Intramolecular Through-Space Double-Electron Transfer Between A Pair of Redox-Active Guanidine Units Aligned by Dithiolate Bridges.

Using unconventional synthesis protocols, two redox-active triguanidine units are connected by a dithiolate bridge, aligning the two redox-active units in close proximity. The reduced, neutral and the tetracationic redox states with two dicationic triguanidine units are isolated and fully characterized. Then, the dicationic redox states are prepared by mixing the neutral and tetracationic molecules. At low temperatures, the dications are diamagnetic (singlet ground state) with two different triguanidine units (neutral and dicationic). At room temperature, the triplet state with two radical monocationic triguanidine units is populated. At low temperature (210 K), chemical exchange by intramolecular through-space electron-transfer between the two triguanidine units is evidenced by EXSY NMR spectroscopy. Intramolecular through-space transfer of two electrons from the neutral to the dicationic triguanidine unit is accompanied by migration of the counterions in opposite direction. The rate of double-electron transfer critically depends on the bridge. No electron-transfer is measured in the absence of a bridge (in a mixture of one dicationic and one neutral triguanidine), and relatively slow electron transfer if the bridge does not allow the two triguanidine units to approach each other close enough. The results give detailed, quantitative insight into the factors that influence intramolecular through-space double-electron-transfer processes.

Osteocytes Influence on Bone Matrix Integrity Affects Biomechanical Competence at Bone-Implant Interface of Bioactive-Coated Titanium Implants in Rat Tibiae.

Osseointegration is a prerequisite for the long-term success of implants. Titanium implants are preferred for their biocompatibility and mechanical properties. Nonetheless, the need for early and immediate loading requires enhancing these properties by adding bioactive coatings. In this preclinical study, extracellular matrix properties and cellular balance at the implant/bone interface was examined. Polyelectrolyte multilayers of chitosan and gelatin or with chitosan and Hyaluronic acid fabricated on titanium alloy using a layer-by-layer self-assembly process were compared with native titanium alloy. The study aimed to histologically evaluate bone parameters that correlate to the biomechanical anchorage enhancement resulted from bioactive coatings of titanium implants in a rat animal model. Superior collagen fiber arrangements and an increased number of active osteocytes reflected a significant improvement of bone matrix quality at the bone interface of the chitosan/gelatin-coated titan implants over chitosan/hyaluronic acid-coated and native implants. Furthermore, the numbers and localization of osteoblasts and osteoclasts in the reparative and remodeling phases suggested a better cellular balance in the chitosan/Gel-coated group over the other two groups. Investigating the micro-mechanical properties of bone tissue at the interface can elucidate detailed discrepancies between different promising bioactive coatings of titanium alloys to maximize their benefit in future medical applications.

Evaluation of the Synthetic Scope and the Reaction Pathways of Proton-Coupled Electron Transfer with Redox-Active Guanidines in C-H Activation Processes.

Proton-coupled electron transfer (PCET) is currently intensively studied because of its importance in synthetic chemistry and biology. In recent years it was shown that redox-active guanidines are capable PCET reagents for the selective oxidation of organic molecules. In this work, the scope of their PCET reactivity regarding reactions that involve C-H activation is explored and kinetic studies carried out to disclose the reaction mechanisms. Organic molecules with potential up to 1.2 V vs. ferrocenium/ferrocene are efficiently oxidized. Reactions are initiated by electron transfer, followed by slow proton transfer from an electron-transfer equilibrium.

Redox-Active Guanidines in Proton-Coupled Electron-Transfer Reactions: Real Alternatives to Benzoquinones?

Guanidino-functionalized aromatics (GFAs) are readily available, stable organic redox-active compounds. In this work we apply one particular GFA compound, 1,2,4,5-tetrakis(tetramethylguanidino)benzene, in its oxidized form in a variety of oxidation/oxidative coupling reactions to demonstrate the scope of its proton-coupled electron transfer (PCET) reactivity. Addition of an excess of acid boosts its oxidation power, enabling the oxidative coupling of substrates with redox potentials of at least +0.77 V vs. Fc+ /Fc. The green recyclability by catalytic re-oxidation with dioxygen is also shown. Finally, a direct comparison indicates that GFAs are real alternatives to toxic halo- or cyano-substituted benzoquinones.

Oxidation of Organic Molecules with a Redox-Active Guanidine Catalyst.

Herein, we report the first examples of the use of redox-active guanidines as catalysts in the green oxidation of organic molecules with dioxygen. In one half-reaction, the oxidized form of the redox-active guanidine is converted into the reduced, protonated state, thereby enabling dehydrogenative oxidation of the substrate (3,5-di-tert-butylcatechol→ortho-benzoquinone, benzoin→benzil, and 2,4-di-tert-butylphenol→biphenol). In the other half-reaction, efficient re-oxidation of the guanidine to the oxidized state is achieved with dioxygen in the presence of a copper catalyst. These results pave the way for the broader use of redox-active guanidines as oxidation catalysts.

Dehydrogenative Coupling Reactions with Oxidized Guanidino-Functionalized Aromatic Compounds: Novel Options for σ-Bond Activation.

We present a new option for metal-free σ-bond activation, making use of oxidized, guanidino-functionalized aromatic compounds (GFAs). We demonstrate this new option by the homocoupling reactions of thiols and phosphines. The kinetics and the reaction pathway were studied by a number of experiments (including heterocoupling of thiols and phosphines), supported by quantum-chemical computations. Reaction of the oxidized GFA with p-dihydrobenzoquinone to give p-benzoquinone shows that typical proton-coupled electron-transfer reactions are also possible.

Redox-controlled hydrogen bonding: turning a superbase into a strong hydrogen-bond donor.

Herein the synthesis, structures and properties of hydrogen-bonded aggregates involving redox-active guanidine superbases are reported. Reversible hydrogen bonding is switched on by oxidation of the hydrogen-donor unit, and leads to formation of aggregates in which the hydrogen-bond donor unit is sandwiched by two hydrogen-bond acceptor units. Further oxidation (of the acceptor units) leads again to deaggregation. Aggregate formation is associated with a distinct color change, and the electronic situation could be described as a frozen stage on the way to hydrogen transfer. A further increase in the basicity of the hydrogen-bond acceptor leads to deprotonation reactions.

Tetracyanoquinodimethane reduction by complexed guanidinyl-functionalized aromatic compounds.

In this work, we report on the reduction of tetracyanoquinodimethane (TCNQ) with dicationic complexes of guanidinyl-functionalized aromatic (GFA) electron donors. In contrast to reduction with free GFAs, milder reduction conditions were achieved, and this led to semiconducting materials with extended TCNQ π stacking. The charge on the TCNQ units was estimated from the structural data obtained by single-crystal X-ray diffraction analysis and from IR spectroscopic data. The electrical conductivity was studied and the activation energy of the semiconducting materials was estimated from the temperature dependence of the conductivity.

Mono- and diprotonation of the superbasic bisguanidine 1,2-Bis(N,N,N',N'-tetramethylguanidino)benzene (btmgb) and Pt II and Pt IV complexes of chelating bisguanidines and guanidinates.

New Pt complexes of chelating bisguanidines and guanidinate ligands were synthesized and characterized. 1,2-Bis(N,N,N',N'-tetramethylguanidino)benzene (btmgb) was used as a neutral chelating bisguanidine ligand, and 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate (hpp(-)) as a guanidinate ligand. The salts [btmgbH]+[HOB(C6F5)3](-) and [btmgbH2]Cl2 and the complexes [(btmgb)PtCl2], [(btmgb)PtCl(dmso)]+[PtCl3(dmso)](-), and [(btmgb)PtCl(dmso)]+[Cl(-)] were synthesized and characterized. In the [btmgbH]+ cation the proton is bound to only one N atom. In the other complexes, both imine N atoms are coordinated to the Pt II, thus adopting a eta2-coordinational mode. The hpp(-) anion, which usually prefers a bridging binding mode in dinuclear complexes, is eta2-coordinated in the Pt IV complex [(eta2-hpp)(hppH)PtCl2(N(H)C(O)CH3)], which is formed (in low yield) by reaction between cis-[(hppH)2PtCl2] and H2O2 in CH3CN.

Ligand functionality as a versatile tool to control the assembly behavior of preformed titania nanocrystals.

Nanoparticle powders composed of surface-functionalized anatase crystals with diameters of about 3 nm self-organize into different structures upon redispersion in water. The assembly is directed by a small amount of a low-molecular-weight functional ligand (the "assembler") adsorbed on the surface of the nanoparticles. The ligand functionality determines the anisotropy of the resulting structures. Multidentate ligands, such as trizma ((HOCH(2))(3)CNH(2)) and serinol ((HOCH(2))(2)CNH(2)), with a chargeable terminal group preferentially induce the formation of anisotropic nanostructures several hundreds of nanometers in total length, whereas all the other investigated ligands (ethanolamine H(2)N(CH(2))(2)OH, glycine hydroxamate H(2)NCH(2)CONHOH, dopamine (OH)(2)C(6)H(3)(CH(2))(2)NH(3)Cl, tris (HOCH(2))(3)CCH(3)) mainly lead to uncontrolled agglomeration. Experimental data suggests that the anisotropic assembly is a consequence of the water-promoted desorption of the organic ligands from the {001} faces of the crystalline building blocks together with the dissociative adsorption of water on these crystal faces. Both processes induce the preferred attachment of the titania nanoparticles along the [001] direction. The use of polydentate and charged ligands to functionalize the surface of nanoparticles thus provides a versatile tool to control their arrangement on the nanoscale.

Gold-nanoparticle/organic linker films: self-assembly, electronic and structural characterisation, composition and vapour sensitivity.

Gold-nanoparticle/organic films were prepared via layer-by-layer self-assembly using dodecylamine-stabilised Au-nanoparticles and poly(propyleneimine) (PPI) dendrimers of generation one to five (G1-G5) or hexadecanedithiol (HDT) as linker compounds. TEM and FE-SEM images revealed that the bulk of the films consisted of nanoparticles with diameters of about 4 nm. XPS was used to study the chemical composition of the films. The C 1s and N 1s signals of an AuPPI-G4 film were interpreted qualitatively according to the dendrimer structure. The absence of the nitrogen signal in case of an AuHDT film indicated that the dodecylamine ligands were quantitatively exchanged during film assembly. About 76% of the sulfur atoms were bound to the nanoparticles. the remainder being present as free thiol (S H) groups. All films displayed linear current voltage characteristics and Arrhenius-type activation of charge transport. The conductivities of the AuPPI films decreased exponentially over approximately two orders of magnitude (6.8 x 10(-2) to 1.0 x 10(-3) ohms(-1) cm(-1)) with a five-fold increase of the dendrimer generation number. Dosing the films with solvent vapours caused their resistances to increase. Using different solvent vapours demonstrated that the sensitivity of this response was determined by the solubility properties of the linker compounds. Microgravimetric measurements showed that absorption of analyte was consistent with a Langmuir adsorption model. These measurements also revealed a linear correlation between the electrical response (deltaR/Rini) and the concentration of absorbed analyte. The absorption of d4-methanol from a saturated vapour atmosphere was studied by neutron reflectometry with an AuPPI-G4 film. This measurement indicated condensation of methanol on top of the film and a uniform distribution of the analyte across the film thickness.

Structure and dynamics of the interface between a Ag single crystal electrode and an aqueous electrolyte.

The aim of this work is to elucidate the initial steps of the electrochemical oxidation of Ag(111) in alkaline electrolytes. We use electrochemical as well as ex situ (XPS) and in situ (SHG) spectroscopic techniques to reconstruct the Ag(111)/electrolyte interface as a complex dynamic entity. Moving in the direction from negative to positive potentials we first observe specific adsorption of hydroxide ions, which starts at ca. -1.1 V vs. Ag/Ag2O in 0.1 M NaOH. SHG data prove that hydroxide retains its negative charge. At -0.3 V oxidation of the surface sets in with the formation of negatively charged adsorbed oxygen species and Ag+ ions, which give rise to peaks at 528.2 +/- 0.2 eV and at 367.7 eV in the O 1s and the Ag 3d(5/2) XP spectra, respectively. Around -0.1 V the adlayer is transformed into an ordered surface oxide phase which grows via a nucleation and growth mechanism. Above the reversible Ag/Ag2O potential the 2D Ag(I) oxide transforms into a 3D Ag(I) oxide. The electrochemical oxidation is compared with the previously studied gas-phase process, demonstrating both remarkable similarities as well as some differences.