Oscillopolarographic reduction of white phosphorus

Reduction of white phosphorus in ethanol solutions on a dropping mercury electrode was studied by oscillopolarography. The effect of the type of electrolyte and potential sweep rate on the peak potential was demonstrated.     

Electrochemical reactions of white phosphorus

The main electrochemical transformations of elemental (white) phosphorus were considered. Special attention was given to the recently developed processes of preparation of organophosphorus compounds (OPCs) with phosphorus-carbon bonds. The electrochemical approaches to the synthesis of OPCs from white phosphorus using organonickel and organozinc reagents are described. The importance of using the electrochemical methods for the generation of highly reactive phosphorus intermediates was shown for phosphine oxide H3PO obtained for the first time. This provides significant prospects for the electrochemical approaches that could be applied for the development of technologies of the chlorine-free synthesis of OPCs from white phosphorus.     

Synthesis and solid-state dynamics of molecular dirotors

We report the design, synthesis, thermal properties, and dynamic NMR characterization of four new molecular dirotors, each containing two rotary 1,4-diethynyl phenylene units, a bridging 1,3-bis(diphenylmethyl) benzene stator, and two capping triphenylmethyl groups. These compounds are variations of the well-known wheel and axle structures. Ambient temperature quadrupolar echo ²H NMR spectra of phenylene-deuterated samples revealed line shapes consistent with dynamic processes having exchange rates >10⁸ s⁻¹, and more than the two minima typically observed for most phenylene rotators. Reasonable line-shape simulations were obtained with a model that involves four minima related by angular displacements of 0°, 30°, 180°, and 230°. Cooling the samples to 260 K revealed spectra consistent with samples that contain two or more components. These results indicate that novel wheel and axle structures do create a certain amount of free volume that allows for the desired mobility of the rotating units.     

Solution and solid-state variation of cupric phenanthroline complexes

The 1:1 cupric-phenanthroline complexes, [Cu(5,6-Me2-phen)(MeCN)2(BF4)](BF4) (1), [Cu(o-phen)(MeCN)2(H2O)](BF4)2 (2), and [Cu(5-Cl-phen)(MeCN)2(BF4)](BF4) (3), have been prepared and characterized by X-ray crystallography. The structures of 1 and 3 are characterized by an equatorial plane about the copper center consisting of a phenanthroline ligand and two acetonitrile ligands. The copper units are connected by bridging counterions in the axial positions of the pseudo-octahedral metal centers to form one-dimensional solid-state linkages. The structure of 2 contains the same equatorial plane as 1 and 3, but an axial water ligand completes a square pyramidal geometry for each discrete metal unit. Although the solid-state structures vary for the three complexes, characterization through electronic spectroscopy and cyclic voltammetry reveals similar behavior for all three complexes in solution.     

Solution and solid-state structures of lithiated cyclic phosphonates

Spectroscopic analysis of lithiated cyclic phospho-nates belonging to the 1,3,2-dioxaphosphorinane 2-oxide class have been examined by NMR spectroscopy in THF and by single-crystal X-ray crystallography. Lithio P-benzyl-1,3,2-dioxaphosphorinane 2-oxide (Li⁺7⁻) and lithio P-isopropyl-1,3,2-dioxaphosphorinane 2-oxide (Li⁺8⁻) are characterized by freely rotating, sp²-hybridized anions devoid of lithium–carbon contacts. The anions are most likely dimers linked through oxygen-lithium bridges. The P-isopropyl compound crystallized from TMEDA/THF as a C-lithiated dimer in which each lithium bridges a carbon and an oxygen and is solvated by one TMEDA molecule. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:209–218, 1998     

Electrocatalytic eco-efficient functionalization of white phosphorus

The novel eco-efficient methods to transform white phosphorus into the esters of phosphoric, phosphorous and phosphonic acids, tertiary phosphines and other organophosphorus compounds under conditions of electrochemical catalysis were elaborated. The mechanism of these processes was investigated using the method of cyclic voltammetry and preparative electrolysis.     

Solution and solid-state NMR studies of some cadmium-selenone complexes

Cadmium(II) complexes of Imidazolidine-2-selenone (ImSe) and its derivatives have been prepared with the general formula Cd(RImSe)2Cl2 (where R=Me, Et, Pr, etc.). These complexes are characterized by elemental analysis, IR and NMR (1H, 13C, 77Se and 113Cd) spectroscopy. An upfield shift in C=Se resonance of selenones in 13C NMR and in 77Se and high-frequency shifts in N-H resonances in 1H are consistent with the selenium coordination to Cd(II). The 77Se nucleus in Cd(ImSe)2Cl2 is shielded by 38 ppm on coordination, relative to the free ligand. The principal components of the 77Se, 113Cd and 13C shielding tensors for the complexes were determined from solid-state NMR data. Large selenium chemical shift anisotropies were observed for these complexes.     

Tunable construction of transition metal-coordinated helicene cages

We report a facile and tailored method to prepare globally twisted chiral molecular cages through tunable coordination of bis-bipyridine-terminated helicene ligands to a series of transition metals including Fe(II), Co(II), Ni(II) and Zn(II). This system shows an efficient remote transfer of stereogenecity from the helicene core to the bipyridine-metal coordination sites and subsequently the entire cages. While the Fe(II), Co(II) and Ni(II)-derived M₂L₃ (M for metal and L for ligand) cages exhibit quasi-reversible redox features, the Zn(II) analogues reveal prominent yellow circularly polarized luminescence. Interestingly, with the addition of Na₂SO₄, the Zn₂L₃ cages reassemble into sextuple-stranded Zn₆L₆(SO₄)₄ cages in which three Zn₂L₂ units are bound together by four sulfates and further coalesced by offset inter-ligand π-π interactions.     

Photoinduced electron transfer between metal-coordinated cyclodextrin assemblies and viologens

Two novel tris(bipyridine)ruthenium(II) complexes bearing two and six beta-cyclodextrin binding sites on their ligands have been synthesised and characterised. Complex 1, bearing two cyclodextrins, adopts a conformation in aqueous solution where parts of the aromatic ligands are self-included into the cyclodextrin moieties. This results in a loss of symmetry of the complex and gives rise to a much more complicated 1H NMR spectrum than expected. Photophysical studies indicate that the appended cyclodextrins protect the luminescent ruthenium core from quenching by oxygen, which results in longer excited state lifetimes and higher emission quantum yields compared with the reference compound, the unsubstituted ruthenium tris(bipyridine). Inclusion of suitable guests such as dialkyl-viologens leads to a quenching of the luminescence of the central unit. In these supramolecular donor-acceptor dyads an efficient photoinduced electron transfer from the excited ruthenium moiety (the donor) to the viologen unit (the acceptor) is observed. The alkyl chain length of the acceptor plays an important role on the binding properties; when it exceeds a certain limit the binding becomes strong enough for electron transfer to occur. Interestingly, a viologen with only one long alkyl tail instead of two shows no efficient quenching; this indicates that cooperative interactions between two cyclodextrins binding one viologen are essential to raise the binding constant of the supramolecular dyad.     

Solution versus solid-state structure of ytterbium heterobimetallic catalysts

The solution structures of the ytterbium heterobimetallic complexes Na(3)[Yb((S)-BINOL)(3)] (1), K(3)[Yb((S)-BINOL)(3)] (2), and Li(3)[Yb((S)-BINOL)(3)] (3), belonging to a family of well-known enantioselective catalysts, are studied by means of NMR and circular dichroism (CD) in the UV and near-IR regions. The experimental NMR paramagnetic shifts were employed to obtain a refined solution structure of 1. NMR analysis demonstrated that 1, 2, and 3 have the same solution geometry but different magnetic susceptibility anisotropy D factors. By comparing XRD and NMR structures of 1, we demonstrate that, upon dissolution, this complex experiences a rearrangement from the crystalline C(3) symmetry into the solution D(3) symmetry. Remarkably, Yb is not bound to water in solution, and Ln-BINOL bonds are labile as demonstrated through EXSY. NIR-CD is confirmed especially sensitive to changes in the ytterbium coordination sphere.     

Solution and solid-state NMR analysis of phenolic resin cure kinetics

Phenolic condensate resins, produced from the reaction of phenol with formaldehyde, are important components in the chemical industry and have a wide variety of applications. Resins of this type are used in the aerospace industry to produce rocket motor components with strength and rigidity. While methods currently exist (e.g., DMA, DEA, calorimetry) for the examination of phenolic-based systems during the curing process, none are available to directly measure the functionality and chemical degree of resin advancement during this process. Solution and solid-state ¹³C NMR spectroscopy can be implemented to examine the functional group content of the resin. Reactant and product moieties are spectrally resolved in both techniques and can be monitored during resin cure. ¹H wide-line solid-state NMR can be used to measure the rate of sample curing in situ by monitoring the sample T₂ relaxation time as a function of sample heating time. Comparison of these results with the ¹³C NMR data will establish the degree of resin cure on a functional group basis.     

Mechanically induced solid-state generation of phosphorus ylides and the solvent-free Wittig reaction

We describe the nearly quantitative preparation of phosphorus ylides and the Wittig reaction occurring in the solid sate during high-energy mechanochemical processing. Initial insights into the details of the discovered chemical transformations indicate that high-energy mechanical processing supports the interaction of reacting centers by breaking crystallinity of the reactants and by providing mass transfer without a solvent.     

O-nitrobenzyl photochemistry: Solution vs. solid-state behavior

The efficiency of various 2-nitrobenzyl groups in the photogeneration of carboxylic acid from nitrobenzyl esters has been determined, and the solution vs. solid polymer matrix photochemistry is compared. Generally, the quantum yield for photoreaction is lower in the solid state, probably due to decreased conformational mobility. The substitution that affords the largest increase in quantum yield (?) is at the α-carbon atom. The value of ? for α-methyl-2-nitrobenzyl trimethylacetate in solution is 0.65 vs. 0.13 for the parent 2-nitrobenzyl ester.     

Easy hydrolysis of white phosphorus coordinated to ruthenium

The P4 molecule bound to ruthenium as an eta1-ligand in [CpRu(PPh3)2(eta1-P4)]Y (Y = PF6, CF3SO3) undergoes an easy reaction with water in exceedingly mild conditions to yield PH3, which remains coordinated to the [CpRu(PPh3)2] fragment, and oxygenated derivatives.     

Oxidation of white phosphorus by peroxides in water

A mixture of hypophosphorous, phosphorous, and phosphoric acids is formed during the anaerobic oxidation of white phosphorus by peroxides [ROOН; R = Н, 3-ClC₆H₄CO, (СН₃)₃С] in water. The rate of reactions grows considerably upon adding nonpolar organic solvents. The activity series of peroxides and solvents are determined experimentally. NMR spectroscopy shows that the main product of the reaction is phosphorous acid, regardless of the nature of the peroxide and solvent. A radical mechanism of oxidation of white phosphorus by peroxides in water is proposed. It is initiated by the homolysis of peroxide with the formation of НO^? radicals that are responsible for the homolytic opening of phosphoric tetrahedrons. Further oxidation and stages of the hydrolysis of intermediate phosphorus-containing compounds yield products of the reaction.     

Solution and solid-state interactions in a supramolecular ruthenium photosensitizer-polyoxometalate aggregate

The intermolecular interactions between a ruthenium-based photosensitizer ([Ru(tbbpy)(2)(biH(2))](2+)) and a molecular metal oxide ([β-Mo(8)O(26)](4-)) are investigated in solution and in the solid state. The supramolecular interactions were studied using (1)H-NMR, UV-Vis and emission spectroscopy, ESI mass spectrometry and single-crystal X-ray diffraction. The formation of supramolecular aggregates was observed both in the crystal lattice and in solution. In addition, it is shown that aggregation in solution can be controlled by the competitive formation of ion pairs.     

Investigations of metal-coordinated peptides as supramolecular synthons

This article describes the synthesis and controlled assembly of four model biological-hybrid scaffolds via coordination of a metal complex to four new tripeptides. Each model tripeptide investigated has either a central pyridyl glycyl or a pyridyl alanyl residue between two terminally protected glycines. All tripeptides were coordinated to their complementary recognition unit, a p-methoxy SCS-Pd pincer complex. The assembly events were fully characterized and investigated by 1H NMR, ES-MS, and isothermal titration calorimetry (ITC) to elucidate how the substitution and spatial distance of the pyridyl moiety to the peptide backbone affects the metal coordination. Using these characterization techniques, we have shown that the metal-coordination events in all cases are fast and quantitative and that the peptide backbones do not interfere with the self-assembly. The ITC analyses showed that the 4-pyridyl tripeptides are the tightest binding ligands toward the palladated pincer complexes with the alanyl derivative being the strongest overall, demonstrating the superiority of the 4-pyridyl peptides over their 3-pyridyl analogues. The measured association constants are comparable to other pincer-pyridine systems in DMSO suggesting that the controlled coordination of the metalated pincer/pyridine interaction is an interesting biological synthon and will allow for the future development of important noncovalent peptide-based hybrid materials.     

Solution and solid-state models of peptide CH...O hydrogen bonds

Fumaramide derivatives were analyzed in solution by (1)H NMR spectroscopy and in the solid state by X-ray crystallography in order to characterize the formation of CH...O interactions under each condition and to thereby serve as models for these interactions in peptide and protein structure. Solutions of fumaramides at 10 mM in CDCl(3) were titrated with DMSO-d(6), resulting in chemical shifts that moved downfield for the CH groups thought to participate in CH...O=S(CD(3))(2) hydrogen bonds concurrent with NH...O=S(CD(3))(2) hydrogen bonding. In this model, nonparticipating CH groups under the same conditions showed no significant change in chemical shifts between 0.0 and 1.0 M DMSO-d(6) and then moved upfield at higher DMSO-d(6) concentrations. At concentrations above 1.0 M DMSO-d(6), the directed CH...O=S(CD(3))(2) hydrogen bonds provide protection from random DMSO-d(6) contact and prevent the chemical shifts for participating CH groups from moving upfield beyond the original value observed in CDCl(3). X-ray crystal structures identified CH...O=C hydrogen bonds alongside intermolecular NH...O=C hydrogen bonding, a result that supports the solution (1)H NMR spectroscopy results. The solution and solid-state data therefore both provide evidence for the presence of CH...O hydrogen bonds formed concurrent with NH...O hydrogen bonding in these structures. The CH...O=C hydrogen bonds in the X-ray crystal structures are similar to those described for antiparallel beta-sheet structure observed in protein X-ray crystal structures.