Non-photochemical synthesis of Re(diimine)(CO)2(L)Cl (L = phosphine or phosphite) compounds

Anticancer and Antibiotic Rhenium Tri- and Dicarbonyl Complexes: Current Research and Future Perspectives

Organometallic compounds are increasingly recognized as promising anticancer and antibiotic drug candidates. Among the transition metal ions investigated for these purposes, rhenium occupies a special role. Its tri- and dicarbonyl complexes, in particular, attract continuous attention due to their relative ease of preparation, stability and unique photophysical and luminescent properties that allow the combination of diagnostic and therapeutic purposes, thereby permitting, e.g., molecules to be tracked within cells. In this review, we discuss the anticancer and antibiotic properties of rhenium tri- and dicarbonyl complexes described in the last seven years, mainly in terms of their structural variations and in vitro efficacy. Given the abundant literature available, the focus is initially directed on tricarbonyl complexes of rhenium. Dicarbonyl species of the metal ion, which are slowly gaining momentum, are discussed in the second part in terms of future perspective for the possible developments in the field.

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.

In-Depth Study of the Electronic Properties of NIR-Emissive κ3N Terpyridine Rhenium(I) Dicarbonyl Complexes

The structure-properties relationship in a series of carbonyl rhenium(I) complexes based on substituted terpyridine ligands of general formula [Re(κ^xN-Rtpy)(CO)_yL]ⁿ⁺ is explored by both experimental and theoretical methods. In these compounds, the terpyridine ligands adopt both bidentate (κ²N) and terdentate (κ³N) coordination modes associated with three or two carbonyls, respectively. Conversion from the κ²N to the κ³N coordination mode leads to large changes in the absorption spectra and oxidation potentials due to destabilization of the HOMO level of each complex. The absorption profiles of the κ³N complexes cover the whole visible spectra with lower maxima around 700 nm, tailing out to 800 nm, while no emission is observed with Br^- as the axial ligand L. When the axial ligand is modified from the native halide to pyridine or triphenylphosphine, the lowest absorption band is blue-shifted by 60 and 90 nm, respectively. These cationic complexes are near-infrared emitters with emission maxima between 840 and 950 nm for the pyridine compounds and 780-800 nm for the triphenylphosphine compounds.

Targeted Synthesis of NIR Luminescent Rhenium Diimine cis,trans-[Re( N N )(CO)2 (L)2 ]n+ Complexes Containing N-Donor Axial Ligands: Photophysical, Electrochemical, and Theoretical Studies

The combined action of ultraviolet irradiation and microwave heating onto acetonitrile solution of [Re( N N )(CO)₃ (NCMe)]OTf ( N N =phenantroline and neocuproine) afforded cis,trans-Re( N N )(CO)₂ (NCMe)₂ ]⁺ acetonitrile derivatives. Substitution of relatively labile NCMe with a series of aromatic N-donor ligands (pyridine, pyrazine, 4,4'-bipyridine, N-methyl-4,4'-bipyridine) gave a novel family of the diimine cis,trans-[Re( N N )(CO)₂ (L)₂ ]⁺ complexes. Photophysical studies of the obtained compounds in solution revealed unusually high absorption across the visible region and NIR phosphorescence with emission band maxima ranging from 711 to 805 nm. The nature of emissive excited states was studied using DFT calculations to show dominant contribution of ³ MLCT (dπ(Re)→π*( N N )) character. Electrochemical (CV and DPV) studies of the monocationic diimine complexes revealed one reduction and one oxidation wave assigned to reduction of the diimine moiety and oxidation of the rhenium center, respectively.

Aquation and Anation Kinetics of Rhenium(I) Dicarbonyl Complexes: Relation to Cell Toxicity and Bioavailability

The aquation reactions of four rhenium(I) dicarbonyl complexes, [Re(CO)₂(NN)(PR₃)(Cl)], where NN = 1,10-phenanthroline (Phen) and 2,9-dimethyl-1,10-phenanthroline (DMPhen) and PR₃ = 1,3,5-triaza-7-phosphaadamantane (PTA) and 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA). Additionally, the anation reactions of the corresponding aqua complexes with Cl^- were investigated. Single crystals of [Re(CO)₂(DMPhen)(PTA)(Cl)]·DMF and [Re(CO)₂(DMPhen)(DAPTA)(Cl)] were obtained, and their structures were determined using X-ray diffraction. The Re-Cl interatomic distances are 2.4991(13) and 2.4922(6) Å, respectively, indicating a mild trans influence effect of the phosphine ligands. The rate constants, k_aq, for the aquation reactions of these complexes spanned a range of (3.7 ± 0.3) × 10^-4 to (15.7 ± 0.3) × 10^-4 s^-1 with the two Phen complexes having rate constants that are 2.5 times greater than those of the DMPhen complexes at 298 K. Similarly, the second-order anation rate constants (k_Cl) of the resulting aqua complexes, [Re(CO)₂(NN)(PR₃)(H₂O)]⁺, with Cl^- ions at 298 K varied between (2.99 ± 0.05) × 10^-3 and (6.79 ± 0.09) × 10^-3 M^-1 s^-1. Likewise, these rate constants for the Phen complexes were almost 2 times faster than those of the DMPhen complexes. The pK_a values of the four aqua complexes were determined to be greater than 9.0 for all of the complexes with [Re(CO)₂(Phen)(PTA)(H₂O)]⁺ having the highest pK_a value of 9.28 ± 0.03. From the pK_a values and the ratios of the aquation and anation rate contants, which give thermodynamic Cl^- binding constants, the speciation of the rhenium(I) complexes in blood plasma, the cytoplasm, and the cell nucleus were estimated. The data suggest that the aqua complexes would be the dominant species in all three environments. This result may have important implications on the potential biological activity of these complexes.

Inter-ligand intramolecular through-space anisotropic shielding in a series of manganese carbonyl phosphorous compounds

Eight novel manganese carbonyl complexes of the type [Mn(bpy-tBu)(CO)3PR3]+ (bpy-tBu = 4,4'-di-tert-butyl-2,2'-bipyridine; R = Cy, nBu, Me, p-tol, Ph, p-F-Ph, OEt, and OMe), have been synthesized and characterized by 1H NMR, FTIR, UV/Vis, HRMS and CV. X-ray crystallographic structures of [Mn(bpy-tBu)(CO)3(PCy3)]+ and [Mn(bpy-tBu)(CO)3(PPh3)]+ were obtained. The short Mn-P bond length allows for close proximity of the bipyridine ligand and the phosphine R groups, resulting in strong anisotropic shielding of certain bipyridine protons by aryl R groups (reordering the bipyridine 1H NMR pattern in the most extreme case). Electrochemical analysis of the compound series reveals that while each is a competent precatalyst for electrochemical carbon dioxide reduction (to carbon monoxide), the lability of the PR3 ligand results in similar catalytic performance amongst the series.

Photoactivated in Vitro Anticancer Activity of Rhenium(I) Tricarbonyl Complexes Bearing Water-Soluble Phosphines

Fifteen water-soluble rhenium compounds of the general formula [Re(CO)3(NN)(PR3)]+, where NN is a diimine ligand and PR3 is 1,3,5-triaza-7-phosphaadamantane (PTA), tris(hydroxymethyl)phosphine (THP), or 1,4-diacetyl-1,3,7-triaza-5-phosphabicylco[3.3.1]nonane (DAPTA), were synthesized and characterized by multinuclear NMR spectroscopy, IR spectroscopy, and X-ray crystallography. The complexes bearing the THP and DAPTA ligands exhibit triplet-based luminescence in air-equilibrated aqueous solutions with quantum yields ranging from 3.4 to 11.5%. Furthermore, the THP and DAPTA complexes undergo photosubstitution of a CO ligand upon irradiation with 365 nm light with quantum yields ranging from 1.1 to 5.5% and sensitize the formation of 1O2 with quantum yields as high as 70%. In contrast, all of the complexes bearing the PTA ligand are nonemissive and do not undergo photosubstitution upon irradiation with 365 nm light. These compounds were evaluated as photoactivated anticancer agents in human cervical (HeLa), ovarian (A2780), and cisplatin-resistant ovarian (A2780CP70) cancer cell lines. All of the complexes bearing THP and DAPTA exhibited a cytotoxic response upon irradiation with minimal toxicity in the absence of light. Notably, the complex with DAPTA and 1,10-phenanthroline gave rise to an IC50 value of 6 μM in HeLa cells upon irradiation, rendering it the most phototoxic compound in this library. The nature of the photoinduced cytotoxicity of this compound was explored in further detail. These data indicate that the phototoxic response may result from the release of both CO and the rhenium-containing photoproduct, as well as the production of 1O2.