Reported herein is the coordination of rhenium complexes to tetrazine ligand in [ReCl(CO)3(TzPy)] [1] (TzPy = 3-(2-pyridyl)-1,2,4,5-tetrazine) and the rates of addition of different dienophiles to the tetrazine.

The doubly deprotonated bridging ligand L12- derived from 2,6-bis(2-pyridyl)-1,5-dihydro-1’,4’-benzoquinono[2’,3’-d:5’,6’-d’]diimidazole H2L1 forms coordination compounds with two bis(2,2’-bipyridine)osmium(II) complex fragments in anti ([1](ClO4)2) and syn configuration ([2](ClO4)2) of {(μ-L1)[Os(bpy)2]2}(ClO4)2, as evident from...

We present herein the synthesis of three new bis(corrolato-ruthenium(III)) complexes containing unsupported Ru-Ru bonds and their characterization in different redox states. The H NMR spectra of the bis(corrolato-ruthenium(III)) complexes displayed "normal" chemical shifts and the compounds proved to be EPR-silent. Crystallographic characterization of the dimers indicated Ru-Ru distances of 2.175 Å, consistent with a triple bond between the two ruthenium centers. All of the synthesized complexes undergo two successive reversible oxidations and a single reversible reduction. A combination of UV/Vis/NIR/EPR spectroelectrochemical studies and DFT calculations established the redox state distributions in these ruthenium-ruthenium-bonded dimers. Whereas reduction of the dimers is metal-based and leads to metal-metal-bonded mixed-valent Ru -Ru species, one-electron oxidation largely retains the Ru -Ru situation with the generation of metal-bound corrolato radicals. The present study thus concerns the first UV/Vis/NIR/EPR spectroelectrochemical characterization and DFT calculations of ruthenium-ruthenium-bonded rotationally ordered corrole dimers. The mean plane separation between the two corrole units in these dimers is around 3.543 Å, which is in close agreement to that in the "special pair" in chlorophyll. Oxidation of these ruthenium-ruthenium-bonded dimers gives rise to two new electronic absorption bands in the NIR region (similar to those of the special pair), which have apparently not been mentioned/observed in earlier reports on ruthenium-ruthenium-bonded corrole dimers. These bands mainly originate from inter-corrole transitions.

Recently, anionic charges were found to negatively influence the in vitro gastrin-releasing peptide receptor (GRPR) binding parameters of dually radioisotope and fluorescent dye labeled GRPR-specific peptide dimers. From this, the question arose if this adverse impact on in vitro GRP receptor affinities could be mitigated by a higher valency of peptide multimerization. For this purpose, we designed two different hybrid multimodal imaging units (MIUs), comprising either one or two click chemistry-compatible functional groups and reacted them with PESIN (PEG3-BBN7–14, PEG = polyethylene glycol) dimers to obtain a dually labeled peptide homodimer or homotetramer. Using this approach, other dually labeled peptide monomers, dimers, and tetramers can also be obtained, and the chelator and fluorescent dye can be adapted to specific requirements. The MIUs, as well as their peptidic conjugates, were evaluated in terms of their photophysical properties, radiolabeling efficiency with 68Ga and 64Cu, hydrophilicity, and achievable GRP receptor affinities. Here, the hydrophilicity and the GRP receptor binding affinities were found to be especially strongly influenced by the number of negative charges and peptide copies, showing logD (1-octanol-water-distribution coefficient) and IC50 (half maximal inhibitory concentration) values of −2.2 ± 0.1 and 59.1 ± 1.5 nM for the homodimer, and −1.9 ± 0.1 and 99.8 ± 3.2 nM for the homotetramer, respectively. From the obtained data, it can be concluded that the adverse influence of negatively charged building blocks on the in vitro GRP receptor binding properties of dually labeled PESIN multimers can, at least partly, be compensated for by the number of introduced peptide binding motives and the used molecular design.

Under oxidative conditions the complexes M(bpy) 2 Cl 2 , M = Ru or Os, react with 1,2-dipivaloylhydrazine H 2 L = tBuC(O)-NH-NH-C(O)-tBu to yield the paramagnetic compounds [M(L)(bpy) 2 ](PF 6 ) {1(PF 6 ), M = Ru and 2(PF 6 ), M = Os}.Crystal structures of 1(ClO 4 ) and 2(PF 6 ) reveal an NNCO chelate coordination of L with d(NN) ≈ 1.39 Å. EPR spectroscopy suggests a metal/ligand mixed situation for the unpaired electron, with 29 % (Ru, 1 + ) or 46 % (Os, 2 + ) metal spin contribution according to DFT calculations. Both complex cations exhibit reversible one-electron oxidation and reduction (K c ≈ 10 19 for 1 + and 10 12 for 2 + ) which could also be monitored by IR (ν CO ) and UV/Vis-NIR spectroelectrochemistry. These data confirm a [a] Scheme 3. Chemical structures of singly charged mononuclear complexes. Scheme 4. Representation of differently charged species [M(bpy) 2 (L)] 0/+/2+ incorporating different ligand redox states (L 2-/·-/0 ). Results and Discussion Synthesis and CharacterizationScheme 5 illustrates the reactions leading to the compounds from Scheme 3. Deprotonation of the 1,2-dipivaloylhydrazine Scheme 5. Reaction scheme for mononuclear ruthenium complexes (top) and for the mononuclear osmium complex (bottom).