Ein neutrales frustriertes Silicium/Phosphor‐Lewis‐Paar

Dihydrogen Activation with a Neutral, Intermolecular Silicon(IV)-Amine Frustrated Lewis Pair

The heterolytic cleavage of dihydrogen constitutes the hallmark reaction of frustrated Lewis pairs (FLP). While being well-established for planar Lewis acids, such as boranes or silylium ions, the observation of the primary H₂ splitting products with non-planar Lewis acid FLPs remained elusive. In the present work, we report bis(perfluoro-N-phenyl-ortho-amidophenolato)silane and its application in dihydrogen activation to a fully characterized hydridosilicate. The strict design of the Lewis acid, the limited selection of the Lewis base, and the distinct reaction conditions emphasize the narrow tolerance to achieve this fascinating process with a tetrahedral Lewis acid.

Frustrated Lewis pair chemistry of hydride sponges

An improved synthesis for 1,8-bis(dimethylboranyl)naphthalene (1, hydride sponge) was developed avoiding the use of tin(IV) reagents. The related 1,2-bis(dimethylboranyl)benzene (2) was prepared. 1 combined with 1,8-bis(dimethylamino)naphthalene (3, proton sponge) is a Frustrated Lewis pair (FLP) that forms adducts [1-EH_n-1][3-H] with the protic compounds EH_n = H₂O, NH₃, H₂S, PH₃, H₂Se, HCN. Their structures show the chelation of the deprotonated substrates, except for CN^- (binds to one B atom of 1). The mechanisms of formation of [1-EH_n-1][3-H] were explored by NMR spectroscopy. Similar reactions took places for the FLP system 2 + 3, but the adducts [2-EH_n-1][3-H] are less stable; only the PH₃ adduct was isolated and AsH₃ forms a chelated adduct [2-NC(CD₃)AsH₂][3-H]. FLP 1 + 3 does not react with molecular hydrogen, but the formal adduct [1-H][3-H] was isolated via salt metathesis from K[1-H] and [3-H]Cl; it is stable towards water, atmospheric oxygen and up to 100 °C; its thermal decomposition proceeds without formation of H₂. The stabilities of both, the mixture 1 + 3 + H₂ and the formal adduct [1-H][3-H], allow concluding that hydrogen activation of FLP 1 + 3 is kinetically prevented.

Reactivity of the Bicyclic Amido-Substituted Silicon(I) Ring Compound Si4 {N(SiMe3 )Mes}4 with FLP-Type Character

The bicyclic amido‐substituted silicon(I) ring compound Si₄{N(SiMe₃)Mes}₄2 (Mes=Mesityl=2,4,6‐Me₃C₆H₂) features enhanced zwitterionic character and different reactivity from the analogous compound Si₄{N(SiMe₃)Dipp}₄1 (Dipp=2,6‐ⁱPr₂C₆H₃) due to the smaller mesityl substituents. In a reaction with the N‐heterocyclic carbene NHCMe4 (1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene), we observe adduct formation to give Si₄{N(SiMe₃)Mes}₄ ⋅ NHCMe4 (3). This adduct reacts further with the Lewis acid BH₃ to yield the Lewis acid–base complex Si₄{N(SiMe₃)Mes}₄ ⋅ NHCMe4  ⋅ BH₃ (4). Coordination of AlBr₃ to 2 leads to the adduct 5. Calculated proton affinities and fluoride ion affinities reveal highly Lewis basic and very weak Lewis acidic character of the low‐valent silicon atoms in 1 and 2. This is confirmed by protonation of 1 and 2 with Brookharts acid yielding 6 and 7. Reaction with diphenylacetylene only occurs at 111 °C with 2 in toluene and is accompanied by fragmentation of 2 to afford the silacyclopropene 8 and the trisilanorbornadiene species 9.

Bis(pertrifluoromethylcatecholato)silane: Extreme Lewis Acidity Broadens the Catalytic Portfolio of Silicon

Given its earth abundance, silicon is ideal for constructing Lewis acids of use in catalysis or materials science. Neutral silanes were limited to moderate Lewis acidity, until halogenated catecholato ligands provoked a significant boost. However, catalytic applications of bis(perhalocatecholato)silanes were suffering from very poor solubility and unknown deactivation pathways. In this work, the novel per(trifluoromethyl)catechol, H₂ cat^CF3 , and adducts of its silicon complex Si(cat^CF3 )₂ (1) are described. According to the computed fluoride ion affinity, 1 ranks among the strongest neutral Lewis acids currently accessible in the condensed phase. The improved robustness and affinity of 1 enable deoxygenations of aldehydes, ketones, amides, or phosphine oxides, and a carbonyl-olefin metathesis. All those transformations have never been catalyzed by a neutral silane. Attempts to obtain donor-free 1 attest to the extreme Lewis acidity by stabilizing adducts with even the weakest donors, such as benzophenone or hexaethyl disiloxane.

Exploring the Reactivity of a Frustrated Sn/P Lewis Pair: The Highly Selective Complexation of the cis-Azobenzene Photoisomer

The reactivity of the geminal frustrated Lewis pair (FLP) (F5 C2 )3 SnCH2 P(tBu)2 (1) was explored by reacting it with a variety of small molecules (PhOCN, PhNCS, PhCCH, tBuCCH, H3 CC(O)CH=CH2 , Ph[C(O)]2 Ph, PhN=NPh and Me3 SiCHN2 ), featuring polar or non-polar multiple bonds and/or represent α,β-unsaturated systems. While most adducts are formed readily, the binding of azobenzene requires UV-induced photoisomerization, which results in the highly selective complexation of cis-azobenzene. In the case of benzil, the reaction does not lead to the expected 1,2- or 1,4-addition products, but to the non-stereoselective (tBu)2 PCH2 -transfer to a prochiral keto function of benzil. All adducts of 1 were characterised by means of multinuclear NMR spectroscopy, elemental analyses and X-ray diffraction experiments.

Easy Access to Enantiomerically Pure Heterocyclic Silicon-Chiral Phosphonium Cations and the Matched/Mismatched Case of Dihydrogen Release

Phosphonium ions are widely used in preparative organic synthesis and catalysis. The provision of new types of cations that contain both functional and chiral information is a major synthetic challenge and can open up new horizons in asymmetric cation-directed and Lewis acid catalysis. We discovered an efficient methodology towards new Si-chiral four-membered CPSSi* heterocyclic cations. Three synthetic approaches are presented. The stereochemical sequence of anchimerically assisted cation formation with B(C6 F5 )3 and subsequent hydride addition was fully elucidated and proceeds with excellent preservation of the chiral information at the stereogenic silicon atom. Also the mechanism of dihydrogen release from a protonated hydrosilane was studied in detail by the help of Si-centered chirality as stereochemical probe. Chemoselectivity switch (dihydrogen release vs. protodesilylation) can easily be achieved through slight modifications of the solvent. A matched/mismatched case was identified and the intermolecularity of this reaction supported by spectroscopic, kinetic, deuterium-labeling experiments, and quantum chemical calculations.

A Zwitterionic Phosphonium Stannate(II) via Hydrogen Splitting by a Sn/P Frustrated Lewis-Pair and Reductive Elimination

The reactivity of the frustrated Lewis pair (FLP) (F₅C₂)₃SnCH₂P(tBu)₂ (1) was investigated with respect to the activation of elemental hydrogen. The reaction of 1 at elevated hydrogen pressure afforded the intramolecular phosphonium stannate(II) (F₅C₂)₂SnCH₂PH(tBu)₂ (3). It was characterized by means of multinuclear NMR spectroscopy and single crystal X‐ray diffraction. NMR experiments with the two isotopologues H₂ and D₂ showed it to be formed via an H₂ adduct (F₅C₂)₃HSnCH₂PH(tBu)₂ (2) and the subsequent formal reductive elimination of pentafluoroethane; this is supported by DFT calculations. Parahydrogen‐induced polarization experiments revealed the formation of a second product of the reaction of 1 with H₂, [HP(tBu)₂Me][Sn(C₂F₅)₃] (4), in ¹H NMR spectra, whereas 2 was not detected due to its transient nature.

An Adduct of Sulfur Monoxide to a Frustrated Sn/P Lewis Pair

The geminal frustrated Lewis pair (F5 C2 )3 SnCH2 P(tBu)2 (1) reacted with N-sulfinylaniline PhNSO to afford the first sulfur monoxide adduct of a main group metal, (F5 C2 )3 SnCH2 P(tBu)2 ⋅SO (2), which contains a SnCPSO ring. The second product is a phenylnitrene adduct of 1. The surprising stability of 2 was compared with the stabilities of the so far inaccessible O2 and S2 adducts of 1. Attempts to prepare these from 1 and the elemental chalcogens (O2 , S8 , Se∞ , Te∞ ) led to four-membered SnCPE ring systems. Quantum-chemical investigations of 2 demonstrate the bond polarity of the SO unit to stabilize 2.

Triple Frustrated Lewis Pair-Type Reactivity on a Single Rare-Earth Metal Center

Rare-earth metal cations have been used rarely as Lewis-acidic components in the chemistry of frustrated Lewis pairs (FLPs). Herein, we report the first cerium/phosphorus system (2) employing a heptadentate N₄ P₃ ligand, which exhibits triple FLP-type reactivity towards a series of organic substrates, including isocyanates, isothiocyanates, diazomethane, and azides on a single rare-earth Lewis acidic Ce center. This result shows that the Ce center and three P atoms in 2 could simultaneously activate three equivalents of small molecules under mild conditions. This study broadens the diversity of FLPs and demonstrates that rare earth based FLP exhibit unique properties compared with other FLP systems.

Designing Metal-Free Frustrated Lewis Pairs Catalyst for the Efficient Dehydrogenation of Ammonia Borane

Ammonia borane (AB) has been in the spotlight for the chemical storage of hydrogen over the past decade. However, the development of methods for efficient and controlled hydrogen release from AB under mild conditions is still underway. Herein, using density functional theory (DFT) computations, we designed a metal-free frustrated Lewis pair (FLP) catalyst o-(BPh₂ )C₆ H₄ (NiPr₂ ) (M1) that can efficiently dehydrogenate AB to release more than two equivalents of H₂ under mild conditions. Catalyst M1 can dehydrogenate not only AB to H₂ N=BH₂ (AOB) and H₂ , but also oligomers of AOB with rather low free-energy barriers. The high dehydrogenation activity of M1 is the key of new oligomerization routes to the efficient dehydrogenation of AB to borazine (BZ) or H₂ B-(NH=BH)_n -NH₂ (PIB) and finally to polyborazylene (PBZ) so that more than two equivalents of H₂ can be released. A first-principle kinetic Monte Carlo (KMC) study reveals that the activity of our catalytic system can be tuned by varying the initial concentration of M1 and AB. This work can guide the design of catalyst for the highly efficient utilization of AB as a hydrogen storage material.

Bis(perchlorocatecholato)silane-A Neutral Silicon Lewis Super Acid

No neutral silicon Lewis super acids are known to date. We report on the synthesis of bis(perchlorocatecholato)silane and verify its Lewis super acidity by computation (DLPNO-CCSD(T)) and experiment (fluoride abstraction from SbF₆^- ). The exceptional affinity towards donors is further demonstrated by, for example, the characterization of an unprecedented SiO₄ F₂ dianion and applied in the first hydrodefluorination reaction catalyzed by a neutral silicon Lewis acid. Given the strength and convenient access to this new Lewis acid, versatile applications might be foreseen.

Transition-Metal-Free Cleavage of CO

Tertiary silane 1H , 2-[(diphenylsilyl)methyl]-6-methylpyridine, reacts with tris(pentafluorophenyl)borane (BCF) to form the intramolecular pyridine-stabilized silylium 1+ -HBCF. The corresponding 2-[(diphenylsilyl)methyl]pyridine, lacking the methyl-group on the pyridine ring, forms classic N(py)→B adduct 2H -BCF featuring an intact silane Si-H fragment. Complex 1+ -HBCF promotes cleavage of the C≡O triple bond in carbon monoxide with double C-Csp2 bond formation, leading to complex 3 featuring a B-(diarylmethyl)-B-aryl-boryloxysilane fragment. Reaction with pinacol generates bis(pentafluorophenyl)methane 4 as isolable product, proving the transition-metal-free deoxygenation of carbon monoxide by this main-group system. Experimental data and DFT calculations support the existence of an equilibrium between the silylium-hydroborate ion pair and the silane-borane mixture that is responsible for the observed reactivity.

Improved reactivity of a cyclic 13/15 compound by increased steric demand

The reactivity of a masked flp was investigated for the sterically overloaded group 13/15 ring compound [tBu₂GaP(H)tBu₂Ph]₂, as can be seen from its reaction with phenyl-isocyanate.

Contrasting Frustrated Lewis Pair Reactivity with Selenium- and Boron-Based Lewis Acids

The activation of π-bonds in diynyl esters has been investigated by using soft and hard Lewis acids. In the case of the soft selenium Lewis acid PhSeCl, sequential activation of the alkyne bonds leads initially to an isocoumarin (1 equiv PhSeCl) and then to a tetracyclic conjugated structure with the isocoumarin subunit fused to a benzoselenopyran (3 equiv PhSeCl). Conversely, the reaction with the hard Lewis acidic borane B(C6 F5 )3 initiates a cascade reaction to yield a complex π-conjugated system containing phthalide and indene subunits.