ESR studies on reactions of dicyclopentadienyl dicarbonyl titanium with organic halides

ESR method was used to elucidate the mechanism of the reactions of alkyl, allyl or benzyl halides with dicyclopentadienyldicarbonyl titanium. The paramagnetic [intermediates of the reactions were identified during the course of the reactions. The reaction mechanism based on ESR findings and the products analyses is postulated to operate on radical pathways. When alkyl halides were used to react with the organometallic compound 1, the intermediate found was [Cp₂Ti(CO)X] (C), and the main product was identified to be dicyclopentadienyl-acyl-halo titanium (3), an insertion of TiCO into R-X, i.e. [Cp₂Ti-C(0)R] X. When allyl or benzyl halides were used, the intermediate found was [Cp₂TiX] (B), and the main products were identified to be the dicyclopentadienyl titanium dihalides and the coupling products of allyl or benzyl groups.     

Density functional and molecular orbital study of physical process of inversion of nitrogen trifluoride (NF3) molecule

The physical process of the umbrella inversion of the nitrogen trifluoride molecule has been studied invoking the formalisms of the density functional theory, the frontier orbital theory, and the molecular orbital theory. An intuitive structure and dynamics of evolution of the transition state for the event of inversion is suggested. The physical process of dynamic evolution of the molecular conformations between the equilibrium (C_3v) shape and the planar (D_3h) transition state has been followed by a number of molecular orbital and density functional parameters like the total energy, the eigenvalues of the frontier orbitals, the highest occupied molecular orbital and lowest unoccupied molecular orbital, the (HOMO–LUMO) gap, the global hardness and softness, and the chemical potential. The molecular conformations are generated by deforming the ∠FNF angle through steps of 2° from its equilibrium value, and the cycle is continued till the planar transition state is reached, and the geometry of each conformation is optimized with respect to the length of the N? F bond. The geometry optimization demonstrates that the structural evolution entails an associated slow decrease in the length of the N? F bond. The dipole moment at the equilibrium form is small and that at the transition state is zero and shows a strange behavior with the evolution of conformations. As the molecular structure begins to distort from its equilibrium shape by opening of the ∠FNF angle, the dipole moment starts increasing very sharply, and the trend continues very near to the transition state but abruptly vanishes at the transition state. A rationale of the strange variation of dipole moment as a function of evolution of conformations could be obtained in terms of quantum mechanical hybridization of the lone pair on the N atom. The pattern of charge density reorganization as a function of geometry evolution is a continuous depletion of charge from the F center and piling up of charge on the N center. The continuous shortening of bond length and the pattern of variation of net charge densities on atomic sites with evolution of molecular conformations predicts that the bond moment would decrease continuously. The quantum mechanical hybridization of the lone pair of the central N atom shows that the percentage of s character of the lone‐pair hybrid on the N atom decreases at a very accelerated rate, and the lone pair at the transition state is accommodated in a pure p orbital. The result of the continued destruction of asymmetry of charge distribution in the lone pair on the central N atom due to the elimination of contribution of the s orbital with evolution of molecular conformations is the sharp decrease in lone‐pair moment. The decrease in bond moment is overcompensated by the sharp fall of its offsetting component, the lone‐pair moment, resulting in a net gain in dipole moment with the evolution of molecular geometry. Since the offsetting component decreases very sharply, the net effect is a sharp rise of dipole moment with the evolution of molecular conformations just before the transition state. The lone‐pair moment is zero by virtue of the symmetry of the pure p orbital, the lone pair of the central atom in the transition state, and the sum of the bond moments is zero by symmetry of the geometry. The barrier height is quite high at ～65.45 kcal/mol, which is close to values computed through more sophisticated methods. It is argued that an earlier suggestion regarding the development of high barrier value of NF₃ system seems to be misleading and confronting with the conclusions of the density functional theory. An analysis and a comparative study of the physical components of the one‐ and two‐center energy terms reveals that the pattern of the charge density reorganization has the principal role in deciding the origin and the magnitude of barrier of inversion of the molecule and the barrier originates not from a particular energetic effect localized in a particular region of the molecule, rather the barrier originates from a subtle interplay of one‐ and two‐center components of the total energy. The decomposed energy components show that the F?F nonbonded interaction and N? F bonded interaction favor the formation of transition state, while the one‐center energy terms prohibit the formation of the transition state. The barrier principally develops from the one‐center energy components. The profile of the HOMO is isomorphic and that of the LUMO is homomorphic with the potential energy curve for the physical process of the event of umbrella inversion of the molecule. The variation of the HOMO–LUMO gap, ?ε, the global hardness, η, and the softness, S, as a function of the reaction coordinates of angular deformation of NF₃ molecule are quite consistent with the predictions of the molecular orbital and the density functional theories in connection with the deformation of molecular geometry. The profiles of ?ε, η, and S, as a function of reaction coordinates, mimic the potential energy curve of the molecule. The eigenvalues of the frontier orbitals, and the ?ε, η, S parameters are found to be equally effective theoretical parameters, like the total energy, to monitor the physical process of the inversion of pyramidal molecules. The nature of the variation of the global hardness parameter between the equilibrium shape and the transition state form for the inversion is in accordance with the principle of maximum hardness (PMH). © 2002 John Wiley & Sons, Inc. Int J Quantum Chem, 2002     

New Types of Stable Aldehydes: Formylphosphane and Formylphosphane Oxide

The reactivity of the formyl group in α-phosphorus-substituted aldehydes 1 can be tuned by adjusting the coordination number of the phosphorus atom. When X is a lone pair, the formylphosphanes obtained are stable compounds, whereas when X=O a relatively unstable formylphosphane oxide is produced. R=iPr₂N, cHex₂N.     

Synthetic and Structural Studies of 1,8‐Chalcogen Naphthalene Derivatives

Four novel 1,8‐disubstituted naphthalene derivatives 4 – 7 that contain chalcogen atoms occupying the peri positions have been prepared and fully characterised by using X‐ray crystallography, multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular distortion due to noncovalent substituent interactions was studied as a function of the bulk of the interacting chalcogen atoms and the size and nature of the alkyl group attached to them. X‐ray data for 4 – 7 was compared to the series of known 1,8‐bis(phenylchalcogeno)naphthalenes 1 – 3 , which were themselves prepared from novel synthetic routes. A general increase in the E???E′ distance was observed for molecules containing bulkier atoms at the peri positions. The decreased S???S distance from phenyl‐ 1 and ethyl‐ 4 analogues is ascribed to a weaker chalcogen lone pair–lone pair repulsion acting in the ethyl analogue due to the presence of two equatorial S(naphthyl) ring conformations. Two novel peri‐substituted naphthalene sulfoxides of 1 , Nap(O?SPh)(SPh) 8 and Nap(O?SPh)₂ 9 , which contain different valence states of sulfur, were prepared and fully characterised by using X‐ray crystallography and multinuclear NMR spectroscopy, IR spectroscopy and MS. Molecular structures were analysed by using naphthalene ring torsions, peri‐atom displacement, splay angle magnitude, S???S interactions, aromatic ring orientations and quasi‐linear O?S???S arrangements. The axial S(naphthyl) rings in 8 and 9 are unfavourable for S???S contacts due to stronger chalcogen lone pair–lone pair repulsion. Although quasi‐linear O?S???S alignments suggest attractive interaction is conceivable, analysis of the B3LYP wavefunctions affords no evidence for direct bonding interactions between the S atoms.     

Manipulating Lone-Pair-Driven Luminescence in 0D Tin Halides by Pressure-Tuned Stereochemical Activity from Static to Dynamic

The excellent luminescence properties and structural dynamics driven by the stereoactivity of the lone pair in a variety of low-dimensional ns² metal halides have attracted growing investigations for optoelectronic applications. However, the structural and photophysical aspects of the excited state associated with the lone pair expression are currently open questions. Herein, zero-dimensional Sn-based halides with static stereoactive 5 s² lone pairs are selected as a model system to understand the correlations between the distinctive lone pair expression and the excited-state structural relaxation and charge carrier dynamics by continuous lattice manipulation. Lattice compression drives 5 s² lone pair active switching and self-trapped exciton (STE) redistribution by suppressing excited-state structural deformation of the isolated SnBr₄²⁻ units. Our results demonstrate that the static expression of the 5 s² lone pair results in a red broadband triplet STE emission with a large Stokes shift, while its dynamic expression creates a sky-blue narrowband emission dominated by the radiative recombination of singlet STEs. Our findings and the photophysical mechanism proposed highlight the stereochemical effects of lone pair expression in controlling light emission properties and offer constructive guidelines for tuning the optoelectronic properties in diverse ns² metal halides.     

14N and 35Cl NQR studies of organophosphorus compounds

¹⁴N and ³⁵Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the N? P bond are calculated according to the Townes and Dailey method. The experimental data are interpreted assuming a partial double bond character of the N? P bond due to the p_π? d_π interaction and p_π? σ conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the N? P bond populations is observed in X ? PR_n (R₂N)_3-n molecules (where X is O, S, Se, or lone pair electrons and n = 0, 1, 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence P?Se > P?S > P?O > P.     

Synthesis and Crystal Structures of Phosphorus-and Selenium-Containing Medium-Sized Heterocycles and Macrocycles

Abstract

New phosphorus- and selenium-containing heterocyclyes, 1,3,2,6-dioxa-phosphaselenacyclooctanes (la-lo), were synthesized in ca. 10% yields from 3-selena-l,5-penta-diol and RP(X)Cl₂ where R = alkoxyl, aroxyl, aryl and X = 0 or lone pair. Three macrocycles 3, 4 and 5, which were expected to be heterodinucleating ligands, were obtained in moderate yields (20–30%) from bis(o-bromomethylpheny1)phenylphosphine oxide 2 and corresponding selenium-containing materials.     

Polymerization of methyl methacrylate initiated by tri-n-butylborane–organic halide systems

The polymerization of methyl methacrylate (MMA) initiated by tri-n-butylborane (TBB) was studied in the presence of various organic halides (R′X). It was found that R′X accelerated the polymerization of MMA. Aliphatic halides were more effective than aromatic halides. Cocatalytic effects of butyl halides decreased in the order: n -BuI > n -BuBr > n -BuCl; n -BuBr ? sec-BuBr > i-BuBr > tert-BuBr. In the polymerization of MMA by TBB- n -BuI, the initial rate of polymerization was found to be proportional to the concentration of MMA and to the square root of the concentration of TBB-n-BuI. The apparent activation energy was 5.3 kcal/mole. From this and other results, it was assumed that the polymerization of MMA by this initiator system proceeds by a radical mechanism via a weak complex between TBB and R′X; alkyl radicals are formed by the interaction of R′X with TBB. The TBB–R′X system can initiate the polymerization of MMA and AN, but is ineffective in the polymerization of styrene.     

Palladium(II) halide complexes with dithioesters derived from sarcosine

Summary The compounds EtO₂CCH₂(Me)NCS₂R (R = Me, ESDTM; R = Et, ESDTE) were prepared from sarcosine ethyl ester hydrochloride, CS₂ and alkyl iodide in EtOH-H₂O. These ligands react with palladium halides in benzene to yield the benzene solvates [Pd(ESDTR)X₂]·nC₆H₆ (R = Me or Et; X = Cl or Br; n < 1), in which the dithioester molecule coordinates through both sulphur atoms. Ligands and complexes have been characterized by i.r. and ¹H n.m.r. spectroscopy and by thermal analysis (t.g., d.t.g. and d.t.a.). The low stability of the adducts in both solution and solid phase is discussed on the basis of proton n.m.r. spectra. Thermal degradation of the 1∶1 complexes has been examined up to 1000° C. The first decomposition step involves release of alkyl halide to form the [Pd(ESDT)X] _n (X = Cl or Br) intermediates, which successively decompose, finally giving palladium.     

SrSn3 – eine supraleitende Legierung mit freien Elektronenpaaren

SrSn₃ – a Superconducting Alloy with Non‐bonding Electron Pairs SrSn₃ was synthesized from the elements in a welded niobium ampoule. The crystal structure was determined from X‐ray single crystal data. Space group R3m, a = 6,940(2) Å, c = 33,01(1) Å, Z = 12, Pearson symbol hR48. SrSn₃ shows an ordered atomic distribution on four crystallographic sites. The structure is build up from two closed packed atom layers (Sn1/Sr1 and Sn2/Sr2) each with the composition Sr : Sn = 1 : 3 and with hexagonal symmetry of the Sr atoms. The Sn atoms are shifted with respect to the ideal positions of a closed packed layer in a way that Sn triangles, which are separated by Sr atoms, result. Translational symmetry along the c axis arises from a 12‐layer stacking sequence with hexagonal and cubic closest packing motives. Due to the layer sequence ABABCACABCBC… units of three face‐sharing Sn octahedra result (condensation through Sn2 atoms) which form the Sn partial structure. The octahedra chains run parallel to the c axis and are connected by exclusively vertex sharing Sn octahedra (Sn1 atoms). Temperature dependent susceptibility measurements reveal superconducting properties. LMTO band structure calculations verify the metallic behavior. An analysis of the density of states with the help of the electron localization function (ELF) shows, that two kinds of lone pairs occur in this intermetallic phase: non‐bonding electron pairs with the shape of a sp² orbital hybrid are located at the Sn2 atoms and lone pairs with p orbital character are located at Sn1 atoms. The role of lone pairs with respect to the superconducting property is discussed.     

Exploration of Chiral Organic–Inorganic Hybrid Semiconducting Lead Halides

Organic–inorganic lead halides have recently emerged as promising alternatives to conventional optoelectronic materials, considering their intriguing physical properties. However, organic–inorganic lead halides featuring chirality are seldom explored. Here, a pair of enantiomorphic organic–inorganic hybrid semiconducting lead halides, (R‐C₅H₁₄N₂)PbBr₄ ( 1R ) and (S‐C₅H₁₄N₂)PbBr₄ ( 2S ), were successfully obtained with the templating of chiral amines. These compounds adopt distinct one‐dimensional infinite quantum helices formed by edge‐shared transformative lead bromide octahedra. Notably, 1R and 2S present mirror circular dichroism (CD) signals due to the chirality transfer of the enantiopure amines. Furthermore, 1R and 2S exhibit phase‐matchable quadratic nonlinear response and typical semiconducting behaviours. This work highlights the potential of lead halides as a new kind of chiral semiconducting materials in spintronic and chiral optical applications.     

LSn(OCH2CH2)2NR (L = lone pair,W(CO)5; R = Me,t‐Bu). The Molecular Structures of 5‐Aza‐2,8‐dioxa‐1‐stannabicyclo[3.3.0]1.5octanes and Their Tungstenpentacarbonyl Complexes

Single crystal X‐ray diffraction analyses of LSn(OCH₂CH₂)₂NR [ 1 , R = Me, L = lone pair; 2 , R = Me, L = W(CO)₅; 3 , R = t‐Bu, L = W(CO)₅] reveal these compounds to be dimeric and cis‐configurated. The dimerization is realized by intramolecular O→Sn interactions to give four‐membered Sn₂O₂‐rings. In addition, there are intramolecular N→Sn interactions ranging in between 2.356(5) ( 2 ) and 2.549(4) Å ( 3 ).     

Synthesis of 2,3-Disubstituted Thiophenes from 1,3-Dimetallated Acetylenes and Non-Enolizable Thiocarbonyl Compounds

Reaction of 1,3-dimetallated acetylenes KC°C-CH(K)R (R=alkyl, O-alkyl, S-alkyl, aryl, N(alkyl)₂) with non-enolizable thiocarbonyl compounds XCSSCH₃ (X=aryl, heteroaryl, t-Bu, CH₃O, t-BuO, CH₃S, N(alkyl)₂) gives 2,3-disub-stituted thiophenes (6) having X in the 2-position and R in the 3-position.     

Selenium-77 chemical shifts of cyclodiphosphazane selenides

Isomeric cyclodiphosphazane selenides, R(X)P(NBu¹)₂P(Y)R (R=OMe or NMe₂, X=Se, Y=lone pair; R=OMe or NMe₂, X=Y=Se) display large differences (34–122 ppm) in ⁷⁷Se chemical shift. The ⁷⁷Se shifts of these and related amino derivatives are well to low field of the ⁷⁷Se shifts of analogous acyclic phosphorus selenides.     

Tailoring of Electronic Structure and Thermoelectric Properties of a Topological Crystalline Insulator by Chemical Doping

Topological crystalline insulators (TCIs) are a new quantum state of matter in which linearly dispersed metallic surface states are protected by crystal mirror symmetry. Owing to its vanishingly small bulk band gap, a TCI like Pb_0.6Sn_0.4Te has poor thermoelectric properties. Breaking of crystal symmetry can widen the band gap of TCI. While breaking of mirror symmetry in a TCI has been mostly explored by various physical perturbation techniques, chemical doping, which may also alter the electronic structure of TCI by perturbing the local mirror symmetry, has not yet been explored. Herein, we demonstrate that Na doping in Pb_0.6Sn_0.4Te locally breaks the crystal symmetry and opens up a bulk electronic band gap, which is confirmed by direct electronic absorption spectroscopy and electronic structure calculations. Na doping in Pb_0.6Sn_0.4Te increases p‐type carrier concentration and suppresses the bipolar conduction (by widening the band gap), which collectively gives rise to a promising zT of 1 at 856 K for Pb_0.58Sn_0.40Na_0.02Te. Breaking of crystal symmetry by chemical doping widens the bulk band gap in TCI, which uncovers a route to improve TCI for thermoelectric applications.     

Theoretical Study on the Mechanism of Ni‐Catalyzed Alkyl–Alkyl Suzuki Cross‐Coupling

Ni‐catalyzed cross‐coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl–alkyl bonds. The mechanism of this reaction with the Ni/ L1 ( L1 =trans‐N,N′‐dimethyl‐1,2‐cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a Ni^I–Ni^III catalytic cycle with three main steps: transmetalation of [Ni^I( L1 )X] (X=Cl, Br) with 9‐borabicyclo[3.3.1]nonane (9‐BBN)R₁ to produce [Ni^I( L1 )(R₁)], oxidative addition of R₂X with [Ni^I( L1 )(R₁)] to produce [Ni^III( L1 )(R₁)(R₂)X] through a radical pathway, and C? C reductive elimination to generate the product and [Ni^I( L1 )X]. The transmetalation step is rate‐determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol^?1). On the other hand, the cross‐coupling of alkyl chlorides can be catalyzed by Ni/ L2 ( L2 =trans‐N,N′‐dimethyl‐1,2‐diphenylethane‐1,2‐diamine) because the activation barrier of transmetalation with L2 is lower than that with L1 . Importantly, the Ni⁰–Ni^II catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [Ni^II( L1 )(R₁)(R₂)] is very difficult.     

Sodium iodine(V) oxyfluoride,NaIO2F2

As an extension of a general structural study concerning fluorides and oxyfluorides of cations presenting a stereochemically active electronic lone pair, until now limited to tellurium(IV) phases, the previously unknown structure of NaIO₂F₂ corresponds to a new structure type based on isolated IO₂F₂⁻ polyhedra forming sheets separated by Na⁺ layers. The sodium ion is octahedrally coordinated with 2/m site symmetry, while the I^V atom has m2m symmetry with a stereochemically active lone electron pair. The O and F atoms (both with m symmetry) are bonded to the I^V atoms in a fully ordered manner. A comparison with the structure of ferroelastic KIO₂F₂ and with structures based on hexagonal close packing of anions, mainly rutile‐type and FeTeO₃F‐type, reveals differences that are attributed to the smaller ionic radius of Na⁺ and the ordering of the Na and I cations.     

1,2-Metallobridging and the Antiperiplanar Effect. Thermodynamic Preference for the Z-Configuration in Imidoyl(Alkali Metals) and their Phosphorus Analogs

We showed that imidoyl- and phosphaethenyl(alkali metals) would thermodynamically prefer the Z-configuration. The bond model analysis of the electronic structures showed that the Z-preference should originate from 1,2-metallobridging by the delocalization of lone pairs on N or P to vacant p-orbitals of the alkali metals and from the antiperiplanar effect of the delocalization from σ _C—M to σ? _{N(P)—R ²} and from n _N(P) to the C—R¹. The Z-preference increases by more electron-withdrawing groups at the carbon atom of the double bond. However, substitution at the nitrogen/phosphorus results in E-preference because of 1,4-chelation of the lone pair of the substituents to alkali metals. Most of halogen derivatives were not stable and eliminate metal halides.     

Structure-Property Correlation Relation: XII. Simple Expression for a Property of Any Organic Compound Containing Unbranched Alkyl Substituents

A simple expression that allows accurate calculation of physicochemical properties of organic compounds like RX (R is an unbranched alkyl substituent C _n H_{2 n + 1} and X is a functional group) was proposed. The potential of the proposed approach is demonstrated by the estimation of the boiling points and heat capacities at constant pressure of alkyl benzenes and carboxylic esters.     

Intermolecular interactions: Elaboration on an additive procedure including an explicit charge-transfer contribution

An additive procedure is derived for the computation of intermolecular interactions, in which an explicit expression for the charge-transfer energy contribution E_CT is implemented. In the total interaction energy, the electrostatic terms E_MTP and E_pol are calculated as in our previous treatment. The dispersion contribution is calibrated by reference to variation-perturbation computations on model systems and the repulsion contribution is computed as a sum of bond—bond, bond—lone pair, and lone pair—lone pair interactions. Tests of the procedure are given for representative hydrogen-bonded systems.