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Pyramidane molecules have attracted chemists for many decades due to their regular shape, high symmetry and their correspondence in the macroscopic world. Recently, experimental access to a number of examples has been reported, in particular the rarely reported square pyramidal bora[4]pyramidanes. To describe the bonding situation of the nonclassical structure of pyramidanes, we present solid-state Nuclear Magnetic Resonance (NMR) as a versatile tool for deciphering such bonding properties for three now accessible bora[4]pyramidane and dibora[5]pyramidane molecules. 11 B solid-state NMR spectra indicate that the apical boron nuclei in these compounds are strongly shielded (around -50 ppm vs. BF 3 -Et 2 O complex) and possess quadrupolar coupling constants of less than 0.9 MHz pointing to a rather high local symmetry. 13 C- 11 B spin-spin coupling constants have been explored as a measure of the bond covalency in the borapyramidanes. While the carbon-boron bond to the -B(C 6 F 5 ) 2 substituents of the base serves as an example for a classical covalent 2-center-2-electron (2c-2e) sp 2 -carbon-sp 2 -boron σ-bond with 1 J( 13 C- 11 B) coupling constants in the order of 75 Hz, those of the boron(apical)-carbon(basal) bonds in the pyramid are too small to measure. These results suggest that these bonds have a strongly ionic character, which is also supported by quantum-chemical calculations., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The study of 3D architectures at a molecular scale has fascinated chemists for generations. This includes molecular pyramids with all-carbon frameworks, such as trigonal, tetragonal and pentagonal pyramidal geometries. A small number of substituted tetrahedranes and all-carbon [4]-[5]pyramidanes have been experimentally generated and studied. Although the hypothetical unsubstituted parent [3]-[6]pyramidanes have only been explored computationally, the formal replacement of carbon vertices with isolobal main group element fragments has provided a number of examples of stable hetero[m]pyramidanes, which have been isolated and amply characterized. In this Review, we highlight the synthesis and chemical properties of [3]-[6]pyramidanes and summarize the progress in the development of chemistry of pyramid-shaped molecules., (© 2023. Springer Nature Limited.)

The neutral N-heterocyclic carbene stabilized bora-alkene 1, conveniently prepared by a BH borenium/hydroboration route, forms stable copper, gold or palladium π-complexes. The polar bora-alkene B=C system undergoes regioselective hydroboration reactions with the (C 6 F 5 ) 2 BH or C 6 F 5 BH 2  ⋅ SMe 2 boranes. The latter reaction involves a subsequent rearrangement that leads to internal hydride vs. isothiocyanate substituent exchange at the borane pair., (© 2023 Wiley-VCH GmbH.)

Hydroboration of styrene or vinylcyclohexane with the IMes(C 6 F 5 )BH + cation followed by deprotonation provided a convenient synthetic entry to the [B]=CHCH 2 R boraalkenes 9 a and 9 b. The in situ generated IMes(SCN)BH + system reacted similarly with 1,1-diphenylethene followed by deprotonation to give the isothiocyanato substituted boraalkene 9 c. The boraalkenes underwent [2+2] cycloaddition reactions with a small series of heterocumulenes to give the respective four-membered heterocycles. The [B]=CHCH 2 R+CO 2 cycloadducts 13 a and 13 b added the borane HB(C 6 F 5 ) 2 with cleavage of the central B-C σ-bond. CS 2 underwent an unusual reaction with the boraalkenes, namely insertion into the B=C bond with formation of the borylated dithioketene acetal under complete rupture of the strong B=C double bond. The intermediate dithiobora-β-lactone type intermediate was isolated in the case of the isothiocyanato-boraalkene reaction and characterized by X-ray diffraction., (© 2022 Wiley-VCH GmbH.)

Examples of isolated boron substituted nitroxide radicals are rare. The reaction of the reactive cyclic boraalkene 3 with nitrosobenzene yields a mixture of the [2 + 2] cycloaddition product 4a, the B -nitroxide radicals 5a and 6a and the azoxybenzene co-product 7a via a bora nitroso ene reaction pathway, the boron analogue of the nitroso ene reaction. The products were separated by flash chromatography, and the B -nitroxide radicals were characterized by X-ray diffraction and EPR spectroscopy. Radical 5a was shown to be a hydrogen atom abstractor. Both the B -nitroxide radicals are more easily oxidized compared to e.g. TEMPO, as shown by cyclic voltammetry., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The B(C 6 F 5 ) 2 -substituted borole 2⋅SMe 2 was obtained from the phenyl bis(trimethylsilylethynyl) borane SMe 2 adduct (1⋅SMe 2 ) by a synthetic sequence containing a rare 1,1-hydroboration reaction. Subsequent thermolysis at 50 °C converted it to the bicyclic borenium/borate zwitterion 3⋅SMe 2 . Photolysis of 3⋅SMe 2 gave the diboracyclohexadiene derivative 4⋅SMe 2 , which after ligand exchange with N,N-dimesitylimidazolylidene and deprotonation gave the 1-bora-3-boratabenzene anion derivative as its potassium salt 6. Some unique follow-up reactions of the unsaturated diboron containing six-membered heterocycles were investigated., (© 2022 Wiley-VCH GmbH.)

The BH compounds IMes(Ar F )BH(NTf 2 ) (Ar F : C 6 F 5 or FpXyl) were converted to the IMes(Ar F )BCH 2 boraalkenes in a three step reaction sequence of B-methylation with methyllithium, hydride abstraction and deprotonation. The BCH 2 boraalkenes reacted with elemental sulfur to give a thiaborirane product. They underwent [2+2] cycloaddition reactions with carbon dioxide or sulfur dioxide to give four-membered boron containing heterocycles. The boraalkenes added strongly Lewis acidic boranes at their CH 2 carbon atoms. The corresponding HB(C 6 F 5 ) 2 /boraalkene adduct reduced carbon monoxide to a -OCH(C 6 F 5 )- moiety inside a five-membered heterocycle at the B-CH 2 -B template. The boraalkenes reacted with the [(Me 2 S)AuCl] reagent to form the corresponding (boraalkene)AuCl complexes.

Boroles and borapyramidanes are classical and nonclassical constitutional isomers, respectively. It is here shown that they can indeed be interconverted. Treatment of the bis(alkynyl)B(C 6 F 5 ) SMe 2 adduct 3 ·SMe 2 with HB(C 6 F 5 ) 2 gave borole 1· SMe 2 , featuring trimethylsilyl substituents in both α positions to boron, by means of a 1,1-hydroboration/alkenylboration sequence. Photolysis of the classical borole adduct 1· SMe 2 resulted in rearrangement to its nonclassical structural isomer, borapyramidane 2 , in high yield, which exhibits a vicinal pair of trimethylsilyl substituents at the square pyramidane base. Neutral borapyramidane 2 is a rare example of an isoster of the (CH) 5 + pyramidane cation. Thermolysis of borapyramidane 2 in the presence of SMe 2 at 60 °C re-formed borole 1· SMe 2 , which converted at 100 °C to 2,3-bis-silyl-substituted borole isomer 8· SMe 2 . Its photolysis also gave borapyramidane 2 . Prolonged photolysis of 2 at elevated temperatures converted this to borapyramidane isomer 10 containing a pair of trimethylsilyl groups in 1,3-position at its square C 4 -pyramidal base. The borole and borapyramidane isomers were characterized by X-ray diffraction, and the system was analyzed by density functional theory (DFT) calculations.

The P-stereogenic bis(phosphanes) 7 and 9, featuring pairs of P(Mes)-ethynyl or vinyl substituents at the dimethyl xanthene backbone show rather low barriers of stereochemical inversion at phosphorus. π-Conjugative effects are probably causing these low inversion barriers. Compound 7 reacted with B(C 6 F 5 ) 3 to form the nine-membered heterocyclic product 10, featuring a [P]-C≡C-B(C 6 F 5 ) 3 substituent. Compound 7 was converted to the bis[P(Mes)vinyl] xanthene derivative 9, which gave the zwitterionic P(H)(Mes)-CH=CH-B(C 6 F 5 ) 3 containing product 16 upon treatment with B(C 6 F 5 ) 3 . Thermally induced epimerization barriers at phosphorus of ca. 20 to 27 kcal mol -1 were calculated by DFT for the alkenyl- and alkynyl-P derived systems 6 to 9, 15 and 16 and experimentally determined for the examples 7 and 16., (© 2022 Wiley-VCH GmbH.)

Proton-detected solid-state NMR enables atomic-level insight in solid-state reactions, for instance in heterogeneous catalysis, which is fundamental for deciphering chemical reaction mechanisms. We herein introduce a phosphorus-31 radiofrequency channel in proton-detected solid-state NMR at fast magic-angle spinning. We demonstrate our approach using solid-state 1 H/ 31 P and 1 H/ 13 C correlation experiments at high magnetic fields (850 and 1200 MHz) and high spinning frequencies (100 kHz) to characterize four selected PH-containing compounds from the chemistry of phosphane-borane frustrated Lewis pairs. Frustrated Lewis pairs have gained high interest in the past years, particularly due to their capabilities of activating and binding small molecules, such as di-hydrogen, however, their analytical characterization especially in the solid state is still limited. Our approach reveals proton-phosphorus connectivities providing important information on spatial proximity and chemical bonding within such compounds. We also identify protons that show strongly different chemical-shift values compared to the solution state, which we attribute to intermolecular ring-current effects. The most challenging example presented herein is a cyclotrimeric frustrate Lewis pair-associate comprising three crystallographically distinct phosphonium entities that are unambiguously distinguished by our approach. Such 31 P spin-filtered proton-detected NMR can be easily extended to other material classes and can strongly impact the structural characterization of reaction products of hydrogen-activated phosphane/borane FLPs, heterogeneous catalysts and solid-state reactions in general.

Reaction of the borane FmesBH 2 ·SMe 2 [Fmes: 2,4,6-tris(trifluoromethyl)phenyl] with two molar equiv. of a small series of 1-alkenes followed by treatment with two molar equivalents of the bulky isonitrile CN-Xyl (Xyl: 2,6-dimethylphenyl) gives 1,2-azaborolidine derivatives that exhibit an exocyclic alkene unit and a NHXyl substituent at the heterocyclic core.

Deprotonation of [(NHC)(Fmes)B-allyl] + borenium cations (NHC, IMes ( a ) or IMe 2 ( b ); Fmes, 2,4,6-(CF 3 ) 3 C 6 H 2 ) provides an easy entry to the NHC-stabilized 1-bora-1,3-butadienes. They feature a planar s-trans-conformation just like 1,3-butadiene. The 1-borabutadiene 7a undergoes hydroboration reactions; the HB(C 6 F 5 ) 2 hydroboration product is trapped with CO or an isonitrile to give the respective cyclic zwitterionic borenium-borate enolate or enamide products. 1-Borabutadiene 7b undergoes 1,4-chalcogenation with elemental sulfur or selenium, and it gives the six-membered heterocyclic 1,4-addition product with the S═O bond of sulfur dioxide. Compound 7b served as a precursor for the formation of a borylated η 3 -allyl ligand at Ru. 7b formed a Rh complex by reaction with [Rh(ethylene) 2 Cl] 2 . It subsequently underwent an intramolecular C-H activation reaction to a mixture of η 3 -methyl-boraallyl Rh complex isomers.

The borane FmesBH 2 reacts in a three-component reaction with an isonitrile and a small series of organonitriles to give rare examples of the class of dihydro-1,4,2-diazaborole derivatives. In a related way, annulated BN-indolizine derivatives became conveniently available, as were dihydro-1,4,2-oxaza- or thiazaborole derivatives. The nucleophilic framework of a dihydro-1,4,2-diazaborole example allowed for an uncatalyzed acylation reaction. It also served as a 1,3-dipolar reagent and underwent a [3+2] cycloaddition/[4+2] cycloreversion sequence when treated with methyl propiolate to give the respective pyrrole product. The [3+2] cycloaddition product of a dihydro-1,4,2-diazaborole derivative with N-phenylmaleimide was isolated and its heterobicyclo[2.2.1]heptane derived structure characterized by X-ray diffraction., (© 2021 Wiley-VCH GmbH.)

The synthesis and characterization of the bis(η 6 -benzene)lithium cation, the benzene metallocene of the lightest metal, is reported. The boron compound FmesBCl 2 [Fmes: 2,4,6-tris(trifluoromethyl)phenyl] reacted with three molar equivalents of the lithio-acetylene reagent Li-C≡C-Fmxyl [Fmxyl: 3,5-bis(trifluoromethyl)phenyl]. Subsequent crystallization from benzene gave the [bis(η 6 -benzene)Li] + cation with the [{FmesB(-C≡C-Fmxyl) 3 } 2 Li] - anion. This parent [(arene) 2 Li] + cation shows an eclipsed arrangement of the pair of benzene ligands at the central lithium cation with uniform carbon-lithium bond lengths. The corresponding [(η 6 -toluene) 2 Li] + and [(η 6 -durene) 2 Li] + containing salts were similarly prepared. The bis(arene)lithium cations were characterized by X-ray diffraction, by solid-state 7 Li MAS NMR spectroscopy and their bonding features were analyzed by DFT calculations., (© 2021 Wiley-VCH GmbH.)

The η 2 -formyl borane system 3 is readily available by carbonylation of the vicinal P/BH frustrated Lewis pair (FLP) 1 . It serves as a frustrated C/B Lewis pair toward carbon dioxide or phenylisocyanate. In the presence of B(C 6 F 5 ) 3 , it forms the coupling product between two CO-derived units. The resulting compound 13 rearranged to a doubly O -borylated endiolate, with both of the central carbon atoms originating from carbon monoxide. The subsequent treatment with a silane gave a rare macrocyclic silicon endiolate.

The synthesis of element-carbon double bonds is of great importance for the development and understanding of reactive π-bonded systems in chemistry. The seven-membered heterocyclic system 4 b is readily made by internal C-H activation at a pendent isopropyl methyl group of the respective [(IPr)C 6 F 5 BH] + borenium ion. Subsequent deprotonation with the IMes carbene gives the neutral cyclic boraalkene system 5 b. The B=C double bond in compound 5 b adds carbon dioxide, CS 2 , sulfur dioxide, phenyl isocyanate, an acetylenic ester or two NO molecules to give the corresponding four-membered ring annulated heterocycles. With sulfur or selenium 5 b gives the respective three-membered ring systems. N 2 O reacts with 5 b to give a mixture of the related oxaborirane 18 and a unique [B]OH containing diazoalkane product 19., (© 2021 Wiley-VCH GmbH.)

Attachment of a pair of P-stereogenic mesityl(alkynyl)phosphanyl groups at the 4- and 5-positions of a 9,9-dimethylxanthene framework gave mixtures of the respective rac- and meso-bisphosphanyl diastereoisomers. They slowly epimerized in a thermally induced reaction with Gibbs activation barriers of about 25 kcal mol -1 at room temperature (measured and DFT calculated). The reaction of the meso-mesityl(tert-butylethynyl)phosphanyl derivative with two molar equivalents of Piers' borane [HB(C 6 F 5 ) 2 ] led to the formation of the alkylidene-bridged geminal bisphosphane/borane-frustrated Lewis pair system. The compound was obtained enriched (>85 %) in the rac diastereoisomer. With a variety of bifunctional donor substrates, the rac-bis-P/B FLP formed macrocyclic compounds. They were all formally derived from meso-configurated diastereoisomers of the bisphosphanylxanthene backbone., (© 2021 Wiley-VCH GmbH.)

We report on the synthesis of the first examples of 1,3-azaborinine derived oxindole systems, the BN-isosteres of the important compound class of the oxindoles. Hydroboration of terminal aryl acetylenes with FmesBH2, followed by treatment with 2 equiv. of a glycine ester derived isonitrile gave a small series of 1,3-azaborinine derived oxindoles. The new BN-oxindoles show interesting photophysical behavior.

Cyclohexene reacts with the (Fmes)BH2·SMe2 borane reagent and three molar equivalents of the isonitrile CN-Xyl to give the five membered 1,3-BN heterocyclic product 7 that contains a zwitterionic borata-amidinium moiety and a cyclohexenyl substituent. The analogous five-component coupling between cyclopentene, (Fmes)BH2·SMe2 and CN-Xyl in a 1 : 1 : 3 molar ratio gives the related cyclic amidino-borate derivative 10. The reaction of the (Fmes)BH2 derived frustrated Lewis pair 12, in situ generated or employed as the isolated dimer, reacts with 3 CN-Xyl equivs. at elevated temperature (60 °C) to yield the analogous coupling product 13.

The reaction of 2,4,6-tri(tert-butyl)phenyl vinyl phosphane with Piers' borane [HB(C6F5)2] gave the ethylene-bridged PH/B frustrated Lewis pair (FLP) system. It is a monomer at high temperature (>323 K), but exists as an associated 12-membered macrocyclic trimer below 273 K. The PH/B FLP splits dihydrogen and serves as a metal-free hydrogenation catalyst. It adds carbon dioxide. It serves as a PH/B template for the reduction of carbon monoxide by the HB(C6F5)2 borane to the formyl stage. The resulting six membered P/B/O containing heterocycle is opened upon treatment with pyridine and it reacts with benzaldehyde in a boron mediated Claisen-Tishchenko reaction.

The Mes 2 P-C≡C-SiMe 3 alkyne reacts with the borane H 2 B-Fmes by means of a rare 1,1-hydroboration reaction to give an unsaturated C 2 -bridged frustrated P/B-H Lewis pair. Most of its reactions are determined by the presence of the B-H functionality at the FLP function and the activated connecting carbon-carbon double bond. It reduces carbon monoxide to the formyl stage. With nitriles it reacts in an extraordinary way: it undergoes a reaction sequence that eventually results in the formation of a P-substituted dihydro-1,2-azaborole derivative. Several similar examples were found. In one case a P-ylide was isolated that was related to an intermediate of the reaction sequence. It subsequently opened in an alternative way to give an alkenyl borane product., (© 2020 Wiley-VCH GmbH.)

The (Fmes)BH 2 ·SMe 2 reagent ( 7 ) reacts sequentially with an acetylene and, e.g., xylylisonitrile in a convenient three-component reaction to give a series of unprecedented dihydro-1,3-azaborole derivatives 16 . The tolane-derived example 16a was deprotonated and used as a ligand in organometallic chemistry. Compounds 16 served as the starting materials for the straightforward synthesis of various dihydro-1,3-azaborinine derivatives by treatment with an isonitrile. Several diaryldihydro-1,3-azaboroles showed interesting photophysical properties such as aggregation-induced emission and high fluorescence quantum yields.

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH 2 C 6 F 5 (NHC: IMes or IMe 4 ) gave the B-H containing [(NHC)B(H)C 6 F 5 ] + borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C 6 F 5 )CO] + boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives., (© 2020 Wiley-VCH GmbH.)

Insertion of sulfur into the B-H bond of the BH borenium salt [IMes(C 6 F 5 )BH] + followed by deprotonation gave the thioxoborane IMes(C 6 F 5 )B═S. Subsequent treatment with additional sulfur gave the corresponding boron persulfide, a NHC-stabilized boradithiirane. The B═S compound reacted with carbon dioxide in the presence of the lithium salt Li[B(C 6 F 5 ) 4 ] by formal [2+2] cycloaddition to give a boron thiocarbonate-type product. The boron persulfide formally inserted phenyl acetylene into the B-S bond in the presence of Li[B(C 6 F 5 ) 4 ] to give the respective five-membered heterocycle.

A dimeric ethylene-bridged PH/BH system reduced carbon monoxide to the -CH 2 -O- state. In the presence of B(C 6 F 5 ) 3 , the frustrated PH/BH Lewis pair reacted with carbon monoxide by reductive coupling of two CO molecules at the template. Removal of the B(C 6 F 5 ) 3 borane with pyridine liberated one equiv of carbon monoxide to give a cyclic five-membered P(═O)-CH 2 -B compound, the product of reductive cleavage of carbon monoxide. It reacted as a borylated Horner P(═O)CH 2 B carbon nucleophile with carbon dioxide to give a bicyclic product by P-CH 2 attack on CO 2 combined with internal P═O to boron coordination.

The reactive borane reagent FpXylBH2•SMe2 was prepared from 1,4-bis(trifluoromethyl)benzene by treatment with n-BuLi, followed by H3B·SMe2 and subsequent removal of hydride. It undergoes a regioselective hydroboration reaction with 1,2-bis(trimethylsilylethynyl)benzene to give the "dimeric" product 13a featuring a conjugated 14-membered core heterocyclic structure that contains a pair of FpXylB units.

(2,6-Dimesitylphenyl)P(vinyl) 2 5d reacts with HB(C 6 F 5 ) 2 in a sequence involving a rare example of a 1,1-carboboration of an olefin to give the borylated tetrahydrophosphole derivative 6d. Compound 6d is an active frustrated Lewis pair that splits dihydrogen under mild conditions and serves as a metal-free hydrogenation catalyst. It also adds to carbon dioxide. Compound 6d serves as an intermediate in the HB(C 6 F 5 ) 2 catalyzed aryl(divinyl)phosphane (5d) to dihydrophosphole conversion.

Reaction of the secondary alkynyl(Mes*)PH phosphane 2 with (Fmes)BH 2 ⋅SMe 2 gives the geminal PH/BH frustrated Lewis pair (FLP) 3. The PH and the BH functions are jointly used in the reduction of carbon monoxide under mild reaction conditions to give the [P]-CH 2 -O-[B] product. A subsequent cycloaddition sequence results in the liberation of formaldehyde. The FLP 3 reacts with benzonitrile to give a P-benzamidine, and it couples two isonitriles at the FLP framework., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Borata-alkenes can serve as anionic olefin equivalent ligands in transition metal chemistry. A chelate ligand of this type is described and used for metal coordination. Deprotonation of the Mes 2 P(CH 2 ) 2 B(C 6 F 5 ) 2 frustrated Lewis pair in the α-CH[B] position gave the methylene-bridged phosphane/borata-alkene anion. It reacted with the [Rh(nbd)Cl] or [Rh(CO) 2 Cl] dimers to give the respective neutral chelate [P/C[double bond, length as m-dash]B][Rh] complexes. The reaction of the [P/C[double bond, length as m-dash]B] - anion with [Ir(cod)Cl] 2 proceeded similarly, only that the complex underwent a subsequent oxidative addition reaction at the mesityl substituent. Both the resulting Ir(iii)hydride complex 15 and the P/borata-alkene Rh system 12 were used as hydrogenation catalysts. The [P/C[double bond, length as m-dash]B(C 6 F 5 ) 2 ]Rh(nbd) complex 12 served as a catalyst for arylacetylene polymerization., (This journal is © The Royal Society of Chemistry 2020.)

Modern solid-state NMR techniques offer a wide range of opportunities for the structural characterization of frustrated Lewis pairs (FLPs), their aggregates, and the products of cooperative addition reactions at their two Lewis centers. This information is extremely valuable for materials that elude structural characterization by X-ray diffraction because of their nanocrystalline or amorphous character, (pseudo-)polymorphism, or other types of disordering phenomena inherent in the solid state. Aside from simple chemical shift measurements using single-pulse or cross-polarization/magic-angle spinning NMR detection techniques, the availability of advanced multidimensional and double-resonance NMR methods greatly deepened the informational content of these experiments. In particular, methods quantifying the magnetic dipole-dipole interaction strengths and indirect spin-spin interactions prove useful for the measurement of intermolecular association, connectivity, assessment of FLP-ligand distributions, and the stereochemistry of adducts. The present review illustrates several important solid-state NMR methods with some insightful applications to open questions in FLP chemistry, with a particular focus on supramolecular associates.

The active six-membered cyclo-FLP 6 undergoes a rapid P/B addition reaction to carbon dioxide. At elevated temperature, the resulting heterobicyclo[2.2.2]octane derived product 7 undergoes ring opening and equilibrates with the cyclotetramer (7) 4 . In the large macrocyclic structure, four monomeric six-membered cyclo-FLP units are connected by four CO 2 molecules to form the supramolecular ring system. The P/B cyclo-FLP 6 undergoes a variety of additional cycloaddition reactions., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The cyclic five-membered frustrated phosphane/borane Lewis pairs 11 a, b featuring the bulky octaethylhydrindacenyl- (Eind) substituent or its mono-bromo derivative ( Br Eind) at phosphorus are monomeric at room temperature. The reactive frustrated Lewis pairs (FLPs) cleave dihydrogen. The cyclic FLP 11 b ( Br Eind) undergoes 1,2-P/B addition to ethylene to give the zwitterionic heteronorbornane derivative 14 b. It reacts similarly with the carbon-carbon double bond of norbornene. With a variety of organic π-reagents, the cyclic FLP 11 b often undergoes reaction sequences reminiscent of the Alder-Rickert reaction: the cycloaddition reaction is followed by rapid cycloreversion to form new five-membered heterocyclic FLP products with extrusion of ethene. Reactions of 11 b with benzaldehyde or with acetylenes follow this reaction pattern., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The trimethylene-linked bis-allene 3a reacts with Piers' borane [HB(C 6 F 5 ) 2 ] by a hydroboration/allylboration sequence to generate the cyclization product 5a . Its pyridine adduct was isolated and characterized by X-ray diffraction. Compound 5a undergoes a typical frustrated Lewis pair 1,2-P/B alkene addition reaction with PPh 3 to give the heterobicyclic bridged olefinic zwitterionic product 9a . The tetramethylene-linked bis-allene 3b and its phenylene annulated analogue 3c react with HB(C 6 F 5 ) 2 to give the analogous seven-membered ring products 5b,c under mild conditions. The cyclization product 5a undergoes a series of sequential allylboration reactions with two equivalents of allene followed by ring-closure to give the four-component coupling product 12a . It undergoes FLP addition to an exo-methylene group upon treatment with PPh 3 . Compound 12a is oxidatively converted to the boron-free alcohol., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The twofold hydroboration products of (Fmes)BH 2 ⋅SMe 2 with a series of alkynes (2-butyne, arylethynes) react with two molar equiv of 2,6-dimethylphenyl isocyanide (CN-Xyl) at 80 °C to give rare examples of 1,3-azaborinine derivatives. A mechanistic study revealed a reaction course involving insertion of one isonitrile followed by a bora-Nazarov type ring-closure reaction and subsequent isonitrile insertion to give the respective 1,3-dihydro-1,3-azaborinines 5., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The three-component reactions of bulky diarylbutenylphosphanes with dimethyl acetylenedicarboxylate (1a) and the strong boron Lewis acid B(C 6 F 5 ) 3 give the ylide-substituted cyclopropane derivatives 26. The acetylenic ester 1a and ketone 1b react with oligomethylene bridged frustrated phosphane/borane Lewis pairs to give the bicyclic C-H insertion/C-C coupling reaction products. The formal carbene products are formed by means of stepwise sequences involving polar intermediate.

Phenylallene reacts rapidly with ClB(C6F5)2 to give the respective 2-borylindene. Several substituted allenylarenes form the respective 2-B(C6F5)2 boryl-substituted indenes upon treatment with ClB(C6F5)2 or BrB(C6F5)2 as well. Bis- and tris-allenylarenes form the corresponding products featuring multiple five-membered ring annulations, including a symmetrical tris-borylated dihydro-1H-trindene derivative. The B(C6F5)2 borylindenes show fluorescence properties.

Four hydrogenated intramolecular phosphane-borane frustrated Lewis pair (B/P FLP) compounds bearing unsaturated cyclic or aromatic carbon backbones have been synthesized and structurally characterized using 11 B, 31 P, 1 H and 2 H solid-state NMR spectroscopy. A comparison of the spectra with those of the corresponding free B/P FLPs shows that both 11 B isotropic chemical shifts as well as nuclear electric quadrupolar coupling constants decrease significantly upon FLP hydrogenation, revealing the breakage of the partial B-P bond present in the starting materials. Likewise, the 31 P isotropic chemical shift, the chemical shift anisotropy, and the asymmetry parameter decrease significantly upon FLP hydrogenation, reflecting the formation of a more symmetric, C 3v -like local environment. 11 B{ 31 P} rotational echo double resonance (REDOR) experiments can be used to measure the B-P internuclear distance (about 3.2 Å) of these compounds. Observation of the hydrogen atoms bound to the Lewis centers is best accomplished via 31 P{ 1 H} and 11 B{ 1 H} cross-polarization-heteronuclear correlation experiments or by direct observation of the 2 H MAS NMR signals on especially prepared FLP-D 2 adducts. For accurately measuring the phosphorus-deuterium distance via 31 P{ 2 H} rotational echo adiabatic passage double resonance (REAPDOR), it is essential to take the secondary dipolar coupling of 31 P with the boron-bonded 2 H nuclei explicitly into consideration, by simulating a 2 H P - 31 P- 2 H B three-spin system based on structural input. All of the experimental NMR interaction parameters are found in excellent agreement with values calculated by DFT methods, using the geometries obtained either by energy optimization or from single-crystal structures., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Treatment of 1,3,5-tris(trifluoromethyl)benzene with n-BuLi, followed by H 3 B⋅SMe 2 and subsequent hydride removal gave the (Fmes)BH 2 reagent, which was isolated as a SMe 2 stabilized monomer or a ligand free (μ-H) 2 -bridged dimer. Reaction with Mes 2 P(vinyl) gave the respective ethylene-bridged P/B(Fmes)H FLP. It reduced carbon monoxide to the formyl stage and carbon dioxide to the formaldehyde oxidation state. Most new compounds were characterized by X-ray diffraction., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The P/B FLP 6, formed by the 1,1-hydroboration of dimesitylphosphino(trimethylsilyl)acetylene with HB(C6F5)2 forms the Au(i)X complexes 9a (X: Cl) and 10a (X: NTf2), respectively. Both show marked AuB interactions. The P/B FLP isomer 8, featuring the bulky SiMe3 substituent at the carbon adjacent to boron forms gold complexes 9b and 10b, both of which show weaker AuB interactions. Complex 10a was employed in the catalytic hydroamination of a series of alkynes with p-toluidine. Complexes 9 and 10 were characterized by X-ray diffraction.

1-(Trimethylsilyl)propyne 2a reacts with halogenoboranes XB(C6F5)2 (X: Cl, Br) in a 1 : 1 molar ratio at r.t. to give the respective alkenylboranes 3. These undergo a subsequent 1,1-alkenylboration reaction with additional alkyne 2a to give the persubstituted 3-boryl-2,4-hexadiene products 4. The conjugated diyne bis(trimethylsilyl)butadiyne 2d reacts with ClB(C6F5)2 in a 2 : 1 molar ratio by a sequence of consecutive 1,1-carboboration reactions to give the persubstituted borylated octadiendiyne product 6.

Proton spectroscopy in solid-state NMR on catalytic materials offers new opportunities in structural characterization, in particular of reaction products of catalytic reactions such as hydrogenation reactions. Unfortunately, the 1 H NMR line widths in magic-angle spinning solid-state spectra are often broadened by an incomplete averaging of 1 H- 1 H dipolar couplings. We herein discuss two model compounds, namely the H 2 -splitting products of two phosphane-borane Frustrated Lewis Pairs (FLPs), to study potentials and limitations of proton solid-state NMR experiments employing magic-angle spinning frequencies larger than 100 kHz at a static magnetic field strength of 20.0 T. The 1 H lines are homogeneously broadened as illustrated by spin-echo decay experiments. We study two structurally similar materials which however show significant differences in 1 H line widths which we explain by differences in their 1 H- 1 H dipolar networks. We discuss the benefit of fast MAS experiments up to 110 kHz to detect the resonances of the H + /H - pair in the hydrogenation products of FLPs., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Tame d 0 phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d 1 precursors with [Cp 2 Fe][BPh 4 ]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d 0 complexes, whereas dπ-pπ repulsion prevents such interactions in the d 1 complexes. In addition, CH-π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d 0 complexes were found to be light sensitive, and decompose through Ti-P bond homolysis to give Ti III species. A naked d 0 phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new six-membered cyclic frustrated phosphane/borane Lewis pair was liberated from its HB(C 6 F 5 ) 2 adduct by treatment with vinylcyclohexane. The system is an active frustrated Lewis pair that undergoes cycloaddition reactions with suitable π reagents and it splits dihydrogen. At room temperature in solution the new compound is a monomer, however, in the crystal and in solution at low temperature it aggregates to a thermodynamically favoured supramolecular macrocyclic cyclooctamer., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The halogenoboranes XB(C 6 F 5 ) 2 (X: Cl, Br) react with allene to give a mixture of the cyclotrimer 1,3,5-trimethylenecyclohexane ( 1 ) and the related halogenoborylated cyclotetramerization products 3a and 3b . Alkyl-substituted allenes were catalytically cyclotrimerized metal-free by XB(C 6 F 5 ) 2 (X: Cl, Br) to give cis , trans -2,4,6-trialkyl-1,3,5-trimethylenecyclohexanes under mild conditions.

A series of phosphanyl substituted alkynes R12P-C[triple bond, length as m-dash]C-R2 [R12P: Mes2P or tBu2P; R2: SiMe3, Ph, tBu] reacts with the X-B(C6F5)2 reagents [X: Cl, Br] to give thermally rather stable neutral zwitterionic phosphirenium borate products. Five examples of this class of compounds were characterized by X-ray diffraction.

The biphenylene derived triborane(7) derivatives 7 featuring an internal phosphane donor group can readily be prepared from the respective in situ generated aryllithium compound with excess BH3·SMe2. The internal donor group assisted assembly procedure was also applied to the synthesis of the phenylene bonded triborane(7) compound 11. These aryl-triborane(7) systems can further react with HB(C6F5)2 to give the doubly C6F5 functionalized derivatives 12 and 13. The products were characterized spectroscopically and by X-ray diffraction.

The β-borylborole 3 adds phenylacetylene in a [4+2]-cycloaddition to give the boryl-substituted 7-boranorbornadiene derivative 4 b. Thermolysis (60 °C) results in a rearrangement with B-B coupling to give the tetraaryldiborane(4) 6 b. It splits dihydrogen with cleavage of the B-B bond. With the terminal acetylene 1-pentyne the diborane(4) compound 6 b undergoes a rare 1,1-diboration reaction to yield the respective geminal diborylalkene product 14., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A series of arylallenes react with HB(C 6 F 5 ) 2 in a 2:1 molar ratio to give the tail-to-tail 1,6-diaryl-2-boryl-hexa-1,5-diene coupling products. The reaction of the phenylallene substrate with HB(C 6 F 5 ) 2 was shown to initially give the 2-boryl-3,4-diphenyl-1,5-hexadiene head-to-head coupling product which then rearranged, by a thermally induced Cope rearrangement, into the final product., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

In the presence of two molar equiv. of B(C 6 F 5 ) 3 p -benzoquinone reacts with persistent radicals TEMPO, trityl or decamethylferrocene by single electron transfer to give doubly O -borylated benzosemiquinone radical anions with TEMPO + , trityl or ferrocenium counter cations. All three [(C 6 F 5 ) 3 B] 2 -semiquinone radical anion salts were characterized by X-ray diffraction. The addition of donor reagent THF or DMSO induced rapid back electron transfer, in the case of the [(C 6 F 5 ) 3 B] 2 -semiquinone radical anion oxoammonium salt giving rise to the formation of the (C 6 F 5 ) 3 B-DMSO (or THF) Lewis adduct, p -benzoquinone and the TEMPO radical. The reaction of 9,10-anthraquinone or acenaphthenequinone with either the Gomberg dimer or in 1 : 1 stoichiometry in the presence of two molar equiv. of B(C 6 F 5 ) 3 gave the respective two-fold O-B(C 6 F 5 ) 3 containing 9,10-anthrasemiquinone or acenaphthene-semiquinone radical anion salts with either Ph 3 C + or counter cations. These products were also characterized by X-ray diffraction. The salts showed analogous back electron shuttling behavior upon treatment with DMSO. 9,10-Phenanthrenequinone reacted analogously with B(C 6 F 5 ) 3 and the electron rich ferrocene. The [(C 6 F 5 ) 3 B] 2 -9,10-phenanthrene-semiquinone salt was characterized by X-ray diffraction. The radical anions were characterized by ESR spectroscopy.