Synthesis and Reactivity of the Phosphinoboranes R2PB(C6F5)2

B[double bond, length as m-dash]P double bonds relieved from steric encumbrance: matrix-isolation infrared spectroscopy of the phosphaborene F2B-P[double bond, length as m-dash]BF and the triradical B[double bond, length as m-dash]PF3

Free phosphaborenes have a labile boron-phosphorus double bond and therefore require extensive steric shielding by bulky substituents to prevent isomerisation and oligomerisation. In the present work, the small free phosphaborene F2B-P[double bond, length as m-dash]BF was isolated by matrix-isolation techniques and was characterised by infrared spectroscopy in conjunction with quantum-chemical methods. In contrast to its sterically hindered relatives, this small phosphaborene exhibits an acute BPB angle of 83° at the CCSD(T) level. An alternative orbital structure for the B[double bond, length as m-dash]P double bond is found in the triradical B[double bond, length as m-dash]PF3, the direct adduct of laser-ablated atomic B and PF3. The single-bonded isomer F2B-PF and the dimer F3P-B[triple bond, length as m-dash]B-PF3 are also tentatively assigned.

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.

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.

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.

Synergistic Catalysis by Brønsted Acid/Carbodicarbene Mimicking Frustrated Lewis Pair-Like Reactivity

Carbodicarbene (CDC), unique carbenic entities bearing two lone pairs of electrons are well-known for their strong Lewis basicity. We demonstrate herein, upon introducing a weak Brønsted acid benzyl alcohol (BnOH) as a co-modulator, CDC is remolded into a Frustrated Lewis Pair (FLP)-like reactivity. DFT calculation and experimental evidence show BnOH loosely interacting with the binding pocket of CDC via H-bonding and π-π stacking. Four distinct reactions in nature were deployed to demonstrate the viability of proof-of-concept as synergistic FLP/Modulator (CDC/BnOH), demonstrating enhanced catalytic reactivity in cyclotrimerization of isocyanate, polymerization process for L-lactide (LA), methyl methacrylate (MMA) and dehydrosilylation of alcohols. Importantly, the catalytic reactivity of carbodicarbene is uniquely distinct from conventional NHC which relies on only single chemical feature of nucleophilicity. This finding also provides a new spin in diversifying FLP reactivity with co-modulator or co-catalyst.

A Novel Reactivity of Phosphanylalumane (>P-Al<): Reversible Addition of a Saturated Interelement Bond to Olefins

The reversible addition of olefins to a phosphanylalumane, P-Al single-bond species, was investigated. The P-Al bond added to ethylene and relatively small terminal alkenes (propylene and hex-1-ene) at room temperature to give the corresponding alkene adducts. Heating the terminal alkene adducts released the corresponding alkenes and regenerated the P-Al bond, but no release of ethylene was observed even under vacuum conditions. The reactivity of ethylene adduct as a new saturated C₂ vicinal P/Al-based FLP was also investigated. The ethylene adduct was found to undergo complexation with nitriles to give the corresponding nitrile adducts to the Al center, which retained the ethylene tether as in the case of the corresponding P/B-based FLP. However, the reactivity of ethylene toward CO₂ and benzaldehyde differed from that of the P/B system giving the corresponding adducts.

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.

New Reactivity Patterns in 3H‐Phosphaallene Chemistry [Aryl‐P=C=C(H)‐ t Bu]: Hydroboration of the C=C Bond, Deprotonation and Trimerisation

3H‐Phosphaallenes, R−P=C=C(H)C−R’ (3), are accessible in a multigram scale on a new and facile route and show a fascinating chemical reactivity. BH₃(SMe₂) and 3 a (R=Mes*, R’=tBu) afforded by hydroboration of the C=C bonds of two phosphaallene molecules an unprecedented borane (7) with the B atom bound to two P=C double bonds. This compound represents a new FLP based on a B and two P atoms. The increased Lewis acidity of the B atom led to a different reaction course upon treatment of 3 a with H₂B‐C₆F₅(SMe₂). Hydroboration of a C=C bond of a first phosphaallene is followed in a typical FLP reaction by the coordination of a second phosphaallene molecule via B−C and P−B bond formation to yield a BP₂C₂ heterocycle (8). Its B−P bond is short and the B‐bound P atom has a planar surrounding. Treatment of 3 a with tBuLi resulted in deprotonation of the β‐C atom of the phosphaallene (9). The Li atom is bound to the P atom as demonstrated by crystal structure determination, quantum chemical calculations and reactions with HCl, Cl‐SiMe₃ or Cl‐PtBu₂. The thermally unstable phosphaallene Ph−P=C=C(H)‐tBu gave a unique trimeric secondary product by P−P, P−C and C−C bond formation. It contains a P₂C₄ heterocycle and was isolated as a W(CO)₄ complex with two P atoms coordinated to W (15).

Bis(pentafluorophenyl)phenothiazylborane - an intramolecular frustrated Lewis pair catalyst for stannane dehydrocoupling

We synthesized a novel Lewis acidic aminoborane containing a phenothiazyl substituent and demonstrated its potential to catalytically promote the dehydrocoupling of tin hydrides. The observed reactivity would imply a homolytic frustrated Lewis pair type mechanism, however computational analysis suggests a heterolytic mechanism for this reaction. This result represents one of the first frustrated Lewis pair systems to dehydrocouple stannanes in a heterolytic fashion.

Phosphanylalanes and Phosphanylgallanes Stabilized only by a Lewis Base

The synthesis and characterization of the first parent phosphanylalane and phosphanylgallane stabilized only by a Lewis base (LB) are reported. The corresponding substituted compounds, such as IDipp⋅GaH2 PCy2 (1) (IDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene) were obtained by the reaction of LiPCy2 with IDipp⋅GaH2 Cl. However, the LB-stabilized parent compounds IDipp⋅GaH2 PH2 (3) and IDipp⋅AlH2 PH2 (4) were prepared via a salt metathesis of LiPH2 ⋅DME with IDipp⋅E'H2 Cl (E'=Ga, Al) or by H2 -elimination reactions of IDipp⋅E'H3 (E'=Ga, Al) and PH3 , respectively. The compounds could be isolated as crystalline solids and completely characterized. Supporting DFT computations gave insight into the reaction pathways as well as into the stability of these compounds with respect to their decomposition behavior.

Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation

Ph₂PBH₂ is generated and trapped on-metal by addition of B(C₆F₅)₃ to a β-diketiminato magnesium phospidoborane complex, similar reactivity is meanwhile prevented for the analogous calcium-based system by formation of a stable η⁶-toluene complex.

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.

Dehydrogenation of Amine-Boranes Using p-Block Compounds

Amine-boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition- and main-group metal catalysts for the dehydrogenation of amine-boranes. During the past decade, new systems started to emerge solely based on p-block elements that promote the dehydrogenation of amine-boranes through hydrogen-transfer reactions, polymerization initiation, and main-group catalysis. In this review, we highlight the development of these p-block based systems for stoichiometric and catalytic amine-borane dehydrogenation and discuss the underlying mechanisms.

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.

1,1-Phosphinoboration of diazomethanes

The reactions of the phosphinoboranes Ph₂PBMes₂, Ph₂PBpin, and Ph₂PBcat with the diazomethanes Ph₂CN₂, C₁₂H₈CN₂, and EtO₂CCHN₂ are shown to give products of 1,1-phosphinoboration.

Coordination of Asymmetric Diborenes towards Cationic Coinage Metals (Au, Ag, Cu)

Synthesis of a series of cationic coinage metal (Au: 2-4, Ag: 5, and Cu: 6) complexes of asymmetric diborenes (1 a, 1 b) is reported. X-ray diffraction analysis reveals that the metal center interacts unsymmetrically with the B₂ moiety and the terminal boron atom exhibits pronounced pyramidal geometry. A computational study shows that in complexes 2-6, the bonding between the B₂ units and the metal center is dominated by electrostatic interactions concomitant with non-negligible covalent contributions.

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.

Frustrated Lewis Pair Catalyzed C-H Activation of Heteroarenes: A Stepwise Carbene Mechanism Due to Distance Effect

This study presents new mechanistic insights into the frustrated Lewis pairs (FLPs) catalyzed C-H activation of heteroarenes. Besides the generally accepted concerted C-H activation, a novel stepwise carbene type pathway is proposed as an alternative mechanism. The reaction mechanisms can be varied by tuning the distance between Lewis acid and Lewis base due to catalyst-substrate match. These results should expand the understanding of the structure and function of FLPs for catalyzed C-H activation.

Triple-Bonded Boron≡Phosphorus Molecule: Is That Possible?

The effect of substitution on the potential energy surfaces of RB≡PR (R = H, F, OH, SiH₃, and CH₃) is studied using density functional theories (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP, and B3LYP/LANL2DZ+dp). There is significant theoretical evidence that RB≡PR compounds with smaller substituents are fleeting intermediates, so they would be difficult to be detected experimentally. These theoretical studies using the M06-2X/Def2-TZVP method demonstrate that only the triply bonded R'B≡PR' molecules bearing sterically bulky groups (R' = Tbt (=C₆H₂-2,4,6-{CH(SiMe₃)₂}₃), SiMe(SitBu₃)₂, Ar* (=C₆H₃-2,6-(C₆H₂-2,4,6-i-Pr₃)₂), and SiiPrDis₂) are significantly stabilized and can be isolated experimentally. Using the simple valence-electron bonding model and some sophisticated theories, the bonding character of R'B≡PR' should be viewed as R'BI PR'. The present theoretical observations indicate that both the electronic and the steric effect of bulkier substituent ligands play a key role in making triply bonded R'B≡PR' species synthetically accessible and isolable in a stable form.

Phosphaborenes: Accessible Reagents for the Synthesis of C-C/P-B Isosteres

Formal exchange of C=C units with isoelectronic B=N or B=P units can provide access to molecules with unique electronic or chemical properties. Herein, we report the simple solution-phase generation of highly reactive phosphaborenes, RP=BR, and demonstrate their use for the introduction of P=B units into organic systems. Ring opening of a P-B-containing cyclobutene isostere provided access to unique 1,4-boraphosphabutadiene systems with conjugated main-group multiple bonds.

The phosphinoboration of carbodiimides, isocyanates, isothiocyanates and CO2

The transition metal-free addition of phosphinoboronate ester Ph₂PBpin (pin = 1,2-O₂C₂Me₄) to heterocumulenes including carbodiimides, isocyanates, isothiocyanates and carbon dioxide proceeds with remarkable selectivity to give products in high yield.

Probing the reactivity of pentaphenylborole with N–H, O–H, P–H, and S–H bonds

The reactions of molecules containing E–H functionalities (E = Group 15 or 16 element) and pentaphenylborole were investigated revealing diverse outcomes.