Heterolysis of H2 Across a Classical Lewis Pair, 2,6-Lutidine⋠BCl3 : Synthesis, Characterization, and Mechanism.

We report that 2,6-lutidine⋅trichloroborane (Lut⋅BCl3 ) reacts with H2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut⋅BHCl2 . The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut⋅BCl3 was calculated to react with H2 and form the ion pair, [LutH(+) ][HBCl3 (-) ], with a barrier of ΔH(≠) =24.7 kcal mol(-1) (ΔG(≠) =29.8 kcal mol(-1) ). Metathesis with a second molecule of Lut⋅BCl3 produced Lut⋅BHCl2 and [LutH(+) ][BCl4 (-) ]. The overall reaction is exothermic by 6.0 kcal mol(-1) (Δr G°=-1.1). Alternate pathways were explored involving the borenium cation (LutBCl2 (+) ) and the four-membered boracycle [(CH2 {NC5 H3 Me})BCl2 ]. Barriers for addition of H2 across the Lut/LutBCl2 (+) pair and the boracycle BC bond are substantially higher (ΔG(≠) =42.1 and 49.4 kcal mol(-1) , respectively), such that these pathways are excluded. The barrier for addition of H2 to the boracycle BN bond is comparable (ΔH(≠) =28.5 and ΔG(≠) =32 kcal mol(-1) ). Conversion of the intermediate 2-(BHCl2 CH2 )-6-Me(C5 H3 NH) to Lut⋅BHCl2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl3 . Intramolecular protodeboronation, which could form Lut⋅BHCl2 directly, is prohibited by a high barrier (ΔH(≠) =52, ΔG(≠) =51 kcal mol(-1) ).

Synthesis and hydride transfer reactions of cobalt and nickel hydride complexes to BX3 compounds.

Hydrides of numerous transition metal complexes can be generated by the heterolytic cleavage of H(2) gas such that they offer alternatives to using main group hydrides in the regeneration of ammonia borane, a compound that has been intensely studied for hydrogen storage applications. Previously, we reported that HRh(dmpe)(2) (dmpe = 1,2-bis(dimethylphosphinoethane)) was capable of reducing a variety of BX(3) compounds having a hydride affinity (HA) greater than or equal to the HA of BEt(3). This study examines the reactivity of less expensive cobalt and nickel hydride complexes, HCo(dmpe)(2) and [HNi(dmpe)(2)](+), to form B-H bonds. The hydride donor abilities (ΔG(H(-))°) of HCo(dmpe)(2) and [HNi(dmpe)(2)](+) were positioned on a previously established scale in acetonitrile that is cross-referenced with calculated HAs of BX(3) compounds. The collective data guided our selection of BX(3) compounds to investigate and aided our analysis of factors that determine favorability of hydride transfer. HCo(dmpe)(2) was observed to transfer H(-) to BX(3) compounds with X = H, OC(6)F(5), and SPh. The reaction with B(SPh)(3) is accompanied by the formation of dmpe-(BH(3))(2) and dmpe-(BH(2)(SPh))(2) products that follow from a reduction of multiple B-SPh bonds and a loss of dmpe ligands from cobalt. Reactions between HCo(dmpe)(2) and B(SPh)(3) in the presence of triethylamine result in the formation of Et(3)N-BH(2)SPh and Et(3)N-BH(3) with no loss of a dmpe ligand. Reactions of the cationic complex [HNi(dmpe)(2)](+) with B(SPh)(3) under analogous conditions give Et(3)N-BH(2)SPh as the final product along with the nickel-thiolate complex [Ni(dmpe)(2)(SPh)](+). The synthesis and characterization of HCo(dedpe)(2) (dedpe = Et(2)PCH(2)CH(2)PPh(2)) from H(2) and a base is also discussed, including the formation of an uncommon trans dihydride species, trans-[(H)(2)Co(dedpe)(2)][BF(4)].

Thermochemistry of Lewis adducts of BH3 and nucleophilic substitution of triethylamine on NH3BH3 in tetrahydrofuran.

The thermochemistry of the formation of Lewis base adducts of BH(3) in tetrahydrofuran (THF) solution and the gas phase and the kinetics of substitution on ammonia borane by triethylamine are reported. The dative bond energy of Lewis adducts were predicted using density functional theory at the B3LYP/DZVP2 and B3LYP/6-311+G** levels and correlated ab initio molecular orbital theories, including MP2, G3(MP2), and G3(MP2)B3LYP, and compared with available experimental data and accurate CCSD(T)/CBS theory results. The analysis showed that the G3 methods using either the MP2 or the B3LYP geometries reproduce the benchmark results usually to within ~1 kcal/mol. Energies calculated at the MP2/aug-cc-pVTZ level for geometries optimized at the B3LYP/DZVP2 or B3LYP/6-311+G** levels give dative bond energies 2-4 kcal/mol larger than benchmark values. The enthalpies for forming adducts in THF were determined by calorimetry and compared with the calculated energies for the gas phase reaction: THFBH(3) + L → LBH(3) + THF. The formation of NH(3)BH(3) in THF was observed to yield significantly more heat than gas phase dative bond energies predict, consistent with strong solvation of NH(3)BH(3). Substitution of NEt(3) on NH(3)BH(3) is an equilibrium process in THF solution (K ≈ 0.2 at 25 °C). The reaction obeys a reversible bimolecular kinetic rate law with the Arrhenius parameters: log A = 14.7 ± 1.1 and E(a) = 28.1 ± 1.5 kcal/mol. Simulation of the mechanism using the SM8 continuum solvation model shows the reaction most likely proceeds primarily by a classical S(N)2 mechanism.

Thermodynamic studies and hydride transfer reactions from a rhodium complex to BX3 compounds.

This study examines the use of transition-metal hydride complexes that can be generated by the heterolytic cleavage of H(2) gas to form B-H bonds. Specifically, these studies are focused on providing a reliable and quantitative method for determining when hydride transfer from transition-metal hydrides to three-coordinate BX(3) (X = OR, SPh, F, H; R = Ph, p-C(6)H(4)OMe, C(6)F(5), (t)Bu, Si(Me)(3)) compounds will be favorable. This involves both experimental and theoretical determinations of hydride transfer abilities. Thermodynamic hydride donor abilities (DeltaG(o)(H(-))) were determined for HRh(dmpe)(2) and HRh(depe)(2), where dmpe = 1,2-bis(dimethylphosphinoethane) and depe = 1,2-bis(diethylphosphinoethane), on a previously established scale in acetonitrile. This hydride donor ability was used to determine the hydride donor ability of [HBEt(3)](-) on this scale. Isodesmic reactions between [HBEt(3)](-) and selected BX(3) compounds to form BEt(3) and [HBX(3)](-) were examined computationally to determine their relative hydride affinities. The use of these scales of hydride donor abilities and hydride affinities for transition-metal hydrides and BX(3) compounds is illustrated with a few selected reactions relevant to the regeneration of ammonia borane. Our findings indicate that it is possible to form B-H bonds from B-X bonds, and the extent to which BX(3) compounds are reduced by transition-metal hydride complexes forming species containing multiple B-H bonds depends on the heterolytic B-X bond energy. An example is the reduction of B(SPh)(3) using HRh(dmpe)(2) in the presence of triethylamine to form Et(3)N-BH(3) in high yields.

Lewis acidities and hydride, fluoride, and X- affinities of the BH(3-n)Xn compounds for (X = F, Cl, Br, I, NH2, OH, and SH) from coupled cluster theory.

Atomization energies at 0 K and enthalpies of formation at 0 and 298 K are predicted for the BH(4-n)X(n)(-) and the BH(3-n)X(n)F(-) compounds for (X = F, Cl, Br, I, NH(2), OH, and SH) from coupled cluster theory (CCSD(T)) calculations with correlation-consistent basis sets and with an effective core potential on I. To achieve near chemical accuracy (+/-1.0 kcal/mol), additional corrections were added to the complete basis set binding energies. The hydride, fluoride, and X(-) affinities of the BH(3-n)X(n) compounds were predicted. Although the hydride and fluoride affinities differ somewhat in their magnitudes, they show very similar trends and are both suitable for judging the Lewis acidities of compounds. The only significant differences in their acidity strength orders are found for the boranes substituted with the strongly electron withdrawing and back-donating fluorine and hydroxyl ligands. The highest H(-) and F(-) affinities are found for BI(3) and the lowest ones for B(NH(2))(3). Within the boron trihalide series, the Lewis acidity increases monotonically with increasing atomic weight of the halogen, that is, BI(3) is a considerably stronger Lewis acid than BF(3). For the X(-) affinities in the BX(3), HBX(2), and H(2)BX series, the fluorides show the highest values, whereas the amino and mercapto compounds show the lowest ones. Hydride and fluoride affinities of the BH(3-n)X(n) compounds exhibit linear correlations with the proton affinity of X(-) for most X ligands. Reasons for the correlation are discussed. A detailed analysis of the individual contributions to the Lewis acidities of these substituted boranes shows that the dominant effect in the magnitude of the acidity is the strength of the BX(3)(-)-F bond. The main contributor to the relative differences in the Lewis acidities of BX(3) for X, a halogen, is the electron affinity of BX(3) with a secondary contribution from the distortion energy from planar to pyramidal BX(3). The B-F bond dissociation energy of X(3)B-F(-) and the distortion energy from pyramidal to tetrahedral BX(3)(-) are of less importance in determining the relative acidities. Because the electron affinity of BX(3) is strongly influenced by the charge density in the empty p(z) lowest unoccupied molecular orbital of boron, the amount of pi-back-donation from the halogen to boron is crucial and explains why the Lewis acidity of BF(3) is significantly lower than those of BX(3) with X = Cl, Br, and I.

Predicting the UV-vis spectra of Tetraarylcyclopentadienones: Using DFT molecular orbital energies to model electronic transitions of organic materials.

Tetraphenylcyclopentadienone, due to its intrinsically low HOMO-LUMO gap, has been suggested as a valuable repeat unit in conducting polymers for nanoscale electronics. The HOMO and LUMO of tetraphenylcyclopentadienone appear to be associated with the relevant pi orbitals of unsubstituted cyclopentadienone. Using previously developed carbonylative coupling reactions, a series of tetraarylcyclopentadienones was synthesized, accessing a range of substituents not previously available. The UV-vis spectra of these molecules were compared to their calculated wave functions and predicted transitions. A quantitative structure-activity relationship was discovered that may greatly simplify prediction of band gaps for oligomers and polymers built from these tetraarylcyclopentadienones.

Synthesis of poly(para-phenylene)(2-isocyano-2-tosylpropane-1,3-diyl), poly(para-phenylene)(2-oxopropane-1,3-diyl) and oligo(cyclopentadienones) via carbonylative coupling of alpha,alpha'-dibromoxylene.

Poly(para-phenylene)(2-oxopropane-1,3-diyl), a potential precursor of linear graphene, is generated in low yield from hydrolysis of a previously unknown poly(para-phenylene)(2-isocyano-2-tosylpropane-1,3-diyl) generated from inexpensive, commercially available starting materials.

Synthesis of heterosubstituted hexaarylbenzenes via asymmetric carbonylative couplings of benzyl halides.

[structure: see text]. Asymmetric carbonylative couplings of benzyl halides have been shown to give heterosubstituted 1,3-diarylacetones in moderate to high yields. These asymmetric ketones were converted via Knoevenagel condensations to tetraarylcyclopentadienones, and further conversion via dehydro-Diels-Alder cycloadditions gave highly heterofunctionalized hexaarylbenzenes with uniquely functionalized aryl groups at the para positions of the central benzene. This method allows control of the substituents on each of four unique pendent aryl group positions, giving rise to substitution patterns not available using symmetrical 1,3-diarylacetones.