Third Generation Buchwald Precatalysts with XPhos and RuPhos: Multigram Scale Synthesis,Solvent-Dependent Isomerization of XPhos Pd G3 and Quality Control by 1H- and 31P-NMR Spectroscopy

The third generation Buchwald precatalysts Pd(ABP)(Phos)(OMs) (also known as Phos Pd G3)) with XPhos and RuPhos were prepared in multigram scale by a modified procedure (ABP = fragment of C-deprotonated 2-aminobiphenyl, XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl, RuPhos = 2-dicyclohexylphosphino-2′,6′-diisopropoxybiphenyl, OMs⁻ = CH₃SO₃⁻). The ¹H- and ³¹P-NMR spectra of the title complexes and some impurities, measured by various 1D and 2D techniques, were analyzed in detail. The solvent-dependent isomerization of Pd(ABP)(XPhos)(OMs) was studied by NMR, and the X-ray structures of two isomers were determined. The impurities in precatalysts, such as Pd(ABP)(HABP)(OMs) (HABP—neutral 2-aminobiphenyl coordinated to Pd²⁺ in N-monodentate mode) and PdCl₂(XPhos)₂, were identified and characterized by single crystal X-ray diffraction. A simple method for the quick quality control (QC) of the precatalysts, suitable for routine use, was proposed. The method was based on the assessment of the impurity content on the basis of the ¹H-NMR spectra analysis.     

Generation of a Hetero Spin Complex from Iron(II) Iodide with Redox Active Acenaphthene-1,2-Diimine

The reaction of the redox active 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-BIAN) and iron(II) iodide in acetonitrile led to a new complex [(dpp-BIAN)Fe^III₂] (1). Molecular structure of 1 was determined by the single crystal X-ray diffraction analysis. The spin state of the iron cation in complex 1 at room temperature and the magnetic behavior of 1 in the temperature range of 2–300 K were studied using Mossbauer spectroscopy and magnetic susceptibility measurements, respectively. The neutral character of dpp-BIAN in 1 was confirmed by IR and UV spectroscopy. The electrochemistry of 1 was studied in solution and solid state using cyclic voltammetry. The generation of the radical anion form of the dpp-BIAN ligand upon reduction of 1 in a CH₂Cl₂ solution was monitored by EPR spectroscopy.     

Application of fluorescence correlation spectroscopy: a study of flocculation of rigid rod-like biopolymer (schizophyllan) and colloidal particles

The flocculation between the rod-like biopolymer Schizophyllan and two types of colloidal particles (latex with diameter 40 nm and alumina with diameter 60 nm) has been investigated by means of fluorescence correlation spectroscopy (FCS). The concentration ratio of Schizophyllan/particle q was varied in the range 0.1 approximately 20. Under conditions of pH about 5.7, 1 mmol.L(-1) NaCl, and room temperature (22+/-0.5 degrees C), the particles are strongly charged (alumina particles positively charged, latex negatively), while Schizophyllan is neutral. We observed that Schizophyllan chains flocculate with both types of particles, which suggests that the charge neutralization does not play a decisive role in these interactions. The ratio of fluorescence intensity of one floc over that of one particle, Q(f)/Q(p), and the corresponding hydrodynamic radius (r(h)) of the flocs have been measured. For a Schizophyllan-latex system, Q(f)/Q(p) reached a maximum value of 5 for q=3 indicating that the flocs contained five particles on average. The corresponding value of r(h) was r(h)=455 nm. The flocculation kinetic of latex particles with Schizophyllan was too fast to be measurable by FCS. For the Schizophyllan-alumina system, Q(f)/Q(p) was stable at about 1 in the whole studied range of q but r(h) increased with q suggesting that many Schizophyllan chains are adsorbed on individual particles. The flocculation kinetic of this system was studied by FCS and the obtained results were compatible with those of photon correlation spectroscopy.     

Bending of DNA by asymmetric charge neutralization: all-atom energy simulations

DNA dodecamers of the alternating d(CG).d(CG) sequence with six phosphate groups either charge-neutralized or substituted by neutral methylphosphonates across the major or minor groove have been subjected to energy minimization to determine the conformational effect of the asymmetric elimination of phosphate charge. We report bending angles, directions of bending, and detailed structural characteristics such as groove widths and local base-pair parameters. Our principal results are that charge neutralization on one face of the DNA induces significant bending toward the neutralized face, in agreement with theoretical predictions on a simplified model and experimental data on a similar base-pair sequence, and that the DNA conformation averaged over all stereospecific methylphosphonate substitutions is nearly the same as the conformation produced by charge neutralization of the phosphates. Individual isomers, however, cover a wide range of structures, with the magnitude and direction of overall bending sensitive to the precise stereochemical pattern of neutralization. Our simulation does not explicitly contain counterions, and the results therefore suggest that counterions can influence DNA structure by neutralizing the phosphate charge. These data provide new hints into the molecular mechanisms which underlie the deformations of DNA structure induced by the binding of positively charged proteins and other tightly associated cationic species.     

4,10‐Dinitro‐2,6,8,12‐tetraoxa‐4,10‐diazaisowurtzitane (TEX): a nitramine with an exceptionally high density

The title compound (systematic name: 4,10‐di­nitro‐2,6,8,12‐tetraoxa‐4,10‐di­aza­tetra­cyclo­[5.5.0.0^3,11.0^5,9]­do­decane), C₆H₆N₄O₈, exhibits the highest density among known N‐nitramines, due to its close‐packed crystal structure. It may be regarded as consisting of a distorted hexagonal close‐packed lattice formed by the isowurtzitane cages, with the nitro groups occupying the free space between the cages.     

Diastereoselective synthesis of 3-tert-butyl-3,4-dihydropyrazolo[5,1-c][1,2,4]triazine-3,4-diyl dicarboxylates

The reactions of 3-tert-butyl-7-R¹-8-R²-pyrazolo[5,1-c][1,2,4]triazines (R¹ = H, Br; R² = Me, n-Bu) with N-bromosuccinimide in the presence of R³CO₂H (R³ = Me, t-Bu, Ph) afforded novel diastereomerically pure 3-tert-butyl-7-R¹-8-R²-3,4-dihydropyrazolo[5,1-c][1,2,4]triazine-3,4-diyl dicarboxylates. The structures of the isolated products were established on the basis of IR, ¹H, ¹³C, 2D NOESY NMR, high resolution mass spectrometry and X-ray single-crystal analysis. The steric and mechanistic origins of the observed regio- and stereoselectivity were also discussed.     

Cascade reaction of 2,3-naphthalenediol with benzene in the presence of aluminum halides

2,3-Naphthalenediol on superelectrophilic activation with aluminium halides smoothly reacts with benzene to give 4-((3-phenyl-1H-inden-1-yl)methyl)benzene-1,2-diol, which in turn undergoes intramolecular cyclization to form 5,10-methano-5-phenyldibenzo[a,d]cycloheptane-2,3-diol. The mechanistic aspects of these unusual transformations are discussed.     

A quantitative determination of the polymerization of benzoxazine thin coatings by time-of-flight secondary ion mass spectrometry

Phenol-paraphenylenediamine (P-pPDA) benzoxazines exhibit excellent barrier properties, adequate to protect aluminum alloys from corrosion, and constitute interesting candidates to replace chromate-containing coatings in the aeronautical industry. For the successful application of P-pPDA coatings, it is necessary to decrease the curing temperature to avoid the delamination of the coating while preserving the mechanical properties of the alloy, as well as the barrier properties of the coating. However, decreasing the curing temperature leads to less polymerized films, the extent of which requires a quantitative assessment. While the conversion rate of the polymerization reaction is commonly evaluated for bulk samples using differential scanning calorimetry (DSC), a tool for its evaluation in thin films is missing. Therefore, a new approach was developed for that matter using time-of-flight secondary ion mass spectrometry (ToF-SIMS). The relation between the SIMS data integrated from inside thin films and the DSC results obtained on bulk samples with the same curing cycle allowed to calibrate the SIMS data. With this preliminary calibration of the technique, the polymerization of P-pPDA coatings can be locally determined, at the surface and along the depth of the coating, using dual-beam depth profiling with large argon cluster beam sputtering.     

Infrared matrix isolation and quantum chemical studies of the nitrous acid complexes with water

The 1:1 and 2:1 complexes between water and trans- and cis-isomers of nitrous acid have been isolated in argon matrices and studied using FTIR spectroscopy and DFT(B3LYP) calculations with a 6-311++G(2d,2p) basis set. The analysis of the experimental spectra indicate that 1:1 complexes trapped in solid argon involve very strong hydrogen bond in which acid acts as the proton donor and water as the proton acceptor. The perturbed OH stretches are −248, −228 cm⁻¹ red shifted from their free-molecules values in complexes formed by trans- and cis-HONO isomers, respectively. The calculated spectral parameters for the two complexes are in good agreement with experimental data. The calculations also predict stability of two more 1:1 weakly bound complexes formed by each isomer. In these the water acts as the proton donor and one of the two oxygen atoms of the acid as the acceptor. The experimental spectra demonstrate also formation of 2:1 complex between water and trans-HONO isomer in an argon matrix. The performed calculations indicate that the complex involves a seven-membered ring in which OH group of HONO forms very strong hydrogen bond with the oxygen atom of one water molecule and nitrogen atom acts as a weak proton acceptor for the hydrogen atom of the second water molecule of the water dimer. The observed perturbations of the OH stretch of trans-HONO (750 cm⁻¹ red shift) is much larger than that predicted by calculations (556 cm⁻¹ red shift); this difference is attributed to strong solvation effect of argon matrix on very strong hydrogen bond.     

Molecular “Multi‐rod Cable” {[Co(NC5H4–CH=CH–C6H4–CH=CH–C5H4N)(HCOO)2]8}n: A Novel Supramolecular Architecture by Cooperation of Coordination and Stacking Interactions

The cobalt‐formate coordination polymers {[Co(bpyph)(HCOO)₂]₈}_n ( 1 ) (bpyph = 1,4‐bis(2‐(4‐pyridyl)ethenyl)benzene) and {[Co(HCONH₂)₂(HCOO)₂]}_n ( 2 ) have been prepared by interaction of Co(NO₃)₂ · 6 H₂O in formamide solution with generation of formate anion by hydrolysis of the solvent. Coordination polymer 1 reveals an unprecedented example of “molecular multi‐rod cable” architecture, in which eight single “molecular wires” {[Co(bpyph)]}_n are interlinked by bridging formate anions to give infinite octameric chains. The formate groups adopt mono‐, and bi‐ and tridentate bridging and chelate modes of coordination (Co–O 1.966–2.134 Å). The coordination geometry around the cobalt atoms is essentially dominated by the demands for most effective packing of parallel situated polycyclic aromatic ligands, with extensive CH…π, or edge‐to‐face stacking interactions within the single octameric chain as well as between the closest neighbours (C…C separations within this stack are ca. 3.50 Å).