A Mixed-Ring Sandwich Complex from Unexpected Ring Contraction in [Re(η6-C6H5Br)(η6-C6R6)](PF6)

[Tc(NO)(Cp)(PPh3)Cl] and [Tc(NO)(Cp)(PPh3)(NCCH3)](PF6), and Their Reactions with Pyridine and Chalcogen Donors

Reactions of the technetium(I) nitrosyl complex [Tc(NO)(Cp)(PPh3)Cl] with triphenylphosphine chalcogenides EPPh3 (E = O, S, Se), and Ag(PF6) in a CH2Cl2/MeOH mixture (v/v, 2/1) result in an exchange of the chlorido ligand and the formation of [Tc(NO)(Cp)(PPh3)(EPPh3)](PF6) compounds. The cationic acetonitrile complex [Tc(NO)(Cp)(PPh3)(NCCH3)]+ is formed when the reaction is conducted in NCCH3 without additional ligands. During the isolation of the corresponding PF6- salt a gradual decomposition of the anion was detected in the solvent mixture applied. The yields and the purity of the product increase when the BF4- salt is used instead. The acetonitrile ligand is bound remarkably strongly to technetium and exchange reactions readily proceed only with strong donors, such as pyridine or ligands with 'soft' donor atoms, such as the thioether thioxane. Substitutions on the cyclopentadienyl ring do not significantly influence the ligand exchange behavior of the starting material. 99Tc NMR spectroscopy is a valuable tool for the evaluation of reactions of the complexes of the present study. The extremely large chemical shift range of this method allows the ready detection of corresponding ligand exchange reactions. The observed 99Tc chemical shifts depend on the donor properties of the ligands. DFT calculations support the discussions about the experimental results and provide explanations for some of the unusual findings.

Unlocking Air- and Water-Stable Technetium Acetylides and Other Organometallic Complexes

The first technetium complexes containing anionic alkynido ligands in an end-on coordination mode have been prepared by using the nonprotic, cationic precursor mer,trans-[Tc(SMe₂)(CO)₃(PPh₃)₂]⁺. This cation acts as a functional analogue of the highly reactive 16-electron metallo Lewis acid {Tc(CO)₃(PPh₃)₂}⁺ in reactions with alkynes, acetylides, and other organometallic reagents. Such reactions give a variety of organometallic technetium complexes in excellent yields and enable the preparation of [Tc(CH₃)(CO)₃(PPh₃)₂], [Tc(Ph)(CO)₃(PPh₃)₂], [Tc(Cp)(CO)₂(PPh₃)], [Tc(═CCH₂CH₂CH₂O)(CO)₃(PPh₃)₂]⁺, [Tc(═CCH₂CH₂CH₂CH₂O)(CO)₃(PPh₃)₂]⁺, [Tc(C≡C-H)(CO)₃(PPh₃)₂], [Tc(C≡C-Ph)(CO)₃(PPh₃)₂], [Tc(C≡C-^tBu)(CO)₃(PPh₃)₂], [Tc(C≡C-ⁿBu)(CO)₃(PPh₃)₂], [Tc(C≡C-SiMe₃)(CO)₃(PPh₃)₂], and [Tc{C≡C-C₆H₃(CF₃)₂}(CO)₃(PPh₃)₂]. The bonding situation in the alkynyl complexes is compared to that in corresponding alkyl- and arylnitrile and -isonitrile complexes. [Tc(N≡C-Ph)(CO)₃(PPh₃)₂](BF₄), [Tc(C≡N-Ph)(CO)₃(PPh₃)₂](BF₄), [Tc(N≡C-^tBu)(CO)₃(PPh₃)₂](BF₄), and [Tc(C≡N-^tBu)(CO)₃(PPh₃)₂](BF₄) were prepared in high yields by ligand exchange reactions starting from mer,trans-[Tc(OH₂)(CO)₃(PPh₃)₂](BF₄). The novel complexes were characterized by single-crystal X-ray diffraction and spectroscopic methods. In particular, ⁹⁹Tc nuclear magnetic resonance spectroscopy proved to be an invaluable and sensitive tool for the characterization of the complexes. Density functional theory calculations strongly suggest similar bonding situations for the related alkynyl, nitrile, and isonitrile complexes of technetium.

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.

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...

[Tc(OH2)(CO)3(PPh3)2]+: A Synthon for Tc(I) Complexes and Its Reactions with Neutral Ligands

A scalable synthesis of the novel and highly reactive [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ cation is described. The ligand-exchange chemistry of this compound with neutral ligands coordinating through C, N, O, S, Se, and Te has been explored systematically. The complexes either retain the original mer-trans tricarbonyl core under exclusive exchange of the aqua ligand or form dicarbonyl complexes by thermal decarbonylation. Ligand exchange reactions starting from [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ proceed under mild conditions and are generally almost quantitative. Some of the formed complexes are remarkably stable and inert, while others provide products with one labile ligand for further reactions. The derived complexes of the type [Tc(L)(CO)₃(PPh₃)₂]⁺ and [Tc(L)₂(CO)₂(PPh₃)₂]⁺ represent an interesting opportunity for the development of ^99mTc complexes with potential use in radiopharmacy. The ready displacement of the aqua ligand highlights the synthetic value of [Tc(OH₂)(CO)₃(PPh₃)₂]⁺ as a reactive entry point for further studies in the little explored field of the organometallic chemistry of technetium.

As Nice as π: Aromatic Reactions Activated by π-Coordination to Transition Metals

π‐Coordination of aromatic molecules to metals dramatically alters their reactivity. For example, coordinated carbons become more electrophilic and C−H bonds of coordinated rings become more acidic. For many years, this change in reactivity has been used to trigger reactions that would not take place for uncoordinated arenes, however, there has been a recent resurgence in use of this technique, in part due to the development of catalytic reactions in which π‐coordination is transient. In this Minireview, we describe the key reaction chemistry of arenes coordinated to a range of transition metals, including stereoselective reactions and industrially relevant syntheses. We also summarise outstanding examples of catalytic processes. Finally, we give perspectives on the future direction of the field, with respect to both reactions that are stoichiometric in activating metals and those employing catalytic metal.

Chemoselective C-C σ-Bond Activation of the Most Stable Ring in Biphenylene*

The chemoselective cleavage of a six-membered aromatic ring in biphenylene is reported using an aluminum(I) complex. This type of selectivity is unprecedented. In every example of transition metal mediated C-C σ-bond activation reported to date, the reaction occurs at the central four-membered ring of biphenylene. Insight into the origin of chemoselectivity was obtained through a detailed mechanistic analysis (isolation of an intermediate, DFT studies, activation strain analysis). In conclusion, the divergent reactivity can be attributed to differences in both the symmetry and radial extension of the frontier molecular orbitals of the aluminum(I) fragment compared to that of common transition metal fragments.

Fully Solvated, Monomeric ReII Complexes: Insights into the Chemistry of [Re(NCCH3)6]2

The oxidation of [Re(η⁶-C₁₀H₈)₂]⁺ with Ag^I in acetonitrile yields [Re(NCCH₃)₆]²⁺. This fully solvated Re^II compound was characterized by spectroscopic methods and X-ray structure analyses. We show that [Re(NCCH₃)₆]²⁺ acts as a precursor complex for a variety of substitution reactions. Treatment with monodentate triphenylphosphine (PPh₃) and bidentate 1,2-bis(diphenylphosphino)ethane (dppe) yields the complexes [trans-Re(PPh₃)₂(NCCH₃)₄]²⁺ and [trans-Re(dppe)₂(NCCH₃)₂]⁺, respectively. [trans-Re(dppe)₂(NCCH₃)₂]⁺ is oxidized under mild conditions by Ag^I to its Re^II analogue [trans-Re(dppe)₂(NCCH₃)₂]²⁺. Reactions of [Re(NCCH₃)₆]²⁺ with a halide mixture consisting of NaX and AgX (X = Cl, I) result in the formation of the corresponding Re^III complexes [trans-ReX₂(NCCH₃)₄]⁺. [trans-ReBr₂(NCCH₃)₄]⁺ can be obtained directly from [Re(η⁶-C₁₀H₈)₂]⁺ by oxidation with FeBr₃ in acetonitrile. The title compound is thus a convenient starting material for Re^II and Re^III complexes by simple solvent exchange, which are otherwise difficult to access.

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.

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.