Chemistry

Metallic-free dehydropolymerisation of phosphine-boranes utilizing cyclic (alkyl)(amino)carbenes as hydrogen acceptors

Reactivity of carbenes with phosphine-boranes

The synthesis of monomeric aminoborane-NHC adducts (NHC-BH2NHR) has been reported each by means of using an NHC for ambient temperature dehydrogenation of amine-boranes (RNH2·BH3; R = H, Me)43 and NHC-induced depolymerisation of poly(N-methylaminoborane)44. Extra not too long ago, analogous species that includes using NHCs to stabilise phosphinoborane monomers have been remoted utilizing NHC-induced thermal depolymerisation of polyphosphinoboranes45. Consequently, previous to investigating the reactivity of phosphine-boranes with CAACs, we explored the dehydrogenation potential of NHCs.

Upon addition of 1 equal of IDipp to an answer of PhPH2·BH3 in tetrahydrofuran (THF), a homogeneous answer was shaped after 10 min, and evaluation of the response combination by 31P and 11B nuclear magnetic resonance (NMR) spectroscopy confirmed full conversion to a brand new species (δP = −84.2ppm (br), δB = −33.4ppm (dq) in THF) (Supplementary Figs. 2 and three). The similarity of those spectral options to these noticed for Li[PhPHBH3] (δP = −93.8ppm (d), δB = −34.6ppm (dq) in THF)46, an identical compound with a distinct cation, is according to deprotonation of PhPH2·BH3 by IDipp to yield the salt [IDippH][PhHPBH3] (1a) (Fig. 2a). The formation of this salt was additional confirmed by an impartial synthesis by way of a metathesis response in THF between [IDippH]Cl and Li[PhHPBH3]. This confirmed 11B and 31P NMR spectral options that matched these assigned to 1a together with precipitation of LiCl (Fig. 2b). The 13C NMR spectrum of 1a confirmed no 1JCP couplings involving the iminium carbon atom, which, along with the downfield chemical shift within the 1H NMR spectrum of the imidazolium proton (δH = 10.0ppm) (Supplementary Fig. 1), helps an ionic formulation for this species in answer. When Ph2PH·BH3 was reacted with IDipp, the analogous salt [IDippH][Ph2PBH3] (1b) was shaped (Supplementary Figs. four–6) and subsequently characterised utilizing X-ray crystallography (Supplementary Fig. 7 and Supplementary Desk 6).

Fig. 2Fig. 2

Reactivity of IDipp and CAACMe with phosphine-boranes. a Synthesis of 1a and 1b by deprotonation of the phosphine-borane utilizing IDipp; b synthesis of 1a and 1b utilizing salt metathesis route; c synthesis of 3a by means of oxidative addition of PhPH2·BH3; d synthesis of 3a by means of stepwise response of PhPH2, then BH3·THF; and e synthesis of 3a by means of salt metathesis route

Subsequent, we tried the analogous response with a CAAC because the smaller HOMO–LUMO separation of CAACs renders them doubtlessly higher candidates for E–H bond activations. The P–H activation of PhPH2·BH3 by one equal of CAACMe (Fig. 2c) occurred readily at 22 °C in THF to offer 3a, which exists as two diastereomers (3a′ and 3a″). The identification of 3a was initially established primarily based on a particular doublet of quartet of doublets coupling sample noticed within the 1H NMR spectrum for the P–H protons (Supplementary Figs. eight–10). This task was additional corroborated by an impartial synthesis by way of a stepwise process involving oxidative addition of PhPH2 to the carbene centre in CAACMe to yield 2a (as a combination of diastereomers every with indistinguishable enantiomers by NMR), adopted by the addition of BH3·THF to offer 3a (Fig. 2nd). The 2 diastereomers of 3a have been additionally shaped instantly upon combining Li[PhHPBH3] and [CAACMeH]Cl by means of elimination of LiCl (Fig. 2e). In distinction to the outcomes obtained within the response of [IDippH]Cl and Li[PhPHBH3] above (Fig. 2b), the decrease steric hindrance and better π-acidity36 of the cation [CAACMeH]+ results in the formation of a molecular species with a definite P–C bond, quite than the corresponding iminium salt [CAACMeH][PhPHBH3]. The molecular formulation of 3a is supported by the remark of each 1JCP (1JCP = 41.zero Hz (3a′), 1JCP = 38.Three Hz (3a″)) and 2JHP (2JHP = four.2 Hz (3a′), 2JHP = 5.eight Hz (3a″)) coupling constants within the 13C and 1H NMR spectra.

Makes an attempt to crystallographically characterise 3a have been unsuccessful as options in THF (zero.10 M) spontaneously decomposed to a combination of poly(phenylphosphinoborane) [PhHPBH2]n and (CAACMe)H2 as proven by 1H, 11B and 31P NMR spectroscopy. Though solely delicate to low molar mass fractions47, electrospray ionisation-mass spectrometry (ESI-MS) confirmed the formation of [PhHPBH2]n (as much as n = 22) by figuring out repeat items of ∆(m/z) = 122.05 (molecular weight of [PhHPBH2] = 122.05 g mol−1). Isolation of pure [PhHPBH2]n was achieved by means of precipitation of the response combination into chilly (−40 °C) hexanes to take away the hydrogenated carbene, (CAACMe)H2 (Supplementary Figs. 25 and 26), which was additionally characterised by X-ray crystallography (Supplementary Fig. 27 and Supplementary Desk 6). Within the current case, the eradicated phosphinoborane monomer [PhHPBH2] polymerises, presumably because of the small measurement of the substituents at P and B.

The affect of temperature, solvent and focus upon the molar mass of the poly(phenylphosphinoborane) obtained was systematically investigated with a view of optimising the polymerisation circumstances (Desk 1; Supplementary Desk 1; and Supplementary Figs. 18–21, 23 and 24). In every case, ESI-MS and gel permeation chromatography (GPC) analyses have been carried out (Supplementary Figs. 11–17, 22 and 23). ESI-MS clearly confirmed the presence of the [PhHPBH2] monomeric repeat unit in every case and allowed us to detect the presence of both BH3 or PPhH2 finish teams (Supplementary Fig. 22). Nonetheless, attributable to solely the low molar mass fraction being detected by the strategy, it’s not potential to attract hyperlinks between the response circumstances and the diploma of polymerisation utilizing these information47. In distinction, GPC evaluation permitted optimisation of the polymerisation circumstances as this method reveals the entire molar mass distribution (Desk 1 and Supplementary Desk 1). Rising the temperature (run 1 vs. Three, and 5 vs. 6) lowered the response time, however has no vital impact on the molar mass of the polymer obtained. Utilizing a non-polar solvent (toluene) quite than THF (runs Three vs. 5) additionally had no vital impact on the polymer molar mass. It was discovered that at larger concentrations (run 2 vs. Three vs. four), a bigger amount of polymeric relative to oligomeric materials was shaped (Supplementary Fig. 17). This remark is according to head-to-tail polymerisation of transiently generated phenylphosphinoborane, [PhHPBH2]. The response was additionally tried underneath solvent-free, soften circumstances at 110 °C (run 7), and, though excessive molar mass materials was shaped, the molar mass was no better than that obtained utilizing a concentrated answer at 60 °C. On account of considerations concerning the homogeneity of the response on account of poor mixing, subsequent research have been carried out in concentrated options quite than within the soften section.

Desk 1 Affect of temperature, solvent and focus on the formation of poly(phenylphosphinoborane), [PhHPBH2]n, in a closed system

Mechanistic research

A collection of experimental and density practical idea (DFT) research have been undertaken to probe the mechanism of the dehydrogenation of PhPH2·BH3 with CAACMe. A number of mechanisms for the era of monomeric [PhHPBH2] have been thought-about and subsequently discounted, primarily based on experimental and computational proof (for a full dialogue see ‘Proposed and subsequently discounted mechanisms for phosphine-borane dehydrogenation mediated by CAACMe’ within the Supplementary Data; Supplementary Figs. 32 and 33; and Supplementary Tables Three and 5), earlier than the ultimate mechanism proven beneath was proposed and supported (Fig. Three). Makes an attempt to entice the launched monomer with both cyclohexene48 or 1,Three-cyclohexadiene49 proved unsuccessful.

Fig. ThreeFig. 3

DFT examine. Simplified schematic response profile calculated for the response of PhPH2·BH3 (A) with N-phenyl CAAC (B) on the PBE0/6-31 + G(d,p)/IEFPCM(THF) stage of idea; Gibbs free energies for the second diastereomer are given in spherical brackets (for a complete depiction of the response profile see Supplementary Fig. 31)

Kinetic research have been carried out to evaluate the proposed mechanisms (Supplementary Desk 2 and Supplementary Fig. 28). A plot of ln[3a] vs. response time confirmed equal half-lives of 1.5 h (Supplementary Fig. 29) for a number of preliminary concentrations between zero.Three and zero.7 M at 50 °C, indicating a first-order course of in 3a. Monitoring the response at a number of temperatures between 22 and 60 °C allowed the enthalpy and entropy of activation to be calculated as 21.5 kcal mol−1 and −9.5 cal Okay−1 mol−1, respectively, according to a considerable vitality barrier involving a comparatively ordered transition state (Supplementary Fig. 30).

DFT calculations have been carried out on the PBE0/6-31 + G(d,p)/IEFPCM(THF) stage of idea50,51,52 with an N-phenyl mannequin system for the CAACMe (B) to additional elucidate the dehydrogenation mechanism (Fig. Three). An preliminary deprotonation of the P–H bond of PhPH2·BH3 (A) with B to offer a [CAAC(H)]+ and [PhPH(BH3)]− ion pair (Cpair) was essentially the most favoured first response step with a low Gibbs free vitality of activation of four.9 kcal mol−1 (see ‘DFT calculations’ within the Supplementary Data). Subsequent nucleophilic assault on the iminium carbon of the [CAAC(H)]+ cation by the phosphorus centre of the [PhPH(BH3)]− anion leads by way of TS4 to the SP,S (F) diastereomer of the P–H activation product, or by way of TS4′, to the opposite RP,S (F′) diastereomer. The calculation of the activation barrier for this step was hampered by the inherently flat development of the potential vitality hypersurface between TS4 or TS4′ and Cpair, which suggests, in settlement with the experimentally discovered speedy formation of 3a, that this step happens with a really small activation barrier. F and F′ are kinetic merchandise of the response. Considerably, this step is reversible by way of P–C dissociation, for which a most activation barrier of 18.7 kcal mol−1 was calculated from F to TS4. This opens up a second response pathway from Cpair resulting in (CAAC)H2 (G) and [PhHPBH2] (H) by way of B–H hydride abstraction from the [PhPH(BH3)]− anion by the π-acidic (CAAC-H)+ cation with a low activation barrier of 5.Three kcal mol−1 (by way of TS5). Thus, the formation of [PhHPBH2]n from 3a might be rationalised by the formation of transient [CAAC(H)]+ and [PhPH(BH3)]− ions by way of consecutive P–C bond scission and B–H hydride abstraction resulting in (CAACMe)H2 and [PhHPBH2], the latter present process head-to-tail polymerisation to thermodynamically favoured [PhHPBH2]n (Supplementary Fig. 31). For a dialogue of the proposed polymerisation mechanism, see ‘Supplementary dialogue of the polymerisation mechanism from phosphinoborane monomers’ within the Supplementary Data. The truth that the response between PhPH2·BH3 and IDipp stops on the [IDipp(H)]+ and [PhPH(BH3)]− ions (Fig. 2a) might be traced again to the better π-acidity of the [CAAC(H)]+ in comparison with the analogous [NHC(H)]+ cation, as prompt by the excessive exergonicity of the isodesmic response [CAAC(H)]+ + (NHC)H2 → [NHC(H)]+ + (CAAC)H2 (ΔG0 = −66.eight kcal mol−1; N-phenyl mannequin methods) (Supplementary Desk four).

In line with the calculations, the dissociation of the P–H activation merchandise F or F′ by way of the transition states TS4 or TS4′ requires the best activation vitality within the general mechanism, which is in settlement with the first-order price regulation discovered for 3a by the kinetic measurements. As well as, the calculated enthalpy of activation for this step (ΔH0 = 20.2 (TS4), 19.2 (TS4′) kcal mol−1) is in good settlement with the substantial experimentally derived enthalpy of activation for the general response (ΔH0 = 21.5 kcal mol−1). The upper Gibbs free vitality of activation required for the dissociation of the SP,S diastereomer F (ΔG0 = 18.7 kcal mol−1) in comparison with the RP,S diastereomer F′ (ΔG0 = 16.6 kcal mol−1) accounts for the experimentally noticed sooner conversion of 1 diastereomer throughout the response. Furthermore, the noticed enhanced response charges in THF (see Supplementary Desk 2) might be rationalised by the higher stabilisation of the [CAAC(H)]+ and [PhPH(BH3)]− ions in THF than in toluene, which is additional corroborated by the calculations (Supplementary Fig. 31).

Substrate scope

Given the success with PhPH2·BH3, the scope of the CAACMe-mediated dehydropolymerisation was prolonged with the goal of focusing on hitherto inaccessible excessive molar mass P-disubstituted polyphosphinoboranes. CAACMe(H)Ph2PBH3 (3b) was synthesised from Ph2PH·BH3 and CAACMe in THF (Fig. 4a and Supplementary Figs. 34 and 35). Formation of 3b was additionally detected instantly upon combining Li[Ph2PBH3] and [CAACMeH]Cl, and in addition by means of the stepwise addition of Ph2PH adopted by BH3·THF to an answer of CAACMe.

Fig. fourFig. 4

Reactions of Ph2PH·BH3 and rac-Ph(Et)PH·BH3 with CAACMe and CAACCy. a CAACMe-mediated dehydrocoupling of Ph2PH·BH3; b CAACCy-mediated dehydrocoupling of Ph2PH·BH3; c CAACMe-mediated dehydrocoupling of rac-Ph(Et)PH·BH3; d CAACCy-mediated dehydrocoupling of rac-Ph(Et)PH·BH3; and e thermal ellipsoid plot of 3c. H atoms apart from these sure to C9 and B1 have been omitted for readability. Ellipsoids are proven on the 30% likelihood stage

Heating a concentrated answer of 3b (2.5 M, 60 °C, 1 h, THF or toluene) effected full conversion to (CAACMe)H2, the linear dimer Ph2PHBH2PPh2BH3, cyclic oligomers [Ph2PBH2]x (x = Three, four), and the polymer [Ph2PBH2]n as noticed by 1H and 31P NMR (Fig. 4a and Supplementary Fig. 40a)53. Elimination of (CAACMe)H2 and cyclic oligomers was achieved by precipitation into hexanes, however makes an attempt to separate Ph2PHBH2PPh2BH3 and [Ph2PBH2]n proved unsuccessful (for particulars see ‘Dehydropolymerisation of Ph2PH·BH3’ within the Supplementary Data). ESI-MS evaluation of the product after precipitation however confirmed the presence of the repeat unit ∆(m/z) = 198.08 (molecular weight of [Ph2PBH2] = 198.08 g mol−1, most worth of n = 10) (Supplementary Fig. 36). Nonetheless, GPC evaluation confirmed solely a really small quantity of excessive molar mass materials. Apparently, when toluene, quite than THF, is used because the solvent, a a lot smaller amount of linear dimer is shaped (Supplementary Figs. 37 and 40b). Underneath these circumstances, GPC evaluation on the precipitated materials confirmed a majority of low molar mass materials (Mn = ca. 1,300; polydispersity index (PDI)  = 1.31) and a small quantity (ca. 10%) of excessive molar mass materials (Mn = 54,300; PDI = 1.12) (Supplementary Fig. 38).

With the goal of accelerating the yield and quantity of excessive molar mass materials, we investigated using the extra reactive CAACCy, exemplified by its potential to activate dihydrogen underneath gentle circumstances37. The initially shaped P–H activation compound is consumed inside 1 h at 22 °C (Fig. 4b). Nonetheless, GPC evaluation once more confirmed solely a small quantity (ca. 12%) of excessive molar mass materials (Mn = 59,600; PDI = 1.08) with the bulk being low molar mass materials (Mn = ca. 1100; PDI = 1.28) (Supplementary Figs. 39, 40c and 41–45).

In an try and additional lengthen the scope of the dehydropolymerisation to different P-disubstituted phosphine-boranes the reactivity of rac-Ph(Et)PH·BH3 with CAACMe and CAACCy was investigated. CAACMe(H)PhEtPBH3 (3c) was shaped by means of direct response of CAACMe with rac-PhEtPH·BH3 (Supplementary Figs. 46–50). In contrast to with the mono- and di-phenyl derivatives, 3c is steady at 22 °C, which allowed the construction to be confirmed by X-ray diffraction (Fig. 4e and Supplementary Desk 6). Upon heating remoted 3c to 100 °C, the focused dehydropolymerisation occurred to offer [PhEtPBH2]n and (CAACMe)H2 (Fig. 4c and Supplementary Fig. 51). Pure [PhEtPBH2]n was obtained in 23% yield as a wonderful white powder following precipitation. ESI-MS evaluation of the precipitated pattern confirmed the presence of the repeat unit of [PhEtPBH2]n (∆(m/z) = 150.08, molar mass of [PhEtPBH2] = 150.08 g mol−1) and n = 33 (Supplementary Fig. 52); nonetheless, there was no convincing excessive molar mass materials noticed utilizing GPC. Within the analogous response utilizing CAACCy, the yield was additionally low (19%); nonetheless, a GPC peak akin to excessive molar mass materials (Mn = 62,600, PDI = 1.19, n = ca. 400) was noticed (Supplementary Figs. 53–58). Once more this was solely a small quantity (ca. 18 %) in comparison with the low molar mass fraction (Mn = ca. 1900; PDI = 1.47, n = ca. 13). Upon nearer evaluation of the ESI-MS spectra for every synthesis of [Ph2PBH2]n and [PhEtBH2]n, the tip group of the main distribution was detected as being both CAACMe or CAACCy (Supplementary Figs. 36, 37, 44, 52 and 56). These outcomes counsel that hint quantities of CAACs might react with the polymer chain in some unspecified time in the future throughout the polymerisation (for additional dialogue see ‘Supplementary dialogue of the polymerisation mechanism from phosphinoborane monomers’ within the Supplementary Data).

When the reactivity of CAACMe with bulkier P-disubstituted phosphine-borane substrates (R = R′ = tBu or R = R′ = Mes) was explored, an enlightening divergence in reactivity was famous. For full conversion of those substrates, two equivalents of CAACMe are required. In situ 1H NMR reveals that an equimolar combination of (CAACMe)H2 and the brand new species 4a/4b are shaped (Fig. 5a and Supplementary Figs. 59–68).

Fig. 5Fig. 5

Synthesis and construction of cyclic (alkyl)(amino)carbene-phosphinoborane adducts 4a and 4b. a Synthesis of 4a and 4b; b thermal ellipsoid plot of 4a; and c thermal ellipsoid plot of 4b. For each 4a and 4b ellipsoids are proven on the 30% likelihood stage, and H atoms apart from these on the B1 centre have been omitted for readability

The constructions of 4a and 4b have been confirmed by X-ray crystallography revealing that in each compounds the CAACMe C-donor was sure to the boron of the phosphinoborane moiety (Fig. 5b, c and Supplementary Desk 7). Apparently, species 4a and 4b are analogous to the beforehand talked about NHC–phosphinoborane adducts which were not too long ago reported45.

The reactivity with the cumbersome, P-disubstituted phosphine-boranes contrasts with that noticed with PhPH2·BH3, Ph2PH·BH3 and PhEtPH·BH3 as an preliminary P–H oxidative-addition product analogous to compounds 3a–c isn’t noticed. The monomeric phosphinoborane generated upon dehydrogenation doesn’t bear head-to-tail polymerisation, as an alternative it’s trapped by a second equal of carbene. The absence of an observable P–H activation compound might be defined by the better steric bulk across the phosphorus centre. The trapping, nonetheless, offers additional proof for the discharge of monomeric phosphinoboranes within the proposed polymerisation mechanism. It’s noteworthy that when Ph2PH·BH3 is reacted with two equivalents of CAACMe clear conversion to the species analogous to 4a and 4b isn’t noticed; nonetheless, peaks for the short-chain oligomers CAAC(BH2PPh2)x (x = 1–four) have been recognized utilizing ESI-MS (Supplementary Fig. 69).


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