Douglas W. Stephan

Reversible, Metal-Free Hydrogen Activation

Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100°C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25°C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.

Frustrated Lewis Pair Chemistry: Development and Perspectives

Frustrated Lewis pairs (FLPs) are combinations of Lewis acids and Lewis bases in solution that are deterred from strong adduct formation by steric and/or electronic factors. This opens pathways to novel cooperative reactions with added substrates. Small-molecule binding and activation by FLPs has led to the discovery of a variety of new reactions through unprecedented pathways. Hydrogen activation and subsequent manipulation in metal-free catalytic hydrogenations is a frequently observed feature of many FLPs. The current state of this young but rapidly expanding field is outlined in this Review and the future directions for its broadening sphere of impact are considered.

Reversible Metal-Free Carbon Dioxide Binding by Frustrated Lewis Pairs†

Frustrated Lewis pairs comprising phosphine and borane react to reversibly bind and release CO2, offering a rare example of metal-free CO2 sequestration. The mechanism of formation of CO2 derivatives by almost simultaneous P-C and O-B bond formation was characterized by quantum chemical calculations.

Room temperature reduction of CO2 to methanol by Al-based frustrated Lewis pairs and ammonia borane.

AlX(3) (X = Cl or Br) and PMes(3) (Mes = 2,4,6-C(6)H(2)Me(3)) react to form weak Lewis adducts but also react with CO(2) to give Mes(3)P(CO(2))(AlX(3))(2) (X = Cl; Br). These species react rapidly with excess ammonia borane at room temperature to give CH(3)OH upon quenching with water.

Frustrated Lewis Pairs: From Concept to Catalysis

CONSPECTUS: Frustrated Lewis pair (FLP) chemistry has emerged in the past decade as a strategy that enables main-group compounds to activate small molecules. This concept is based on the notion that combinations of Lewis acids and bases that are sterically prevented from forming classical Lewis acid-base adducts have Lewis acidity and basicity available for interaction with a third molecule. This concept has been applied to stoichiometric reactivity and then extended to catalysis. This Account describes three examples of such developments: hydrogenation, hydroamination, and CO2 reduction. The most dramatic finding from FLP chemistry was the discovery that FLPs can activate H2, thus countering the long-existing dogma that metals are required for such activation. This finding of stoichiometric reactivity was subsequently evolved to employ simple main-group species as catalysts in hydrogenations. While the initial studies focused on imines, subsequent studies uncovered FLP catalysts for a variety of organic substrates, including enamines, silyl enol ethers, olefins, and alkynes. Moreover, FLP reductions of aromatic anilines and N-heterocycles have been developed, while very recent extensions have uncovered the utility of FLP catalysts for ketone reductions. FLPs have also been shown to undergo stoichiometric reactivity with terminal alkynes. Typically, either deprotonation or FLP addition reaction products are observed, depending largely on the basicity of the Lewis base. While a variety of acid/base combinations have been exploited to afford a variety of zwitterionic products, this reactivity can also be extended to catalysis. When secondary aryl amines are employed, hydroamination of alkynes can be performed catalytically, providing a facile, metal-free route to enamines. In a similar fashion, initial studies of FLPs with CO2 demonstrated their ability to capture this greenhouse gas. Again, modification of the constituents of the FLP led to the discovery of reaction systems that demonstrated stoichiometric reduction of CO2 to either methanol or CO. Further modification led to the development of catalytic systems for the reduction of CO2 by hydrosilylation and hydroboration or deoxygenation. As each of these areas of FLP chemistry has advanced from the observation of unusual stoichiometric reactions to catalytic processes, it is clear that the concept of FLPs provides a new strategy for the design and application of main-group chemistry and the development of new metal-free catalytic processes.

Early-late heterobimetallics

Synthese de composes heterobimetalliques. Un des metaux est souvent carbonyle. La structure moleculaire et la conformation sont donnees

Terminal Alkyne Activation by Frustrated and Classical Lewis Acid/Phosphine Pairs

Frustrated and classical Lewis pairs arising from combinations of Lewis acids and phosphines react with terminal alkynes either via C-H activation forming an alkynylborate salt or by addition to alkyne giving a zwitterionic phosphonium borate.

Lutidine/B(C6F5)3: At the Boundary of Classical and Frustrated Lewis Pair Reactivity

Classical Lewis acid-base adducts, previously thought to be unreactive, can provide access to the unique reactivity of frustrated Lewis pairs. This was demonstrated with a mixture of 2,6-lutidine and B(C(6)F(5))(3) where an equilibrium affords the adduct and yet also effects the heterolytic activation of H(2) and the ring opening of THF.

Complexation of Nitrous Oxide by Frustrated Lewis Pairs

Frustrated Lewis pairs comprised of a basic yet sterically encumbered phosphine with boron Lewis acids bind nitrous oxide to give intact PNNOB linkages. The synthesis, structure, and bonding of these species are described.

Reversible, metal-free, heterolytic activation of H2 at room temperature.

The combination of the borane B(p-C(6)F(4)H)(3) and (o-C(6)H(4)Me)(3)P activates H(2) to give [(o-C(6)H(4)Me)(3)PH][HB(p-C(6)F(4)H)(3)]. This salt when placed under vacuum releases H(2) at room temperature.