Functionalized fullerenes in self-assembled monolayers

Anisotropy of intermolecular and molecule-substrate interactions holds the key to controlling the arrangement of fullerenes into 2D self-assembled monolayers (SAMs). The chemical reactivity of fullerenes allows functionalization of the carbon cages with sulfur-containing groups, thiols and thioethers, which facilitates the reliable adsorption of these molecules on gold substrates. A series of structurally related molecules, eight of which are new fullerene compounds, allows systematic investigation of the structural and functional parameters defining the geometry of fullerene SAMs. Scanning tunnelling microscopy (STM) measurements reveal that the chemical nature of the anchoring group appears to be crucial for the long-range order in fullerenes: the assembly of thiol-functionalized fullerenes is governed by strong molecule-surface interactions, which prohibit formation of ordered molecular arrays, while thioether-functionalized fullerenes, which have a weaker interaction with the surface than the thiols, form a variety of ordered 2D molecular arrays owing to noncovalent intermolecular interactions. A linear row of fullerene molecules is a recurring structural feature of the ordered SAMs, but the relative alignment and the spacing between the fullerene rows is strongly dependent on the size and shape of the spacer group linking the fullerene cage and the anchoring group. Careful control of the chemical functionality on the carbon cages enables positioning of fullerenes into at least four different packing arrangements, none of which have been observed before. Our new strategy for the controlled arrangement of fullerenes on surfaces at the molecular level will advance the development of practical applications for these nanomaterials.     

Interactions of hafnocene and zirconocene dihydrides with acetylenes. Synthesis of the first acetylene complex of hafnium

The first acetylene complex of hafnium, Cp₂Hf[Me₃SiC=CHf(H)Cp₂], was synthesized by the reaction of hafnocene dihydride Cp₂HfH₂ with bis(trimethylsilyl)acetylene in benzene. The reaction is accompanied by elimination of the Me₃Si group from the molecule of the initial acetylene, as a result of which the acetylenide derivative of hafnium Cp₂Hf(C=CSiMe₃)(H) acts as an acetylene ligand in the complex. Under analogous conditions, the reaction of zirconocene dihydride Cp₂ZrH₂ with bis(trimethylsilyl)acetylene affords an analogous acetylene complex of zirconium Cp₂Zr(M₃SiC=CZr(H)Cp₂]. Reactions of Cp₂HfH₂ with tolane and 3-hexyne proceed differently than the reaction with bis(trimethylsilyl)acetylene. Here the corresponding hafnacyclopentadiene metallacycles are the final products. For preliminary communication, see Ref. 3. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 853–856, April, 1997.     

A rigid body on a surface with random roughness

Consider an interval on a horizontal line with random roughness. With probability one it is supported at two points: one on the left, and another on the right from its center. We compute the probability distribution of the support points provided the roughness is fine grained. We also solve an analogous problem where a circle or a disk lies on a rough plane. Some applications in static are given.     

Electrophilic reactivity of the zwitterionic titanocene monohalides Cp[η-C5H4B(C6F5)3]TiX (X = Cl, Br): Reactions with water and methanol

The paper reports new data evidencing for a high electrophilicity of the positively charged titanium atom in the previously described zwitterionic titanocene monochloride Cp[η⁵-C₅H₄B(C₆F₅)₃]TiCl (1) and titanocene monobromide Cp[η⁵-C₅H₄B(C₆F₅)₃]TiBr (2), containing a B(C₆F₅)₃ group in one of the C₅ rings. It has been established that on a contact of a toluene solution of these zwitterions with water vapour at 20 °C under Ar, a rapid protolytic cleavage of the otherwise inert B-C₆F₅ bond in the tris(pentafluorophenyl)borane moiety occurs to afford pentafluorobenzene and the corresponding halogenide hydroxide complex of titanocene Cp[η⁵-C₅H₄B(C₆F₅)₂]TiX(μ-OH), where X = Cl (3), Br (4). An X-ray diffraction study of the complexes has shown that the hydroxide group in 3 and 4 is bonded via the oxygen atom both to the titanium and boron atoms. Under similar conditions, the interaction of zwitterion 1 with methanol gives rise to pentafluorobenzene and the chloride methoxide complex of titanocene Cp[η⁵-C₅H₄B(C₆F₅)₂]TiCl(μ-OCH₃). It has been suggested that the driving force of the protolysis of the B-C₆F₅ bond in 1 and 2 is a sharp increase in the acidity of water or methanol molecule as a result of their complexation with the positively charged titanium centre in the starting zwitterion.     

The Influence of the Ligands Cp*(η5‐C5Me5) and Cp(η5‐C5H5) on the Stability and Reactivity of Titanocene and Zirconocene Complexes: Reactions of the Bis(trimethylsilyl)acetylene Permethylmetallocene Complexes (η5‐C5Me5)2M(η2‐Me3SiC2SiMe3), M = Ti,Zr, with H2O and CO2

The reactions of the bis(trimethylsilyl)acetylene permethylmetallocene complexes CpM(η²‐Me₃SiC₂SiMe₃) (M = Ti ( 1 ), M = Zr ( 2 )) with H₂O and CO₂ were studied and compared to those of the corresponding metallocene complexes Cp₂M(L)(η²‐Me₃SiC₂SiMe₃) (M = Ti ( 3 ), L = – ; M = Zr, L = THF ( 4 )) to understand the influence of the ligands Cp(η⁵‐C₅H₅) and Cp*(η⁵‐C₅Me₅) as well as the metals titanium and zirconium on the reaction pathways and the obtained products. In the reaction of the permethyltitanocene complex 1 with water the dihydroxy complex CpTi(OH)₂ ( 5 ) was formed. This product differs from the well‐known titanoxane Cp₂TiOTiCp₂ which was obtained by the reaction of the corresponding titanocene complex 3 with water. The reaction of the permethylzirconocene complex 2 with water gives the mononuclear alkenyl zirconocene hydroxide 6 . An analogous product was assumed as the first step in the reaction of the corresponding zirconocene complex 4 with water which ends up in a dinuclear zirconoxane. In the conversion of the permethylzirconocene complex 2 with carbon dioxide the mononuclear insertion product 7 was formed by coupling of carbon dioxide and the acetylene. In contrast, the corresponding zirconocene complex 4 affords, by an analogous reaction, a dinuclear complex. In additional experiments the known complex CpZr(η²‐PhC₂SiMe₃) ( 8 ) was prepared, starting from CpZrCl₂ and Mg in the presence of PhC≡CSiMe₃. This complex reacts with carbon dioxide resulting in a mixture of the regioisomeric zirconafuranones 9 a and 9 b . From these in the complex 9 a , having the SiMe₃ group in β‐position to the metal, the Zr–C bond was quickly hydrolyzed by water to give the complex CpZr(OH)OC(=O)–C(SiMe₃)=CHPh ( 10 a ) compared to complex ( 9 b ) which gives slowly the complex CpZr(OH)OC(=O)–CPh=CH(SiMe₃) ( 10 b ).     

Intramolecular interactions and nature of the lowest electronically excited states in compounds modeling the structural unit of lignin. II. Phenolate anions

Quantum-chemical calculations of the electronic structures of the phenolate ions of compounds modeling lignin in the ground and electronically excited states have been made by the CNDO/S method. The intramolecular electron donor-acceptor interactions in the phenolate anion on excitation and the nature of the lowest electronically excited states are discussed on the basis of the results obtained.Siberian Scientific-Research Institute of Pulp and Board, Bratsk. A. A. Zhdanov Irkutsk State University. Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 275–282, March–April, 1988.