Copper‐Mediated Aromatic 1,1‐Difluoroethylation with (1,1‐Difluoroethyl)trimethylsilane (TMSCF2CH3)

A new method for the formation of 1,1‐difluoroethyl copper species (“CuCF₂CH₃”) with 1,1‐difluoroethylsilane (TMSCF₂CH₃) has been developed. The “CuCF₂CH₃” species can be applied to the efficient 1,1‐difluoroethylation of diaryliodonium salts under mild conditions, affording (1,1‐difluoroethyl)arenes in good to excellent yields. This convenient procedure tolerates a wide range of functional groups and thus serves as a practical synthetic tool for the introduction of CF₂CH₃ group(s) into complex molecules.     

A Density Functional Theory (DFT) Quantum‐Chemical Approach to the Real Structure of Poly(methylaluminoxane)

DFT quantum chemical calculations have been performed in order to optimize the geometric and electronic cage structure of poly(methylaluminoxane) (MAO) with oligomerization degree n = 9–15, and to find such structures that fit most closely the existing experimental data on the MAO composition and structure. The following peculiarities of the MAO structure were found: i) In “classic” MAO (n = 9, 12, 15; Al : CH₃ : O = 1 : 1 : 1), which has a triple‐layer cage structure, the inner layer contains highly reactive Al‐O bonds. ii) The reaction between “classic MAO” and trimethylaluminium (TMA) proceeds by the concerted mechanism, with the insertion of Al‐CH₃ groups into these Al‐O bonds producing “true” MAO (Al : CH₃ : O = 1 : 1.5 : 0.75). The calculated geometric and electronic structures of “true” MAO with n = 6, 9, 12 are presented. iii) “True” MAO and “classic” MAO exist in equilibrium. The driving force for the formation of “true” MAO is the decrease in enthalpy, and of “classic” MAO the increase in entropy, in the equilibrium reaction between “classic” MAO and TMA.     

Molecular mechanics of organic halides. III. Fluorinated olefins

A molecular mechanical force field for fluorinated ethylenes and propylenes is described. The field computes geometries, dipole moments, and energy differences between isomers and rotamers. Component analysis indicates that “classical” strains alone (skeletal, nonbonded, electrostatic) cannot account for energy relationships. The discussion is supplemented by a semiempirical quantum chemical comparison of cis- and trans-FCH?CHF and of gauche- and anti-FCH₂CH₂F.     

Parallel ab initio and molecular mechanics investigation of polycoordinated Zn(II) complexes with model hard and soft ligands: Variations of binding energy and of its components with number and charges of ligands

In this study we compare the binding energies of polycoordinated complexes of Zn²⁺ within cavities composed of model “hard” (H₂O, OH⁻) or “soft” (CH₃SH, CH₃S⁻) ligands. Ab initio supermolecule computations are performed at the HF and MP2 levels using extended basis sets to determine the binding energies and their components as a function of: the number of ligands, ranging from three to six; the net charge of the cavity; and the “hard” versus “soft” character of the ligands. These ab initio computations are used to test the reliability of the SIBFA molecular mechanics procedure, originally formulated and calibrated on the basis of ab initio computations, for such charged systems. The SIBFA intermolecular interaction energies match the corresponding ab initio values using a coreless effective potential split‐valence basis set with a relative error of ≤3%. Extensions to binuclear Zn²⁺ complexes, such as those that occur in the Zn‐binding sites of Gal4 and β‐lactamase proteins, are performed to test the applicability of the methodology for such systems. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 1011–1039, 2000     

Anti‐Electrostatic Hydrogen Bonds

Ab initio and hybrid density functional techniques were employed to characterize a surprising new class of H‐bonded complexes between ions of like charge. Representative H‐bonded complexes of both anion–anion and cation–cation type exhibit appreciable kinetic stability and the characteristic theoretical, structural, and spectroscopic signatures of hydrogen bonding, despite the powerful opposition of Coulomb electrostatic forces. All such “anti‐electrostatic” H‐bond (AEHB) species confirm the dominance of resonance‐type covalency (“charge transfer”) interactions over the inessential (secondary or opposing) “ionic” or “dipole–dipole” forces that are often presumed to be essential for numerical modeling or conceptual explanation of the H‐bonding phenomenon.     

Interaction of acetonitrile with alkaline metal cations: A density functional,coupled‐cluster,and quadratic configuration interaction study

A systematic quantum chemical study of CH₃CN and its CH₃CN?M⁺ 1:1 model adducts (M⁺∈{Li⁺, Na⁺}) is presented, with respect to binding energetics, structural and vibrational force field changes. Several gradient‐corrected density functional levels of theory were employed (of both “pure” and “hybrid” character), together with the coupled cluster including double substitutions from the Hartree–Fock determinant (CCD) and quadratic configuration interaction including single and double substitutions (QCISD) methods [with the rather large 6‐311G(d,p) basis set], and their performances compared. The binding energy decompositions according to the Kitaura–Morokuma scheme and the reduced variational space self‐consistent field (RVS‐SCF) method have shown that the electrostatic plus polarization interaction terms are primarily responsible for overall stabilization, while the charge‐transfer term is negligibly small and virtually identical for both adducts. The computed harmonic vibrational frequencies for acetonitrile correlate excellently with the experimental ones (r²>0.9998 for almost all cases, while for the BLYP level, r²=1). It is shown for the first time that the experimentally observed blue shifts of the ν_CN mode are caused even by formation of 1:1 adducts, contrary to the previously accepted opinions. The CCD and QCISD, as well as the BPW91 and BP86 levels of theory predict almost excellently the ν_CN mode blue shift upon adduct formation, while the BLYP and B3LYP levels perform significantly poorer. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Lone pairs mapping by Laplacian of 3He NMR chemical shift

Results of approbation of a new quantum mechanical approach of lone pairs (LPs) visualization, its optimization and testing on a range of model molecules are presented. The main idea of proposed methodology is using ³He atom as a probe for investigating electronic shells of species with LPs. As model objects, we consider “classical” examples of hydrogen cyanide, methanimine, ammonia, phosphine, formaldehyde, water, and hydrogen sulfide. It is shown that LPs can be visualized by means of 3D maps of Laplacian of ³He chemical shift ∇²δ_He. NMR calculations could be performed using level of theory as low as B3LYP/6-31G, allowing for the reduction of computational time without significant loss of quality. Advantages of our approach are discussed in comparison with usual methods of lone pairs visualization (electron localization function, molecular electrostatic potential).     

Molekülstruktur und thermische Stabilität des metallacyclischen Platin(II)‐Komplexes [Li(TMEDA)]2Pt(CH2CMe2CMe2CH2)2

Molecular Structure and Thermal Stability of the Metallacyclic Platinum(II) Complex [Li(TMEDA)]₂Pt(CH₂CMe₂CMe₂CH₂)₂ The X‐ray investigation at the “ate‐complex” [Li(TMEDA)]₂Pt(CH₂CMe₂CMe₂CH₂)₂ ( 1 ) revealed a new structure type of homoleptic organometallic compounds of platinum(II). Differences of the molecular structure of the “ate‐complex” [Li(TMEDA)]₂Pt(CH₂CH₂CH₂CH₂)₂ ( 2 ) as well as similarities to the structure of the homologeous “ate‐complex” of nickel(II) [Li(TMEDA)]₂Ni(CH₂CMe₂CMe₂CH₂)₂ ( 3 ) are described. A possible mechanism of the thermal decomposition of the complex 1 is discussed.     

Interactions Between Polymers and Nanoparticles: Formation of “Supermicellar” Hybrid Aggregates

Abstract

When polyelectrolyte‐neutral block copolymers are mixed in solutions to oppositely charged species (e.g., surfactant micelles, macromolecules, proteins, etc.), there is the formation of stable “supermicellar” aggregates combining both components. The resulting colloidal complexes exhibit a core‐shell structure, and the mechanism yielding to their formation is electrostatic self‐assembly. In this contribution, we report on the structural properties of “supermicellar” aggregates made from yttrium‐based inorganic nanoparticles (radius 2 nm) and polyelectrolyte‐neutral block copolymers in aqueous solutions. The yttrium hydroxyacetate particles were chosen as a model system for inorganic colloids, and also for their use in industrial applications as precursors for ceramic and opto‐electronic materials. The copolymers placed under scrutiny are the water‐soluble and asymmetric poly(sodium acrylate)‐b‐poly(acrylamide) diblocks. Using static and dynamical light‐scattering experiments, we demonstrate the analogy between surfactant micelles and nanoparticles in the complexation phenomenon with oppositely charged polymers. We also determine the sizes and the aggregation numbers of the hybrid organic–inorganic complexes. Several additional properties are discussed, such as the remarkable stability of the hybrid aggregates and the dependence of their sizes on the mixing conditions.     

Collisionally induced dissociative ionization of neutral products from unimolecular ion fragmentations. 1—Neutral product structures

A method is described for the investigation of the structure of neutral products from the unimolecular (metastable) dissociative ionizations of mass selected ions, by means of the collisionally induced dissociative ionization of the neutral species themselves. The neutral species, with kilovolt translational energies, enter a positively charged collision cell situated in the second field free region of a standard ZAB-2F mass spectrometer. Dissociative ionization of the neutrals results therein from their collisions with He target gas. The resulting ions are analysed by means of the electric sector and the relative ion abundances are shown to be structure characteristic. For such experiments the neutral flux should be c. ≥ 0.5% of the selected precursor ion flux; the collision gas pressure must be insufficient to cause significant precursor ion fragmentation in the field free region preceding the collision cell. It was shown that HNC is generated in the fragmentation of aniline molecular ions, whereas HCN is the neutral product in the dissociative ionizations of pyridine, benzonitrile and benzyl cyanide. The neutral radical [C, H₃, O˙] formed together with [CH₃CO]⁺ from ionized methyl acetate has the structure ˙CH₂OH, but that from the analogous fragmentation of the methyl propanoate molecular ion has the structure CH₃O˙. Allyl radicals were shown to be generated from [(CH₃)₂CHCH₂OH]⁺˙ together with [CH₃OH₂]⁺ ions.     

Syntheses and Structures of Xenon Trioxide Alkylnitrile Adducts

The potent oxidizer and highly shock‐sensitive binary noble‐gas oxide XeO₃ interacts with CH₃CN and CH₃CH₂CN to form O₃XeNCCH₃, O₃Xe(NCCH₃)₂, O₃XeNCCH₂CH₃, and O₃Xe(NCCH₂CH₃)₂. Their low‐temperature single‐crystal X‐ray structures show that the xenon atoms are consistently coordinated to three donor atoms, which results in pseudo‐octahedral environments around the xenon atoms. The adduct series provides the first examples of a neutral xenon oxide bound to nitrogen bases. Raman frequency shifts and Xe?N bond lengths are consistent with complex formation. Energy‐minimized gas‐phase geometries and vibrational frequencies were obtained for the model compounds O₃Xe(NCCH₃)_n (n=1–3) and O₃Xe(NCCH₃)_n?[O₃Xe(NCCH₃)₂]₂ (n=1, 2). Natural bond orbital (NBO), quantum theory of atoms in molecules (QTAIM), electron localization function (ELF), and molecular electrostatic potential surface (MEPS) analyses were carried out to further probe the nature of the bonding in these adducts.     

NMR Diffusion Measurements as a Simple Method to Examine Solvent–Solvent and Solvent–Solute Interactions in Mixtures of the Ionic Liquid [Bmim][N(SO2CF3)2] and Acetonitrile

The self‐diffusion coefficients of each component in mixtures of 1‐butyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide ([Bmim][N(SO₂CF₃)₂]) and acetonitrile were determined. The results suggest that the hydrodynamic boundary conditions change from “stick” to “slip” as the solvent composition transitions from “ionic liquid dissolved in acetonitrile” (χ_IL<0.4) to “acetonitrile dissolved in ionic liquid” (χ_IL>0.4). At higher χ_IL, the acetonitrile species are affected by “cage” and “jump” events, as the acetonitrile molecules reside nearer to the charged centre on the ions than in the “non‐polar” regions. The self‐diffusion coefficients of hexan‐1‐amine, dipropylamine, 1‐hexanol and dipropylether in mixtures of [Bmim][N(SO₂CF₃)₂] and acetonitrile were determined. In general, the nitrogen‐containing solutes were found to diffuse slower than the oxygen‐containing solutes; this indicates that there are greater ionic liquid–N interactions than ionic liquid–O interactions. This work demonstrates that the self‐diffusion coefficients of species can provide valuable information about solvent–solvent and solvent–solute interactions in mixtures containing an ionic liquid.     

H2S‐Mediated Thermal and Photochemical Methane Activation

Sustainable, low‐temperature methods for natural gas activation are critical in addressing current and foreseeable energy and hydrocarbon feedstock needs. Large portions of natural gas resources are still too expensive to process due to their high content of hydrogen sulfide gas (H₂S) mixed with methane, deemed altogether as sub‐quality or “sour” gas. We propose a unique method of activation to form a mixture of sulfur‐containing hydrocarbon intermediates, CH₃SH and CH₃SCH₃, and an energy carrier such as H₂. For this purpose, we investigated the H₂S‐mediated methane activation to form a reactive CH₃SH species by means of direct photolysis of sub‐quality natural gas. Photoexcitation of hydrogen sulfide in the CH₄+H₂S complex resulted in a barrierless relaxation by a conical intersection to form a ground‐state CH₃SH+H₂ complex. The resulting CH₃SH could further be coupled over acidic catalysts to form higher hydrocarbons, and the resulting H₂ used as a fuel. This process is very different from conventional thermal or radical‐based processes and can be driven photolytically at low temperatures, with enhanced control over the conditions currently used in industrial oxidative natural gas activation. Finally, the proposed process is CO₂ neutral, as opposed to the current industrial steam methane reforming (SMR).     

Efficient Room‐Temperature,Au+‐Mediated Coupling of a Carbene Ligand with Methane To Generate C2Hx (x=4, 6)

The thermal reactions of the closed‐shell, “naked” gold–carbene complex [Au(CH₂)]⁺ with methane have been explored by using FTICR mass spectrometry complemented by quantum chemical (QC) calculations at the CCSD(T)//BMK level of theory. Mechanistic aspects for this unprecedentedly efficient carbene insertion in the C? H bond of methane have been addressed and the origin of the counterintuitive high reactivity of [Au(CH₂)]⁺ towards this most inert hydrocarbon is discussed.     

Scanning force microscopic study of protein adsorption on the surface of organosilane monolayers prepared by the Langmuir‐Blodgett method

The n‐octadecyltrichlorosilane (OTS, CH₃(CH₂)₁₇SiCl₃), 18‐nonadecenyltrichlorosilane (NTS, CH₂=CH(CH₂)₁₇SiCl₃), [2‐(perfluorooctyl)ethyl] trichlorosilane (FOETS, CF₃(CF₂)₇CH₂CH₂SiCl₃) monolayers, and their mixed monolayers were used as the model substrates for the study of protein adsorption mechanism. Surface plasmon resonance (SPR) spectroscopy was applied to analyze the protein adsorption behavior onto the surface of the monolayers. Atomic force microscope (AFM) was used to observe the monolayer surfaces after exposure of these monolayers to bovine serum albumin (BSA) and γ‐globulin(IgG) solution. AFM observation revealed that the charged protein either below or above the isoelectric point was preferentially adsorbed onto the FOETS phase of the (OTS/FOETS) mixed monolayer. SPR revealed that the amount of adsorbed protein in the charged state was lower than that in the neutral state. These results indicate that the preferential adsorption of protein onto the FOETS phase for the mixed monolayer systems at either below or above pI is due to (1) the minimization of interfacial free energy between the monolayer surface and the buffer solution, and (2) the electrostatic repulsion among protein molecules bearing charges.     

Electrostatic vs. inductive effects in phosphine ligand donor properties and reactivity

Enhanced rates and selectivity in enzymes are enabled in part by precisely tuned electric fields within active sites. Analogously, the use of charged groups to leverage electrostatics in molecular systems is a promising strategy to tune reactivity. However, separation of the through space and through bond effects of charged functional groups is a long standing challenge that limits the rational application of electric fields in molecular systems. To address this challenge we developed a method using the phosphorus selenium coupling value (J_P–Se) of anionic phosphine selenides to quantify the electrostatic contribution of the borate moiety to donor strength. In this analysis we report the synthesis of a novel anionic phosphine, PPh₂CH₂BF₃K, the corresponding tetraphenyl phosphonium and tetraethyl ammonium selenides [PPh₄][SePPh₂CH₂BF₃] and [TEA][SePPh₂CH₂BF₃], and the Rh carbonyl complex [PPh₄][Rh(acac)(CO)(PPh₂(CH₂BF₃))]. Solvent-dependent changes in J_P–Se were fit using Coulomb''s law and support up to an 80% electrostatic contribution to the increase in donor strength of [PPh₄][SePPh₂CH₂BF₃] relative to SePPh₂Et, while controls with [TEA][SePPh₂CH₂BF₃] exclude convoluting ion pairing effects. Calculations using explicit solvation or point charges effectively replicate the experimental data. This J_P–Se method was extended to [PPh₄][SePPh₂(2-BF₃Ph)] and likewise estimates up to a 70% electrostatic contribution to the increase in donor strength relative to SePPh₃. The use of PPh₂CH₂BF₃K also accelerates C–F oxidative addition reactivity with Ni(COD)₂ by an order of magnitude in comparison to the comparatively donating neutral phosphines PEt₃ and PCy₃. This enhanced reactivity prompted the investigation of catalytic fluoroarene C–F borylation, with improved yields observed for less fluorinated arenes. These results demonstrate that covalently bound charged functionalities can exert a significant electrostatic influence under common solution phase reaction conditions and experimentally validate theoretical predictions regarding electrostatic effects in reactivity.

A detailed analysis of donor properties in solution reveals a major, even dominant, electrostatic effect from charged substituents.     

Organometallic Antitumor Compounds: Ferrocifens as Precursors to Quinone Methides

The synthesis and chemical oxidation profile of a new generation of ferrocifen derivatives with strong antiproliferative behavior in vitro is reported. In particular, the hydroxypropyl derivative HO(CH₂)₃C(Fc)=C(C₆H₄OH)₂ ( 3 b ) exhibited exceptional antiproliferative activity against the cancer cell lines HepG2 and MDA‐MB‐231 TNBC, with IC₅₀ values of 0.07 and 0.11 μM , respectively. Chemical oxidation of 3 b yielded an unprecedented tetrahydrofuran‐substituted quinone methide (QM) via internal cyclization of the hydroxyalkyl chain, whereas the corresponding alkyl analogue CH₃CH₂‐C(Fc)=C(C₆H₄OH)₂ merely formed a vinyl QM. The ferrocenyl group in 3 b plays a key role, not only as an intramolecular reversible redox “antenna”, but also as a stabilized carbenium ion “modulator”. The presence of the oxygen heterocycle in 3 b‐QM enhances its stability and leads to a unique chemical oxidation profile, thus revealing crucial clues for deciphering its mechanism of action in vivo.