Diaminophosphinoboranes: effective reagents for phosphinoboration of CO2

Phosphinoborenium cations stabilized by N-heterocyclic carbenes: synthesis, structure, and reactivity

Phosphinoborenium cations stabilized by N-heterocyclic carbenes (NHCs) were synthesized via the reaction of bromo(phosphino)boranes with NHCs. Their structures were investigated by heteronuclear magnetic resonance spectroscopy, X-ray diffraction, and density functional theory calculations. They possess a planar trigonal boron center directly bonded with the pyramidal phosphanyl group (PR2) and can be treated as cationic phosphinoboranes. The reactivity of the selected NHC-phosphinoborenium cation was tested toward AuCl·SMe2 and Ph2PCl. In both reactions, the titled compound acted as a phosphido group donor under heterolytic cleavage of the P-B bond. Control experiments with parent phosphinoborane emphasized differences between the reactivity of low-coordinate neutral and cationic species with P-B functionality.

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.

Reactivity of triphosphinoboranes towards H3B·SMe2: access to derivatives of boraphosphacycloalkanes with diverse substituents

Triphosphinoboranes activated the B-H bond in the BH₃ molecule without any catalysts at room temperature. Hydroboration reactions led to boraphosphacyloalkanes with diverse structures. The outcomes of reactions depend on the size of the phosphanyl substituent on the boron atom of the parent triphosphinoborane, where derivatives of boraphosphacyclobutane and boraphosphacyclohexane were obtained. Furthermore, the precursor of triphosphinoboranes, namely bromodiphosphinoborane, also exhibited high reactivity towards H₃B·SMe₂, yielding bromo-substituted boraphosphacyclobutane. The obtained products were characterized by heteronuclear NMR spectroscopy, single crystal X-ray diffraction, and elemental analysis.

Reactivity of tetrel functionalized heptapnictogen clusters towards heteroallenes

Despite being known for decades, the solution-state molecular chemistry of heptapnictogen ([Pn₇]^3-; Pn = P, As) clusters is not well established. Here we study heavy element derivatives of tetrel functionalized heptapnictogen clusters towards heteroallene capture, specifically isocyanates, an isothiocyanate and CO₂ are probed. Clusters (Me₃Ge)₃P₇ (1), (Et₃Ge)₃P₇ (2), (ⁿBu₃Sn)₃P₇ (3), and (Me₃Si)₃As₇ (4) were all found to capture isocyanates between all three of their tetrel-pnictogen bonds. In the case of phenyl isocyanate insertion, tetrel coordination at the isocyanate nitrogen atoms is preferred, while in the case of p-toluenesulfonyl isocyanate insertion, tetrel coordination at oxygen is preferred. Furthermore, the reaction of (Me₃Si)₃P₇ with CO₂ gave NMR spectra consistent with the capture of the greenhouse gas. Heteroallene insertion at these clusters was also studied using density functional theory.

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.

A Zintl Cluster for Transition Metal-Free Catalysis: C═O Bond Reductions

The first fully characterized boron-functionalized heptaphosphide Zintl cluster, [(BBN)P7]2- ([1]2-), is synthesized by dehydrocoupling [HP7]2-. Dehydrocoupling is a previously unprecedented reaction pathway to functionalize Zintl clusters. [Na(18-c-6)]2[1] was employed as a transition metal-free catalyst for the hydroboration of aldehydes and ketones. Moreover, the greenhouse gas carbon dioxide (CO2) was efficiently and selectively reduced to methoxyborane. This work represents the first examples of Zintl catalysis where the transformation is transition metal-free and where the cluster is noninnocent.

Phosphinoborinium cation: a synthon for cationic B-P bond systems

Herein, we report access to phosphinoborinium cations via heterolytic cleavage of the boron-bromide bond in bromophosphinoborane. The product of the reaction was isolated as a dimeric dication possessing a planar B₂P₂ core. Activation of the C-H and C-P bonds in the dication led to the formation of the borinium-phosphaborene adduct. Reactivity studies revealed that the title cation exhibits ambiphilic properties and intramolecular frustrated Lewis pair features.

Carbene-Anchored Boryl- and Stibanyl-Phosphaalkenes as Precursors for Bis-Phosphaalkenyl Dichlorogermane and Mixed-Valence AgI/AgII Phosphinidenide

Cyclic alkyl(amino) carbene (cAAC)-anchored boryl- and stibanyl-phosphaalkenes with general formula cAAC = P-ER₂ [E = B, R = (NⁱPr₂)₂ (3a-c); E = Sb, R = 2,4,6-triisopropylphenyl (5a-b)] have been synthesized and utilized as precursors for the bis-phosphaalkenyl dichlorogermane [(cAAC = P)₂GeCl₂] (6) and the first molecular example of a neutral polymeric mixed-valence Ag^I/Ag^II phosphinidenide complex [(cAACP)₂Ag₄^IAg^IICl₄]_n (7). All compounds have been characterized by single-crystal X-ray diffraction and further investigated by nuclear magnetic resonance (NMR), mass spectrometric analysis, and UV-vis/fluorescence measurements. The paramagnetic complex 7 has been characterized by ESR spectroscopy. Cyclic voltammetry studies of compounds 3/5 have suggested possible one-electron quasi-reversible reductions, indicating their redox noninnocent behavior in solution. Quantum chemical studies revealed the electron-sharing nature of the P-B and P-Sb σ bonds in compounds 3 and 5, and the polar C_cAAC = P bonds in compounds 3, 5, and 6 prevailing their phosphaalkene structures over phosphinidenes.

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.

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.

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.