Three new ligands based on the alloxazine core appended with pyridyl coordinating groups have been designed, synthesized, and characterized. The ligands are revealed to be redox-active in DMF ...solution, as attested to by CV and combined CV/EPR studies. The spin of the reduced species appears to be delocalized on the alloxazine core, as attested to by DFT calculations. The coordination abilities of one of the ligands toward Cu2+ or Ni2+ 3d cations revealed the formation of the first alloxazine-based 3D coordination polymers, presenting strong π–π stacking and substantial cavities. Preliminarily charge/discharge experiments in Li batteries evidence Li+ insertion in such systems.
Potential inversion refers to the situation where a protein cofactor or a synthetic molecule can be oxidized or reduced twice in a cooperative manner; that is, the second electron transfer is easier ...than the first. This property is very important regarding the catalytic mechanism of enzymes that bifurcate electrons and the properties of bidirectional redox molecular catalysts that function in either direction of the reaction with no overpotential. Cyclic voltammetry is the most common technique for characterizing the thermodynamics and kinetics of electron transfer to or from these molecules. However, a gap in the literature is the absence of analytical predictions to help interpret the values of the voltammetric peak potentials when potential inversion occurs; the cyclic voltammograms are therefore often analyzed by simulating the data, with no discussion of the possibility of overfitting and often no estimation of the error on the determined parameters. Here we formulate the theory for the voltammetry of freely diffusing or surface-confined two-electron redox species in the experimentally relevant irreversible limit where the peak separation depends on the scan rate. We explain why the model is intrinsically underdetermined, and we illustrate this conclusion by analysis of the voltammetry of a nickel complex with redox-active iminosemiquinone ligands. Being able to characterize the thermodynamics of two-electron electron-transfer reactions will be crucial for designing more efficient catalysts.
Structure−activity relationships for inhibition of DNA-dependent protein kinase (DNA-PK) have been defined for substituted chromen-4-ones. For the 2-amino-substituted benzohchromen-4-ones, a ...morpholine substituent at this position was essential for activity. Small libraries of 6- and 7-alkoxy-substituted chromen-4-ones showed that a number of 7-alkoxy-substituted chromenones displayed improved activity. Focused libraries incorporating 6-, 7-, and 8-aryl and heteroaryl substituents were prepared. In these cases, 6- and 7-substitution was disfavored, whereas 8-substitution was largely tolerated. Surprisingly, two compounds, 2-N-morpholino-8-dibenzofuranyl-chromen-4-one (NU7427, 32{38}) and the 2-N-morpholino-8-dibenzothiophenyl-chromen-4-one (NU7441, 32{26}) were excellent inhibitors (IC50 vs DNA-PK = 40 and 13 nM, respectively). The ring-saturated analogue 2-N-morpholino-8-(6‘,7‘,8‘,9‘-tetrahydrodibenzothiophene)chromen-4-one, 36, retained potent activity (IC50 vs DNA-PK = 23 nM). The dibenzothiophene 32{38} sensitized HeLa cells to ionizing radiation in vitro, with dose modification factors of 2.5 at 10% survival being observed at 0.5 μM. The cytotoxicity of the topoisomerase II inhibitor etoposide was also potentiated.
Metalloenzymes use earth-abundant non-noble metals to perform high-fidelity transformations in the biological world. To ensure chemical efficiency, metalloenzymes have acquired evolutionary ...reactivity-enhancing tools. Among these, the entatic state model states that a strongly distorted geometry induced by ligands around a metal center gives rise to an energized structure called entatic state, strongly improving the reactivity. However, the original definition refers both to the transfer of electrons or chemical groups, whereas the chemical application of this concept in synthetic systems has mostly focused on electron transfer, therefore eluding chemical transformations. Here we report that a highly strained redox-active ligand enables a copper complex to perform catalytic nitrogen- and carbon-group transfer in as fast as 2 min, thus exhibiting a strong increase in reactivity compared with its unstrained analogue. This report combines two reactivity-enhancing features from metalloenzymes, entasis and redox cofactors, applied to group-transfer catalysis.
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•We design a catalyst interfacing two reactivity-enhancing tools from metalloenzymes•This work merges redox-active cofactors and entatic state reactivity•The modifications in the coordination sphere lead to enhanced catalytic behavior•These results open perspectives in bioinspired catalysis and group-transfer reactions
Inorganic Chemistry; Molecular Inorganic Chemistry; Catalysis
A permethylated analogue 1 of the mushroom pigment norbadione A was synthesized through a double Suzuki–Miyaura coupling of 1,7‐diboronated naphthalene 2 and triflate 3. ESI‐MS studies showed similar ...constants for the formation of the complexes of the norbadione A dipotassium salt and of 1 with a cesium ion, thus suggesting that cesium complexation involves the two pulvinic acid parts of these molecules. Tf=trifluoromethanesulfonyl.
The reactivity of a stable copper(II) complex bearing fully oxidized iminobenzoquinone redox ligands towards nucleophiles is described. In sharp contrast with its genuine low‐valent counterpart ...bearing reduced ligands, this complex performs high‐yielding C−N bond formations. Mechanistic studies suggest that this behavior could stem from a mechanism akin to reductive elimination occurring at the metal center but facilitated by the ligand: it is proposed that a masked high oxidation state of the metal can be stabilized as a lower copper(II) oxidation state by the redox ligands without forfeiting its ability to behave as a high‐valent copper(III) center. These observations are substantiated by a combination of advanced EPR spectroscopy techniques with DFT studies. This work sheds light on the potential of redox ligands as promoters of unusual reactivities at metal centers and illustrates the concept of masked high‐valent metallic species.
CuIII spielt Verstecken: Ein beständiger Kupfer(II)‐Komplex mit vollständig oxidierten Iminobenzochinon‐Liganden reagiert wie eine Kupfer(III)‐Spezies und vermittelt C‐N‐Kupplungen mit hohen Ausbeuten. Mechanistische Studien führen dieses Verhalten auf einen Prozess zurück, welcher der reduktiven Eliminierung am Metallzentrum ähnlich ist, aber durch den redoxaktiven Ligand erleichtert wird.
The reaction of a copper complex bearing iminosemiquinone ligands with a CF3+ source provides an unprecedented CuII-CF3 complex through ligand-based oxidation. Reactivity of this complex leads to ...nucleophilic trifluoromethylation of the ligand, suggesting an electronic interplay that results in a formal umpolung of the initial CF3+.
The reaction of a copper complex bearing iminosemiquinone ligands with a CF3(+) source provides an unprecedented Cu(II)-CF3 complex through ligand-based oxidation. Reactivity of this complex leads to ...nucleophilic trifluoromethylation of the ligand, suggesting an electronic interplay that results in a formal umpolung of the initial CF3(+).
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