Preparation of high molecular weight polyurethane particles by nonaqueous emulsion polyaddition

A versatile nonaqueous emulsion polyaddition process for the one-step fabrication of spherical polyurethane nanoparticles is presented. Three different emulsion systems were used consisting of N,N′-dimethylformamide (DMF) dispersed in n-hexane, acetonitrile dispersed in cyclohexane, and acetonitrile dispersed in tetradecane. After successful stabilization of the emulsion systems by using a poly(isoprene)-poly(methylmethacrylate) block copolymer, the fabrication of the polyurethanes was carried out within the dispersed polar phase. The polyurethane particles showed average diameters as small as 35 nm. Additionally, infrared (IR) characterization revealed that the formation of any urea, which decreases the mechanical properties of the polyurethanes, was prevented during the polyaddition. This was attributed to the anhydrous reaction conditions. Gel permeation chromatography (GPC) analysis demonstrated the average molecular weights (M _n) of the polyurethanes to be as high as 16,500 g/mol, corresponding to conversions of 0.98. Comparable molecular weights and conversions have not previously been achieved without the formation of urea.     

1-Lithio-2,2-diphenyl-1-(phenylsulfonyl)ethen: Synthese und Kristallstruktur

Syntheses and Crystal Structure of 1-Lithio-2,2-diphenyl-1-(phenylsulfonyl)ethene Crystals of [1-lithio-2,2-diphenyl-1-(phenylsulfonyl)ethene]– N,N,N′,N′ -tetramethylethylenediamine (2/2) ( 2 ) were prepared by addition of BuLi to 1,1-diphenyl-2-(phenylsulfonyl)ethene ( 1 ) in the presence of N,N,N′,N′ -tetramethylethylenediamine (TMEDA) at low temperature. The X-ray structure analysis shows a centrosymmetric dimer bridged over an eight-membered (Li? O? S? O)₂ ring. There are no Li–C contacts to the C(α) atoms. Both Li cations are tetracoordinated via the sulfonyl O-atoms and the N-atoms of the TMEDA ligand. The X-ray structure analysis of 1,1-diphenyl-2-(phenylsulfonyl)ethene ( 1 ) also was determined to compare interatomic distances and angles.     

Photoreduction of Triplet Benzophenone by Tertiary Amines: Amine molecular structure and ketyl radical yield

The photoreduction of triplet benzophenone by 14 tertiary amines was investigated. The ketyl radical yields do not correlate with the quenching rate constants, nor with the electron donor propensity of the amines. Individual structural features of the amines seem to determine the photoreduction yields.     

The Uracil C(5) Position as a Metal Binding Site: Solution and X-ray Crystal Structure Studies of Pt(II) and Hg(II) Compounds

1,3-Dimethyluracil (1,3-DimeU) reacts with trans-[(CH(3)NH(2))(2)Pt(H(2)O)(2)](+) to give trans-[(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)(H(2)O)]X (X = NO(3)(-), 1a, ClO(4)(-), 1b) and subsequently with NaCl to give trans-(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)Cl (2) or with NH(3) to yield trans-[(CH(3)NH(2))(2)Pt(1,3-DimeU-C5)(NH(3))]ClO(4) (3). In a similar way, (dien)Pt(II) forms [dienPt(1,3-DimeU-C5)](+) (4). Reactions leading to formation of 1 and 4 are slow, taking days. In contrast, Hg(CH(3)COO)(2) reacts fast with 1,3-DimeU to give (1,3-DimeU-C5)Hg(CH(3)COO) (5). Both 1-methyluracil (1-MeUH) and uridine (urdH) react with (dien)Pt(II) initially at N(3) and subsequently with either (dien)Pt(II) or Hg(CH(3)COO)(2) also at C(5) to give the diplatinated species 7 and 9 or the mixed PtHg complex 8. C(5) binding of either Pt(II) or Hg(II) is evident from coupling of uracil-H(6) with either (195)Pt or (199)Hg nuclei and (3)J values of 47-74 Hz (for Pt compounds) and 185-197 Hz (for Hg compounds). J values of Pt compounds are influenced both by the ligands trans to the uracil C(5) position and by the number of metal entities bound to a uracil ring. Both 2 and 5 were X-ray structurally characterized. 2: monoclinic system, space group P2(1)/c, a = 15.736(6) ?, b = 11.481(6) ?, c = 25.655 (10) ?, beta = 145.55(3) degrees, V = 2621.9(28) ?(3), Z = 4. 5: monoclinic system, space group P2(1)/c, a = 4.905(2) ?, b = 18.451(6) ?, c = 11.801(5) ?, beta = 94.47(3) degrees, V = 1064.77(72) ?(3), Z = 4.     

Photochemical reactions. 152nd Communication. Photochemistry of acylsilanes; photolysis and thermolysis of cyclopropyl silyl ketones

The photolysis and thermolysis of the Cyclopropyl silyl ketones 3, 4 , and 5 are described. On n,π* excitation, the silyl ketones 3 and 4 undergo a Norrish-type-II reaction involving γ-H abstraction, cyclopropyl ring cleavage followed by retro-enolization to the acylsilanes 6 and (E/Z)- 12 , respectively. As a common product of 3 and 4 , the dihydrofuran 7 is formed via the alternative C(α)-C(β) cleavage of the cyclopropyl moiety. Compounds 6 , 7 , and (E/Z)- 12 are new types of acylsilane photoproducts. The irradiation of acylsilane 5 gave the analogous dihydrofuran 15 as the only product. On photolysis of 3 and 4 , products 8A + B and 13A + B , derived from a siloxy carbene intermediate, were found as well. On thermolysis of 3 and 4 , the acylsilanes 6 (80%), and (E)- 12 (33%) and (Z)- 12 (34%), respectively, are formed as the only products. Their formation may occur via a [1, 5] sigmatropic H-shift. The thermolysis of 5 gave the diene 16 whose formation can be explained by insertion of a siloxycarbene into the neighboring cyclopropane leading to the cyclobutene 28 as thermally unstable intermediate.     

Bis (2,4,6,8-cyclononatetraen-1-yl)methane

Bis (2,4,6,8-cyclononatetraen-1-yl)methanes Bis (2,4,6,8-cyclononatetraen-1-yl)methanes ( 2a–c ) have been prepared by reaction of all-cis-cyclononatetraenide with 1,1-dichlorodimethyl ether as well as with carbenium ion precursors 9b and 9c . The title compounds 2 are attractive precursors of highly delocalised nonafulvenes of type 3 ; however, elimination experiments 2→3 failed so far.     

Nitrogen fixation under mild ambient conditions: part I--the initial dissociation/association step at molybdenum triamidoamine complexes

In several recent studies Schrock and collaborators demonstrated for the first time how molecular dinitrogen can be catalytically transformed under mild and ambient conditions to ammonia by a molybdenum triamidoamine complex. In this work, we investigate the geometrical and electronic structures involved in this process of dinitrogen activation with quantum chemical methods. Density functional theory (DFT) has been employed to calculate the coordination energies of ammonia and dinitrogen relevant for the dissociation/association step in which ammonia is substituted by dinitrogen. In the DFT calculations the triamidoamine chelate ligand has been modeled by a systematic hierarchy of increasingly complex substituents at the amide nitrogen atoms. The most complex ligand considered is an experimentally known ligand with an HMT = 3,5-(2,4,6-Me3C6H2)2C6H3 substituent. Several assumptions by Schrock and collaborators on key reaction steps are confirmed by our calculations. Additional information is provided on many species not yet observed experimentally. Particular attention is paid to the role of the charge of the complexes. The investigation demonstrates that dinitrogen coordination is enhanced for the negatively charged metal fragment, that is, coordination is more favorable for the anionic metal fragment than for the neutral species. Coordination of N2 is least favorable for the cationic metal fragment. Furthermore, ammonia abstraction from the cationic complex is energetically unfavorable, while NH3 abstraction is less difficult from the neutral and easily feasible from the anionic low-spin complex.     

Sublimation enthalpy of dye molecules measured using fluorescence

We present an in-flight fluorescence detection scheme for molecular beams which is applied to determine the enthalpy of sublimation of dye molecules. We investigate tetraphenylporphyrin (TPP), porphine, and nile red, which are believed to be suitable candidates for molecular de Broglie wave interferometry. The measured values are H(sub)(TPP)=142+/-3 kJ/mol, H(sub)(porphine)=87+/-3 kJ/mol, and H(sub)(nile red)=66+/-2 kJ/mol. For TPP, sublimation enthalpies differ in the literature by more than a factor of 2. Our measurements confirm a value at the lower end of this scale. We discuss changes in the character of the molecular flow with the source temperature as a prime reason for discrepancies in the published data.     

Synthesis and Characterization of Three Homoleptic Bismuth Silanolates: [Bi(OSiR3)3] (R = Me,Et, iPr)

The bismuth tris(triorganosilanolates) [Bi(OSiR₃)₃] ( 1 , R = Me; 2 , R = Et; 3 , R = iPr) were prepared by reaction of R₃SiOH with [Bi(OtBu)₃]. Compound 1 crystallizes in the triclinic space group with Z = 2 and the lattice constants a = 10.323(1) Å, b = 13.805(1) Å, c = 21.096(1) Å and α = 91.871(4)°, β = 94.639(3)°, γ = 110.802(3)°. In the solid state compound 1 is a trimer as result of weak intermolecular bismuth‐oxygen interactions with Bi–O distances in the range 2.686(6)–3.227(3) Å. The coordination at the bismuth atoms Bi(1) and Bi(3) is best described as 3 + 2 coordination whereas Bi(2) shows a 3 + 3 coordination. The intramolecular Bi–O distances fall in the range 2.041(3)–2.119(3) Å. Compound 3 crystallizes in the orthorhombic space group Pbcm with Z = 4 and the lattice constants a = 7.201(1) Å, b = 23.367(5) Å and c = 20.893(1) Å, whereas the triethylsilyl‐derivative 2 is liquid. In contrast to [Bi(OSiMe₃)₃] ( 1 ) compound 3 is monomeric in the solid state, but shows similar intramolecular Bi–O distances in the range 1.998(2)–2.065(5) Å. The bismuth silanolates are highly soluble in common organic solvents and strongly moisture sensitive. Compound 1 shows the lowest thermal stability.