Pi-stacking induced NMR spectrum splitting in enantiomerically enriched Ru(II) complexes: evaluation of enantiomeric excess

Several chiral octahedral complexes of the general formula [Ru(bpy)2 (Lig)][PF6]2 (Lig = a ligand that can participate in pi-stacking interactions such as eilatin, isoeilatin, and tpphz) were synthesized in both the racemic and enantiomerically pure/enriched forms. Nonracemic mixtures of enantiomers of all these complexes exhibit splitting of the 1H NMR spectra (NMR nonequivalence); i.e., each spectrum contains a major and a minor set of peaks. The origin of this phenomenon is attributed to a fast equilibrium between monomers and discrete dimers held together by pi-stacking interactions, and it is observed for a wide range of pi-stacking interaction strengths. The NMR spectrum splitting exhibited by these complexes can be exploited for the evaluation of their enantiomeric excess simply from the integral ratio, without addition of chiral shift reagents.     

Eilatin complexes of ruthenium and osmium. synthesis, electrochemical behavior, and near-IR luminescence

The synthesis and characterization of new Ru(II) and Os(II) complexes of the ligand eilatin (1) are described. The new complexes [Ru(bpy)(eil)(2)](2+) (2), [Ru(eil)(3)](2+) (3), and [Os(eil)(3)](2+) (4) (bpy = 2,2'-bipyridine; eil = eilatin) were synthesized and characterized by NMR, fast atom bombardment mass spectrometry, and elemental analysis. In the series of complexes [Ru(bpy)(x)(eil)(y)()](2+) (x + y = 3), the effect of sequential substitution of eil for bpy on the electrochemical and photophysical properties was examined. The absorption spectra of the complexes exhibit several bpy- and eil-associated pi-pi and metal-to-ligand charge-transfer (MLCT) transitions in the visible region (400-600 nm), whose energy and relative intensity depend on the number of ligands bound to the metal center (x and y). On going from [Ru(bpy)(2)(eil)](2+) (5) to 2 to 3, the d(pi)(Ru) --> pi(eil) MLCT transition undergoes a red shift from 583 to 591 to 599 nm, respectively. Electrochemical measurements performed in dimethyl sulfoxide reveal several ligand-based reduction processes, where each eil ligand can accept up to two electrons at potentials that are significantly anodically shifted (by ca. 1 V) with respect to the bpy ligands. The complexes exhibit near-IR emission (900-1100 nm) of typical (3)MLCT character, both at room temperature and at 77 K. Along the series 5, 2, and 3, upon substitution of eil for bpy, the emission maxima undergo a blue shift and the quantum yields and lifetimes increase. The radiative and nonradiative processes that contribute to deactivation of the excited level are discussed in detail.     

Formation of hypervalent complexes of trifluorosilanes with pyridine and with 4-methoxypyridine, through intermolecular silicon...nitrogen interactions

Herein we report that trifluorohexylsilane (2), trifluorophenylsilane (3) and trifluoro(pentafluorophenyl)silane (4) form new hypervalent complexes with pyridine (py) and with 4-methoxypyridine (MeO-py), through intermolecular Si...N interactions. In general, stronger and more efficient binding is observed for the more electron poor (Si atom in) silane 4 and for the more electron rich (N atom in) 4-methoxypyridine. Binding constants of 15+/-2, 25+/-5, and 550+/-100 M(-1) at 25 degrees C in benzene were measured for the formation of the pentacoordinate 2.py, 3.py, and 3.MeO-py complexes, respectively. In addition, silane 3 also forms the hexacoordinate 3-2py and 3-2MeO-py complexes at low temperatures and silane 4 forms the 4-2py complex below room temperature and the 4-2MeO-py complex already at room temperature and in a high concentration. The various types of hypervalent complexes and different binding strengths described here for silanes 2-4 and previously for trifluoro(phenylethynyl)silane (1) and the possibility to modulate the binding modes (penta- vs hexacoordination) of these complexes (by the type of amine used, concentration, and the temperature applied) suggest that such new intermolecular Si...N interactions could be used as efficient and versatile binding motifs in supramolecular chemistry.     

On surface elasticity theory for plane interfaces

A closed system of surface elasticity equations was derived in terms of surface quantities defined as integrals of respective excess bulk quantities normal to the interface. The equations were consistently linearized for the case of small strains. It is shown that these equations are more general than the Shuttleworth equations. Equations of this type were also derived for the particular case of an interface formed by a thin layer with constant properties. The derived equations were used to consider bending of a plate under pressure applied to both sides.     

Torsion of cylindrically anisotropic nano/microtubes from seven-constant tetragonal crystals. Poynting’s effect

In the paper presented, we continue the research in the elastic properties of seven-constant tetragonal crystals and nano/microtubes. Our previous study concerned tension of this type of structures, and here we are dealing with their torsion, Poynting’s effect or axial extension under torsion with no tensile force, and torsional stiffness. It is demonstrated that there exists inverse Poynting’s effect: the tubes experience torsion under tension without applying a torque.     

Solvent Dependence of H-F Coupling Parameters in Fluorobenzene

Solvent dependence of NMR spectral parameters has been the object of considerable interest in recent years (see Ref. 1 for a brief summary). More recently attention has been devoted to the solvent dependence of NMR parameters in fluorine-containing compounds. In their investigation of fluorobenzene, Mohanty and Venkateswarlu observed the effect of solvents on the proton and fluorine chemical shifts, but did not report any dependence of the H-K or H-F couplings on the solvent medium.² For 1,2-dichlorofluoroethylene Bell and Danyluk found a linear correlation between the directly bonded ¹³C-H and ¹³C-F couplings and the H-F couplings.³ These results, along with linear correlations between the ¹³C-H (or ¹³C-F) coupling and the proton (or fluorine) chemical shift, led to the inference that the medium effect acts to change the coupling parameters and shieldings by a similar mechanism.     

Semimicro determination of carbon in organic compounds by closed flask combustion: Evaluation of the method

Summary A procedure is described for the determination of carbon by closed flask combustion. It involves electrical ignition of the sample (15–20 mg) wrapped in a high melting glass fiber, absorption of the CO₂ in NaOH-BaCl₂ solution, separation and dissolution of the BaCO₃ formed in HCl and back titration with NaOH. This method has been tested with organic compounds containing nitrogen, sulfur and chlorine.

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Zusammenfassung Ein Verfahren zur Kohlenstoffbestimmung mit Hilfe der Kolbenverbrennung wurde beschrieben. 15 bis 20 mg der in hochschmelzender Glasfaser verpackten Probe werden elektrisch gezündet, das Kohlendioxid in einer Natronlauge-Bariumchloridlösung absorbiert, das Bariumcarbonat abgetrennt, in Salzsäure gelöst und mit Natronlauge zurücktitriert. Das Verfahren wurde mit organischen Verbindungen überprüft, die Stickstoff, Schwefel bzw. Chlor enthielten.