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Base-stabilized phosphinidene boranes via silylium-ion abstractionAmy N. Price and Michael J. Cowley*

Abstract: We report the preparation of NHC-stabilized compounds containing P=B double bonds. The reaction of the highly functionalized phosphinoborane Mes*(SiMe3)P-B(Cl)Cp* with Lewis bases allows access to base-stabilized phosphinidene boranes, Mes*P=B(L)Cp* (L = DMAP, NHC) via Me3SiCl elimination. The formation of these species is shown to proceed via transient boryl-phosphide anions generated by Me3Si abstraction.

The synthesis and reactivity of heavier analogues of alkynes such as disilynes, digermynes and distannynes have received considerable attention within the last decade, in particular with regard to small molecule activation.[1] Power has demonstrated that distannynes can reversibly bind ethene or activate H2.[2]

Amido digermynes are also capable of H2 activation.[3] The presence of closely-spaced frontier molecular orbitals at single or adjacent centers in these compounds is key to their ability in small molecule activation. Indeed, the possible resonance forms for heavy alkyne analogues show the significant degree of electronic flexibility these compounds possess, with charge separated, singlet diradical or 1,2-dimetalylene forms all readily constructed.[4]

In this context, the chemistry of the isoelectronic mixed group 13/15 compounds R–E≡B–R (E = group 15 element) is of interest. Whilst the iminoboranes R–N≡B–R have been extensively studied, their phosphorus analogues remain elusive. The chemistry of iminoboranes is dominated by the polarity of their B≡N bonds, a result of the large electronegativity difference between N and B (N = 3.07, B = 2.01). In contrast, boron and phosphorus have very similar electronegativity (P = 2.06, B = 2.01). Phosphinidene boranes R–P=B–R should thus display significantly different reactivity. Indeed, theoretical studies of the parent compound HP=BH revealed a bent geometry with R-P-B angles close to 90°, and B–P bond distances indicative of P=B double bonds.[5] The predicted bent structure results from the localisation of the lone pair at the phosphorus centre [6] enforcing similar frontier molecular orbitals to heavier group 14 alkynes analogues and resulting in a maximum P=B bond order of 2.

Previous attempts to generate monomeric phosphinidene boranes have largely resulted in oligomerisation, with the formation of four-membered rings (diphosphadiboretanes) or higher oligomers observed.[7] However, stabilization strategies exploiting the coordination of a Lewis acid or base can enable the isolation of phosphinidene borane derivatives. Thus, Paine, Nöth, and coworkers prepared the phosphinidene borane

derivative A by coordination of the phosphorus centre to a metal centre.[8] Power, Rivard et al employed the coordination of the Lewis base dimethylaminopyridine (DMAP) to the boron center to stabilize the phosphinidene borane B.[9]

Recent studies by the groups of Bertrand, Braunschweig and Stephan have revealed the diverse coordination chemistry of iminoboranes, including those bearing functionalized B or N centers (C, D, E).[10]

Recently, Rivard et al described the isolation of ‘encapsulated’ iminoborane, stabilized by simultaneous coordination of a Lewis acid and a base at the nitrogen and boron centers respectively.[11]

Figure 1. Examples of Lewis acid (A) or base (B) coordinated phosphinidene boranes and carbene coordinated iminoboranes (C - F). Ar* = C6H3-2,6-(C6H2-2,4,6-iPr3)2, Dipp = 2,6-diisopropylphenyl. ArF = 3,5-C6H3(CF3)2.

Here, we report a generalizable method for the synthesis of P=B doubly bonded species, which are accessed by Me3SiCl elimination from phosphinoboranes. We demonstrate that Me3SiCl proceeds by a surprising base-promoted silylium-ion abstraction mechanism, and intercept a key intermediate in the process.

We first targeted the synthesis of the phosphinoborane 1, a highly-functionalized phosphinoborane bearing trimethylsilyl and chloride substituents at the phosphorus and boron atoms. Thermally promoted trimethylsilyl-halide elimination has been widely exploited to prepare iminoboranes.[12] We reasoned that, with suitable substituents, phosphinidene boranes (e.g. Mes*P=BCp*) would be accessible via Me3SiCl elimination from 1, and that the bulky Mes* and Cp* substituents should prevent the oligomerisation which has hindered previous attempts to isolate phosphinidene boranes.

[*] Ms Amy N. Price, Dr Michael Cowley,EaStCHEM, School of Chemistry, University of EdinburghEdinburgh, EH9 3FJEmail: michael.cowley@ed.ac.uk

Supporting information for this article is given via a link at the end of the document

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Scheme 1. Synthesis of E and Z isomers of 1 from Mes*P(SiMe3)Li and Cp*BCl2. (Mes* = 2,4,6-tritertbutylphenyl; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl).

Reaction of LiP(SiMe3)Mes* with Cp*BCl2 at low temperatures affords the phosphinoborane 1 in good yields.[13] In solution, compound 1 exists as a mixture of E and Z isomers in the ratio 3:1, as evidenced by the observation of two broad resonances at = -20.2 and -32.7 ppm in the 31P{1H} NMR spectrum at 300 K. Interconversion of the two isomers is rapid in solution at room temperature; freshly prepared solutions of pure 1-E contained both isomers. In the 11B NMR spectrum, a broad signal at = 61.1 ppm, which is characteristic of monomeric phosphinoboranes,[6b, 14] is observed. On cooling, the broad signals in the 31P NMR spectrum decrease in linewidth (55.4 and 44.8 Hz at 193 K), indicative of rotation about the P-B bond becoming slow on the NMR timescale.[15]

The identity of 1 as a monomeric phosphinoborane was confirmed by a single crystal X-ray structure.[13] Compound 1 crystallizes exclusively as the E isomer, and possesses planar geometry about the P-B bond (sum of angles at P = 358.58(14)º, B = 359.91(28)º). The P–B bond distance, at 1.834(2) Å is in accord with other planar phosphinoboranes and consistent with a degree of multiple bonding between the P and B centers.[7]

We first attempted thermal elimination of Me3SiCl from 1. Heating solutions of 1 did result in the formation of Me3SiCl, but multiple uncharacterized phosphorus- and boron-containing products resulted. We thus turned our attention to base-promoted reactivity.

Scheme 2. Reaction of 1 (both isomers) with dimethylaminopyridine, yielding the DMAP coordinated phosphinidene-borane 2a. (Mes* = 2,4,6-tritertbutylphenyl; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl).

Mindful of the DMAP-stabilized phosphinidene borane B generated in low yields by dehydrochlorination,[9] we treated a benzene solution of phosphinoborane 1 with DMAP, immediately obtaining a bright orange solution. New low-field resonances in the 31P and 11B NMR spectra were observed at = 96.7 and 52.4 ppm respectively, significantly downfield from those of 1 indicating the quantitative formation of the base-stabilized phosphinidene borane 2a.[9] Compound 2a crystallizes as the benzene solvate from C6H6 and was isolated in 56 % yield.

A single crystal X-ray structure of compound 2a revealed a short P–B bond of 1.8211(16) Å, indicative of multiple bonding and significantly shorter than that observed for the monomeric phosphinoborane 1-E (1.834(2) Å). The phosphorus center is two-coordinate and has a small C11-P1-B1 bond angle of 110.14(10)º despite the steric bulk of the Mes* substituent, indicating the presence of a non-bonding lone pair at

phosphorus. The P–B bond distance in 2a, 1.795(3) Å is comparable to that observed in B (1.8092(17) Å).[9] The boron center of 2a is trigonal planar (sum of angles = 360.0(6)º), and the dative N-B interaction has a bond distance of 1.556(4) Å.

Figure 2. Molecular structure of 2a in the solid state. Ellipsoids are set to 50 % probability; hydrogen atoms are omitted for clarity. Selected both lengths [Å] and angles [º]: P1–B1 1.795(3), B1–N1 1.556(4), B1–C1 1.636(4), B1–P1–C11 106.12(13), P1–B1–N1 125.0(2), N1–B1–C1 114.3(2), C1–B1–P1 120.8(2).

The room-temperature base-promoted Me3SiCl elimination from 1 intrigued us. The thermal elimination of Me3SiCl from amino-boranes only occurs at very high temperatures and in the gas phase.[12] Indeed, most main-group halides would be expected to undergo substitution in the presence of a nucleophile, though base promoted Me3SiCl elimination has been observed in the reaction of cyclic alkyl-amino carbenes with Me3SiCl substituted aminoboranes, forming C.[10a] Migration of Me3Si groups is an important step in the synthesis of many doubly-bonded species, including silenes (Si=C)[16], phosphaalkenes[17] (P=C) and phosphaalkynes[18] (P≡C), which rely on the formation of strong Si-O bonds. Seeking to establish the generality of base-promoted Me3SiCl elimination from 1, we decided to probe its reactivity towards other Lewis bases.

Iminoboranes form adducts with NHCs that are unstable at room temperature and rearrange readily,[10c] but to our knowledge there are no previously reported examples of NHC adducts of phosphinidene boranes. Reaction of 1,3,4,5 tetramethylimidazol-2-ylidene 3 with 1 at room temperature led to quantitative (by NMR) formation of the NHC coordinated phosphinidene borane 2b.

Compound 2b was isolated in 38 % yield by crystallization from pentane. In the 31P and 11B NMR spectra, signals for 2b are observed at = 192.9 and 48.5 ppm respectively. A resonance at = 153.1 ppm in the 13C NMR spectrum confirms the coordination of the NHC moiety to the boron center. Notably, the 31P NMR chemical shift of 2b, at = 192.9 ppm, is shifted considerably downfield from those observed for 2a and B (= 96.7 and57.3 ppm respectively).

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Scheme 3. Reaction of 1 (both isomers) with NHC 3, yielding the NHC coordinated phosphinidene-borane 2b. (Mes* = 2,4,6-tritertbutylphenyl; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl).

Figure 3. Molecular structure of 2b in the solid state. Ellipsoids are set to 50 % probability; hydrogen atoms are omitted for clarity. Selected both lengths [Å] and angles [º]: P1–B1 1.8067(15), B1–C1 1.6516(19), B1–C2 1.5821(18), B1–P1–C3 108.94(6), P1–B1–C1 117.71(9), C1–B1–C2 117.19(11), C2–B1–P1 125.10(10).

An X-ray crystal structure of 2b confirms its assignment. The structural characteristics of phosphinidene borane 2b are consistent with those of 2a and B. The P–B bond distance is short at 1.8067(15) Å. The geometry about the boron centre is planar (sum of angles = 360.0(2)º), although the P–B double bond is noticeably twisted (twist angle = 10.98º). The dative bond between the boron centre and carbene carbon is significantly shorter (1.5821(18) Å) than that between the boron and the Cp* carbon (1.6516(19) Å).

When we monitored the reaction of phosphinoborane 1 with NHC 3 by 31P NMR spectroscopy, we observed initial formation of an intermediate species with a signal at δ77.5. Notable features in the 1H NMR spectrum included a high field singlet at = 0.59 ppm, which was assigned to the Me3Si group. The absence of any coupling of this signal to phosphorus suggested complete cleavage of the P–Si bond. Also revealed was the consumption of the majority of free NHC 3, and the formation of a new NHC-containing species. We thus assigned 1H resonances at δ= 3.75, 2.24, and 0.59, to the trimethylsilylimidazolium cation 5b. Formal abstraction of trimethylsilylium cation from 1 by the NHC 3 suggested that the intermediate observed at δ= 77.5 in the 31P NMR spectrum was the boryl-phosphide anion 4, and indeed, both the 31P (δ=

77.5 ppm) and 11B (δ= 54.9 ppm) NMR chemical shifts are consistent with this, falling in the range of previously prepared aryl-subsituted boryl-phosphides.[19]

The observation of 4 by NMR spectroscopy reveals the initial step in the mechanism of formation of base-stabilized phosphinidene boranes 2. Following formation of 4, we propose that reaction with remaining free ligand (DMAP or NHC 3) to form the base stabilized species 2 occurs. This reaction may occur by direct attack of the base at the boron center of 4, or alternatively, by chloride dissociation from the anion 4 and base-trapping of the transient phosphinidene borane that results. We also investigated the effect of treating 1 with [5b]Cl (generated in situ from 3 and Me3SiCl). We observed immediate conversion to 2b, indicating that once formed by initial reaction of 1 with base, the salts 5a/5b may also promote the desilylation of 1 to form 2 in a parallel reaction pathway.

Scheme 4. Reaction of 1 (both isomers) with NHC 6, yielding the boryl-phosphide salt [5c][4] (Mes* = 2,4,6-tritertbutylphenyl; Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl).

We were able to isolate a meta-stable salt of the boryl-phosphide anion 4. When we treated a C6D6 solution of 1 with the sterically bulky NHC 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr, 6), we observed immediate consumption of 1 and formation of the boryl-phosphide anion 4, (resonances at δ= 75.2 and 55.0 in the 31P and 11B NMR spectra respectively). In the 1H NMR spectrum, the distinctive high field chemical shift for the SiMe3 groups of the trimethylsilyl imidazolium cation 5c was observed. Single crystals of the trimethylsilyl imidazolium salt [5c][4] precipitated from benzene. Despite very rapid loss of crystallinity on removal from the benzene mother liquor, speedy mounting of a single crystal enabled an X-ray structural determination. The NMR assignment of 4 as the boryl-phosphide anion was immediately confirmed.

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Figure 4. Molecular structure of [5c][4] (co-crystallised C6H6 not shown) in the solid state. Ellipsoids are set to 50 % probability; hydrogen atoms are omitted for clarity. Selected both lengths [Å] and angles [º]: P1–B1 1.8039(16), B1–Cl1 1.8332(15), B1–C1 1.639(2), B1–P1–C11 105.35(6), P1–B1–Cl1 122.76(9), Cl1–B1–C1 115.15(10), C1–B1–P1 122.09(10).

The anion 4 could conceivably be considered a chloride adduct of a phosphinidene borane, akin to compounds 2a and 2b. A more appropriate formulation is as the boryl-phosphide anion, [RP–BR2]⁻. Although phosphide anions normally resonate at high field in the 31P NMR spectrum (e.g. Mes*(SiMe3)PLi δ= -147.4), the low field resonances in the 31P NMR of 4 are a result of delocalization of a phosphorus lone pair into the p orbital at boron.[19] This interaction results in multiple-bond character between the phosphorus and boron centers, a conclusion which is supported by the structural parameters of 4. The P–B bond, at 1.8039(16) is short, though in the range of other, aryl substituted boryl-phosphides [RPBR2]⁻.[19] DFT calculations (B3LYP/6-31G(d,p) level) support our interpretation of the experimental results. Examination of the frontier molecular orbitals in the anion 4 reveals that the HOMO has significant B-P -bonding character (figure S22). An NBO analysis reveals that although the P=B -bond is relatively non-polar, the P-B -bond is heavily polarized towards the phosphorus centre (71 %). The Wiberg bond index (WBI), at 1.67, supports the presence of P-B multiple-bond character.

Salts of 4 are not stable. In benzene, the reaction of 1 with NHC 6 halts at the trimethylsilyl imidazolium salt of 4, which precipitates from solution. Any attempts to isolate this salt from the mother liquor led to decomposition to an oily solid, which when analyzed by NMR spectroscopy proved to be an intractable mixture of products. When we repeated the reaction between 1 and the NHC 5 in d8-THF, we again observed formation of the boryl-phosphide anion 4, which remains in solution but over the course of 2-3 days decomposes to multiple uncharacterized products. We ascribe the instability of the boryl-phosphide anion 4 to facile chloride elimination, which generates the transient phosphinidene borane Mes*P=BCp* (not observed) which rapidly decomposes. Steric bulk prevents trapping of the phosphinidene borane by coordination of the NHC 6, and neither could the proposed intermediate Mes*P=BCp* be trapped in the presence of alkynes or butadienes. In contrast to 4, the DMAP and NHC adducts 2a and 2b are stable against dissociation.

Calculations at the B3LYP/6-31G(d,p) level reveal that G for dissociation of the DMAP or NHC ligands from 2a and 2b is +59.2 kJ mol-1 and +118.6 kJ mol-1 respectively.

In conclusion, we have prepared and structurally characterized new NHC-stabilized phosphinidene boranes. We have demonstrated that base-promoted Me3SiCl elimination proceeds initially via Me3Si cation abstraction, and have isolated the boryl-phosphide 4. The base-stabilized phosphinidene boranes 2a and 2b are examples of a new and growing group of compounds in which base-coordinated BR fragments replace CR2 units, and may be considered boron-containing analogues of phosphaalkenes. The observation of 4, a ‘snapshot’ in the process of Me3SiCl elimination from 1, offers insight into the mechanism of thermally-promoted Me3SiCl elimination, for example in the solution phase synthesis of boron-phosphide.[20]

Acknowledgement

We thank the European Commission (Marie Curie CIG) and the University of Edinburgh (including a Principal’s Career Development Scholarship for ANP) for funding. We thank Dr Gary S. Nichol for assistance with crystallographic work. We also thank the referees for their helpful comments.

Keywords: boron • phosphorus • multiple bonds • carbenes

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[10] a) F. Dahcheh, D. Martin, D. W. Stephan, G. Bertrand, Angew. Chem. Int. Ed. 2014, 53, 13159–13163; b) F. Dahcheh, D. W. Stephan, G. Bertrand, Chem. Eur. J. 2015, 21, 199–204; c) H. Braunschweig, W. C. Ewing, K. Geetharani, M. Schäfer, Angew. Chem. Int. Ed. 2015, 54, 1662–1665.

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[13] Full experimental details, including crystallographic data, are provided in the Supporting Information. CCDC 1443233 (1), 1443234 (2a), 1443235 (2b) and 1443236 ([5c][4].C6H6) contain the supplementary crystallographic data for this paper. These can be obtained free of

COMMUNICATION charge from the CCDC via www.ccdc.cam.ac.uk/Community/Requestastructure/Pages/DataRequest.aspx.

[14] a) X. Feng, M. M. Olmstead, P. P. Power, Inorg. Chem. 1986, 25, 4615–4616; b) S. J. Geier, T. M. Gilbert, D. W. Stephan, J. Am. Chem. Soc. 2008, 130, 12632–12633.

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5014.

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Intermediate intercepted: The syntheses of a range of base-stabilized P=B doubly bonded compounds proceeds via SiMe3 abstraction. The interception of an anionic boryl phosphide intermediate offers insight into thermal Me3SiCl elimination, for example in chemical vapor deposition processes.

Amy N. Price and Michael J. Cowley*

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Base-stabilized phosphinidene boranes via silylium-ion abstraction