Evidence of AlII Radical Addition to Benzene

Electrophilic Al^III species have long dominated the aluminum reactivity towards arenes. Recently, nucleophilic low-valent Al^I aluminyl anions have showcased oxidative additions towards arenes C-C and/or C-H bonds. Herein, we communicate compelling evidence of an Al^II radical addition reaction to the benzene ring. The electron reduction of a ligand stabilized precursor with KC₈ in benzene furnishes a double addition to the benzene ring instead of a C-H bond activation, producing the corresponding cyclohexa-1,3(orl,4)-dienes as Birch-type reduction product. X-ray crystallographic analysis, EPR spectroscopy, and DFT results suggest this reactivity proceeds through a stable Al^II radical intermediate, whose stability is a consequence of a rigid scaffold in combination with strong steric protection.

(PNSiMe3 )4 (NMe)6 : A Robust Tetravalent Phosphaza-adamantane Scaffold for Molecular and Macromolecular Construction

Tetraarylmethanes and adamantanes are important rigid covalent connectors that play a four-way scaffolding role in molecular and materials chemistry. We report the synthesis of a new tetravalent phosphaza-adamantane cage, (PNSiMe₃ )₄ (NMe)₆ (2), that shows high thermal, air, and redox stability due to its geometry. It nevertheless participates in covalent four-fold functionalization reactions along its periphery. The combination of a robust core and reactive corona makes 2 a convenient inorganic scaffold upon which tetrahedral molecular and macromolecular chemistry can be constructed. This potential is demonstrated by the synthesis of a tetrakis(bis(phosphine)iminium) ion (in compound 3) and the first all P/N poly(phosphazene) network (5).

Frustrated Lewis Pair Chelation as a Vehicle for Low-Temperature Semiconductor Element and Polymer Deposition

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB, ⁱ Pr₂ P(C₆ H₄ )BCy₂ (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH₂ }] and [PB{GeH₂ }] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine-borane ligand (PB) after element deposition, and 2) the single-source precursor [PB{SiH₂ }] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe₂ }] was also prepared, and shown to release poly(dimethylsilane) [SiMe₂ ]_n upon heating. Overall, this study introduces a "closed loop" deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.

Hydrostibination of Alkynes: A Radical Mechanism*

The addition of Sb-H bonds to alkynes was reported recently as a new hydroelementation reaction that exclusively yields anti-Markovnikov Z-olefins from terminal acetylenes. We examine four possible mechanisms that are consistent with the observed stereochemical and regiochemical outcomes. A comprehensive analysis of solvent, substituent, isotope, additive, and temperature effects on hydrostibination reaction rates definitively refutes three ionic mechanisms involving closed-shell charged intermediates. Instead the data support a fourth pathway featuring open-shell neutral intermediates. Density-functional theory (DFT) calculations are consistent with this model, predicting an activation barrier that is in agreement with the experimental value (Eyring analysis) and a rate limiting step that is congruent with the experimental kinetic isotope effect. We therefore conclude that hydrostibination of arylacetylenes is initiated by the generation of stibinyl radicals, which then participate in a cycle featuring Sb^II and Sb^III intermediates to yield the observed Z-olefins as products. This mechanistic understanding will enable rational evolution of hydrostibination as a synthetic methodology.

Orthogonally Bimetallized Phosphane-ene Photopolymer Networks

The development of batteries and fuel cells has brought to light a need for carbon anode materials doped homogeneously with electrocatalytic metals. In particular, combinations of electrocatalysts in carbon have shown promising activity. A method to derive functional carbon materials is the pyrolysis of metallopolymers. This work describes the synthesis of a bifunctional phosphonium-based system derived from a phosphane-ene network. The olefin functionality can be leveraged in a hydrogermylation reaction to functionalize the material with Ge. Unaffected by this radical addition, the bromide counterion of the phosphonium cation can be used to subsequently incorporate a second metal in an ion-complexation reaction with CuBr₂ . The characterization of the polymers and the derived ceramics are discussed.