Facile Synthesis of an Octagermacubane with Two Three-Coordinate Germanium(0) Atoms and its Unique Radical Anion.

Thermolysis of a 1 : 1 mixture of tris(di-tert-butylmethylsilyl)germane 9 and bis(di-tert-butylmethylsilyl)germane 17 at 100 °C produces unexpectedly octagermacubane 18, having two 3-coordinate Ge0 atoms (40 % yield). 18 was characterized by X-ray crystallography and it is a singlet biradical (according to DFT quantum mechanical calculations and the absence of an EPR signal). Reactions of 18 with CH2 Cl2 and H2 O yield the novel dichloro-octagermacubane 24 and hydroxy-octagermacubane 25, respectively. Reduction of 18 with tBuMe2 SiNa in THF produces an isolable octagermacubane radical anion 26-Na. Based on X-ray crystallography, EPR spectroscopy and DFT quantum mechanical calculations, 26-Na is classified as a Ge-centered radical anion.

Geometrically constrained square pyramidal phosphoranide.

Geometrical constriction of main group elements leading to a change in the reactivity of these main group centers has recently become an important tool in main group chemistry. A lot of focus on using this modern method is dedicated to group 15 elements and especially to phosphorus. In this work, we present the synthesis, isolation and preliminary reactivity study of the geometrically constrained, square pyramidal (SP) phosphoranide anion (1-). Unlike, trigonal bipyramidal (TBP) phosphoranides that were shown to react as nucleophiles while their redox chemistry was not reported, 1- reacts both as a nucleophile and reductant. The chemical oxidation of 1- leads to a P-P dimer (1-1) that is formed via the dimerization of unstable SP phosphoranyl radical (1˙), an unprecedented decay pathway for phosphoranyl radicals. Reaction of 1- with benzophenone leads via a single electron transfer (SET) to 1-OK and corresponding tetraphenyl epoxide (4).

Synthesis of Genuine Germenyl Lithiums and the First Persistent Germenyl Radicals.

The first isolated genuine germenyl lithiums (R3 Si)(1-Ad)C=Ge(SiMetBu2 )(Li⋅2 L) (R3 Si=tBu2 MeSi, L=THF (1 a), or L=12-crown-4 (1 b) and R3 Si=tBuMe2 Si, L=THF (2 a), or L=12-crown-4 (2 b)), were synthesized by reaction of the corresponding acyl germanes 3 and 4, respectively, with tBu2 MeSiLi in THF at 70 °C. The novel 1 a and 2 b were characterized by NMR and UV/Vis spectroscopy, and also by X-ray crystallography (r(C=Ge)=1.865 Šfor 1 a and 1.877 Šfor 2 b). Nucleophilic addition reaction of 1 a with MeI and a C-H insertion reactions to the C=Ge bond of 1 a, 2 a and 2 b, are reported. Oxidation of 1 a and 2 b (toluene, 230 K) produces the first persistent germenyl radicals (R3 Si)(1-Ad)C=Ge⋅-(SiMetBu2 ) (R3 Si=tBu2 MeSi (13 a), R3 Si=tBuMe2 Si (13 b)), which were characterized by EPR spectroscopy (t1/2 ≈30 min at 230 K, g=2.029, aav (73 Ge) is 55.0G for 13 a and 60.2G for 13 b). The experimental EPR parameters and DFT calculations indicate that 13 a and 13 b have a strongly bent structure at Ge (calc. ∡(C=Ge-Si)=136.7° (13 a), 135.9° (13 b)), and that the unpaired electron has a substantial s-character.

A chromophore-supported structural and functional model of dinuclear copper enzymes, for facilitating mechanism of action studies.

Type III dicopper centres are the heart of the reactive sites of enzymes that catalyze the oxidation of catechols. Numerous synthetic model complexes have been prepared to uncover the fundamental chemistry involved in these processes, but progress is still lagging much behind that for heme enzymes. One reason is that the latter gain very much from the informative spectroscopic features of their porphyrin-based metal-chelating ligand. We now introduce sapphyrin-chelated dicopper complexes and show that they may be isolated in different oxidation states and coordination geometries, with distinctive colors and electronic spectra due to the heme-like ligands. The dicopper(i) complex 1-Cu2 was characterized by 1H and 19F NMR spectroscopy of the metal-chelating sapphyrin, the oxygenated dicopper(ii) complex 1-Cu2O2 by EPR, and crystallographic data was obtained for the tetracopper(ii)-bis-sapphyrin complex [1-Cu2O2]2. This uncovered a non-heme [Cu4(OH)4]4- cluster, held together with the aid of two sapphyrin ligands, with structural features reminiscent of those of catechol oxidase. Biomimetic activity was demonstrated by the 1-Cu2O2 catalyzed aerobic oxidation of catechol to quinone; the sapphyrin ligand aided very much in gaining information about reactive intermediates and the rate-limiting step of the reaction.

Low-Power Laser Ignition of an Antenna-Type Secondary Energetic Copper Complex: Synthesis, Characterization, Evaluation, and Ignition Mechanism Studies.

In recent years, development of new energetic compounds and formulations, suitable for ignition with relatively low-power lasers, is a highly active and competitive field of research. The main goal of these efforts is focused on achieving and providing much safer solutions for various detonator and initiator systems. In this work, we prepared, characterized, and studied thermal and ignition properties of a new laser-ignitable compound, based on the 5,6-bis(ethylnitroamino)-N'2,N'3-dihydroxypyrazine-2,3-bis(carboximidamide) (DS3) proligand. This new energetic proligand was prepared in three steps, starting with 5,6-bis(ethylamino)-pyrazine-2,3-dicarbonitrile. Crystallography studies of the DS3-derived Cu(II) complex (DS4) revealed a unique stacked antenna-type structure of the latter compound. DS4 has an exothermal temperature of 154.5 °C and was calculated to exhibit a velocity of detonation of 6.36 km·s-1 and a detonation pressure of 15.21 GPa. DS4 showed properties of a secondary explosive, having sensitivity to impact, friction, and electrostatic discharge of 8 J, 360 N, and 12 mJ, respectively. In order to study the mechanism of ignition by a laser (using a diode laser, 915 nm), we conducted a set of experiments that enabled us to characterize a photothermal ignition mechanism. Furthermore, we found that a single pulse, with a time duration of 1 ms and with a total energy of 4.6 mJ, was sufficient for achieving a consistent and full ignition of DS4. Dual-pulse experiments, with variable time intervals between the laser pulses, showed that DS4 undergoes ignition via a photothermal mechanism. Finally, calculating the chemical mechanism of the formation of the complex DS4 and modeling its anhydrous and hydrated crystal structures (density functional theory calculations using Gaussian and HASEM software) allowed us to pinpoint a more precise location of water molecules in experimental crystallographic data. These results suggest that DS4 has potential for further development to a higher technology readiness level and for integration into small-size safe detonator systems as for many civil, aerospace, and defense applications.

Radicals and Anions of Siloles and Germoles.

Invited for the cover of this issue is the group of Thomas Müller at the University of Oldenburg. The image shows the calculated spin distribution of a persistent silacyclopentadienyl radical. Read the full text of the article at 10.1002/chem.202101415.

Carborane Stabilized "19-Electron" Molybdenum Metalloradical.

Paramagnetic metal complexes gained a lot of attention due to their participation in a number of important chemical reactions. In most cases, these complexes are dominated by 17-e metalloradicals that are associatively activated with highly reactive paramagnetic 19-e species. Molybdenum paramagnetic complexes are among the most investigated ones. While some examples of persistent 17-e Mo-centered radicals have been reported, in contrast, 19-e Mo-centered radicals are illusive species and as such could rarely be detected. In this work, the photodissociation of the [Cp(CO)3Mo]2 dimer (1) in the presence of phosphines was revisited. As a result, the first persistent, formally 19-e Mo radical with significant electron density on the Mo center (22%), Cp(CO)3Mo•PPh2(o-C2B10H11) (5b), was generated and characterized by EPR spectroscopy and MS as well as studied by DFT calculations. The stabilization of 5b was likely achieved due to a unique electron-withdrawing effect of the o-carboranyl substituent at the phosphorus center.

Radicals and Anions of Siloles and Germoles.

The synthesis of persistent sila- and germacyclopentadienyl (silolyl- and germolyl-) radicals by careful stoichiometric reduction of the corresponding halides with potassium is reported. The radicals were characterized by EPR spectroscopy and trapping reactions. The reduction of tris(trimethylsilyl)silyl-substituted halides was successful while smaller substituents (i. e., t-Butyl, Ph) gave the corresponding dimers. The EPR spectroscopic parameter of the synthesized tetrolyl radicals indicate only small spin delocalization to the butadiene unit due to cross-hyperconjugation. Silolyl- and germolyl anions are unavoidable byproducts and are isolated in the form of their potassium salts and characterized by X-ray crystallography. The comparison of the molecular structures of two closely related potassium silolides provided an example for different coordination of the potassium cation to the silolyl anion (η1 vs. η5 coordination) that triggers the switch between delocalized and localized states.

Isolable cyclic (alkyl)(amino)carbene-phosphonyl radical adducts.

Phosphonyl radicals ([R2P[double bond, length as m-dash]O]˙) and their adducts are proposed as intermediates in a number of important chemical and biological processes. Despite the great interest in these species, there are no examples of stable, isolated phosphonyl radicals or their adducts reported in the literature. Here we report the synthesis, EPR and theoretical study of stable, isolable cyclic (alkyl)(amino)carbene (cAAC)-phosphonyl radical adducts, [cAAC-P(O)R2]˙ (R = OPri, Ph).

The Reactions of Carbon Monoxide with Silyl and Silenyl Lithium - Synthesis and Isolation of the First Stable Tetra-Silyl Di-Ketyl Biradical and 1-Silaallenolate Lithium.

Reactions of carbon monoxide (CO) with tBu2 MeSiLi and (E)-(tBu2 MeSi)(tBuMe2 Si)C=Si(SiMetBu2 )Li⋅2 THF (4) were studied both experimentally and computationally. Reaction of tBu2 MeSiLi with CO in hexane yields the first stable tetra-silyl di-ketyl biradical [(tBu2 MeSi)2 COLi]. 2 (3). Reaction of 4 with CO yields selectively and quantitatively the first reported 1-silaallenolate, (tBu2 MeSi)(tBuMe2 Si)C=C=Si(SiMetBu2 )OLi⋅THF (5). Both 3 and 5 were characterized by X-ray crystallography and biradical 3 also by EPR spectroscopy. Silaallenolate 5 reacts with Me3 SiCl to produce siloxy substituted 1-silaallene (tBu2 MeSi)(tBuMe2 Si)C=C=Si(SiMetBu2 )OSiMe3 . The reaction of 4 with CO provides a new route to 1-silaallenes. The mechanisms of the reactions of tBuMe2 SiLi and of 4 with CO were studied by DFT calculations.

Corroles and corrole/transferrin nanoconjugates as candidates for sonodynamic therapy.

We report the utility of water-soluble corroles and also protein-coated nanoparticles (NPs) of lipophilic corroles as potent candidates for sonodynamic therapy (SDT), through the detection and quantification of the singlet oxygen that is produced by the ultrasonic irradiation of their aqueous solutions. Preliminary results on a cancer cell line provide evidence for the true utility of the NPs for SDT.

First α-deuterium nitroxides; synthesis and EPR study.

Herein is reported the first preparation of stable α-deuterium nitroxides of the IAPNO family. The confirmation and characteristics of the α-deuterium nitroxides and their α-hydrogen analogues are compared and analyzed. Such α-deuterium nitroxides may find use in biology, medicine and physical chemistry.

Generation and EPR Spectroscopy of the First Silenyl Radicals, R2 C=Si. -R: Experiment and Theory.

The first two persistent silenyl radicals (R2 C=Si. -R), with a half-life (t1/2 ) of about 30 min, were generated and characterized by electron paramagnetic resonance (EPR) spectroscopy. The large hyperfine coupling constants (hfccs) (a(29 Siα )=137.5-148.0 G) indicate that the unpaired electron has substantial s character. DFT calculations, which are in good agreement with the experimentally observed hfccs, predict a strongly bent structure (∡C=Si-R=134.7-140.7°). In contrast, the analogous vinyl radical, R2 C=C. -R (t1/2 ≈3 h), exhibits a small hfcc (a(13 Cα )=26.6 G) and has a nearly linear geometry (∡C=C-R=168.7°).

Manganese(iii) acetate-mediated activation of C-H bonds of weak CH-acids; addition of o-carborane, its derivatives, and some other CH-acids to [60]-fullerene.

o-Carborane, 9-I-o-carborane, 1-Me-o-carborane, and several other CH-acids, 9H-fluorene, 2-Br-9H-fluorene, and trimethylsylylacetylene, have been shown to react with C60 affording their monoadducts with fullerene, the reaction being mediated by Mn(OAc)3·2H2O. In the case of o-carborane, when the molar ratio of C60 : o-carborane : Mn(OAc)3·2H2O was 1 : 21 : 20, polyaddition occurred to furnish adducts bearing between one and six o-C2HB10H10 groups. A distinguishing characteristic of the carboranyl derivatives of C60 obtained appeared to be that the carboranyl moieties were connected to the fullerenyl one by their carbon atom. No such fullerene derivatives have been known so far. Based on the results previously obtained for the phosphonylation of fullerenes, an oxidative-ion-transfer (OIT) mechanism was suggested for the reactions. The mechanism involves the replacement of an acetate group in Mn(OAc)3 with the corresponding R of a CH-acid (RH), oxidation of C60 by Mn(OAc)2R with the formation of a pair [C60˙+, Mn(OAc)2R˙-] followed by the transfer of R- to C60˙+ to furnish the radical RC60˙. The radical abstracts a hydrogen atom from another molecule of RH or from the solvent. The polyaddition occurs in an analogous way. This mechanism found support in this study owing to the fact that the reactions of both o-carborane and 9H-fluorene did not proceed with perfluorobenzophenone as a substrate, because it was unable to undergo oxidation. C70 reacted with o-carborane in a similar way to give the corresponding monoadduct. No reaction of C60 with m-carborane was observed and this was explained by its insufficient CH-acidity.

Generation and Characterization of the First Persistent Platinum(I)-Centered Radical.

The first persistent platinum(I)-centered radical was generated by homolytic cleavage of a Pt-HgSiR3 bond of a mercury-substituted platinum(II) complex. The PtI radical was characterized by EPR spectroscopy, chemical trapping experiments, and density functional theory (DFT) calculations.

Reactive Intermediates Involved in Cobalt Corrole Catalyzed Water Oxidation (and Oxygen Reduction).

A detailed investigation of the cobalt corrole Co(tpfc) as molecular catalyst for electrochemical water oxidation uncovered many important mechanism-of-action details that are crucial for the design of optimally performing systems. This includes the identification of the redox states that do and do not participate in catalysis and very significant axial ligand effects on the activity of the doubly oxidized complex. Specifics deduced for the electrocatalysis under homogeneous conditions include the following: the one-electron oxidation of the cobalt(III) corrole is completely unaffected by reaction conditions; catalysis coincides with the second oxidation event; two catalytic waves develop in the presence of OH-, and the one at lower overpotential is dominant under more basic conditions. Comparative spectroelectrochemical measurements performed for Co(tpfc) and Al(tpfc), the analogous corrole chelated by the nonredox active aluminum, revealed that the second oxidation process of Co(tpfc) is much more significantly metal-centered than the first one. EPR studies revealed that shift from fully corrole-centered to partially metal-centered in the singly oxidized complex [Co(tpfc)]+ is achievable with fluoride as axial ligand. The analogous experiment, but with hydroxide instead of fluoride, could not be performed because of a surprising phenomenon: formation of a cobalt-superoxide complex that is actually relevant to oxygen reduction rather than to water oxidation. Nevertheless, fluoride and hydroxide induce very similar effects in terms of the appearance of two catalytic waves, lowering of onset potentials, and increasing the catalytic activity. The main conclusions from the accumulated data are that the apparent pH effect is actually due to hydroxide binding to the cobalt center and that π-donating axial ligands play pivotal and beneficial roles regarding the main factors that are important for facilitating the oxidation of water.

Effect of Selective CF3 Substitution on the Physical and Chemical Properties of Gold Corroles.

Gold corroles are not readily accessible and they display no interesting physical or chemical properties. A facile methodology has now been developed for obtaining selectively CF3 -substituted gold(III) corroles and the introduction of these groups has been found to have an immense effect on the structures of the complexes, their photophysical and redox properties, and on their ability to participate in catalytic processes.

Spectroscopic Observation of the Triplet Diradical State of a Cyclobutadiene.

Tetrakis(trimethylsilyl)cyclobuta-1,3-diene (1) was subjected to a temperature-dependent EPR study to allow the first spectroscopic observation of a triplet diradical state of a cyclobutadiene (2). From the temperature dependent EPR absorption area we derive a singlet→triplet (1→2) energy gap, EST , of 13.9 kcal mol-1 , in agreement with calculated values. The zero-field splitting parameters D=0.171 cm-1 , E=0 cm-1 are accurately reproduced by DFT calculations. The triplet diradical 2 is thermally accessible at moderate temperatures. It is not an intermediate in the thermal cycloreversion of cyclobutadiene to two acetylene molecules.

A mechanistic study of manganese(iii) acetate-mediated phosphonyl group additions to [60]- and [70]-fullerenes: the oxidative-ion-transfer mechanism vs. free radical addition.

The phosphonylation of C60 with HP(O)(OAlk)2 and Mn(OAc)3·2H2O has been considered to occur via a free radical (FR) path involving intermediate radicals ˙P(O)(OAlk)2. The present study provides evidence in support of another mechanism for the reactions, oxidative-ion-transfer (OIT). The mechanism involves the change of an acetate group in Mn(OAc)3 for the phosphonate group and oxidation of C60 by the Mn(OAc)2P(O)(OAlk)2 formed to a pair: (C60˙+, Mn(OAc)2P(O)(OAlk)2˙-) followed by the transfer of the phosphonate anion to give the monophposphonylfullerenyl radical. It undergoes reversible dimerization. The polyaddition occurs analogously. Moreover, the compounds Mn(OAc)2P(O)(OAlk)2 (Alk = Et and i-Pr) obtained make novel reagents for phosphonylation of fullerenes working by the OIT mechanism. The reactions of C60 in benzene with equimolar amounts of Mn(OAc)2P(O)(OPr-i)2 or Hg[P(O)(OPr-i)2]2 which is known as working by the FR mechanism since it produces radical ˙P(O)(OPr-i)2 under UV-irradiation, furnished the same radical ˙C60P(O)(OPr-i)2. However, at a 20-fold molar excess of the reagent toward C60, a single derivative C60[P(O)(OPr-i)2]4 and a mixture of derivatives bearing between two and eight phosphonyls were obtained in the former and latter cases, respectively. With C70, the change of the mechanism produced a change in the regioselectivity: 5 and 3 isomers of ˙C70P(O)(OPr-i)2 were obtained, respectively. DFT-calculations provided the hyperfine coupling (hfc) constants of the isomers and explained the regioselectivity change.