Relevance of the electronegativity of boron in eta5-coordinating ligands: regioselective monoalkylation and monoarylation in cobaltabisdicarbollide [3,3'-Co(1,2-C2B9H11)2]- clusters

Regioselective monoalkylation and monoarylation in cobaltabisdicarbollide clusters has been achieved starting from Cs[8-I-3,3'-Co(1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))] by cross-coupling reactions between a B-I fragment and an appropriate Grignard reagent in the presence of a Pd catalyst and CuI. A considerable number of monoalkylated and monoarylated derivatives have been synthesized, which allowed study of the influence of boron in metallocene-type ligands and the effect of alkyl and aryl substituents on boron in boron anionic clusters. Experimental data from UV/Vis spectroscopy, E(1/2) measurements, and X-ray diffraction analysis, and supported by EHMO and ab initio analyses, indicate that the participation of metal d orbitals in the HOMO is less than that in typical metallocene complexes. This can be explained in terms of the lower electronegativity of boron compared to carbon. Related to this is the -I character of alkyl groups when bonded to boron in boron anionic clusters, contrary to the common belief that alkyl groups are generally electron-releasing moieties.     

Syntheses; 77Se, 203Tl, and 205Tl NMR; and theoretical studies of the Tl2Se6(6-), Tl3Se6(5-), and Tl3Se7(5-) anions and the X-ray crystal structures of [2,2,2-crypt-Na]4[Tl4Se8].en and [2,2,2-crypt-Na]2[Tl2Se4](infinity)1.en

The 2,2,2-crypt salts of the Tl4Se8(4-) and [Tl2Se4(2-)]infinity1 anions have been obtained by extraction of the ternary alloy NaTl0.5Se in ethylenediamine (en) in the presence of 2,2,2-crypt and 18-crown-6 followed by vapor-phase diffusion of THF into the en extract. The [2,2,2-crypt-Na]4[Tl4Se8].en crystallizes in the monoclinic space group P2(1)/n, with Z = 2 and a = 14.768(3) angstroms, b = 16.635(3) angstroms, c = 21.254(4) angstroms, beta = 94.17(3) degrees at -123 degrees C, and the [2,2,2-crypt-Na]2[Tl2Se4]infinity1.en crystallizes in the monoclinic space group P2(1)/c, with Z = 4 and a = 14.246(2) angstroms, b = 14.360(3) angstroms, c = 26.673(8) angstroms, beta = 99.87(3) degrees at -123 degrees C. The TlIII anions, Tl2Se6(6-) and Tl3Se7(5-), and the mixed oxidation state TlI/TlIII anion, Tl3Se6(5-), have been obtained by extraction of NaTl0.5Se and NaTlSe in en, in the presence of 2,2,2-crypt and/or in liquid NH3, and have been characterized in solution by low-temperature 77Se, 203Tl, and 205Tl NMR spectroscopy. The 1J(203,205Tl-77Se) and 2J(203,205Tl-203,205Tl) couplings of the three anions have been used to arrive at their solution structures by detailed analyses and simulations of all spin multiplets that comprise the 205,203Tl NMR subspectra arising from natural abundance 205,203Tl and 77Se isotopomer distributions. The structure of Tl2Se6(6-) is based on a Tl2Se2 ring in which each thallium is bonded to two exo-selenium atoms so that these thalliums are four-coordinate and possess a formal oxidation state of +3. The Tl4Se8(4-) anion is formally derived from the Tl2Se6(6-) anion by coordination of each pair of terminal Se atoms to the TlIII atom of a TlSe+ cation. The structure of the [Tl2Se4(2-)]infinity1 anion is comprised of edge-sharing distorted TlSe4 tetrahedra that form infinite, one-dimensional [Tl2Se42-]infinity1 chains. The structures of Tl3Se6(5-) and Tl3Se7(5-) are derived from Tl4Se4-cubes in which one thallium atom has been removed and two and three exo-selenium atoms are bonded to thallium atoms, respectively, so that each is four-coordinate and possesses a formal oxidation state of +3 with the remaining three-coordinate thallium atom in the +1 oxidation state. Quantum mechanical calculations at the MP2 level of theory show that the Tl2Se6(6-), Tl3Se6(5-), Tl3Se7(5-), and Tl4Se8(4-) anions exhibit true minima and display geometries that are in agreement with their experimental structures. Natural bond orbital and electron localization function analyses were utilized in describing the bonding in the present and previously published Tl/Se anions, and showed that the Tl2Se6(6-), Tl3Se6(5-), Tl3Se7(5-), and Tl4Se8(4-) anions are electron-precise rings and cages.     

Conformations and energetics of sulfur and selenium diimides

The geometries and energetics of different conformations of sulfur and selenium diimides E(NR)(2) (E = S, Se; R = H, Me, (t)Bu, C(6)H(3)Me(2)-2,6, SiMe(3)) have been studied by using various ab initio and DFT molecular orbital techniques. The syn,syn conformation is found to be most stable for parent E(NH)(2), but in general, the preferred molecular conformation for substituted chalcogen diimides is syn,anti. In the case of E(NH)(2) the present calculations further confirm that syn,syn and syn,anti conformations lie energetically close to each other. From the three different theoretical methods used, B3PW91/6-31G proved to be the most suitable method for predicting the geometries of chalcogen diimides. The optimized geometrical parameters are in a good agreement with all available experimental data. While qualitative energy ordering of the different conformations is independent of the level of theory, the quantitative energy differences are dependent on the method used. The performance and reliability of higher level ab initio calculations and DFT methods using large basis sets were tested and compared with experimental information where available. All of the higher level ab inito methods give very similar results, but the use of large basis sets with the B3PW91 method does not increase the reliability of the results. The combination of CCSD(T)/cc-pVDZ with the B3PW91/6-31G-optimized geometries is found to be the method of choice to study energetic properties of chalcogen diimides.     

The syntheses of carbocations by use of the noble-gas oxidant, [XeOTeF5][Sb(OTeF5)6]: the syntheses and characterization of the CX3+ (X = Cl, Br, OTeF5) and CBr(OTeF5)2+ cations and theoretical studies of CX3+ and BX3 (X = F, Cl, Br, I, OTeF5)

The CCl(3)(+) and CBr(3)(+) cations have been synthesized by oxidation of a halide ligand of CCl(4) and CBr(4) at -78 degrees C in SO(2)ClF solvent by use of [XeOTeF(5)][Sb(OTeF(5))(6)]. The CBr(3)(+) cation reacts further with BrOTeF(5) to give CBr(OTeF(5))(2)(+), C(OTeF(5))(3)(+), and Br(2). The [XeOTeF(5)][Sb(OTeF(5))(6)] salt was also found to react with BrOTeF(5) in SO(2)ClF solvent at -78 degrees C to give the Br(OTeF(5))(2)(+) cation. The CCl(3)(+), CBr(3)(+), CBr(OTeF(5))(2)(+), C(OTeF(5))(3)(+), and Br(OTeF(5))(2)(+) cations and C(OTeF(5))(4) have been characterized in SO(2)ClF solution by (13)C and/or (19)F NMR spectroscopy at -78 degrees C. The X-ray crystal structures of the CCl(3)(+), CBr(3)(+), and C(OTeF(5))(3)(+) cations have been determined in [CCl(3)][Sb(OTeF(5))(6)], [CBr(3)][Sb(OTeF(5))(6)].SO(2)ClF, and [C(OTeF(5))(3)][Sb(OTeF(5))(6)].3SO(2)ClF at -173 degrees C. The CCl(3)(+) and CBr(3)(+) salts were stable at room temperature, whereas the CBr(n)(OTeF(5))(3-n)(+) salts were stable at 0 degrees C for several hours. The cations were found to be trigonal planar about carbon, with the CCl(3)(+) and CBr(3)(+) cations showing no significant interactions between their carbon atoms and the fluorine atoms of the Sb(OTeF(5))(6)(-) anions. In contrast, the C(OTeF(5))(3)(+) cation interacts with an oxygen of each of two SO(2)ClF molecules by coordination along the three-fold axis of the cation. The solid-state Raman spectra of the Sb(OTeF(5))(6)(-) salts of CCl(3)(+) and CBr(3)(+) have been obtained and assigned with the aid of electronic structure calculations. The CCl(3)(+) cation displays a well-resolved (35)Cl/(37)Cl isotopic pattern for the symmetric CCl(3) stretch. The energy-minimized geometries, natural charges, and natural bond orders of the CCl(3)(+), CBr(3)(+), CI(3)(+), and C(OTeF(5))(3)(+) cations and of the presently unknown CF(3)(+) cation have been calculated using HF and MP2 methods have been compared with those of the isoelectronic BX(3) molecules (X = F, Cl, Br, I, and OTeF(5)). The (13)C and (11)B chemical shifts for CX(3)(+) (X = Cl, Br, I) and BX(3) (X = F, Cl, Br, I) were calculated by the GIAO method, and their trends were assessed in terms of paramagnetic contributions and spin-orbit coupling.     

Alkoxo, chlorido, and methyl derivatives of oxidomolybdenum(VI) complexes with tetradentate [O3N]-type ligands

Dioxomolybdenum(VI) complex [MoO₂(Heg)₂] (H₂eg = 1,2-ethanediol) reacts with phenolic ligand precursors tris(2-hydroxy-3,5-dimethylbenzyl)amine (H₃L^Me) and tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine (H₃L^tBu) to form oxomolybdenum(VI) complexes of type [MoO(L^R) (Heg)]. The Heg ligand can be replaced by other alcohols (i.e. 2-aminoethanol, 2-amino-2-methylpropan-1-ol, 2-(dimethylamino)ethanol or allyl alcohol) in the reaction at refluxing toluene or at neat alcohol. Treatment of [MoO(L^R)(Heg)] with Me₃SiCl yields corresponding chlorido complexes [MoO(L^R)Cl]. These are also formed in the reaction of H₃L^R with [MoO₂Cl₂(dmf)₂]. The reaction of [MoO(L^R)Cl] with MeMgI yields air-stable monomethyl derivatives [MoO(L^R)(Me)]. X-ray analyses of [MoO(L^tBu)X] (X = Heg, 2-methyl-2-aminopropanolate anion or Cl) reveal that the ligand L^R has a tetradentate coordination through three oxygen donors and one nitrogen donor, which is located trans to the terminal oxo group. The sixth coordination site is occupied by an oxygen donor, a chlorido ligand or a methyl group.     

Synthesis, vibrational and NMR spectroscopic characterization of [N(CH3)4][IO2F2] and X-ray crystal structure of [N(CH3)4]2[IO2F2][HF2]

The salt, [N(CH₃)₄][IO₂F₂], was prepared from [N(CH₃)₄][IO₃] and 49% aqueous HF, and characterized by Raman, infrared, and ¹⁹F NMR spectroscopy. Crystals of [N(CH₃)₄]₂[IO₂F₂][HF₂] were obtained by reduction of [N(CH₃)₄][cis-IO₂F₄] in the presence of [N(CH₃)₄][F] in CH₃CN solvent and were characterized by Raman spectroscopy and single-crystal X-ray diffraction: C2/m, a = 14.6765(2) Å, b = 8.60490(10) Å, c = 13.9572(2) Å, β = 120.2040(10)°, V = 1523.35(3) Å³, Z = 4 and R = 0.0192 at 210 K. The crystal structure consists of two IO₂⁻F₂ anions that are symmetrically bridged by two H⁻F₂ anions, forming a [F₂O₂I(FHF)₂IO₂F₂]⁴⁻ dimer. The symmetric bridging coordination for the H⁻F₂ anion in this structure represents a new bonding modality for the bifluoride anion.     

Chemoselective,Substrate‐directed Fluorination of Functionalized Cyclopentane β‐Amino Acids

This work describes a substrate‐directed fluorination of some highly functionalized cyclopentane derivatives. The cyclic products incorporating CH₂F or CHF₂ moieties in their structure have been synthesized from diexo‐ or diendo‐norbornene β‐amino acids following a stereocontrolled strategy. The synthetic study was based on an oxidative transformation of the ring carbon–carbon double bond of the norbornene β‐amino acids, followed by transformation of the resulted ?all cis“ and ?trans“ diformyl intermediates by fluorination with ?chemodifferentiation“.     

Water dynamics and proton transfer in nafion fuel cell membranes

The dynamics of water and its effect on proton transport kinetics in Nafion membranes are compared at several hydration levels. Nafion is the most widely used polyelectrolyte membrane in fuel cells. Ultrafast infrared spectroscopy of the O-D stretch of dilute HOD in H2O provides a probe of the local environment and hydrogen bond network dynamics of water confined in the hydrophilic regions of Nafion. The kinetics of proton transfer in Nafion are tracked by following the excited-state proton transfer and recombination kinetics of a molecular probe, pyranine (HPTS). The hydrophilic domains of Nafion grow with increased hydration, and the interfacial regions reorganize, leading to a changing local environment for water near the interface. Swelling is not uniform throughout the membrane, and heterogeneity is observed in the fluorescence anisotropy decays of the methoxy derivative of pyranine. Measurements of the time-dependent anisotropy of water in Nafion provide a direct probe of the hydrogen bond network dynamics. These dynamics, as well as the rate of proton transport over nanoscopic distances, are observed to slow significantly as the hydration level of the membrane decreases. The results provide insights into the influence of changes in the dynamics of water on the proton-transfer processes.     

Designed synthesis of new ortho-carborane derivatives: from mono- to polysubstituted frameworks

The use of nucleophilic and electrophilic processes allow the designed synthesis of several B-iodinated derivatives of o-carborane. Because of the straightforward Pd-catalyzed conversion of B-I to B-C bond with Grignard reagents, such as methylMgBr and biPhenylMgBr, both, symmetrical 3,6-R 2-1,2- closo-C 2B 10H 10 and asymmetrical 3-I-6-Me-1,2- closo-C 2B 10H 10 could be obtained. Not only conventional reactions in solution have been studied but also a highly efficient, clean and fast solvent-free procedure has provided successful results to regioselectively produce B-iodinated o-carborane derivatives by a careful control of the reaction conditions. The high number of nonequivalent leaving groups in boron iodinated o-carborane derivatives opens the possibility through B-C coupling to materials with novel possibilities and to self-assembling due to the enhanced polarizability of the C-H bond.     

Stereocontrolled synthesis of spiroketals via a remarkable methanol-induced kinetic spirocyclization reaction

A methanol-induced kinetic spiroketalization reaction has been developed for the stereocontrolled target- and diversity-oriented synthesis of spiroketals. In contrast to existing methods for spiroketal synthesis, this reaction does not depend on thermodynamic product stability or require axial attack of an oxygen nucleophile. Stereodiverse glycals are alkylated at the C1 position with side chains bearing protected hydroxyl groups. After alcohol deprotection, the glycal is epoxidized stereoselectively, then the side chain hydroxyl is spirocyclized with inversion of configuration at the anomeric carbon by addition of excess MeOH at -63 degrees C. This spirocyclization reaction appears to proceed by MeOH hydrogen-bonding catalysis and has been used to form five- and six-membered rings with stereoisomeric substituents. In some cases, the stereocomplementary spiroketals can be also obtained by classical acid-catalyzed equilibration.