Light-Driven Ring Slippage in [Re(η7-C7H7)(η5-C7H9)]+ and the Inertness of Its Technetium Homologue

Here, we present the light-driven reactions of [Re(η7-C7H7)(η5-C7H9)]+ (1+) with nitriles, phosphines, and isocyanides, which are added to 1+ via a ring slippage of the tropylium cation from η7 to η3, forming [Re(η3-C7H7)(η5-C7H9)(L)2]+ (L= acetonitrile 2+; 2-phenylacetonitrile 3+; 1,3,5-triaza-5-phosphoadamantane (PTA) 4+; tert-butyl isocyanide 6+; benzyl isocyanide 7+) and [Re(η3-C7H7)(η5-C7H9)(L)]+ with L = (ethane-1,2-diyl)bis(diphenylphosphane) (dppe) 5+. To compare the reactivities of rhenium and technetium, we also investigated the synthesis of [99Tc(η6-C10H8)2]+, its substitution of naphthalene with cyclohepta-1,3,5-triene to obtain [99Tc(η7-C7H7)(η5-C7H9)]+, and its reactivity (or lack thereof) with light.

Induced fac-mer rearrangements in {M(CO)3}+ complexes (M = Re, 99(m)Tc) by a PNP ligand

The fac-mer rearrangements in [MX3(CO)3]2- (M = Re, 99Tc) induced by a pincer-type ligand (PNP) and a "halide scavenger" are reported. The reactions of fac-[99mTc(CO)3(OH2)3]+ or [99mTcO4]- in saline both yield mer-[99mTc(PNP)(CO)3]+, the first example of a mer-{99mTc(CO)3}+ type complex. In contrast, reactions with terpyridine (terpy) only gave the facial κ2-terpy complexes with Re and 99Tc.

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.

In vivo and in vitro studies of [M(η6-pseudoerlotinib)2]+ sandwich complexes (M = Re, 99mTc)

The pursuit of molecular imaging for tumors has led to endeavors focused on targeting epidermal growth factor receptors (EGFR) through monoclonal antibodies or radionuclide-labelled EGF analogs with 99mTc, 111In, or 131I. In this context, various 99mTc-labeled EGFR inhibitors using quinazoline structures have been reported based on the so-called pendant approach and on two types of complexes and labelling strategies: "4 + 1" mixed ligand complexes and fac-tricarbonyl complexes. Apart from this approach, which alters lead structures by linking pharmacophores to chelator frameworks through different connectors, the integrated incorporation of topoisomerase and tyrosine kinase inhibitors into Re and 99mTc complexes has not been explored. Here we present [M(η6-inhibitor)2]+ (M = Re, 99mTc) and [Re(η6-bz)(η6-inhibitor)]+ complexes, where the core structure of an EGFR tyrosine kinase inhibitor binds directly to the metal center. These complexes exhibit potential for tumor imaging: initial biological investigations highlight the influence of one versus two bound inhibitors on the metal center.

Binding Small Molecules to a cis-Dicarbonyl 99TcI-PNP Complex via Metal-Ligand Cooperativity

Metal-ligand cooperativity is a powerful tool for the activation of various bonds but has rarely, if ever, been studied with the radioactive transition metal ⁹⁹Tc. In this work, we explore this bond activation pathway with the dearomatized PNP complex cis-[⁹⁹Tc^I(^PyrPNP^tBu*)(CO)₂] (4), which was synthesized by deprotonation of trans-[⁹⁹Tc^I(^PyrPNP^tBu)(CO)₂Cl] with KO^tBu. Analogous to its rhenium congener, the dearomatized compound reacts with CO₂ to form the carboxy complex cis-[⁹⁹Tc^I(^PyrPNP^tBu-COO)(CO)₂] and with H₂ to form the mono-hydride complex cis-[⁹⁹Tc^I(^PyrPNP^tBu)(CO)₂H] (7). Substrates with weakly acidic protons are deprotonated by the Brønsted basic pincer backbone of 4, yielding a variety of intriguing complexes. Reactions with terminal alkynes enable the isolation of acetylide complexes. The deprotonation of an imidazolium salt results in the in situ formation and coordination of a carbene ligand. Furthermore, a study with heterocyclic substrates allowed for the isolation of pyrrolide and pyrazolide complexes, which is uncommon for Tc. The spectroscopic analyses and their solid-state structures are reported.

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.

Two Novel Dinuclear Cobalt Polypyridyl Complexes in Electro- and Photocatalysis for Hydrogen Production: Cooperativity Increases Performance

Syntheses and mechanisms of two dinuclear Co-polypyridyl catalysts for the H₂ evolution reaction (HER) were reported and compared to their mononuclear analogue (R1). In both catalysts, two di-(2,2'-bipyridin-6-yl)-methanone units were linked by either 2,2'-bipyridin-6,6'-yl or pyrazin-2,5-yl. Complexation with Co^II gave dinuclear compounds bridged by pyrazine (C2) or bipyridine (C1). Photocatalytic HER gave turnover numbers (TONs) of up to 20000 (C2) and 7000 (C1) in water. Electrochemically, C1 was similar to the R1, whereas C2 showed electronic coupling between the two Co centers. The E(Co^II/I ) split by 360 mV into two separate waves. Proton reduction in DMF was investigated for R1 with [HNEt₃ ](BF₄ ) by simulation, foot of the wave analysis, and linear sweep voltammetry (LSV) with in-line detection of H₂ . All methods agreed well with an (E)ECEC mechanism and the first protonation being rate limiting (≈10⁴  m^-1  s^-1 ). The second reduction was more anodic than the first one. pK_a values of around 10 and 7.5 were found for the two protonations. LSV analysis with H₂ detection for all catalysts and acids with different pK_a values [HBF₄ , pK_a (DMF)≈3.4], intermediate {[HNEt₃ ](BF₄ ), pK_a (DMF)≈9.2} to weak [AcOH, pK_a (DMF)≈13.5] confirmed electrochemical H₂ production, distinctly dependent on the pK_a values. Only HBF₄ protonated Co^I intermediates. The two metals in the dualcore C2 cooperated with an increase in rate to a competitive 10⁵  m^-1  s^-1 with [HNEt₃ ](BF₄ ). The overpotential decreased compared to R1 by 100 mV. Chronoamperometry established high stabilities for all catalysts with TON_lim of 100 for R1 and 320 for C1 and C2.

High-Yield 99mTc Labeling of Gold Nanoparticles Carrying Atropine and Adrenaline

This work focuses on the synthesis, purification, and analytical characterization of novel multifunctional Au NPs radiolabeled with ^99mTc. These mixed-ligand shell Au NPs represent pharmacologically relevant samples for potential application in theragnostics. A ligand using a plain linker with a rather long chain consisting of 10 CH₂ groups and a thiol moiety along with the PADA chelator has been used for both the attachment to the Au NP surface and for the ^99mTc(CO)₃⁺ complexation. We have combined this with our approach of stabilizing Au NP without any PEG or other stabilizing groups. Thus, monoligand shell Au NPs were radiolabeled by different strategies (prelabeling and postlabeling). Additionally, pharmacologically relevant Au NPs were synthesized carrying both a biofunctionalization with either atropine or adrenaline and the ^99mTc radiolabel. All samples were obtained in very good yields (up to 80% of the total activity loaded onto the column) and completely/particularly purified using desalting columns. Detailed analytical characterization of the Au NPs before and after radiolabeling has proven the NPs' robustness throughout the process. Their intact functionalization, shape, and stability was confirmed by transmission electron microscopy (TEM), ultraviolet/visible (UV/vis) spectroscopy, dynamic light scattering (DLS), and infrared (IR) spectroscopy. The presented strategy represents a versatile building block system that can be adapted to a variety of bioactive molecules and may be of high relevance for theragnostic applications.

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...

Organometallic small molecule kinase inhibitors - direct incorporation of Re and 99mTc into Opaganib®

[(η5-Cp)Re^I(CO)₃] was incorporated into the kinase inhibitor Opaganib®. The resulting bioorganometallic complex showed a similar anti-cancer activity to Opaganib® against PC-3 cancer cells. The IC₅₀ value for the kinase SK2 is 30x higher than that of Opaganib®. The ^99mTc homologue was synthesized, completing a matched-pair for molecular theranostics.

Replacing an adamantyl unit in the protein kinase inhibitor Opaganib® with an integrated [(η⁵-Cp)M(CO)₃] (M = Re, ^99mTc) unit retains the lead's bioactivity and yields a true matched-pair pharmacomimetic.

Excited-state structure of copper phenanthroline-based photosensitizers

Cu diimine complexes present a noble metal free alternative to classical Ru, Re, Ir and Pt based photosensitizers in solution photochemistry, photoelectrochemical or dye-sensitized solar cells. Optimization of these dyes requires understanding of factors governing the key photochemical properties: excited state lifetime and emission quantum yield. The involvement of exciplex formation in the deactivation of the photoexcited state is a key question. We investigate the excited-state structure of [Cu(dmp)₂]⁺ and [Cu(dsbtmp)₂]⁺ (dmp = 2,9-dimethyl-1,10-phenanthroline, dsbtmp = 2,9-di-sec-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) using pump-probe X-ray absorption spectroscopy (XAS) and DFT. Features of XAS that distinguish flattened tetrahedral site and 5-coordinated geometry with an additional solvent near Cu(II) center are identified. Pump-probe XAS demonstrates that for both complexes the excited state is 4-coordinated. For [Cu(dmp)₂]⁺ the exciplex is 0.24 eV higher in energy than the flattened triplet state, therefore it can be involved in deactivation pathways as a non-observable state that forms slower than it decays. For [Cu(dsbtmp)₂]⁺ the excited-state structure is characterized by Cu-N distances of 1.98 and 2.07 Å and minor distortions, leading to a 3 orders of magnitude longer excited-state lifetime.

New approach for the synthesis of water soluble fac-[MI(CO)3]+ bis(diarylphosphino)alkylamine complexes (M = 99Tc, Re)

A novel proof-of-concept is reported to modify the water solubility and potential biological effects of a bis(diphenylphosphino)alkylamine (PNP) ligand and the corresponding metal complex, by introducing an amine group on the outer periphery of the pendant ligand arm. Thus, a tertiary butoxycarbonyl protected N'-Boc-ethylenediamine-N,N-bis(diphenylphosphino) (N'-Boc-PNP) ligand (1) was synthesized by reacting the protected ethylenediamine and chlorodiphenylphosphine in a 1 : 2 molar ratio. The corresponding fac-[Re(CO)₃(N'-Boc-PNP)Br] (1A) complex was then obtained by reacting N'-Boc-PNP (1) with (Et₄N)₂fac-[Re(CO)₃Br₃] in equimolar amounts in DCM at 50 °C. De-protection of the N'-Boc pendant amine group in 1A with TFA leads to fac-[Re(NH₃⁺-PNP)(CO)₃Br]·CF₃COO^- (1B) which is soluble in D₂O (>0.05 M). Treating 1B with saturated aqueous NaHCO₃ yields fac-[Re(NH₂-PNP)(CO)₃Br]·MeOH (1C) in near quantitative yield. Although both 1A and 1C are not soluble in D₂O, addition of TFA easily generates 1B (³¹P NMR), confirming the formation of the protonated amine. Isolation of fac-[⁹⁹Tc(CO)₃(N-Boc-PNP)(Cl)] (1D) confirmed that the rhenium and technetium (⁹⁹Tc) can be easily interchanged in this process. Reported are hence the unique rhenium series of compounds 1A, 1B and 1C and the corresponding technetium complex 1D, unequivocally characterized by single crystal XRD, as well as IR and ¹H NMR spectroscopy. Preliminary antimicrobial evaluation indicates that ligand 1 and its respective rhenium complexes (1A-1C) were not active against selected fungi (Candida albicans and Cryptococcus neoformans) and bacteria (Escherichia coli, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Staphylococcus aureus). These types of ligands and complexes therefore present themselves as excellent radio models for further evaluation using ¹⁸⁶Re, ¹⁸⁸Re and ^99mTc to potentially study the radiotoxicity of appropriately designed complexes.

Naphthalene Exchange in [Re(η6 -napht)2 ]+ with Pharmaceuticals Leads to Highly Functionalized Sandwich Complexes [M(η6 -pharm)2 ]+ (M=Re/99m Tc)

Bis‐arene sandwich complexes are generally prepared by the Fischer‐Hafner reaction, which conditions are incompatible with most O‐ and N‐ functional groups. We report a new way for the synthesis of sandwich type complexes [Re(η⁶‐arene)₂]⁺ and [Re(η⁶‐arene)(η⁶‐benzene)]⁺ from [Re(η⁶‐napht)₂]⁺ and [Re(η⁶‐napht)(η⁶‐benzene)]⁺, with functionalized arenes and pharmaceuticals. N‐methylpyrrolidine (NMP) facilitates the substitution of naphthalene with the incoming arene. A series of fully characterized rhenium sandwich complexes with simple arenes, such as aniline, as well as with active compounds like lidocaine and melatonin are presented. With these rhenium compounds in hand, the radioactive sandwich complexes [^99mTc(η⁶‐pharm)₂]⁺ (pharm=pharmaceutical) can be unambiguously confirmed. The direct labelling of pharmaceuticals with ^99mTc through η⁶‐coordination to phenyl rings and the confirmation of the structures with the rhenium homologues opens a path into molecular theranostics.

Efficient Alkaline Water Oxidation with a Regenerable Nickel Pseudo-Complex

Efficient and robust electrocatalysts are required for the oxygen evolution reaction (OER). Photosystem II-inspired synthetic transition metal complexes have shown promising OER activity in water-poor or mild conditions, yet challenges remain in the improvement of current density and performance stability for practical applications in alkaline electrolytes in contrast to solid-state oxide catalysts. Here, we report that a nickel pseudo-complex (bpy)_zNiO_xH_y (bpy = 2,2'-bipyridine) catalyst, which bridges solid oxide and molecular catalysts, exhibits the highest OER activity among nickel-based catalysts with a turnover frequency of 1.1 s^-1 at an overpotential of 0.30 volts, even outperforming iron-incorporated nickel (oxy)hydroxide under an identical nickel mass load. Benefiting from the strong coordination between bpy and nickel, this (bpy)_zNiO_xH_y catalyst exhibits long-term stability in highly alkaline media at 1.0 mA cm^-2 for over 200 h and at 20 mA cm^-2 for over 60 h. Our findings indicate that dynamically coordinating a small amount of bpy in the catalyst layer efficiently sustains highly active nickel sites for water oxidation, demonstrating a general strategy for improving the activity of transition metal sites with active ligands beyond the incorporation of metal cations to form double-layered hydroxides.

The crystal structure of cis-diaqua-bis (N-butyl-N-(pyridin-2-yl)pyridin-2-amine-κ 2 N,N′)cobalt(II)] dichloride trihydrate, C28H44Cl2N6O5Co

C28H44Cl2N6O5Co, triclinic, P 1 ‾ $P‾{1}$ (no. 2), a = 11.20(2) Å, b = 12.70(3) Å, c = 12.80(4) Å, α = 100.30(12)°, β = 103.18(9)°, γ = 106.77(6)°, V = 1638(8) Å3, Z = 2, R gt (F) = 0.0372, wR ref (F 2) = 0.0890, T = 100 K.

Relativity as a Synthesis Design Principle: A Comparative Study of [3 + 2] Cycloaddition of Technetium(VII) and Rhenium(VII) Trioxo Complexes with Olefins

The difference in [3 + 2] cycloaddition reactivity between fac-[MO₃(tacn)]⁺ (M = Re, ⁹⁹Tc; tacn = 1,4,7-triazacyclononane) complexes has been reexamined with a selection of unsaturated substrates including sodium 4-vinylbenzenesulfonate, norbornene, 2-butyne, and 2-methyl-3-butyn-2-ol (2MByOH). None of the substrates was found to react with the Re cation in water at room temperature, whereas the ⁹⁹Tc reagent cleanly yielded the [3 + 2] cycloadducts. Interestingly, a bis-adduct was obtained as the sole product for 2MByOH, reflecting the high reactivity of a ⁹⁹TcO-enediolato monoadduct. On the basis of scalar relativistic and nonrelativistic density functional theory calculations of the reaction pathways, the dramatic difference in reactivity between the two metals has now been substantially attributed to differences in relativistic effects, which are much larger for the 5d metal. Furthermore, scalar-relativistic ΔG values were found to decrease along the series propene > norbornene > 2-butyne > dimethylketene, indicating major variations in the thermodynamic driving force as a function of the unsaturated substrate. The suggestion is made that scalar-relativistic effects, consisting of greater destabilization of the valence electrons of the 5d elements compared with those of the 4d elements, be viewed as a new design principle for novel ^99mTc/Re radiopharmaceuticals, as well as more generally in heavy-element coordination chemistry.

Room temperature cycloaddition reactivity of fac-[⁹⁹TcO₃(tacn)]⁺ (tacn = 1,4,7-triazacyclononane) with a variety of unsaturated substrates and the lack of such reactivity for fac-[ReO₃(tacn)]⁺ appears largely attributable to much stronger relativistic effects for Re relative to Tc, based on relativistic density functional theory calculations.

cis-Locked Ru(II)-DMSO Precursors for the Microwave-Assisted Synthesis of Bis-Heteroleptic Polypyridyl Compounds

We describe a synthetic strategy for the preparation of bis-heteroleptic polypyridyl Ru(II) complexes of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands) from well-defined Ru(II) precursors. For this purpose, a series of six neutral, anionic, and cationic cis-locked Ru(II)-DMSO complexes (2–7) of the general formula [Y] fac-[RuX(DMSO–S)₃(O–O)]ⁿ (where O–O is a symmetrical chelating anion: oxalate (ox), malonate (mal), acetylacetonate (acac); X = DMSO–O or Cl^–; n = −1/0/+1 depending on the nature and charge of X and O–O; when present, Y = K⁺ or PF₆^–) were efficiently prepared from the well-known cis-[RuCl₂(DMSO)₄] (1). When treated with diimine chelating ligands (L1 = bpy, phen, dpphen), the compounds 2–7 afforded the target [Ru(L1)₂(O–O)]^0/+ complex together with the undesired (and unexpected) [Ru(L1)₃]²⁺ species. Nevertheless, we found that the formation of [Ru(L1)₃]²⁺can be minimized by carefully adjusting the reaction conditions: in particular, high selectivity toward [Ru(L1)₂(O–O)]^0/+ and almost complete conversion of the precursor was obtained within minutes, also on a 100–200 mg scale, when the reactions were performed in absolute ethanol at 150 °C in a microwave reactor. Depending on the nature of L1 and concentration, with the oxalate and malonate precursors, the neutral product [Ru(L1)₂(O–O)] can precipitate spontaneously from the final mixture, in pure form and acceptable-to-good yields. When spontaneous precipitation of the disubstituted product does not occur, purification from [Ru(L1)₃]²⁺ can be rather easily accomplished by column chromatography or solvent extraction. By comparison, under the same conditions, compound 1 is much less selective, thus demonstrating that locking the geometry of the precursor through the introduction of O–O in the coordination sphere of Ru is a valid strategic approach. By virtue of its proton-sensitive nature, facile and quantitative replacement of O–O in [Ru(L1)₂(O–O)]^0/+ by L2, selectively affording [Ru(L1)₂(L2)]²⁺, was accomplished in refluxing ethanol in the presence of a slight excess of trifluoroacetic acid or HPF₆.

cis-Locked Ru(II)-DMSO complexes bearing a symmetrical chelating anion, such as [K] fac-[RuCl(DMSO−S)₃(η²-mal)] (2), are suitable precursors for the two-step selective preparation of bis-heteroleptic polypyridyl compounds of the type [Ru(L1)₂(L2)]²⁺ (L1 and L2 = diimine ligands).

Cobalt Complexes of Polypyridyl Ligands for the Photocatalytic Hydrogen Evolution Reaction

The reductive part of artificial photosynthesis, the reduction of protons into H₂, is a two electron two proton process. It corresponds basically to the reactions occurring in natural photosystem I. We show in this review a selection of involved processes and components which are mandatory for making this light-driven reaction possible at all. The design and the performances of the water reduction catalysts is a main focus together with the question about electron relays or sacrificial electron donors. It is shown how an original catalyst is developed into better ones and what it needs to move from purely academic homogeneous processes to heterogeneous systems. The importance of detailed mechanistic knowledge obtained from kinetic data is emphasized.

Exploring preliminary structural relationships and mitochondrial targeting of fac-[MI(CO)3]-bis(diarylphosphino)alkylamine complexes (M = 99Tc, Re)

Preliminary structural relationships in fac-[MI(CO)3]-bis(diarylphosphino)alkylamine complexes (M = 99Tc, Re), antimicrobial and mitochondrial targeting are reported.

Bioorganometallic Technetium and Rhenium Chemistry: Fundamentals for Applications

Due to its long half-life of 2.111×10⁵ y, technetium, i.e. ⁹⁹Tc, offers the excellent opportunity of combining fundamental and ' classical ' organometallic or coordination chemistry with all methodologies of radiochemistry. Technetium chemistry is inspired by the applications of its short-lived metastable isomer ^99mTc in molecular imaging and radiopharmacy. We present in this article examples about these contexts and the impact of purely basic oriented research on practical applications. This review shows how the chemistry of this element in the middle of the periodic system inspires the chemistry of neighboring elements such as rhenium. Reasons are given for the frequent observation that the chemistries of ⁹⁹Tc and ^99mTc are often not identical, i.e. compounds accessible for ^99mTc, under certain conditions, are not accessible for ⁹⁹Tc. The article emphasizes the importance of macroscopic technetium chemistry not only for research but also for advanced education in the general fields of radiochemistry.

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.

The "Carbonyl Story" and Beyond; Experiences, Lessons and Implications

The complex [^99m Tc(OH₂ )₃ (CO)₃ ]⁺ has become a versatile building block in radiopharmaceutical chemistry, applied by many groups worldwide. However, despite widespread efforts, only one compound has made it right the way through clinical trials. Along the way from its discovery to its development into an eventual product, the author experienced issues that he would handle differently in retrospect. In this article, these experiences are turned into "lessons" that might be helpful for young researchers finding themselves in similar situations. Beside issues with patenting and company strategies, the carbonyl story has provided scientific implications beyond its own story, and insights from which any future ^99m Tc-based chemistry for radiopharmacy or molecular imaging might benefit.

Expanding the Cyclopentadienyl Framework: 99mTc/Re Complexes with Orthogonal Functions for Bioconjugation

A series of bifunctional cyclopentadienes of the type 1,3-EtOCO-HCp-linker-NH₂ were synthesized. In this series, the linker length (distance between the amine functionalities and the cyclopentadiene) has been systematically varied (CH₂)_n (n = 1-3). The corresponding Re complexes [(η⁵-C₅H₃RR')Re(CO)₃] (R = -COOEt, R' = -linker-NH₂) were synthesized and structurally characterized. They exhibit extraordinary stability toward water and air. All bifunctional cyclopentadienes have been labeled with the [^99mTc(CO)₃]⁺ moiety. Whereas the reactions with ethylene and propylene linked cyclopentadiene under mild reaction conditions led to the products in high radiochemical purity (>96%) without applying further purification protocols, harsher reaction conditions were required for the synthesis of the methylene-linked cyclopentadiene compound. Masking the amine in the methylene-linked cyclopentadiene by an amide bond bypasses this problem. The very hydrophilic characters of these complexes were assessed by K_OW analysis. The reported cyclopentadienes and their complexes offer a robust and versatile platform for (radio)metal incorporation into biologically active lead structures.

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.

Synergizing hole accumulation and transfer on composite Ni/CoOx for photoelectrochemical water oxidation

Ni/CoOx sites were supported on an around 2 nm-TiOx modified hematite photoanode for water oxidation. TiOx demonstrates insignificant hole accumulation and a catalytically inactive surface that serves as an ideal platform. We reveal that the NiOx favors the extraction of holes from the TiOx surface, which are efficiently transferred to active CoOx for water oxidation.

[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.

Crystal structure of hexacarbonyl-(μ2-methanoato-k2 O:O′)-(μ2–bis(di-p-tolylphosphino)cyclohexylamine-κ2 P:P′)dirhenium(I), C42H45NO8P2Re2

C42H45NO8P2Re2, monoclinic, C2/c (no. 15), a = 24.1612(7) Å, b = 13.0229(3) Å, c = 14.9180(4) Å, β = 121.383(3)°, V = 4007.2(2) Å3, Z = 4, R gt(F) = 0.0235, wR ref(F 2) = 0.0596, T = 173(2) K.