Dications Based on a Hydrotris(phosphonio)borate Skeleton

Boron Polycation Supported by Cyclic Bis(carbodiphosphorane)

A novel cyclic bis(carbodiphosphorane) ligand was prepared and investigated in coordination with group 13 elements, B, Al, and Ga. Al and Ga afforded dinuclear adduct where two metal centers were bridged by the bis(carbodiphosphorane) ligand. In contrast, the reaction with boron trichloride afforded a monomeric dicationic three-coordinate boron species composed of one boron moieties and one ligand. The structures of these products were determined by X-ray crystallography. In the dicationic boron compound, the sterically constrained cyclic structure enforced the boron center to acquire strained trigonal geometry with wide C-B-C angle of 140°. Furthermore, theoretical investigation with DFT and NBO suggested a significant contribution of tricationic two-coordinate boron resonance structure supported by two CDP ligands.

Cationic Boron Formazanate Dyes*

Incorporation of cationic boron atoms into molecular frameworks is an established strategy for creating chemical species with unusual bonding and reactivity but is rarely thought of as a way of enhancing molecular optoelectronic properties. Using boron formazanate dyes as examples, we demonstrate that the wavelengths, intensities, and type of the first electronic transitions in BN heterocycles can be modulated by varying the charge, coordination number, and supporting ligands at the cationic boron atom. UV-vis absorption spectroscopy measurements and density-functional (DFT) calculations show that these modulations are caused by changes in the geometry and extent of π-conjugation of the boron formazanate ring. These findings suggest a new strategy for designing optoelectronic materials based on π-conjugated heterocycles containing boron and other main-group elements.

Cationic Complexes of Boron and Aluminum: An Early 21st Century Viewpoint

Boron and aluminum are lighter Group 13 elements, found in daily life commodities, and considered environmentally benign. Nevertheless, they markedly differ in their elemental properties (e.g., metal character, atomic radius). The use of Lewis acidic complexes of boron and aluminum for methods of bond activation and catalysis (e.g., hydrogenation of unsaturated substrates, polymerization of olefins and epoxides) is quickly expanding. The introduction of cationic charge may boost the metalloid-centered Lewis acidity and allow for its fine-tuning particularly with regard to preference for "hard" or "soft" Lewis bases (i.e., substrates). Especially the isolation of low-coordinate cations (number of ligand atoms smaller than four) demands elaborate techniques of thermodynamic and kinetic stabilization (i.e., electronic saturation and steric shielding) by a ligand system. Furthermore, the properties of the solvent and the counteranion must be considered with care. Here, selected examples of boron and aluminum cations are described.

Synthetic and structural studies of phosphine coordinated boronium salts

The reaction of BrH₂B·SMe₂ with primary and secondary phosphines affords a range of boronium salts of the form [H₂B(PR₃)₂]Br (PR₃ = PHCy₂1; PHPh₂, 2; PH₂Cy, 3), which have been fully characterised including solid-state determinations. Reactions of bulky tertiary phosphines, e.g., PCy₃ and PPh₃, with BrH₂B·SMe₂ do not proceed beyond the phosphine-stabilised bromoborane adducts, however, the smaller tertiary phosphine PMe₂Ph readily proceeds to form [H₂B(PMe₂Ph)₂]Br (4). The formation of the unsymmetrical boronium salts [H₂B(PH₂Cy)(PHCy₂)]Br (5) and [H₂B(PHCy₂)(PHPh₂)]Br (6) was observed by in situ NMR spectroscopy, however, the compounds were found to spontaneously disproportionate to their respective homophosphine boronium cations, even on prolonged storage in solution at -78 °C. Di- and triphosphines were found to form ring-closed boronium salts to afford [H₂B(κ²-P,P'-diphosphine)]Br (diphosphine = dppe, 7; dcpe, 8; dmpe, 9; dppf, 10; dppf with [AsF₆]^- counterion, 11; amphos, 12). The analogous methodology with Br₂HB·SMe₂ proved less generally applicable due to an accessible decomposition pathway being available to boronium salts bearing primary and secondary phosphines, leading to the formation of phosphonium salts, although [BrHB(dcpe)]Br (13), [BrHB(PMe₂Ph)₂]Br (14) and [BrHB(amphos)]Br (15) were synthesised with varying degrees of success. Reaction of BrH₂B·SMe₂ with diphars afforded the boronium salt [H₂B(κ²-P,P'-diphars)]Br (16), which featured two pendant arsine arms. Similarly, triphos was found to react with BrH₂B·SMe₂ to give [H₂B(κ²-P,P'-triphos)]Br (17), which featured a pendant phosphine arm. Substitution of the bromide counter anion with either hexafluoroarsenate or hexfluoroantimonate anions revealed weak hydrogen bonds between the P-H bonds of the boronium cations and the anions, that appeared through NMR studies to be retained in solution (where hydrogen bonding order was determined to be Br^- > [SbF₆]^-/[AsF₆]^-). This was further demonstrated by comparison of solid-state structures and solution NMR data of 1 with [H₂B(PHCy₂)₂][SbF₆] (18), 4 with [H₂B(PMe₂Ph)₂][AsF₆] (19) and 17 with [H₂B(triphos)][AsF₆] (20).

Lewis base activation of borane–dimethylsulfide into strongly reducing ion pairs for the transformation of carbon dioxide to methoxyboranes

The hydroboration of carbon dioxide into methoxyboranes by borane–dimethylsulfide using different base catalysts, including notably non-nucleophilic proton sponge, is described.

Boron di- and tri-cations

Previous work on boron di- and tri-cations is reviewed. The structural chemistry of representative examples of these classes of compound has been probed by determination of the single-crystal X-ray structures of [(4-Mepy)4B]Br3 and [py3BH]Br2. The electronic structures of the polycations [(py)3BH]2+, [(py)3BBr]2+, [(4-Mepy)3BH]2+, [(4-Mepy)4B]3+, [(Me3P)3BH]2+ and [(Me3P)4B]3+ have been examined by DFT methods. The atomic charges on these cations were evaluated by Mulliken, natural population analysis (NPA), Hirschfeld and Voronoi deformation density (VDD) methods.