Rhenium(I) complexes with NNS donor thioarylazoimidazole ligands with the cis-{Re(CO)2}+ core: Synthesis, characterization, electrochemical study and DFT calculation

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.