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Article

Extended Synthetic Pathways Towards Dialkyl-Substituted Phosphanylboranes

by
Mehdi Elsayed Moussa
†,
Oliver Hegen
†,
Christoph Riesinger
and
Manfred Scheer
*
Department of Inorganic Chemistry, University of Regensburg, 93040 Regensburg, Germany
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Inorganics 2025, 13(7), 239; https://doi.org/10.3390/inorganics13070239
Submission received: 18 June 2025 / Revised: 2 July 2025 / Accepted: 8 July 2025 / Published: 11 July 2025
(This article belongs to the Special Issue State-of-the-Art Inorganic Chemistry in Germany)
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Abstract

Phosphine–boranes have garnered growing interest for their potential in catalysis and as building blocks for inorganic polymers. While various synthetic methods exist, flexibility to introduce diverse substituents on the P centers remains limited. Our group reported routes to monoalkylated phosphanylboranes starting from primary phosphanylboranes or sodium phosphide. In this work, we extend these strategies to enable the synthesis of dialkylated phosphanylboranes bearing either identical or different substituents on the P atoms. This expanded methodology provides access to a broader scope of diverse P centers, a key factor influencing the reactivity and applications of phosphine–borane derivatives.

Graphical Abstract

1. Introduction

In the last two decades, an increasing interest has been witnessed in the synthesis and reactivity study of phosphine–borane compounds, particularly due to their potential as starting materials for the synthesis of inorganic polymers through dehydrocoupling reactions [1,2,3,4]. Given that P−B bonds are isoelectronic with C−C single bonds, poly(phosphine–boranes) are considered an alternative class of inorganic analogs to organic polymers, such as polyolefins, albeit with significantly different properties [5,6,7,8,9,10]. Phosphine–borane monomers are also highly versatile molecules. They have been widely employed in synthesis and catalysis [11,12,13,14,15,16,17,18,19,20,21,22] and, to a lesser extent, in biomedical applications [23,24,25], as well as in the activation and transfer of H2 and other small molecules [26,27,28,29,30]. Interestingly, these compounds are easily accessible, highly modular, and offer valuable insights via multi-nuclear NMR spectroscopy. Over time, a number of methods have been developed for the synthesis of these compounds. Typical approaches involve the reaction of phosphines (PR3, R = H, alkyl, aryl) with diboranes, borane complexes, or borohydrides [31]. Alternatively, chlorophosphines can be reduced with sodium or lithium borohydrides to avoid the use of primary and secondary phosphines, which are often challenging to handle [13,31]. Additionally, a number of secondary and tertiary phosphine–boranes have been prepared from direct reduction of phosphine oxides [31,32,33]. One of our group’s contributions to this field involves the synthesis and reactivity of Lewis base-stabilized pnictogenylboranes R2E-BH3•NMe3 (E = P, As; R = H, alkyl, aryl) [34,35]. Consequently, their coordination [36], oxidation [37], oligomerization [38], and polymerization [39] chemistry have been investigated. Moreover, these compounds have been utilized in the synthesis of cationic and anionic chain compounds, built up by R2P-BH2 units [40,41]. Considering that even minor modifications of the substituents on the P or B atoms can significantly impact the reactivity of phosphanylborane [42,43], we have become highly interested in developing new routes to facilitate such modifications. Accordingly, we created a straightforward route to high-yield mono-alkylated phosphonium iodides, [RPH2-BH2•NMe3]I (R = Me, Et, nPr; Scheme 1a), from the reaction of iodoalkanes with PH2-BH2•NMe3. The methyl phosphonium iodide was further treated with lithium diisopropylamide, yielding the methyl-substituted phosphanylborane (HMeP-BH2•NMe3), which was subsequently used as a precursor to poly(methylphosphanylborane) via simple thermolysis [44]. More recently, another more general method was developed. This method involves the synthesis of mono-alkylated phosphines from NaPH2 and iodoalkanes, followed by a one-pot metalation of the phosphine by NaNH2. Finally, a salt metathesis of the resulting phosphanide NaRHP with IBH2NMe3 yields the targeted mono-P-substituted phosphanylboranes in good yields and on a gram scale (Scheme 1b) [45].
In view of these results, the question arises whether it is possible to broaden the scope of these methods towards dialkylated phosphanylboranes, especially with two different substituents on the P atoms. Herein, we report the extension of our first method towards dialkyl-substituted phosphanylborane derivatives (RR’PH2-BH2•NMe3) proceeding via phosphonium iodide intermediates. In addition, we adapted the salt metathesis step of our second approach to access similar disubstituted compounds. The first method, which involves a stepwise alkylation pathway from primary to tertiary phosphanylboranes, provides flexibility in introducing different substituents on the P atom, allowing for the formation of unprotected chiral phosphanylboranes, which are not accessible by other reported methods in the field. While herein these species are obtained as racemic mixtures, related phosphanylboranes could be prepared enantiomerically pure, which holds promise for future investigations [46,47,48,49,50].

2. Results and Discussion

To demonstrate the feasibility of our reported methods toward new dialkylated phosphanylboranes, we selected, in a first approach, the secondary phosphanylborane tBuPHBH2NMe3 (1) [39] and reacted it with alkyl halides (MeI; nPrI; 1,3-dibrompropane). These reactions were carried out in toluene by stirring the reaction mixture for 20 h at room temperature. After evaporation of the solvents, white solids of the corresponding alkylated phosphonium salts (tBu(Me)PHBH2NMe3)I (2), (tBu(nPr)PHBH2NMe3)I (3), and [{(Me3NH2BHPtBu)nPr(tBuPHBH2NMe3)}(Br)2] (4), respectively, were obtained (Scheme 2) after washing them with n-hexane (2, 3) or toluene (4) to remove any traces of the used alkyl halides. Based on their 31P NMR spectra, elemental analysis, and the weight of the isolated powders, compounds 2 and 3 were formed selectively and in quantitative yields.
However, the 31P and 11B NMR spectra of the crude reaction mixture of 4 revealed unidentified side products, reducing its yield to about 40%. Compounds 2–4 are highly soluble in common organic solvents, including n-alkanes. Their 31P NMR spectra show signals centered at −15.7, −7.4, and −10.4 ppm, respectively, all of which are downfield-shifted compared to the phosphanylborane salt 1 (−67.6 ppm) [39]. However, in the 11B NMR spectra, the signals attributed to the BH2-moiety (−10.3 (2), −11.1 (3), and −10.9 (4) ppm) are found upfield-shifted compared to that of 1 (−6.0 ppm, Table 1) [39]. In the ESI-MS spectra recorded in CD3CN, molecular ion peaks were observed in the positive ion mode for all the products 2–4. Additionally, peaks attributed to the cations [(3)2(I)]+ and [(4)(Br)]+ were detected for 3 and 4, respectively.
Single crystals of compounds 2–4 were obtained by layering their THF/MeCN (2, 3) or CH2Cl2 (4) solutions with n-hexane. Their molecular structures in the solid state were determined by single crystal X-ray diffraction analysis (Figure 1). Compound 2 crystallizes in the monoclinic space group P21/n, compound 3 in the triclinic space group P 1 ¯ , and compound 4 in the orthorhombic space group Pna21. The unit cells of 2 and 3 contain both the R and the S enantiomers of the respective compound, whereas only the meso-form of 4 is observed in its unit cell. All bond lengths in 2–4 are consistent with single bonds.
The B–P (2: 1.968(5) Å, 3: 1.956(6)/1.966(5)Å, 4: 1.90(5)/2.01(2) Å) and B–N (2: 1.594(5) Å, 3: 1.599(7)/1.602(8) Å, 4: 1.55(3)/1.69(4) Å) bond lengths in 2–4 are similar to those observed for 1 (1.985(2) and 1.621(2) Å, respectively) [42]. The P atoms in compounds 2–4 exhibit slightly disordered tetrahedral environments, with each connected to two C atoms, one H atom, and one B atom (Table 2). In both 2 and 3, the B–P axis shows an antiperiplanar arrangement, whereas it shows an anticlinal arrangement in 4.
Deprotonation of 2–4 with KH in CH3CN leads to the dialkylated neutral phosphanylboranes tBuPRBH2NMe3 (R = Me(5), nPr (6)) and [(Me3NBH2PtBu)2nPr] (7), respectively (Scheme 3). According to the 11B and 31P NMR spectroscopy, all the reactions proceeded to completion. However, minor impurities were still present after extracting the products with n-hexane due to the very good solubility of trace impurities and 5–7 in common organic solvents, including n-alkanes. Only compound 5 could be crystallized and, consequently, purely isolated in good yields (55%). In the 31P NMR spectra of 5–7, the signals of the P atoms (−53.38, −36.18 and −39.27 ppm, respectively) are upfield-shifted compared to those of 2–4. In contrast, the resonances of the B atoms in the 11B NMR spectra are downfield-shifted (δ = −1.58 (5), −2.40 (6), and −2.73 (7) ppm) as compared to those observed for 2–4.
As expected, the molecular structure of 5 in the solid state (Figure 2) reveals a neutral compound with the absence of the proton on the P center and the I− counterion. The P atom remains chiral, as it is bonded to three different groups and possesses a lone pair of electrons, but both enantiomers are found within the unit cell in the form of a 50:50 disorder. Similar to 2, an antiperiplanar arrangement along the B–P bond is observed in 5. The P–B bond length in 5 (2.002(2) Å) reveals a single bond and is slightly elongated compared to those found in 2 (1.969(5)/1.953(5) Å).
In a second approach, the reaction of IBH2NMe3 with LiPCy2 (Cy = cyclohexyl) and LiPtBu2 [51], respectively, was investigated in THF at −80 °C. This reaction yielded the Lewis base (LB) stabilized dialkylated phosphanylboranes Cy2PBH2NMe3 (8) and tBu2PBH2NMe3 (9), respectively (Scheme 4). The synthesis of 8 and 9 proceeded to completion as revealed by the crude 31P NMR spectra. However, the corresponding phosphines, HPCy2 and HPtBu2, respectively, could be identified as side products. Thus, yields of 55% and 30%, respectively, were estimated for 8 and 9 based on the comparison of the integrals of the signals with side products. Due to the similar solubility of 8/9 and the phosphines, it was not possible to analytically isolate pure compounds.
Nevertheless, both compounds were characterized by single crystal X-ray diffraction analysis (Figure 2). They present an anticlinal arrangement along the P–B axis with ∢(N-B–P) = 111.00(16)° and 111.72(17)° and ∢(C1-P-C2) = 102.52(10)° and 109.952°, respectively. Additionally, their P–B (1.983(3) and 1.987(3) Å, respectively) and the B–N (1.645(3) and 1.636(4) Å, respectively) bond lengths are quite similar. The reaction of IBH2SMe2 with LiPCy2 under similar reaction conditions yields the cyclic phosphanylborane (Cy2PBH2)2 (10) featuring a four-membered ring motif in moderate yields (50%). The reaction probably includes the formation of Cy2PBH2SMe2 as an intermediate, which undergoes dimerization and elimination of the weak LB SMe2.
In contrast to 8 and 9, compound 10 can be purely isolated, as it is not soluble in n-alkanes and thus can be separated from any impurities present in the crude reaction mixture. The resonance signals of the B and P nuclei are broad in both the 11B and 31P NMR spectra of 10. Its 31P{1H} NMR spectrum at room temperature reveals a signal at δ = −10.88 ppm (ω1/2 = 483 Hz), which is further broadened in its 31P NMR spectrum (ω1/2 = 530 Hz). In the 11B NMR spectrum, the signal of the B atoms (δ = −32.57 ppm) is observed in the typical range of oligomeric or polymeric compounds [52]. In the 1H NMR spectrum of 10, signals of the cyclohexyl substituents appear at δ = 1.06–2.36 ppm and those of the BH2 moieties at δ = 1.90 ppm as a broad multiplet. The molecular structure of 10 was determined by single crystal X-ray diffraction analysis (Figure 2). The P–B bond distance (1.9874(16) Å) corresponds to a P–B single bond. The slightly distorted square-planar conformation shows a P–B–P angle of ∢(P–B–P) = 88.144° and a B–P–B angle of ∢(B–P–B) = 91.856°. The P∙∙∙P (2.766 Å) and the B∙∙∙B (2.858 Å) distances are below the van der Waals radii (Σ r(P)vdW = 3.60 Å; r(B)vdW = 3.84 Å) [53]. In addition, irradiation of 10, whether as a solid or in solution, with UV light (λ = 366 nm) shows visible blue luminescence. Emission measurements will be conducted in the near future to obtain further insights into the electronic properties of 10, thus opening the door for a possible unexplored area of application for this class of compounds. It is worth mentioning that all compounds 1–10 are air- and moisture-sensitive, and their solids can only be stored for extended periods under an inert atmosphere, such as in a Schlenk tube or a glovebox.

3. Materials and Methods

3.1. General Information

All the manipulations were carried out under a dry argon or dinitrogen atmosphere using glovebox or standard Schlenk techniques. The solvents were taken from a solvent purification system of the MB-SPS-800 type from the company MBRAUN (Garching, Germany) and degassed by standard procedures. The starting materials, tBuPHBH2NMe3 [39], LiPCy2 [51], LitBu2P [51], IBH2NMe3 [41], and IBH2SMe2 [41], were prepared according to procedures outlined in the literature. The compounds MeI, nPrI, C3H6Br2, and KH were purchased from the commercial suppliers Sigma-Aldrich Chemie GmbH (Eschenstraße 5, 82024 Taufkirchen, Germany), Thermo Fisher Scientific GmbH (Im Steingrund 4-6, 63303 Dreieich, Germany), and Merck Chemicals GmbH (Frankfurter Straße 133, 64293 Darmstadt, Germany), respectively. All the nuclear magnetic resonance (NMR) spectra were recorded on a Bruker Avance 400 spectrometer (Brucker Instruments, Ettlingen, Germany) (1H: 400.13 MHz, 31P:161.976 MHz, 11B: 128.378 MHz, 13C{1H}: 100.623 MHz) with δ [ppm] referenced to external SiMe4 (1H, 13C), H3PO4 (31P), and BF3·Et2O (11B). The IR spectra were measured on a DIGILAB (FTS 800) FT-IR spectrometer (Digilab Inc., Marlborough, MA, USA) and on a Thermo Scientific Nicolet iS5 (Thermo Fisher Scientific Inc., Madison, WI, USA). The mass spectra were recorded on a Micromass LCT ESI-TOF (Waters Corporation, Wexford, Ireland) and a Jeol AccuTOF GCX (LIFDI) (JEOL Ltd., Tokyo, Japan). Elemental analyses (C, H, N) were determined on a Vario EL III instrument (Elementar Analysensysteme GmbH, Donaustraße 7, 63452 Hanau, Germany).

3.2. Synthesis and Characterization of Compounds 2–10

3.2.1. Synthesis and Characterization of 2

To a stirred solution of 365 mg tBuPHBH2NMe3 (2.28 mmol) in 15 mL toluene, 0.3 mL MeI (3.32 mmol) was added at room temperature. A white solid precipitated immediately. The solution was stirred for 20 h, then the supernatant was decanted, and the remaining solid was washed with 30 mL n-hexane. The solid was dissolved in a THF/MeCN (5:1) solution and layered with three volumes of n-hexane, from which compound 2 crystallized as colorless blocks. Yield of 2: 640 mg (93%). 1H NMR (400.13 MHz, CD3CN): δ[ppm] = 1.24 (d, 9H, 3JHP = 16 Hz, C(CH3)3), 1.58 (dd, 3H, 2JHP = 12 Hz, 3JHH = 6 Hz, CH3), 2.15 (m, 2H, BH2), 2.83 (s, 9H, N(CH3)3), 5.47 (m, 1H, 1JPH = 396 Hz, 3JHH = 6 Hz, PH). 31P{1H} NMR (161.975 MHz, CD3CN): δ[ppm] = −15.70 (bq, 1JPH = 396 Hz). 31P NMR (161.975 MHz, CD3CN): δ[ppm] = −15.70 (bq, 1JPB = 78 Hz). 11B{1H} NMR (128.378 MHz, CD3CN): δ[ppm] = −10.30 (d, 1JBP = 78 Hz). 11B NMR (128.378 MHz, CD3CN): δ[ppm] = −10.30 (dt, 1JBP = 78 Hz, 1JBH = 113 Hz). 13C{1H} NMR (100.613 MHz, CD3CN): δ[ppm] = 26.24 (s, CH3), 26.25 (s, C(CH3)3), 27.71 (d, 1JCP = 40 Hz, C(CH3)3), 54.98 (d, 3JCP = 6 Hz), N(CH3)3). IR (KBr): ῦ/cm−1 = νCO: 2963 (m, C-H), 2949 (m, C-H), 2918 (w), 2863 (m), 2460 (m, P-H), 2414 (m), 2323 (m, B-H), 1481 (m), 1412 (w), 1375 (vw), 1298 (w), 1249 (w), 1192 (vw), 1165 (vw), 1142 (s), 1092 (m), 1034 (w), 1015 (w), 987 (vw), 945 (vw), 911 (w), 876 (w), 856 (m), 821 (w), 755 (vw), 702 (vw). EI-MS (CH3CN): m/z (%) = 176 [(tBu(Me)PHBH2NMe3)+]. Elemental analysis, calcd. for C8H24BPNI (303.08 g/mol): C: 31.71%, H: 7.98%, N: 4.62%. Found C: 31.92%, H: 7.67%, N: 4.59%.

3.2.2. Synthesis and Characterization of 3

To a stirred solution of 400 mg tBuPHBH2NMe3 (2.48 mmol) in 20 mL toluene, 0.3 mL nPrI (3.08 mmol) was added at room temperature. The solution was stirred for 20 h, after which all the volatiles were removed in vacuo, and the remaining solid was washed with 15 mL n-hexane. The solid was dissolved in a THF/MeCN (5:1) solution and layered with three volumes of n-hexane, from which compound 3 crystallized as colorless prisms. Yield of 3: 630 mg (70%). 1H NMR (400.13 MHz, CD3CN): δ[ppm] = 1.07 (td, 3H, 3JHH = 7 Hz, 4JHH = 1 Hz, CH3), 1.27 (d, 9H, 3JHP = 16 Hz, C(CH3)3), 1.57–2.13 (m, 4H, CH2), 2.40 (m, 2H, BH2), 2.81 (d, 9H, 4JHP = 1 Hz, N(CH3)3), 5.29 (m, 1H, 1JPH = 380 Hz, 3JHH = 3 Hz, PH). 31P{1H} NMR (161.975 MHz, CD3CN): δ[ppm] = −7.35 (bq, 1JPH = 380 Hz). 31P NMR (161.975 MHz, CD3CN): δ[ppm] = −7.35 (q, 1JPB = 73 Hz). 11B{1H} NMR (128.378 MHz, CD3CN): δ[ppm] = −11.05 (d, 1JBP = 73 Hz). 11B NMR (128.378 MHz, CD3CN): δ[ppm] = −11.05 (dt, 1JBP = 73 Hz, 1JBH = 110 Hz). 13C{1H} NMR (100.613 MHz, CD3CN): δ[ppm] = 14.74 (d, 2JCP = 14 Hz, CH2), 18.02 (d, 1JCP = 35 Hz, H2C-P), 18.28 (d, 3JCP = 4 Hz, CH3), 25.84 (d, 2JCP = 1 Hz, C(CH3)3), 27.75 (d, 1JCP = 38 Hz, C(CH3)3), 54.09 (d, 3JCP = 6 Hz), N(CH3)3). IR (KBr): ῦ/cm−1 = νCO: 2994(w, C-H), 2958 (s, C-H), 2870 (m), 2736 (vw), 2452 (m, P-H), 2399 (w), 2286 (m, B-H), 1473 (s), 1417 (w), 1373 (w), 1247 (w), 1196 (vw), 1162 (w), 1138 (s), 1096 (m), 1054 (m), 1029 (w), 978 (w), 910 (vw), 861 (s), 820 (w), 771 (vw), 719 (vw). EI-MS(CH3CN): m/z (%) = 535 [{(tBu(nPr)PHBH2NMe3)2(I)}+], 204 [(tBunPrPHBH2NMe3)+]. Elemental analysis, calcd. for C10H28BPNI (331.11 g/mol): C: 36.28%, H: 8.52%, N: 4.23%. Found C: 36.05%, H: 8.33%, N: 3.99%.

3.2.3. Synthesis and Characterization of 4

To a stirred solution of 240 mg tBuPHBH2NMe3 (1.49 mmol) in 15 mL toluene, 0.075 mL (0.75 mmol) C3H6Br2 was added at room temperature. The solution was stirred for 3 days, and a white solid precipitated. All the volatiles were removed in vacuo, and the remaining solid was washed with 15 mL toluene. The solid was dissolved in 10 mL CH2Cl2 and layered with four volumes of n-hexane, from which compound 4 crystallized as colorless plates. Yield of 4: 155 mg (40%). 1H NMR (400.13 MHz, CD3CN): δ[ppm] = 1.29 (d, 18H, 3JHP = 16 Hz, C(CH3)3), 2.01–2.41 (m, 6H, CH2), 2.27–2.61 (m, 4H, BH2), 2.89 (s, 9H, N(CH3)3), 6.41 (m, 1H, 1JPH = 390 Hz, PH). 31P{1H} NMR (161.975 MHz, CD3CN): δ[ppm] = −10.35 (m, br, 1JPH = 390 Hz). 31P NMR (161.975 MHz, CD3CN): δ[ppm] = −10.35 (m, br, 1JPB = 70 Hz). 11B{1H} NMR (128.378 MHz, CD3CN): δ[ppm] = −10.87 (s, br). 11B NMR (128.378 MHz, CD3CN): δ[ppm] = −10.87 (s, br). 13C{1H} NMR (100.613 MHz, CD3CN): δ[ppm] = 18.60 (d, 1JCP =35 Hz, H2C-P), 21.81 (t, 3JCP = 3 Hz, CH2), 26.84 (d, 2JCP = 1 Hz, C(CH3)3), 28.79 (d, 1JCP = 38 Hz, C(CH3)3), 55.12 (d, 3JCP = 6 Hz), N(CH3)3). ES-MS (CH3CN): m/z (%) = 443 [{(Me3NH2BHPtBu)nPr(tBuPHBH2NMe3)Br}+], 363 [{(Me3NH2BPtBu)nPr(tBuPHBH2NMe3)(Br)}+], 182 [(Me3NH2BHPtBu)nPr(tBuPHBH2NMe3)2+].

3.2.4. Synthesis and Characterization of 5

To a solution of 528 mg 2 (3.00 mmol) in 25 mL CH3CN, 120 mg (2.99 mmol) KH were added as a solid at −40 °C. The solution was stirred at −40 °C until all the solid disappeared. All the volatiles were removed in vacuo, and the remaining white solid was dissolved in 20 mL n-hexane. After filtration over diatomaceous earth, the solvent was concentrated until saturation in vacuo and stored at −30 °C, from which compound 5 crystallized as colorless blocks. Yield of 5: 287 mg (55%). 1H NMR (400.13 MHz, C6D6): δ[ppm] = 1.34 (s, br, 3H, CH3), 1.38 (d, 9H, 3JHP = 11 Hz, C(CH3)3), 1.95 (s, 9H, N(CH3)3), 2.45 (m, 2H, BH2). 31P{1H} NMR (161.975 MHz, C6D6): δ[ppm] = −53.38 (q, br, 1JPB = 53 Hz). 31P NMR (161.975 MHz, C6D6): δ[ppm] = −53.38 (m, br). 11B{1H} NMR (128.378 MHz, C6D6): δ[ppm] = −1.58 (d, 1JBP = 53 Hz). 11B NMR (128.378 MHz, C6D6): δ[ppm] = −1.58 (dt, 1JBP = 53 Hz, 1JBH = 102 Hz). 13C{1H} NMR (100.613 MHz, C6D6): δ[ppm] = 8.40 (d, 1JCP = 17 Hz, CH3), 26.80 (d, 3JCP = 5 Hz, C(CH3)3), 29.41 (d, 2JCP = 5 Hz, C(CH3)3), 52.05 (d, 3JCP = 12 Hz), N(CH3)3). IR (KBr): ῦ/cm−1 = νCO: 2941 (m, C-H), 2897 (m, C-H), 2854 (m), 2374 (m, B-H), 2288 (w), 2187 (vw), 1653 (vw), 1485 (m), 1460 (m), 1422 (w), 1400 (w), 1383 (w), 1356 (w), 1253 (w), 1187 (vw), 1152 (w), 1123 (m), 1069 (s), 1014 (w), vw), 954 (vw), 884 (vw), 868 (vw), 842 (s), 817 (w), 681 (w), 637 (vw), 581 (vw), 472 (vw). LIFDI-MS (toluene): m/z (%) = 524 [{Me3N(H2BPMetBu)4}+], 408 [{Me3N(H2BPMetBu)3}+]. Elemental analysis, calcd. for C8H23BPN (175.17 g/mol): C: 54.80%, H: 13.23%, N: 7.99%. Found C: 54.66%, H: 12.90%, N: 7.90%.

3.2.5. Synthesis and Characterization of 6

To a solution of 612 mg 3 (3.00 mmol) in 25 mL CH3CN, 120 mg (2.99 mmol) KH were added as a solid at −40 °C. The solution was stirred at −40 °C until all the solids almost disappeared. All the volatiles were removed in vacuo, and the remaining white solid was dissolved in 20 mL n-hexane. After filtration over diatomaceous earth, the solvent was removed in vacuo. Compound 6 was isolated as a white solid, which was a bit waxy at room temperature. Yield of 6: (38%, based on 31P NMR). 1H NMR (400.13 MHz, C6D6): δ[ppm] = 1.04 (t, 3H, 3JHH = 7 Hz, 4JHH = 1 Hz, CH3), 1.37 (d, 9H, 3JHP = 11 Hz, C(CH3)3), 1.35–1.44 (m, 4H, CH2), 2.38 (m, 2H, BH2), 2.01 (s, 9H, N(CH3)3). 31P{1H} NMR (161.975 MHz, C6D6): δ[ppm] = −36.19 (bd, br, 1JPH = 162 Hz). 31P NMR (161.975 MHz, C6D6): δ[ppm] = −36.21 (bd, 1JPB = 79 Hz). 11B{1H} NMR (128.378 MHz, C6D6): δ[ppm] = −2.33 (bd, 1JBP = 52 Hz). 11B NMR (128.378 MHz, C6D6): δ[ppm] = −2.53 (m, 1JBP = 52 Hz, 1JBH = 88 Hz). Note: signals corresponding to impurities were observed in the NMR spectra (see Figures S25–S29 in the ESI).

3.2.6. Synthesis and Characterization of 7

To a solution of 507 mg 4 (1.50 mmol) in 25 mL CH3CN, 120 mg (2.99 mmol) KH were added as a solid at −40 °C. The solution was stirred at −40 °C until all the solids almost disappeared. All the volatiles were removed in vacuo, and the remaining white solid was dissolved in 20 mL n-hexane. After filtration over diatomaceous earth, the solvent was removed in vacuo. Compound 7 was isolated as a white solid, which was a bit waxy at room temperature. Yield of 7: (24%, based on 31P NMR). 1H NMR (400.13 MHz, C6D6): δ[ppm] = 1.22 (d, 18H, 3JHP = 13 Hz, C(CH3)3), 1.41–1.45 (m, 6H, CH2), 2.15–2.91 (m, 4H, BH2), 2.26 (s, 9H, N(CH3)3). 31P{1H} NMR (161.975 MHz, C6D6): δ[ppm] = −35.77 (bd, 1JPB = 161 Hz). 31P NMR (161.975 MHz, C6D6): δ[ppm] = −35.56 (bd, 1JPB = 160 Hz). 11B{1H} NMR (128.378 MHz, C6D6): δ[ppm] = −2.87 (s, br). 11B NMR (128.378 MHz, C6D6): δ[ppm] = −2.66 (s, br). Note: signals corresponding to impurities were observed in the NMR spectra (see Figures S30–S34 in the ESI).

3.2.7. Synthesis and Characterization of 8

To a solution of 120 mg (0.59 mmol) LiPCy2 in 10 mL THF, a solution of 100 mg (0.50 mmol) IBH2NMe3 in 10 mL THF was added at −80 °C. The color of the solution changed from yellow to colorless, and the solution was allowed to reach room temperature over 18 h. All the volatiles were removed in vacuo. The remaining solid was suspended in 10 mL n-hexane and filtrated over diatomaceous earth. The volume was reduced to 3 mL. Compound 8 crystallized as colorless plates upon storing a saturated solution of n-hexane at −30 °C. Due to the similar solubility of the decomposition products of 8, it was not possible to obtain analytically pure compound 8 (see Figures S35–S37). Yield of 8: (30%, based on 31P NMR). 31P NMR (161.975 MHz, C6D6 capillary in THF): δ[ppm] = −37.29 (m, br). 11B{1H} NMR (128.378 MHz, C6D6 capillary in THF): δ[ppm] = −4.25 (m, br, ω1/2 = 200 Hz). 11B NMR (128.378 MHz, C6D6 capillary in THF): δ[ppm] = −4.25 (m, br, ω1/2 = 350 Hz).

3.2.8. Synthesis and Characterization of 9

To a solution of 304 mg (2.00 mmol) LiPtBu2 in 10 mL THF, a solution of 388 mg (1.95 mmol) IBH2NMe3 in 10 mL THF was added at −80 °C. The color of the solution changed from yellow to colorless, and the solution was allowed to reach room temperature over 18 h. All the volatiles were removed in vacuo. The remaining white solid was suspended in 45 mL n-pentane and filtrated over diatomaceous earth. Compound 9 crystallized as colorless blocks upon storing a saturated solution in n-pentane at −30 °C. Due to the similar solubility of decomposition products of 9, it was not possible to obtain analytically pure compound 9 (see Figures S38–S41). Compound 9 crystallized as colorless plates. Yield of 9: (55%, based on 31P NMR). 1H NMR (400.13 MHz, C6D6): δ[ppm] = 1.21 (m, br, 2H, BH2), 1.50 (d, 18H, 3JHP = 11 Hz, C(CH3)3), 1.96 (s, 9H, N(CH3)3). 31P{1H} NMR (161.975 MHz, C6D6): δ[ppm] = 8.92 (q, br, 1JBP = 60 Hz). 31P NMR (161.975 MHz, C6D6): δ[ppm] = 8.92 (m, br). 11B{1H} NMR (128.378 MHz, C6D6): δ[ppm] = −4.18 (d, 1JBP = 60 Hz). 11B NMR (128.378 MHz, C6D6): δ[ppm] = −4.18 (m, 1JBP = 60 Hz, 1JBH = 106 Hz).

3.2.9. Synthesis and Characterization of 10

To a solution of 204 mg (1.00 mmol) LiPCy2 in 10 mL THF 1 mL, a 1 M solution of IBH2SMe2 in toluene was added at −80 °C. The solution was allowed to reach room temperature over 18 h, and the color changed from yellow to colorless. All the volatiles were removed in vacuo, and the remaining white solid was dried for 60 min. The solid was suspended in 20 mL n-hexane and filtrated over diatomaceous earth. All the volatiles were removed, and the remaining solid was dried under a vacuum. Compound 10 crystallized as colorless needles upon storing a saturated solution in toluene at −30 °C. Yield of 10: 200 mg (50%). 1H NMR (400.13 MHz, C6D6): δ[ppm] = 1.06–2.36 (m, 44H, Cy), 1.90 (m, 4H, BH2). 31P{1H} NMR (161.975 MHz, C6D6): δ[ppm] = −10.88 (s, br). 31P NMR (161.975 MHz, C6D6): δ[ppm] = −10.92 (s, br). 11B{1H} NMR (128.378 MHz, C6D6): δ[ppm] = −3.34 (m, br). 11B NMR (128.378 MHz, C6D6): δ[ppm] = −3.34 (m, br). LIFDI-MS (toluene): m/z (%) = 420 [((Cy2PBH2)2)+]. Elemental analysis, calcd. for C24H48B2P2 (420.34 g/mol): C: 68.51%, H: 11.24%. Found C: 68.04%, H: 11.24%.

4. Conclusions

In summary, our approach towards the alkylation of phosphanylboranes was extended to dialkylated phosphanylboranes using a similar strategy through phosphonium salt intermediates. The phosphonium salts, as well as the formed products, were all obtained as racemic mixtures of chiral compounds due to the presence of chiral P centers. The developed process may allow high flexibility in synthesizing phosphanylboranes with selected but different alkyl groups on the P atoms. These compounds could be used as potential ligands for catalytic processes, as it is well known for phosphines in general. Further, we synthesized dialkylated phosphanylboranes possessing similar alkyl groups on the P atoms from a salt metathesis reaction of LiP(alkyl)2 (alkyl = tBu, cyclohexyl) with IH2B•LB (LB = NMe3, SMe2). Although some of the isolated compounds contain impurities arising from similar solubilities, these synthetic pathways are still generally worth extending to phosphanylboranes with a large variety of substituents and to investigate their limitations as the resulting products cannot, until today, be accessed by other methods.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics13070239/s1, Figure S1: 1H NMR spectrum of 2, Figure S2: 31P{1H} NMR spectrum of 2, Figure S3: 31P NMR spectrum of 2, Figure S4: 11B{1H} NMR spectrum of 2, Figure S5: 11B NMR spectrum of 2, Figure S6: 13C{1H} NMR spectrum of 2, Figure S7: 1H NMR spectrum of 3, Figure S8: 31P{1H} NMR spectrum of 3, Figure S9: 31P NMR spectrum of 3, Figure S10: 11B{1H} NMR spectrum of 3, Figure S11: 11B NMR spectrum of 3, Figure S12: 13C{1H} NMR spectrum of 3, Figure S13: 1H NMR spectrum of 4, Figure S14: 31P NMR spectrum of 4, Figure S15: 31P{1H} NMR spectrum of 4, Figure S16: 11B{1H} NMR spectrum of 4, Figure S17: 11B NMR spectrum of 4, Figure S18: 13C{1H} NMR spectrum of 4, Figure S19: 1H NMR spectrum of 5, Figure S20: 31P{1H} NMR spectrum of 5, Figure S21: 31P NMR spectrum of 5, Figure S22: 11B{1H} NMR spectrum of 5, Figure S23: 11B NMR spectrum of 5, Figure S24: 13C{1H} NMR spectrum of 5, Figure S25: 1H NMR spectrum of 6, Figure S26: 31P{1H} NMR spectrum of 6, Figure S27: 31P NMR spectrum of 6, Figure S28: 11B{1H} NMR spectrum of 6, Figure S29: 11B NMR spectrum of 6, Figure S30: 1H NMR spectrum of 7, Figure S31: 31P{1H} NMR spectrum of 7, Figure S32: 31P NMR spectrum of 7, Figure S33: 11B{1H} NMR spectrum of 7, Figure S34: 11B NMR spectrum of 7, Figure S35: 31P NMR spectrum of 8, Figure S36: 11B NMR spectrum of 8, Figure S37: 11B{1H} NMR spectrum of 8, Figure S38: 1H NMR spectrum of 9, Figure S39: 11B NMR spectrum of 9, Figure S40: 11B{1H} NMR spectrum of 9, Figure S41: 31P NMR spectrum of 9, Figure S42: 31P{1H} NMR spectrum of 9, Figure S43: 1H NMR spectrum of 10, Figure S44: 11B NMR spectrum of 10, Figure S45: 11B{1H} NMR spectrum of 10, Figure S46: 31P NMR spectrum of 10, Figure S47: 31P{1H} NMR spectrum of 10, Figure S48: Molecular structure of 2 in the solid state. Thermal ellipsoids are drawn with 50% probability. Counter-anions are omitted for clarity. Figure S49: View of the asymmetric unit of 3 in the solid state. Thermal ellipsoids are drawn with 50% probability. Counter-anions are omitted for clarity. Figure S50: Molecular structure of 4 in the solid state. Thermal ellipsoids are drawn with 50% probability. Counter-anions are omitted for clarity. Figure S51: Molecular structure of 5 in the solid state. Thermal ellipsoids are drawn with 50% probability. Counter-anions are omitted for clarity. Figure S52: Molecular structure of 8 in the solid state. Thermal ellipsoids are drawn with 50% probability. Counter-anions are omitted for clarity. Figure S53: Molecular structure of 9 in the solid state. Thermal ellipsoids are drawn with 50% probability. Counter-anions are omitted for clarity. Figure S54: Molecular structure of 10 in the solid state. Thermal ellipsoids are drawn with 50% probability. Counter-anions are omitted for clarity. Table S1: Crystallographic data for compounds 2–5, Table S2: Crystallographic data for compounds 8–10. References [54,55,56,57] are cited in the supplementary materials.

Author Contributions

Synthesis and characterization of compounds 6 and 7, literature study, and manuscript preparation, M.E.M.; synthesis of all other compounds and their characterizations, O.H.; preparation of final CIF files for all compounds and revising the X-ray part, C.R.; correction of and finalizing the manuscript, supervision of the whole project, and funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Deutsche Forschungsgemeinschaft within the project Sche 384/41-2.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

NMR spectra and crystal data are given in the Supporting Information. Crystallographic data for compounds 2–10 have been deposited with the Cambridge Crystallographic Data Center (CCDC 2464405-2464411).

Acknowledgments

The authors thank the Deutsche Forschungsgemeinschaft for the support within the project Sche 384/41-2. C.R. is grateful for the Studienstiftung des Deutschen Volkes e. V. for his PhD fellowship.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 13 00239 sch001
Scheme 1. Two approaches recently reported towards the synthesis of mono-alkyl-substituted phosphanylboranes. (a) Synthesis of mono-alkylated phosphonium salts and methylphosphanylborane; (b) General route for the synthesis of mono-alkyl-substituted phosphanylboranes via salt metathesis of mono-alkylated phosphines.
Scheme 1. Two approaches recently reported towards the synthesis of mono-alkyl-substituted phosphanylboranes. (a) Synthesis of mono-alkylated phosphonium salts and methylphosphanylborane; (b) General route for the synthesis of mono-alkyl-substituted phosphanylboranes via salt metathesis of mono-alkylated phosphines.
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Scheme 2. Syntheses of compounds 2–4 (yields are given in parenthesis).
Scheme 2. Syntheses of compounds 2–4 (yields are given in parenthesis).
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Figure 1. Molecular structures of the phosphanylborane salts 2–4 in the solid state.
Figure 1. Molecular structures of the phosphanylborane salts 2–4 in the solid state.
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Scheme 3. Syntheses of compounds 5–7.
Scheme 3. Syntheses of compounds 5–7.
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Scheme 4. Syntheses of compounds 8–10. Isolated yields are given in parenthesis.
Scheme 4. Syntheses of compounds 8–10. Isolated yields are given in parenthesis.
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Inorganics 13 00239 g002
Figure 2. Molecular structures of 5 (top left), 8 (top right), 9 (bottom left), and 10 (bottom right) in the solid state.
Figure 2. Molecular structures of 5 (top left), 8 (top right), 9 (bottom left), and 10 (bottom right) in the solid state.
Inorganics 13 00239 g002
Table 1. NMR data for the phosphonium halides 1–4 at 300 K.
Table 1. NMR data for the phosphonium halides 1–4 at 300 K.
δ [ppm] (J [Hz])
1H 31P11B
PH (1JPH)BH2 (1JBH)N(CH3)3PtBuR (1JPB)BH2
12.60 (197)2.67 (104)1.94−67.6 (48)−6.0
25.47 (396)2.15 (113)2.83−15.7 (78)−10.3
35.29 (380)2.40 (110)2.81−7.4 (73)−11.1
46.41 (390)2.27–2.612.89−10.4 (70)−10.9
Table 2. Selected bond distances [Å] and angles [°] for compounds 1–4.
Table 2. Selected bond distances [Å] and angles [°] for compounds 1–4.
Compound1234
P−B1.985(2)1.968(5)1.956(6)–1.966(5)1.90(2)–2.01(2)
B−N1.621(2)1.594(5)1.599(7)–1.602(8)1.55(3)–1.69(4)
P−C1.890(2)1.804(5)–1.832(4)1.818(5)–1.863(5)1.72(2)–1.86(2)
P−B−N108.9(1)112.4(3)114.0(3)–115.4(3)116.9(2)–118.2(2)
C−P−C-109.7(2)108.7(2)–109.2(2)102.8(1)–106.5(9)
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Moussa, M.E.; Hegen, O.; Riesinger, C.; Scheer, M. Extended Synthetic Pathways Towards Dialkyl-Substituted Phosphanylboranes. Inorganics 2025, 13, 239. https://doi.org/10.3390/inorganics13070239

AMA Style

Moussa ME, Hegen O, Riesinger C, Scheer M. Extended Synthetic Pathways Towards Dialkyl-Substituted Phosphanylboranes. Inorganics. 2025; 13(7):239. https://doi.org/10.3390/inorganics13070239

Chicago/Turabian Style

Moussa, Mehdi Elsayed, Oliver Hegen, Christoph Riesinger, and Manfred Scheer. 2025. "Extended Synthetic Pathways Towards Dialkyl-Substituted Phosphanylboranes" Inorganics 13, no. 7: 239. https://doi.org/10.3390/inorganics13070239

APA Style

Moussa, M. E., Hegen, O., Riesinger, C., & Scheer, M. (2025). Extended Synthetic Pathways Towards Dialkyl-Substituted Phosphanylboranes. Inorganics, 13(7), 239. https://doi.org/10.3390/inorganics13070239

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