Iron-Catalyzed Oxidative C-O and C-N Coupling Reactions Using Air as Sole Oxidant.

We describe the oxygenation of tertiary arylamines, and the amination of tertiary arylamines and phenols. The key step of these coupling reactions is an iron-catalyzed oxidative C-O or C-N bond formation which generally provides the corresponding products in high yields and with excellent regioselectivity. The transformations are accomplished using hexadecafluorophthalocyanine-iron(II) (FePcF16 ) as catalyst in the presence of an acid or a base additive and require only ambient air as sole oxidant.

Electrophilic Trifluoromethylation of Dimetallofullerene Anions en Route to Air-Stable Single-Molecule Magnets with High Blocking Temperature of Magnetization.

Lanthanide dimetallofullerenes with single-electron M-M bonds are an important class of single molecular magnets and qubit candidates, but stabilization of their unique electronic and spin structure in the form of a neutral molecule requires functionalization of the fullerene cage with a single radical group. The lack of selectivity of the currently available procedure results in a complicated and tedious separation process. Here we demonstrate that electrophilic trifluoromethylation of a mixture of metallofullerene anions with Umemoto reagent II is highly selective toward M2@C80- (M = Tb, Y) anions, yielding M2@C80(CF3) monoadducts as the main reaction product. Single-crystal X-ray diffraction study proved attachment of the CF3 group to the pentagon/hexagon/hexagon junction and revealed that positions of metal atoms inside the fullerene cage in the cocrystal with NiOEP are strongly related to the position of the porphyrin moieties. Magnetic characterization of Tb2@C80(CF3) showed that it is a robust single-molecule magnet with broad magnetic hysteresis, 100 s blocking temperature of 25 K, and the relaxation barrier of 801(4) K, corresponding to the flipping of the Tb magnetic moment in the strongly ferromagnetically coupled [Tb3+-e-Tb3+] spin system.

Persistent peri-Heptacene: Synthesis and In Situ Characterization.

n-peri-Acenes (n-PAs) have gained interest as model systems of zigzag-edged graphene nanoribbons for potential applications in nanoelectronics and spintronics. However, the synthesis of n-PAs larger than peri-tetracene remains challenging because of their intrinsic open-shell character and high reactivity. Presented here is the synthesis of a hitherto unknown n-PA, that is, peri-heptacene (7-PA), in which the reactive zigzag edges are kinetically protected with eight 4-tBu-C6 H4 groups. The formation of 7-PA is validated by high-resolution mass spectrometry and in situ FT-Raman spectroscopy. 7-PA displays a narrow optical energy gap of 1.01 eV and exhibits persistent stability (t1/2 ≈25 min) under inert conditions. Moreover, electron-spin resonance measurements and theoretical studies reveal that 7-PA exhibits an open-shell feature and a significant tetraradical character. This strategy could be considered a modular approach for the construction of next-generation (3 N+1)-PAs (where N≥3).

Photoinduced Charge Accumulation and Prolonged Multielectron Storage for the Separation of Light and Dark Reaction.

The utilization of solar energy is restricted by the intermittent nature of solar influx. We present novel noble-metal free complexes that can be photochemically charged in the presence of sacrificial electron donors and remain stable in its charged form for over 14 h. This allows the doubly reduced Cu(I) 4H-imidazolate complex to be stored after photochemical charging and used as a reagent in dark reactions, such as the reduction of methyl viologen or oxygen. Combined UV-vis/EPR spectroelectrochemistry indicates that a two-electron reduction is induced by introducing sacrificial electron donors that facilitate proton-coupled electron transfer. Repeated photochemical reduction and chemical oxidation reveals that the complex retained a charging capacity of 72% after four cycles. We demonstrate a chemical system that can decouple photochemical processes from the day-night cycle, which has been a barrier to realizing utilization of solar energy in photochemical processes on a global scale.

(Electrochemical) Properties and Computational Investigations of Ferrocenyl-substituted Fe3(µ3-PFc)2(CO)9 and Co4(µ4-PFc)2(CO)9 Clusters and Their Reduced Species.

The formation of ferrocenyl-functionalized iron and cobalt carbonyl clusters is reported, based on a reaction of FcPCl2 (3) (Fc = Fe(η5-C5H5)(η5-C5H4)) with Fe2(CO)9 and Co2(CO)8, respectively. Therein, nido-Fe3(CO)9(µ3-PFc)2 (4) and nido-Co4(CO)10(µ3-PFc)2 (5) clusters were obtained as the first diferrocenyl-substituted carbonyl clusters with a symmetrical cluster core. Cluster 4 shows two reversible one-electron processes within the anodic region, based on Fc/Fc+ redox events, as well as two processes in the cathodic region. In situ IR and electron paramagnetic resonance (EPR) measurements of all electronic states confirmed an Fc-based oxidation and a core-based reduction. On the basis of the results of a single-crystal X-ray analysis of structures of 4 and 5, computational studies of the highest occupied molecular orbital-lowest unoccupied molecular orbital energies, the spin density, quantum theory of atom-in-molecule delocalization indices, and the atomic charges were performed to explain the experimental results. The latter revealed a reorganization of the cluster core upon reduction and the existence of weak P···P interactions in 4 and 5. Ferrocenyl-related redox processes, occurring reversibly in case of 4, were absent for 5, due to a different distribution of the HOMO energies. EPR measurements furthermore confirmed the core-based radical anion and the formation of a decomposition product at potentials lower than [M]2- (M = Fe, Co).

Quinoidal Azaacenes: 99 % Diradical Character.

Quinoidal azaacenes with almost pure diradical character (y=0.95 to y=0.99) were synthesized. All compounds exhibit paramagnetic behavior investigated by EPR and NMR spectroscopy, and SQUID measurements, revealing thermally populated triplet states with an extremely low-energy gap ΔEST' of 0.58 to 1.0 kcal mol-1 . The species are persistent in solution (half-life≈14-21 h) and in the solid state they are stable for weeks.

Synthesis and Isolation of the Titanium-Scandium Endohedral Fullerenes-Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 and Sc2 TiC2 @Ih -C80 : Metal Size Tuning of the Ti(IV) /Ti(III) Redox Potentials.

The formation of endohedral metallofullerenes (EMFs) in an electric arc is reported for the mixed-metal Sc-Ti system utilizing methane as a reactive gas. Comparison of these results with those from the Sc/CH4 and Ti/CH4 systems as well as syntheses without methane revealed a strong mutual influence of all key components on the product distribution. Whereas a methane atmosphere alone suppresses the formation of empty cage fullerenes, the Ti/CH4 system forms mainly empty cage fullerenes. In contrast, the main fullerene products in the Sc/CH4 system are Sc4 C2 @C80 (the most abundant EMF from this synthesis), Sc3 C2 @C80 , isomers of Sc2 C2 @C82 , and the family Sc2 C2 n (2 n=74, 76, 82, 86, 90, etc.), as well as Sc3 CH@C80 . The Sc-Ti/CH4 system produces the mixed-metal Sc2 TiC@C2 n (2 n=68, 78, 80) and Sc2 TiC2 @C2 n (2 n=80) clusterfullerene families. The molecular structures of the new, transition-metal-containing endohedral fullerenes, Sc2 TiC@Ih -C80 , Sc2 TiC@D5h -C80 , and Sc2 TiC2 @Ih -C80 , were characterized by NMR spectroscopy. The structure of Sc2 TiC@Ih -C80 was also determined by single-crystal X-ray diffraction, which demonstrated the presence of a short Ti=C double bond. Both Sc2 TiC- and Sc2 TiC2 -containing clusterfullerenes have Ti-localized LUMOs. Encapsulation of the redox-active Ti ion inside the fullerene cage enables analysis of the cluster-cage strain in the endohedral fullerenes through electrochemical measurements.

(19)F NMR-, ESR-, and vis-NIR-spectroelectrochemical study of the unconventional reduction behaviour of a perfluoroalkylated fullerene: dimerization of the C70(CF3)10(-) radical anion.

The most abundant isomer of C70(CF3)10 (70-10-1) is a rare example of a perfluoroalkylated fullerene exhibiting electrochemically irreversible reduction. We show that electrochemical reversibility at the first reduction step is achieved at scan rates higher than 500 V s(-1). Applying ESR-, vis-NIR-, and (19)F NMR-spectroelectrochemistry, as well as mass spectrometry and DFT calculations, we show that the (70-10-1)(-) radical monoanion is in equilibrium with a singly-bonded diamagnetic dimeric dianion. This study is the first example of (19)F NMR spectroelectrochemistry, which promises to be an important method for the elucidation of redox mechanisms of fluoroorganic compounds. Additionally, we demonstrate the importance of combining different spectroelectrochemical methods and quantitative analysis of the transferred charge and spin numbers in the determination of the redox mechanism.

Methane as a Selectivity Booster in the Arc-Discharge Synthesis of Endohedral Fullerenes: Selective Synthesis of the Single-Molecule Magnet Dy2TiC@C80 and Its Congener Dy2TiC2@C80.

The use of methane as a reactive gas dramatically increases the selectivity of the arc-discharge synthesis of M-Ti-carbide clusterfullerenes (M=Y, Nd, Gd, Dy, Er, Lu). Optimization of the process parameters allows the synthesis of Dy2TiC@C80-I and its facile isolation in a single chromatographic step. A new type of cluster with an endohedral acetylide unit, M2TiC2@C80, is discovered along with the second isomer of M2TiC@C80. Dy2TiC@C80-(I,II) and Dy2TiC2@C80-I are shown to be single-molecule magnets (SMM), but the presence of the second carbon atom in the cluster Dy2TiC2@C80 leads to substantially poorer SMM properties.

Nd─Nd Bond in Ih and D5h Cage Isomers of Nd2 @C80 Stabilized by Electrophilic CF3 Addition.

Synthesis of molecular compounds with metal-metal bonds between 4f elements is recognized as one of the fascinating milestones in lanthanide metallochemistry. The main focus of such studies is on heavy lanthanides due to the interest in their magnetism, while bonding between light lanthanides remains unexplored. In this work, the Nd─Nd bonding in Nd-dimetallofullerenes as a case study of metal-metal bonding between early lanthanides is demonstrated. Combined experimental and computational study proves that pristine Nd2 @C80 has an open shell structure with a single electron occupying the Nd─Nd bonding orbital. Nd2 @C80 is stabilized by a one-electron reduction and further by the electrophilic CF3 addition to [Nd2 @C80 ]- . Single-crystal X-ray diffraction reveals the formation of two Nd2 @C80 (CF3 ) isomers with D5h -C80 and Ih -C80 carbon cages, both featuring a single-electron Nd─Nd bond with the length of 3.78-3.79 Å. The mutual influence of the exohedral CF3 group and endohedral metal dimer in determining the molecular structure of the adducts is analyzed. Unlike Tb or Dy analogs, which are strong single-molecule magnets with high blocking temperature of magnetization, the slow relaxation of magnetization in Nd2 @Ih -C80 (CF3 ) is detectable via out-of-phase magnetic susceptibility only below 3 K and in the presence of magnetic field.

Covalency versus magnetic axiality in Nd molecular magnets: Nd-photoluminescence, strong ligand-field, and unprecedented nephelauxetic effect in fullerenes NdM2N@C80 (M = Sc, Lu, Y).

Nd-based nitride clusterfullerenes NdM2N@C80 with rare-earth metals of different sizes (M = Sc, Y, Lu) were synthesized to elucidate the influence of the cluster composition, shape and internal strain on the structural and magnetic properties. Single crystal X-ray diffraction revealed a very short Nd-N bond length in NdSc2N@C80. For Lu and Y analogs, the further shortening of the Nd-N bond and pyramidalization of the NdM2N cluster are predicted by DFT calculations as a result of the increased cluster size and a strain caused by the limited size of the fullerene cage. The short distance between Nd and nitride ions leads to a very large ligand-field splitting of Nd3+ of 1100-1200 cm-1, while the variation of the NdM2N cluster composition and concomitant internal strain results in the noticeable modulation of the splitting, which could be directly assessed from the well-resolved fine structure in the Nd-based photoluminescence spectra of NdM2N@C80 clusterfullerenes. Photoluminescence measurements also revealed an unprecedentedly strong nephelauxetic effect, pointing to a high degree of covalency. The latter appears detrimental to the magnetic axiality despite the strong ligand field. As a result, the ground magnetic state has considerable transversal components of the pseudospin g-tensor, and the slow magnetic relaxation of NdSc2N@C80 could be observed by AC magnetometry only in the presence of a magnetic field. A combination of the well-resolved magneto-optical states and slow relaxation of magnetization suggests that Nd clusterfullerenes can be useful building blocks for magneto-photonic quantum technologies.

Combined spectroelectrochemical and theoretical study of electron-rich dendritic 2,5-diaminothiophene derivatives: N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine.

The in situ spectroelectrochemical and electron spin resonance (ESR) behavior of the recently prepared N,N,N',N'-tetrakis-(4-diphenylamino-phenyl)-thiophene-2,5-diamine 11 is presented. The results are compared to the ones of the parent 2,5-bis-diphenylamino-thiophene 41 as well as to the corresponding high-molar third dendrimer generation 8 containing the same thiophene-2,5-diamine core. The dendritic compound 11 can be reversibly oxidized in three separated steps to yield the corresponding stable monocation 11(•+), dication 11(2+), and tetracation 11(4+). A well resolved ESR spectrum of the corresponding cation radical 11(•+) with dominating splittings from two nitrogen atoms and two hydrogen atoms was observed at the first oxidation peak similar to 41(•+). The shape of the SOMOs orbitals very well correlates with the proposed distribution of the unpaired electron mainly on the thiophene center and neighboring nitrogen atoms. The spin delocalization on the central thiophene moiety in the monocations for all three model compounds 41(•+), 11(•+), and 8(•+) was confirmed. The computed single occupied molecular orbital (SOMO) for trication 11(•3+) is completely different compared to the SOMO of the corresponding monocation 11(•+), and it confirms a largely delocalized unpaired spin density. Dominating diamagnetic product was determined at the third oxidation peak, confirming the formation of a tetracation by a two electron oxidation of ESR silent dication. The positive charge is fully delocalized over the lateral parts of the molecule leading to the high stability of tetracation 11(4+). The estimated theoretical limit energy of the lowest optical transition S0 → S1 is 2.90 eV, and it can be achieved for the 3D dendrimer generation.

Formation of free radicals in human hair under strain: Combined electron paramagnetic resonance (EPR) - Strain technique.

In this work, the formation of free radicals in human hair and the evolution of the radical concentration under strain, using a combined electron paramagnetic resonance (EPR) - strain technique, has been investigated. The radicals formed in the hair as a result of homolytic bond cleavage in cystine residues of polypeptide chains were identified. Stability of the radicals formed in dry hair and in the presence of water were studied. The spin-strain curves for the grey human hair in dry state and in water are presented and compared with conventional hair stress vs. strain curves. The evolution of sulfur radical species has been found to only occur at strains of above ca. 25%, corresponding with the Post-Yield region of the stress vs. strain behaviour for hair; this indicates that the matrix of the hair in this region behaves like a highly crosslinked gel and helps to explain the reversibility in hair mechanical behaviour below the Post-Yield region.

Unusually large hyperfine structure of the electron spin levels in an endohedral dimetallofullerene and its spin coherent properties.

We report the synthesis, ESR spectroscopic and spin coherent properties of the dimetallofullerene Sc2@C80(CH2Ph). The single-electron metal-metal bond of the Sc2 dimer inside the fullerene's cage is stabilized with the electron spin density being fully localized at the metal bond. This results in an extraordinary strong hyperfine interaction of the electron spin with the 45Sc nuclear spins with a coupling constant a = 18.2 mT (∼510 MHz) and yields a fully resolved hyperfine-split ESR spectrum comprising 64 lines. The splitting is present even at low temperatures where the molecular dynamics are completely frozen. The large extent and the robustness of the hyperfine-split spectra enable us to identify and control the well-defined transitions between specific electron-nuclear quantum states. This made it possible to demonstrate in our pulse ESR study the remarkable spin coherent dynamics of Sc2@C80(CH2Ph), such as the generation of arbitrary superpositions of the spin states in a nutation experiment and the spin dephasing times above 10 µs at temperatures T < 80 K reaching the value of 17 µs at T ≤ 20 K. These observations suggest Sc2@C80(CH2Ph) as an interesting qubit candidate and motivate further synthetic efforts to obtain fullerene-based systems with superior spin properties.

Syntheses and Molecular Structures of Liquid Pyrophoric Hydridosilanes.

Trisilane, isotetrasilane, neopentasilane, and cyclohexasilane have been prepared in gram scale. In-situ cryo crystallization of these pyrophoric liquids in sealed capillaries on the diffractometer allows access to the single crystal structures of these compounds. Structural parameters are discussed and compared to gas-phase electron diffraction structures from literature and with the results from quantum chemical calculations. Significantly higher packing indices are found for the silanes compared to the corresponding alkanes. Radiation with ultraviolet light (365 nm) and parallel ESR (EPR) measurement shows that cyclohexasilane is easily split into radicals, which subsequently leads to the formation of branched and chain-like oligomers. The other compounds form no radicals under these conditions. NMR spectra of all four compounds have been recorded.

Addition of CF2 group to endohedral fullerene Sc3N@Ih-C80.

We report the first successful synthesis of a CF2 derivative of the stable endohedral fullerene Sc3N@Ih-C80. Reaction with CF2ClCOONa yields a single Cs-symmetric Sc3N@C80(CF2) adduct where the CF2 group is inserted into a [6,6]-bond and opens it to 2.3 Å between the bridgehead carbon atoms. As evidenced by absorption and fluorescence spectroscopy as well as cyclic voltammetry, both the HOMO and the LUMO level of Sc3N@C80(CF2) are slightly (ca. 0.1 eV) downshifted with respect to the parent Sc3N@Ih-C80, so the HOMO-LUMO gap remains essentially unchanged. The DFT calculations suggest that the reaction mechanism is not the previously assumed [2 + 1]-cycloaddition of :CF2 carbene but rather nucleophilic addition of CF2Cl- anion followed by elimination of Cl- and closing of the CF2 bridge via intramolecular nucleophilic substitution. Selective formation of the [6,6]-Sc3N@C80(CF2) turns out to be kinetically controlled and promoted by a particular orientation of the endohedral Sc3N cluster with respect to the CF2Cl- addition site. In its turn, the CF2 addend partly hampers the rotation of Sc3N the endohedral cluster compared to its quasi-free reorientations in the parent Sc3N@Ih-C80.

Air-stable redox-active nanomagnets with lanthanide spins radical-bridged by a metal-metal bond.

Engineering intramolecular exchange interactions between magnetic metal atoms is a ubiquitous strategy for designing molecular magnets. For lanthanides, the localized nature of 4f electrons usually results in weak exchange coupling. Mediating magnetic interactions between lanthanide ions via radical bridges is a fruitful strategy towards stronger coupling. In this work we explore the limiting case when the role of a radical bridge is played by a single unpaired electron. We synthesize an array of air-stable Ln2@C80(CH2Ph) dimetallofullerenes (Ln2 = Y2, Gd2, Tb2, Dy2, Ho2, Er2, TbY, TbGd) featuring a covalent lanthanide-lanthanide bond. The lanthanide spins are glued together by very strong exchange interactions between 4f moments and a single electron residing on the metal-metal bonding orbital. Tb2@C80(CH2Ph) shows a gigantic coercivity of 8.2 Tesla at 5 K and a high 100-s blocking temperature of magnetization of 25.2 K. The Ln-Ln bonding orbital in Ln2@C80(CH2Ph) is redox active, enabling electrochemical tuning of the magnetism.

A solvent- and temperature-dependent intramolecular equilibrium of diamagnetic and paramagnetic states in Co complexes bearing triaryl amines.

Complexes [Co(L)2](ClO4)2 (L = o-substituted 2-(pyridine-2-yl)-1,10-phenanthrolines 1a-c) containing three redox active centres (a Co2+ ion and two triaryl amine (Tara) units) have been synthesised. The order of oxidation steps in [Co(L)2](ClO4)2 (L = 1a-c) was determined using cyclic voltammetry and EPR/UV-vis-NIR spectroelectrochemistry. In acetonitrile solutions, at room temperature, the first oxidation is Co-centred followed by the Tara oxidation at more anodic potentials. The order of oxidation is inverted in solutions of the less polar solvent dichloromethane. The Co3+/2+-centred redox event leads to a spin transition between the paramagnetic high-spin (HS) Co2+ and the diamagnetic low-spin (LS) Co3+ state, which was proven using 1H NMR and EPR spectroscopy. After one-electron oxidation of [Co(L)2](ClO4)2, an equilibrium between the diamagnetic [Co3+(L)]3+ and paramagnetic [Co2+(L)(L+)]3+ state in [Co(L)2]3+ (L = 1a-c) was found. Cyclic voltammetry showed enhanced intermolecular electron transfer between the [Co2+(L)2]2+ and [Co3+(L)2]3+ redox states mediated by [Co2+(L)(L+)]3+. Variable temperature vis-NIR spectroscopy of in situ generated [Co(L)2]3+ revealed a temperature-dependent redox equilibrium between the [Co3+(L)2]3+ and the [Co2+(L+)(L)]3+ states (L = 1a-c). Magnetic coupling between the HS-Co2+ ion and the Tara+ radical in [HS-Co2+(L+)(L)]3+ (L = 1a,c) was deduced from broad and undetectable lines observed in the corresponding EPR spectra. Complete oxidation to [LS-Co3+(L+)2]5+ (L = 1a,c) leads to characteristic EPR spectra of Tara biradicals with non-interacting spins.

Redox properties and electron transfer in a triarylamine-substituted HS-Co2+/LS-Co3+ redox couple.

A new tridentate phenanthroline-pyridyl-based ligand 1 containing a redox active Tara (triaryl amine) unit has been developed (1 = 4-((6-(1,10-phenanthrolin-2-yl)pyridin-2-yl)oxy)-N,N-di-p-tolylaniline). The complex [Co2+(1)2](ClO4/BF4)2 was prepared and the order of the oxidation steps was analysed by cyclic voltammetry and EPR/UV-vis-NIR spectroelectrochemistry. Oxidation of [Co2+(1)2]2+ to [Co3+(1+)2]5+ proceeds in two steps. The first step is the Co2+/3+ centred oxidation to [Co3+(1)2]3+ (E°'(M2+/3+) = 284 mV vs. Fc/Fc+) followed by oxidation of the Tara0/+ centres (E°'(Tara) = 531 mV). Both kinds of oxidation processes were independently investigated in the analogous complexes [Zn(1)2](ClO4)2 and [Co(2)2](BF4)2 allowing an assignment of changes in the electronic spectra to the redox states (2 = 2-(6-phenoxypyridin-2-yl)-1,10-phenanthroline). Although spectroelectrochemistry did not indicate substantial coupling between the redox centres the Tara unit is an efficient mediator for the self-exchange in the [Co2+/3+(1)2]2+/3+ couple. The electron transfer by self-exchange in [Co2+/3+(1)2]2+/3+ was further investigated by variable temperature (VT) 1H NMR spectroscopy. In addition, the resonances found in the paramagnetic proton NMR spectra were assigned by using COSY, T1 and EXSY measurements in combination with the Co-N distances obtained from X-ray analysis. [Co(1)2]2+ is found in the HS state. In contrast, the Fe2+ species [Fe(1)2](ClO4)2 is a spincrossover system. The SCO was analysed in solution by VT 1H NMR and VT/vis spectroscopy.

Single molecule magnet with an unpaired electron trapped between two lanthanide ions inside a fullerene.

Increasing the temperature at which molecules behave as single-molecule magnets is a serious challenge in molecular magnetism. One of the ways to address this problem is to create the molecules with strongly coupled lanthanide ions. In this work, endohedral metallofullerenes Y2@C80 and Dy2@C80 are obtained in the form of air-stable benzyl monoadducts. Both feature an unpaired electron trapped between metal ions, thus forming a single-electron metal-metal bond. Giant exchange interactions between lanthanide ions and the unpaired electron result in single-molecule magnetism of Dy2@C80(CH2Ph) with a record-high 100 s blocking temperature of 18 K. All magnetic moments in Dy2@C80(CH2Ph) are parallel and couple ferromagnetically to form a single spin unit of 21 µB with a dysprosium-electron exchange constant of 32 cm-1. The barrier of the magnetization reversal of 613 K is assigned to the state in which the spin of one Dy centre is flipped.