Monomeric phosphinoboranes

A General Pathway towards NHC ⋅ GaH2 (OTf) Adducts - The Key for the Synthesis of NHC-Stabilized Cationic 13/15 Chain Compounds of Gallium

A general pathway towards NHC (NHC=N-heterocyclic carbene)-stabilized galliummonotriflates NHC ⋅ GaH2 (OTf) (NHC=IDipp, 1 a; IPr2 Me2 , 1 b; IMes, 1 c; IDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene, IPr2 Me2 =1,3-bis-(diisopropyl)-4,5-dimethyl-imidazolin-2-ylidene, IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene) is reported. Quantum chemical calculations give detailed insight into the underlying reaction pathway. The obtained NHC ⋅ GaH2 (OTf) compounds were employed in reactions with donor-stabilized pnictogenylboranes to synthesize the elusive cationic parent 13/15/13 chain compounds [IDipp ⋅ GaH2 ER2 E'H2  ⋅ D][OTf] (3 a: D=IDipp, E=P, E'=B, R=H; 3 b: D=NMe3 , E=P, E'=B, R=H, 3 c: D=NMe3 , E=P, E'=B, R=Ph, 3 d: D=IDipp, E=P, E'=Ga, R=H). Supporting computational studies highlight the electronic features of the products.

Phosphorus-Boron Multiple Bonding in the π Radical HBP

The HBP radical was generated via the reaction of laser ablated boron atom with PH₃ in a solid neon matrix, which is identified via IR spectroscopy with isotopic substitutions and quantum chemical calculations. The results show that HBP has a ² Π electronic ground state with a short B-P bond. Bonding analysis indicates that besides an electron-sharing σ bond, there are two degenerate π bonding orbitals that are occupied by three electrons, resulting in a bond order of two and half between P and B. This is in sharp contrast to the bonding properties of the isovalent HNB, which was characterized to be a N≡B triply bonded σ radical with the unpaired electron locating on the B atom.

Three- and Five-Membered Anionic Chains of Pnictogenylboranes

An unprecedented family of three- and five-membered substituted anionic derivatives of parent pnictogenylboranes is herein reported. Reacting various combinations of the pnictogenylboranes H₂ E'-BH₂ -NMe₃ (E'=P, As) with pnictogen-based nucleophiles MER1R2 (E=P, As; R1=H, R2=^t Bu; R1=R2=Ph; M=Na, K) allows for the isolation of the unsymmetrical products [Na(18-crown-6)][H₂ E'-BH₂ -EH^t Bu] (3: E=E'=P; 4: E=E'=As; 5: E=As, E'=P) and [M(C)][H₂ E'-BH₂ -EPh₂ ] (7: E=E'=P, M=Na, C=18-crown-6; 8: E=E'=As; M=K, C=[2.2.2]cryptand; 9: E=P, E'=As, M=Na, C=[2.2.2]cryptand; 10: E=As, E'=P, M=K, C=[2.2.2]cryptand). [Na(18-crown-6)][H₂ As-BH₂ -^t BuPH-BH₃ ] (6) is only accessible by a different pathway, using ^t BuPH₂ , BH₃  ⋅ SMe₂ and NaNH₂ as starting materials. Additionally, the synthesis of symmetrical diphenyl-substituted compounds [M(18-crown-6)][Ph₂ E-BH₂ -EPh₂ ] (11: E=P, M=Na; 12: E=As, M=K) is reported which can be regarded as isostructural inorganic, negatively charged analogs of dppm (1,1-bis(diphenylphosphino)methane) and dpam (1,1-bis(diphenylarsino)methane). Furthermore, an elongation of the pnictogen boron backbone in compounds 3, 7 and 9' (similar compound to 9, stabilized however by 18-crown-6), is attainable by reacting them with the pnictogenylboranes H₂ E'-BH₂ -NMe₃ leading to corresponding five-membered chain-like compounds [Na(18-crown-6)][H₂ E-BH₂ -R1R2P-BH₂ -E'H₂ ] (E=E'=P, R1=H, R2=^t Bu (13); E=E'=P, R1=R2=Ph (14); E=E'=As, R1=R2=Ph (15); E=P, E'=As, R1=R2=Ph (16)). Finally, the thermodynamics of the reaction pathways were evaluated by quantum chemical computations.

Phosphinophosphoranes: Mixed-Valent Phosphorus Compounds with Ambiphilic Properties

Herein, we present a simple synthesis of mixed-valent phosphinophosphoranes bearing three- and five-coordinate phosphorus centers. Compounds with phosphorus-phosphorus bonds were synthesized via a reaction of lithium phosphides RR'PLi with cat₂PCl (cat = catecholate), whereas derivatives with methylene-linked phosphorus centers were obtained via a reaction of phosphanylmethanides RR'CH₂Li with cat₂PCl. The presence of accessible lone-pair electrons on the P-phosphanyl atom of phosphinophosphoranes during the reaction of the title compounds with H₃B·SMe₂, where phosphinophosphorane-borane adducts were formed quantitatively, was confirmed. Furthermore, the Lewis basic and Lewis acidic properties of the phosphinophosphoranes in reactions with phenyl isothiocyanate were tested. Depending on the structure of the starting phosphinophosphorane, phosphinophosphorylation of PhNCS or formation of a five-membered zwitterionic adduct was observed. The structures of the isolated compounds were unambiguously determined by heteronuclear nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction. Moreover, by applying density functional theory calculations, we compared the Lewis basicity and nucleophilicity of diversified trivalent P-centers.

The phosphinoboration of thiosemicarbazones

This study reports on the exploration of the phosphinoboration reaction with several thiosemicarbazones (R5R4NC(S)NR3N=CR1R2). Reactions between either Ph2PBpin (pin = 1,2-O2C2Me4) or Ph2PBcat (cat = 1,2-O2C6H4) with thiosemicarbazones containing a terminal primary or secondary amine afforded boron-containing heterocyclic 1,3,4-thiadiazoline products in excellent yield. The addition of Ph2PBpin to thiosemicarbazones containing an NMe2 group in the terminal position generated novel five-membered heterocycles in moderate yield, which included boron, sulfur, and nitrogen atoms. Heterocyclization of the thiosemicarbazones occurs preferentially in the presence of functional groups such as acetyl and pyridyl groups.

Exploring the Reactivity of Unsymmetrical Diphosphanes toward Heterocumulenes: Access to Phosphanyl and Phosphoryl Derivatives of Amides, Imines, and Iminoamides

We present a comprehensive study on the diphosphanation of iso(thio)cyanates by unsymmetrical diphosphanes. The reactions involving unsymmetrical diphosphanes and phenyl isocyanate or phenyl thioisocyanate gave rise to phosphanyl, phosphoryl, and thiophosphoryl derivatives of amides, imines, and iminoamides. The structures of the diphosphanation products were confirmed through NMR spectroscopy, IR spectroscopy, and single-crystal X-ray diffraction. We showed that unsymmetrical diphosphanes could be used as building blocks to synthesize phosphorus analogues of important classes of organic molecules. The described transformations provided a new methodology for the synthesis of organophosphorus compounds bearing phosphanyl, phosphoryl, or thiophosphoryl functional groups. Moreover, theoretical studies on diphosphanation reactions explained the influence of the steric and electronic properties of the parent diphosphanes on the structures of the diphosphanation products.

We provided synthetic access to phosphanyl, phosphoryl, or thiophosphoryl derivatives of amides, imines, and iminoamides starting from simple building blocks such as unsymmetrical diphosphanes and heterocumulenes.

Monomeric Triphosphinoboranes: Intramolecular Lewis Acid–Base Interactions between Boron and Phosphorus Atoms

Herein, we present the synthesis of the first fully characterized monomeric triphosphinoboranes. The simple reaction of boron tribromide with 3 equiv of bulky lithium phosphide tBu₂PLi yielded triphosphinoborane (tBu₂P)₃B. Triphosphinoboranes with diversified phosphanyl substituents were obtained via a two-step reaction, in which isolable bromodiphosphinoborane (tBu₂P)₂BBr is first formed and then reacts with 1 equiv of less bulky phosphide R₂PLi (R₂P = Cy₂P, iPr₂P, tBuPhP, or Ph₂P). By utilizing this method, we obtained a series of triphosphinoboranes with the general formula (tBu₂P)₂BPR₂. On the basis of structural and theoretical studies, two main types of triphosphinoborane structures can be distinguished. In the first type, all three electron lone pairs interact with the formally empty p orbital of the central boron atom, resulting in delocalized π bonding, whereas in the second type, one localized P=B bond and two P–B bonds are observed. The Lewis acidic–basic properties of triphosphinoboranes during the reaction of (tBu₂P)₂BPiPr₂ with H₃B·SMe₂ were analyzed. The P–B bond-containing compound mentioned above not only formed an adduct with BH₃ but also activated the B–H bond of the borane molecule, resulting in the incorporation of the BH₂ unit into two phosphorus atoms and migration of a hydride to the boron atom of the parent triphosphinoborane. The structures of the triphosphinoboranes were confirmed by single-crystal X-ray analysis, multinuclear nuclear magnetic resonance spectroscopy, and elemental analysis.

The synthesis and structural analysis of the first fully characterized monomeric triphosphinoboranes are presented.

Synthesis and structure of a phosphinoboronic ester in a fused bicyclic framework

The first phosphinoboronic ester bearing a fused bicyclic framework was synthesised by either deprotonation and hydride abstraction or Rh-catalysed dehydrogenation of a hydrophosphineboronic ester. The phosphinoboronic ester reacted as a Lewis acid with KF/18-crown-6, pyridine and DMAP to give the corresponding adducts. Furthermore, its crystal structure shows a remarkably short P-B bond in comparison with other P-B bonded derivatives in spite of the trigonal pyramidal geometry of the phosphorus. Consistent with the phosphorus pyramidality, the π-type donor-acceptor interaction of the P-B bond is small as revealed by the DFT calculations. The P-B bond shared within the fused six-membered rings has to shorten because of the geometrical requirement and high s-character of the boron.

Diphosphinoboranes as Intramolecular Frustrated Lewis Pairs: P-B-P Bond Systems for the Activation of Dihydrogen, Carbon Dioxide, and Phenyl Isocyanate

Herein, we present the first example of the activation of small molecules by P-B-P bond systems. The reactivity study involves reactions of two selected diphosphinoboranes, (t-Bu2P)2BPh (1') and (Cy2P)2BNiPr2 (2), that differ in terms of their structural and electronic properties for the activation of dihydrogen, carbon dioxide, and phenyl isocyanate. Diphosphinoborane 1' activates H2 under very mild conditions in the absence of a catalyst with the formation of the dimer (t-Bu2PB(Ph)H)2 and t-Bu2PH. Conversely, diphosphinoborane 2 did not react with H2 under the same conditions. The reaction of 1' with CO2 led to the formation of a compound with an unusual structure, where two phosphinoformate units were coordinated to the PhBOBPh moiety. In addition, 2 reacted with CO2 to insert two CO2 molecules into the P-B bonds of the parent diphosphinoborane. Both diphosphinoboranes activated PhNCO, yielding products resulting from the addition of two and/or three PhNCO molecules and the formation of new P-C, B-O, B-N, and C-N bonds. The products of the activation of small molecules by diphosphinoboranes were characterized with nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy, single-crystal X-ray diffraction, and elemental analysis. Additionally, the reaction mechanisms of the activation of small molecules by diphosphinoboranes were elucidated by theoretical methods.

A new family of silver(i) complexes stabilised by the phosphanylborane (C6H5)2PBH2·N(CH3)3

By the reaction of the phosphanylborane Ph2PBH2·NMe3 with Ag[FAl{OC6F10(C6F5)}3] or Ag[BF4] first Ag(i) coordination compounds stabilized by phosphine-boranes were obtained.

Activation of N2O and SO2 by the P-B Bond System. Reversible Binding of SO2 by the P-O-B Geminal Frustrated Lewis Pair

Herein, we present the first transformation of borylphosphine into borylphosphinite using nitrous oxide. Borylphosphine reacts with N2O via insertion of a single oxygen atom into the P-B bond and formation of a P-O-B bond system. Borylphosphine and borylphosphinite capture SO2 and activate it in an irreversible and reversible manner, respectively.

The phosphinoboration of acyl chlorides

This investigation examines the reactivity of phosphinoboronate esters Ph₂PBpin (pin = 1,2-O₂C₂Me₄) and Ph₂PBcat (cat = 1,2-O₂C₆H₄), as well as other phosphinoboron species, with various aryl and aliphatic acyl chlorides.

Synthesis and structures of gallaarsenes LGaAsGa(X)L featuring a Ga-As double bond

Three equivalents of LGa {L = HC[C(Me)N(2,6-i-Pr₂C₆H₃)]₂} react with AsX₃ (X = Cl, Br) by insertion into two As-X bonds, followed by the elimination of LGaX₂ and formation of LGaAsGa(Cl)L (1) and LGaAsGa(Br)L (2). According to single crystal X-ray analysis, 1 and 2 each exhibit one Ga-As single bond and one Ga-As double bond. The π-bonding contribution (9.71 kcal mol^-11 and 9.44 kcal mol^-12) was proved by variable temperature (VT) ¹H NMR spectroscopy, while the electronic structure of 1' was studied by quantum chemical calculations.

Hydrophosphination of boron-boron multiple bonds

Five compounds containing boron–boron multiple bonds are shown to undergo hydrophosphination reactions with diphenylphosphine in the absence of a catalyst. With diborenes, the products obtained are highly dependent on the substitution pattern at the boron atoms, with both 1,1- and 1,2-hydrophosphinations observed. With a symmetrical diboryne, 1,2-hydrophosphination yields a hydro(phosphino)diborene. The different mechanistic pathways for the hydrophosphination of diborenes are rationalised with the aid of density functional theory calculations.

Compounds containing boron–boron double and triple bonds are shown to undergo uncatalysed hydrophosphination reactions with diphenylphosphine.

Syntheses and Structures of Novel λ3,λ3-Phosphanylalumanes Fully Bearing Carbon Substituents and Their Substituent Effects

The novel phosphanylalumanes, Al–P single-bond species, fully bearing carbon protecting groups on aluminum and phosphorus atoms, are synthesized by the reactions of aluminum monohalides [(t-Bu)2AlBr and (C6F5)2AlCl·0.5(toluene)] with Mes2PLi. Regarding the t-Bu system, λ3,λ3-phosphanylalumane is obtained. Concerning the C6F5 system, on the other hand, the corresponding LiCl complex, λ4,λ4-phosphanylalumane, is obtained. The Al–P bond lengths of C6F5-substituted λ3,λ4-, and λ4,λ4-derivatives are much shorter than those of the reported λ3,λ4-phosphanylalumane derivatives and comparable to that observed for the previously reported λ3,λ3-phosphanylalumanes. Theoretical calculations reveal that the binding of the C6F5 groups to Al results in a large contribution of Al and a large s-character in the Al–P bond of phosphanylalumanes. Considering t-Bu-substituted phosphanylalumanes, the Al–P bond lengths reflect the coordination number of Al, showing a longer Al–P bond length in the case of λ4-Al as compared with that of λ3-Al. Combining the structural, spectroscopic, and theoretical results, the t-Bu-substituted λ3,λ3-phosphanylalumane has well separated vacant p orbital and lone pairs, which is suitable for reactivity studies.

Structural and spectroscopic analysis of a new family of monomeric diphosphinoboranes

We present a series of amino- and aryl(diphosphino)boranes R₂PB(R'')PR'₂, where R₂P, R'₂P = tBu₂P, tBuPhP, Ph₂P, Cy₂P, and R'' = iPr₂N, Ph, which were obtained via the metathesis reaction of iPr₂NBBr₂ or PhBBr₂ with selected lithium phosphides. The structures of isolated diphosphinoboranes were characterized in the solid state and in solution by means of X-ray diffraction and NMR spectroscopy, respectively. The utility of these P-B-P species as ligands for transition metal complexes was tested in the reaction with [(COD)PtMe₂]. Moreover, we carried out DFT calculations to elucidate bonding interactions and philicity of the reactive centers as well as to analyze conformations of the studied species. Electronic and steric properties of substituents on P and B atoms were found to have a strong influence on the structures of the obtained compounds. Three main types of diphosphinoboranes were distinguished, based on the strength of P-B π-interaction within the molecule: (i) application of strong electron-donating substituents on P-atoms and electron-accepting phenyl groups on B atoms led to the structure with one double P[double bond, length as m-dash]B and one single P-B bond and diverse planar and pyramidal geometry of phosphanyl groups; (ii) reduction of the donor ability of phosphanyl groups gave diphosphanylboranes with delocalized P-B-P π-interactions; (iii) introduction of amino groups with strong donor abilities on B atoms canceled P-B π-interactions and allowed compounds with two very long P-B bonds and two pyramidal phosphanyl groups to be obtained.

Diaminophosphinoboranes: effective reagents for phosphinoboration of CO2

The monomeric diaminophosphinoboranes readily react with CO₂ under mild conditions to cleanly form products of the general formula in the absence of a catalyst. The isolated products from the CO₂-phosphinoboration were fully characterized by NMR spectroscopy, IR spectroscopy, and X-ray diffraction. The mechanism of CO₂ phosphinoboration with diaminophosphinoboranes was elucidated by DFT calculations.

We present the activation of CO₂ by aminophosphinoboranes along with the mechanism of CO₂ phosphinoboration.

Inorganic Triphenylphosphine

A completely inorganic version of one of the most famous organophosphorus compounds, triphenylphosphine, has been prepared. A comparison of the crystal structures of inorganic triphenylphosphine, PBaz₃ (where Baz=B₃ H₂ N₃ H₃ ) and PPh₃ shows that they have superficial similarities and furthermore, the Lewis basicities of the two compounds are remarkably similar. However, their oxygenation and hydrolysis reactions are starkly different. PBaz₃ reacts quantitatively with water to give PH₃ and with the oxidizing agent ONMe₃ to give the triply-O-inserted product P(OBaz)₃ , an inorganic version of triphenyl phosphite; a corresponding transformation with PPh₃ is inconceivable. Thermodynamically, what drives these striking differences in the chemistry of PBaz₃ and PPh₃ is the great strength of the B-O bond.

PBP bridged [3]ferrocenophane: a bisphosphanylborane with a redox trigger

Stereospecific access to the unprecedented P-B-P bridged [3]ferrocenophane Fe(C₅H₄PtBu)₂BMes is presented. In contrast to acyclic ferrocenylphosphanes, we find evidence for a shift of spin-density from Fe to P upon pyramidal inversion of the P centers in the mono-cation; the latter has a lifetime of τ = 0.31(4) s.

Trihydroborates and Dihydroboranes Bearing a Pentacoordinated Phosphorus Atom: Double Ring Expansion To Balance the Coordination States

The first trihydroborate bearing a pentacoordinated phosphorus atom was synthesized as a new P-B bonded compound. Hydride abstraction of the trihydroborate gave an intermediary dihydroborane, which showed hydroboration reactivity and was trapped with pyridine whilst maintaining the P-B bond. The dihydroborane underwent a rearrangement, which involved a double ring expansion to compensate for the unbalanced coordination states of the phosphorus and boron atoms, to give a new fused bicyclic phosphine-boronate.

Dehydrocoupling of phosphine-boranes using the [RhCp*Me(PMe3)(CH2Cl2)][BArF4] precatalyst: stoichiometric and catalytic studies

Detailed experimental and computational studies are reported on the fundamental B–H and P–H bond activation steps involved in the dehydrocoupling/dehydropolymerization of primary and secondary phosphine–boranes, H₃B·PPhR′H (R = Ph, H), using the [RhCp*(PMe₃)Me(ClCH₂Cl)][BAr^F₄] catalyst.

We report a detailed, combined experimental and computational study on the fundamental B–H and P–H bond activation steps involved in the dehydrocoupling/dehydropolymerization of primary and secondary phosphine–boranes, H₃B·PPhR′H (R = Ph, H), using [RhCp*(PMe₃)Me(ClCH₂Cl)][BAr^F₄], to either form polyphosphino-boranes [H₂B·PPhH]_n (M_n ∼ 15 000 g mol^–1, PDI = 2.2) or the linear diboraphosphine H₃B·PPh₂BH₂·PPh₂H. A likely polymer-growth pathway of reversible chain transfer step-growth is suggested for H₃B·PPhH₂. Using secondary phosphine–boranes as model substrates a combined synthesis, structural (X-ray crystallography), labelling and computational approach reveals: initial bond activation pathways (B–H activation precedes P–H activation); key intermediates (phosphido-boranes, α-B-agostic base-stabilized boryls); and a catalytic route to the primary diboraphosphine (H₃B·PPhHBH₂·PPhH₂). It is also shown that by changing the substituent at phosphorus (Ph or Cy versus^tBu) different final products result (phosphido-borane or base stabilized phosphino-borane respectively). These studies provide detailed insight into the pathways that are operating during dehydropolymerization.

Metal-Free Addition/Head-to-Tail Polymerization of Transient Phosphinoboranes, RPH-BH2: A Route to Poly(alkylphosphinoboranes)

Mild thermolysis of Lewis base stabilized phosphinoborane monomers R(1)R(2)P-BH2⋅NMe3 (R(1),R(2)=H, Ph, or tBu/H) at room temperature to 100 °C provides a convenient new route to oligo- and polyphosphinoboranes [R(1)R(2)P-BH2]n. The polymerization appears to proceed via the addition/head-to-tail polymerization of short-lived free phosphinoborane monomers, R(1)R(2)P-BH2. This method offers access to high molar mass materials, as exemplified by poly(tert-butylphosphinoborane), that are currently inaccessible using other routes (e.g. catalytic dehydrocoupling).