Extending Chirality in Group XIV Metallatranes.

The syntheses of the racemic amino alcohol rac-N(CH2CMe2OH)(CMe2CH2OH)(CH2CHMeOH) (L22'1*H3, 2) and its representative N(CH2CMe2OH)(CMe2CH2OH)(CH2C(R)HMeOH) (L22'1RH3, 3) with the stereogenic carbon center being R-configured are reported. Also reported are the stannatranes L22'1*SnOt-Bu (4) L22'1RSnOt-Bu (6) and germatranes L22'1*GeOEt (5) and L22'1RGeOEt (7) as well as the trinuclear tin oxocluster [(µ3-O)(µ3-O-t-Bu){SnL22'1R}3] (8). NMR and IR spectroscopy, electrospray ionization mass spectrometry (ESI MS), and single crystal X-ray diffraction analysis characterize these compounds. Computational studies accompany the experimental work and help understand the diastereoselectivity observed in the course of the metallatrane syntheses.

Electrooxidation of Hypercoordinated Derivatives of Silicon and Reactivity of Their Electrogenerated Cation Radicals: 1-Substituted Silatranes.

Electrochemical oxidation of 1-R-substituted silatranes 1 (R = Me, vinyl, (CH2)2CN, CH2Ph, CH2(C10H7), Ph, C6H4Me, p-Cl-C6H4, Cl)-classical representatives of pentacoordinated silicon compounds-and the formation of their short living cation radicals upon reversible or quasi-reversible one-electron withdrawal were studied by means of cyclic and square-wave voltammetry, faradaic impedance spectroscopy and real-time temperature-dependent EPR spectroelectrochemistry supported by DFT B3PW91/6-311++G(d,p) (C-PCM, acetonitrile) calculations. The main reaction responsible for the decay of 1+• is shown to be their deprotonation, and ways of increasing the stability of these species are proposed.

Molecular Design of Silicon-Containing Diazenes: Absorbance of E and Z Isomers in the Near-Infrared Region.

The effective use of photochromic systems based on azo compounds in a number of applications, especially biomedical and pharmacological ones, is impeded by the unresolved problem of their E⇆Z isomerization in the near-IR region, NIR (780-1400 nm). We have demonstrated at the TD-DFT, STEOM-DLPNO-CCSD and CASSCF-NEVPT2 levels of theory that the presence of a silylated diazene core -Si-N=N-Si- with three-, tetra- or five-coordinated silicon atoms practically guarantees the absorption of the E and Z forms of such derivatives in NIR and the amazing (185-400 nm) separation of their first absorption bands. In particular, the maximum λ1 of the first n→π* band of the E isomer of azosilabenzene ASiB is at ∼1030 nm, while for the Z isomer λ1 ≅1340 nm. Based on the found bistable azo compounds (ASiB, bis(silyl)- SiD and bis(silatranyl)- SaD diazenes) and their derivatives with E and Z absorption in NIR, unique photoswitches can be created for a number of applications, in particular, for photothermal therapy.

Oligosilanylated Silocanes.

A number of mono- and dioligosilanylated silocanes were prepared. Compounds included silocanes with 1-methyl-1-tris(trimethylsilyl)silyl, 1,1-bis[tris(trimethylsilyl)silyl], and 1,1-bis[tris(trimethylsilyl)germyl] substitution pattern as well as two examples where the silocane silicon atom is part of a cyclosilane or oxacyclosilane ring. The mono-tris(trimethylsilyl)silylated compound could be converted to the respective silocanylbis(trimethylsilyl)silanides by reaction with KOtBu and in similar reactions the cyclosilanes were transformed to oligosilane-1,3-diides. However, the reaction of the 1,1-bis[tris(trimethylsilyl)silylated] silocane with two equivalents of KOtBu leads to the replacement of one tris(trimethylsilyl)silyl unit with a tert-butoxy substituent followed by silanide formation via KOtBu attack at one of the SiMe3 units of remaining tris(trimethylsilyl)silyl group. For none of the silylated silocanes, signs of hypercoordinative interaction between the nitrogen and silicon silocane atoms were detected either in the solid state. by single crystal XRD analysis, nor in solution by 29Si-NMR spectroscopy. This was further confirmed by cyclic voltammetry and a DFT study, which demonstrated that the N-Si distance in silocanes is not only dependent on the energy of a potential N-Si interaction, but also on steric factors and through-space interactions of the neighboring groups at Si and N, imposing the orientation of the pz(N) orbital relative to the N-Si-X axis.

Electrochemical Oxidation and Radical Cations of Structurally Non-rigid Hypervalent Silatranes: Theoretical and Experimental Studies.

Using 18 silatranes XSi(OCH2 CH2 )3 N (1) as examples, the potentials of electrochemical oxidation E0 of the hypervalent compounds of Si were calculated for the first time at the ab initio and DFT levels. The experimental peak potentials Ep (acetonitrile) show an excellent agreement (MAE=0.03) with the MP2//B3PW91 calculated E0 (C-PCM). Radical cations of 1 reveal a stretch isomerism of the N→Si dative bond. Localization of the spin density (SD) on the substituent X and the short (s) coordination contact Si⋅⋅⋅N (dSiN <2.13 Å) along with the high five-coordinate character of Si are typical for the first isomer 1+.(s) , whereas the second one, 1+.(l) , has a longer (l) Si⋅⋅⋅N distance (dSiN >3.0 Å), the four-coordinate Si and the SD localized on the silatrane nitrogen atom Ns . The vertical model of adiabatic ionization (1→1+.(s) or 1→1+.(l) ) was developed. It allows, in accordance with an original experimental test (electrooxidation of 1 in the presence of ferrocene), a reliable prediction of the most probable pathways of the silatrane oxidation. The reliable relationships of E0 (1) with the strength characteristics of the dative contact N→Si were revealed.

Different Complexation Behavior of P-Functionalized Ferrocene Derivatives Towards SnCl2 , SnCl4 and SnPh2 Cl2 : Auto-ionization and Redox-Type Reactions.

The novel phosphonyl-substituted ferrocene derivatives [Fe(η(5) -Cp)(η(5) -C5 H3 {P(O)(O-iPr)2 }2 -1,2)] (Fc(1,2) ) and [Fe{η(5) -C5 H4 P(O)(O-iPr)2 }2 ] (Fc(1,1') ) react with SnCl2 , SnCl4 , and SnPh2 Cl2 , giving the corresponding complexes [(Fc(1,2) )2 SnCl][SnCl3 ] (1), [{(Fc(1,1') )SnCl2 }n ] (2), [(Fc(1,1') )SnCl4 ] (3), [{(Fc(1,1') )SnPh2 Cl2 }n ] (4), and [(Fc(1,2) )SnCl4 ] (5), respectively. The compounds are characterized by elemental analyses, (1) H, (13) C, (31) P, (119) Sn NMR and IR spectroscopy, (31) P and (119) Sn CP-MAS NMR spectroscopy, cyclovoltammetry, electrospray ionization mass spectrometry, and single-crystal as well as powder X-ray diffraction analyses. The experimental work is accompanied by DFT calculations, which help to shed light on the origin for the different reaction behavior of Fc(1,1') and Fc(1,2) towards tin(II) chloride.

Covalent grafting of silatranes to carbon interfaces.

Covalent Si-C grafting of a silatrane cage to a carbon-based interface provides a truly conjugated benzyl-type system in which the 3 c-4 e orbital of the silatrane interacts with the macroscopic π-type substituent (graphite Csp2 network) through hyperconjugation. This process, studied by voltammetry, EIS, FTIR, SEM and DFT modeling, allows one to build carbon-based conducting interfaces with electronically conjugated molecular extensions. Non-conjugated covalent grafting of an alkyl silatrane moiety provides chemically stable functional interfaces that have good promise for electrochemically-driven applications, for example, electrochemical spin-writing.

Formation and properties of a bicyclic silylated digermene.

In the presence of PMe3 or N-heterocyclic carbenes, the reaction of oligosilanylene dianions with GeCl2⋅dioxane gives germylene-base adducts. After base abstraction, the free germylenes can dimerize by formation of a digermene. An electrochemical and theoretical study of a bicyclic tetrasilylated digermene revealed formation of a comparably stable radical anion and a more reactive radical cation, which were characterized further by UV/Vis and ESR spectroscopy.

Efficient anodic allylation and benzylation of carbons using allyl and benzyl trimethylsilanes.

An easy process for allylation and benzylation of different carbon materials, primarily of glassy carbon, in acetonitrile solutions containing tetraalkyammonium salts is described. The method relies on the capability of C(sp(2)) zones of glassy carbon (graphite and fullerene-like inclusions) to be anodically charged at potentials >1.5 V versus Ag/AgCl to form electrophilic centers reacting with substituted trimethylsilanes RSiMe3. Great propensity of the trimethylsilyl group (TMS(+)) to act as a cationic leaving group facilitates electrophilic reactions of the charged anodic surface with R-carrying silylated precursors, permitting efficient grafting of a large variety of R groups. The present preliminary work focuses only on the efficient grafting of benzyl and allyl moieties.

Novel method for grafting alkyl chains onto glassy carbon. Application to the easy immobilization of ferrocene used as redox probe.

Primary alkyl iodides (RI) have been found to react with a cathodically charged glassy carbon surface at potentials more negative than -1.7 V vs Ag/AgCl. In aprotic solvents, this reaction results in grafting of the alkyl chains onto carbon. It is proposed that the process corresponds to the cathodic charge of graphitized and fullerenized zones present in carbon followed by a displacement reaction (analogous to a nucleophilic attack) toward alkyl iodides. This new mode of grafting is applied to the immobilization of ferrocene used as an electrochemical probe. The present work points out the reaction of ω-iodoalkylferrocenes and quantifies the level of grafting of alkyl chains via this promising method for modification of carbon surfaces. Coverage levels were found to be high, reaching the apparent surface concentrations of 8 × 10(-9) mol cm(-2). These large values are explained on the basis of swelling of the interface provoked by progressive charging of the carbon surface via insertion of tetraalkylammonium cations concomitantly with the substitution process. Alkylferrocene layers deposited onto carbon were found to be chemically and electrochemically stable.

Novel stannatranes of the type N(CH2CMe2O)3SnX (X = OR, SR, OC(O)R, SP(S)Ph2, halogen). Synthesis, molecular structures, and electrochemical properties.

The syntheses of the stannatrane derivatives of the type N(CH(2)CMe(2)O)(3)SnX (1, X = Ot-Bu; 2, X = Oi-Pr; 3, X = 2,6-Me(2)C(6)H(3)O; 4, X = p-t-BuC(6)H(4)O; 5, X = p-NO(2)C(6)H(4)O; 6, X = p-FC(6)H(4)O; 7, X = p-PPh(2)C(6)H(4)O; 8, X = p-MeC(6)H(4)S; 9, X = o-NH(2)C(6)H(4)O; 10, X = OCPh(2)CH(2)NMe(2); 11, X = Ph(2)P(S)S; 12, X = p-t-BuC(6)H(4)C(O)O; 13, X = Cl; 14, X = Br; 15, X = I; 16, X = p-N(CH(2)CMe(2)O)(3)SnOSiMe(2)C(6)H(4)SiMe(2)O) are reported. The compounds are characterized by X-ray diffraction analyses (3-8, 11-16), multinuclear NMR spectroscopy, (13)C CP MAS (14) and (119)Sn CP MAS NMR (13, 14) spectroscopy, mass spectrometry and osmometric molecular weight determination (13). Electrochemical measurements show that anodic oxidation of the stannatranes 4 and 8 occurs via electrochemically reversible electron transfer resulting in the corresponding cation radicals. The latter were detected by cyclic voltammetry (CV) and real-time electron paramagnetic resonance spectroscopy (EPR). DFT calculations were performed to compare the stannatranes 4, 8, and 13 with the corresponding cation radicals 4(+•), 8(+•), and 13(+•), respectively.

Efficient covalent capping of carbon and gold with TEMPO for catalysis and spin writing.

Covalent immobilization of TEMPO at carbon and gold via an aliphatic -(CH2)6- linker was achieved via cathodic grafting of a diamagnetic precursor, tetramethylpiperidine, with subsequent >NH to >NO oxidation to give TEMPO-capped paramagnetic interfaces (ΓTEMPO = 5.2 × 10-10 mol cm-2); catalytic and spin switching potential of the thus prepared systems was demonstrated.

2-Carboxyethylgermanium Sesquioxide as A Promising Anode Material for Li-Ion Batteries.

Various forms of germanium and germanium-containing compounds and materials are actively investigated as energy-intensive alternatives to graphite as the anode of lithium-ion batteries. The most accessible form-germanium dioxide-has the structure of a 3D polymer, which accounts for its rapid destruction during cycling, and requires the development of further approaches to the production of nanomaterials and various composites based on it. For the first time, we propose here the strategy of using 2-carboxyethylgermanium sesquioxide ([O1.5 GeCH2 CH2 CO2 H]n , 2-CEGS), in lieu of GeO2 , as a promising, energy-intensive, and stable new source system for building lithium-ion anodes. Due to the presence of the organic substituent, the formed polymer has a 1D or a 2D space organization, which facilitates the reversible penetration of lithium into its structure. 2-CEGS is common and commercially available, completely safe and non-toxic, insoluble in organic solvents (which is important for battery use) but soluble in water (which is convenient for manufacturing diverse materials from it). This paper reports the preparation of micro- (flower-shaped agglomerates of ≈1 µm thick plates) and nanoformed (needle-shaped nanoparticles of ≈500×(50-80) nm) 2-CEGS using methods commonly available in laboratories and industry such as vacuum and freeze-drying of aqueous solutions of 2-CEGS. Lithium half-cell anodes based on 2-CEGS show a capacity of ≈400 mAh g-1 for microforms and up to ≈700 mAh g-1 for nanoforms, which is almost two times higher than the maximal theoretical capacity of graphite. These anodes are stable during the cycling at various rates. The results of DFT simulations suggest that Li atoms form the stable Li2 O with the oxygen atoms of 2-CEGS, and actual charge-discharge cycles involve deoxygenated GeC3 H5 molecules. Thus, C3 chains loosen the anode structure compared to pure Ge, improving its ability to accommodate Li ions.

Halogen-free GeO2 conversion: electrochemical reduction vs. complexation in (DTBC)2Ge[Py(CN)n] (n = 0 2) complexes.

3,5-di-tert-Butylcatecholate (DTBC) germanium complexes (DTBC)2Ge[Py(CN)n]2 (n = 0…2) have been synthesized from GeO2, 3,5-di-tert-butylcatechol and cyano-substituted pyridines Py(CN)n and characterized by elemental analysis, NMR, IR and UV-VIS spectroscopy. The structure of 1 (with 4-cyanopyridine) has been determined by X-ray single crystal analysis. UV-VIS spectra have shown that these complexes are stable in CH3CN, toluene and CH2Cl2 solutions; in contrast, they are rapidly decomposed by dimethylformamide and tetrahydrofuran. Complexes 1 and 2 (with 4-cyano and 3-cyanopyridine) are electrochemically reducible in toluene/1 M Bu4NPF6 at E = -1.3…-1.7 V vs. AgCl. The quantum-chemical study of these complexes is in accordance with the unsuccessful attempts to obtain analogous derivatives with 2-cyanopyridine and 2,6-dicyanopyridine.

A NADPH substitute for selective photo-initiation of reductive bioprocesses via two-photon induced electron transfer.

A NADPH substitute where the nicotinamide moiety is replaced by a chromophoric unit having much larger two-photon absorption cross-section and able to transfer electrons to flavins only upon excitation is described as an effective two-photon nanotrigger for selective photo-activation of electron transfer in bioreductive processes.

Radical cations of phenyl silatrane.

Electrochemical oxidation of phenylsilatrane (1) in CH3CN/0.1 M Bu4NPF6 has been studied by voltammetry, UV-Vis and EPR-coupled spectroelectrochemistry supported by DFT calculations. One-electron withdrawal from the HOMO of 1, formed with a predominant contribution of the atrane N atom to the 3c-4e system, results in a short lived radical cation, in which the atrane nitrogen atom is almost planar and carries most of the spin density showing strong coupling with the protons of the axially directed C-H bonds of the three adjacent α-methylene groups (g = 2.0037, aαHax = 37.93 G, aαHlat = 0.23 G and aßH = 1.8 G). EPR spectroscopy and DFT calculations attest that the unpaired electron in the radical cation does not reside at the Si atom.

Electron Transfer and Modification of Oligosilanylsilatranes and Related Derivatives.

Several silatranyl -substituted oligosilanes were prepared starting from bis(trimethylsilyl)silatranylsilanide. Electrochemical and theoretical investigations of some oligosilanes revealed that electrooxidation occurs by formation of a short-lived cation radical. This species undergoes structural relaxation to form a pair of conformers, with endo and exo relationships with respect to the Si-N interaction. Reaction of a 1,4-disilatranyl-1,4-disilanide with 1,2-dichlorotetramethyldisilane gave a mixture of cis and trans diastereomers of a cyclohexasilane with the trans isomer showing a diminished Si-N distance.

Superoxide-stable ionic liquids: new and efficient media for electrosynthesis of functional siloxanes.

The electrogeneration of diorganylsilanones from difunctional precursors Y(CH(2))(3)(Me)SiX(2) and Ph(2)SiX(2)(Y = NH(2), CF(3), CN; X = Cl, OEt, OMe), performed in the presence of hexamethyldisiloxane or hexamethylcyclotrisiloxane (D(3)) in the ionic liquids [C(5)H(5)NC(8)F(18)].NTf(2), [C(5)H(5)NC(18)H(38)].NTf(2) and Me(3)BuN.NTf(2), which reveal high solubility of oxygen and are inert toward superoxide anion, allows functionalized siloxanes to be produced selectively in good isolated yields.

Simultaneous control of emission localization and two-photon absorption efficiency in dissymmetrical chromophores.

The aim of the present work is to demonstrate that combined spatial tuning of fluorescence and two-photon absorption (TPA) properties of multipolar chromophores can be achieved by introduction of slight electronic chemical dissymmetry. In that perspective, two model series of structurally related chromophores have been designed and investigated. One is based on rod-like quadrupolar chromophores bearing either two identical or different electron-donating (D) end groups and the other on three-branched octupolar chromophores built from a trigonal donating moiety bearing identical or different acceptor (A) peripheral groups. The influence of the electronic dissymmetry is investigated by combined experimental and theoretical studies of the linear and nonlinear optical properties of dissymmetrical chromophores compared to their symmetrical counterparts. In both types of systems (i.e., quadrupoles and octupoles), experiments and theory reveal that excitation is essentially delocalized and that excitation involves synchronized charge redistribution (i.e., concerted intramolecular charge transfer) between the different D and A moieties within the multipolar structure. In contrast, the emission stems only from a particular dipolar subunit bearing the strongest D or A moiety due to fast excitation localization after excitation, prior to emission. Hence, control of emission characteristics (polarization and emission spectrum), can be achieved, in addition to localization, by controlled introduction of electronic dissymmetry (i.e., replacement of one of the D or A end-groups by a slightly stronger D' or A' unit). Interestingly, slight dissymmetrical functionalization of both quadrupolar and octupolar compounds does not lead to significant loss in TPA responses and can even be beneficial due to the spectral broadening and peak position tuning that it allows. This study thus reveals an original molecular engineering route allowing TPA enhancement in multipolar structures, due to concerted core-to-periphery or periphery-to-core intramolecular charge redistribution upon excitation, while providing for control of emission localization. Such a route could be extended to more intricate (dendritic) and multipolar (3D) systems.

Reversed redox generation of silyl radicals in a four-electrode flow-through EPR spectroelectrochemical cell.

A flow-through four-electrode EPR spectroelectrochemical cell was developed which allowed the observation of silyl radical formation in apparently multielectron electrochemical processes, in which these species could not be detected directly because of the high driving force of their further reduction/oxidation leading to non-paramagnetic products. Silyl radicals thus generated were characterized by spin trapping with alpha-phenyl-N-tert-butyl nitrone (PBN), intramolecular spin trapping or by direct detection. The overall multielectron process is realized in the first, generating, compartment of the cell and the ionic species formed are then transformed into the corresponding radicals in the second compartment via a one-electron redox process in the opposite direction, e.g. two-electron reductions of Ph(3)SiCl or Et(3)SiCl followed by one-electron oxidation of the resulting Ph(3)Si(-) or Et(3)Si(-) anions (+2e/-e process). These radical species were then identified as their secondary paramagnetic products or by their spin trapping with PBN. Using (2-[cyclohex-3-enyl]ethyl)dimethyl chlorosilane in this process, the formation of the silicon-centered radical and its intramolecular addition across the internal double bond were evidenced by spin trapping. The reduction of electrophilic silicon intermediates issued from the oxidation of Ph(3)SiSiPh(3) (-2e/+2e process) resulted in Ph(3)Si* radicals trapped with PBN. The reduction of the electrochemically prepared persistent dication of a stable disilene, thiatetrasilacyclopentene, allowed generation of a disilene cation radical characterized by EPR.