Bis-Arene Complexes [Re(η6-arene)2]+ as Highly Stable Bioorganometallic Scaffolds

Bifunctional chelators for radiorhenium: past, present and future outlook

Targeted radionuclide therapy (TRNT) is an ever-expanding field of nuclear medicine that provides a personalised approach to cancer treatment while limiting toxicity to normal tissues. It involves the radiolabelling of a biological targeting vector with an appropriate therapeutic radionuclide, often facilitated by the use of a bifunctional chelator (BFC) to stably link the two entities. The radioisotopes of rhenium, 186Re (t 1/2 = 90 h, 1.07 MeV β-, 137 keV γ (9%)) and 188Re (t 1/2 = 16.9 h, 2.12 MeV β-, 155 keV γ (15%)), are particularly attractive for radiotherapy because of their convenient and high-abundance β--particle emissions as well as their imageable γ-emissions and chemical similarity to technetium. As a transition metal element with multiple oxidation states and coordination numbers accessible for complexation, there is great opportunity available when it comes to developing novel BFCs for rhenium. The purpose of this review is to provide a recap on some of the past successes and failings, as well as show some more current efforts in the design of BFCs for 186/188Re. Future use of these radionuclides for radiotherapy depends on their cost-effective availability and this will also be discussed. Finally, bioconjugation strategies for radiolabelling biomolecules with 186/188Re will be touched upon.

Watching Hydrogens Migrate: Step by Step from [ReI(η6-C6H6)2]+ to [ReIII(η3-C6H9)(η6-C6H6)(NCCH3)2]2

Arene substitution reactions in [M(η⁶-arene)₂]^0/2+ are well documented for Groups 6 and 8 but are essentially unknown for the manganese triad. Aiming to replace benzene in [Re^I(η⁶-C₆H₆)₂]⁺, we altered the hapticity of one coordinated benzene, which we found to be tunable stepwise from an η⁶ to an η³-allyl coordination mode. Reduction of [Re^I(η⁶-C₆H₆)₂]⁺ with hydrides gives [Re^I(η⁵-C₆H₇)(η⁶-C₆H₆)]. Subsequent addition of acid yields [Re^IIIH(η⁵-C₆H₇)(η⁶-C₆H₆)]⁺, which converts to [Re^I(η⁴-C₆H₈)(η⁶-C₆H₆)NCCH₃]⁺ in acetonitrile. Further protonation gives the title complex [Re^III(η³-C₆H₉)(η⁶-C₆H₆)(NCCH₃)₂]²⁺ by a rhenium-mediated, intramolecular hydride shift. Herein, we present a full mechanistic elucidation of these transformations based on NMR studies, isolation of reaction intermediates, and their full characterizations. The structural feature {Re^III(η⁶-C₆H₆)} is unprecedented. Direct arene exchange from [Re^I(η⁶-C₆H₆)₂]⁺ to [Re^I(η⁶-arene)(η⁶-C₆H₆)]⁺ was found only under strongly acidic conditions in neat arene. The analogous chemistry of the lighter homologue technetium (⁹⁹Tc) is distinctly different. Treatment of [Tc^I(η⁵-C₆H₇)(η⁶-C₆H₆)] with acid in acetonitrile yields only mixtures of [Tc^I(η⁶-C₆H₆)₂]⁺ and [Tc^II(NCCH₃)₆]²⁺.

An Isoindoline Bridged [M(η6-arene)2]+ (M = Re, 99mTc) ansa-Arenophane and its Dinuclear Macrocycles with Axial Chirality

This work presents a straightforward method for the preparation of an isoindoline bridged [M(arene)2]+ (M = Re, 99mTc) ansa-[3]arenophane. This intramolecular formation of an ansa-complex is accompanied by the intermolecular...

Cationic Niobium-Sandwich and Piano-Stool Complexes

The syntheses of the homoleptic bis(arene) niobium cations [Nb(arene)2 ]+ (arene = C6 H3 Me3 , C6 H5 Me) with 16 valence electrons and heteroleptic arene-carbonyl cations [(CO)Nb(arene)2 ]+ (arene = C6 H3 Me3 , C6 H5 Me) and [(arene)M(CO)4 ]+ (arene = C6 H3 Me3 , C6 H6 ) obeying 18 valence electrons are described. Stabilization of these complexes was achieved by using the weakly coordinating anions [Al(ORF )4 ]- or [F{Al(ORF )3 }2 ]- (RF = C(CF3 )3 ). The limits of two synthesis routes starting from neutral Nb(arene)2 (arene = C6 H3 Me3 , C6 H5 Me) or [NEt4 ][M(CO)6 ] (M = Nb, Ta) were investigated. All compounds were analyzed by single crystal X-ray determination, vibrational and NMR spectroscopy. DFT calculations were executed to support the experimental data.

Dynamic dimer-monomer equilibrium in a cycloruthenated complex of [Re(η6-C6H6)2]. Chem Commun (Camb) 2020;56:10658-10661. [PMID: 32785303 DOI: 10.1039/d0cc04180g]

Cycloruthenation is a well known process in organometallic ruthenium chemistry. In this work, we report unprecedented cycloruthenated rhenium bis-arene compounds with planar chirality. In a two-step process, the reaction of acetyl-pyridine with [Re(η6-C6H6)2]+ introduced a pyridinyl-methanol ligand at one of the arene rings. Coordination of [Ru(CO)2Cl2] led to cycloruthenation, and the products were obtained as two diastereomeric pairs of enantiomers. Under basic pH conditions, the two pairs of enantiomers undergo spontaneous and reversible dimerization. The cycloruthenated monomers were fully characterized, and the dimerization process was studied by NMR, IR spectroscopy, and DFT calculations.

[Re(η6-arene)2]+ as a highly stable ferrocene-like scaffold for ligands and complexes

Ferrocenes are versatile ligand scaffolds, complexes of which have found numerous applications in catalysis. Structurally similar but of higher redox stabilites are sandwich complexes of the [Re(η6-arene)2]+ type. We report herein routes for conjugating potential ligands to a single or to both arenes in this scaffold. Since the arene rings can freely rotate, the [Re(η6-arene)2]+ has a high degree of structural flexibility. Polypyridyl ligands were successfully introduced. The coordination of Co(ii) to such a model tetrapyridyl-Re(i)-bis-benzene complex produced a bimetallic Re(i)-Co(ii) complex. To show the stability of the resulting architecture, a selected complex was subjected to photocatalytic reactions. It showed good activity in proton reduction over a long time and did not decompose, corroborating its extraordinary stability even under light irradiation. Its activity compares well with the parent catalyst in turn over numbers and frequencies. The supply of electrons limits catalytic turnover frequency at concentrations below ∼10 μM. We also show that other ligands can be introduced along these strategies. The great diversity offered by [Re(η6-arene)2]+ sandwich complexes from a synthetic point allows this concept to be extended to other catalytic processes, comparable to ferrocenes.

Expanding medicinal chemistry into 3D space: metallofragments as 3D scaffolds for fragment-based drug discovery

Fragment-based drug discovery (FBDD) is a powerful strategy for the identification of new bioactive molecules. FBDD relies on fragment libraries, generally of modest size, but of high chemical diversity. Although good chemical diversity in FBDD libraries has been achieved in many respects, achieving shape diversity - particularly fragments with three-dimensional (3D) structures - has remained challenging. A recent analysis revealed that >75% of all conventional, organic fragments are predominantly 1D or 2D in shape. However, 3D fragments are desired because molecular shape is one of the most important factors in molecular recognition by a biomolecule. To address this challenge, the use of inert metal complexes, so-called 'metallofragments' (mFs), to construct a 3D fragment library is introduced. A modest library of 71 compounds has been prepared with rich shape diversity as gauged by normalized principle moment of inertia (PMI) analysis. PMI analysis shows that these metallofragments occupy an area of fragment space that is unique and highly underrepresented when compared to conventional organic fragment libraries that are comprised of orders of magnitude more molecules. The potential value of this metallofragment library is demonstrated by screening against several different types of proteins, including an antiviral, an antibacterial, and an anticancer target. The suitability of the metallofragments for future hit-to-lead development was validated through the determination of IC50 and thermal shift values for select fragments against several proteins. These findings demonstrate the utility of metallofragment libraries as a means of accessing underutilized 3D fragment space for FBDD against a variety of protein targets.

Chelators and metal complex stability for radiopharmaceutical applications

Diagnostic and therapeutic nuclear medicine relies heavily on radiometal nuclides. The most widely used and well-known radionuclide is technetium-99m (99mTc), which has dominated diagnostic nuclear medicine since the advent of the 99Mo/99mTc generator in the 1960s. Since that time, many more radiometals have been developed and incorporated into potential radiopharmaceuticals. One critical aspect of radiometal-containing radiopharmaceuticals is their stability under in vivo conditions. The chelator that is coordinated to the radiometal is a key factor in determining radiometal complex stability. The chelators that have shown the most promise and are under investigation in the development of diagnostic and therapeutic radiopharmaceuticals over the last 5 years are discussed in this review.

Access to the Bis-benzene Cobalt(I) Sandwich Cation and its Derivatives: Synthons for a "Naked" Cobalt(I) Source?

The synthesis and structural characterization of the hitherto unknown parent Co(bz)₂⁺ (bz=benzene) complex and several of its derivatives are described. Their synthesis starts either from a CoCO₅⁺ salt, or directly from Co₂ (CO)₈ and a Ag⁺ salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(OR^F )₄ ]^- and [F{Al(OR^F )₃ }₂ ]^- {R^F =C(CF₃ )₃ } and the solvent ortho-difluorobenzene (o-DFB). The magnetic properties of Co(bz)₂⁺ were measured and compared in the condensed and gas phases. The weakly bound Co(o-dfb)₂⁺ salts are of particular interest for the preparation of further Co^I salts, for example, the structurally characterized low-coordinate 12 valence electron Co(P^t Bu₃ )₂⁺ and Co(NHC)₂⁺ salts.

Structure and reactivities of rhenium and technetium bis-arene sandwich complexes [M(η6-arene)2]+

Rhenium and ⁹⁹Tc bis-arene complexes for a molecule-based theranostic approach are presented. Conjugation of biovectors to benzene or substitution of naphthalene allows integration of {Re(η⁶-C₆H₆)}⁺ in pharmaceutical lead structures.