Syntheses and Characterization of (C2F5)3BCO and (C3F7)3BCO

The new tris(perfluoroalkyl)borane carbonyls, (C₂F₅)₃BCO and (C₃F₇)₃BCO, were prepared by means of a novel synthetic route using commercially available precursors by reacting K[(C₂F₅)₃BCOOH] and K[(C₃F₇)₃BCOOH] with concentrated sulfuric acid in the last step. The carboxylic acids, K[(C₂F₅)₃BCOOH] and K[(C₃F₇)₃BCOOH], were prepared by oxidative cleavage of the C?C triple bonds in Cs[(C₂F₅)₃BC?CPh] and Cs[(C₃F₇)₃BC?CPh] in a two‐step process to yield K[(C₂F₅)₃BCO? COPh] and K[(C₃F₇)₃BCO? COPh] as isolable intermediates. Crystal structures were obtained of K[(C₂F₅)₃BCO? COPh], K[(C₂F₅)₃BCOOH] ? H₂O, (C₂F₅)₃BCO, K[(C₃F₇)₃BCOOH] ? 2 H₂O, and (C₃F₇)₃BCO. In the crystal structures of (C₂F₅)₃BCO and (C₃F₇)₃BCO the C?O bond lengths are 1.109(2) and 1.103(5) Å, respectively, which are among the shortest observed to date. Tris(pentafluoroethyl)borane carbonyl and (C₃F₇)₃BCO slowly decompose at room temperature to yield CO, difluoroperfluoroalkylboranes and perfluoroalkenes. The decomposition of (C₂F₅)₃BCO was found to follow a first‐order rate law with E_a=107 kJ mol^?1.     

Adducts of (C6F5)3B with Selected Nitrogen

The Lewis acid (C₆F₅)₃B was reacted with ICN, NH₂CN, C₃N₃X₃ (X = H, Cl, F). The resulting Lewis acid base adducts ( 1—5 ) were fully characterized by analytic and spectroscopic methods. Additionally, the structures of the adducts 1—4 were determined by single crystal X‐ray analyses. It has been qualitatively shown, that a high field shift of the ¹¹B as well as the ¹⁹F NMR resonances of the o‐F atoms of the C₆F₅‐substituents suggests a longer B—N distance.     

B(C6F5)3-Enabled Synthesis of a Cyclic cis-Arsaphosphene

The synthesis and characterization of an (arsino)phosphaketene, As(PCO){[N(Dipp)](CH₂)}₂ (Dipp=2,6-diisopropylphenyl) is reported along with its subsequent reactivity with B(C₆F₅)₃. When reacted in a stoichiometric ratio, B(C₆F₅)₃ drove the insertion of the P=C bond of the phosphaketene into one of the As−N bonds of the arsino functionality, affording an acid-stabilized, seven-membered, cyclic arsaphosphene. In contrast, when catalytic amounts of B(C₆F₅)₃ were employed, dimeric species, which formed through a formal [2+2] cycloaddition of the cyclic arsaphosphene, were generated. The cyclic arsaphosphene product represents the first example of such a compound in which the two substituents are arranged in a cis-configuration.     

Solution structure, dynamics and speciation of perfluoroaryl boranes through H, B and F NMR spectroscopy

Multinuclear NMR spectroscopy has been used to study the solution properties of several perfluoroaryl borane derivatives. The information obtained from all the NMR active isotopes present in these molecules made it possible to establish not only chemical identity, structure and dynamics of their reaction products, but also the complexity of the solution speciation. In other words, multinuclear NMR helped in unravelling the real forms in which they are present in solution, that in some cases can change dramatically according to even slight changes of the solution conditions. Examples will be presented related to the chemistry of tris(pentafluorophenyl)borane, B(C₆F₅)₃, and bis(pentafluorophenyl)borinic acid, B(C₆F₅)₂OH.     

Interaction of Metal Oxido Compounds with B(C6F5)3

Lewis acid–base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C₆F₅)₃. It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.     

B(C6F5)3- vs Al(C6F5)3-derived metallocenium ion pairs. Structural, thermochemical, and structural dynamic divergences

The thermodynamic and structural characteristics of Al(C6F(5)3-derived vs B(C6F5)3-derived group 4 metallocenium ion pairs are quantified. Reaction of 1.0 equiv of B(C6F5)3 or 1.0 or 2.0 equiv of Al(C6F5)3 with rac-C2H4(eta5-Ind)2Zr(CH3)2 (rac-(EBI)Zr(CH3)2) yields rac-(EBI)Zr(CH3)(+)H3CB(C6)F5)(3)(-) (1a), rac-(EBI)Zr(CH3)+H3CAl(C6F5)(3)(-) (1b), and rac-(EBI)Zr2+[H3CAl(C6F5)3](-)(2) (1c), respectively. X-ray crystallographic analysis of 1b indicates the H3CAl(C6F5)(3)(-) anion coordinates to the metal center via a bridging methyl in a manner similar to B(C6F5)3-derived metallocenium ion pairs. However, the Zr-(CH3)(bridging) and Al-(CH3)(bridging) bond lengths of 1b (2.505(4) A and 2.026(4) A, respectively) indicate the methyl group is less completely abstracted in 1b than in typical B(C6F5)3-derived ion pairs. Ion pair formation enthalpies (DeltaH(ipf)) determined by isoperibol solution calorimetry in toluene from the neutral precursors are -21.9(6) kcal mol(-1) (1a), -14.0(15) kcal mol(-1) (1b), and -2.1(1) kcal mol(-1) (1b-->1c), indicating Al(C6F5)3 to have significantly less methide affinity than B(C6F5)3. Analogous experiments with Me2Si(eta5-Me4C5)(t-BuN)Ti(CH3)2 indicate a similar trend. Furthermore, kinetic parameters for ion pair epimerization by cocatalyst exchange (ce) and anion exchange (ae), determined by line-broadening in VT NMR spectra over the range 25-75 degrees C, are DeltaH++(ce) = 22(1) kcal mol(-1), DeltaS++(ce) = 8.2(4) eu, DeltaH++(ae) = 14(2) kcal mol(-1), and DeltaS++(ae) = -15(2) eu for 1a. Line broadening for 1b is not detectable until just below the temperature where decomposition becomes significant ( approximately 75-80 degrees C), but estimation of the activation parameters at 72 degrees C gives DeltaH++(ce) approximately 22 kcal mol(-1)and DeltaH++(ae) approximately 16 kcal mol(-1), consistent with the bridging methide being more strongly bound to the zirconocenium center than in 1a.     

B(C6F5)3: A Lewis Acid that Brings the Light to the Solid State

The straightforward coordination of the Lewis acid B(C₆F₅)₃ to classical, non‐emitting aldehydes results in solid‐state photoluminescence. Variation of the electronic properties of the carbonyl moieties lead to the modulation of the solid‐state emission colors, covering the entire visible spectrum with quantum yields up to 0.64. Steady‐state spectroscopy in combination with X‐ray diffraction analysis and DFT calculations confirm that intermolecular interactions between the Lewis adducts are responsible for the observed luminescence. Alteration of the latter interactions induces, moreover, remarkable solid‐state phenomena such as piezochromism. The versatility and simplicity of our approach facilitate the future development of solid‐state emitting materials.     

How Frustrated Lewis Acid/Base Systems Pass through Transition‐State Regions: H2 Cleavage by [tBu3P/B(C6F5)3]

We investigate the transition‐state (TS) region of the potential energy surface (PES) of the reaction tBu₃P+H₂+B(C₆F₅)₃→tBu₃P‐H⁽⁺⁾+^(?)H?B(C₆F₅)₃ and the dynamics of the TS passage at room temperature. Owing to the conformational inertia of the phosphane???borane pocket involving heavy tBu₃P and B(C₆F₅)₃ species and features of the PES E(P???H, B???H | B???P) as a function of P???H, B???H, and B???P distances, a typical reactive scenario for this reaction is a trajectory that is trapped in the TS region for a period of time (about 350 fs on average across all calculated trajectories) in a quasi‐bound state (scattering resonance). The relationship between the timescale of the TS passage and the effective conformational inertia of the phosphane???borane pocket leads to a prediction that isotopically heavier Lewis base/Lewis acid pairs and normal counterparts could give measurably different reaction rates. Herein, the predicted quasi‐bound state could be verified in molecular collision experiments involving femtosecond spectroscopy.     

The Formation of Imidazolium Salt Intimate (Contact) Ion Pairs in Solution

1‐n‐Butyl‐2,3‐dimethylimidazolium (BMMI) ionic liquids (ILs) associated with different anions undergo H/D exchange preferentially at 2‐Me group of the imidazolium in deuterated solvents. This process is mainly related to the existence of ion pairs rather than the anion basicity. The H/D exchange occurs in solvents (CDCl₃ and MeCN for instance) in which intimate contact ion pairs are present and the anion possesses a labile H in its structure, such as hydrogen carbonate and prolinate. In D₂O, separated ion pairs are formed and the H/D exchange does not occur. A plausible catalytic cycle is that the IL behaves as a neutral base in the course of all H/D exchange processes. NMR experiments, density functional calculations, and molecular dynamics simulations corroborate these hypotheses.     

Multiple‐pathways of carbon dioxide binding by a Lewis acid [B(C6F5)3] and a lewis base [P(tBu)3]: The energy landscape perspective

Using the reaction‐relevant two‐dimensional potential energy surface, an accurate reaction‐pathway mapping and ab inito molecular dynamics, it is shown that CO₂ capture by P(tBu)₃ and B(C₆F₅)₃ species has many nearly degenerate reaction‐routes to take. The explanation of that is in the topography of the transition state (saddle) area. An ensemble of asynchronous reaction‐routes of CO₂ binding is described in fine detail. © 2013 Wiley Periodicals, Inc.     

1H, (19)F, and (31)P PGSE NMR diffusion studies on chiral organic salts: ion pairing and the dependence of a diffusion value on diastereomeric structure

1H, (19)F and (31)P pulsed field gradient spin-echo (PGSE) diffusion studies on chiral organic salts that contain hexacoordinate phosphate anions, namely tris(tetrachlorobenzenediolato)phosphate(V) (TRISPHAT) and bis(tetrachlorobenzenediolato)mono([1,1']binaphthalenyl-2,2'diolato)-phosphate(V) (BINPHAT), are reported. The first example of the dependence of a diffusion value on diastereomeric structure is presented. Marked solvent and concentration effects on the diffusion constants (D) of these salts are noted and the question of ion pairing is discussed.     

Steric and electronic effects on the heterolytic H2‐splitting by phosphine‐boranes R3B/PR′3 (R = C6F5, Ph; R′ = C6H2Me3, tBu,Ph, C6F5, Me,H): A computational study

The steric and electronic effects exerted by the substituents R/R′ on the heterolytic H₂‐splitting by phosphine‐boranes R₃B/PR′₃ [R = C₆F₅ ( 1 ), Ph ( 2 ); R′ = C₆H₂Me₃ ( a ), tBu ( b ), Ph ( c ), C₆F₅ ( d ), Me ( e ), H ( f )] have been studied by performing quantum mechanical density functional theory and RI‐MP2 calculations. Energy decomposition analyses based on the block‐localized wavefunction method show that the nature of the interaction between R₃B and PR′₃ is strongly dependent on the B? P distance. With short B? P distances (～2.1 Å), the strength of Lewis pairs results from the balance among various energy terms, and both strong and weak dative bonds can be found in this group. However, at long B? P distances (>4.0 Å), the correlation and dispersion energy (ΔE_corr) dominates. In other words, the van der Waals (vdW) interaction rules these weakly bound complexes. No ion‐pair structures of 1f and 2c – 2f can be located as they instantly converge to vdW complexes R₃B···H₂···PR′₃. We thus propose a model, which predicts that when the sum (E_hp) of the hydride affinity (HA) of BR₃ and the proton affinity (PA) of PR′₃ is higher than 340.0 kcal/mol, the ion‐pair [R₃BH^?][HPR′] can be observed, whereas with E_hp below this value, the ion pair would instantly undergo the combination of proton and hydride with the release of H₂. The overall reaction energies ( 1a – 1e and 2a – 2b ) can be best described by a fitting equation with HA(BR₃), PA(PR′₃), and the binding energy ΔE_b(BR₃/PR′₃) as predictor variables: ΔE_R([R₃BH^?][HPR′]) = ?0.779HA(BR₃) ? 0.695PA(PR′₃) ? 1.331 ΔE (BR₃/PR′₃) + 245.3 kcal/mol. The fitting equation provides quantitative insights into the steric and electronic effects on the thermodynamic aspects of the heterolytic H₂‐splitting reactions. The electronic effects are reflected by HA(BR₃) and PA(PR′₃), and ΔE_b can be significantly influenced by the steric overcrowding. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

19F NMR Investigations of the Reaction of B(C6F5)3 with Different Tri(alkyl)aluminum Compounds

Tris(pentafluorophenyl)borane, B(C₆F₅)₃ reacts with triethylaluminum, AlEt₃ to a mixture of Al(C₆F₅)_3−nEt_n and Al₂(C₆F₅)_6−nEt_n compounds depending on the B/Al ratio. From excess borane to excess AlEt₃ the species Al(C₆F₅)₃ → Al(C₆F₅)₂Et Al₂(C₆F₅)₄Et₂ → Al₂(C₆F₅)₃Et₃ → Al₂(C₆F₅)₂Et₄ → Al₂(C₆F₅)Et₅ are formed and differentiated by their para-F signal in ¹⁹F NMR. The reaction between B(C₆F₅)₃ and the higher aluminum alkyls, tri(iso-butyl)aluminum and tri(n-hexyl)aluminum AlR₃ (R = i-Bu, n-C₆H₁₃) is slower and requires AlR₃ excess to shift the C₆F₅ R exchange equilibria to almost complete formation of Al(C₆F₅)R₂ and BR₃. At equimolar ratio the equilibrium lies on the side of the unchanged borane together with its boranate [B(C₆F₅)₃R]⁻ anion. For tri(n-octyl)aluminum even at large Al(n-C₈H₁₇)₃ excess no C₆F₅ alkyl exchange can be observed, but boranate anions form.     

Copolymerization of propylene and 1,5‐hexadiene with stereospecific metallocene/Al(i‐Bu)3/[Ph3C][B(C6F5)4]

Copolymerizations of propylene (P) with 1,5‐hexadiene (1,5‐HD) were carried out with isospecific rac‐1,2‐ethylenebis(1‐indenyl)Zr(NMe₂)₂ [rac‐(EBI)Zr(NMe₂)₂, 1] and syndiospecific isopropylidene(cyclopentadienyl)(9‐fluorenyl)ZrMe₂ [i‐Pr(Cp)(Flu)ZrMe₂, 2] compounds combined with Al(i‐Bu)₃/[Ph₃C][B(C₆F₅)₄] as a cocatalyst system. Microstructures of poly(propylene‐co‐1,5‐HD) were determined by ¹H NMR, ¹³C NMR, Raman spectroscopies and X‐ray powder diffraction. The isospecific 1/Al(i‐Bu)₃/[Ph₃C][B(C₆F₆)₄] catalyst showed much higher polymerization rate than 2/Al(i‐Bu)₃/[Ph₃C][B(C₆F₆)₄] system, however, the latter system showed higher incorporation of 1,5‐HD (r_P = 8.85, r_1,5‐HD = 0.274) than the former system (r_P = 16.25, r_1,5‐HD = 0.34). The high value of r_P × r_1,5‐HD far above 1 demonstrated that the copolymers obtained by both catalysts are somewhat blocky. The insertion of 1,5‐HD proceeded by enantiomorphic site control; however, the diastereoselectivity of the intramolecular cyclization reaction of 1,2‐inserted 1,5‐HD was independent of the stereospecificity of metallocene compounds, but dependent on the concentration of 1,5‐HD in the feed. The insertion of the monomers by enantiomorphic site control could also be realized by Raman spectroscopy and X‐ray powder diffraction of the polymers. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1590–1598, 2000     

Synthesis and reactivity of N-heterocycle-B(C6F5)3 complexes. 4. Competition between pyridine- and pyrrole-type substrates toward B(C6F5)3: structure and dynamics of 7-B(C6F5)3-7-azaindole and [7-Azaindolium]+[HOB(C6F5)3]-

Reaction between 7-azaindole and B(C6F5)3 quantitatively yields 7-(C6F5)3B-7-azaindole (4), in which B(C6F5)3 coordinates to the pyridine nitrogen of 7-azaindole, leaving the pyrrole ring unreacted even in the presence of a second equivalent of B(C6F5)3. Reaction of 7-azaindole with H2O-B(C6F5)3 initially produces [7-azaindolium]+[HOB(C6F5)3]- (5) which slowly converts to 4 releasing a H2O molecule. Pyridine removes the borane from the known complexes (C6F5)3B-pyrrole (1) and (C6F5)3B-indole (2), with formation of free pyrrole or indole, giving the more stable adduct (C6F5)3B-pyridine (3). The competition between pyridine and 7-azaindole for the coordination with B(C6F5)3 again yields 3. The molecular structures of compounds 4 and 5 have been determined both in the solid state and in solution and compared to the structures of other (C6F5)3B-N-heterocycle complexes. Two dynamic processes have been found in compound 4. Their activation parameters (DeltaH = 66 (3) kJ/mol, DeltaS = -18 (10) J/mol K and DeltaH = 76 (5) kJ/mol, DeltaS = -5 (18) J/mol K) are comparable with those of other (C6F5)3B-based adducts. The nature of the intramolecular interactions that result in such energetic barriers is discussed.