Neutral rhenium(I) tricarbonyl complexes with sulfur-donor ligands: anti-proliferative activity and cellular localization

Rhenium(I) tricarbonyl complexes are widely studied for their cell imaging properties and anti-cancer and anti-microbial activities, but the complexes with S-donor ligands remain relatively unexplored. A series of six fac-[Re(NN)(CO)3(SR)] complexes, where (NN) is 2,2'-bipyridyl (bipy) or 1,10-phenanthroline (phen), and RSH is a series of thiocarboxylic acid methyl esters, have been synthesized and characterized. Cellular uptake and anti-proliferative activities of these complexes in human breast cancer cell lines (MDA-MB-231 and MCF-7) were generally lower than those of the previously described fac-[Re(NN)(CO)3(OH2)]+ complexes; however, one of the complexes, fac-[Re(CO)3(phen)(SC(Ph)CH2C(O)OMe)] (3b), was active (IC50 ∼ 10 μM at 72 h treatment) in thiol-depleted MDA-MB-231 cells. Moreover, unlike fac-[Re(CO)3(phen)(OH2)]+, this complex did not lose activity in the presence of extracellular glutathione. Taken together these properties show promise for further development of 3b and its analogues as potential anti-cancer drugs for co-treatment with thiol-depleting agents. Conversely, the stable and non-toxic complex, fac-[Re(bipy)(CO)3(SC(Me)C(O)OMe)] (1a), predominantly localized in the lysosomes of MDA-MB-231 cells, as shown by live cell confocal microscopy (λex = 405 nm, λem = 470-570 nm). It is strongly localized in a subset of lysosomes (25 μM Re, 4 h treatment), as shown by co-localization with a Lysotracker dye. Longer treatment times with 1a (25 μM Re for 48 h) resulted in partial migration of the probe into the mitochondria, as shown by co-localization with a Mitotracker dye. These properties make complex 1a an attractive target for further development as an organelle probe for multimodal imaging, including phosphorescence, carbonyl tag for vibrational spectroscopy, and Re tag for X-ray fluorescence microscopy.

Rhenium(I)-tricarbonyl complexes with methimazole and its selenium analogue: Syntheses, characterization and cell toxicity

This study explores the effect of a thione/selone ligand on the cell toxicity (in vitro) and light activity of diimine Re(CO)₃⁺ complexes. Six rhenium(I) complexes with general formula fac-[Re(CO)₃(N,N')X]⁺ were prepared, where X = 2-mercapto-1-methylimidazole (methimazole; MMI), and 1-methylimidazole-2-selone (MSeI); N,N' = 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen) and 2,9-dimethyl-1,10-phenanthroline (dmphen). Their triflate salts were characterized using single-crystal X-ray diffraction, ¹H, ¹³C and 2D NMR, UV-vis and vibrational spectroscopy. Their cytotoxic properties were tested, showing significant cytotoxicity (IC₅₀ = 8.0-55 μM) towards the human breast cancer cell line MDA-MB-231. The half-inhibitory concentration (IC₅₀) for fac-[Re(CO)₃(dmphen)(MMI)]⁺, the most toxic complex in this series (8.0 ± 0.2 μM), was comparable to that of the corresponding aqua complex fac-[Re(CO)₃(dmphen)(H₂O)]⁺ with IC₅₀ = 6.0 ± 0.1 μM. The fac-[Re(CO)₃(bpy)(MMI/MSeI)]⁺ complexes were somewhat less toxic towards the human embryonic kidney cell line HEK-293 T after 48 h of exposure. The stability of the complexes upon irradiation was monitored using UV-vis spectroscopy, with no CO released when exposed to UV-A light (λ = 365 nm).

Aerobically stable and substitutionally labile α-diimine rhenium dicarbonyl complexes

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes are described. The molecules are prepared in high yield from the cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br₂]⁻ anion (2, where ^tBu₂bpy is 4,4′-di-tert-butyl-2,2′-bipyridine), which can be isolated from the one electron reduction of the corresponding 17-electron complex (1). Compound 2 is stable in the solid state, but in solution it is oxidized by molecular oxygen back to 1. Replacement of a single bromide of 2 by σ-donor monodentate ligands (Ls) yields stable neutral 18-electron cis–cis–trans-[Re(CO)₂(^tBu₂bpy)Br(L)] species. In coordinating solvents like methanol the halide is replaced giving the corresponding solvated cations. [Re(CO)₂(^tBu₂bpy)Br(L)] species can be further reacted with Ls to prepare stable cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes in good yield. Ligand substitution of Re(i) complexes proceeds via pentacoordinate intermediates capable of Berry pseudorotation. In addition to the cis–cis–trans-complexes, cis–cis–cis- (all cis) isomers are also formed. In particular, cis–cis–trans-[Re(CO)₂(^tBu₂bpy)(L)₂]⁺ complexes establish an equilibrium with all cis isomers in solution. The solid state crystal structure of nearly all molecules presented could be elucidated. The molecules adopt a slightly distorted octahedral geometry. In comparison to similar fac-[Re(CO)₃]⁺complexes, Re(i) diacarbonyl species are characterized by a bend (ca. 7°) of the axial ligands towards the α-diimine unit. [Re(CO)₂(^tBu₂bpy)Br₂]⁻ and [Re(CO)₂(^tBu₂bpy)Br(L)] complexes may be considered as synthons for the preparation of a variety of new stable diamagnetic dicarbonyl rhenium cis-[Re(CO)₂]⁺ complexes, offering a convenient entry in the chemistry of the core.

New synthetic routes to aerobically stable and substitutionally labile α-diimine rhenium(i) dicarbonyl complexes offer a convenient entry in the chemistry of the cis-[Re(CO)₂]⁺ core.

The effect of sodium thiosulfate on cytotoxicity of a diimine Re(I) tricarbonyl complex

Recently, diimine Re(i) tricarbonyl complexes have attracted great interest due to their promising cytotoxic effects. Here, we compare the cytotoxicity and cellular uptake of two Re(i) compounds fac-[(Re(CO)3(bpy)(H2O)](CF3SO3) (1) and Na(fac-[(Re(CO)3(bpy)(S2O3)])·H2O (bpy = 2,2'-bipyridine) (2). The Re-thiosulfate complex in 2 was characterized in two solvated crystal structures {Na(fac-[Re(CO)3(bpy)(S2O3)])·1.75H2O·C2H5OH}4 (2 + 0.75H2O + C2H5OH)4 and (fac-[Re(CO)3(bpy)(H2O)]) (fac-[Re(CO)3(bpy)(S2O3)])·4H2O (3). The cytotoxicity of 1 and 2 was tested in the MDA-MB-231 breast cancer cell line and compared with that of cisplatin. The cellular localization of the Re(i) complexes was investigated using synchrotron-based X-ray fluorescence microscopy (XFM). The results show that replacement of the aqua ligand with thiosulfate renders the complex less toxic most likely by distrupting its cellular entry. Therefore, thiosulfate could potentially have a similar chemoprotective effect against diimine fac-Re(CO)3 complexes as it has against cisplatin.

Rhenium versus cadmium: an alternative structure for a thermally stable cadmium carbonyl compound

An alternative description is provided for the previously reported novel tetranuclear cadmium carbonyl compound, [Cd(CO)3(C6H3Cl)]4. Specifically, consideration of single crystal X-ray diffraction data indicates that the compound is better formulated as the rhenium compound, [Re(CO)3(C4N2H3S)]4. Furthermore, density functional theory calculations predict that, if it were to exist, [Cd(CO)3(C6H3Cl)]4 would have a very different structure to that reported. While it is well known that X-ray diffraction may not reliably distinguish between atoms of similar atomic number (e.g. N/C and Cl/S), it is not generally recognized that two atoms with very different atomic numbers could be misassigned. The misidentification of two elements as diverse as Re and Cd (ΔZ = 27) is unexpected and serves as an important caveat for structure determinations.

Coordination driven self-assembly of [2 + 2 + 2] molecular squares: synthesis, crystal structures, catalytic and luminescence properties

Ten molecular squares were prepared through coordination-driven self-assembly and their catalytic activity and luminescence properties are reported.

Photofunctional multinuclear rhenium(i) diimine carbonyl complexes

In this review, we summarize the synthesis, photophysical properties and applications of a wide variety of multinuclear complexes consisting of Re(i)-diimine-carbonyl units.

The First Carbonyl-Substituted Derivative of [Mn2(CO)6(µ-pyS)2]

Reaction of [Mn2(CO)6(μ-pyS)2] (1) with (Ph3P)2Ni(CO)2 at room temperature affords [Mn2(CO)5(PPh3)(μ-pyS)2] (3) which is the first carbonyl-substituted derivative of 1. A mononuclear complex fac-[Mn(CO)3(PPh3)(κ2-pyS)] (4) is also isolated as a minor product in this reaction. The formation of 3 allows us to propose a different mechanism operating in this reaction. A similar reaction between [Re2(CO)6(μ-pyS)2] (2) and (Ph3P)2Ni(CO)2 gives only mononuclear fac-[Re(CO)3(PPh3)(κ2-pyS)] (5). Both 4 and 5 undergo CO substitution to produce [M(CO)2(PPh3)2(κ2-pyS)] (6, M = Mn; 7, M = Re) when treated with (Ph3P)2Ni(CO)2 in boiling THF. Complex 3 also reacts with further (Ph3P)2Ni(CO)2 at room temperature to give 4 and 6. The molecular structures of 3 and 4 have been established by single crystal X-ray diffraction analyses.