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.

Coordination of Pnictogenylboranes Towards Tl(I) Salts and a Tl- Mediated P-P Coupling

The coordination chemistry of only Lewis-base (LB)-stabilized pnictogenylboranes EH₂ BH₂ ⋅NMe₃ (E=P, As) towards Tl(I) salts has been studied. The reaction of Tl[BAr^Cl ] (BAr^Cl =[B(3,5-C₆ H₃ Cl₂ )₄ ]^- ) with the corresponding pnictogenylborane results in the formation of [Tl(EH₂ BH₂ ⋅NMe₃ )][BAr^Cl ] (1 a: E=P; 1 b: E=As). Whereas the Tl ion in 1 a/b is monocoordinated, the exchange of the weakly coordinating anion (WCA) in the Tl(I) salt leads to the formation of a trigonal pyramidal coordination mode at the Tl atom by coordination of three equivalents of EH₂ BH₂  ⋅ NMe₃ in [Tl(EH₂ BH₂  ⋅ NMe₃ )₃ ][WCA] (2 a: E=P, WCA=TEF^Cl ; 2 b: E=As, WCA=TEF) (TEF=[Al{OC(CF₃ )₃ }₄ ]^- , TEF^Cl =[Al{(OC(CF₃ )₂ (CCl₃ )}₄ ]^- ). Furthermore, by using two equivalents of PH₂ BH₂ ⋅NMe₃ , a Tl(I)-mediated P-P coupling takes place in CH₂ Cl₂ as solvent resulting in [Me₃ N⋅BH₂ PH₂ PHBH₂ ⋅NMe₃ ][WCA] (WCA=TEF, 3 a; BAr^Cl , 3 b; TEF^Cl , 3 c). In contrast, for the arsenic derivatives 1 b and 2 b, no coupling reaction is observed. The underlying chemical processes are elucidated by quantum chemical computations.

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.

Synthesis and Application of Alkali Metal Antimonide-A New Approach to Antimony Chemistry

Alkali metal dihydrogen-antimonides [M(L)_x SbH₂ ], short: alkali metal antimonides (M=Li, Na, K, Rb, Cs; 1: L=pmdta; 2: L=crown-ether), were prepared from stibine and n-Butyllithium, M(hmds) (hmds=hexamethyldisilazane) or MOtBu, respectively. We developed a generally applicable synthesis route for these compounds and the obtained compounds were examined on their stability depending on the alkali metal and stabilizing additives used, whereby the use of appropriate crown-ethers allowed their isolation and characterization at room temperature. Moreover, the 1,4-dioxane adduct [Na(dioxane)_x SbH₂ ] was the appropriate starting compound for the synthesis of the first primary silylstibane (Me₃ Si)₃ SiSbH₂ (3) which was characterized by NMR and IR spectroscopy. Reaction of 3 with (Dipp₂ NacNac)Ga (Dipp₂ NacNac=HC{C(Me)N(Dipp)}₂ ; Dipp=2,6-iPr₂ C₆ H₃ ) resulted in the formation of (Dipp₂ NacNac)GaH(SbHSi(SiMe₃ )₃ ) (4) which was furthermore characterized by single crystal x-ray diffraction.

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.

Recent advances in the domain of Cyclic (alkyl)(amino) carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

An NHC-Stabilized H2 GeBH2 Precursor for the Preparation of Cationic Group 13/14/15 Hydride Chains

The synthesis, characterization and reactivity studies of the NHC‐stabilized complex IDipp ⋅ GeH₂BH₂OTf (1) (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) are reported. Nucleophilic substitution of the triflate (OTf) group in 1 by phosphine or arsine donors provides access to the cationic group 13/14/15 chains [IDipp ⋅ GeH₂BH₂ERR¹R²]⁺ (2 E=P; R, R¹=H; R²=^tBu; 3 E=P; R=H; R¹, R²=Ph; 4 a E=P; R, R¹, R²=Ph; 4 b E=As; R, R¹, R²=Ph). These novel cationic chains were characterized by X‐ray crystallography, NMR spectroscopy and mass spectrometry. Moreover, the formation of the parent complexes [IDipp ⋅ GeH₂BH₂PH₃][OTf] (5) and [IDipp ⋅ GeH₃][OTf] (6) were achieved by reaction of 1 with PH₃. Accompanying DFT computations give insight into the stability of the formed chains with respect to their decomposition.

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.

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.

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.

Bidentate Phosphanyl- and Arsanylboranes

A new class of neutral bidentate ligands with pnictogenyl-functional sites has been obtained. The reaction of tmeda⋅(BH₂ I)₂ (1, tmeda=tetramethylethylendiamine) with different phosphanides yields the corresponding bidentate phosphanylboranes tmeda⋅(BH₂ PH₂ )₂ (2 a), tmeda⋅(BH₂ PPh₂ )₂ (2 b), and tmeda⋅(BH₂ tBuPH)₂ (2 c). This reaction strategy could be further extended to synthesize the first bidentate arsanylborane tmeda⋅(BH₂ AsPh₂ )₂ (3). Depending on the substituents on the phosphorus, these compounds form different Au^I complexes, to build either polymeric tmeda⋅(BH₂ PH₂ AuCl)₂ (4 a), or monomeric tmeda⋅(BH₂ PPh₂ AuCl)₂ (4 b) products. These compounds form also neutral oligomeric group 13/15 chain-like molecules by coordination to a boron moiety such as tmeda⋅(BH₂ PH₂ BH₃ )₂ (5 a) and tmeda⋅(BH₂ AsPh₂ BH₃ )₂ (5 b). DFT calculations provide insight into the differences between the syntheses of mono- and bidentate pnictogenylboranes.

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.

Gold(I) Complexes Containing Phosphanyl- and Arsanylborane Ligands

The structural and photophysical properties of a series of new Au^I compounds have been studied. The reactions of AuCl(tht) with the phosphanyl- and arsanylboranes RR^' EBH₂ NMe₃ (E=P, As; R=H, Ph; R'=H, Ph, tBu) afford the complexes [AuCl(RR^' EBH₂ NMe₃ )]. In the solid state, [AuCl(H₂ PBH₂ NMe₃ )]₂ (2 a) is a dimer showing unsupported intermolecular aurophilic interactions with short Au⋅⋅⋅Au distances. In contrast, [AuCl(H₂ AsBH₂ NMe₃ )]_n (2 b) aggregates to form 1D chains. Organic substituents on the pnictogen atoms lead to discrete molecules in [AuCl(RR^' PBH₂ NMe₃ )] (2 c: R=H, R'=tBu; 2 d: R=R'=Ph). To increase the aurophilicity, the ionic homoleptic complexes [Au(RR^' EBH₂ NMe₃ )₂ ][AlCl₄ ] (3 a-d) have been synthesized, for which 3 a,b form chains in the solid state and exhibit luminescence. The emissions show a drastic redshift with temperature decrease, correlating with decreasing Au⋅⋅⋅Au distances. DFT calculations provide insight into the bonding situation of the products.

From Monodisperse Thienyl- and Furylborane Oligomers to Polymers: Modulating the Optical Properties through the Hetarene Ratio

The application of our newly developed B-C coupling method by catalytic Si/B exchange is demonstrated for the synthesis of a series of triarylboranes (1), monodisperse thienyl- and furylborane dimers (2) and trimers (9), extended oligomers (3) and polymers (3'), as well as mixed (oligo)thienyl-/furylboranes. The structures of 1 aa^Tip , 1 bb^Tip , and 2 bbb^Mes* , determined by X-ray crystallography, reveal largely coplanar hetarene rings and BR₃ environments, which are most pronounced in the furylborane species. Photophysical investigations, supported by TD-DFT calculations, revealed pronounced π-electron delocalization over the hetarene backbones including the boron centers. With an extended series of derivatives of varying chain lengths available, we were able to determine the effective conjugation lengths (ECL) of poly(thienylborane)s and poly(furylborane)s, which have been reached with the highest-molecular-weight derivatives of our study. Through variation of the furan-to-thiophene ratio, the photophysical properties of these materials are effectively modulated. Significantly, higher furan contents lead to considerably increased fluorescence intensities. Compounds 1 aa^Tip , 1 bb^Tip , and 3 a^Tip showed the ability to bind fluoride anions. The binding process is signaled by a distinct change in their optical absorption characteristics, thus rendering these materials attractive targets for sensory applications.

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.

Polymers and the p-block elements

A survey of the state-of-the-art in the development of synthetic methods to incorporate p-block elements into polymers is given. The incorporation of main group elements (groups 13-16) into long chains provides access to materials with fascinating chemical and physical properties imparted by the presence of inorganic groups. Perhaps the greatest impedance to the widespread academic and commercial use of p-block element-containing macromolecules is the synthetic challenge associated with linking inorganic elements into long chains. In recent years, creative methodologies have been developed to incorporate heteroatoms into polymeric structures, with perhaps the greatest advances occurring with hybrid organic-inorganic polymers composed of boron, silicon, phosphorus and sulfur. With these developments, materials are currently being realized that possess exciting chemical, photophysical and thermal properties that are not possible for conventional organic polymers. This review focuses on highlighting the most significant recent advances whilst giving an appropriate background for the general reader. Of particular focus will be advances made over the last two decades, with emphasis on the novel synthetic methodologies employed.