Easy Access to Phosphine‐Borane Building Blocks

Abstract

In this paper, we highlight the synthesis of a variety of primary phosphine‐boranes (RPH₂⋅BH₃) from the corresponding dichlorophosphines, simply by using Li[BH₄] as reductant and provider of the BH₃ protecting group. The method offers facile access not only to alkyl‐ and arylphosphine‐boranes, but also to aminophosphine‐boranes (R₂NPH₂⋅BH₃) that are convenient building blocks but without the protecting BH₃ moiety thermally labile and notoriously difficult to handle. The borane‐protected primary phosphines can be doubly deprotonated using n‐butyllithium to provide soluble phosphanediides Li₂[RP⋅BH₃] of which the phenyl‐derivative Li₂[PhP⋅BH₃] was structurally characterized in the solid state.

A range of differently substituted primary phosphine‐boranes (RPH₂⋅BH₃) are now accessible that as unprotected phosphines difficult to handle or even unstable. In particular, the synthesis of the aminophosphane‐boranes (R₂NPH₂⋅BH₃) offers new opportunities to use as versatile building blocks in the syntheses of organophosphorus compounds.

Introduction

Primary phosphines (RPH₂) ^[1] are the archetypical building blocks that are used for the creation of a plethora of functionalized phosphorus compounds via addition (hydrophosphination) ^[2] or substitution reactions. The latter proceeds typically by in situ mono or double deprotonation affording the reactive phosphanide M[RPH] ^[3] and phosphanediide M₂[RP] intermediates. ^[4] Notable examples include the pyrophoric phenylphosphine (PhPH₂) that provides the useful, but poorly characterized, phenylphosphanediide M₂[PhP], ^[5] and the pyrophoric and hazardous 1,2‐bis‐phosphinobenzene 1,2‐(PH₂)₂C₆H₄ ^[6] that is required to produce the privileged DuPhos ligand class. ^[7] A clear drawback of these primary phosphines is their pyrophoric nature, high sensitivity to oxidation and noxious character, ^[1c] which requires experienced chemists with dedicated lab facilities to handle these compounds safely. ^[8] Furthermore, the appealing primary (dialkylamino)phosphines (R₂NPH₂) that are suitable for further functionalization (treatment with HCl provides the corresponding chlorophosphines) ^[9] are, in addition, thermally labile. For example, iPr₂NPH₂ decomposes upon attempted isolation. ^[10]

Protecting the phosphorus lone pair with a borane (BH₃) moiety offers advantages, ^[11] as the corresponding primary phosphine‐boranes (RPH₂⋅BH₃) are easier to handle, ^[12] but also provide access to interesting polyphosphinoborane material [RPH‐BH₂]_n by catalytic dehydrogenation/dehydrocoupling. ^[13] Typically, primary phosphine‐boranes are prepared in a two‐step procedure by Li[AlH₄] reduction of the corresponding dichlorophosphines and subsequent treatment of the resulting primary phosphine with BH₃⋅THF or BH₃⋅SMe₂.[ ¹² , ¹⁴ ] Yet, Nöth and later Manners et al. described that the direct synthesis of some selected primary phosphine‐boranes is also feasible by using Li[BH₄], ^[15] which does not require the handling of primary phosphine intermediates.

To further advance this field, we were keen on developing a general protocol for the synthesis of primary alkyl‐ and arylphosphine‐boranes that are also applicable for primary aminophosphine‐boranes (R₂NPH₂⋅BH₃). We were further curious if the stabilizing BH₃ moiety would also offer the possibility for characterizing phosphanediides M₂[RP] in the solid state. Herein, we report on a robust protocol for the synthesis of a range of substituted phosphine‐borane building blocks (RPH₂⋅BH₃), including the bis‐borane protected 1,2‐bis‐phosphinobenzene 1,2‐(PH₂⋅BH₃)₂C₆H₄, and present the first single‐crystal structure of the phenylphosphanediide Li₂[PhP⋅BH₃].

Results and Discussion

Treatment of dichlorophosphines 1 a–c (R=Ph (a), Mes (b), tBu (c)) with 2 equiv of Li[BH₄] in diethyl ether afforded after work‐up easy access to the colorless primary phosphine‐boranes 2 a–c[ ^14a , ^14b , ^14c , ^15d ] on a multigram scale (85–98 %; Scheme 1, Table 1). This procedure is not only convenient (max. 60 minutes reaction time), it is also higher yielding than the two‐step procedure that requires an excess of Li[AlH₄] (cf. PhPCl₂ reduction to PhPH₂, max. 55 % yield (lit. value) ^[16] ).

Scheme 1.

Synthesis of primary phosphine‐boranes RPH₂⋅BH₃ 2. TMP=2,2,6,6‐tetramethylpiperidyl.

Table 1.

³¹P{¹H} and ¹¹B NMR chemical shifts including ¹ J(P,B) and ¹ J(B,H) coupling constants of compounds 2 a–h, 3 a, and 4 a,e.

	δ 31P{1H} [ppm]	1 J(P,B) [Hz]	δ 11B NMR [ppm]	1 J(B,H) [Hz]
2 a	−47.4	34.8	−42.2	101.2
2 b	−68.3	40.5	−40.7	101.3
2 c	−10.7	37.0	−43.1	100.4
2 d	−52.4	34.4	−41.5	103.3
2 e	−18.6	50.8	−41.4	99.9
2 f	−13.2	45.9	−41.0	96.8
2 g	−15.9	48.9	−34.5	99.2
2 h	2.1	51.6	−36.7	98.2
3 a	−166.4	–	−34.5	84.5
4 a	−95.6	32.4	−40.1	98.3
4 e	1.1	26.6	−35.4	98.8

Interestingly, treatment of 1,2‐bis(dichlorophosphino)benzene (1 d; 1,2‐(PCl₂)₂C₆H₄) with Li[BH₄] in diethyl ether afforded the novel bis‐phosphine‐borane 1,2‐(PH₂⋅BH₃)₂C₆H₄ (2 d, δ ³¹P{¹H}=−52.4 (¹ J(P,B)=34.4 Hz), δ ¹¹B NMR=−41.5 (¹ J(B,H)=103.29 Hz, ¹ J(B,P)=25.0 Hz); Scheme 1) as a colorless crystalline solid in 65 % isolated yield. Recrystallization from a mixture of DCM and pentane at −78 °C afforded colorless crystals suitable for an X‐ray crystal structure determination. The molecular structure of 2 d (Figure 1) displays typical P−B (P1−B1 1.9182(17), P2−B2 1.9193(18) Å) and P−C (P1−C1 1.8080(14), P2−C2 1.8050(14) Å) bond lengths for an arylphosphine‐borane. The molecular structure of 2 d also shows intermolecular P−H^δ+⋅⋅⋅^δ−H−B and C−H^δ+⋅⋅⋅^δ−H−B interactions in the solid state as a result of the oppositely charged hydrogen atoms (Table 2), ^[13b] where the BH₃ moiety functions as the acceptor.

Figure 1.

Molecular structure of 1,2‐(PH₂⋅BH₃)₂C₆H₄ (2 d) in the crystal (displacement ellipsoids are set at 50 % probability). Selected bond lengths [Å], angles and torsion angles [°]: P1−B1 1.9182(17), P2−B2 1.9193(18), P1−C1 1.8080(14), P2−C2 1.8050(14), B1−H1B 1.108(19), B1−H2B 1.071(19), B1−H3B 1.062(19), B2−H4B 1.096(19), B2−H5B 1.07(2), B2−H6B 1.05(2), C1‐P1‐B1 117.53(7), C2‐P2‐B2 119.03(8), C2‐C1‐P1 123.44(10), C1‐C2‐P2 123.51(10), B1‐P1‐C1‐C2 165.66(12), B2‐P2‐C2‐C1 166.77(12).

Table 2.

Potential intermolecular dihydrogen bonds with BH₃ as acceptor in the crystal structure of 2 d.

	d‐H [Å]	H⋅⋅⋅A [Å]	D⋅⋅⋅A [Å]	d‐H⋅⋅⋅A [°]
P1−H1P⋅⋅⋅H1Bi	1.292(18)	2.33(2)	3.491(18)	148.1(13)
C4−H4⋅⋅⋅H4Bii	0.94(2)	2.38(3)	3.308(19)	168.3(17)

Symmetry codes i: x‐$1/2$ , y, $1/2$ −z; ii: x+$1/2$ , $1/2$ −y, −z. D=donor, A=acceptor BH₃.

Next, we turned our attention to the primary aminophosphines in the hope that the accessibility of these building blocks can also be improved by the direct reduction of dichloroaminophosphines with Li[BH₄]. Indeed, treatment of 1 e–h (R=iPr₂N (e), Cy₂N (f), TMP (g), (Me₃Si)₂N (h)) with 2 equiv of Li[BH₄] in diethyl ether at 0 °C afforded after work‐up the colorless primary aminophosphine‐boranes 2 e–h (81–98 %; δ ³¹P{¹H}=−18.6 (2 e), −13.2 (2 f), −15.9 (2 g), 2.1 (2 h); Scheme 1). We obtained suitable crystals for iPr₂NPH₂⋅BH₃ (2 e) from diethyl ether at −70 °C of which the molecular structure could be determined by X‐ray crystal structure analysis (Figure 2). In the solid state, the molecules 2 e sit on a crystallographic mirror plane with a planar nitrogen N1; the angle sum is 360.0(2)°. Interestingly, in this case there are no intermolecular distances that are shorter than the sum of van der Waals radii. This crystal structure therefore belongs to the rare collection of “loosely packed” crystals. ^[17] The loose packing can also be seen in the low crystal density of only 0.970 g cm⁻³, which also explains the low melting point of 1.0–2.0 °C.

Figure 2.

Molecular structure of iPr₂NPH₂⋅BH₃ in the crystal (2 e; symmetry code i: x, 1−y, z. Displacement ellipsoids are set at 50 % probability). Selected bond lengths [Å] and angles [°]: P1−B1 1.898(2), P1−N1 1.6297(15), N1−C1 1.482(2), N1−C3 1.489(2), B1−H1A 1.15(3), B1−H1B 1.055(19); N1‐P1‐B1 120.03(9), C1‐N1‐C3 118.49(14), C1‐N1‐P1 120.34(12), C3‐N1‐P1 121.17(11).

We then selected P‐phenyl and P‐(diisopropyl)amino substituted 2 a,e to investigate their applicability as synthons in organophosphorus chemistry, in particular by studying the double deprotonation and subsequent quenching of the intermediate phosphanediide 3 a,e with trimethylsilylchloride to afford the bis(trimethylsilyl)phosphine‐boranes 4 (Scheme 2). In both cases, phosphine‐borane 2 could be readily deprotonated with 2 equiv of n‐butyllithium in THF at −78 °C (3 a: δ ³¹P{¹H}=−166.4, δ ⁷Li=0.6). Subsequent treatment with Me₃SiCl and work‐up afforded PhP(SiMe₃)₂⋅BH₃ (4 a; 98 %; δ ³¹P{¹H}=−95.6 (¹ J(P,B)=32.4 Hz), δ ²⁹Si=6.0 (¹ J(Si,P)=48.5 Hz), δ ¹¹B=−40.1 (¹ J(B,H)=98.3 Hz, ¹ J(B,P)=24.7 Hz)) and iPr₂NP(SiMe₃)₂⋅BH₃ (4 e; 89 %; δ ³¹P{¹H}=1.1 (¹ J(P,B)=26.6 Hz), δ ²⁹Si=2.2 (¹ J(Si,P)=72.2 Hz); δ ¹¹B=−35.4 ppm (¹ J(B,H)=98.8 Hz, ¹ J(B,P)=34.7 Hz) as colorless solids. Single crystals of 4 e were obtained from a mixture of diethyl ether and pentane at −30 °C. The crystal structure of 4 e (Figure 3) contains two independent molecules in the asymmetric unit of which one was well ordered, while the other was refined with a disorder model. The structure of 4 e contains intramolecular C−H⋅⋅⋅H dihydrogen bonds where the BH₃ groups function as acceptors (Table 3). The packing of the molecules, however, does not contain intermolecular distances that are shorter than the sum of van der Waals radii, as is the case for 2 e. In addition, a PLATON calculation ^[18] detects the presence of four small, symmetry related voids in the unit cell with a volume of 21 ų each (minor disorder component ignored). This loose packing results in a low crystal density of only 0.976 g cm⁻³, and a low melting point (28.7–29.4 °C).

Scheme 2.

Synthesis of phosphanediides 3 and bis(trimethylsilyl)phosphine‐boranes 4.

Figure 3.

Molecular structure of iPr₂NP(SiMe₃)₂⋅BH₃ in the crystal (4 e; displacement ellipsoids were drawn at the 30 % probability level, only one of two independent molecules is shown). Selected bond lengths [Å] and angles [°] (the values of only one of two independent molecules are given): P1−B1 1.9685(16), P1−N1 1.6987(10), P1−Si11 2.2853(4), P1−Si12 2.2905(5), N1−C11 1.4797(15), N1−C14 1.4763(14), B1−H1A 1.113(19), B1−H1B 1.07(2), B1−H1C 1.07(2); N1‐P1‐B1 118.52(6), N1‐P1‐Si11 107.29(4), N1‐P1‐Si12 110.41(4), Si11‐P1‐Si12 111.523(18), B1‐P1‐Si11 104.89(6), B1‐P1‐Si12 104.12(7), P1‐N1‐C11 125.17(8), P1‐N1‐C14 118.24(8), C11‐N1‐C14 116.37(9).

Table 3.

Potential intramolecular dihydrogen bonds with BH₃ as acceptor in crystal structure of 4 e. The asymmetric unit contains two independent molecules. The minor disorder component is ignored.

	d‐H [Å]	H⋅⋅⋅A [Å]	D⋅⋅⋅A [Å]	d‐H⋅⋅⋅A [°]
C12−H12C⋅⋅⋅H1C[*]	0.98	2.23	3.131(19)	152
C13−H13A⋅⋅⋅H1B[*]	0.98	2.17	3.065(19)	151
C22−H22C⋅⋅⋅H2A[**]	0.98	2.22	3.098(16)	148
C23−H23A⋅⋅⋅H2C[**]	0.98	2.20	3.090(18)	151

[*] Molecule 1. [**] Molecule 2. D=donor, A=acceptor BH₃.

During our investigations, ^[19] Oulyadi, Gaumont, Harrison‐Marchanda et al. reported on the characterization of the gem‐dilithium phosphido‐borane Li₂[PhP⋅BH₃] intermediate 3 a in THF solution, which matches with our findings, but they did not provide structural characterization in the solid state. ^[20] We anticipated that the BH₃ moiety might assist the crystallization of this unique dianion and indeed found it to be possible to obtain the highly reactive phosphanediide 3 a as a colorless solid (91 %), which can be recrystallized from a mixture of THF, DME and hexanes at −78 °C to provide colorless crystals suitable for X‐ray analysis. The molecular structure of 3 a features the dianionic [PhP⋅BH₃] fragment, of which the phosphorus atom connects to four lithium ions in the polymeric chain in the direction of the crystallographic b‐axis (P1−Li1 2.550(3), P1−Li2 2.524(4), P1−Li1ⁱⁱ 2.540(4), P1−Li2ⁱⁱ 2.561(4) Å; Figure 4). The polymeric chain is supported by the BH₃ moiety which also interacts with the lithium ions (Li1−H2Bⁱ 1.83(3), Li2−H3B 1.96(3) Å), and by the DME molecule which is also bridging the lithium centers (O1−Li1 2.307(4), O1−Li2ⁱⁱ 1.995(4), O2−Li1 2.015(4), O2−Li2ⁱⁱ 2.520(4) Å), with no short intermolecular contacts between the two chains. 3 a was twinned in the crystal structure and a twofold rotation about hkl=(1,0,−1) was used in its crystal structure analysis (see the Experimental Section). In the monoclinic system this is equivalent to a twofold rotation about the a,c‐diagonal in direct space (for a view along the a,c‐diagonal, see Figure 5). ^[21] Overall, the asymmetric unit contains two independent Li centers, one [PhP⋅BH₃] dianion, and one DME molecule. The presence of H₂B−H⋅⋅⋅Li interactions has been reported for related lithium borane phosphanides Li[R₂P⋅BH₃],[ ²² , ²³ ] but the crystal structure of 3 a represents, in fact, the first structural characterization of an arylphosphanediide M₂[ArP] in the solid state.

Figure 4.

Part of the polymeric chain in the crystal structure of Li₂[PhP⋅BH₃](DME) (3 a). Displacement ellipsoids were drawn at the 30 % probability level. C−H hydrogen atoms are omitted for clarity. Symmetry codes i: 3/2−x, y‐1/2, 1/2−z; ii: 3/2−x, y+1/2, 1/2−z. Selected bond lengths [Å] and angles [°] in the crystal structure of 3 a: P1−B1 1.996(2), P1−Li1 2.550(3), P1−Li2 2.524(4), P1−Li1ii 2.540(4), P1−Li2ⁱⁱ 2.561(4), B1−H1B 1.13(3), B1−H2B 1.17(3), B1−H3B 1.16(3), O1−Li1 2.307(4), O1−Li2ⁱⁱ 1.995(4), O2−Li1 2.015(4), O2−Li2ⁱⁱ 2.520(4), Li1−H2Bⁱ 1.83(3), Li2−H3B 1.96(3); B1‐P1‐Li1 113.61(11), B1‐P1‐Li2 60.62(11), B1‐P1‐Li1ⁱⁱ 64.42(10), B1‐P1‐Li2ⁱⁱ 117.10(11), P1ⁱ‐Li1‐P1 111.56(13), O1‐Li1‐P1 94.92(13), O2‐Li1‐P1 98.87(15), O1‐Li1‐P1ⁱ 153.28(16), O2‐Li1‐P1ⁱ 106.45(16), P1‐Li2‐P1ⁱ 111.71(13), O1ⁱ‐Li2‐P1 144.57(18), O2ⁱ‐Li2‐P1 107.13(15), O1ⁱ‐Li2‐P1ⁱ 102.97(15), O2ⁱ‐Li2‐P1ⁱ 86.60(12).

Figure 5.

One‐dimensional coordination chains in b‐direction in the crystal structure of Li₂[PhP⋅BH₃](DME) (3 a). View approximately along the a,c‐diagonal. Shown are two of the one‐dimensional coordination chains which are related by the inversion center of the space group.

Conclusion

We herein provide a facile protocol that gives easy access to a range of substituted primary phosphine‐boranes (RPH₂⋅BH₃). These stable borane‐protected phosphines are well behaved in contrast to the free unprotected ones that are difficult to handle or even unstable. In particular, the readily accessible aminophosphine‐boranes (R₂NPH₂⋅BH₃) offer new opportunities as versatile building block in the synthesis of organophosphorus compounds, which can be easily deprotected using common procedures. ^[12] Furthermore, we showed that the use of BH₃ also offers increased solubility and stability of reactive intermediates, which allowed the first structural characterization of an arylphosphanediide in the solid state.

Experimental Section

General methods and materials

All manipulations were carried out under an atmosphere of dry nitrogen, using standard Schlenk and drybox techniques. Solvents were purified, dried and degassed according to standard procedures and stored under 3 Å molecular sieves or a sublimed sodium mirror. ¹H and ¹³C{¹H} NMR spectra were recorded on a Bruker Avance 250, Bruker Avance 400 or Bruker Avance 500 and internally referenced to the residual solvent resonances (CDCl₃: ¹H δ=7.26, ¹³C{¹H} δ=77.2; THF‐D8: ¹H δ=3.58, 1.72, ¹³C{¹H} δ=67.2, 5.3 ppm or TMS; ³¹P{¹H}, ³¹P, ¹¹B{¹H}, ¹¹B, ⁷Li and ²⁹Si NMR spectra were recorded on a Bruker Avance 250 or Bruker Avance 400 and externally referenced (85 % H₃PO₄, BF₃⋅OEt₂ and LiCl, respectively). Chemical shifts are reported in ppm. High resolution mass spectra were recorded on a Bruker MicroTOF with ESI nebulizer (ESI). Melting points were measured in sealed capillaries and are uncorrected. PhPCl₂, tBuPCl₂, PCl₃, iPr₂NH, Cy₂NH, TMP, (Me₃Si)₂NNa (1.0 m in THF), Li[BH₄] (2 m in THF) and nBuLi (1.6 m in hexanes) were purchased from Sigma and used as received. Me₃SiCl was bought from Sigma and freshly distilled before use. The various dichlorophosphines RPCl₂ (R=Mes, ^[24] iPr₂N, ^[25] Cy₂N, ^[26] TMP, ^[27] and (Me₃Si)₂  n) ^[28] were prepared according to known literature procedures.

Synthesis and characterization

PhPH₂⋅BH₃ (2 a): A solution of PhPCl₂ (6.8 mL, 50.0 mmol) in Et₂O (100 mL) was added dropwise in about 30 minutes to a solution of Li[BH₄] (50.0 mL, 2.0 m in THF, 100.0 mmol) in Et₂O (200 mL) at 0 °C. A colorless precipitate was formed and the reaction mixture was stirred for another 30 minutes during which the temperature was allowed to warm to room temperature. The solvent was removed in vacuo, the product was extracted into pentane (3×100 mL) and then filtered over Celite. Removal of pentane in vacuo afforded a colorless solid. Recrystallization from pentane at −30 °C yielded PhPH₂⋅BH₃ (2 a)[ ^13b , ^14b ] as colorless crystals (5.29 g, 42.7 mmol, 85.3 %). ¹H NMR (400.1 MHz, CDCl₃): δ=0.90 (br. q, ¹ J(H,B)=100.2 Hz, 3 H; BH ₃), 5.52 (dq, ¹ J(H,P)=371.5 Hz, ³ J(H,H)=7.8 Hz, 2 H; PH ₂), 7.43–7.59 (m, 3 H; m,p‐PhH), 7.65–7.77 (m, 2 H; o‐PhH); ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.90 (dt, ² J(H,P)=16.1, ³ J(H,H)=8.0 Hz, 3 H; BH ₃), 5.52 (dq, ¹ J(H,P)=371.5 Hz, ³ J(H,H)=7.8 Hz, 2 H; PH ₂), 7.43–7.59 (m, 3 H; m,p‐PhH), 7.65–7.77 (m, 2 H; o‐PhH); ¹¹B NMR (128.4 MHz, CDCl₃ ): δ=−42.2 (dq, ¹ J(B,H)=101.2 Hz, ¹ J(B,P)=36.2 Hz); ¹³C NMR (100.6 MHz, CDCl₃): δ=119.9 (d, ¹ J(C,P)=57.7 Hz; ipso‐PhC), 129.2 (d, ³ J(C,P)=10.6 Hz; m‐PhC), 132.0 (d, ⁴ J(C,P)=2.7 Hz; p‐PhC), 133.7 (d, ² J(C,P)=9.1 Hz; o‐PhC); ³¹P{¹H} NMR (101.3 MHz, CDCl₃): δ=−47.4 ppm (q, ¹ J(P,B)=34.8 Hz).

MesPH₂⋅BH₃ (2 b): A solution of MesPCl₂ (1.10 g, 4.98 mmol) in Et₂O (10 mL) was added dropwise in about 30 minutes to a solution of Li[BH₄] (5.0 mL, 2.0 m in THF, 10.0 mmol) in Et₂O (20 mL) at 0 °C. A colorless precipitate was formed and the reaction mixture was stirred for another 30 minutes during which the temperature was slowly warmed to room temperature. The solvent was removed in vacuo, the product was extracted into pentane (3×10 mL) and then filtered over Celite. Removal of pentane in vacuo afforded MesPH₂⋅BH₃ (2 b) ^[14a] as a colorless solid (0.81 g, 4.88 mmol, 97.6 %). Colorless crystals were obtained by crystallization from pentane at −30 °C. H NMR (250.1 MHz, CDCl₃): δ=0.85 (br. q, ¹ J(H,B)=98.2 Hz, 3 H; BH ₃), 2.30 (s, 3 H; p‐PhCH ₃), 2.45 (s, 6 H; o‐PhCH ₃), 5.46 (dq, ¹ J(H,P)=370.4 Hz, ³ J(H,H)=7.6 Hz, 2 H; PH ₂), 6.94 (d, ⁴ J(H,P)=3 Hz, 2 H; m‐PhH); ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.84 (dt, ² J(H,P)=15.6, ³ J(H,H)=7.8 Hz, 3 H; BH ₃), 2.30 (s, 3 H; p‐PhCH ₃), 2.45 (s, 6 H; o‐PhCH ₃), 5.46 (dq, ¹ J(H,P)=370.4 Hz, ³ J(H,H)=7.6 Hz, 2 H; PH ₂), 6.94 (d, ⁴ J(H,P)=3 Hz, 2 H; m‐PhH); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−40.7 (dq, ¹ J(B,H)=101.3 Hz, ¹ J(B,P)=34.8 Hz); ¹³C NMR (100.6 MHz, CDCl₃): δ=21.1 (d, ⁵ J(C,P)=1.0 Hz; p‐PhCH₃), 21.6 (d, ³ J(C,P)=8.5 Hz; o‐PhCH₃), 116.8 (d, ¹ J(C,P)=57.9 Hz; ipso‐PhC), 129.4 (d, ³ J(C,P)=8.3 Hz; m‐PhC), 141.0 (d, ² J(C,P)=8.0 Hz; o‐PhC), 141.3 (d, ⁴ J(C,P)=2.3 Hz; p‐PhC); ³¹P{¹H} NMR (101.3 MHz, CDCl₃): δ=−68.3 ppm (br. d, ¹ J(P,B)=40.5 Hz).

t BuPH₂⋅BH₃ (2 c): A solution of tBuPCl₂ (3.15 g, 19.8 mmol) in Et₂O (40 mL) was added dropwise in about 30 minutes to a solution of Li[BH₄] (20 mL, 2.0 m in THF, 20.0 mmol) in Et₂O (80 mL) at 0 °C. A colorless precipitate was formed and the reaction mixture was stirred for another 30 min during which the temperature was allowed to warm to room temperature. The solvent was removed in vacuo, the product was extracted into pentane (3×20 mL) and then filtered over Celite. After removal of all volatiles, subsequent distillation (89–91 °C, 10 mbar) yielded tBuPH₂⋅BH₃ (2 c)[ ^14c , ^15d ] as a colorless oil (1.82 g, 17.5 mmol, 88.4 %). ¹H NMR (250.1 MHz, CDCl₃): δ=0.51 (br. q, ¹ J(H,B)=100.1 Hz, 3 H; BH ₃), 1.25 (d, ³ J(H,P)=15.5 Hz, 9 H; C(CH ₃)₃), 4.38 (dq, ¹ J(H,P)=353.1 Hz, ³ J(H,H)=7.6 Hz, 2 H; PH ₂); ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.51 (dt, ² J(H,P)=15.7, ³ J(H,H)=7.8 Hz, 3 H; BH ₃), 1.25 (d, ³ J(H,P)=15.5 Hz, 9 H; C(CH ₃)₃), 4.38 (dq, ¹ J(H,P)=353.1 Hz, ³ J(H,H)=7.6 Hz, 2 H; PH ₂); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−43.1 (dq, ¹ J(B,H)=100.4 Hz, ¹ J(B,P)=35.5 Hz); ¹³C NMR (100.6 MHz, CDCl₃): δ=24.9 (d, ¹ J(C,P)=36.2 Hz; C(CH₃)₃), 27.7 (d, ² J(C,P)=2.8 Hz; C(CH₃)₃); ³¹P{¹H} NMR (101.3 MHz, CDCl₃): δ=−10.7 ppm (q, ¹ J(P,B)=37.0 Hz).

1,2‐(PH₂⋅BH₃)₂C₆H₄ (2 d): Step  1. Synthesis of 1,2‐(P[O]Cl₂)₂C₆H₄: A mixture of 1,2‐(P[O]OMe₂)₂C₆H₄ (5.88 g, 20.0 mmol) and PCl₅ (16.66 g, 80.0 mmol) was heated for 16 hours at 120 °C in a three‐necked flask with reflux condenser under a nitrogen atmosphere. Caution: this is a very exothermic reaction. The mixture liquefied and slowly turned from pale yellow to dark yellow. Upon cooling to room temperature, the mixture solidified. Subsequently, all volatiles were removed in vacuo and the remaining pale brown solid was distilled (140 °C at 5.1×10⁻² mbar) yielding 1,2‐(P[O]Cl₂)₂C₆H₄ ^[29] as a colorless solid (5.55 g, 17.8 mmol, 89.0 %), 98 % pure according to ³¹P{¹H} NMR spectroscopy, which was used without further purification. ¹H NMR (500.2 MHz, CDCl₃): δ=7.89–7.95 (m, 2 H; m‐PhH), 8.33–8.45 (m, 2 H; o‐PhH); ¹³C NMR (125.8 MHz, CDCl₃): δ=134.1–134.4 (m; m‐PhC), 134.6 (t, ^2, 3 J(C,P)=13.6 Hz; o‐PhC), 136.1 (dd, ¹ J(C,P)=156.2 Hz, ² J(C,P)=11.4 Hz; ipso‐PhC); ³¹P{¹H} NMR (101.3 MHz, CDCl₃): δ=31.2 ppm (s). Step  2. Synthesis of 1,2‐(PCl₂)₂C₆H₄: A mixture of 1,2‐(P[O]Cl₂)₂C₆H₄ (5.31 g, 98 % pure, 17.0 mmol) and Ph₃P (9.83 g, 37.5 mmol) was heated for 16 hours at 230 °C in a three‐necked flask with reflux condenser under a nitrogen atmosphere. The mixture liquefied and slowly turned from pale yellow to brown. Upon cooling to room temperature, the mixture solidified. All volatiles were removed in vacuo and the product was separated from the Ph₃PO byproduct using a Schlenk to Schlenk distillation (100 °C at 4.0×10⁻² mbar). Subsequently, the resulting yellow oil was purified by fractional distillation (81–84 °C at 2.9×10⁻² mbar) yielding 1,2‐(PCl₂)₂C₆H₄ as a pale yellow oil (1.80 g, 6.43 mmol, 37.7 %), 96 % pure according to ³¹P{¹H} NMR spectroscopy. Alternatively, 1,2‐(PCl₂)₂C₆H₄ can be synthesized on large scale in a one‐pot protocol, without purification of the intermediate 1,2‐(P[O]Cl₂)₂C₆H₄ as follows: A mixture of 1,2‐(P[O]Cl₂)₂C₆H₄ (44.13 g, 150.0 mmol) and PCl₅ (124.93 g, 600 mmol) was heated for 16 hours at 120 °C in a three‐necked flask with reflux condenser under a nitrogen atmosphere. Caution: this is a very exothermic reaction. All volatiles were removed in vacuo and immediately Ph₃P (78.69 g, 300.0 mmol) was added. The mixture was heated for another 16 hours at 230 °C. Upon cooling to room temperature, all volatiles were removed in vacuo and the resulting mixture was purified by Schlenk to Schlenk distillation (140 °C, 4.0×10⁻² mbar) yielding 1,2‐(PCl₂)₂C₆H₄ as a yellow oil (39.38 g, 140.7 mmol, 93.8 %), 85 % pure according to ³¹P {¹H} NMR spectroscopy. Further purification of 1,2‐(PCl₂)₂C₆H₄ can be achieved by fractional distillation using a spinning band (60×2.5 cm, 4 bladed), yielding 1,2‐(PCl₂)₂C₆H₄ ^[30] as a colorless oil (22.38 g, 79.98 mmol, 53.3 %). ¹H NMR (500.2 MHz, CDCl₃): δ=7.70–7.77 (m, 2 H; m‐PhH), 8.20–8.27 (m, 2 H; o‐PhH); ¹³C NMR (125.8 MHz, CDCl₃): δ=130.6 (t, ^3, 4 J(C,P)=6.4 Hz; m‐PhC), 133.4 (s, o‐PhC), 144.3 (t, ^1, 2 J(C,P)=12.7 Hz; ipso‐PhC); ³¹P{¹H} NMR (101.3 MHz, CDCl₃): δ=151.6 ppm (s). Step  3. Synthesis of 1,2‐(PH₂⋅BH₃)₂C₆H₄ (2 d): A solution of Li[BH₄] (1.81 mL, 2.0 m in THF, 3.62 mmol) in Et₂O (5 mL) was added dropwise in about 30 minutes to a solution of 1,2‐(PCl₂)₂C₆H₄ (0.29 g, 1.04 mmol) in Et₂O (5 mL) at 0 °C. A colorless precipitate was formed and the reaction mixture was stirred for another 30 minutes at 0 °C. The reaction mixture was concentrated at 0 °C and filtered over a glass filter. The remaining solvents where removed in vacuo at 0 °C, the product was extracted into Et₂O (3×10 mL) followed by a second filtration. Removal of Et₂O at 0 °C in vacuo yielded 1,2‐(PH₂⋅BH₃)₂C₆H₄ (2 d) as small colorless needles, 99 % pure according to ³¹P NMR spectroscopy (0.11 g, 0.65 mmol, 64.7 %). Recrystallization from a mixture of DCM and pentane at −78 °C yielded colorless crystals suitable for X‐ray analysis. M.p. 81.7–82.8 °C; ¹H NMR (400.1 MHz, CDCl₃): δ=0.96 (br. q, ¹ J(H,B)=99.44 Hz, 6 H; BH ₃), 5.73 (dq, ¹ J(H,P)=373.9 Hz, ³ J(H,H)=8.1 Hz, 4 H; PH ₂), 7.66–7.73 (m, 2 H; m‐PhH), 7.92–8.01 (m, 2 H; o‐PhH); ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.96 (dt, ² J(H,P)=15.65 Hz, ³ J(H,H)=7.83, 6 H; BH ₃), 5.73 (dq, ¹ J(H,P)=373.9 Hz, ³ J(H,H)=8.1 Hz, 4 H; PH ₂), 7.66–7.73 (m, 2 H; m‐PhH), 7.92–8.01 (m, 2 H; o‐PhH); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−41.5 (dq, ¹ J(B,H)=103.29 Hz, ¹ J(B,P)=25.0 Hz); ¹³C NMR (125.8 MHz, CDCl₃): δ=125.8 (dd, ¹ J(C,P)=52.7 Hz, ² J(C,P)=3.6 Hz; ipso‐PhC), 132.6 (dd, ³ J(C,P)=11.8 Hz, ⁴ J(C,P)=2.7 Hz; m‐PhC), 136.1 (dd, ³ J(C,P)=15.4 Hz, ² J(C,P)=7.3 Hz; o‐PhC); ³¹P{¹H} NMR (101.3 MHz, CDCl₃): δ=−52.4 ppm (br. d, ¹ J(P,B)=34.4 Hz); HR ESI‐MS: m/z (%): $\tilde{\nu }$ =169.07 (2.8) [M−H]⁻, 167.05 (2.2) [M−H₂₋H]⁻, 155.04 (2.0) [M−BH₃₋H]⁻; Elemental analysis calcd (%) for C₆H₁₃B₂P₂ (M−H): 169.0686, found 169.0684.

i Pr₂NPH₂⋅BH₃ (2 e): A solution of iPr₂NPCl₂ (4.6 mL, 25.0 mmol) in Et₂O (50 mL) was added dropwise in about 30 minutes to a solution of Li[BH₄] (25 mL, 2.0 m in THF, 50.0 mmol) in Et₂O (100 mL) at 0 °C. A colorless precipitate was formed and the reaction mixture was stirred for another 30 minutes during which the temperature was allowed to warm to room temperature. The solvent was removed in vacuo, the product was extracted into pentane (3×100 mL) and then filtered over Celite. Removal of pentane in vacuo afforded iPr₂NPH₂⋅BH₃ (2 e) as a colorless oil (3.35 g, 22.8 mmol, 91.3 %). Crystals suitable for X‐ray analysis were obtained from diethyl ether at −70 °C. M.p. 1.0–2.0 °C; ¹H NMR (250.1 MHz, CDCl₃): δ=1.12 (d, ³ J(H,H)=6.8 Hz, 12 H; CH(CH ₃)₂), 3.57 (sept, ³ J(H,H)=6.8 Hz, 2 H; CH(CH₃)₂), 5.93 (dq, ¹ J(H,P)=379.4 Hz, ³ J(H,H)=6.3 Hz, 2 H; PH₂), signals for BH ₃ were unresolved; ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.67 (dt, ² J(H,P)=15.2, ³ J(H,H)=6.4 Hz, 3 H; BH ₃), 1.12 (d, ³ J(H,H)=6.8 Hz, 12 H; CH(CH ₃)₂), 3.57 (sept, ³ J(H,H)=6.8 Hz, 2 H; CH(CH₃)₂), 5.93 (dq, ¹ J(H,P)=379.4 Hz, ³ J(H,H)=6.3 Hz, 2 H; PH₂); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−41.4 (dq, ¹ J(B,H)=99.9 Hz, ¹ J(B,P)=50.9 Hz); ¹³C NMR (100.6 MHz, CDCl₃): δ=21.5 (d, ³ J(C,P)=3.2 Hz; CH(CH₃)₂), 47.2 (d, ² J(C,P)=3.7 Hz; CH(CH₃)₂); ³¹P{¹H} NMR (101.3 MHz, CDCl₃): δ=−18.6 ppm (q, ¹ J(P,B)=50.8 Hz); HR ESI‐MS: m/z (%): $\tilde{\nu }$ =146.13 (0.2) [M−H]⁻, 144.11 (0.3) [M−H₂₋H]⁻; Elemental analysis calcd (%) for C₆H₁₈BNP (M−H)⁻: 146.1277, found 146.1274.

Cy₂NPH₂⋅BH₃ (2 f): A solution of Cy₂NPCl₂ (1.41 g, 5.0 mmol) in Et₂O (50 mL) was slowly added to a solution of Li[BH₄] (5.0 mL, 2.0 m in THF, 10.0 mmol) in Et₂O (20 mL) at 0 °C. A colorless precipitate was formed and the reaction mixture was stirred for another 30 minutes during which the temperature was allowed to warm to room temperature. The solvent was removed in vacuo, the product was extracted into pentane (3×30 mL) and then filtered over Celite. Removal of pentane in vacuo yielded a colorless solid, which after recrystallization from a mixture of diethyl ether and pentane at −70 °C yielded Cy₂NPH₂ ⋅BH₃ (2 f) as colorless crystals (0.87 g, 4.1 mmol, 81.6 %). M.p. 48.2–49.9 °C; ¹H NMR (400.1 MHz, CDCl₃): δ=0.71 (br. q, ¹ J(H,B)=94.7 Hz, 3 H; BH ₃), 0.97–1.11 (m, 2 H; CyH), 1.22–1.36 (m, 4 H; CyH), 1.42–1.54 (m, 4 H; CyH), 1.57–1.67 (m, 6 H; CyH), 1.74–1.82 (m, 4 H; CyH), 2.99–3.13 (m, 2 H; CyHN), 6.02 (dq, ¹ J(H,P)=379.8 Hz, ³ J(H,H)=6.3 Hz, 2 H; PH ₂); ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.71 (dt, ² J(H,P)=15.2 Hz, ³ J(H,H)=6.3 Hz, 3 H; BH ₃), 0.97–1.11 (m, 2 H; CyH), 1.22–1.36 (m, 4 H; CyH), 1.42–1.54 (m, 4 H; CyH), 1.57–1.67 (m, 6 H; CyH), 1.74–1.82 (m, 4 H; CyH), 2.99–3.13 (m, 2 H; CyHN), 6.02 (dq, ¹ J(H,P)=379.8 Hz, ³ J(H,H)=6.3 Hz, 2 H; PH ₂); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−41.0 (dq, ¹ J(B,H)=96.8 Hz, ¹ J(B,P)=54.9 Hz); ¹³C NMR (68.9 MHz, CDCl₃): δ=25.3 (s, NCHCH₂CH₂ CH₂), 25.9 (s, NCHCH₂ CH₂CH₂), 32.3 (d, ³ J(C,P)=3.0 Hz; NCHCH₂CH₂CH₂), 56.5 (d, ² J(C,P)=3.2 Hz; NCHCH₂CH₂CH₂); ³¹P{¹H} NMR (162.0 MHz, CDCl₃): δ=<M‐>13.2 ppm (br. q, ¹ J(P,B)=45.9 Hz); HR ESI‐MS: m/z (%): $\tilde{\nu }$ =242.18 (64.0) [M−H+O]⁻, 226.19 (100.0) [M−H]⁻; Elemental analysis calcd (%) for C₁₂H₂₆BNP (M−H)⁻: 226.1904, found 226.1901.

(CH₂)₃(CMe₂)₂NPH₂⋅BH₃ (2 g): A solution of TMPPCl₂ (1.21 g, 5.0 mmol; TMP=2,2,6,6‐tetramethylpiperidine) in Et₂O (10 mL) was added slowly to a solution of Li[BH₄] (5.0 mL, 2.0 m in THF, 10.0 mmol) in Et₂O (20 mL) at 0 °C. The reaction mixture was stirred for 30 minutes at 0 °C and another 30 minutes during which the mixture was allowed to warm to room temperature. All volatiles were removed in vacuo, the product was extracted into Et₂O (3×30 mL) and then filtered over Celite. Removal of Et₂O in vacuo afforded TMPPH₂ ⋅BH₃ (2 g) as a colorless powder (0.92 g, 4.9 mmol, 98.4 %). Recrystallization from a mixture of Et₂O and pentanes at −70 °C yielded colorless needles. M.p. >20 °C (decomp); ¹H NMR (400.1 MHz, CDCl₃): δ=1.14 (s, 12 H; NC(CH ₃)₂CH₂CH₂), 1.13–1.21 (m, 4 H; NC(CH₃)₂CH ₂CH₂), 1.23–1.31 (m, 2 H; NC(CH₃)₂CH₂CH ₂), 1.82 (br. q, ¹ J(H,B)=99.3 Hz, 3 H; BH ₃), 5.79 (dq, ¹ J(H,P)=369.6 Hz, ¹ J(H,H)=6.8 Hz, 2 H; PH ₂); ¹H{¹¹B) NMR (400.1 MHz, CDCl₃): δ=1.14 (s, 12 H; NC(CH ₃)₂CH₂CH₂), 1.13–1.21 (m, 4 H; NC(CH₃)₂CH ₂CH₂), 1.23–1.31 (m, 2 H; NC(CH₃)₂CH₂CH ₂), 1.82 (dt, ¹ J(H,P)=13.6 Hz, ³ J(H,H)=6.9 Hz, 3 H; BH ₃), 5.79 (dq, ¹ J(H,P)=369.6 Hz, ¹ J(H,H)=6.8 Hz, 2 H; PH ₂); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−34.5 (dq, ¹ J(B,H)=99.2 Hz, ¹ J(B,P)=52.7 Hz); ¹³C NMR (125.8 MHz, C₆D₆): δ=17.0 (s; NC(CH₃)₂CH₂ CH₂), 29.0 (d, ³ J(C,P)=4.3 Hz; NC(CH₃)₂CH₂CH₂), 40.9 (d, ³ J(C,P)=4.9 Hz; NC(CH₃)₂ CH₂CH₂), 55.6 (s, NC(CH₃)₂CH₂CH₂); ³¹P{¹H} NMR (162.0 MHz, CDCl₃): δ=−15.9 ppm (br. q, ¹ J(P,B)=48.9 Hz); HR ESI‐MS: m/z (%): $\tilde{\nu }$ =202.15 (22.0) [M−H+O]⁻, 186.16 (36.0) [M−H]⁻; Elemental analysis calcd (%) for C₉H₂₂BNP (M−H)⁻: 186.1590, found 186.1587.

(Me₃Si)₂NPH₂⋅BH₃ (2 h): Step  1. Synthesis of (Me₃Si)₂NPCl₂: (Me₃Si)₂NNa (25.0 mL, 1.0 m in THF, 25.0 mmol) was added slowly to a solution of PCl₃ (2.18 mL, 25.0 mmol) in THF (74 mL) at 0 °C. The reaction mixture was stirred for 30 minutes at 0 °C and subsequently 30 minutes at room temperature after which the solvent was removed in vacuo, the product extracted into diethyl ether (3×30 mL) and then filtered. Removal of diethyl ether yielded (Me₃Si)₂NPCl₂ as a colorless oil. Step  2. Synthesis of (Me₃Si)₂NPH₂⋅BH₃: The resulting (Me₃Si)₂NPCl₂ was dissolved in Et₂O (50 mL) and added slowly to a solution of Li[BH₄] (25.0 mL, 2.0 m in THF, 50.0 mmol) in Et₂O at 0 °C and subsequently stirred for 60 minutes at the same temperature. The following purification steps where all carried out at 0 °C. The solvent was removed in vacuo, the product was extracted into Et₂O (3×30 mL) and then filtered over Celite. Removal of Et₂O in vacuo yielded (Me₃Si)₂NPH₂ ⋅BH₃ (2 h) as a pale yellow solid (4.36 g, 21.2 mmol, 84.6 %; 92.3 % pure according to ³¹P NMR spectroscopy). Recrystallization from a mixture of diethyl ether and pentane at −70 °C afforded (Me₃Si)₂NPH₂ ⋅BH₃ (2 h) as small needless (3.28 g, 15.9 mmol, 63.6 %). M.p. 5–7 °C; ¹H NMR (400.1 MHz, CDCl₃): δ=0.26 (br. s, 18 H; (CH ₃)₃Si), 0.67 (br. q, ¹ J(H,B)=94.3 Hz, 3 H; BH ₃), 6.06 (dq, ¹ J(H,P)=371.7 Hz, ³ J(H,H)=6.7 Hz, 2 H; PH ₂); ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.26 (br. s, 18 H; (CH ₃)₃Si), 0.67 (dt, ² J(H,P)=15.3 Hz, ³ J(H,H)=6.7 Hz, 3 H; BH ₃), 6.06 (dq, ¹ J(H,P)=371.6, ³ J(H,H)=6.7 Hz, 2 H; PH ₂); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−36.7 (dq, ¹ J(B,H)=98.2, ¹ J(B,P)=54.4 Hz); ¹³C NMR (62.9 MHz, CDCl₃): δ=2.1 (d, ³ J(C,P)=3.4 Hz; (CH₃)₃Si); ²⁹Si NMR (79.5 MHz, CDCl₃): δ=12.9 (br. s); ³¹P{¹H} NMR (162.0 MHz, CDCl₃): δ=2.1 ppm (q, ¹ J(P,B)=51.6 Hz); HR ESI‐MS: m/z (%): $\tilde{\nu }$ =206.11 (1.0) [M−H]⁻, 160.10 (100.0) [M−PH₂BH₃]⁻; Elemental analysis calcd (%) for C₆H₂₂BNPSi₂ (M−H): 206.1129, found 206.1124.

Li₂[PhP⋅BH₃] (3 a): A stock solution of PhPH₂⋅BH₃ (2 a; 14.6 mL, 0.25 m in THF, 5.0 mmol) was cooled to −78 °C, subsequently 2 equiv of nBuLi (6.25 mL, 1.6 m in hexanes, 10.0 mmol) was slowly added and the mixture was stirred for 30 minutes, after which the mixture was slowly warmed to room temperature and stirred for another 30 minutes. The reaction mixture was concentrated to 10 % of its volume and washed with hexanes (2×20 mL). Evaporation of the residue to dryness at 0 °C yielded Li₂[PhP⋅BH₃] (3 a) as a (thermally unstable) colorless powder (1.1 g, 4.6 mmol, 91 %). Recrystallization from a mixture of THF, DME and hexanes at −78 °C provided colorless crystals suitable for X‐ray analysis. ¹H NMR (400.1 MHz, THF‐D₈): δ=0.68 (br. q, ¹ J(H,B)=84.9 Hz, 3 H; BH ₃), 0.87 (m, 12 H; Hex), 1.30 (m, 16 H; Hex), 1.72 (br. s, 2 H; THF‐D₈), 1.78 (m, 3 H; THF), 3.56 (br. s, 2 H; THF‐D₈), 3.61 (m, 3 H; THF), 6.24–6.28 (m, 1 H; p‐PhH), 6.53–6.57 (m, 2 H; m‐PhH), 7.28–7.31 (m, 2 H; o‐PhH); ¹H{¹¹B} NMR (400.1 MHz, THF‐D₈): δ=0.68 (d, ² J(H,P)=6.8 Hz, 3 H; BH ₃), 0.87 (m, 12 H; Hex), 1.30 (m, 16 H; Hex), 1.72 (br. s, 2 H; THF‐D₈), 1.78 (m, 3 H; THF), 3.56 (br. s, 2 H; THF‐D₈), 3.61 (m, 3 H; THF), 6.24–6.28 (m, 1 H; p‐PhH), 6.53–6.57 (m, 2 H; m‐PhH), 7.28–7.31 (m, 2 H; o‐PhH); ⁷Li NMR (155.5 MHz, THF‐D₈): δ=0.62 (s); ¹¹B NMR (128.4 MHz, THF‐D₈): δ=−34.0 (br. q, ¹ J(B,H)=84.5 Hz); ¹³C NMR (100.6 MHz, THF‐D₈): δ=15.3 (s; C_1,6‐Hex), 24.41 (s; C_2,5‐Hex), 26.2 (m; C_3,4‐THF), 27.3 (s; C_3,4‐THF‐complex), 33.4 (s; C_3,4‐Hex), 68.3 (m; C_2,5‐THF), 69.1 (s; C_2,5‐THF‐complex), 117.1 (s; p‐PhC), 126.7 (d, ³ J(C,P)=4.6 Hz; m‐PhC), 133.6 (d, ² J(C,P)=12.1 Hz; o‐PhC), 165.9 (d, ¹ J(C,P)=43.9 Hz; ipso‐PhC); ³¹P{¹H} NMR (101.3 MHz, THF‐D₈): δ=−166.4 ppm (br. m).

PhP(SiMe₃)₂⋅BH₃ (4 a): 2 equiv of nBuLi (6.88 mL, 1.6 m in hexanes, 11.0 mmol) was added dropwise to a solution of PhPH₂⋅BH₃ (2 a; 0.62 g, 5.0 mmol) in THF (25 mL) at −78 °C. The reaction mixture was stirred for 30 minutes at the same temperature followed by the slow addition of freshly distilled Me₃SiCl (1.40 mL, 11.0 mmol). Subsequently, the reaction mixture was stirred for another 60 minutes during which the temperature was slowly warmed to room temperature. The solvent was removed in vacuo and the residue was extracted into pentanes (3×20 mL). Removal of pentane in vacuo yielded PhP(SiMe₃)₂⋅BH₃ (4 a) as a colorless solid (1.25 g, 4.7 mmol, 98.2 %). M.p. 65.8–66.9 °C. ¹H NMR (250.1 MHz, CDCl₃): δ=0.34 (d, ³ J(H,P)=5.7 Hz, 18 H; Si(CH₃)₃), 7.32–7.41 (m, 3 H; m,p‐PhH), 7.56–7.67 (m, 2 H; o‐PhH), signals for BH ₃ were unresolved; ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.34 (d, ³ J(H,P)=5.7 Hz, 18 H; Si(CH₃)₃), 0.88 (d, ² J(H,P)=10.4 Hz; 3 H, BH ₃), 7.32–7.41 (m, 3 H; m,p‐PhH), 7.56–7.67 (m, 2 H; o‐PhH); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−40.1 (dq, ¹ J(B,H)=98.3 Hz, ¹ J(B,P)=24.7 Hz); ¹³C NMR (62.9 MHz, CDCl₃): δ=−1.3 (d, ² J(C,P)=8.6 Hz; Si(CH₃)₃), 126.5 (d, ¹ J(C,P)=31.3 Hz; ipso‐PhC), 128.6 (d, ³ J(C,P)=8.9 Hz; m‐PhC), 129.3 (d, ⁴ J(C,P=2.5 Hz; p‐PhC), 134.3 (d, ² J(C,P)=6.7 Hz; o‐PhC); ²⁹Si NMR (79.5 MHz, CDCl₃): δ=6.0 (d, ¹ J(Si,P)=48.5 Hz); ³¹P{¹H} NMR (162.0 MHz, CDCl₃): δ=−95.6 ppm (br. d, ¹ J(P,B)=32.4 Hz).

i Pr₂NP(SiMe₃)₂⋅BH₃ (4 e): 2 equiv of nBuLi (10.46 mL, 1.6 m in hexanes, 19.74 mmol) was added slowly to a solution of iPr₂NPH₂ ⋅BH₃ (2 e; 1.23 g, 8.37 mmol) in THF (40 mL) at −78 °C. The reaction mixture was stirred for 30 minutes at the same temperature followed by the slow addition of freshly distilled Me₃SiCl (2.12 mL, 16.74 mmol). Subsequently, the reaction mixture was stirred for another 60 minutes during which the temperature was slowly warmed to room temperature. The solvent was removed in vacuo and the residue was extracted into pentanes (3×30 mL). Removal of pentane in vacuo yielded iPr₂NP(SiMe₃)₂⋅BH₃ (4 e) as a colorless solid (2.18 g, 7.48 mmol, 89.3 %). Colorless crystals were obtained by crystallization from a mixture of diethyl ether and pentane at −30 °C. M.p. 28.7–29.4 °C; ¹H NMR (250.1 MHz, CDCl₃): δ=0.39 (d, ³ J(H,P)=5.3 Hz, 18 H; Si(CH ₃)₃), 1.29 (d, ³ J(H,H)=6.8 Hz, 12 H; CH(CH ₃)₂), 3.29 (d. sept, ³ J(H,P)=14.7 Hz, ³ J(H,H)=6.8 Hz, 2 H; CH(CH₃)₂), signals for BH ₃ were unresolved; ¹H{¹¹B} NMR (400.1 MHz, CDCl₃): δ=0.39 (d, ³ J(H,P)=5.3 Hz, 18 H; Si(CH ₃)₃), 1.03 (d, ² J(H,P)=7.9 Hz, 3 H; BH ₃), 1.29 (d, ³ J(H,H)=6.8 Hz, 12 H; CH(CH ₃)₂), 3.29 (d. sept, ³ J(H,P)=14.7 Hz, ³ J(H,H)=6.8 Hz, 2 H; CH(CH₃)₂); ¹¹B NMR (128.4 MHz, CDCl₃): δ=−35.4 (dq, ¹ J(B,H)=98.8 Hz, ¹ J(B,P)=34.7 Hz); ¹³C NMR (125.8 MHz, CDCl₃): δ=−0.3 (d, ² J(C,P)=8.6 Hz; Si(CH₃)₃), 24.1 (d, ³ J(C,P)=1.3 Hz; CH(CH₃)₂), 51.6 (d, ² J(C,P)=0.9 Hz; CH(CH₃)₂); ²⁹Si NMR (79.5 MHz, CDCl₃): δ=2.2 (d, ¹ J(Si,P)=72.2 Hz); ³¹P{¹H} NMR (162.0 MHz, CDCl₃): δ=1.1 ppm (br. d, ¹ J(P,B)=26.6 Hz).

X‐ray crystal structure determinations

2 d: C₆H₁₄B₂P₂, Fw=169.73, colorless needle, 0.60×0.18×0.18 mm³, orthorhombic, Pbca (no. 61), a=9.2604(4), b=8.0989(2), c=26.4819(7) Å, V=1986.12(11) ų, Z=8, D_x=1.135 g cm⁻³, μ=0.37 mm⁻¹. The diffraction experiment was performed on a Nonius KappaCCD diffractometer with rotating anode and graphite monochromator (λ=0.71073 Å) at a temperature of 150(2) K up to a resolution of (sin θ/λ)_max=0.65 Å⁻¹. The intensity integration was performed with the EvalCCD software. ^[31] A multi‐scan absorption correction and scaling was performed with SADABS ^[32] (correction range 0.73–0.94). A total of 22295 reflections was measured, 2274 reflections were unique (R _int=0.028), 1921 reflections were observed [I>2σ(I)]. The structure was solved with direct methods using SHELXS‐97. ^[33] Structure refinement was performed with SHELXL‐2018 ^[34] on F² of all reflections. Non‐hydrogen atoms were refined freely with anisotropic displacement parameters. All hydrogen atoms were located in difference Fourier maps and refined freely with isotropic displacement parameters. 147 Parameters were refined with no restraints. R1/wR2 [I>2σ(I)]: 0.0293/ 0.0714. R1/wR2 [all refl.]: 0.0395/ 0.0771. S=1.051. Residual electron density between −0.22 and 0.32 e Å⁻³. Geometry calculations and checking for higher symmetry was performed with the PLATON program. ^[18]

2 e: C₆H₁₉BNP, Fw=147.00, colorless block, 0.60×0.50×0.50 mm³, monoclinic, C2 m⁻¹ (no. 12), a=11.7959(3), b=9.8864(2), c=8.6272(2) Å, β=90.1828(10) °, V=1006.09(4) ų, Z=4, D_x=0.970 g cm⁻³, μ=0.21 mm⁻¹. The diffraction experiment was performed on a Nonius KappaCCD diffractometer with rotating anode and graphite monochromator (λ=0.71073 Å) at a temperature of 150(2) K up to a resolution of (sin θ/λ)_max=0.65 Å⁻¹. The intensity integration was performed with the HKL2000 software. ^[35] A multi‐scan absorption correction and scaling was performed with SADABS ^[32] (correction range 0.65–0.90). A total of 8401 reflections was measured, 1203 reflections were unique (R _int=0.033), 1030 reflections were observed [I>2σ(I)]. The structure was solved with direct methods using SHELXS‐97. ^[33] Structure refinement was performed with SHELXL‐2018 ^[34] on F² of all reflections. Non‐hydrogen atoms were refined freely with anisotropic displacement parameters. All hydrogen atoms were located in difference Fourier maps. The P‐H hydrogen atom was kept fixed at the located position. All other hydrogen atoms were refined freely with isotropic displacement parameters. 86 Parameters were refined with no restraints. R1/wR2 [I>2σ(I)]: 0.0371/ 0.0971. R1/wR2 [all refl.]: 0.0454/ 0.1027. S=1.090. Residual electron density between −0.42 and 0.28 e Å⁻³. Geometry calculations and checking for higher symmetry was performed with the PLATON program. ^[18]

3 a: C₁₀H₁₈BLi₂O₂P, Fw=225.90, yellow needle, 0.60×0.24×0.09 mm³, monoclinic, P2₁/n (no. 14), a=12.4001(9), b=7.2198(7), c=14.7720(8) Å, β=94.983(3) °, V=1317.47(17) ų, Z=4, D_x=1.139 g cm⁻³, μ=0.19 mm⁻¹. The diffraction experiment was performed on a Nonius KappaCCD diffractometer with rotating anode and graphite monochromator (λ=0.71073 Å) at a temperature of 125(2) K up to a resolution of (sin θ/λ)_max=0.65 Å⁻¹. The crystal appeared to be twinned with a twofold rotation about hkl=(1,0,‐1) as twin operation. Consequently, two orientation matrices were used for the integration with the Eval15 software. ^[36] A multi‐scan absorption correction and scaling was performed with TWINABS ^[32] (correction range 0.64–1.00). A total of 16125 reflections was measured, 3014 reflections were unique (R _int=0.068), 2693 reflections were observed [I>2σ(I)]. The structure was solved with direct methods using SIR97. ^[37] Structure refinement was performed with SHELXL‐2018 ^[34] on F² of all reflections based on an HKLF‐5 file. ^[38] Non‐hydrogen atoms were refined freely with anisotropic displacement parameters. All hydrogen atoms were located in difference Fourier maps. The B‐H hydrogen atoms were refined freely with isotropic displacement parameters. C−H hydrogen atoms were refined with a riding model. 160 Parameters were refined with no restraints. R1/wR2 [I>2σ(I)]: 0.0447/ 0.1255. R1/wR2 [all refl.]: 0.0524/ 0.1313. S=1.264. Twin fraction BASF=0.507(3). Residual electron density between −0.29 and 0.45 e Å⁻³. Geometry calculations and checking for higher symmetry was performed with the PLATON program. ^[18]

4 e: C₁₂H₃₅BNPSi₂, Fw=291.37, colorless block, 0.60×0.60×0.42 mm³, monoclinic, P2₁/c (no. 14), a=17.1577(4), b=12.5115(3), c=18.7430(6) Å, β=99.706(1) °, V=3965.94(18) ų, Z=8, D_x=0.976 g cm⁻³, μ=0.25 mm⁻¹. The diffraction experiment was performed on a Nonius KappaCCD diffractometer with rotating anode and graphite monochromator (λ=0.71073 Å) at a temperature of 150(2) K up to a resolution of (sin θ/λ)_max=0.65 Å⁻¹. The intensity integration was performed with the Eval15 software. ^[36] A multi‐scan absorption correction and scaling was performed with SADABS ^[32] (correction range 0.74–0.90). A total of 103227 reflections was measured, 9124 reflections were unique (R _int=0.032), 7809 reflections were observed [I>2σ(I)]. The structure was solved with direct methods using SHELXS‐97. ^[33] Structure refinement was performed with SHELXL‐2018 ^[34] on F² of all reflections. The asymmetric unit contains two independent molecules. Non‐hydrogen atoms of the first molecule were refined freely with anisotropic displacement parameters. The second molecule was refined with a disorder model (ratio 0.87:0.13 between the components). The major disorder form was refined anisotropically, the minor form isotropically. The B−H hydrogen atoms were located in difference Fourier maps and refined freely with isotropic displacement parameters. The C−H hydrogen atoms were introduced in calculated positions and refined with a riding model. 395 Parameters were refined with 455 restraints (distances and angles for handling the disorder). R1/wR2 [I>2σ(I)]: 0.0321/ 0.0832. R1/wR2 [all refl.]: 0.0396/ 0.0885. S=1.041. Residual electron density between −0.19 and 0.35 e Å⁻³. Geometry calculations and checking for higher symmetry was performed with the PLATON program. ^[18]

Deposition Numbers 2001504, 2001505, 2001507, and 2001506 (2 d, 2 e, 3 a, 4 e, respectively) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

G. B. de Jong, N. Ortega, M. Lutz, K. Lammertsma, J. C. Slootweg, Chem. Eur. J. 2020, 26, 15944.