Generation of aminoborane monomers RR'N=BH2 from amine-boronium cations [RR'NH-BH2L](+): metal catalyst-free formation of polyaminoboranes at ambient temperature

Dehydropolymerization of Amine-Boranes using Bis(imino)pyridine Rhodium Pre-Catalysis: σ-Amine-Borane Complexes, Nanoparticles, and Low Residual-Metal BN-Polymers that can be Chemically Repurposed

The sigma amine-borane complexes [Rh(L1)(η2 :η2 -H3 B⋅NRH2 )][OTf] (L1=2,6-bis-[1-(2,6-diisopropylphenylimino)ethyl]pyridine, R=Me, Et, n Pr) are described, alongside [Rh(L1)(NMeH2 )][OTf]. Using R=Me as a pre-catalyst (1 mol %) the dehydropolymerization of H3 B ⋅ NMeH2 gives [H2 BNMeH]n selectively. Added NMeH2 , or the direct use of [Rh(L1)(NMeH2 )][OTf], is required for initiation of catalysis, which is suggested to operate through the formation of a neutral hydride complex, Rh(L1)H. The formation of small (1-5 nm) nanoparticles is observed at the end of catalysis, but studies are ambiguous as to whether the catalysis is solely nanoparticle promoted or if there is a molecular homogeneous component. [Rh(L1)(NMeH2 )][OTf] is shown to operate at 0.025 mol % loadings on a 2 g scale of H3 B ⋅ NMeH2 to give polyaminoborane [H2 BNMeH]n [Mn =30,900 g/mol, Ð=1.8] that can be purified to a low residual [Rh] (6 μg/g). Addition of Na[N(SiMe3 )2 ] to [H2 BNMeH]n results in selective depolymerization to form the eee-isomer of N,N,N-trimethylcyclotriborazane [H2 BNMeH]3 : the chemical repurposing of a main-group polymer.

Cp* non-innocence and the implications of (η4-Cp*H)Rh intermediates in the hydrogenation of CO2, NAD+, amino-borane, and the Cp* framework - a computational study

In hydrogenation mediated by half-sandwich complexes of Rh, Cp*Rh(III)-H intermediates are critical hydride-delivery agents. For bipyridine-supported complexes, a unique transformation named 'Cp* non-innocence' leads to the appearance of (Cp*H)Rh(I) intermediates, which are purported to exhibit enhanced hydride-delivery capabilities. In this work, DFT calculations performed to compare the role of these complexes in hydrogenation reveal that (Cp*H)Rh(I) intermediates do not compete with the conventional pathway (involving Cp*Rh(III)-H); instead they can lead to sequential hydrogenation of the Cp* framework, and potentially, catalyst degradation. Thus, caution is warranted when invoking the truly homogeneous nature of hydrogenation catalysis mediated by this popular class of complexes.

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.

Dehydropolymerization of H3B·NMeH2 Mediated by Cationic Iridium(III) Precatalysts Bearing κ3-iPr-PNRP Pincer Ligands (R = H, Me): An Unexpected Inner-Sphere Mechanism

The dehydropolymerization of H₃B·NMeH₂ to form N-methylpolyaminoborane using neutral and cationic catalysts based on the {Ir(ⁱPr-PN^HP)} fragment [ⁱPr-PN^HP = κ³-(CH₂CH₂PⁱPr₂)₂NH] is reported. Neutral Ir(ⁱPr-PN^HP)H₃ or Ir(ⁱPr-PN^HP)H₂Cl precatalysts show no, or poor and unselective, activity respectively at 298 K in 1,2-F₂C₆H₄ solution. In contrast, addition of [NMeH₃][BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃) to Ir(ⁱPr-PN^HP)H₃ immediately starts catalysis, suggesting that a cationic catalytic manifold operates. Consistent with this, independently synthesized cationic precatalysts are active (tested between 0.5 and 2.0 mol % loading) producing poly(N-methylaminoborane) with M_n ∼ 40,000 g/mol, Đ ∼1.5, i.e., dihydrogen/dihydride, [Ir(ⁱPr-PN^HP)(H)₂(H₂)][BAr^F₄]; σ-amine-borane [Ir(ⁱPr-PN^HP)(H)₂(H₃B·NMe₃)][BAr^F₄]; and [Ir(ⁱPr-PN^HP)(H)₂(NMeH₂)][BAr^F₄]. Density functional theory (DFT) calculations probe hydride exchange processes in two of these complexes and also show that the barrier to amine-borane dehydrogenation is lower (22.5 kcal/mol) for the cationic system compared with the neutral system (24.3 kcal/mol). The calculations show that the dehydrogenation proceeds via an inner-sphere process without metal–ligand cooperativity, and this is supported experimentally by N–Me substituted [Ir(ⁱPr-PN^MeP)(H)₂(H₃B·NMe₃)][BAr^F₄] being an active catalyst. Key to the lower barrier calculated for the cationic system is the outer-sphere coordination of an additional H₃B·NMeH₂ with the N–H group of the ligand. Experimentally, kinetic studies indicate a complex reaction manifold that shows pronounced deceleratory temporal profiles. As supported by speciation and DFT studies, a key observation is that deprotonation of [Ir(ⁱPr-N^HP)(H)₂(H₂)][BAr^F₄], formed upon amine-borane dehydrogenation, by the slow in situ formation of NMeH₂ (via B–N bond cleavage), results in the formation of essentially inactive Ir(ⁱPr-PN^HP)H₃, with a coproduct of [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺. While reprotonation of Ir(ⁱPr-PN^HP)H₃ results in a return to the cationic cycle, it is proposed, supported by doping experiments, that reprotonation is attenuated by entrainment of the [NMeH₃]⁺/[H₂B(NMeH₂)₂]⁺/catalyst in insoluble polyaminoborane. The role of [NMeH₃]⁺/[H₂B(NMeH₂)]⁺ as chain control agents is also noted.

Aminoboranes via Tandem Iodination/Dehydroiodination for One-Pot Borylation

A rapid synthesis of aminoboranes from amine-boranes utilizing an iodination/dehydroiodination sequence is described. Monomeric aminoboranes are generated exclusively from several substrate adducts, following an E2-type elimination, with the added base playing a critical role in monomer vs dimer formation. Diisopropylaminoborane formed using this methodology has been applied to a one-pot palladium-catalyzed conversion of iodo- and bromoarenes to the corresponding boronates. Additionally, modification of the workup allows for isolation of the boronic acid and recovery of the utilized amine.

NHC induced radical formation via homolytic cleavage of B–B bonds and its role in organic reactions

New borylation methodologies have been reported recently, wherein diboron(4) compounds apparently participate in free radical couplings via the homolytic cleavage of the B–B bond. We report herein that bis-NHC adducts of the type (NHC)₂·B₂(OR)₄, which are thermally unstable and undergo intramolecular ring expansion reactions (RER), are sources of boryl radicals of the type NHC–BR₂˙, exemplified by Me₂Im^Me·Bneop˙ 1a (Me₂Im^Me = 1,3,4,5-tetramethyl-imidazolin-2-ylidene, neop = neopentylglycolato), which are formed by homolytic B–B bond cleavage. Attempts to apply the boryl moiety 1a in a metal-free borylation reaction by suppressing the RER failed. However, based on these findings, a protocol was developed using Me₂Im^Me·B₂pin₂3 for the transition metal- and additive-free boryl transfer to substituted aryl iodides and bromides giving aryl boronate esters in good yields. Analysis of the side products and further studies concerning the reaction mechanism revealed that radicals are likely involved. An aryl radical was trapped by TEMPO, an EPR resonance, which was suggestive of a boron-based radical, was detected in situ, and running the reaction in styrene led to the formation of polystyrene. The isolation of a boronium cation side product, [(Me₂Im^Me)₂·Bpin]⁺I⁻7, demonstrated the fate of the second boryl moiety of B₂pin₂. Interestingly, Me₂Im^Me NHC reacts with aryl iodides and bromides generating radicals. A mechanism for the boryl radical transfer from Me₂Im^Me·B₂pin₂3 to aryl iodides and bromides is proposed based on these experimental observations.

Bis-NHC adducts of the type (NHC)₂·B₂(OR)₄ are sources of boryl radicals of the type NHC–BR₂˙, which are formed by homolytic B–B bond cleavage.

Controlled Synthesis of Well-Defined Polyaminoboranes on Scale Using a Robust and Efficient Catalyst

The air tolerant precatalyst, [Rh(L)(NBD)]Cl ([1]Cl) [L = κ³-(ⁱPr₂PCH₂CH₂)₂NH, NBD = norbornadiene], mediates the selective synthesis of N-methylpolyaminoborane, (H₂BNMeH)_n, by dehydropolymerization of H₃B·NMeH₂. Kinetic, speciation, and DFT studies show an induction period in which the active catalyst, Rh(L)H₃ (3), forms, which sits as an outer-sphere adduct 3·H₃BNMeH₂ as the resting state. At the end of catalysis, dormant Rh(L)H₂Cl (2) is formed. Reaction of 2 with H₃B·NMeH₂ returns 3, alongside the proposed formation of boronium [H₂B(NMeH₂)₂]Cl. Aided by isotopic labeling, Eyring analysis, and DFT calculations, a mechanism is proposed in which the cooperative "PNHP" ligand templates dehydrogenation, releasing H₂B═NMeH (ΔG^‡_calc = 19.6 kcal mol^-1). H₂B═NMeH is proposed to undergo rapid, low barrier, head-to-tail chain propagation for which 3 is the catalyst/initiator. A high molecular weight polymer is formed that is relatively insensitive to catalyst loading (M_n ∼71 000 g mol^-1; Đ, of ∼ 1.6). The molecular weight can be controlled using [H₂B(NMe₂H)₂]Cl as a chain transfer agent, M_n = 37 900-78 100 g mol^-1. This polymerization is suggested to arise from an ensemble of processes (catalyst speciation, dehydrogenation, propagation, chain transfer) that are geared around the concentration of H₃B·NMeH₂. TGA and DSC thermal analysis of polymer produced on scale (10 g, 0.01 mol % [1]Cl) show a processing window that allows for melt extrusion of polyaminoborane strands, as well as hot pressing, drop casting, and electrospray deposition. By variation of conditions in the latter, smooth or porous microstructured films or spherical polyaminoboranes beads (∼100 nm) result.

Reactivity and mechanisms of hydridic hydrogen of B–H in ammonia borane towards acetic acids: the ammonia B-monoacyloxy boranes

B-monoacyloxy boranes are first obtained by moderate reactions of ammonia borane with acetic acids.

Dehydropolymerisation of methylamine borane using highly active rhodium(iii) bis(thiophosphinite) pincer complexes: catalytic and mechanistic insights

The bis(thiophosphinite) pincer complexes [(^RPSCSP^R)Rh(py)(H)(Cl)] (^RPSCSP^R = C₆H₄–2,6-(SPR₂)₂ with R = ⁱPr, 2a and R = Ph, 2b) are highly active precatalysts for the dehydropolymerisation of methylamine borane.

A general method for the synthesis of covalent and ionic amine borane complexes containing trinitromethyl fragments

A general approach for the synthesis of covalent and ionic amine borane complexes containing trinitromethyl fragments has been developed through metathesis reactions between amine chloroborane complexes and potassium salt of trinitromethyl (K[C(NO₂)₃]). Five covalent and ionic trinitromethyl amine borane complexes have been synthesized in good yields with high purity and it is found that the ionic complex, [H₂B(NH₃)₂][C(NO₂)₃], might be a promising energetic material on the basis of the investigation of its thermal decomposition behaviour.

A general approach has been developed through which five covalent and ionic amine borane complexes containing trinitromethyl fragments were synthesized.

Influence of Multisite Metal-Ligand Cooperativity on the Redox Activity of Noninnocent N2S2 Ligands

Metal-ligand cooperativity (MLC) relies on chemically reactive ligands to assist metals with small-molecule binding and activation, and it has facilitated unprecedented examples of catalysis with metal complexes. Despite growing interest in combining ligand-centered chemical and redox reactions for chemical transformations, there are few studies demonstrating how chemically engaging redox active ligands in MLC affects their electrochemical properties when bound to metals. Here we report stepwise changes in the redox activity of model Ru complexes as zero, one, and two BH₃ molecules undergo MLC binding with a triaryl noninnocent N₂S₂ ligand derived from o-phenylenediamine (L1). A similar series of Ru complexes with a diaryl N₂S₂ ligand with ethylene substituted in place of phenylene (L2) is also described to evaluate the influence of the o-phenylenediamine subunit on redox activity and MLC. Cyclic voltammetry (CV) studies and density functional theory (DFT) calculations show that MLC attenuates ligand-centered redox activity in both series of complexes, but electron transfer is still achieved when only one of the two redox-active sites on the ligands is chemically engaged. The results demonstrate how incorporating more than one multifunctional reactive site could be an effective strategy for maintaining redox noninnocence in ligands that are also chemically reactive and competent for MLC.

Dehydropolymerisation of Methylamine Borane and an N-Substituted Primary Amine Borane Using a PNP Fe Catalyst

Dehydropolymerisation of methylamine borane (H3 B⋅NMeH2 ) using the well-known iron amido complex [(PNP)Fe(H)(CO)] (PNP=N(CH2 CH2 PiPr2 )2 ) (1) gives poly(aminoborane)s by a chain-growth mechanism. In toluene, rapid dehydrogenation of H3 B⋅NMeH2 following first-order behaviour as a limiting case of a more general underlying Michaelis-Menten kinetics is observed, forming aminoborane H2 B=NMeH, which selectively couples to give high-molecular-weight poly(aminoborane)s (H2 BNMeH)n and only traces of borazine (HBNMe)3 by depolymerisation after full conversion. Based on a series of comparative experiments using structurally related Fe catalysts and dimethylamine borane (H3 B⋅NMe2 H) polymer formation is proposed to occur by nucleophilic chain growth as reported earlier computationally and experimentally. A silyl functionalised primary borane H3 B⋅N(CH2 SiMe3 )H2 was studied in homo- and co-dehydropolymerisation reactions to give the first examples for Si containing poly(aminoborane)s.

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.

A simple cobalt-based catalyst system for the controlled dehydropolymerisation of H3B·NMeH2 on the gram-scale

A simple Co-based catalyst system promotes the efficient and controlled dehydropolymerisation of amine–boranes on scale.

The role of neutral Rh(PONOP)H, free NMe2H, boronium and ammonium salts in the dehydrocoupling of dimethylamine-borane using the cationic pincer [Rh(PONOP)(η2-H2)]+ catalyst

The σ-amine-borane pincer complex [Rh(PONOP)(η1-H3B·NMe3)][BArF4] [2, PONOP = κ3-NC5H3-2,6-(OPtBu2)2] is prepared by addition of H3B·NMe3 to the dihydrogen precursor [Rh(PONOP)(η2-H2)][BArF4], 1. In a similar way the related H3B·NMe2H complex [Rh(PONOP)(η1-H3B·NMe2H)][BArF4], 3, can be made in situ, but this undergoes dehydrocoupling to reform 1 and give the aminoborane dimer [H2BNMe2]2. NMR studies on this system reveal an intermediate neutral hydride forms, Rh(PONOP)H, 4, that has been prepared independently. 1 is a competent catalyst (2 mol%, ∼30 min) for the dehydrocoupling of H3B·Me2H. Kinetic, mechanistic and computational studies point to the role of NMe2H in both forming the neutral hydride, via deprotonation of a σ-amine-borane complex and formation of aminoborane, and closing the catalytic cycle by reprotonation of the hydride by the thus-formed dimethyl ammonium [NMe2H2]+. Competitive processes involving the generation of boronium [H2B(NMe2H)2]+ are also discussed, but shown to be higher in energy. Off-cycle adducts between [NMe2H2]+ or [H2B(NMe2H)2]+ and amine-boranes are also discussed that act to modify the kinetics of dehydrocoupling.

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.

Dehydropolymerization of H3B·NMeH2 Using a [Rh(DPEphos)]+ Catalyst: The Promoting Effect of NMeH2

[Rh(κ²-PP-DPEphos){η²η²-H₂B(NMe₃)(CH₂)₂^tBu}][BAr^F₄] acts as an effective precatalyst for the dehydropolymerization of H₃B·NMeH₂ to form N-methylpolyaminoborane (H₂BNMeH)_n. Control of polymer molecular weight is achieved by variation of precatalyst loading (0.1–1 mol %, an inverse relationship) and use of the chain-modifying agent H₂: with M_n ranging between 5 500 and 34 900 g/mol and Đ between 1.5 and 1.8. H₂ evolution studies (1,2-F₂C₆H₄ solvent) reveal an induction period that gets longer with higher precatalyst loading and complex kinetics with a noninteger order in [Rh]_TOTAL. Speciation studies at 10 mol % indicate the initial formation of the amino–borane bridged dimer, [Rh₂(κ²-PP-DPEphos)₂(μ-H)(μ-H₂BN=HMe)][BAr^F₄], followed by the crystallographically characterized amidodiboryl complex [Rh₂(cis-κ²-PP-DPEphos)₂(σ,μ-(H₂B)₂NHMe)][BAr^F₄]. Adding ∼2 equiv of NMeH₂ in tetrahydrofuran (THF) solution to the precatalyst removes this induction period, pseudo-first-order kinetics are observed, a half-order relationship to [Rh]_TOTAL is revealed with regard to dehydrogenation, and polymer molecular weights are increased (e.g., M_n = 40 000 g/mol). Speciation studies suggest that NMeH₂ acts to form the  precatalysts [Rh(κ²-DPEphos)(NMeH₂)₂][BAr^F₄] and [Rh(κ²-DPEphos)(H)₂(NMeH₂)₂][BAr^F₄], which were independently synthesized and shown to follow very similar dehydrogenation kinetics, and produce polymers of molecular weight comparable with [Rh(κ²-PP-DPEphos){η²-H₂B(NMe₃)(CH₂)₂^tBu}][BAr^F₄], which has been doped with amine. This promoting effect of added amine in situ is shown to be general in other cationic Rh-based systems, and possible mechanistic scenarios are discussed.

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.

On the primary structure of polysilenes and polygermenes

Materials derived from the anionic polymerization of a silene or a germene have a regular, alternating structure without any significant rearrangement taking place during polymerization.

Step-growth titanium-catalysed dehydropolymerisation of amine-boranes

Precatalysts active for the dehydropolymerisation of primary amine-boranes are generally based on mid or late transition metal. We have found that the activity of the precatalyst system formed from Cp^R₂TiCl₂ and 2nBuLi towards the dehydrogenation of the secondary amine-borane Me₂NH·BH₃, to yield the cyclic diborazane [Me₂N-BH₂]₂, increases dramatically with increasing electron-donating character of the cyclopentadienyl rings (Cp^R). Application of the most active precatalyst system (Cp^R = η-C₅Me₅) to the primary amine-borane MeNH₂·BH₃ enabled the first synthesis of high molar mass poly(N-methylaminoborane), [MeNH-BH₂] _n , the BN analogue of polypropylene, by an early transition metal such as catalyst. Significantly, unlike other dehydropolymerization precatalysts for MeNH₂·BH₃ such as [Ir(POCOP)H₂], skeletal nickel, and [Rh(COD)Cl]₂, the Ti precatalyst system was also active towards a range of substrates including BzNH₂·BH₃ (Bz = benzyl) yielding high molar mass polymer. Moreover, in contrast to the late transition metal catalysed dehydropolymerisation of MeNH₂·BH₃ and also the Ziegler-Natta polymerisation of olefins, studies indicate that the Ti-catalyzed dehydropolymerization reactions proceed by a step-growth rather than a chain-growth mechanism.

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.

Formation of high-molecular weight polyaminoborane by Fe hydride catalysed dehydrocoupling of methylamine borane

The complex [(PNHP)Fe(H)(CO)(HBH₃)] (PNHP = HN(CH₂CH₂Pi-Pr₂)₂) serves as a catalyst precursor for the selective dehydrocoupling of methylamine borane at room temperature, tentatively via an off-metal polymerisation pathway.

The Simplest Amino-borane H2 B=NH2 Trapped on a Rhodium Dimer: Pre-Catalysts for Amine-Borane Dehydropolymerization

The μ-amino-borane complexes [Rh2 (L(R) )2 (μ-H)(μ-H2 B=NHR')][BAr(F) 4 ] (L(R) =R2 P(CH2 )3 PR2 ; R=Ph, (i) Pr; R'=H, Me) form by addition of H3 B⋅NMeR'H2 to [Rh(L(R) )(η(6) -C6 H5 F)][BAr(F) 4 ]. DFT calculations demonstrate that the amino-borane interacts with the Rh centers through strong Rh-H and Rh-B interactions. Mechanistic investigations show that these dimers can form by a boronium-mediated route, and are pre-catalysts for amine-borane dehydropolymerization, suggesting a possible role for bimetallic motifs in catalysis.

The Synthesis, Characterization and Dehydrogenation of Sigma-Complexes of BN-Cyclohexanes

The coordination chemistry of the 1,2-BN-cyclohexanes 2,2-R2 -1,2-B,N-C4 H10 (R2 =HH, MeH, Me2 ) with Ir and Rh metal fragments has been studied. This led to the solution (NMR spectroscopy) and solid-state (X-ray diffraction) characterization of [Ir(PCy3 )2 (H)2 (η(2) η(2) -H2 BNR2 C4 H8 )][BAr(F) 4 ] (NR2 =NH2 , NMeH) and [Rh(iPr2 PCH2 CH2 CH2 PiPr2 )(η(2) η(2) -H2 BNR2 C4 H8 )][BAr(F) 4 ] (NR2 =NH2 , NMeH, NMe2 ). For NR2 =NH2 subsequent metal-promoted, dehydrocoupling shows the eventual formation of the cyclic tricyclic borazine [BNC4 H8 ]3 , via amino-borane and, tentatively characterized using DFT/GIAO chemical shift calculations, cycloborazane intermediates. For NR2 =NMeH the final product is the cyclic amino-borane HBNMeC4 H8 . The mechanism of dehydrogenation of 2,2-H,Me-1,2-B,N-C4 H10 using the {Rh(iPr2 PCH2 CH2 CH2 PiPr2 )}(+) catalyst has been probed. Catalytic experiments indicate the rapid formation of a dimeric species, [Rh2 (iPr2 PCH2 CH2 CH2 PiPr2 )2 H5 ][BAr(F) 4 ]. Using the initial rate method starting from this dimer, a first-order relationship to [amine-borane], but half-order to [Rh] is established, which is suggested to be due to a rapid dimer-monomer equilibrium operating.

Diverse reactivity of borenium cations with >N–H compounds

The P-stabilized borenium 1 displays rich reactivity towards >N–H compounds: substitution reaction at a borenium center with Ph₂NH, Lewis adduct formation and subsequent deprotonation with NH₃, B–Mes cleavage and formation of a dicationic species with HNTf₂.

Use of crown ethers to isolate intermediates in ammonia-borane dehydrocoupling reactions

The presence of 18-crown-6 in the Lewis acid-promoted dehydrocoupling reaction of ammonia borane permits isolation of [(THF)BH₂NH₃]⁺ cation.

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.