Counterion Dependence on the Synthetic Viability of NHC-stabilized Dichloroborenium Cations

Synthesis of P-bridged, planar bis(NHC) BCl3 adducts

Planar PV- or PIII-bridged bis(NHCs), which have only been employed in transition metal complex chemistry so far, were subjected to BCl3-containing solutions targeting the corresponding bis(NHC) BCl3 adducts. While the P(O)NEt2-bridged bis(NHC) showed the expected adduct formation, the PNEt2-bridged bis(NHC) reacted not only at the carbene moiety but also at the P-NEt2 functional group. The latter enabled access to the first 1,4-diphosphinine bis(NHC) main group adduct; its formation and properties were investigated by DFT calculations. Through the same route, a 1,4-diphosphinine bis(imidazolium) scaffold was generated and explored theoretically and experimentally. The new 1,4-diphosphinines are shown to possess high global aromaticity and a unique P-centred reactivity, allowing the formation of hitherto inaccessible [4 + 2]-cycloaddition products, thus suggesting potential new applications compared to previously known 1,4-diphosphinine derivatives.

Phosphinoborenium cations stabilized by N-heterocyclic carbenes: synthesis, structure, and reactivity

Phosphinoborenium cations stabilized by N-heterocyclic carbenes (NHCs) were synthesized via the reaction of bromo(phosphino)boranes with NHCs. Their structures were investigated by heteronuclear magnetic resonance spectroscopy, X-ray diffraction, and density functional theory calculations. They possess a planar trigonal boron center directly bonded with the pyramidal phosphanyl group (PR2) and can be treated as cationic phosphinoboranes. The reactivity of the selected NHC-phosphinoborenium cation was tested toward AuCl·SMe2 and Ph2PCl. In both reactions, the titled compound acted as a phosphido group donor under heterolytic cleavage of the P-B bond. Control experiments with parent phosphinoborane emphasized differences between the reactivity of low-coordinate neutral and cationic species with P-B functionality.

Substitution at sp3 boron of a six-membered NHC·BH3: convenient access to a dihydroxyborenium cation

Herein, we have undertaken the synthesis and investigated the reactivity of a 6-membered saturated NHC borane adduct (1). Direct electrophilic halogenation of 1 with a stoichiometric amount of I₂ led to NHC boryl iodides, 6-SIDipp·BH₂I (2) and 6-SIDipp·BHI₂ (3), which were further reacted with various nucleophiles to give novel 6-SIDipp based mono and disubstituted boranes with OTf (4 and 6) or ONO₂ (5 and 7) functional groups. The addition of Br₂/H₂O to 1 smoothly results in a dihydroxyborenium cation (8).

Hydrosilylation of Carbonyls Catalyzed by Hydridoborenium Borate Salts: Lewis Acid Activation and Anion Mediated Pathways

The electronically unsaturated three-coordinated hydridoborenium cations [LBH]⁺[HB(C₆F₅)₃]^- (1) and [LBH]⁺[B(C₆F₅)₄]^- (2), supported by a bis(phosphinimino)amide ligand, were found to be excellent catalysts for hydrosilylation of a range of aliphatic and aromatic aldehydes and ketones under mild reaction conditions (L = [{(2,4,6-Me₃C₆H₂N)P(Ph₂)}₂N]). The key steps of the catalytic cycle for hydrosilylation of PhCHO were monitored via in situ multinuclear NMR measurements for catalysts 1 and 2. The combined effect of carbonyl activation via the Lewis acidic hydridoborenium cation and the hydridic nature of the borate counteranion in 1 makes it a more efficient catalyst in comparison to that of carbonyl activation via the predominant Lewis acid activation pathway operating with catalyst 2. The catalytic cycle of 1 showed hydride transfer from the borate moiety [HB(C₆F₅)₃]^- to PhCHO in the first step, forming [PhCH₂-O-B(C₆F₅)₃]^-, which subsequently underwent σ-bond metathesis with Et₃SiH to form the product, PhCH₂-O-SiEt₃. Quantum chemical calculations also support the borate anion mediated mechanism with 1. In contrast, the reaction catalyzed by 2 proceeds predominantly via the Lewis acid activation of the carbonyl group involving [LB(H)←OC(H)Ph]⁺[B(C₆F₅)₄]^- as the transition state and [LBOCH₂Ph]⁺[B(C₆F₅)₄]^- as the intermediate.

Group 13 Element Trihalide Complexes of Anionic N-Heterocyclic Carbenes

A series of group 13 complexes of the general type [{(WCA-IDipp)EX3 }Li(solv)] (E=B, Al, Ga, In; X=Cl, Br) that bear an anionic N-heterocyclic carbene ligand with a weakly coordinating borate moiety (WCA-IDipp, WCA=B(C6 F5 )3 and IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by the reaction of the respective group 13 trihalides (EX3 ) with the lithium salt [(WCA-IDipp)Li ⋅ toluene]. The molecular structures of the BBr3 , AlCl3 , AlBr3 , GaCl3 and InCl3 adducts were established by X-ray diffraction analyses, revealing the formation of coordination polymers linked by halide-lithium interactions, except for the indium derivative, which consists of isolated [Li(THF)4 ]+ and [(WCA-IDipp)InCl3 ]- ions in the solid state.

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.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Probing the nature of peripheral boryl groups within luminescent tellurophenes

In this article our attempts to tune the color of luminescence within a new class of aggregation-induced emission (AIE) active tellurophenes is reported along with computational details that include spin–orbit coupling effects so as to better understand the nature of emission in the phosphorescent tellurophene (B-Te-6-B). Despite not meeting some of the initial synthetic targets, the emission within a borylated tellurophene can be altered with the addition of an N-heterocyclic carbene.

Fine-Tuning of Lewis Acidity: The Case of Borenium Hydride Complexes Derived from Bis(phosphinimino)amide Boron Precursors

Reactions of bis(phosphinimino)amines LH and L'H with Me2 S⋅BH2 Cl afforded chloroborane complexes LBHCl (1) and L'BHCl (2), and the reaction of L'H with BH3 ⋅Me2 S gave a dihydridoborane complex L'BH2 (3) (LH=[{(2,4,6-Me3 C6 H2 N)P(Ph2 )}2 N]H and L'H=[{(2,6-iPr2 C6 H3 N)P(Ph2 )}2 N]H). Furthermore, abstraction of a hydride ion from L'BH2 (3) and LBH2 (4) mediated by Lewis acid B(C6 F5 )3 or the weakly coordinating ion pair [Ph3 C][B(C6 F5 )4 ] smoothly yielded a series of borenium hydride cations: [L'BH](+) [HB(C6 F5 )3 ](-) (5), [L'BH](+) [B(C6 F5 )4 ](-) (6), [LBH](+) [HB(C6 F5 )3 ](-) (7), and [LBH](+) [B(C6 F5 )4 ](-) (8). Synthesis of a chloroborenium species [LBCl](+) [BCl4 ](-) (9) without involvement of a weakly coordinating anion was also demonstrated from a reaction of LBH2 (4) with three equivalents of BCl3 . It is clear from this study that the sterically bulky strong donor bis(phosphinimino)amide ligand plays a crucial role in facilitating the synthesis and stabilization of these three-coordinated cationic species of boron. Therefore, the present synthetic approach is not dependent on the requirement of weakly coordinating anions; even simple BCl4 (-) can act as a counteranion with borenium cations. The high Lewis acidity of the boron atom in complex 8 enables the formation of an adduct with 4-dimethylaminopyridine (DMAP), [LBH⋅(DMAP)](+) [B(C6 F5 )4 ](-) (10). The solid-state structures of complexes 1, 5, and 9 were investigated by means of single-crystal X-ray structural analysis.

Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the - naturally subjective -"reactive"-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

Synthesis and reactivity of N,N′-1,4-diazabutadiene derived borocations

A new route to generate borocations has been established. By utilising readily synthesised diimines derived from commercially available materials, a new family of borenium- and boronium-cations can be synthesised from haloboranes.

Synthesis, structural studies and ligand influence on the stability of aryl-NHC stabilised trimethylaluminium complexes

Treatment of a series of aromatic NHCs with trimethylaluminium produced their corresponding NHC·AlMe₃ adducts displaying different stabilities. Their stabilities are rationalized using topographic steric maps and NHC % V Bur.

syn-1,2-carboboration of alkynes with borenium cations

The reaction of 8-(trimethylsiloxy)quinoline (QOTMS) with BCl₃ and (aryl)BCl₂ forms QOBCl₂ and QOBCl(aryl). The subsequent addition of stoichiometric AlCl₃ follows one of two paths, dependent on the steric demands of the QO ligand and the electrophilicity of the resulting borenium cation. The phenyl- and 5-hexylthienylborenium cations, QOBPh⁺ and QOBTh⁺, are formed, whereas QOBCl⁺ is not. Instead, AlCl₃ preferentially binds with QOBCl₂ at oxygen, forming QOBCl₂⋅AlCl₃, rather than abstracting chloride. A modest increase in the steric demands around oxygen, by installing a methyl group at the 7-position of the quinolato ligand, switches the reactivity with AlCl₃ back to chloride abstraction, allowing formation of QOBCl⁺. All the prepared borenium cations are highly chlorophilic and exhibit significant interaction with AlCl₄⁻ resulting in an equilibrium concentration of Lewis acidic “AlCl₃” species. The presence of “AlCl₃^” species limits the alkyne substrates compatible with these borenium systems, with reaction of [QOBPh][AlCl₄] with 1-pentyne exclusively yielding the cyclotrimerised product, 1,3,5-tripropylbenzene. In contrast, QOBPh⁺ and QOBTh⁺ systems effect the syn-1,2-carboboration of 3-hexyne. DFT calculations at the M06-2X/6-311G(d,p)/PCM(DCM) level confirm that the higher migratory aptitude of Ph versus Me leads to a lower barrier to 1,2-carboboration relative to 1,1-carboboration.

Dihaloborenium cations stabilized by a four-membered N-heterocyclic carbene: electron deficiency compensation by asymmetric structural changes

Coordination of the four membered carbene ligand to electron deficient dihaloborenium moieties has resulted in asymmetric structural changes for the ligand's CN₂P cyclic fragment.