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Jens Geier

Publications and source records attributed to Jens Geier.

12 recordsLinked to original sources
PubMed2006

Cesium hydridotris(trifluoromethyl)borate, Cs[(CF3)3BH].

Treatment of Cs[(CF3)3BNH2] with the aminating agent H2NOSO3H in aqueous solution allowed the isolation of pure Cs[(CF3)3BH], which is stable up to 300 degrees C. Due to the strong electron-withdrawing effect of the CF3 substituents, the [(CF3)3BH]- anion behaves as a very unreactive hydride. It is stable in concentrated hydrochloric acid for many days but reacts cleanly with F2, Cl2, and Br2 to the corresponding haloborates. The molecular structure was determined by single-crystal X-ray diffraction. Crystal data: orthorhombic, space group Pnma; a = 11.4296(5) A, b = 7.9510(4) A, c = 9.7268(5) A; V = 883.94(7) A(3), Z = 4; R1 = 0.0294, wR2 = 0.0818. The anions exhibit only Cs symmetry in the lattice. The natural and deuterated anions were characterized by IR, Raman, and multinuclear NMR spectroscopy; vibrational assignments were supported by DFT calculations. QTAIM charges derived from the B3LYP electron density are given for [(CF3)3BH]- and several related anions.

Journal Article↗
PubMed2006

Spatial shape of electron delocalization: structure of the laplacian of the negative exchange-correlation density.

The Laplacian of the negative exchange-correlation density (with respect to coordinate r(2)), nabla<(r)2>(2)[-Gamma(sigma1)(sigma2)(XC) (r(1),r(2))] = nabla(r)2(2)X(sigma1)(sigma2)(r(1),r(2)), is proposed as an instrument for the analysis of electron delocalization in real space. It determines local concentrations in the amount of electrons that are delocalized from a reference point r(1) over space. Integration of the reference coordinate r(1) over an atomic basin Omega(n) gives the function nabla(2)X(sigma1)(sigma2)(Omega(n);r), which contains detailed information about the spatial shape of the delocalization that originates from an atom in a molecule. Its isosurface representations are richly structured and resemble molecular orbitals in their complexity and partly also in their shape. The sum over all nabla(2)X(sigma1)(sigma2)(Omega(n);r) functions of a molecule equals the Laplacian of the electron density nabla(2)rho(r), for which it provides a meaningful partitioning into atomic contributions.

Journal Article↗
PubMed2005

Sodium compounds of the benzophenone dianion (diphenyloxidomethanide).

The benzophenone dianion [diphenyloxidomethanide, (Ph(2)CO)(2-)], which occurs in the well known deeply violet sodium/benzophenone tetrahydrofuran solutions, was crystallised with sodium cations in form of the two polymeric chain compounds [Na(2)(Ph(2)CO)(tetraglyme)](infinity) and [Na(2)(Ph(2)CO)(thf)(2)](infinity). It was found to aggregate with its conjugated acid, the alcoholate (Ph(2)CHO)(-), around a central unit of sodium hydroxide, resulting in the mixed cage compound [Na(13)(Ph(2)CO)(4)(Ph(2)CHO)(4)(OH)(mtbe)(4)].mtbe. The structural parameters of the benzophenone dianion indicate that a considerable amount of its negative charge is located within the phenyl rings, rather than on the formally anionic benzylic carbon atom. The topological analysis of the electron density of the monomeric model structure [Na(2)(Ph(2)CO)] reveals an even positive charge for this particular atom, hence (Ph(2)CO)(2-) is, despite its usual representation, not a vicinal dianion.

Journal Article↗
PubMed2005

A stable aminyl radical metal complex.

Metal-stabilized phenoxyl radicals appear to be important intermediates in a variety of enzymatic oxidations. We report that transition metal coordination also supports an aminyl radical, resulting in a stable crystalline complex: [Rh(I)(trop2N.)(bipy)]+OTf- (where trop is 5-H-dibenzo[a,d]cycloheptene-5-yl, bipy is 2,2'-bipyridyl, OTf- is trifluorosulfonate). It is accessible under mild conditions by one-electron oxidation of the amide complex [Rh(I)(trop2N)(bipy)], at a potential of -0.55 volt versus ferrocene/ferrocenium. Both electron paramagnetic resonance spectroscopy and density functional theory support 57% localization of the unpaired spin at N. In reactions with H-atom donors, the Rh-coordinated aminyl behaves as a nucleophilic radical.

Journal Article↗
PubMed2004

Alkylating agents stronger than alkyl triflates.

A new class of potent electrophilic "R(+)" alkylating agents has been developed using weakly nucleophilic carborane anions as leaving groups. These reagents, R(CHB(11)Me(5)X(6)) (R = Me, Et, and i-Pr; X = Cl, Br), are prepared via metathesis reactions with conventional alkylating agents such as alkyl triflates, using the high oxophilicity of silylium ion-like species, Et(3)Si(carborane), as the driving force to obtain increased alkyl electrophilicity. The crystal structure of the isopropyl reagent, i-Pr(CHB(11)Me(5)Br(6)), has been determined, revealing covalence in the alkyl-carborane bonding. This contrasts with the free i-Pr(+) carbocation observed when the anion is less coordinating (e.g. Sb(2)F(11)(-)) or with tertiary alkyl centers, as in [tert-butyl][carborane] salts. In solution, the reagents exist as equilibrating isomers with the alkyl group at the 7-11 or 12 halide positions of the CB(11) icosahedral carborane anion. These alkylating agents are so electrophilic that they (a) react with alkanes at or below room temperature via hydride extraction to produce carbenium ions, (b) alkylate benzene without a Friedel-Crafts catalyst to give arenium ions, and (c) alkylate electron-deficient phosphorus compounds that are otherwise inert to conventional alkylating agents such as methyl triflate.

Alkylating Agents↗
PubMed2004

Olefins as steering ligands for homogeneously catalyzed hydrogenations.

Iridium(I) complexes containing a (5H-dibenzo[a,d]cyclohepten-5-yl)-phosphane (tropp(R); R = phosphorus-bound substituent = Ph, Cyc) as a rigid, concave-shaped, mixed phosphane olefin ligand were prepared and tested as catalyst precursors in the hydrogenation of imines. With the complex [Ir(tropp(Cyc))(cod)]OTf, turnover frequencies (TOFs) of >6000 h(-1) were reached in the hydrogenation of N-phenyl-benzylidenamine, PhN==CHPh. Lower activities (TOF>80 h(-1)) are observed with N-phenyl-(1-phenylethylidene)amine, PhN==CMePh. Chiral tropp-type ligands were prepared in few simple steps. Monosubstitution of the olefinic unit in the dibenzo[a,d]cycloheptenyl moiety with (R)- or (S)-mentholate gave mixtures of diastereomers that could be separated and isolated in enantiomerically pure form. Iridium(I) complexes with these ligands are rare examples of side-on bonded enolether complexes. In catalytic imine hydrogenations, complete conversion (>98 %) was reached in all cases (conditions: p[H(2)] = 50 bar, T = 50 degrees C, t = 2 h, substrate/catalyst 100:1). The best enantiomeric excess (ee = 86 % S isomer) was reached with PhN==CMePh as substrate and the R,R form of the (10-menthyloxy-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ligand. The iridium(I) complex containing the same phosphane gave a 60 % ee (S isomer) with the enamide N-(1-phenylvinyl)acetamide as substrate (conditions: p[H(2)] = 4 bar, T = 50 degrees C, t = 18 h, substrate/catalyst = 50:1). These reactions constitute the first examples in which chiral olefins have been used as steering ligands in catalytic enantioselective hydrogenations.

Journal Article↗
PubMed2003

Synthesis and structure of [Na11(OtBu)10(OH)]: 1H NMR shift of a hydroxide ion encapsulated in a 21-vertex alcoholate cage.

[Na11(OtBu)10(OH)], a hydroxide enclosing 21-vertex cage compound, was found to crystallize from mixtures of sodium tert.butanolate with sodium hydroxide. Its structure can be derived from the known (NaOtBu)6-hexaprismane by replacing one butanolate unit with OH- and capping the latter with five additional units of NaOtBu. The hydroxide shows a signal at -3.21 ppm in the 1H NMR spectrum.

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