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

Role of Pure Technetium Chemistry: Are There Still Links to Applications in Imaging?

The discovery and development of new 99mTc-based radiopharmaceuticals or labeled drugs in general is based on innovative, pure chemistry and subsequent, application-targeted research. This was the case for all currently clinically applied imaging agents. Most of them were market-introduced some 20 years ago, and the few more recent ones are based on even older chemistry, albeit technetium chemistry has made substantial progress over the last 20 years. This progress though is not mirrored by new molecular imaging agents and is even accompanied by a steady decrease in the number of groups active in pure and applied technetium chemistry, a contrast to the trends in most other fields in which d-elements play a central role. The decrease in research with technetium has been partly counterbalanced by a strong increase of research activities with homologous, cold rhenium compounds for therapy, disclosing in the future eventually a quite unique opportunity for theranostics. This Viewpoint analyzes the pathways that led to radiopharmaceuticals in the past and their underlying fundamental contributions. It attempts to tackle the question of why new chemistry still does not lead to new imaging agents, i.e., the question of whether pure technetium chemistry is still needed at all.

Exploring Rhenium Arene Piano-Stool Chemistry with [Re(η6-C6H6)(NCCH3)3]+: A Powerful Semi-Solvated Precursor

Thermal treatment of the Re^III hydride complex [ReH(η⁵-C₆H₇)(η⁶-C₆H₆)]⁺ in CH₃CN results in the formation of [Re(η⁶-C₆H₆)(NCCH₃)₃]⁺. This semi-solvated complex is remarkably stable under an ambient atmosphere and exhibits a fast CH₃CN self-exchange, which facilitates substitution reactions. The CH₃CN ligands are replaced by σ-donating phosphines such as trimethyl phosphine (PMe₃), triphenyl phosphine (PPh₃), or the bidentate 1,2-bis(diphenylphosphino)ethane (dppe) to afford [Re(η⁶-C₆H₆)(NCCH₃)_3-x(PR₃)_x]⁺ (if R = Me, then x = 2; if R = Ph, then x = 1 or 2) or [Re(η⁶-C₆H₆)(dppe)(NCCH₃)]⁺, respectively. [Re(η⁶-C₆H₆)(NCCH₃)₃]⁺ also reacts with π-acceptors such as 2,2'-bipyridine (bipy), 1,10-phenanthroline (phen), or CO (1 atm) to give [Re(η⁶-C₆H₆)(L)(NCCH₃)]⁺ (L = bipy or phen) and [Re(η⁶-C₆H₆)(CO)(NCCH₃)₂]⁺, respectively. The latter does not show any signs of decomposition after being exposed to an ambient atmosphere for multiple days. Additionally, [Re(η⁶-C₆H₆)(NCCH₃)₃]⁺ reacts with π-donors such as the dienes 2,3-dimethyl-1,3-butadiene (DMBD), norbornadiene (NBD), or 1,5-cyclooctadiene (COD) to give [Re(η⁶-C₆H₆)(η⁴-diene)(NCCH₃)]⁺ (diene = DMBD, NBD, and COD). All three complexes are extremely stable and do not decompose during purification by preparative high-performance liquid chromatography (aqueous acidic gradient). In the presence of 18-crown-6, [Re(η⁶-C₆H₆)(NCCH₃)₃]⁺ reacts with lithium cyclopentadienyl to give the sandwich complex [Re(η⁵-C₅H₅)(η⁶-C₆H₆)]. Loss of the coordinated benzene was observed when treating [Re(η⁶-C₆H₆)(NCCH₃)₃]⁺ with diphenylacetylene (PhC≡CPh), yielding the tetra-coordinated [Re(NCCH₃)(η²-PhC≡CPh)₃]⁺.

One Electron Changes Everything: Synthesis, Characterization, and Reactivity Studies of [Re(NCCH3)6]3

Oxidation of [Re(NCCH₃)₆]²⁺ with the thianthrene radical cation results in the formation of [Re(NCCH₃)₆]³⁺, one of the very rare cases of a fully solvated +3 complex. It was fully characterized by spectroscopy and X-ray structure analysis. In contrast to its reduced analogue, [Re(NCCH₃)₆]³⁺ exhibits a much faster CH₃CN exchange. Hence, substitution reactions proceed at 20 °C within minutes. Its potential as a versatile precursor for Re^III chemistry was examined with a series of substitution reactions. The more lipophilic analogue [Re(NCPh)₆]³⁺ was synthesized by nitrile exchange in benzonitrile (NCPh). The Re(II) analogue of [Re(NCPh)₆]³⁺, [Re(NCPh)₆]²⁺, forms by Ag^I-mediated oxidation of in situ formed [Re(η⁶-C₆H₆)(NCPh₃)₃]⁺ in NCPh. The same synthetic strategy is feasible for the synthesis of [Re(NCCH₃)₆]²⁺ as well. [Re(NCCH₃)₆]³⁺ reacts with 1,4,7-trithiacyclononane (C₆H₁₂S₃) to yield sevenfold-coordinated [Re(κ³-C₆H₁₂S₃)₂(NCCH₃)]³⁺. The reaction of [Re(NCCH₃)₆]³⁺ with 1 equiv of (NBu₄)X produces the Re^III monohalide complexes [ReX(NCCH₃)₅]²⁺ (X = Cl, Br, I). Mixed Re^III dihalides (trans-[ReXY(NCCH₃)₄]⁺) were obtained by treating [ReX(NCCH₃)₅]²⁺ with a second equivalent of (NBu₄)Y (if X = Cl, Y = Br, I; if X = Br, Y = I). Because of this fast CH₃CN exchange, [Re(NCCH₃)₆]³⁺ is a very suitable precursor for new Re^III complexes.