Sterically encumbered regioselective cycloaddition of a calixarene derived bis(spirodienone) with 1,2-benzoquinones

A calix[4]arene derived bis(spirodienone) acts as the 2π component in a cycloaddition reaction with two molecules of 3,5-di-tert-butyl-1,2-benzoquinone in the [2+4] manner leading to macrocycles with a benzodioxin moiety. A theoretical rationalization of the results suggested a sterically encumbered regioselective pathway, which gives sterically crowded products.     

Un Nouveau diphosphene stable: Le bis[tris(trimethylsilyl)methyl] diphosphene

A new stable diphosphene (Me₃Si)₃CPPC(SiMe₃)₃ has been isolated and characterized particularly by its NMR data; the ³¹P NMR chemical shift appears to be the largest ever observed.     

Multifrequency EPR and redox reactivity investigations of a bis(mu-thiolato)-dicopper(II,II) complex

From a new tripodal ligand [N2SS'H] with mixed N, S(thioether), and S(thiolate) donor set, the corresponding bis(mu-thiolato)dicopper(II) complex has been prepared and characterized. X-ray crystallographic analysis of the complex [Cu2(N2SS')2](ClO4)2.C4H10O (1) demonstrates that the two five-coordinated Cu atoms are bridged by two thiolates leading to a nearly planar Cu2S2 core with a Cu1...Cu1* distance of 3.418(8) A and a large bridging angle Cu1S1Cu1* of 94.92 degrees. X-band (10 GHz), Q-band (34 GHz), and F-Band (115 GHz) EPR spectra of 1 are consistent with a weakly coupled dicopper(II,II) center attributed to an S = 1 state. Simulations for the three frequencies are obtained with a unique set of electronic parameters. The mean values of the spin Hamiltonian parameters for 1 are D = 0.210(3) cm(-1), E = 0.0295(5) cm(-1), |E/D| = 0.140, gx = 2.030(2), gy = 2.032(2), gz = 2.128(2). The electrochemical one-electron reduction of 1 generates the mixed-valent CuIICuI species. EPR and UV-vis spectra are consistent with a type I localized mixed-valent species, while dinuclear CuA centers of native cytochrome c oxidase (CcO)1-3 or nitrous oxide reductase (N2OR)4 have a delocalized CuIICuI mixed-valent state. After reoxidation of the CuIICuI species, the initial complex 1 is regenerated through a reversible interconversion process.     

Sterically encumbered acyclic diphosphazanes: synthesis, conformations and coordination behavior

The reaction between lambda3-diphosphazane [EtN(PCl2)2] and the sodium salts of substituted phenols affords sterically encumbered diphosphazanes [EtN{P(OR)2}2] (R = -C6H3iPr2-2,6 (1), -C6H3Me2-2,6 (2) and -C6H2Me3-2,4,6 (3)). When the same reaction was carried out with bulky sodium 2,4-di-tert-butyl-4-methylphenoxide, only a monosubstitution takes place to result in the formation of [EtN{PCl(OR)}2] (R = -C6H2tBu2-2,6-Me-4) (4). Further reaction of 2 with [Mo(CO)4(NBD)] produces cis-[(EtN{P(OC6H3Me2-2,6)2}2)Mo(CO)4] (5). Diphosphazanes 1-4 and the metal derivative 5 have been characterized by means of their analytical data and EI-MS, IR and multinuclear NMR (1H and 31P) spectral data. The solid-state structure of the diphosphazanes 1, 2 and 4, and the molybdenum complex 5 have been determined by X-ray diffraction studies. Irrespective of the size of substituent, the bulky groups on the phosphorus and nitrogen are on the same side of the P-N-P skeleton with a local C2v symmetry. The central nitrogen remains almost trigonal planar in all the compounds.     

Design of molecular switches based on transition metal bis(dicarbollide) complexes

The paper presents a review of the state of the art and prospects for the development of rotary molecular switches based on transition metal bis(dicarbollide) complexes.     

Sterically demanding bis(2-silylindenyl)zirconium(IV) dichlorides as polymerisation catalyst precursors

A set of different 1- and 2-silyl-substituted zirconocene dichloride/MAO catalyst systems was investigated with respect to their performance in ethene/1-hexene copolymerisations. In-depth studies of bis(2-dimethylsilylindenyl) zirconium(IV) dichloride ( 1 ) revealed a multi-site behaviour, illustrating sensitivity to the reaction temperature and the comonomer mole fraction. Surprisingly, an upper limit is observed for the latter, leading to complete catalyst inhibition. Analysis of the chain termination processes implies the possibility of a predominant, although in general less favourable, β-hydride elimination route under certain polymerisation conditions.     

Sterically encumbered poly(arylene ether)s containing spiro‐annulated substituents: Synthesis and thermal properties

Four novel 2‐trifluoromethyl‐activated bisfluoro monomers have been synthesized successfully using a Suzuki‐coupling reaction of 3‐trifluoromethyl‐4‐fluoro phenyl boronic acid with 2,7‐dibromofluorene with varied pendants. Four monomers were converted to a series of fluorene‐based poly(arylene ether)s with pendants by nucleophilic displacement of the fluorine atoms on the terminal benzene ring with 4,4′‐hexafluoroisopropylidenediphenol. The polymers obtained by displacement of the fluorine atoms, exhibit weight‐average molecular weight up to 9.89 × 10⁴ g/mol in GPC. Thermal analysis studies indicated that these polymers did not show melting endotherms but did show relatively high T_g values up to 270 °C in DSC and outstanding thermal stability up to 532 °C for 5% weight loss in TGA in a nitrogen atmosphere. The polymers are soluble in a wide range of organic solvents: THF, CHCl₃, NMP, DMAc, DMF, toluene and EAc, and so forth, at room temperature. Transparent and flexible films were easily prepared by solution casting from chloroform solution of each of the polymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Sterically hindered mono- and bis(2,5-diaryloxazoles) containing biphenyl systems

The condensation of hydrochloric salts of -aminomethyl aryl ketones with diphenic anhydride or with the diacid chloride of diphenic acid and subsequent cyclodehydration of the condensation products formed give sterically hindered mono- and bis(2, 5-diaryloxazoles) containing the biphenyl system. The spectroluminescent properties of these compounds were studied. Comparison of the absorption spectra of 2,2-di(5-phenyloxazolyl-2)biphenyl and 2,5-diphenyloxazole indicates the complete lack of conjugation between the diphenyloxazole fragments, their independent behavior, and their retention of the spatial configuration of 2,5-diphenyloxazole. 2,2-Di(5-phenyloxazolyl-2)biphenyl has a large Stokes shift. Steric hindrance is also found in 2-carboxy-2-(5-phenyloxazolyl-2)biphenyl molecules.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 184–188, February, 1985.     

Sterically encumbered hexakis(alkylseleno)benzenes: conformational behavior of hexakis(iso-propylselenomethyl)benzene toward Hg ions on selective recognition

An efficient synthesis and structural aspects of a novel class of hexakis(alkylseleno)benzenes [(RSeCH₂)₆C₆] (R = Me, ⁱPr, ⁿBu, ^sBu, ^tBu, ⁿPn, ⁿHx, ⁿOct, 1-methylnaphthalene) by the reaction of hexakis(bromomethyl)benzene with RSe⁻ ions is demonstrated. Preliminary data on ion-sensing properties reveal that these species may act as selective ionophores for Hg²⁺ ions.     

Tris(bis(trimethylsilyl)amido)samarium: X-ray structure and DFT study

The compound Sm[N(SiMe(3))(2)](3) has been investigated experimentally by X-ray crystallography and computationally by DFT methods. The structure is analogous to that of other tris[bis(trimethylsilyl)amido]lanthanides, featuring positional disorder of the metal atom above and below the plane defined by the three N donor atoms, resulting in a trigonal pyramidal configuration. One of the methyl groups of each amido ligand is placed above the apex of the pyramid at close distance to the metal center suggesting the presence of agostic interactions. The DFT calculations have been carried out on the real molecule and on a Si[N(SiH(3))(SiH(2)Me)](3) model where the unique Me group was placed above the apex of the pyramid to probe the agostic interaction. In both cases, the optimized geometry reproduces very well the experimental structure and indicates the presence of beta-Si-C agostic interactions. A comparison of the optimized geometries obtained in the presence/absence of the Sm d and the Si d orbitals serves to illustrate the relevance of these orbitals for (i). the establishment of the pyramidal configuration at Sm, (ii). the Sm-N bond length, and (iii). the Sm-(beta-Si-C) bond length. The bonding analysis, which was carried out by both Mulliken and NBO methods, not only confirms the importance of the metal d orbitals for the Sm-N and Sm-(beta-Si-C) chemical bonding but also illustrates the relevance of electrostatic terms in the agostic interaction. Sm-N and N-Si pi bonding is present according to the bonding analysis but is not important for enforcing the planar configuration at N, nor the pyramidal configuration at Sm.     

Carbonyl-ene reactions catalyzed by bis(oxazoline) copper (II) complexes proceed by a facile stepwise mechanism: DFT and ONIOM (DFT:PM3) studies

The mechanism of a carbonyl-ene addition reaction catalyzed by a bis(oxazoline) copper (II) complex has been studied using DFT methods. We find that the reaction proceeds by a stepwise mechanism with very low barriers and have identified the role of the metal catalyst. We find that the more computationally economic ONIOM method gives accurate geometries for the stationary structures on the potential energy surface but that accurate energetics must be calculated at the full DFT level.     

High-frequency and -field EPR of a pseudo-octahedral complex of high-spin Fe(II): bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II)

A pseudo-octahedral complex of high-spin Fe(II), bis(2,2'-bi-2-thiazoline)bis(isothiocyanato)iron(II), which has a cis-FeN'2N4 chromophore, has been investigated by high-frequency, high-field electron paramagnetic resonance (HFEPR). Complementary M?ssbauer and DC magnetic susceptibility studies were also performed. HFEPR spectra of powder samples were recorded at frequencies up to 700 GHz and over a magnetic field range of 0-25 T. Analysis of the field-frequency data set yields the following set of spin Hamiltonian parameters for S = 2: D = +12.427(12) cm-1, E = +0.243(3) cm-1; gx = 2.147(3), gy = 2.166(3), gz = 2.01(1). The parameters are analyzed by use of a simple crystal-field model. This study represents the first precise determination by HFEPR of spin Hamiltonian parameters in six-coordinate high-spin Fe(II) and indicates the applicability of HFEPR to the study of high-spin Fe(II) in coordination complexes and biological model compounds.     

Thermal, vibrational and EPR studies of Cu(II) bromide bis(p-methylaniline) and bis(m-methylaniline) complexes

[CuBr(2)(pMA)(2)] and [CuBr(2)(mMA)(2)] complexes (pMA: p-methylaniline, mMA: m-methylaniline) have been prepared and characterized by elemental analyses, thermogravimetric analyses, magnetic moment measurements, and IR, Raman and EPR spectroscopic studies. Coordination effects on the vibrational spectra of the ligands have been investigated. The room temperature EPR spectra of the complexes and their simulated spectra are also discussed in detail. The vibrational and EPR spectral studies suggest that the coordination sphere around Cu(II) consist of a distorted tetragonal structure.     

trans-2-(Azaarylsulfanyl)cyclohexanol derivatives as potential pH-triggered conformational switches

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Spin ground state and magnetic properties of cobalt(II): relativistic DFT calculations guided by EPR measurements of bis(2,4-acetylacetonate)cobalt(II)-based complexes

The spin ground state of the core ion and structure of the bis(2,4-acetylacetonate)cobalt(II) model complex and its synthetic aqua and ethanol derivatives, Co(acac)(2)L(n), (L = EtOH, H(2)O), were examined by means of density functional theory (DFT) calculations supported by electron paramagnetic resonance (EPR) measurements. Geometry optimizations were carried out for low-spin (doublet) and high-spin (quartet) states. For the Co(acac)(2) complex two possible conformations, a square-planar and a tetrahedral one, were taken into account. For all structures relative energies were calculated with both "pure" and hybrid functionals. The calculated data were complemented with the results of the EPR investigations carried out at liquid helium temperature, allowing for definite assignment of the high-spin state for the Co(acac)(2)(EtOH)(2) complex. However, because of the unresolved spectral features, only effective g-values could be assessed, whereas the zero-field splitting parameters (ZFS) were calculated by means of the spin-orbit mean field (SOMF) relativistic DFT method for which direct spin-spin (SS) and spin-orbit coupling (SOC) contributions were quantified.     

EPR study on the ligand-exchange reaction between bis(diethyldiselenocarbamato)copper(II) and bis(octyldithiocarbonato)copper(II)

EPR study on the ligand-exchange reaction between bis(diethyldiselenocarbamato)copper(II), Cu(Et2dsc)2, and bis(octyldithiocarbonato)copper(II), Cu(octxant)2, in CH2Cl2, CHCl3, CCl4, C6H6 and C6H5.CH3 is reported for the first time. Mixing of equimolar amounts of the parents (chromophores CuSe4 and CuS4, respectively) in C6H6, C6H5.CH3 and CH2Cl2 makes EPR signals of both parents superimposed by the spectrum of a mixed-chelate Cu(xant)(dsc) complex (chromophore CuS2Se2). A new additional EPR spectrum appears in CHCl3 or CCl4 due to a five-coordinate mixed-ligand complex with the chromophore Cu(S3Se)S as follows by comparing the g-values of parents and mixed-ligand complexes. The appearance of this complex could be explained having in mind donor-acceptor properties of complexes, solvents and the resultant reaction of Cu(octxant)2 with the ester of diselenocarbamic acid yielded in Cu(Et2dsc)2 destruction by CCl4 or CHCl3.     

NMR and EPR studies of the bis(pyridine) and bis(tert-butyl isocyanide) complexes of iron(III) octaethylchlorin

The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))2 (d(xz)d(yz))3 to (d(xz)d(yz))4 (d(xy))1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large pi spin density on the adjacent carbons which are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (d(xz)d(yz))4 (d(xy))1 and (d(xy))2 (d(xz),d(yz))3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the alpha-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.     

Synthesis of novel aromatic nitroxides as potential DNA intercalators. An EPR spectroscopical and DFT computational study

The synthesis and electron paramagnetic resonance (EPR) spectroscopic properties of three novel aromatic nitroxides and potential DNA intercalators, the carbazole-based 3,6-dimethylcarbazole-9-oxyl, as well as the acridane-based 9-acridanylidenemalonitrile-10-oxyl and 9-ethylacridanylidenecyanoacetate-10-oxyl, are described. The two acridane-based nitroxides can be isolated and are stable in solution as well as in the solid state for several days. Continuous wave X-band EPR measurements and density functional theory (DFT) calculations demonstrated that the spin density is delocalized over the whole molecule in all three cases. Furthermore, the DFT calculations provided insight into the molecular and electronic structures of these nitroxides and yielded hyperfine coupling constants which are in very good agreement with the experimental data allowing therefore an unambiguous assignment of the hyperfine couplings.     

Effect of temperature on the EPR properties of a rhamnose alkoxy radical: a DFT molecular dynamics study

It has been shown previously that two distinctive variants (called RHop and RO4) exist of the radiation-induced rhamnose alkoxy radical. Density functional theory (DFT) calculations of the electron paramagnetic resonance (EPR) properties were found to be consistent with two separate measurements at different temperatures [E. Pauwels, R. Declerck, V. Van Speybroeck, M. Waroquier, Radiat. Res., in press]. However, the agreement between theory and experiment was only of a qualitative nature, especially for the latter radical. In the present work, it is examined whether this residual difference between theoretical and experimental spectroscopic properties can be explained by explicitly accounting for temperature in DFT calculations. With the aid of ab-initio molecular dynamics, a temperature simulation was conducted of the RO4 variant of the rhamnose alkoxy radical. At several points along the MD trajectory, g and hyperfine tensors were calculated, yielding time (and temperature) dependent mean spectroscopic properties. The effect of including temperature is evaluated but found to be within computational error.