogen bonding interaction is observed between H6 and Br1, while three π-interactions are observed in fac-[Re(CO)3(bpy)(Br)]. The photoluminescent properties of eleven rhenium(I) compounds were determined and emission wavelengths ranging from 604 to 650 nm were found for Re(I) tricarbonyl complexes and 610 to 670 nm in Re(I) dicarbonyl complexes. The highest emission wavelength was obtained from the water soluble complex cis-trans-[Re(CO)2(bpy)(PTA)2][NO3] in CH3OH solution. This emission was significantly higher than the same complex in aqueous solution, which displayed an emission wavelength of 642 nm. A second water soluble complex, fac-[Re(CO)3(bpy)(PTA)][NO3], was studied in both CH3OH and aqueous solution. This compound showed a similar trend to cis-trans-[Re(CO)2(bpy)(PTA)2][NO3], namely that the emission of these water soluble complexes is higher in CH3OH than in aqueous solution.