Mobilité conformationnelle et migration des liaisons π dans le [24]annulène

Conformational Mobility and Migration of the π Bonds of the [24]annulene. The configuration and the conformation of [24]annulene have been determined after a detailed analysis of its ¹H-NMR spectrum recorded at −95°. At this temperature, molecular dynamics is practically frozen, and the spectrum can be correctly simulated considering eight magnetic sites with the relevant couplings. The [24]annulene exhibits alternation of the double and the single bonds with the CTTTCTTTCTTT sequence (C=cis, T=trans) expressing the connectivity of the double bonds. The signal of the 9 protons pointing inside the ring is 7.72 ppm at lower field than the signal of the 15 outer protons; this indicates a marked paramagnetic ring current. Molecular dynamics is revealed by the dependence of the spectrum upon the temperature; the simulation of the line shape of these spectra indicates that the [24]annulene in solution exists as an equilibrium of two conformers A and B ( B / A ≤0.05), both having the same configuration. Each of these conformers undergoes two isodynamic processes: a migration of the π bonds on the adjacent single bonds (bond shift) described by V and a conformational mobility described by K. The two conformers interconvert extremely rapidely. Conformer A complies with C_3h symmetry, conformer B with C₃ symmetry. The enthalpy, entropy, and free energy of activation for the processes described by V and K in the major conformer A have been determined: these processes are slower than those observed in [16]annulene. From their values, we could deduce that the resonance energy in the [24]-7³annulene is negative and of the order of −9 to −10 kcal⋅mol⁻¹.     

Macrocyclic Tetraethynylethene Molecular Scaffolding: Perethynylated aromatic dodecadehydro[18]annulenes,antiaromatic octadehydro[12]annulenes,and expanded radialenes

Tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) molecular scaffolding provided access to novel macrocyclic nanometer-sized C-rich molecules with unusual structural and electronic properties. Starting from cis-bis-deprotected cis-bis(trialkylsilyl)protected tetraethynylethenes, the per(silylethynyl)ated octadehydro[12]annulenes 1 and 2 and the corresponding dodecadehydro[18]annulenes 4 and 5 were prepared by oxidative Hay coupling. X-Ray crystal-structure analyses of (i-Pr)₃Si-protected 2 and Me₃Si-protected 4 showed that both annulene perimeters are perfectly planar. Electronic absorption spectral comparisons provided strong evidence that the macro rings in the deep-purple-colored 1 and 2 are antiaromatic (4n π-electrons), whereas those in yellow 4 and 5 are aromatic ((4n + 2) π-electrons). Although unstable in solution, the antiaromatic compound 2 gave high-melting crystals in which the individual octadehydro[12]annulene chromophores are isolated and stabilized in a matrix-type environment formed by the bulky (i-Pr)₃Si groups. Electrochemical studies demonstrated that the antiaromatic octadehydro[12]annulene 2 undergoes two stepwise one-electron reductions more readily that the aromatic chromophore 5 . This redox behavior is best explained by the formation of an aromatic (4n + 2) π-electron dianion from 2 , whereas 5 loses its aromaticity upon reduction. The Me₃Si derivative 4 was deprotected with borax in MeOH/THF to give the highly unstable hexaethynyl-dodecadehydro[18]annulene 6 , a C₃₀H₆ isomer and macrocyclic precursor to a two-dimensional all-C-network. Deprotection of 2 did not give isolable amounts of tetraethynyl-octadehydro[12]annulene 3 due to the extreme instability of the latter. Starting from dimeric and trimeric acyclic tetraethynylethene oligomers, a series of expanded radialenes were obtained. They possess large C-cores with silylethynyl-protected peripheral valences and can be viewed as persilylated C₄₀ ( 7 ), C₅₀ ( 8 ), and C₆₀ ( 9 ) isomers. These expanded C-sheets are high-melting, highly stable, soluble materials which were readily characterized by laser-desorption time-of-flight (LD-TOF) mass spectrometry. Due to inefficient macrocyclic cross-conjugation and/or non-planarity, the extent of π-electron delocalization in 7 – 9 is limited to the longest linearly conjugated π-electron fragment. In agreement with these properties, all three expanded radialenes exhibited similar redox behavior; they are difficult to oxidize but undergo several reversible one-electron reductions in similar potential ranges. Presumably, the reduced π-electron delocalization is also at the origin of the particularly high stability of 7 – 9 .     

A naphtho-tri-t-butyltrisdehydro[16]annulene

Synthesis of a naphtho-tri-t-butyltrisdehydro[16]annulene has been described. Suppression of paratropicity of the 16-membered ring was observed. Comparison of ¹H NMR spectrum of the [16]annulene with that of naphtho-tri-t-butylbisdehydro[14]annulene suggests that the trans double bond adjacent to naphthalene in the [16]annulene is twisted out of the mean molecular plane.     

Investigation on stabilities and spectroscopy of C80O2 based on C80 (D5d) using density functional theory

The relative stabilities of the 17 possible isomers for C₈₀O₂ based on C₈₀ (D_5d) were studied using Becke three parameters plus Lee, Yang, and Parr's (B3LYP) method and 6‐31G (d) basis set in density functional theory. The most stable geometry of C₈₀O₂ was predicted to be 23,24,27,28‐C₈₀O₂ (A) with annulene‐like structures, where the additive bonds are those between two hexagons (6/6 bonds) near the equatorial belt of C₈₀ (D_5d). Electronic spectra of C₈₀O₂ isomers were calculated based on the optimized geometries using intermediate neglect of differential overlap (INDO) calculation. Compared with those of C₈₀ (D_5d), the first absorptions in the electronic spectra of C₈₀O₂ are blue‐shifted owing to the wide energy gaps. ¹³C nuclear magnetic resonance spectra and nucleus independent chemical shifts of the C₈₀O₂ isomers were computed at B3LYP/6‐31G level. The chemical shifts of the bridged carbon atoms in the epoxy structures of C₈₀O₂ compared with those of the bridged carbon atoms in the annulene‐like structures are changed upfield. Generally, the isomers with the annulene‐like structures of C₈₀O₂ are more aromatic than those with the epoxy structures. The addition of the oxygen atoms near the pole of C₈₀ (D_5d) is favorable to improving the aromaticities of C₈₀O₂. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

7,8‐(Dichloromethano)homo[70]fullerene,an Unusual,but Unexceptional,Methanoannulene

The crystal structure of 7,8‐(dichloromethano)homo[70]fullerene shows that addition of the dichlorocarbene (:CCl₂) to C₇₀ gives a surprisingly normal methano‐bridged [9]annulene. The structural changes caused by the addition are highly localized; the C(7)? C(8) distance lengthens by 0.705(2) Å but no other distance changes by more than 0.022(3) Å. Bond angles within the [9]annulene increase by as much as 7.8(1)°, but changes outside the C₉ ring are much smaller. The nearly ellipsoidal molecules form hexagonally close‐packed layers that interact in pairs through short Cl???C, and possibly short Cl???Cl and C???C, interactions.     

Reactions of (η5-C5H5)2Tir compounds with organic cyanides

The syntheses and properties of the titanium(III) complexes Cp₂Tir · R′CN (R = C₆H₅, o-, m-, p-CH₃C₆H₄, CH₂C₆H₅, C₆F₅, Cl; R′ = CH₃, t-C₄H₉, C₆H₅, o-CH₃C₆H₄, 2,6-(CH₃)₂C₆H₃) are described. In the complexes the nitrogen atom of the cyanide ligands is coordinated to the metal. The thermal stabilities of the complexes depend markedly on R and R′; on heating they undergo a novel reaction in which two cyanide ligands are coupled by formation of a CC bond, while the metal is oxidized to titanium(IV).     

7,10,14-Tri-t-butyl-3-amino-1,8-bisdehydro[14]annulene

Tri-t-butylcarboxybisdehydro[14]annulene was converted by the Curtius reaction into unstable aminoannulene, which could be characterized as N-acetyl derivative. The pK_a-value of the aminoannulene reflects aromatic nature of the annulene nucleus.     

Isolierung und struktur des dreikernigen hexamethylbenzolzirkonium-komplexes [(Me6C6)3Zr3Cl6]2+ [(Al2Cl7)−]2

Crystals of the trinuclear complex [(Me₆C₆)₃Zr₃Cl₆][Al₂Cl₇]₂ have been obtained from the reaction of ZrCl₄, hexamethylbenzene, AlCl₃, and Al in benzene. They are monoclinic, space group C2/2, with Z  4 and lattice parameters a 14.167(3), b 27.779(7), c 15.721(3) Å and β 94.27(4)°. The Zr atoms form a regular triangle. Each pair of Zr atoms is bridged by two Cl atoms. The fifth coordination site of each Zr atom is occupied by a h⁶-Me₆C₆ group. The cation is almost isostructural with the known trinuclear cation [(Me₆C₆)₃Nb₃Cl₆]²⁺. Important distances are: ZrZr 3.35, ZrCl 2.56, and Zrcenter of C₆ ring 2.17 Å. One of the two independent [Al₂Cl₇]^? anions occurs in a staggered conformation and one occurs in an eclipsed conformation.     

Synthesis and molecular structures of heptafluoropropylated fullerenes: C70(n-C3F7)8, C70(n-C3F7)6O, and C70(C3F7)4

Ampoule reactions of C₇₀ with n- and i-C₃F₇I were carried out at 250-310 °C. Two step HPLC separations allowed the isolation of several C₇₀(n-C₃F₇)_4-8 and C₇₀(i-C₃F₇)₄ compounds. Crystal and molecular structures of C₇₀(n-C₃F₇)₈-V, C₇₀(n-C₃F₇)₆O, C₇₀(n-C₃F₇)₄, and three isomers of C₇₀(i-C₃F₇)₄ have been determined by X-ray crystallography using synchrotron radiation. Molecular structures of the new compounds were compared with the known examples and discussed in terms of addition patterns and relative energies of their formation.     

Gas phase electron diffraction study of basketene,C10H10

A gas phase electron diffraction study of the cage hydrocarbon, basketene, is reported. A least squares treatment of molecular intensities has been carried out in terms of a geometrically consistent r_α structure. The mean amplitude values and shrinkage corrections have been calculated using the force field parameters estimated from the data on simpler molecules.Structure refinement of the C_2v molecular model yields the following parameter values (bond lengths, r_a, in nm; angles, r_α in degrees): <C₂—C_3, C₄—C₅?_av 0.1609(14); C₃—C₄ 0.1563(6); C₉C₁₀ 0.1360(9); C₁—C₁₀ 0.1511(13); C₁—C₂ 0.1517(9); <C-H>_av. 0.1092(8); <C₃C₄C₇ 88.5(1.0); dihedral angle C₃C₄C₇/C₃C₅C₇ 153.8(1.0). Parenthesized are three times the standard deviation values, 3σ.In addition to the geometric parameters listed, the mean amplitudes for all bonded and C· C nonbonded distances have been determined by the least squares method. All the other amplitudes (C· H and H· H) have been fixed at the values estimated from the spectral data.Comparison of the results obtained with the literature data on similar polycyclic molecules points to the stronger internal strain in the basketene molecule.     

The extended chain compounds Ln12(C2)3I17 (Ln=Pr, Nd, Gd, Dy): Synthesis, structure and physical properties

The title compounds are obtained in high yield from stoichiometric mixtures of Ln, LnI₃ and graphite, heated at 900-950 °C in welded Ta containers. The crystal structures of new Pr and Nd phases determined by single-crystal X-ray diffraction are related to those of other Ln₁₂(C₂)₃I₁₇-type compounds (C 2/c, a=19.610(1) and 19.574(4) Å, b=12.406(2) and 12.393(3) Å, c=19.062(5) and 19.003(5) Å, β=90.45(3)° and 90.41(3)°, for Pr₁₂(C₂)₃I₁₇ and Nd₁₂(C₂)₃I₁₇, respectively). All compounds contain infinite zigzag chains of C₂-centered metal atom octahedra condensed by edge-sharing into the [tcc]_∞ sequence (c=cis, t=trans) and surrounded by edge-bridging iodine atoms as well as by apical iodine atoms that bridge between chains. The polycrystalline Gd₁₂(C₂)₃I₁₇ sample exhibits semiconducting thermal behavior which is consistent with an ionic formulation (Ln³⁺)₁₂(C₂^6-)₃(I⁻)₁₇(e⁻) under the assumption that one extra electron is localized in metal-metal bonding. The magnetization measurements on Nd₁₂(C₂)₃I₁₇, Gd₁₂(C₂)₃I₁₇ and Dy₁₂(C₂)₃I₁₇ indicate the coexistence of competing magnetic interactions leading to spin freezing at T_f=5 K for the Gd phase. The Nd and Dy compounds order antiferromagnetically at T_N=25 and 29 K, respectively. For Dy₁₂(C₂)₃I₁₇, a metamagnetic transition is observed at a critical magnetic field H≈25 kOe.     

Syntheses, crystal structures and Si solubilities of new layered carbides Zr2Al4C5 and Zr3Al4C6

Two types of new ternary carbides, Zr₂Al₄C₅ and Zr₃Al₄C₆, have been synthesized and characterized by X-ray powder diffraction. The crystal structures were refined from laboratory X-ray powder diffraction data (CuKα₁) using the Rietveld method. These carbides form a homologous series with the general formula (ZrC)_mAl₄C₃ (m=2 and 3). The crystal structures can be regarded as intergrowth structures where the Al₄C₃-type [Al₄C₄] layers are the same, while the NaCl-type [Zr_mC_m+1] layers increase in thickness with increasing m value. The new carbides are most probably the end members of continuous solid-solutions (ZrC)_m[Al_4−xSi_x]C₃ with 0?x?0.44.     

Die struktur des dreikernigen hexamethylbenzolniob-komplexes [(Me6C6)3Nb3Cl6]+ Cl- · 3 CHCl3

Well developed crystals of [(Me₆C₆)₃Nb₃Cl₆]⁺ Cl^? · 3 CHCl₃ can be obtained from a solution of [(Me₆C₆)₃Nb₃Cl₆] Cl in CHCl₃ (monoclinic, P2₁/c, a 11.850(3), b 15.906(6), c 28.529(8) Å, β 98.14(3)°, Z  4). An X-ray structure determination shows the structure of the complex cation to be highly symmetric (non-crystallographic D_3h symmetry) and to agree within narrow limits with the known structure of the corresponding 2+ cation. Important distances are: NbNb 3.347(4) and NbCl 2.504(2) Å. The C₆ rings of the hexamethylbenzene rings are not planar. The average folding angle of the C₆ groups is 156.6°. In the crystal the Cl^? anion is bonded by weak H-bridges to three CHCl₃ molecules.     

The preparation of cis- and trans-[PtH(C6Cl5)(PEt3)2 and a study of the “hydride-donor capacity” of the complexes trans- [PtH(C6X5(PEt3)2] (X = F and Cl)

The preparations of cis- and trans-[PtH(C₆Cl₅)(PEt₃)₂] by thermal decomposition of cis- and trans-[Pt(OCHO)(C₆Cl₅)(PEt₃)₂], respectively, are reported. Also described are cis- and trans-[Pt(SnCl₃)(C₆Cl₅)(PEt₃)₂], obtained by treating SnCl₂ with cis- and trans-[PtCl(C₆,Cl₅)(PEt₃)₂], respectively. It is shown that while trans- [PtH(C₆Cl₅)(PEt₃)₂] does not form hydride-bridged complexes in the presence of trans-(PtH(MeOH)(PEt₃)₂]⁺, the corresponding complex trans-[PtH(C₆)(PEt₃)₂] reacts with the same solvento complex, in methanol, giving labile [(PEt₃)₂HPt(-μH)Pt(C₆F₅)(PEt₃)₂]⁺.     

Synthesis and molecular structures of heptafluoroisopropylated fullerenes: C60(i-C3F7)8, C60(i-C3F7)6, and C60(CF3)2(i-C3F7)2

One isomer of C₆₀(i-C₃F₇)₈, three isomers of C₆₀(i-C₃F₇)₆, and the first mixed perfluoroalkylated fullerene, C₆₀(CF₃)₂(i-C₃F₇)₂, have been isolated by HPLC from a mixture prepared by reaction of C₆₀ with heptafluoroisopropyl iodide in a glass ampoule at 260-290 °C. The molecular structures of the four new compounds have been determined by means of X-ray single crystal diffraction partially also by use of synchrotron radiation. Theoretical calculations at the DFT level of theory have been employed to rationalize the energetics of isomers and of C₆₀-R_f binding.     

Polynuclear sandwich derivatives of [10]annulene: A quantum chemical study

Density-functional theory (DFT) calculations at B3LYP/6-311+G(df,p) and M06/6-311+G(df,p) levels of theory predict the stability of new polynuclear sandwich complexes [Cu₅(C₁₀H₁₀)₂]⁺ and Ni₅(C₁₀H₁₀)₂ based on flat [10]annulene cycles.     

Addition of lithium ethyl fluoroacetate to cis and trans α,β-epoxyaldehydes. Access to C2 fluorinated butyrolactones

The aldolisation reaction of lithium ethyl fluoroacetate with cis and trans α,β-epoxyaldehydes in their racemic forms proceeds with good C₃-OH diastereoselectivity and much less at the C₂-F carbon atom. A two-step reaction on the major aldol compounds (iodination, lactonisation) led to racemic functionalised C₂ fluorinated lactones, possessing a C₂/C₃cis relationship between the fluorine and hydroxyl groups.     

Crystal and molecular structure of [{Rh(C5Me5)}3Cl5np3]PF6 · 0.5 C3H8O

The crystal and molecular structure of the monoligand trimetallic complex [{Rh(C₅Me₅)}₃Cl₅np₃]PF₆ · 0.5 C₃H₈O (np₃  tris(2-diphenylphosphinoethyl)-amine) have been established by a single-crystal X-ray diffraction study. The cation of the complex contains two Rh(C₅Me₅)Cl₂ units each bound through the metal to one phosphorus atom of the ligand and a Rh(C₅Me₅)Cl group in which the rhodium is bound to the third phosphorus atom and to the nitrogen of the tetradentate ligand.The crystals are triclinic, space group P$\text{1}$, with cell dimensions a 28.598(8), b 13.757(4), c 10.748(3) Å, α 90.69(4), β 96.67(4), γ 99.71(4)°, D_c 1.38 g cm^?3 for Z  2. The structure was solved by three dimensional Patterson and Fourier syntheses and refined by least-squares techniques to a final conventional R value of 0.098.     

The unusual reactivity of C3F7OCFCF2 with PBu3 and the complex hydrides M[EH4] (M: Li, Na; E: B, Al); preparation of potassium perfluoro-2-propoxyeth-1-enyltrifluoroborate K[C3F7OCFCFBF3]

The nucleophilic hydrodefluorination of C₃F₇OCFCF₂ with the complex hydrides Li[AlH₄], Li[BH₄] or Na[BH₄] proceeded non-stereoselectively and was accompanied by the formation of either cis- and trans-C₃F₇OCHCFH and/or C₃F₇OCHFCF₂H. The reaction of C₃F₇OCFCF₂ with PBu₃ followed by treatment with BF₃·OMe₂ or BF₃·OEt₂ yielded [C₃F₇OCFCFPBu₃] [BF₄] (cis and trans) and, probably, [trans-Bu₃PCFCFPBu₃] [BF₄]₂. The hydrolysis of the latter with pure water proceeded quickly while the former isomeric mixture formed the isomeric olefins C₃F₇OCFCFH slowly. The usage of aqueous NaOH instead of water produced mainly trans-CHFCHF. The metallation of C₃F₇OCFCFH (cis:trans=45:55) to C₃F₇OCFCFLi and its subsequent reaction with B(OMe)₃ and K[HF₂] gave the salt K[C₃F₇OCFCFBF₃] in a different cis to trans ratio (25:75) with satisfactory yield.     

Nickel-catalyzed coupling of bromides of 1,6-methano[10]annulene and azulene. A facile synthesis of biannulene and bi-, ter-, quater-, and polyazulenes

The active nickel complex generated in situ by reduction of NiBr₂(PPh₃)₂ with zinc in the presence of Et₄NI is a useful reagent for the dehalogenative coupling of 2-bromo-1,6- methano[10]annulene and 1-bromo- and 1,3-dibromoazulenes.