Anodic Oxidation of 18 Halogenated and/or Methylated Derivatives of CB11H12.

Anodic oxidation of [CB11H12]- and 18 of its halogenated and/or methylated derivatives was examined. Reversible oxidation was found for four of the anions in liquid SO2 and for four more in 1,1,1,3,3,3-hexafluoroisopropyl alcohol. The oxidation occurred at ∼1 V (for [CB11Me12]-) up to more than 4 V (for [1-H-(2-6)-F5-(7-12)-(CF3)6-CB11]-) relative to ferrocene/ferricinium. The anodic peak potentials are reproduced by a set of additive position-sensitive substituent increments.

Comparison of the electronic properties of diarylamido-based PNZ pincer ligands: redox activity at the ligand and donor ability toward the metal.

This paper presents the synthesis and characterization of a series of pincer ligands and their Ni, Pd, Pt, and Rh complexes. The ligands under examination are based on a diarylamine which is modified either by two phosphino (-PR2) substituents in the ortho-positions (PNP ligands) or by a combination of a phosphino and an iminyl (-CH═NX) substituent (PNN ligands). The ligands can be broken down into three groups: (a) C2v-symmetric PNP ligands with identical side -PR2 donors, (b) Cs-symmetric PNP' ligands with different -PR2 side donors, and (c) PNN ligands containing a -P(i)Pr2 side donor. All of the ligands under study readily formed square-planar complexes of the types (PNZ)PdCl, (PNZ)Pd(OAc), and (PNZ)RhCO, where PNZ is the corresponding anionic tridentate pincer ligand. For select PNP ligands, (PNP)NiCl and (PNP)PtCl were also studied. The (PNZ)MCl complexes (M = Ni, Pd, Pt) underwent quasireversible oxidation in cyclic voltammetry experiments. Based on the close similarity of formal potentials for Ni, Pd, and Pt analogs, and based on the previous literature evidence, these oxidation events are ascribed primarily to the PNZ ligand, and the E1/2 values can be used to compare the ease of oxidation of different ligands. A (PNP)PdCl complex containing methoxy substituents para- to the central nitrogen underwent two quasireversible oxidations. Two mono-oxidized complexes were isolated and structurally characterized in comparison to their neutral analog, revealing minimal changes in the bond distances and angles. Several other neutral complexes were also structurally characterized. The carbonyl stretching frequency in (PNZ)RhCO complexes was used to gauge the donating ability of the various pincer ligands toward the metal. Comparison of E1/2 values for (PNZ)PdCl and νCO values for (PNZ)RhCO revealed that the two are not consistently correlated across all the studied ligands and can be tuned to different degrees through judicious ligand alteration.

Measured and calculated oxidation potentials of 1-X-12-Y-CB11Me10- anions.

Cyclic voltammetry of 31 icosahedral carborane anions 1-X-12-Y-CB(11)Me(10)(-) at a Pt electrode in liquid SO(2) revealed a completely reversible one-electron oxidation even at low scan rates, except for the anions with Y = I, which are oxidized irreversibly up to a scan rate of 5.0 V/s, and the anion with X = COOH and Y = H, whose oxidation is irreversible at scan rates below 1.0 V/s. Relative reversible oxidation potentials agree well with RI-B3LYP/TZVPP,COSMO and significantly less well with RI-BP86/TZVPP,COSMO or RI-HF/TZVPP,COSMO calculated adiabatic electron detachment energies. Correlations with HOMO energies of the anions are nearly as good, even though the oxidized forms are subject to considerable Jahn-Teller distortion. Except for the anion with X = F and Y = Me, the oxidation potentials vary linearly with substituent σ(p) Hammett constants. The slopes (reaction constants) are ~0.31 and ~0.55 V for positions 1 and 12, respectively.

Oxidatively induced concurrent cationic and radical polymerization of isobutylene in the presence of LiCB11Me12.

A solution of a mechanistic puzzle is reported: upon initiation with air at 25 °C or with di-tert-butyl peroxide at 80 °C, isobutylene (IB) polymerizes at 1 atm in weakly coordinating solvents containing 10 wt % LiCB(11)(CH(3))(12) to a mixture of highly branched (b-PIB) and linear (l-PIB) polyisobutylene. The former polymer is separable by solvent extraction and is identical with the b-PIB that is produced from IB as a sole product under similar conditions under nonoxidizing radical initiation with azo-tert-butane. The latter polymer differs from standard l-PIB in that it carries a carborane anion attached at the chain end. The molecular weight of b-PIB ranges up to 26,000, and that of l-PIB, up to 50,000. Evidence is presented that the concurrent polymerization of IB to b-PIB and l-PIB is launched by an initial oxidation of the CB(11)(CH(3))(12)(-) anion to a neutral radical CB(11)(CH(3))(12)(•). This radical is proposed to subsequently transfer a methyl radical to IB, thus launching the formation of b-PIB by the radical mechanism while leaving behind the borenium ylide CB(11)(CH(3))(11), which is a strong Lewis acid and induces simultaneously the formation of l-PIB by the cationic mechanism.

Hydrodefluorination and other hydrodehalogenation of aliphatic carbon-halogen bonds using silylium catalysis.

Trialkylsilylium cation equivalents partnered with halogenated carborane anions (such as Et(3)Si[HCB(11)H(5)Cl(6)]) function as efficient and long-lived catalysts for hydrodehalogenation of C-F, C-Cl, and C-Br bonds with trialkylsilanes as stoichiometric reagents. Only C(sp(3))-halogen bonds undergo this reaction. The range of C-F bond-containing substrates that participate in this reaction is quite broad and includes simple alkyl fluorides, benzotrifluorides, and compounds with perfluoroalkyl groups attached to an aliphatic chain. However, CF(4) has proven immune to this reaction. Hydrodechlorination was carried out with a series of alkyl chlorides and benzotrichlorides, and hydrodebromination was studied only with primary alkyl bromide substrates. Competitive experiments established a pronounced kinetic preference of the catalytic system for activation of a carbon-halogen bond of a lighter halide in primary alkyl halides. On the contrary, hydrodechlorination of C(6)F(5)CCl(3) proceeded much faster than hydrodefluorination of C(6)F(5)CF(3) in one-pot experiments. A solid-state structure of Et(3)Si[HCB(11)H(5)Cl(6)] was determined by X-ray diffraction methods.

Carbon-carbon coupling of C(sp3)-F bonds using alumenium catalysis.

Dialkylalumenium cation equivalents coupled with the hexabromocarborane anion function as efficient and long-lived catalysts for alkylation of aliphatic C-F bonds (alkylative defluorination or AlkDF) by alkylaluminum compounds. Only C(sp(3))-F bonds undergo AlkDF; C(sp(2))-F bonds are unaffected. Examples of compounds undergoing AlkDF include monofluoroalkanes, gem-difluorocyclopentane, and compounds containing a CF(3) group attached to either an aryl or an alkyl substituent. Conversion of C-F bonds to C-Me bonds is accomplished with high fidelity using Me(3)Al as the stoichiometric reagent. In reactions with Et(3)Al, hydrodefluorination of the C-F bonds is competitive with alkylation, indicative presumably of competitive hydride vs alkyl transfer from Et(3)Al. In a trialkylaluminum reagent, 1.1-1.4 alkyl groups per Al can be used to replace C-F bonds. Organoaluminum compounds efficiently remove water from the reaction mixture, obviating the need for rigorously dry solvents. Some organoaluminum compounds, especially methylaluminoxane, are capable of AlkDF with more reactive substrates, but catalysis by alumenium offers an advantage over the uncatalyzed C-F activation in terms of both increased rate and, in some cases, a dramatically increased selectivity.

Hydrodefluorination of perfluoroalkyl groups using silylium-carborane catalysts.

Carbon-fluorine bonds are among the most unreactive functionalities in chemistry. Interest in their activation arises in part from the high global warming potentials of anthropogenic polyfluoroorganic compounds. Conversion to carbon-hydrogen bonds (hydrodefluorination) is the simplest modification of carbon-fluorine bonds, but efficient catalytic hydrodefluorination of perfluoroalkyl groups has been an unmet challenge. We report a class of carborane-supported, highly electrophilic silylium compounds that act as long-lived catalysts for hydrodefluorination of trifluoromethyl and nonafluorobutyl groups by widely accessible silanes under mild conditions. The reactions are completely selective for aliphatic carbon-fluorine bonds in preference to aromatic carbon-fluorine bonds.

Isolating fluorinated carbocations.

Using carboranes as counterions, fluorinated benzyl-type carbocations such as (p-FC(6)H(4))(2)CF(+), (p-FC(6)H(4))(CH(3))CF(+) and fluorinated trityl ions are readily isolated for X-ray and IR structural characterization.