Lewis Acid-base Adducts Research Articles

Using Raman spectroscopy to investigate donor–acceptor reactions in the formamide/dimethylsulfoxide/acetonitrile system

AbstractRaman spectra of binary and ternary solvent systems in different concentrations were carried out. The mixtures containing equimolar quantities of dimethylsulfoxide (DMSO) and acetonitrile (ACN), or those in different compositions, do not show any spectral evidence for the formation of a Lewis acid–base adduct. On the contrary, the formamide (FA), DMSO and ACN mixtures exhibit new bands. In the S stretching region, the band at 1025 cm−1 is assigned to the FA–DMSO adduct whereas, in the C stretching region, the 2257 cm−1 band corresponds to the FA–ACN adduct. The positions of these two new bands in the present system are in agreement with our recent works. In addition, in the NH2 bending region, the 1613 cm−1 band has been described as a mixture of both adducts. Quantitative analyses performed in the S and C stretching regions show that the association extent for the FA–DMSO adduct is greater than for the FA–ACN adduct, and this is in excellent agreement with the acid–base concepts commonly used to nonaqueous solvents. Preferential interactions between FA molecules as well as their dependence on the ternary mixture composition are also discussed. The results described here open up new possibilities to study the acid–base reactions nature involving FA adducts and can be useful for a better description of biological systems in which peptide groups have a special role. Copyright © 2007 John Wiley & Sons, Ltd.

Facile Splitting of Hydrogen and Ammonia by Nucleophilic Activation at a Single Carbon Center

In possessing a lone pair of electrons and an accessible vacant orbital, singlet carbenes resemble transition metal centers and thus could potentially mimic their chemical behavior. Although singlet di(amino)carbenes are inert toward dihydrogen, it is shown that more nucleophilic and electrophilic (alkyl)(amino)carbenes can activate H2 under mild conditions, a reaction that has long been known for transition metals. However, in contrast to transition metals that act as electrophiles toward dihydrogen, these carbenes primarily behave as nucleophiles, creating a hydride-like hydrogen, which then attacks the positively polarized carbon center. This nucleophilic behavior allows these carbenes to activate NH3 as well, a difficult task for transition metals because of the formation of Lewis acid-base adducts.

Reactions of Group 13 Organometallics tBu3M with Distibanes Sb2R4

AbstractTetraalkyldistibanes Sb2R4 (R = nPr, iBu) react with tBu3M (M = Al, Ga) at –30 °C with the formation of the Lewis acid–base adducts [tBu3M]2[Sb2R4] [M = Al, R = nPr (1), iBu (2); M = Ga, R = nPr (3), iPr 4)]. Compounds 1 and 2 are stable in solution whereas 3 undergoes a Sb–Sb bond‐breakage reaction with the subsequent formation of [tBu2GaSb(nPr)2]2 (5). The same trend was observed for the reaction of Sb2(iBu)4 with tBu3Ga, which yielded the heterocyclic stibanylgallane [tBu2GaSb(iBu)2]2 (6). In addition, the simple trialkylstibane adduct tBu3Al–Sb(iBu)3 (7) was obtained as a byproduct from the reaction of Sb2(iBu)4 with tBu3Al. Compounds 1–7 were characterized by multinuclear NMR (1H, 13C{1H}) and mass spectroscopy and elemental analysis. Compounds 1, 3, 5, and 7 were also investigated by single‐crystal X‐ray diffraction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Formation of Polymeric Lewis Acid–Lewis Base Complexes with Well-defined Organoboron Polymers

The binding of Lewis bases to organoboron polymeric Lewis acids has been studied and the parameters that determine the complexation equilibrium have been investigated, which include (i) the strength of the individual Lewis acids and Lewis bases, (ii) concentration, and (iii) temperature. While the strongly Lewis acidic borane polymers poly(4-bis(pentafluorophenyl)borylstyrene) (PS-BPf) and poly(4-(di-2-thienylboryl)styrene) (PS-BTh) form isolable complexes with strong Lewis bases such as 4-t-butylpyridine (tPy), a temperature dependent equilibrium is established with weaker bases such as THF. Similarly, the weakly Lewis acidic boronate polymer poly(4-diethoxyborylstyrene) (PS-BOEt) undergoes a temperature dependent equilibrium with the strong Lewis base 4-dimethylaminopyridine (DMAP), while poly(4-pinacolatoborylstyrene) (PS-BPin) does not significantly bind to pyridine bases. Decomplexation of PS-BTh· t Py is achieved by treatment with the stronger Lewis acid, B(C6F5)3, thereby confirming the reversible nature of the polymeric Lewis acid–base adducts.

Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile formation of phosphine-boranes and cationic phosphonium-boranes

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.

Exploring the chemistry of free CI3+: reactions with the weak Lewis bases PX3 (X = Cl–I), AsI3 and Et2O

What is the preferred coordination site of CI3+? Recent computational work suggested the iodine atoms of the Lewis acid CI3+ to be more electrophilic than the classically expected carbon atom, e.g. the complex with water is of type I2C-I...OH2+ and not the classically expected I3C-OH2+. If this structure is correct, one may also anticipate reactions of CI3+ as an I+ donor. Thus, we were interested in investigating the chemistry of CI3+ in the room-temperature stable salt [CI3]+[pftb](-) ([pftb](-) = [Al(OC(CF3)3)4]-) with weak nucleophiles that i) mimic water (OEt2) or ii) are electronically deactivated weak nucleophiles (PX3, X = Cl-I; AsI3). One question was: is it possible to obtain iodine-coordinated Lewis acid-base adducts of the CI3+ cation? With Et2O as a base, the cation behaves as a strong Lewis acid and cleaves the ether to give I3C-OEt, C2H4 and [H(Et2O)2]+. By contrast PX3 and AsI3 coordinate to the CI3+ cations and the adducts have classical, carbon-bound ethane-like structures, as proven by X-ray single-crystal diffraction, IR, UV-Vis and NMR spectroscopy. From variable temperature 13C NMR studies, it followed for the I3C-AsI3+ salt that the equilibrium between CI3+ and AsI3 is reversible and temperature dependent in solution. The I3C-PI3+ salt decomposes at room temperature giving PI4+ and C2I4, likely through an iodine coordinated I2C-I[dot dot dot]PI3+ intermediate. Thus CI3+ may also act as an I+ donor. All reactions are in agreement with ab initio quantum chemical calculations at the MP2/TZVPP level and assignments of experimental spectra were aided by quantum chemistry.

Transition metal-catalyzed dehydrocoupling of group 13-group 15 Lewis acid–base adducts

Transition metal-catalyzed dehydrocoupling is a relatively new method for main group element–element bond formation, providing a facile alternative to salt metathesis processes. Lewis acid–base adducts of Groups 13 and 15 can undergo dehydrocoupling with both early and late transition metals and complexes thereof to yield rings, chains and macromolecules based on a framework of alternating B–N or B–P atoms. Evidence is also provided for Group 13 hydrides acting as poisons towards heterogeneous dehydrocoupling catalysts.

Salts of the Cobalt(I) Complexes [Co(CO)5]+ and [Co(CO)2(NO)2]+ and the Lewis Acid–Base Adduct [Co2(CO)7COB(CF3)3

The reaction of [Co(2)(CO)(8)] with (CF(3))(3)BCO in hexane leads to the Lewis acid-base adduct [Co(2)(CO)(7)CO--B(CF(3))(3)] in high yield. When the reaction is performed in anhydrous HF solution [Co(CO)(5)][(CF(3))(3)BF] is isolated. The product contains the first example of a homoleptic metal pentacarbonyl cation with 18 valence electrons and a trigonal-bipyramidal structure. Treatment of [Co(2)(CO)(8)] or [Co(CO)(3)NO] with NO(+) salts of weakly coordinating anions results in mixed crystals containing the [Co(CO)(5)](+)/[Co(CO)(2)(NO)(2)](+) ions or pure novel [Co(CO)(2)(NO)(2)](+) salts, respectively. This is a promising route to other new metal carbonyl nitrosyl cations or even homoleptic metal nitrosyl cations. All compounds were characterized by vibrational spectroscopy and by single-crystal X-ray diffraction.

Trimethylphosphine–tris(pentafluorophenyl)borane

The title simple Lewis acid–base adduct, C21H9BF15P or (H3C)3P–B(C6F5)3, is essentially insoluble in most non-coordinating organic solvents. The P—B bond lies along a threefold rotation axis. It crystallizes in the trigonal space group R\overline{3}. Weak intra­molecular C—H⋯F contacts are present between the methyl­phosphine and the ortho-F atoms of the fluoro­aryl­borane, resulting in an eclipsed geometry in the reaction partners. The observed insolubility is likely to be due, in part, to an additional inter­molecular network of weak contacts involving the para-F atoms of the B—C6F5 groups and the P(CH3)3 group.

Lewis acid–base adducts: A quantitative Raman analysis of formamide and dimethylsulfoxide mixtures

Raman spectra of pure liquid dimethylsulfoxide (DMSO) and of binary mixtures of formamide (FA) and DMSO in different compositions were obtained. The vibrations involving the SO functional group in the band envelope at ca. 1050 cm −1 of pure liquid DMSO are assigned to monomers, dimers and higher aggregates of DMSO. The appearance of a new band at 1024 cm −1, whose intensity shows large dependence on the FA concentration, is assigned to a FA–DMSO adduct. This has been possible due to the two H-bond donor sites of FA and the strong donor character of DMSO that become the environment propitious for the donor–acceptor reaction. Quantitative analysis performed in the SO stretching region in the binary mixtures gives a 1:1 stoichiometry in this adduct in the limit of infinite dilution. This proportion is in full agreement with our previous determination for the FA–ACN adduct. The experimental evidence of the 1:1 FA–DMSO adduct is presented for the first time using Raman spectroscopy. The results described here open new possibilities to study the acid–base reactions nature of FA adducts.

Boron–Nitrogen Adducts of 1,3,5‐Triaza‐7‐phosphaadamantane (PTA): Synthesis, Reactivity, and Molecular Structure

AbstractAddition of BH3 to 1,3,5‐triaza‐7‐phosphaadamantane (PTA) leads to the formation of the first coordination complex of PTA in which a nitrogen and not the phosphorus of PTA is involved in the bonding. Multinuclear NMR spectroscopy, X‐ray crystallography, and computational chemistry are utilized in the characterization of the products. Crystal structures of PTA–BH3 (1) and O=PTA–BH3 (2) adducts are described. In addition a coordination polymer, [CpRu(PTA)2Cl(Ph3B3O3)]n (3), resulting from Lewis acid–base adduct formation between the PTA ligands of CpRu(PTA)2Cl and the product of the cyclocondensation of phenylboronic acid is described. The polymeric compound 3 was also characterized by X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Synthesis and structural characterization of the first metal organic thallium antimonide

The first metal organic thallium antimonide, the heterocycle [Me2TlSb(SiMe3)2]3, was synthesized by reaction of [Me2AlSb(SiMe3)2]3 with the Lewis acid-base adduct dmap-TlMe3 (dmap = 4-dimethylaminopyridine). The analogous TlBi heterocycle [Me2TlBi(SiMe3)2]3 couldn't be isolated due to its limited thermal stability in solution.

Boron as a Bridging Ligand

The Lewis acidity of [(eta5-C5R5)Fe(CO)2(BX2)] (1: R = H, X = Cl) and the robust nature of the constituent Fe--B bond have been demonstrated by reaction with 4-methylpyridine to yield the corresponding Lewis acid-base adduct. These properties have subsequently been utilised in the construction of heterodinuclear bridging borylene, bridging boryl and mu(2)-boride complexes from analogues 3 (R = Me, X = Br) and 5 (R = Me, X = Cl). All the bimetallic species so formed exhibited unprecedented coordination modes for boron.

Heterobimetallic bismuth–transition metal coordination complexes as single-source molecular precursors for the formation of advanced oxide materials

Abstract In this manuscript we outline recent results on the synthesis of heterobimetallic coordination complexes involving bismuth and the transition elements using either bifunctional ligands or Lewis acid–base adduct formation as a synthetic strategy. Significant structural features in the molecular heterobimetallic species resulting from these two approaches are highlighted. The ability of these complexes to act as single-source precursors for the formation of advanced oxide nanomaterials has been investigated, and the potential of these complexes versus traditional methods for the formation of advanced ceramics is discussed. To cite this article: T. Ould-Ely et al., C. R. Chimie 8 (2005) .

Functionalization and cleavage of coordinated dinitrogen via hydroboration using primary and secondary boranes

The reaction of the side-on, end-on ditantalum dinitrogen complex ([NPN]Ta)2(µ-η1:η2-N2)(µ-H)2 (where NPN = PhP(CH2SiMe2NPh)2) with a variety of secondary and primary boranes is reported. With 9-BBN, hydroboration of the Ta2N2 unit occurs via B-H addition, which in turn triggers a cascade of reactions that result in N—N bond cleavage, ancillary ligand rearrangement involving silicon group migration, and finally elimination of benzene from the N-Ph group and a B-H moiety to generate the imide–nitride derivative. In the presence of excess 9-BBN, the Lewis acid – base adduct of the imide–nitride ([NPµ–N]Ta(=NBC8H14)(µ-NB(H)C8H14)Ta[NPN]) is formed. A similar set of reactions is observed for dicyclohexylborane (Cy2BH), which hydroborates the dinitrogen complex to generate [NPN]Ta(H)(µ-η1:η2-NNBCy2)(µ-H)2Ta[NPN], followed by loss of H2 and silicon group migration to yield the imide–nitride [NPµ–N]Ta(=NBCy2)(µ-N)(Ta[NPN]. With thexyl borane (H2BCMe2CHMe2), a similar sequence of reactions is suggested starting with hydroboration to generate [NPN]Ta(H)(µ-η1:η2-NNB(H)C6H13)(µ-H)2Ta[NPN], followed by loss of H2 and ancillary ligand rearrangement. When bis(pentafluorophenyl)borane (HB(C6F5)2) is used, no hydroboration of coordinated N2 is observed, rather simple adduct formation to give ([NPN]Ta)2(µ-η1:η2-NN-B(H)(C6F5)2)(µ-H)2 occurs. Key words: dinitrogen, tantalum, hydroboration, N—N bond cleavage.

Hydride Affinities of Borane Derivatives: Novel Approach in Determining the Origin of Lewis Acidity Based on Triadic Formula

The problem of intrinsic Lewis acidities of simple boron compounds (BH3-mXm, m = 0-3, X = F, Cl, Br, CH3, and OH) is assessed by their gas-phase hydride affinities (HAs). A simple and intuitively appealing picture of the interaction process including detachment of an electron from the hydride ion H-, capture of the pruned electron to the investigated Lewis acid (LA), and subsequent formation of the homolytic chemical bond between two newly created radicals is proposed. It enables transparent and straightforward dissection of the initial and final state effects, which taken together with the intermediate relaxation stabilization determine the trend of changes in the hydride affinities. The former effect is reflected in the electron affinities of the neutral Lewis acids given within Koopmans' approximation, while the final state effect involves properties of the formed Lewis acid-base adducts mirrored in the bond dissociation energy of the formed [LA-H]- chemical bond. It is demonstrated that unexpectedly low Lewis acidity of fluoroboranes relative to the corresponding chlorine and bromine derivatives can be traced down to the unfavorable Koopmans' electron affinities. Hence, it is a consequence of the initial state effect. In contrast, chloroboranes are more potent Lewis acids than fluoroboranes, because the relaxation and final state effects decisively influence their Lewis acidity. Finally, bromine-substituted borane compounds provide the most powerful studied Lewis acids. Their hydride affinities are result of a synergic interplay of the initial state, intermediate stabilization via relaxation, and final state effects. It is shown that Pearson's global hardness indices defined within his hard and soft acid-base (HSAB) principle fail to adequately predict and interpret the calculated hydride affinities.

Novel RhCp*/GaCp* and RhCp*/InCp* cluster complexes

The reaction of 6 equivalents of GaCp*(Cp*= pentamethylcyclopentadienyl) with [{Cp*RhCl2}2] yields the complex [Cp*Rh(GaCp*)3(Cl)2] (1) exhibting a cage-like intermetallic RhGa3 center with Ga-Cl-Ga bridges. Treatment of this complex with GaCl3 gives the Lewis acid-base adduct [Cp*Rh(GaCp*)2(GaCl3)]. (2) Reaction of [{Cp*RhCl2}2] with understoichiometric amounts of E(I)Cp*(E = Al, Ga, In) leads to a variety of products strongly dependent on the molecular ratio of the reactants. Thus, the reduction of [{Cp*RhCl2}2] with one equivalent of E(I)Cp*(E = Al, Ga, In) gives the RhII dimer [{Cp*RhCl}2]. The insertion of 3 equivalents of InCp* into the Rh-Cl bonds of [{Cp*RhCl2}2] yields the salt [Cp*2Rh]+[Cp*Rh(InCp*){In2Cl4(mu2-Cp*)}]- (3), the anion exhibiting an intermetallic RhIn(3) center with an intramolecularly bridging Cp* ring. The reaction of [{Cp*RhCl}2] with Cp*Ga yields various insertion products. In trace amount the "all hydrocarbon" cluster complex [(RhCp*)2(GaCp*)3] (6) is obtained. The corresponding ethylene containing cluster complex [{RhCp(GaCp*)(C2H4)}2] (7) can be prepared by treatment of [RhCp*(CH3CN)(C2H4)] with GaCp*.

Structural comparisons between analogously substituted gallane–pnictine adducts

AbstractThe 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.

Toward rational control of metal stoichiometry in heterobimetallic coordination complexes: synthesis and characterization of Pb(Hsal)2(Cu(salen*))2, [Pb(NO3)(Cu(salen*))2](NO3), Pb(OAc)2(Cu(salen*)), and [Pb(OAc)(Ni(salen*))2](OAc).

The ability of the transition metal complex M(salen)* (M = Ni, Cu) to form Lewis acid-base adducts with lead(II) salts has been explored. The new complexes Pb(Hsal)(2)(Cu(salen*))(2) (1), [Pb(NO(3))(Cu(salen*))(2)](NO(3)) (2), Pb(OAc)(2)(Cu(salen*)) (3), and [Pb(OAc)(Ni(salen*)(2)](OAc) (4) (Hsal = O(2)CC(6)H(4)-2-OH, salen* = bis(3-methoxy)salicylideneimine) have been synthesized and characterized spectroscopically and by single-crystal X-ray diffraction. The coordination environment of the lead in the heterobimetallic complex is sensitive both to the initial lead salt and to the transition metal salen* complex that is employed in the synthesis. As a result, we have been able to access both 2:1 and 1:1 adducts by varying either the lead salt or the transition metal in the heterobimetallic coordination complex. In all cases, the salen* complex is associated with the lead center via dative interactions of the phenolic oxygen atoms. The relationship between the coordination requirements of the lead and the chemical nature of the anion is examined. In compound 1, the Pb(2+) ion is chelated by two Cu(salen*) moieties, and both salicylate ligands remain attached to the lead center and bridge to the Cu(2+) ions. The two Cu(salen*) groups are roughly parallel and opposed to each other as required by crystallographic inversion symmetry at lead. In contrast, the two Cu(salen*) groups present in 2 and 4 attached to the lead ion show considerable overlap. Furthermore, only one nitrate ion in 2 and one acetate ion in 4 remain bonded to the lead center. Compound 3 is unique in that only one Cu(salen*) group can bind to lead. Here, both acetate ligands remain attached, although one is chelating bidentate and the other is monodentate.

Intra- and inter-molecular C-H...F-C and N-H...F-C hydrogen bonding in secondary amine adducts of B(C6F5)3: relevance to key interactions in alkene polymerisation catalysis.

The reactions between the cyclic sec. amines pyrrolidine and piperidine with B(C6F5)3 yield Lewis acid-base adducts with both intra- and inter-molecular hydrogen bonding interactions between C-H and N-H groups and aryl-fluorines in the solid state.