Iron-catalyzed dehydropolymerization: a convenient route to poly(phosphinoboranes) with molecular-weight control

B-P Coupling: Metal Stabilized Phosphinoborate Complexes

In an effort to establish B-P coupling reactions without the use of phosphine-borane dehydrocoupling agent, we have developed a new synthetic methodology employing group 8 metal σ-borate complex [{κ3 -H,S,S'-BH2 L2 }Ru{κ3 -H,H,S-BH3 L}] (L=NC5 H4 S), 1. Treatment of 1 with chlorodiphenyl phosphine (PPh2 Cl) yielded 1,5-P,S chelated Ru-dihydridoborate species [PPh2 H{κ3 -H,H,S-BH(OH)L}Ru{κ2 -P,S-(Ph2 P)BH2 L}], 2. The insertion of phosphine moiety (PPh2 ) by the cleavage of 3c-2e σ(Ru… H-B) bonding interaction led to the formation of B-P bond. The κ2 -P,S chelated six-membered ring adopted a boat conformation in complex 2. The heterocycle is made of all different atoms, which is one of the rarest examples of heteroatomic ring systems. Theoretical outcomes demonstrated the electronic insight of B-P coupling and stabilization through transition metal. In order to explore an alternate route of B-P bond formation, we have further explored the reaction of 1 and Ru-bis(dihydridoborate) complex, 5 with secondary phosphine oxide (SPO). Although, thermolysis of 1 with diphenylphosphine oxide yielded analogous σ-borate complex 3, the similar reaction of 5 at room temperature led to the formation of novel phosphinous(III) acid incorporated Ru(σ-borate)(dihydridoborate) complex, 6. In a similar fashion, the reaction of 5 with phosphite ligand generated Ru(σ-borate)(dihydridoborate) complex, 7, which is analogous to 6.

Titanium-Catalyzed Polymerization of a Lewis Base-Stabilized Phosphinoborane

The reaction of the Lewis base-stabilized phosphinoborane monomer tBuHPBH2 NMe3 (2 a) with catalytic amounts of bis(η5 :η1 -adamantylidenepentafulvene)titanium (1) provides a convenient new route to the polyphosphinoborane [tBuPH-BH2 ]n (3 a). This method offers access to high molar mass materials under mild conditions and with short reaction times (20 °C, 1 h in toluene). It represents an unprecedented example of a transition metal-mediated polymerization of a Lewis base-stabilized Group 13/15 compound. Preliminary studies of the substrate scope and a potential mechanism are reported.

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.

Phosphido-borane-supported stannates

The reactions between SnCl₂ and three equivalents of the alkali metal phosphido-borane complexes [R₂P(BH₃)]M yield the corresponding tris(phosphido-borane)stannate complexes [L_nM{R₂P(BH₃)}₃Sn] [R₂ = iPr₂, L_nM = (THF)₃Li (2Li), (Et₂O)Na (2Na), (Et₂O)K (2K); R₂ = Ph₂, L_nM = (THF)Li (3Li), (THF)(Et₂O)Na (3Na), (THF)(Et₂O)K (3K); R₂ = iPrPh, L_nM = (THF)₄Li (4Li)]. In each case X-ray crystallography reveals an anion consisting of a trigonal pyramidal tin centre coordinated by the P atoms of the phosphido-borane ligands. These tris(phosphido-borane)stannate anions coordinate to the alkali metal cations via their BH₃ hydrogen atoms in a variety of modes to give monomers, dimers, and polymers, depending on the alkali metal and the substituents at the phosphorus centres. In contrast, reactions between SnCl₂ and three equivalents of [tBu₂P(BH₃)]M (M = Li, Na) gave the known hydride [M{tBu₂P(BH₃)}₂SnH], according to multinuclear NMR spectroscopy.

Polyphosphinoborane Block Copolymer Synthesis Using Catalytic Reversible Chain-Transfer Dehydropolymerization

An amphiphilic block copolymer of polyphosphinoborane has been prepared by a mechanism-led strategy of the sequential catalytic dehydropolymerization of precursor monomers, H₃ B ⋅ PRH₂ (R=Ph, n-hexyl), using the simple pre-catalyst [Rh(Ph₂ PCH₂ CH₂ PPh₂ )₂ ]Cl. Speciation, mechanism and polymer chain growth studies support a step-growth process where reversible chain transfer occurs, i.e. H₃ B ⋅ PRH₂ /oligomer/polymer can all coordinate with, and be activated by, the catalyst. Block copolymer [H₂ BPPhH]₁₁₀ -b-[H₂ BP(n-hexyl)H]₁₁ can be synthesized and self-assembles in solution to form either rod-like micelles or vesicles depending on solvent polarity.

Transition-Metal-Free Dehydropolymerization of Phosphine-Boranes at Ambient Temperature

Stoichiometric reaction of phosphine-borane adducts RR'PH⋅BH₃ (R=Ph, R'=H, Ph, Et, and R=R'=^t Bu) with the strong acid HNTf₂ (Tf=SO₂ CF₃ ) leads to H₂ elimination and the formation of the triflimido derivatives, RR'PH⋅BH₂ (NTf₂ ). Subsequent deprotonation by using bases, such as diisopropylethylamine or the carbene IPr (IPr=N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), led to the formation of P-mono- or -disubstituted polyphosphinoboranes [RR'P-BH₂ ]_n . Evidence for the intermediacy of transient phosphinoborane monomers, RR'PBH₂ , was provided by trapping reactions.

Calix[4]pyrrolato Aluminate Catalyzes the Dehydrocoupling of Phenylphosphine Borane to High Molar Weight Polymers

High molar weight polyphosphinoboranes represent materials with auspicious properties, but their preparation requires transition metal‐based catalysts. Here, calix[4]pyrrolato aluminate is shown to induce the dehydropolymerization of phosphine boranes to high molar mass polyphosphinoboranes (up to M_n=43 000 Da). Combined GPC and ³¹P DOSY NMR spectroscopic analyses, quantum chemical computations, and stoichiometric reactions disclose a P−H bond activation by the cooperative action of the square‐planar aluminate and the electron‐rich ligand framework. This first transition metal‐free catalyst for P−B dehydrocoupling overcomes the problem of residual d‐block metal impurities in the resulting polymers that might interfere with the reproducibility of the properties for this emerging class of inorganic materials.

Easy Access to Phosphine-Borane Building Blocks

In this paper, we highlight the synthesis of a variety of primary phosphine-boranes (RPH2 ⋅BH3 ) from the corresponding dichlorophosphines, simply by using Li[BH4 ] as reductant and provider of the BH3 protecting group. The method offers facile access not only to alkyl- and arylphosphine-boranes, but also to aminophosphine-boranes (R2 NPH2 ⋅BH3 ) that are convenient building blocks but without the protecting BH3 moiety thermally labile and notoriously difficult to handle. The borane-protected primary phosphines can be doubly deprotonated using n-butyllithium to provide soluble phosphanediides Li2 [RP⋅BH3 ] of which the phenyl-derivative Li2 [PhP⋅BH3 ] was structurally characterized in the solid state.

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.

Synthesis and Post-Polymerization Functionalization of Halogen-Substituted Polyphosphinoboranes to Access Alkyne-Functionalized Derivatives

Catalytic dehydropolymerization of halogen-functionalized phosphine-boranes (4-X-C₆ H₄ )PH₂ ⋅BH₃ (1a: X = Br, 1b: X = I) with [CpFe(CO)₂ (OTf)] at 100 °C provides convenient access to halogen-functionalized polyphosphinoboranes [(4-X-C₆ H₄ )PH-BH₂ ]_n (2a: X = Br, 2b: X = I). These polymers are useful precursors for post-polymerization functionalization, which is demonstrated by Sonogashira coupling under mild conditions to yield the alkynyl-functionalized polyphosphinoborane [(4-PhCC-C₆ H₄ )PH-BH₂ ]_n (3).

Photolytic, radical-mediated hydrophosphination: a convenient post-polymerisation modification route to P-di(organosubstituted) polyphosphinoboranes [RR'PBH2] n

Polymers with a phosphorus-boron main chain have attracted interest as novel inorganic materials with potentially useful properties since the 1950s. Although examples have recently been shown to be accessible via several routes, the materials reported so far have been limited to P-mono(organosubstituted) materials, [RHPBH2] n , containing P-H groups. Here we report a general route for the post-polymerisation modification of such polyphosphinoboranes giving access to a large range of previously unknown examples featuring P-disubstituted units. Insertion of alkenes, R'CH[double bond, length as m-dash]CH2 into the P-H bonds of poly(phenylphosphinoborane), [PhHPBH2] n was facilitated by irradiation under UV light in the presence of the photoinitiator 2,2-dimethoxy-2-phenylacetophenone (DMPAP) and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) under benchtop conditions giving high molar mass, air-stable polymers [PhR'PBH2] n with controlled functionalisation and tunable material properties. The mechanistic explanation for the favourable effect of the addition of TEMPO was also investigated and was proposed to be a consequence of reversible binding to radical species formed from the photolysis of DMPAP. This new methodology was also extended to the formation of crosslinked gels and to water-soluble bottlebrush copolymers showcasing applicability to form a wide range of polyphosphinoborane-based soft materials with tunable properties.

Metal-free dehydropolymerisation of phosphine-boranes using cyclic (alkyl)(amino)carbenes as hydrogen acceptors

The divalent carbene carbon centre in cyclic (alkyl)(amino)carbenes (CAACs) is known to exhibit transition-metal-like insertion into E-H σ-bonds (E = H, N, Si, B, P, C, O) with formation of new, strong C-E and C-H bonds. Although subsequent transformations of the products represent an attractive strategy for metal-free synthesis, few examples have been reported. Herein we describe the dehydrogenation of phosphine-boranes, RR'PH·BH3, using a CAAC, which behaves as a stoichiometric hydrogen acceptor to release monomeric phosphinoboranes, [RR'PBH2], under mild conditions. The latter species are transient intermediates that either polymerise to the corresponding polyphosphinoboranes, [RR'PBH2]n (R = Ph; R' = H, Ph or Et), or are trapped in the form of CAAC-phosphinoborane adducts, CAAC·H2BPRR' (R = R' = tBu; R = R' = Mes). In contrast to previously established methods such as transition metal-catalysed dehydrocoupling, which only yield P-monosubstituted polymers, [RHPBH2]n, the CAAC-mediated route also provides access to P-disubstituted polymers, [RR'PBH2]n (R = Ph; R' = Ph or Et).

Amine-Borane Dehydropolymerization: Challenges and Opportunities

The dehydropolymerization of amine-boranes, exemplified as H2 RB⋅NR'H2 , to produce polyaminoboranes (HRBNR'H)n that are inorganic analogues of polyolefins with alternating main-chain B-N units, is an area with significant potential, stemming from both fundamental (mechanism, catalyst development, main-group hetero-cross-coupling) and technological (new polymeric materials) opportunities. This Concept article outlines recent advances in the field, covering catalyst development and performance, current mechanistic models, and alternative non-catalytic routes for polymer production. The substrate scope, polymer properties and applications of these exciting materials are also outlined. Challenges and opportunities in the field are suggested, as a way of providing focus for future investigations.

Phosphorus-Containing Block Copolymers from the Sequential Living Anionic Copolymerization of a Phosphaalkene with Methyl Methacrylate

Although living polymerization methods are widely applicable to organic monomers, their application to inorganic monomers is rare. For the first time, we show that the living poly(methylenephosphine) (PMP_n ^- ) anion can function as a macroinitiator for olefins. Specifically, the phosphaalkene, MesP=CPh₂ (PA), and methyl methacrylate (MMA) can be sequentially copolymerized using the BnLi-TMEDA initiator system in toluene. A series of PMP_n -b-PMMA_m copolymers with narrow dispersities are accessible (Đ=1.05-1.10). Analysis of the block copolymers provided evidence for -P-CPh₂ -CH₂ -CMe(CO₂ Me)- switching groups. Importantly, this indicates that the -P-CPh₂ ^- anion directly initiates the anionic polymerization of MMA and stands in stark contrast to the isomerization mechanism followed for the homopolymerization of PA. For the first time, the glass transition of a PMP_n homopolymer has been measured (T_g =45.1 °C, n=20). The PMP_n -b-PMMA_m copolymers do not phase separate and show a single T_g which increases with higher PMMA content.

Iridium-catalysed dehydrocoupling of aryl phosphine-borane adducts: synthesis and characterisation of high molecular weight poly(phosphinoboranes)

The thermal dehydrogenative coupling of aryl phosphine-borane adducts with iridium complexes bearing a bis(phosphinite) pincer ligand is reported. This catalysis produces high molecular weight poly(phosphinoboranes) [ArPH-BH2]n (Ar = Ph, (p)Tol, Mes). Furthermore, we investigated the reactivity of these pincer complexes towards primary phosphines and their respective borane adducts on a stoichiometric scale.

Dehydrocoupling of dimethylamine–borane promoted by manganese(ii) m-terphenyl complexes

Low-coordinate m-terphenyl complexes are precatalysts for dehydrocoupling of dimethylamine–borane, where small changes in coordination environment effect significant mechanistic differences.

Depolymerization of Poly(phosphinoboranes): From Polymers to Lewis Base Stabilized Monomers

We report on depolymerization reactions of poly(phosphinoboranes). The cleavage of the polymers [H₂ PBH₂ ]_n (2 a), [tBuHPBH₂ ]_n (2 c), [PhHPBH₂ ]_n (2 e) and the oligomer [Ph₂ PBH₂ ]_n (2 b), with strong Lewis bases (LBs), in particular with NHCs, leads to the corresponding monomeric phosphanylboranes R¹ R² PBH₂ LB. It is observed that the depolymerization depends on the strength and stability of the LBs as well as on the substitution pattern of the poly(phosphinoboranes). The solid state structures of the monomeric phosphinoboranes H₂ PBH₂ NHC^Me (NHC=N-heterocyclic carbene) (4 a), H₂ PBH₂ NHC^dipp (5 a) and tBuHPBH₂ NHC^Me (4 c) were determined. DFT calculations support the experimentally observed reaction behavior.

Iron Catalyzed Dehydrocoupling of Amine- and Phosphine-Boranes

Catalytic dehydrocoupling methodologies, whereby dihydrogen is released from a substrate (or intermolecularly from two substrates) is a mild and efficient method to construct main group element-main group element bonds, the products of which can be used in advanced materials, and also for the development of hydrogen storage materials. With growing interest in the potential of compounds such as ammonia-borane to act as hydrogen storage materials which contain a high weight% of H2, along with the current heightened interest in base metal catalyzed processes, this review covers recent developments in amine and phosphine dehydrocoupling catalyzed by iron complexes. The complexes employed, products formed and mechanistic proposals will be discussed.

Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(C6F5)3-Mediated Formation of P-Si Bonds

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (R¹R²PH, R² = H or R¹) with hydrosilanes (R³R⁴R⁵SiH, R⁴, R⁵ = H or R³) to produce synthetically useful silylphosphines (R¹R²P-SiR³R⁴R⁵) has been achieved using B(C₆F₅)₃ as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C₆F₅)₃ but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a R¹R²HP·B(C₆F₅)₃ adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(C₆F₅)₃ catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (R¹R²P-SiR³R⁴R⁵). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(C₆F₅)₃ at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H₂-acceptor.

Mass spectrometric characterization of oligomeric phosphaalkenes

Oligomeric phosphaalkenes are readily characterized using electrospray ionization mass spectrometry (ESI-MS). The high affinity of phosphines for silver ions permits the detection of the unadulterated polymer as [M + xAg]x+ions (x = 2–3). When the oligomers are oxidized using H2O2, the resulting phosphine oxide polymer may be treated with sodium ions to produce [M + xNa]x+ions (x = 2–3). Both methods predict a similar distribution of oligomers: Mnvalues of 3450 ± 100 Da and a PDI of 1.09 ± 0.01 cover both analyses. This distribution represents oligomers of the general formula Me(PMesCPh2)nH from n = 4–20, maximizing at ∼n = 10.

Mechanism, reactivity, and regioselectivity in rhodium-catalyzed asymmetric ring-opening reactions of oxabicyclic alkenes: a DFT Investigation

The origin of the enantio- and regioselectivity of ring-opening reaction of oxabicyclic alkenes catalyzed by rhodium/Josiphos has been examined using M06-2X density functional theory(DFT). DFT calculations predict a 98% ee for the enantioselectivity and only the 1,2-trans product as one regio- and diastereomer, in excellent agreement with experimental results. The solvent tetrahydrofuran(THF) plays a key role in assisting nucleophilic attack. Orbital composition analysis of the LUMO and the NPA atomic charge calculations were conducted to probe the origins of the regioselectivity. The orbital composition analysis reveals two potential electrophilic sites of the Rh-π-allyl intermediate M3 and the NPA atomic charges demonstrate that Cα carries more positive charges than Cγ, which suggests that Cα is the electrophilic site.