Formation, structural characterization, and reactions of a unique cyclotrimeric vicinal Lewis pair containing (C6F5)2P-Lewis base and (C6F5)BH-Lewis acid components

A unique vicinal Lewis pair cyclotrimer has been synthesized and characterized with respect to its structure and reactivity.

Stoichiometric reductions of alkyl-substituted ketones and aldehydes to borinic esters

A series of alkyl-substituted ketones are shown to activate hydrogen in the presence of B(C₆F₅)₃, affording the corresponding borinic esters RR′CHOB(C₆F₅)₂.

Intramolecular pnicogen interactions in phosphorus and arsenic analogues of proton sponges

A computational study of the intramolecular pnicogen bond in 1,8-bis-substituted naphthalene derivatives (ZXH and ZX₂ with Z = P, As and X = H, F, Cl, and Br), structurally related to proton sponges, has been carried out.

Carbonylation reactions of intramolecular vicinal frustrated phosphane/borane Lewis pairs

The intramolecular frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 4 adds cooperatively to carbon monoxide to form the five-membered heterocyclic carbonyl compound 5. The intramolecular FLP 7 contains an exo-3-B(C6F5)2 Lewis acid and an endo-2-PMes2 Lewis base functionality coordinated at the norbornane framework. This noninteracting FLP adds carbon monoxide in solution at -35 °C cooperatively to yield a five-membered heterocyclic FLP-carbonyl compound 8. In contrast, FLP 7 is carbonylated in a CO-doped argon matrix at 25 K to selectively form a borane carbonyl 9 without involvement of the adjacent phosphanyl moiety. The free FLP 7 was generated in the gas phase from its FLPH2 product 10. A DFT study has shown that the phosphonium hydrido borate zwitterion 10 is formed exergonically in solution but tends to lose H2 when brought into the gas phase.

Peri-substituted (ace)naphthylphosphinoboranes. (Frustrated) Lewis pairs

The synthesis and molecular structures of 1-(diphenylphosphino)-8-naphthyldimesitylborane (1) and 5-(diphenylphosphino)-6-acenaphthyldimesitylborane (2) are reported. The experimentally determined P-B peri distances of 2.162(2) and 3.050(3) Å allow 1 and 2 to be classified as regular and frustrated Lewis pairs. The electronic characteristics of the (non)bonding P-B contacts are determined by analysis of a set of real-space bonding indicators (RSBIs) derived from the theoretically calculated electron and pair densities. These densities are analyzed utilizing the atoms-in-molecules (AIM), stockholder, and electron-localizability-indicator (ELI-D) space partitioning schemes. The recently introduced mapping of the electron localizability on the ELI-D basin surfaces is also applied. All RSBIs clearly discriminate the bonding P-B contact in 1 from the nonbonding P-B contact in 2, which is due to the fact that the acenaphthene framework is rather rigid, whereas the naphthyl framework shows sufficient conformational flexibility, allowing shorter peri interations. The results are compared to the previously known prototypical phosphinoborane Ph3PB(C6F5)3, which serves as a reference for a bonding P-B interaction. The most prominent features of the nonbonding P-B contact in 2 are the lack of an AIM bond critical point, the unaffected Hirshfeld surfaces of the P and B atomic fragments, and the negligible penetration of the electron population of the ELI-D lone pair basin of the P atom into the AIM B atomic basin.

Frustrated Lewis pair chemistry of chiral (+)-camphor-based aminoboranes

Dimethylamino-(+)-camphorenamine reacted with an equimolar amount of Piers' borane, HB(C6F5)2, to give the corresponding iminium-hydroborate zwitterionic salt. Being in equilibrium with the parent enamine-HB(C6F5)2 N-B pair, this salt was able to split hydrogen heterolytically, hydrogenating the iminium group in the molecule. Detailed studies revealed that the hydrogen splitting in this reaction proceeded through an intermolecular pathway leading to a bornylamine-HB(C6F5)2 adduct. When the starting enamine is present in excess over HB(C6F5)2, the produced bornylamine-HB(C6F5)2 adduct breaks up, eliminating free bornylamine and forming the initial camphorenamine-HB(C6F5)2 pair. This results in hydrogenation of the camphorenamine framework in a catalytic fashion.

Metal-free HB(C6F5)2-catalyzed hydrogenation of unfunctionalized olefins and mechanism study of borane-mediated σ-bond metathesis

Out with the metal: Metal-free hydrogenation of unfunctionalized olefins can be achieved by employing HB(C6F5)2 as the catalyst. The key step in the catalytic reaction is believed to involve a novel borane-mediated σ-bond metathesis, which has been investigated both experimentally and theoretically.

The hydride-ion affinity of borenium cations and their propensity to activate H2 in frustrated Lewis pairs

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu(3))](+) has a lower hydride-ion affinity (HIA) than B(C(6)F(5))(3). This resulted in H(2) activation being energetically unfavourable in a FLP with the strong base PtBu(3). However, ligand disproportionation of CatBH(PtBu(3)) at 100 °C enabled trapping of H(2) activation products. DFT calculations at the M06-2X/6-311G(d,p)/PCM (CH(2)Cl(2)) level revealed that replacing catechol with chlorides significantly increases the chloride-ion affinity (CIA) and HIA. Dichloro-borenium cations, [Cl(2)B(amine)](+), were calculated to have considerably greater HIA than B(C(6)F(5))(3). Control reactions confirmed that the HIA calculations can be used to successfully predict hydride-transfer reactivity between borenium cations and neutral boranes. The borenium cations [Y(Cl)B(2,6-lutidine)](+) (Y = Cl or Ph) form FLPs with P(mesityl)(3) that undergo slow deprotonation of an ortho-methyl of lutidine at 20 °C to form the four-membered boracycles [(CH(2){NC(5)H(3)Me})B(Cl)Y] and [HPMes(3)](+). When equimolar [Y(Cl)B(2,6-lutidine)](+)/P(mesityl)(3) was heated under H(2) (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation.

Frustrated Lewis Pairs: from dihydrogen activation to asymmetric catalysis

The non-self-quenched property of Frustrated Lewis Pairs (FLPs) contradicts the classical Lewis acid-base theory, but this peculiarity offers unprecedented possibilities for the activation of small molecules. Among all of their fascinating applications, FLP mediated hydrogen activation and the associated catalytic hydrogenations are currently considered as the most intriguing illustration of their reactivity. The FLPs enabled the catalytic reduction of a wide range of substrates with molecular hydrogen and tuning of the structural properties of the FLP partners allowed broadening of the substrate scope. Based on detailed mechanistic knowledge, FLP based asymmetric hydrogenation of various substrates could be achieved with high enantioselectivities. More importantly, FLP based enantioselective catalysis is not limited to the field of asymmetric hydrogenation, and other exciting catalytic applications have already appeared.

Paracyclophane derivatives in frustrated Lewis pair chemistry

The metal-free activation of hydrogen was achieved using [2.2]paracyclophane-derived bisphosphines as Lewis base in frustrated Lewis pair chemistry. The rigid scaffold allows the orientation of functional groups so that steric aspects can be studied without altering the electronic nature. Depending on the geometry, structurally different phosphonium hydridoborates were generated when the frustrated Lewis pairs were exposed to hydrogen. The bisphosphines were applied in the 1,4-hydrosilylation-hydrogenation domino reaction providing access to secondary silyl-protected alcohols from enones in one step. Additionally, the planar-chiral scaffold was applied for the synthesis of novel enantiopure Lewis acids and Lewis bases.

Activation of hydrogen and hydrogenation catalysis by a borenium cation

The readily prepared borenium salt [(IiPr(2))(BC(8)H(14))][B(C(6)F(5))(4)] (2) [IiPr(2) = C(3)H(2)(NiPr)(2)] is shown to activate H(2) heterolytically in the presence of tBu(3)P. Compound 2 also acts as a catalyst for the metal-free hydrogenation of imines and enamines at room temperature.

Frustrated Lewis pairs: Some recent developments

The chemistry of some reactive frustrated Lewis pairs (FLPs) is reported. This includes intramolecular P/B and N/B FLPs, some of which were used as catalysts for the hydrogenation of electron-rich olefin substrates. Some advanced intermolecular FLPs are reported, which includes systems derived from very bulky alkenyl boranes obtained from 1,1-carboboration reactions of 1-alkynes with tris(pentafluorophenyl)borane. Some such systems activate dihydrogen and transfer the resulting proton/hydride pair even to some electron-poor alkynes. Eventually, we report on the reaction of our intramolecular ethylene-bridged P/B FLP with nitric oxide (NO). N,B-addition of the P-Lewis base/B-Lewis acid combination is observed to form a new type of a persistent aminoxyl radical. Some of the chemistry of the new FLP-NO radicals is presented and discussed.

Zwitterionic half-sandwich Rh and Ir complexes containing a diphosphine nido-carborane ligand: synthesis, structure transformation and application in H2 activation

Several novel zwitterionic half-sandwich complexes and pseudocloso metallacarboranes based on 1,2-(PPh(2))(2)-1,2-C(2)B(10)H(10) were successfully prepared, and further study proved that some of them can activate H(2) to form the metal hydride complexes Cp*M(H)(7,8-(PPh(2))(2)-7,8-C(2)B(9)H(10)) (M = Rh, Ir).