The [(NHC)B(H)C6F5]+ Cations and Their [B](H)−CO Borane Carbonyls

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH₂C₆F₅ (NHC: IMes or IMe₄) gave the B−H containing [(NHC)B(H)C₆F₅]⁺ borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C₆F₅)CO]⁺ boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives.     

Complexation and Versatile Reactivity of a Highly Lewis Acidic Cationic Mg Complex with Alkynes and Phosphines

[(BDI)Mg⁺][B(C₆F₅)₄⁻] ( 1 ; BDI=CH[C(CH₃)NDipp]₂; Dipp=2,6-diisopropylphenyl) was prepared by reaction of (BDI)MgnPr with [Ph₃C⁺][B(C₆F₅)₄⁻]. Addition of 3-hexyne gave [(BDI)Mg⁺ ⋅ (EtC≡CEt)][B(C₆F₅)₄⁻]. Single-crystal X-ray analysis, NMR investigations, Raman spectra, and DFT calculations indicate a significant Mg-alkyne interaction. Addition of the terminal alkynes PhC≡CH or Me₃SiC≡CH led to alkyne deprotonation by the BDI ligand to give [(BDI-H)Mg⁺(C≡CPh)]₂ ⋅ 2 [B(C₆F₅)₄⁻] ( 2 , 70 %) and [(BDI-H)Mg⁺(C≡CSiMe₃)]₂ ⋅ 2 [B(C₆F₅)₄⁻] ( 3 , 63 %). Addition of internal alkynes PhC≡CPh or PhC≡CMe led to [4+2] cycloadditions with the BDI ligand to give {Mg⁺C(Ph)=C(Ph)C[C(Me)=NDipp]₂}₂ ⋅ 2 [B(C₆F₅)₄⁻] ( 4 , 53 %) and {Mg⁺C(Ph)=C(Me)C[C(Me)=NDipp]₂}₂ ⋅ 2 [B(C₆F₅)₄⁻] ( 5 , 73 %), in which the Mg center is N,N,C-chelated. The (BDI)Mg⁺ cation can be viewed as an intramolecular frustrated Lewis pair (FLP) with a Lewis acidic site (Mg) and a Lewis (or Brønsted) basic site (BDI). Reaction of [(BDI)Mg⁺][B(C₆F₅)₄⁻] ( 1 ) with a range of phosphines varying in bulk and donor strength generated [(BDI)Mg⁺ ⋅ PPh₃][B(C₆F₅)₄⁻] ( 6 ), [(BDI)Mg⁺ ⋅ PCy₃][B(C₆F₅)₄⁻] ( 7 ), and [(BDI)Mg⁺ ⋅ PtBu₃][B(C₆F₅)₄⁻] ( 8 ). The bulkier phosphine PMes₃ (Mes=mesityl) did not show any interaction. Combinations of [(BDI)Mg⁺][B(C₆F₅)₄⁻] and phosphines did not result in addition to the triple bond in 3-hexyne, but during the screening process it was discovered that the cationic magnesium complex catalyzes the hydrophosphination of PhC≡CH with HPPh₂, for which an FLP-type mechanism is tentatively proposed.     

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 Me₂S ? BH₂Cl afforded chloroborane complexes LBHCl ( 1 ) and L′BHCl ( 2 ), and the reaction of L′H with BH₃ ? Me₂S gave a dihydridoborane complex L′BH₂ ( 3 ) (LH=[{(2,4,6‐Me₃C₆H₂N)P(Ph₂)}₂N]H and L′H=[{(2,6‐iPr₂C₆H₃N)P(Ph₂)}₂N]H). Furthermore, abstraction of a hydride ion from L′BH₂ ( 3 ) and LBH₂ ( 4 ) mediated by Lewis acid B(C₆F₅)₃ or the weakly coordinating ion pair [Ph₃C][B(C₆F₅)₄] smoothly yielded a series of borenium hydride cations: [L′BH]⁺[HB(C₆F₅)₃]^? ( 5 ), [L′BH]⁺[B(C₆F₅)₄]^? ( 6 ), [LBH]⁺[HB(C₆F₅)₃]^? ( 7 ), and [LBH]⁺[B(C₆F₅)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(C₆F₅)₄]^? ( 10 ). The solid‐state structures of complexes 1 , 5 , and 9 were investigated by means of single‐crystal X‐ray structural analysis.     

Borane‐Catalyzed Cross‐Metathesis Strategy for Facile Transformation of Cyclic (Alkyl)(Amino)Germylenes

A borane B(C₆F₅)₃‐catalyzed metathesis reaction between the Si?C bond in the cyclic (alkyl)(amino)germylene (CAAGe) 1 and the Si?H bond in a silane (R₃SiH; 2 ) is reported. Mechanistic studies propose that the initial step of the reaction involves Si?H bond activation to furnish an ionic species [ 1 ‐SiR₃]⁺[HB(C₆F₅)₃]^?, from which [Me₃Si]⁺[HB(C₆F₅)₃]^? and an azagermole intermediate are generated. The former yields Me₃SiH concomitant with the regeneration of B(C₆F₅)₃ whereas the latter undergoes isomerization to afford CAAGes bearing various silyl groups on the carbon atom next to the germylene center. This strategy allows the straightforward synthesis of eight new CAAGes starting from 1 .     

Frustrated Lewis Pair Chemistry Derived from Bulky Allenyl and Propargyl Phosphanes

The dimesitylpropargylphosphanes mes₂P?CH₂?C≡C?R 6 a (R=H), 6 b (R=CH₃), 6 c (R=SiMe₃) and the allene mes₂P?C(CH₃)=C=CH₂ ( 8 ) were reacted with Piers’ borane, HB(C₆F₅)₂. Compound 6 a gave mes₂PCH₂CH=CH(B(C₆F₅)₂] ( 9 a ). In contrast, addition of HB(C₆F₅)₂ to 6 b and 6 c gave mixtures of 9 b (R=CH₃) and 9 c (R=SiMe₃) with the regioisomers mes₂P?CH₂?C[B(C₆F₅)₂]=CRH 2 b (R=CH₃) and 2 c (R=SiMe₃), respectively. Compounds 2 b , c underwent rapid phosphane/borane (P/B) frustrated Lewis pair (FLP) reactions under mild conditions. Compound 2 c reacted with nitric oxide (NO) to give the persistent FLP NO radical 11 . The systems 2 b , c cleaved dihydrogen at room temperature to give the respective phosphonium/hydridoborate products 13 b , c . Compound 13 c transferred the H⁺/H^? pair to a small series of enamines. Compound 13 c was also a metal‐free catalyst (5 mol %) for the hydrogenation of the enamines. The allene 8 reacted with B(C₆F₅)₃ to give the zwitterionic phosphonium/borate 17 . The ‐PPh₂‐substituted mes₂P‐propargyl system 6 d underwent a typical 1,2‐P/B‐addition reaction to the C≡C triple bond to form the phosphetium/borate zwitterion 20 . Several products were characterized by X‐ray diffraction.     

Cationic Silica‐Supported N‐Heterocyclic Carbene Tungsten Oxo Alkylidene Sites: Highly Active and Stable Catalysts for Olefin Metathesis

Designing supported alkene metathesis catalysts with high activity and stability is still a challenge, despite significant advances in the last years. Described herein is the combination of strong σ‐donating N‐heterocyclic carbene ligands with weak σ‐donating surface silanolates and cationic tungsten sites leading to highly active and stable alkene metathesis catalysts. These well‐defined silica‐supported catalysts, [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)(OTf)] and [(≡SiO)W(=O)(=CHCMe₂Ph)(IMes)⁺][B(Ar^F)₄^?] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)‐imidazol‐2‐ylidene, B(Ar^F)₄=B(3,5‐(CF₃)₂C₆H₃)₄] catalyze alkene metathesis, and the cationic species display unprecedented activity for a broad range of substrates, especially for terminal olefins with turnover numbers above 1.2 million for propene.     

Reactivity of the Latent 12‐Electron Fragment [Rh(PiBu3)2]+ with Aryl Bromides: Aryl?Br and Phosphine Ligand C?H Activation

Oxidative addition of aryl bromides to 12‐electron [Rh(PiBu₃)₂][BAr^F₄] (Ar^F=3,5‐(CF₃)₂C₆H₃) forms a variety of products. With p‐tolyl bromides, Rh^III dimeric complexes result [Rh(PiBu₃)₂(o/p‐MeC₆H₄)(μ‐Br)]₂[BAr^F₄]₂. Similarly, reaction with p‐ClC₆H₄Br gives [Rh(PiBu₃)₂(p‐ClC₆H₄)(μ‐Br)]₂[BAr^F₄]₂. In contrast, the use of o‐BrC₆H₄Me leads to a product in which toluene has been eliminated and an isobutyl phosphine has undergone C? H activation: [Rh{PiBu₂(CH₂CHCH₃C H₂)}(PiBu₃)(μ‐Br)]₂[BAr^F₄]₂. Trapping experiments with ortho‐bromo anisole or ortho‐bromo thioanisole indicate that a possible intermediate for this process is a low‐coordinate Rh^III complex that then undergoes C? H activation. The anisole and thioanisole complexes have been isolated and their structures show OMe or SMe interactions with the metal centre alongside supporting agostic interactions, [Rh(PiBu₃)₂(C₆H₄O Me)Br][BAr^F₄] (the solid‐state structure of the 5‐methyl substituted analogue is reported) and [Rh(PiBu₃)₂(C₆H₄S Me)Br][BAr^F₄]. The anisole‐derived complex proceeds to give [Rh{PiBu₂(CH₂CHCH₃C H₂)}(PiBu₃)(μ‐Br)]₂[BAr^F₄]₂, whereas the thioanisole complex is unreactive. The isolation of [Rh(PiBu₃)₂(C₆H₄O Me)Br][BAr^F₄] and its onward reactivity to give the products of C? H activation and aryl elimination suggest that it is implicated on the pathway of a σ‐bond metathesis reaction, a hypothesis strengthened by DFT calculations. Calculations also suggest that C? H bond cleavage through phosphine‐assisted deprotonation of a non‐agostic bond is also competitive, although the subsequent protonation of the aryl ligand is too high in energy to account for product formation. C? H activation through oxidative addition is also ruled out on the basis of these calculations. These new complexes have been characterised by solution NMR/ESIMS techniques and in the solid‐state by X‐ray crystallography.     

C−F Bond Activation by a Saturated N‐Heterocyclic Carbene: Mesoionic Compound Formation and Adduct Formation with B(C6F5)3

The reaction of SIPr, [1,3‐bis(2,6‐diisopropylphenyl)‐imidazolin‐2‐ylidene] ( 1 ), with C₆F₆ led to the formation of an unprecedented mesoionic compound ( 2 ). The formation of 2 is made accessible by deprotonation of the SIPr backbone with simultaneous elimination of HF. The C?F bond para to the imidazolium ring in 2 is only of 1.258(4) Å, which is the one of the shortest structurally authenticated C?F bonds known to date. The liberation of HF during the reaction is unequivocally proved by the addition of one more equivalent of SIPr, which leads to the imidazolium salt with the HF₂^? anion. To functionalize 2 , the latter reacted with B(C₆F₅)₃ to give an unusual donor–acceptor compound, where the fluoride atom from the C₆F₅ moiety coordinates to B(C₆F₅)₃ and the carbanion moiety remains unaffected. Such coordination susceptibility of the fluoride atom of a nonmetallic system to a main‐group Lewis acid (F_non‐metal→BR₃) is quite unprecedented.     

Deuteration of closo-1,2- and 1,7-C2B10H12 using C6D6/AlCl3: Mechanistic considerations

Regioselective deuteration of 1-X-C₂B₁₀H₁₂ (X = 2, 7) cage systems with C₆D₆/AlCl₃ is correlated to ab initio calculational results on a [C₂B₁₀H₁₃]⁺ intermediate. Full geometry optimizations of pertinent [C₂B₁₀H₁₃]⁺ isomers, derived from each of the two 1-X-C₂B₁₀H₁₂ carborane isomers, result in cage geometries not unlike the (nearly) icosahedral starting carborane. Each isomer contains a BH₂ group having an acute H-B-H angle, long B–H bonds, and a very short H · · · H distance, hinting that the pertinent boron shares the electrons of a hydrogen molecule σ pair. It is suggested that the structural differences between the BH₂ group of [C₂B₁₀H₁₃]⁺ and the CH₂ group of the benzenium ion, [C₆H₇]⁺ (the intermediate strongly intimated upon protonation of benzene), can be explained, in part, by (a) the availability of the π-ring electrons for bonding to the (extra) proton in the latter and (b) the unavailability of π electrons from the carborane. Thus, the C₂B₁₀H₁₂ cage is most probably very reluctant to give up a cage electron pair in order to assist in bonding to an (externally bound) proton. Instead, it is more probable that “hydridic” B–H sigma electrons could very well play the important role in providing bonding to the attacking proton. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:95–102, 1998     

Reaction of Frustrated Lewis Pairs with Ketones and Esters

The frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ ( 1 ) reacts with an enolizable conjugated ynone by 1,4‐addition involving enolate tautomerization to give an eight‐membered zwitterionic heterocycle. The conjugated endione PhCO‐CH?CH‐COPh reacts with the intermolecular FLP tBu₃P/B(C₆F₅)₃ by a simple 1,4‐addition to an enone subunit. The same substrate undergoes a more complex reaction with the FLP 1 that involves internal acetal formation to give a heterobicyclic zwitterionic product. FLP 1 reacts with dimethyl maleate by selective overall addition to the C?C double bond to give a six‐membered heterocycle. It adds analogously to the triple bond of an acetylenic ester to give a similarly structured six‐membered heterocycle. The intermolecular FLP P(o‐tolyl)₃/B(C₆F₅)₃ reacts analogously with acetylenic ester by trans‐addition to the carbon–carbon triple bond. An excess of the intermolecular FLP tBu₃P/B(C₆F₅)₃, which contains a more nucleophilic phosphane, reacts differently with acetylenic ester examples, namely by O? C(alkyl) bond cleavage to give the {R‐CO₂[B(C₆F₅)₃]₂^?}[alkyl‐PtBu₃⁺] salts. Simple aryl or alkyl esters react analogously by using the borane‐stabilized carboxylates as good leaving groups. All essential products were characterized by X‐ray diffraction.     

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

Cationic d⁰ group 6 olefin metathesis catalysts have been recently shown to display in most instances superior activity in comparison to their neutral congeners. Furthermore, their catalytic performance is greatly improved upon immobilization on silica. In this context, we have developed the new family of molecular cationic molybdenum oxo alkylidene complexes stabilized by N-heterocyclic carbenes of the general formula [Mo(O)(CHCMe₃)(IMes)(OR)[X⁻]] (IMes = 1,3-dimesitylimidazol-2-ylidene; R = 1,3-dimesityl-C₆H₃, C₆F₅; X⁻ = B(3,5-(CF₃)₂C₆H₃)₄⁻, B(Ar^F)₄, tetrakis(perfluoro-t-butoxy)aluminate (PFTA)). Immobilization of [Mo(O)(CHCMe₃)(IMes)(O-1,3-dimesityl-C₆H₃)⁺B(Ar^F)₄⁻] on silica via surface organometallic chemistry yields an active alkene metathesis catalyst that shows the highest productivity towards terminal olefins amongst all existing molybdenum oxo alkylidene catalysts.

The first cationic molybdenum oxo complexes were synthesized and immobilized on partially dehydroxylated silica. Vastly enhanced catalytic activity for terminal olefins was found compared to their neutral congeners.     

Thermal Dehydrogenation of Base‐Stabilized B2H5+ Complexes and Its Role in CH Borylation

Thermally induced dehydrogenation of the H‐bridged cation L₂B₂H₅⁺ (L=Lewis base) is proposed to be the key step in the intramolecular C? H borylation of tertiary amine boranes activated with catalytic amounts of strong “hydridophiles”. Loss of H₂ from L₂B₂H₅⁺ generates the highly reactive cation L₂B₂H₃⁺, which in its sp²‐sp³ diborane(4) form then undergoes either an intramolecular C? H insertion with B? B bond cleavage, or captures BH₃ to produce L₂B₃H₆⁺. The effect of the counterion stability on the outcome of the reaction is illustrated by formation of LBH₂C₆F₅ complexes through disproportionation of L₂B₂H₅⁺ HB(C₆F₅)₃^?.     

Thermal Dehydrogenation of Base‐Stabilized B2H5+ Complexes and Its Role in C?H Borylation

Thermally induced dehydrogenation of the H‐bridged cation L₂B₂H₅⁺ (L=Lewis base) is proposed to be the key step in the intramolecular C H borylation of tertiary amine boranes activated with catalytic amounts of strong “hydridophiles”. Loss of H₂ from L₂B₂H₅⁺ generates the highly reactive cation L₂B₂H₃⁺, which in its sp²‐sp³ diborane(4) form then undergoes either an intramolecular C H insertion with B B bond cleavage, or captures BH₃ to produce L₂B₃H₆⁺. The effect of the counterion stability on the outcome of the reaction is illustrated by formation of LBH₂C₆F₅ complexes through disproportionation of L₂B₂H₅⁺ HB(C₆F₅)₃⁻.     

Latent ruthenium initiators containing fluoro aryloxide ligands

A new ligand, methyl 2,3,5,6-tetrafluoro-4-oxybenzoate (C₈H₃F₄O₃), combining an electron withdrawing group (C₆F₄) to tune the reactivity with an anchor group (CO₂Me) for immobilization on supports, was used to prepare four new ruthenium initiators, viz. Ru(C₈H₃F₄O₃)₂(CHPh)(3-Br-C₅H₅N)(H₂IMes) and Ru(C₈H₃F₄O₃)₂XL, where X = C,N-(CHCH₂CH₂-2-C₅H₄N) and L = PⁱPr₃, PCy₃ or H₂IMes. The new ligand greatly reduced the reactivity of the ruthenium centre at room temperature. The ¹H NMR and DSC investigation for the ROMP of norbornene dicarboximide monomers clearly demonstrated that the Ru(C₈H₃F₄O₃)₂XL initiators were inactive at room temperature and required elevated temperatures for their activation.     

Comparison of Characteristic Structural Features among the Triade of Tris(cyclopentadienyl)(Group-4 Metal) Complex Cations: a Combined Theoretical and Experimental Study

A density functional theory computational chemistry study has revealed a fundamental structural difference between [Ti(Cp)₃]⁺ and its congeners [Zr(Cp)₃]⁺ and [Hf(Cp)₃]⁺/(Cp=cyclopentadienyl). Whereas the latter two are found to contain three uniformely η⁵-coordinated Cp ligands (3η⁵-structural type), [Ti(Cp)₃]⁺ is shown to prefer a 2η⁵η² structure. [Ti(Cp)₃]⁺[B(C₆F₅)₃(Me)]⁻ ( 10 ⋅[B(C₆F₅)₃(Me)]⁻) was experimentally generated by treatment of [Ti(Cp)₃(Me)] ( 7a ) with B(C₆F₅)₃ (Scheme 3). Low-temperature ¹H-NMR spectroscopy in CDFCl₂ (143 K, 600 MHz; Fig. 8) showed a splitting of the Cp resonance into five lines in a 2 : 5 : 2 : 5 : 1 ratio which would be in accord with the theoretically predicted 2η⁵η²-type structure of [Ti(Cp)₃]⁺. The precursor [Ti(Cp)₃(Me)] ( 7a ) exhibits two ¹H-NMR Cp resonances in a 10 : 5 ratio in CD₂Cl₂ at 223 K. Treatment of [HfCl(Cp)₂(Me)] ( 6c ) with sodium cyclopentadienide gave [Hf(Cp)₃(Me)] ( 7c ) (Scheme 1). Its reaction with B(C₆F₅)₃ furnished the salt [Hf(Cp)₃]⁺[B(C₆F₅)₃(Me)]⁻ ( 8 ⋅[B(C₆F₅)₃(Me)]⁻), which reacted with tert-butyl isocyanide to give the cationic complex [Hf(Cp)₃(C=N−CMe₃)]⁺ ( 9a ; with counterion [B(C₆F₅)₃(Me)]⁻ (Scheme 2). Complex cation 9a was characterized by X-ray diffraction (Fig. 7). Its Hf(Cp₃) moiety is of the 3η⁵-type. The structure is distorted trigonal-pyramidal with an average D−Hf−D angle of 118.8° and an average D−Hf−C(1) angle of 96.5° (D denotes the centroids of the Cp rings; Table 6). Cation 9a is a typical d⁰-isocyanide complex exhibiting structural parameters of the C≡N−CMe₃ group (d(C(1)−N(2))=1.146 (5) Å; IR: v˜(C≡N) 2211 cm⁻¹) very similar to free uncomplexed isonitrile. Analogous treatment of 8 with carbon monoxide yielded the carbonyl (d⁰-group-4-metal) complex [Hf(Cp)₃(CO)]⁺ ( 9b ; with counterion [B(C₆F₅)₃(Me)]⁻) (Scheme 2) that was also characterized by X-ray crystal-structure analysis (Fig. 6). Complex 9b is also of the 3η⁵-structural type, similar to the peviously described cationic complex [Zr(Cp)₃(CO)]⁺, and exhibits properties of the CO ligand (d(C−O)=1.11 (2) Å; IR: v˜(C≡O) 2137 cm⁻¹) very similar to the free carbon monoxide molecule.     

Ir(PCy3)2(H)2(H2B-NMe2)]+ as a latent source of aminoborane: probing the role of metal in the dehydrocoupling of H3B⋅NMe2H and retrodimerisation of [H2BNMe2]2

The Ir^III fragment {Ir(PCy₃)₂(H)₂}⁺ has been used to probe the role of the metal centre in the catalytic dehydrocoupling of H₃B?NMe₂H ( A ) to ultimately give dimeric aminoborane [H₂BNMe₂]₂ ( D ). Addition of A to [Ir(PCy₃)₂(H)₂(H₂)₂][BAr^F₄] ( 1 ; Ar^F=(C₆H₃(CF₃)₂), gives the amine‐borane complex [Ir(PCy₃)₂(H)₂(H₃B?NMe₂H)][BAr^F₄] ( 2 a ), which slowly dehydrogenates to afford the aminoborane complex [Ir(PCy₃)₂(H)₂(H₂B? NMe₂)][BAr^F₄] ( 3 ). DFT calculations have been used to probe the mechanism of dehydrogenation and show a pathway featuring sequential BH activation/H₂ loss/NH activation. Addition of D to 1 results in retrodimerisation of D to afford 3 . DFT calculations indicate that this involves metal trapping of the monomer–dimer equilibrium, 2 H₂BNMe₂ ? [H₂BNMe₂]₂. Ruthenium and rhodium analogues also promote this reaction. Addition of MeCN to 3 affords [Ir(PCy₃)₂(H)₂(NCMe)₂][BAr^F₄] ( 6 ) liberating H₂B? NMe₂ ( B ), which then dimerises to give D . This is shown to be a second‐order process. It also allows on‐ and off‐metal coupling processes to be probed. Addition of MeCN to 3 followed by A gives D with no amine‐borane intermediates observed. Addition of A to 3 results in the formation of significant amounts of oligomeric H₃B?NMe₂BH₂?NMe₂H ( C ), which ultimately was converted to D . These results indicate that the metal is involved in both the dehydrogenation of A , to give B , and the oligomerisation reaction to afford C . A mechanism is suggested for this latter process. The reactivity of oligomer C with the Ir complexes is also reported. Addition of excess C to 1 promotes its transformation into D , with 3 observed as the final organometallic product, suggesting a B? N bond cleavage mechanism. Complex 6 does not react with C , but in combination with B oligomer C is consumed to eventually give D , suggesting an additional role for free aminoborane in the formation of D from C .     

New Reactivity Patterns in 3H-Phosphaallene Chemistry [Aryl-P=C=C(H)-tBu]: Hydroboration of the C=C Bond,Deprotonation and Trimerisation

3H-Phosphaallenes, R−P=C=C(H)C−R’ ( 3 ), are accessible in a multigram scale on a new and facile route and show a fascinating chemical reactivity. BH₃(SMe₂) and 3 a (R=Mes*, R’=tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane ( 7 ) with the B atom bound to two P=C double bonds. This compound represents a new FLP based on a B and two P atoms. The increased Lewis acidity of the B atom led to a different reaction course upon treatment of 3 a with H₂B-C₆F₅(SMe₂). Hydroboration of a C=C bond of a first phosphaallene is followed in a typical FLP reaction by the coordination of a second phosphaallene molecule via B−C and P−B bond formation to yield a BP₂C₂ heterocycle ( 8 ). Its B−P bond is short and the B-bound P atom has a planar surrounding. Treatment of 3 a with tBuLi resulted in deprotonation of the β-C atom of the phosphaallene ( 9 ). The Li atom is bound to the P atom as demonstrated by crystal structure determination, quantum chemical calculations and reactions with HCl, Cl-SiMe₃ or Cl-PtBu₂. The thermally unstable phosphaallene Ph−P=C=C(H)-tBu gave a unique trimeric secondary product by P−P, P−C and C−C bond formation. It contains a P₂C₄ heterocycle and was isolated as a W(CO)₄ complex with two P atoms coordinated to W ( 15 ).     

Do Rhodium Bis(σ‐amine‐borane) Complexes Play a Role as Intermediates in Dehydrocoupling Reactions of Amine‐boranes?

The recently synthesized rhodium complex [Rh{P(C₅H₉)₂(η²‐C₅H₇)}(Me₂HNBH₃)₂]BAr^F₄ ( 2 ), which incorporates two amine‐boranes coordinated to the rhodium center with two different binding modes, namely η¹ and η², has been used to probe whether bis(σ‐amine‐borane) motifs are important in determining the general course of amine‐boranes dehydrocoupling reactions. DFT calculations have been carried out to explore mechanistic alternatives that ultimately lead to the formation of the amine‐borane cyclic dimer [BH₂NMe₂]₂ ( A ) by hydrogen elimination. Sequential concerted, on‐ or off‐metal, intramolecular dehydrogenations provide two coordinated amine‐borane molecules. Subsequent dimerization is likely to occur off the metal in solution. In spite of the computationally confirmed presence of a BH???NH hydrogen bond between amine‐borane ligands, neither a simple intermolecular route for dehydrocoupling of complex 2 is operating, nor seems [Rh{P(C₅H₉)₂(η²‐C₅H₇)} B ]⁺ to be important for the whole dehydrocoupling process.     

Isolation of a Three‐Coordinate Boron Cation with a Boron–Sulfur Double Bond

The reaction of the bulky bis(imidazolin‐2‐iminato) ligand precursor (1,2‐(L^MesNH)₂‐C₂H₄)[OTs]₂ ( 1 ²⁺ 2[OTs]^?; L^Mes=1,3‐dimesityl imidazolin‐2‐ylidene, OTs=p‐toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2‐(L^MesN)₂‐C₂H₄)BH₂[OTs] ( 2 ⁺ [OTs]^?)_. The boronium cation 2 ⁺ [OTs]^? reacts with elemental sulfur to give the thioxoborane salt (1,2‐(L^MesN)₂‐C₂H₄)BS[OTs] ( 3 ⁺ [OTs]^?). The hitherto unknown compounds 1 ²⁺ 2[OTs]^?, 2 ⁺ [OTs]^?, and 3 ⁺ [OTs]^? were fully characterized by spectroscopic methods and single‐crystal X‐ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2 ⁺ and 3 ⁺. The theoretical, as well as crystallographic studies reveal that 3 ⁺ is the first example for a stable cationic complex of three‐coordinate boron that bears a B?S double bond.     

Hydrolysis of hydro(pyrrolyl-1)borates

The hydrolysis of hydro(pyrrolyl-l)borates ([BH_n(NC₄H₄)_4-n]⁻, n = 1,2,3) can be treated as a kinetically one-step reaction outside of the mildly acidic region. In strongly acidic medium the hydrolysis takes place in a stepwise manner; the intermediates (boranes and the cationic boron compounds) being hydrolyzed more slowly than the borate anion. In the first step of the hydrolysis of [BH₃(NC₄H₄)]⁻ the B---H bond, while in case of [BH₂(NC₄H₄)₂]⁻ and [BH(NC₄H₄)₃]⁻ the B---N bond is breaking.In neutral and mildly alkaline medium, the hydrolysis is a general acid catalyzed reaction (A---S_E2 mechanism). It becomes to a special H⁺-ion catalyzed reaction (A-1 mechanism) in strongly alkaline region since the protonated intermediate can be reversed to the original borate upon reaction with the OH⁻ ion. The hydrolysis presumably takes place through an intermediate which is protonated on the pyrrolyl nitrogen. Concomitant to the hydrolysis an isotopic exchange reaction was observed on the C_α and C_β atoms of the pyrrolyl group in heavy water. In the hydrolysis of the [BH₃(NC₄H₄)]⁻-anion the N-protonated intermediate is assumed to be able to reverse to the original borate even in acidic or neutral region, at least in part.