Lewis Acid-base Adducts Research Articles

Density functional theory study of fullerene–carbene Lewis acid–base adducts: critical role of dispersion interactions

Density functional calculations reveal that dispersion attractions play a critical role in the formation of single-bonded fullerene–carbene Lewis acid–base (LA/LB) adducts reported recently, although they do not make an important contribution to regioselective formation. A stable fullerene-based LA/LB adduct is predicted by enhancing dispersion attractions.

The Methylene-bridged Dialuminium Compound R2Al-CH2-AlR2 [R=CH(SiMe3)2] as an Effective Chelating Lewis Acid – Adducts with Halides, Perchlorate and Isopropylamide

The new methylene-bridged dialuminium compound Aryl2Al-CH2-AlAryl2 1b was synthesised from Cl2Al-CH2-AlCl2 and four equivalents of LiAryl [Aryl=C6H2(2,4,6-i-Pr)3]. A 1 : 2 stoichiometric ratio of the starting compounds afforded the dimeric aluminium halide (Aryl2AlCl)2 (2) by dismutation. The coordination behaviour of the related, but more easily available dialuminium compound R2Al-CH2-AlR2 [R=CH(SiMe3)2] 1a was explored. It yields the Lewis acid base adducts [M][R2Al-CH2-AlR2(μ-X)] [3a, M=Li(TMEN)2, X=Cl; 3b, M=Li(TMEN)2, X=Br; 3c, M=K(18-crown-6), X=ClO4; 3d, M=Li(TMEN)2, X=NH(i-Pr)] with X residing in a bridging position between the two Al atoms as evident from NMR spectroscopy and X-ray crystallography (3b, 3c, 3d). Compound 3c is a unique example of a stable, in organic solvents soluble adduct that features a ClO4 - anion coordinated by an alkylaluminium compound via two of its oxygen atoms.

Synthesis and Structural Characterization of Metallogermylenes, Cp-Substituted Germylene, and a Germanium(II)-Borane Adduct from Pyridyl-1-azaallyl Germanium(II) Chloride

The reaction of [{N(SiMe3)C(Ph)C(SiMe3)(C5H4N-2)}GeCl] (1) with Na[M(η5-C5H5)(CO)3]·2DME (M = Mo, W) afforded the metallogermylenes [{N(SiMe3)C(Ph)C(SiMe3)(C5H4N-2)}Ge-M(η5-C5H5)(CO)3] (M = Mo (2), W (3)). Compounds 2 and 3 have been characterized by X-ray crystallography and NMR and IR spectroscopy. Structural analyses of compounds 2 and 3 are consistent with the presence of lone-pair electrons at the germanium(II) center. The Ge–Mo and Ge–W bond distances of 2.875(1) and 2.852(1) Å are consistent with Ge–metal single bonds. The chlorogermylene 1 was also used in the synthesis of a substituted germylene, [{N(SiMe3)C(Ph)C(SiMe3)(C5H4N-2)}Ge(η1-C5H5)] (4), by reaction with sodium cyclopentadienylide. The reaction of compound 1 with tris(pentafluorophenyl)borane led to the formation of a Lewis acid–base adduct, [{N(SiMe3)C(Ph)C(SiMe3)(C5H4N-2)}Ge(Cl)→B(C6F5)3] (5).

Reactivity of (β‐Diketiminato)zinc Hydride toward Lewis Bases, Heterocumulenes and Cyclohexene Oxide

AbstractThe reaction of LZnH (1) {L = Mesnacnac = [(2,4,6‐Me3C6H2)NC(Me)]2CH} with 4‐(dimethylamino)pyridine (dmap) yielded the Lewis acid–base adduct LZn(H)dmap (2), whereas LZn(Cl)dmap (3) was obtained from the equimolar reaction of LZnCl with dmap. In addition, reactions of 1 with di‐tert‐butylcarbodiimide [C(NtBu)2] and tert‐butyl thioisocyanate (tBuNCS) proceeded with insertion of the heterocumulene into the Zn–H bond and formation of [LZn(tBuNC(H)NtBu)] (4) and [LZn(tBuNC(H)S)] (5), whereas 1 reacted with Me3SiN3 with the elimination of Me3SiH and formation of [LZnN3]2 (6). Moreover, the reaction of 1 with cyclohexene oxide (CHO) occurred with ring opening and formation of [LZnOCy]2 (7) (Cy = cyclohexyl). Complexes 2–7 were characterized by multinuclear NMR (1H, 13C) and IR spectroscopy, elemental analyses and by single‐crystal X‐ray diffraction (2, 3, 6, 7).

Nucleophilic boron strikes back

It has been demonstrated that the interaction of a simple Lewis base with tetraalkoxydiboranes generates an unusual nucleophilic character in one of the boryl moieties. The interaction of amines, N-heterocyclic carbenes (NHCs), and alkoxides with a diboron reagent results in the formation of a Lewis acid–base adduct, in which the formally intact sp2 boryl moiety becomes nucleophilic. We describe in this work the application of this new type of nucleophilic boron synthon in the selective preparation of organoboranes through β-boration and diboration reactions.

Metal-Free Hydrogenations

In a chemical first, an international research team has developed a metal-free reaction that hydrogenates aromatic rings to form cyclohexyl derivatives. The achievement could spark a broader range of applications for industrial hydrogenations, which are widely used for processing petroleum and foods. A metal-free aromatic hydrogenation is surprising, says team leader Douglas W. Stephan of the University of Toronto, because it’s exceedingly hard to overcome the additional stability a molecule gains from aromaticity, even with the best transition-metal catalysts. Stephan and his colleagues have done so by using a chemical construct known as a frustrated Lewis pair, which Stephan introduced in 2006. Lewis acid-base adducts are common in chemistry: An electron-deficient Lewis acid readily shares a Lewis base’s spare pair of electrons. However, when the Lewis acid and base have bulky substituents, their ability to form a close relationship is denied, causing the pair to become “frustrated.” But the pair ...

Chloro- and phenoxy-phosphines in frustrated Lewis pair additions to alkynes

The reaction of tBu(C(6)H(4)O(2))P, with the borane B(C(6)F(5))(3) gives rise to NMR data consistent with the formation of the classical Lewis acid-base adduct tBu(C(6)H(4)O(2))P(B(C(6)F(5))(3)) (1). In contrast, the NMR data for the corresponding reactions of tBu(C(20)H(12)O(2))P and Cl(C(20)H(12)O(2))P with B(C(6)F(5))(3) were consistent with the presence of equilibria between free phosphine and borane and the corresponding adducts. Nonetheless, in each case, the adducts tBu(C(20)H(12)O(2))P(B(C(6)F(5))(3)) (2) and Cl(C(20)H(12)O(2))P(B(C(6)F(5))(3)) (3) were isolable. The species 1 reacts with PhCCH to give the new species tBu(C(6)H(4)O(2))P(Ph)C=CHB(C(6)F(5))(3) (4) in near quantitative yield. In an analogous fashion, the addition of PhCCH to solutions of the phosphines tBu(C(20)H(12)O(2))P, tBuPCl(2) and (C(6)H(3)(2,4-tBu(2))O)(3)P each with an equivalent of B(C(6)F(5))(3) gave rise to L(Ph)C=CHB(C(6)F(5))(3) (L = tBu(C(20)H(12)O(2))P 5, tBuPCl(2)6 and (C(6)H(3)(2,4-tBu(2))O)(3)P 7). X-Ray data for 1, 2, 6 and 7 are presented. The implications of these findings are considered.

Synthesis and X-ray crystal structures of heteroleptic zinc amidinate complexes

The synthesis of heteroleptic zinc amidinate complexes is reported. Reactions of LZnI [L = t -BuC(NDipp) 2 ; Dipp = 2,6- i -Pr 2 -C 6 H 3 ] with diisopropylcarbodiimide ( i -PrN) 2 C yielded the Lewis acid-base adduct LZn(I)-N( i -Pr)CN( i -Pr) 1 . In addition, the synthesis of LZnN(SiMe 3 ) 2 2 and its reaction with ( i -PrN) 2 C, which occurred with formation of the heteroleptic amidinate-guanidinate complex LZnL ′ 3 [L ′ = (Me 3 Si) 2 NC(N i -Pr) 2 ], is reported. Compounds 1 – 3 were characterized by nuclear magnetic resonance ( 1 H, 13 C) and infrared spectroscopy, elemental analysis, and single crystal X-ray diffraction ( 1 and 2 ), respectively.

Investigations on the Interaction of Dichloroaluminum Carboxylates with Lewis Bases and Water: an Efficient Road toward Oxo- and Hydroxoaluminum Carboxylate Complexes

A series of dichloroaluminum carboxylates [Cl(2)Al(O(2)CR)](2) (were R = Ph (1a), (t)Bu (1b), CHCH(2) (1c) and C(11)H(23) (1d)) were prepared and extended investigations on their structure and reactivity toward various Lewis bases and H(2)O performed. Compounds [Cl(2)Al(O(2)CR)](2) and their adducts with Lewis bases show a large structural variety, featuring both molecular and ionic forms with different coordination numbers of the metal center and various coordination modes of the carboxylate ligand. Upon addition of a Lewis base of moderate strength the molecular form [Cl(2)Al(O(2)CR)](2) equilibrates with new ionic forms. In the presences of 4-methylpyridine the six-coordinate Lewis acid-base adducts [Cl(2)Al(λ(2)-O(2)CR)(py-Me)(2)] [R = Ph (3a), (t)Bu (3b)] with a chelating carboxylate ligand were formed. The reactions of 1a, 1b, and 1d with 0.33 equiv of H(2)O in THF-toluene solution lead to oxo carboxylates [(Al(3)O)(O(2)CR)(6)(THF)(3)] [AlCl(4)] [where R = Ph (4a(THF)), (t)Bu (4b(THF)), and C(11)H(23) (4d(THF))] in high yield. The similar reaction of 1c in tetrahydrofuran (THF) afforded the chloro(hydroxo)aluminum acrylate [(ClAl)(2)(OH)(O(2)CC(2)H(3))(2) (THF)(4)][AlCl(4)] (5), while the hydrolysis of 1b in MeCN lead to the hydroxoaluminum carboxylate [Al(2)(OH)(O(2)C(t)Bu)(2)(MeCN)(6)][AlCl(4))(3)] (6). All compounds were characterized by elemental analysis, (1)H, (27)Al NMR, and IR spectroscopy, and the molecular structure of 1a, 3a, 3b, 4a(THF), 4b(THF), 4b(py-Me'), 5, and 6 were determined by single-crystal X-ray diffraction. The study provides a platform for testing transformations of secondary building units in Al-Metal-Organic Frameworks toward H(2)O and neutral donor ligands.

Activation of Diboron Reagents with Brønsted Bases and Alcohols: An Experimental and Theoretical Perspective of the Organocatalytic Boron Conjugate Addition Reaction

Bases play an important role in organocatalytic boron conjugate addition reactions. The sole use of MeOH and a base can efficiently transform acyclic and cyclic activated olefins into the corresponding β-borated products in the presence of diboron reagents. Inorganic and organic bases deprotonate MeOH in the presence of diboron reagents. It is concluded, on the basis of theoretical calculations, NMR spectroscopic data, and ESI-MS experiments, that the methoxide anion forms a Lewis acid-base adduct with the diboron reagent. The sp(2) B atom of the methoxide-diboron adduct gains a strongly nucleophilic character, and attacks the electron-deficient olefin. The methanol protonates the intermediate, generating the product and another methoxide anion. This appears to be the simplest method to activate diboron reagents and make them suitable for incorporation into target organic molecules.

An Overview of Lewis Basicity and Affinity Scales

The impossibility of establishing a universal scale of Lewis basicity does not prevent the determination of the quantitative behavior of Lewis bases, thanks to scales constructed against particular Lewis acids: BF3, 4-FC6H4OH, I2, Li+, Na+, K+, Al+, Mn+, CpNi+, and CH3NH3+. These scales encompass important types of bonds formed in a Lewis acid–base adduct: the dative bond, the conventional and ionic hydrogen bonds, the halogen bond and cation–molecule bonds for metal cations of groups 1, 7, 10, 11, and 13. Moreover, although these scales are generally not interrelated, there exist family-dependent relationships that permit ranking, in a rather general order, of bases belonging to a given chemical family, for example, the family of oxygen bases. Therefore, the skepticism about the quantitative usefulness of the Lewis concept of acids and bases is no longer founded.

Synthesis and X‐ray Crystal Structures of Multinuclear Zinc Amidinate Complexes

AbstractThe reactions of Me2Zn with carbodiimides RN=C=NR (R = Et, Ph) yielded the polynuclear zinc amidinate complexes {[MeZn]4[C(RNCNR)4]} [R = Et (1), Ph (3)] and {[MeZn]2[C(EtNCNEt)2(C2(NEt)3)]} (2). In addition, the reaction of {[ClZn]4[C(iPrNCNiPr)4]} with MCl3 gave the corresponding Lewis acid–base adducts {[MCl3]2[ClZn]4[C(iPrNCNiPr)4]} [M = Al (4), Ga (5)]. Compounds 1–5 were characterized by multinuclear NMR (1H, 13C) and IR spectroscopy, elemental analyses and single‐crystal X‐ray diffraction.

Fullerene–Carbene Lewis Acid–Base Adducts

The reaction between a bulky N-heterocylic carbene (NHC) and C(60) leads to the formation of a thermally stable zwitterionic Lewis acid-base adduct that is connected via a C-C single bond. Low-energy absorption bands with weak oscillator strengths similar to those of n-doped fullerenes were observed for the product, consistent with a net transfer of electron density to the C(60) core. Corroborating information was obtained using UV photoelectron spectroscopy, which revealed that the adduct has an ionization potential ∼1.5 eV lower than that of C(60). Density functional theory calculations showed that the C-C bond is polarized, with a total charge of +0.84e located on the NHC framework and -0.84e delocalized on the C(60) cage. The combination of reactivity, characterization, and theoretical studies demonstrates that fullerenes can behave as Lewis acids that react with C-based Lewis bases and that the overall process describes n-doping via C-C bond formation.

Catalytic Electrochemical Reduction of CO2 in Ionic Liquid EMIMBF3Cl

A novel ionic liquid EMIMBF3Cl that contains a BF3Cl− anion has been synthesized. This is the first report of an ionic liquid with a BF3Cl− anion. It was found that CO2 gas can be dissolved and electrochemically reduced at ambient pressure, and room temperature in the ionic liquid EMIMBF3Cl. The reduction of CO2 occurred at a relatively less negative electrode potential of −1.8 V vs. silver wire, compared to previous reported values that are between −2.4 and −2.0 V. The reduction current density can be as high as ∼5.7 mA/cm2. The BF3Cl− anion may catalyze the CO2 reduction by forming a Lewis acid-base adduct BF3-CO2.

Unwinding Antiaromaticity in 1-Bromo-2,3,4,5-tetraphenylborole

Borole systems tend to undergo various reactions driven by the disruption of its destabilizing antiaromatic character. As a consequence, the isolation and characterization of free boroles is challenging, especially when the substituents around the C4B framework are sterically less demanding. In the present paper we report the synthesis of 1-bromo-2,3,4,5-tetraphenylborole. The title compound readily undergoes a dimerization/rearrangement reaction in analogy to the previously reported 1-chloro-2,3,4,5-tetraphenyborole to form an isostructural product identified by X-ray crystallography. Additionally we present the formation of Lewis acid–base adducts of the title compound with 3,5-lutidine, PCy3, N-hetrocyclic carbene, cyclic (amino)(alkyl)carbene, and THF. The latter compounds were analyzed by single-crystal X-ray diffraction and compared.

Charge Transfer via the Dative N−B Bond and Dihydrogen Contacts. Experimental and Theoretical Electron Density Studies of Four Deltahedral Boranes

In an approach combining high resolution X-ray diffraction at low temperatures with density functional calculations, two closo-borates, B12H12(2-) (1) and B10H10(2-) (2), and two arachno-boranes, B10H12L2 (L = amine (3) or acetonitrile (4)), are studied by means of Atoms In Molecules (AIM) theory and Electron Localizability Indicator (ELI-D). The charge transfer via the dative N-B bonds in the arachno-boranes and via dihydrogen contacts in the closo-borates is quantified. The dative N-B bond in 4 is significantly shorter and stronger than that in 3 and in small N-B Lewis acid base adducts from the literature. It is even shorter in the gas phase than in the crystal environment in contrast to the bond shortening in the crystal generally found for N-B Lewis acid-base adducts. Furthermore, the calculated charge transfer in terms of AIM charges is opposite to the expected N → B direction but still weak as found for all other N-B bonds. The intramolecular charge redistributions due to intermolecular interactions are quantified by the AIM and ELI-D analysis of contact ion pairs. The latter method gives a deeper understanding of delocalization effects in the borane cages as well as in the counterions. Since dihydrogen bonds are rarely found in crystal structures, one focus was directed to the topologies of the large number of 58 experimentally found contacts of this type. The analysis reveals that the electron density at the bond critical point, the corresponding Laplace function, and the curvature along the bond path (λ3) show a behavior that clearly discriminates these interactions from classical hydrogen bonds, confirming earlier theoretical findings.

Chemical Synthesis of Metal Nanoparticles Using Amine–Boranes

The development of new synthetic strategies to obtain monodisperse metal nanoparticles on large scales is an attractive prospect in the context of sustainability. Recently, amine-boranes, the classical Lewis acid-base adducts, have been employed as reducing agents for the synthesis of metal nanoparticles. They offer several advantages over the traditional reducing agents like the borohydrides; for example, a much better control of the rate of reduction and, hence, the particle size distribution of metal nanoparticles; diversity in their reducing abilities by varying the substituents on the nitrogen atom; and solubility in various protic and aprotic solvents. Amine-boranes have not only been used successfully as reducing agents in solution but also in solventless conditions, in which along with the reduction of the metal precursor, they undergo in situ transformation to afford the stabilizing agent for the generated metal nanoparticles, thereby bringing about atom economy as well. The use of amine boranes for the synthesis of metal nanoparticles has experienced an explosive growth in a very short period of time. In this Minireview, recent progress on the use of amine boranes for the synthesis of metal nanoparticles, with a focus towards the development of pathways for sustainability, is discussed.

Synthesis and Reactivity of the Phosphinoboranes R2PB(C6F5)2

The phosphinoboranes [R(2)PB(C(6)F(5))(2)](2) (R = Et 1, Ph 2) and R(2)PB(C(6)F(5))(2) (R = tBu 3, Cy 4, Mes 5) were synthesized from the reaction of (C(6)F(5))(2)BCl and the corresponding lithium phosphide. The relationships between B-P distance, P pyramidality, and the extent of BP multiple bonding were further explored computationally. Natural Bond Order (NBO) analyses of 3 and 4 showed that the π-bonding highest occupied molecular orbitals (HOMOs) were highly polarized. In addition the Lewis acid-base adducts, R(2)(H)P·B(H)(C(6)F(5))(2) (R = Et 6; Ph 7; tBu 8; Cy 9; Mes 10) were prepared via the reaction of the phosphines R(2)PH with the borane HB(C(6)F(5))(2). Compounds 1 and 2 showed no signs of reaction with H(2); however, reaction of compounds 3 and 4 with H(2) was observed to give 8 and 9. In a related set of reactions compounds 3 and 4 were reacted with H(3)NBH(3) or Me(2)(H)NBH(3) also led to the generation of 8 and 9, respectively. The reaction profile of the reaction of (CF(3))(2)BPR(2) with H(2) was examined computationally and shown to be exothermic. Efforts to effect the reverse reaction, that is, dehydrogenation of adducts 6-10 were unsuccessful. Compound 4 was also shown to react with 4-tert-butylpyridine to give Cy(2)PB(C(6)F(5))(2)(4-tBuC(5)H(4)N) 11 while reactions of 3 and 4 with the Lewis acid BCl(3) gave the dimers (R(2)PBCl(2))(2) (R = tBu 12, Cy 13) and the byproduct ClB(C(6)F(5))(2).

Synthesis of Monomeric Divalent Tin(II) Compounds with Terminal Chloride, Amide, and Triflate Substituents

AbstractMonomeric three‐coordinate (amidinato)tin chloride [PhC(NtBu)2SnCl] (1) was prepared by the reaction of N,N′‐di‐tert‐butylcarbodiimide, phenyllithium and SnCl2. Themetathesis reaction of 1 with AgSO3CF3 and (Me3Si)2NLi afforded the formation of PhC(NtBu)2SnOTf (Tf = CF3SO2) (2) and PhC(NtBu)2SnN(SiMe3)2 (3). The reductive dehalogenation of 1 with L‐selectride resulted in the formation of the four‐coordinate homoleptic tin compound Ph2C2(NtBu)4Sn (4). Compounds 1, 2, 3, and 4 were characterized by single‐crystal structural analysis. Furthermore, 1 was treated with Fe2(CO)9 to afford the Lewis acid–base adduct 5.

Late‐Transition‐Metal Complexes as Tunable Lewis Bases

Syntheses of the first heteroleptic N-heterocyclic carbene (NHC)-phosphane platinum(0) complexes and formation of the corresponding Lewis acid-base adducts with aluminum chloride is reported. The influence of N-heterocyclic carbenes on tuning the Lewis basic properties of the metal complexes was judged from spectroscopic, structural, and computational data. Conclusive experimental evidence for the enhanced Lewis basicity of NHC-containing complexes was provided by a transfer reaction.