Small Chains of Main Group Elements by BH3 Adduct Formation of tBu2E-N(H)-EtBu2 (E = P,As)

Borane adducts of bis(di-tert-butylphosphanyl)amine ( 1a ) and bis(di-tert-butylarsino)amine ( 1b ) are reported. Based on quantum-chemical investigations in combination with experimental results, it is demonstrated that the tautomerism known for tBu₂P-N(H)-PtBu₂ ( 1a ), can be observed for the mono adduct tBu₂P-N(H)-P(BH₃)tBu₂ ( 2a ) as well, whereas for the corresponding arsenic compound 2b only one stable isomer is found. The bis-borane adduct tBu₂(BH₃)As-N(H)-As(BH₃)tBu₂ ( 3b ) is a rare example of a structurally characterized, tertiary arsine borane adduct, which can be directly compared with the corresponding phosphorus compound tBu₂(BH₃)P-N(H)-P(BH₃)tBu₂ ( 3a ). Deprotonation of mixtures containing 2a by nBuLi leads to the lithium-containing coordination polymer 4a , in which the actual chain consists only of non-carbon atoms.     

Isolation and Characterization of Lewis Base Stabilized Monomeric Parent Stibanylboranes

The synthesis of the Lewis base stabilized monomeric parent compound of stibanylboranes, “H₂Sb? BH₂”, is reported. Through a salt metathesis route, the silyl‐substituted compounds (Me₃Si)₂Sb? BH₂?LB (LB=NMe₃, NHC^Me) were synthesized as representatives of derivatives with a Sb? B σ bond. Under very mild conditions, they could be transformed into the target compounds Me₃N?H₂B? HSb? BH₂?NMe₃ and H₂Sb? BH₂?NHC^Me, respectively. The products were characterized by X‐ray structure analysis, NMR spectroscopy, IR spectroscopy, and mass spectrometry. DFT calculations give further insight into the stability and bonding of these unique compounds.     

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 (Δ_rG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl₂⁺) and the four‐membered boracycle [(CH₂{NC₅H₃Me})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‐(BHCl₂CH₂)‐6‐Me(C₅H₃NH) 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).     

Platinum Oxoboryl Complexes as Substrates for the Formation of 1:1, 1:2, and 2:1 Lewis Acid–Base Adducts and 1,2‐Dipolar Additions

The oxoboryl complex trans‐[(Cy₃P)₂BrPt(B?O)] ( 2 ) reacts with the Group 13 Lewis acids EBr₃ (E=Al, Ga, In) to form the 1:1 Lewis acid–base adducts trans‐[(Cy₃P)₂BrPt(B?OEBr₃)] ( 6 – 8 ). This reactivity can be extended by using two equivalents of the respective Lewis acid EBr₃ (E=Al, Ga) to form the 2:1 Lewis acid–base adducts trans‐[(Cy₃P)₂(Br₃Al‐Br)Pt(B?OAlBr₃)] ( 18 ) and trans‐[(Cy₃P)₂(Br₃Ga‐Br)Pt(B?OGaBr₃)] ( 15 ). Another reactivity pattern was demonstrated by coordinating two oxoboryl complexes 2 to InBr₃, forming the 1:2 Lewis acid–base adduct trans‐[{(Cy₃P)₂BrPt(B?O)}₂InBr₃] ( 20 ). It was also possible to functionalize the B?O triple bond itself. Trimethylsilylisothiocyanate reacts with 2 in a 1,2‐dipolar addition to form the boryl complex trans‐[(Cy₃P)₂BrPt{B(NCS)(OSiMe₃)}] ( 27 ).     

α‐Hydroxymethylation of Pyridines at a Frustrated Lewis Pair Template

The “η²‐formylborane” moiety formed by CO reduction with HB(C₆F₅)₂ at a P/B frustrated Lewis pair template undergoes a hydroxymethylation reaction at the α‐position to nitrogen in pyridine or isoquinoline. The analogous reaction with pyrimidine revealed a mechanism related to the Tschitschibabin reaction.     

Regioregular Synthesis of Azaborine Oligomers and a Polymer with a syn Conformation Stabilized by NH⋅⋅⋅π Interactions

The regioregular synthesis of the first azaborine oligomers and a corresponding conjugated polymer was accomplished by Suzuki–Miyaura coupling methods. An almost perfectly coplanar syn arrangement of the heterocycles was deduced from an X‐ray crystal structure of the dimer, which also suggested that N? H???π interactions play an important role. Computational studies further supported these experimental observations and indicated that the electronic structure of the longer azaborine oligomers and polymer resembles that of poly(cyclohexadiene) more than poly(p‐phenylene). A comparison of the absorption and emission properties of the polymer with those of the oligomers revealed dramatic bathochromic shifts upon chain elongation, thus suggesting highly effective extension of conjugation.     

π‐Hole Bonds: Boron and Aluminum Lewis Acid Centers

MP2/aug‐cc‐pVTZ calculations were performed on complexes of boron and aluminum trihydrides and trihalides with hydrogen cyanide (ZH₃‐NCH and ZX₃‐NCH; Z=B, Al; X=F, Cl). The complexes are linked through the B???N and Al???N interactions, which are named as triel bonds and which are classified as π‐hole bonds. It was found that they possess numerous characteristics of typical covalent bonds, since they are ruled mainly by processes of the electron charge shift from the Lewis base to the Lewis acid unit. Other configurations of the ZH₃‐NCH and ZX₃‐NCH complexes linked by the dihydrogen, hydrogen, and halogen bonds were found. However, these interactions are much weaker than the corresponding π‐hole bonds. The quantum theory of atoms in molecules and the natural bond orbital approaches were applied to characterize the complexes and interactions analyzed. The crystal structures of triel trihydrides and triel trihalides were also analyzed for comparison with the results of calculations.     

Synthesis and Characterization of the Lewis Acid‐Base Complexes SbCl5 · LB (LB = ICN,BrCN, ClCN, 1/2(CN)2, NH2CN,Pyridine) – a Combined Theoretical and Experimental Investigation. The Crystal Structures of SbCl5 · NCCl and SbCl5 · NCCN · SbCl5

The Lewis acid‐base complexes SbCl₅ · LB (LB = ICN, BrCN, ClCN, ¹/₂(CN)₂, NH₂CN, pyridine) were prepared. The products formed were characterized by Raman and NMR spectroscopy. Density functional theory (B3LYP) was applied to calculate structural and vibrational data. Vibrational assignments of the normal modes for these Lewis acid‐base adducts was made on the basis of their Raman spectra in comparison with computational results. The stability of the complexes was investigated by calculating the bond dissociation enthalpy. SbCl₅ · NCCl and SbCl₅ · NCCN · SbCl₅ were characterized by X‐ray structural analysis. NBO analyses were performed on the crystallographic data.     

Ru–η6‐Arene Cations [{(Ph2PC6H4)2B(η6‐Ph)}RuX]+ (X=Cl,H) as Lewis Acids

The reactivity of [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuCl][B(C₆F₅)₄] ( 1 ) as a Lewis acid was investigated. Treatment of 1 with mono and multidentate phosphorus Lewis bases afforded the Lewis acid–base adducts with the ortho‐carbon atom of the coordinated arene ring. Similar reactivity was observed upon treatment with N‐heterocyclic carbenes; however, adduct formation occurred at both ortho‐ and para‐carbon atoms of the bound arene with the para‐position being favoured by increased steric demands. Interestingly treatment with isocyanides resulted in adduct formation with the B‐centre of the ligand framework. The hydride‐cation [{(Ph₂PC₆H₄)₂B(η⁶‐Ph)}RuH] [B(C₆F₅)₄] was prepared via reaction of 1 with silane. This species in the presence of a bulky phosphine behaves as a frustrated Lewis pair (FLP) to activate H₂ between the phosphorus centre and the ortho‐carbon atom of the η⁶‐arene ring.     

Structural comparisons between analogously substituted gallane–pnictine adducts

The syntheses and solid‐state structures of five Lewis acid–base adducts Et₃Ga–E(SiMe₃)₃ (E = P ( 1 ), As ( 2 ), Sb ( 3 )), i‐Bu(t‐Bu)₂Ga–P(i‐Pr)₃ ( 4 ) and (t‐Bu)₃Ga–As(i‐Pr)₃ ( 5 ) are described and the structural trends observed within these adducts are discussed and compared with those reported for the corresponding alane adducts. Ga? C bond distances and C? Ga? C bond angles were found to be useful structural parameters to estimate the strength of the Lewis acid–base interaction in the solid state. In addition, the solid‐state structure of uncomplexed t‐Bu₃Ga ( 6 ) is reported. Copyright © 2004 John Wiley & Sons, Ltd.     

The nido‐Cage⋅⋅⋅π Bond: A Non‐covalent Interaction between Boron Clusters and Aromatic Rings and Its Applications

Non‐covalent interactions involving multicenter multielectron skeletons such as boron clusters are rare. Now, a non‐covalent interaction, the nido‐cage???π bond, is discovered based on the boron cluster C₂B₉H₁₂^? and an aromatic π system. The X‐ray diffraction studies indicate that the nido‐cage???π bonding presents parallel‐displaced or T‐shaped geometries. The contacting distance between cage and π ring varies with the type and the substituent of the aromatic ring. Theoretical calculations reveal that this nido‐cage???π bond shares a similar nature to the conventional anion???π or π???π bonds found in classical aromatic ring systems. This nido‐cage???π interaction induces variable photophysical properties such as aggregation‐induced emission and aggregation‐caused quenching in one molecule. This work offers an overall understanding towards the boron cluster‐based non‐covalent bond and opens a door to investigate its properties.     

The First Organically Templated Layered Uranium(IV) Fluorides: (H3N(CH2)3NH3)U2F10⋅2 H2O, (H3N(CH2)4NH3)U2F10⋅3 H2O,and (H3N(CH2)6NH3)U2F10⋅2 H2O

A new series of layered organically templated uranium(IV ) fluorides has been exclusively synthesized under hydrothermal conditions. The unprecedented materials contain [U₂F₁₀]²⁻ anionic layers that are separated by charge balancing cationic templates and occluded water molecules (see structure depicted). The templates can be fully ion-exchanged for a variety of metals (Na⁺, K⁺, and Co²⁺) at room temperature.     

Influence of the Li⋅⋅⋅π Interaction on the H/X⋅⋅⋅π Interactions in HOLi⋅⋅⋅C6H6⋅⋅⋅HOX/XOH (X=F,Cl, Br,I) Complexes

The influences of the Li???π interaction of C₆H₆???LiOH on the H???π interaction of C₆H₆???HOX (X=F, Cl, Br, I) and the X???π interaction of C₆H₆???XOH (X=Cl, Br, I) are investigated by means of full electronic second‐order Møller–Plesset perturbation theory calculations and “quantum theory of atoms in molecules” (QTAIM) studies. The binding energies, binding distances, infrared vibrational frequencies, and electron densities at the bond critical points (BCPs) of the hydrogen bonds and halogen bonds prove that the addition of the Li???π interaction to benzene weakens the H???π and X???π interactions. The influences of the Li???π interaction on H???π interactions are greater than those on X???π interactions; the influences of the H???π interactions on the Li???π interaction are greater than X???π interactions on Li???π interaction. The greater the influence of Li???π interaction on H/X???π interactions, the greater the influences of H/X???π interactions on Li???π interaction. QTAIM studies show that the intermolecular interactions of C₆H₆???HOX and C₆H₆???XOH are mainly of the π type. The electron densities at the BCPs of hydrogen bonds and halogen bonds decrease on going from bimolecular complexes to termolecular complexes, and the π‐electron densities at the BCPs show the same pattern. Natural bond orbital analyses show that the Li???π interaction reduces electron transfer from C₆H₆ to HOX and XOH.     

Transfer Hydrocyanation of α‐ and α,β‐Substituted Styrenes Catalyzed by Boron Lewis Acids

A straightforward gram‐scale preparation of cyclohexa‐1,4‐diene‐based hydrogen cyanide (HCN) surrogates is reported. These are bench‐stable but formally release HCN and rearomatize when treated with Lewis acids. For BCl₃, the formation of the isocyanide adduct [(CN)BCl₃]^? and the corresponding Wheland complex was verified by mass spectrometry. In the presence of 1,1‐di‐ and trisubstituted alkenes, transfer of HCN from the surrogate to the C?C double bond occurs, affording highly substituted nitriles with Markovnikov selectivity. The success of this transfer hydrocyanation depends on the Lewis acid employed; catalytic amounts of BCl₃ and (C₆F₅)₂BCl are shown to be effective while B(C₆F₅)₃ and BF₃?OEt₂ are not.