N-(2,4,6-trimethylborazinyl)-substituted phosphanes, arsanes, and stibanes

FULL PAPER

DOI: 10.1002/ejic.200601001

N-(2,4,6-Trimethylborazinyl)-Substituted Phosphanes, Arsanes, and Stibanes^[‡]

Heinrich Nöth,*^[a]Birgit Gemünd,^[a]and Robert T. Paine^[b]

Keywords: Boron / Phosphanes / Arsenic / Antimony

Reaction of the N-lithioborazine LiH₂N₃B₃Me₃·OEt₂(2) in di-

ethyl ether with PCl₃or PBr₃leads to the borazinylphos-

phanes (Me₃B₃N₃H₂)PX₂, (Me₃B₃N₃H₂)₂PX (X = Cl, Br), and

(Me₃B₃N₃H₂)₃P, depending on the initial stoichiometry. The

analogous arsane and stibane derivatives were obtained in a

similar manner. While (Me₃B₃N₃H₂)₂PBr is reduced by

LiAlH₄in diethyl ether/hexane to give the monophosphane

(Me₃B₃N₃H₂)₂PH (13), the reaction of (Me₃B₃N₃H₂)PBr₂(5)

with LiAlH₄in diethyl ether/hexane produces the zwitter-

ionic compound [Me₃B₃N₂(HN)]^–P⁺H₂Et (14). Dehalogena-

tion of (Me₃B₃N₃H₂)₂PBr (6) with Na yields the diphosphane

(Me₃B₃N₃H₂)₂PP(H₂N₃B₃Me₃)₂(18), while the mixed diphos-

phane (Me₃B₃N₃H)₂PPmes₂(17) is obtained from 6 and

LiPmes₂. Dehalogenation of Me₃B₃N₃H₂PBr₂with Na results

in the exclusive formation of (Me₃B₃N₃H₂)₄P₄(19), while the

reaction of 5 with Li₂Pmes leads to a mixture of cyclotet-

raphosphanes (mes)_4–n(Me₃B₃N₃H₂)_nP₄(A–F) along with bi-

cyclic P₄mes₂. The new compounds have been characterized

by NMR and IR spectroscopy and partly by mass spectrome-

try. X-ray structures for compounds, 4, 5, 6, 9, 13, 14, 17, 18,

19, and E have been determined. All show different B–N

bond lengths within the borazine rings, and some have bor-

azine rings in a half-chair conformation.

Germany, 2007)

Introduction

which act as catalysts, because they will change the catalytic

properties of triarylphosphane-based catalysts. Here, we de-

scribe the synthesis, structures, and reactivity of some of

the heavier group 15 element 2,4,6-trimethylborazinyl com-

pounds, with an emphasis on phosphane derivatives.

The deprotonation of borazines R₃B₃N₃H₃by organo-

lithium reagents was first described by Wagner and Brad-

ford,^[2]although none of the resulting N-lithioborazines

were isolated and characterized Ϫ their formation was

based on the observation that the products reacted with

alkyl halides (RЈX) to generate borazines of the type

R₃B₃N₃H_3–nRЈ_n(n = 1–3), thereby indicating that the lithi-

ation is not specific and yields a mixture of N-lithioboraz-

ines. Recently, we reported the isolation and structural char-

acterization of ligand-stabilized N-monolithioborazines

R₃B₃N₃H₂Li·L (e.g. R = Me, iPr, tBu, Ph; L = Et₂O, thf,

tmeda, pmtda).^[3,4]These proved to be versatile reagents for

the preparation of hitherto unknown borazine derivatives

of, for example, B, Al, Ge, or Ti.^[5,6]Here, we describe bora-

zinyl-substituted phosphanes, arsanes, and stibanes. Known

P-containing borazines are derivatives of phosphorus esters

in which either (RO)₂P(O)₂groups are bonded to the B

atoms^[7]or an (RO)PO₃group connects two borazine units

through a B–O–B bond.^[8]Other P-containing borazine de-

rivatives include the interesting borazine cations [Cl_3–nB₃-

(N=PPh₃)₃]⁺ⁿ(Cl^–)_n(n = 1, 2, 3).^[9,10]However, no borazine

derivatives have been reported to date where R₂P groups

are bonded to either B or N. These may be interesting li-

gands for transition metal complexes, particularly those

Results and Discussion

(2,4,6-Trimethylborazinyl)-halogeno-phosphanes, -arsanes,

and -stibanes

Synthesis

2,4,6-Trimethylborazine (1) does not react with PCl₃or

PBr₃in the presence of triethylamine to form borazinyl-

halogeno-phosphanes (Me₃B₃N₃H₂)_nPX_3–n(n = 1, 2, 3; X

= Cl, Br) as the ring N atoms are not sufficiently nucleo-

philic to attack the P atoms of the halogenophosphanes to

allow NEt₃-assisted HX elimination. This contrasts with

the reactivity of dialkylamines with phosphorus ha-

lides.^[11,12]We therefore studied the reactions of phosphorus

trichloride and tribromide, and their arsenic and antimony

analogs, with the diethyl ether adduct of 1-lithio-2,4,6-tri-

methylborazine (2) in diethyl ether solution. Depending on

the molar ratios employed, we obtained the borazinyldi-

halogenophosphanes 3 and 5, as well as the bis(borazinyl)-

halogenophosphanes 4 and 6 in acceptable yields (Ͼ75%).

The tris(borazinyl)phosphane 11 could, however, not be

prepared in pure state; it was always slightly contaminated

with 4. The borazinyl-arsanes 7, 8 and borazinyl-stibanes 9,

10 were prepared analogously. The formation of

[‡] Contribution to the Chemistry of Boron, 263. Contribution

262: Ref.^[1]

[a] Department of Chemistry and Biochemistry, University of Mu-

nich,

Butenandtstr. 5–13, 81377 München, Germany

[b] Department of Chemistry, University of New Mexico,

Albuquerque, New Mexico 87131, USA

4282

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Borazinyl-Substituted Phosphanes, Arsanes and Stibanes

(Me₃B₃N₃H₂)₃E (E = As, Sb) was observed only by NMR the bromides 5 and 6 is only 1.5 ppm but 69.0 ppm for 5

spectroscopy, and these compounds could not be isolated and 11. The corresponding ∆δ³¹P data for the series

in pure state. Compounds 4, 5, 6, and 9 were obtained as Ph_3–nPCl_n(n = 0, 1, 2) are 64 and 91.2 ppm and in the

single crystals.

(Me₂N)_3–nPCl_nseries they are 6.3 and 37 ppm, respec-

tively.^[14]These latter data are similar to those of the

borazinyl-chloro-phosphanes.

Spectra

While the shielding of the boron nuclei is only slightly

affected by the EX_2–ngroups, the methyl protons in the para

position of the borazine rings show differences of 0.06 and

0.09 ppm for 3 and 4 and for 5 and 6, respectively, in con-

trast to a difference of only 0.01 ppm for the ortho methyl

groups. This trend is also observed for the arsenic and anti-

mony compounds 7–10.

Table 1 lists the NMR spectroscopic data of compounds

3–11. Each of these compounds is characterized by two ¹¹

B

NMR signals in a 2:1 ratio. The signal at lower field results

from the B atoms adjacent to the N1(4) atom(s) carrying

the P, As, or Sb atoms (see Figures 1 and 3 for the number-

ing). The signals at higher field result from the para boron

atoms B2(5). The shielding is practically unaffected by the

number of halogen atoms at P, As, or Sb. The replacement

of P by As or Sb leads to an exceptionally small low-field

shift of 1–1.5 ppm, which is only a little bit larger for the

bromides 5 and 6 (approx. 2.6 ppm). This shows that the

charge distribution within the borazine ring is negligibly af-

fected by the N-substitution. Even the replacement of the

Li(OEt₂) group by EX₂or EX groups has little effect. The

The proton of the o-Me groups of compounds 3 to 6

couples with the ³¹P nucleus. The resulting ⁴J

coupling

³¹P,¹H

constants cover the range 3.3–5.7 Hz, with those of the di-

halides being larger than those of the monohalide species.

A similar coupling has been observed in borylamino phos-

phanes.^[15]

Table 2 contains a selection of the typical strong IR

¹¹B NMR shift of compound 2, for example, is δ = bands recorded in Hostaflon/Nujol mulls. There are two

35.4 ppm^[3]and that of the parent borazine 1 is δ = ν(NH) bands for compounds 3 and 5, while three ν(NH)

32.3 ppm.^[13]

bands are observed for 4 and 6, although either two or four

There is a ³¹P chemical shift difference of only 8.6 ppm are to be expected. The two borazinyl groups of 4 and 6 are

between the borazinyl-chloro-phosphanes 3 and 4, whereas obviously not equivalent in the solid state. All of these

this difference is 54.7 ppm for 3 and 11. Similarly, ∆δ³¹P for bands are comparatively sharp, and the bands at highest

Table 1. Chemical shifts of (trimethylborazinyl)-halogeno-phosphanes, -arsanes, and -stibanes (in ppm; solvent C₆D₆).

2·OEt₂

3

4

5

6

7

8

9

10

35.6, 37.9 36.0, 38.3 35.4, 37.8 35.5, 37.7

120.3

11

¹¹B

³¹P

δ

35.4

–

0.47, 0.59

4.66

34.6, 37.1

183.6

34.6, 37.2

175.0

36.1, 38.6

189.3

36.0, 38.8

190.9

35.7, 9.5

120.3

–

δ₁_HCH₃

δ₁_HNH

0.06, 0.66^[a]0.12, 0.65^[b]–0.01, 0.69^[c]

0.10, 0.66^[d]0.18, 0.64 0.15, 0.56 0.05, 0.52 0.11, 0.46 0.18, 0.50

4.48

4.55

4.31

4.41

4.55

4.42

4.41

4.33

4.4

4

31

1

31

1

31

1

[a] J

= 3.6 Hz. [b] J

= 3.6 Hz. [c] J

= 5.7 Hz. [d] J

= 3.3 Hz.

P, H

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4283

H. Nöth, B. Gemünd, R. T. Paine

FULL PAPER

wavenumbers are almost the same for all four compounds,

whereas the others are lower for the bromides than for the

chlorides. BN ring vibrations are found in the regions from

1500 to 1490 cm^–1and 1430 to 1380 cm^–1as strong bands.

Those of the chlorides are observed at somewhat higher

wavenumbers than those of the bromides, which suggests

an inductive effect of the halides. Only one ν(PN) band is

observed for compounds 3 and 5. As expected, there are

two PN₂bands (ν_symPN₂and ν_asymPN₂) for compounds 4

and 6 and their values suggest that the PN bonds of the

chlorides are stronger than those of the bromides. This con-

clusion is also supported by the PN bonds lengths of these

compounds (see below).

X-ray Structures

Single crystals suitable for X-ray structure determi-

nations were obtained for compounds 4, 5, 6, and 9. The

dibromoborazinylphosphane 5 crystallizes in the ortho-

rhombic space group Pnma with Z = 8, which means that

there are two independent molecules in the unit cell (see

Figure 1a). As expected, the structural parameters of the

two molecules do not differ much, but they are nevertheless

significantly different (see Table 3). Both molecules show C_s

symmetry, with the P, N, B, C, and some of the H atoms

occupying special positions on a mirror plane. The sym-

metry-related bromine atoms are perpendicularly oriented

with respect to the ring plane.

The P1–N1–B3 and P2–N4–B4 angles [119.8(6)° and

112.7(5)°, respectively] of compound 5 demonstrate that the

PBr₂groups lie closer to atoms C3 and C6, respectively.

The P–N bond lengths of molecules 5A and 5B are equal

within standard deviations. The B–N bond lengths of the

Figure 1. (a) Molecular structure of compound 5 showing the two

independent molecules 5A (left) and 5B (right). CH hydrogen

atoms have been omitted for clarity. The bonding parameters are

listed in Table 3. (b) Packing of molecules 5A and 5B. These mole-

cules are arranged in strings of alternating 5A and 5B.

borazine units show distortions relative to the parent mole- B3A = 3.971, B6A–B2A = 3.802, N5A–N3A = 3.940, and

cule Me₃B₃N₃H₃.^[16]For example, the P substituent induces N2A–N6A = 3.714 Å. This leads to an average centroid–

a significant B–N bond lengthening for the B–N bonds in- centroid distance of 3.87 Å, which is larger than the dis-

volving the P-bonded atoms N1 and N4. The remaining B– tance between the BN planes of hexagonal boron nitride

N bonds are shorter than those observed for borazines of (3.33 Å). There are no NH···Br interactions and no close

the type (RB=NH)₃(1.42–1.44 Å).^[17]The B–N–B angles at Br···Br contacts.

atoms N1 and N4 [119.8(6)° and 118.6(6)°, respectively] are

The structure of 5 can best be compared with that of

more acute than all other B–N–B bond angles [125.5(6)– the dichlorostibane 9, which crystallizes in the monoclinic

126.7(6)°] while all N–B–N bond angles are smaller than system, space group P2₁/n, also with Z = 8. Relevant struc-

120° (by an average of 4°).

tural parameters are summarized in Table 3. The B–N

Figure 1 (b) shows a plot of the packing of the two inde- bonds to the Sb-substituted atoms N1 and N4 are signifi-

pendent molecules in the unit cell. This packing leads to an cantly longer than all other B–N bonds in the ring, and the

identical orientation of the bromine atoms of 5A and 5B in B–N(1,4)–B bond angles are smaller (av. 120.95°) than all

the stacks of packed borazine rings along the b-axis. These other B–N–B bond angles (av. 125.6°). The Cl–Sb–Cl bond

stacks are formed by alternating 5A and 5B molecules. The angles of the two independent molecules differ by 4°, but

closest interatomic distances between the stacks are: B4– the two pairs of Cl–Sb–N bond angles differ only by 2.7°.

Table 2. Selected IR data [cm^–1] for compounds 3–6, 8, 9, and 10.

Compound

ν(NH)

ν(ring)

ν(EN)

ν(EX)

(Me₃B₃N₃H₂)PCl₂

(Me₃B₃N₃H₂)₂PCl

(Me₃B₃N₃H₂)PBr₂

(Me₃B₃N₃H₂)₂PBr

(Me₃B₃N₃H₂)₂AsCl

(Me₃B₃N₃H₂)SbCl₂

(Me₃B₃N₃H₂)₂SbCl

3429, 3377

3431, 3411, 3402

3423, 3414

3430, 3401, 3391

3419, 3411

3421, 3406

1499, 1402

1295

1285

1290

1282

1285

1262

1261

982, 960

973

975, 940

962

920

889, 802

803

1487, 1435, 1396

1487, 1432, 1394

1483, 1395

1478, 1397

1474, 1407, 1318

1473, 1449, 1400

3438, 3418

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Eur. J. Inorg. Chem. 2007, 4282–4297

Borazinyl-Substituted Phosphanes, Arsanes and Stibanes

Table 3. Selected bond lengths [Å] and bond angles [°] for Me₃B₃N₃H₂PBr₂(5) and Me₃B₃N₃H₂SbCl₂(8).

5A

5B

9A

9B

P1–Br1

2.261(1)

P2–Br2

2.307(2)

Sb1–Cl1

Sb1–Cl2

Sb1–N1

N1–B1

B1–N2

N2–B2

B2–N3

N3–B3

2.387(1)

2.381(1)

2.022(3)

1.470(5)

1.425(6)

1.421(8)

1.430(6)

1.425(5)

1.465(5)

97.50(9)

100.27(9)

91.63(4)

126.0(3)

120.9(3)

113.1(2)

126.6(4)

125.4(4)

115.9(4)

114.3(4)

116.8(3)

Sb2–Cl3

Sb2–Cl4

Sb2–N4

N4–B4

B4–N5

N5–B5

B5–N6

N6–B6

2.365(1)

2.375(1)

2.027(3)

1.455(5)

1.429(5)

1.425(6)

1.412(6)

1.424(6)

1.465(5)

99.73(9)

97.52(9)

95.40(4)

126.5(2)

121.0(3)

112.4(3)

121.0(3)

125.3(4)

116.5(3)

115.2(4)

116.9(4)

P1–N1

1.682(5)

1.482(9)

1.419(9)

1.416(9)

1.422(9)

1.411(9)

1.486(9)

P2–N4

1.674(5)

1.506(9)

1.411(9)

1.406(9)

1.416(9)

1.386(9)

1.495(9)

105.89(1)

N1–B1

B1–N2

N2–B2

B2–N3

N3–B3

B3–N1

N1–P1–Br1

N4–B4

B4–N5

N5–B5

B5–N6

N6–B6

B6–N4

N4–P2–Br2

B3–N1

B6–N4

104.4(1)

N1–Sb1–Cl1

N1–Sb1–Cl2

Cl1–Sb1–Cl2

Sb1–N1–B1

B1–N1–B3

Sb1–N1–B3

B1–N2–B2

B2–N3–B3

N1–B1–N2

N2–B2–N3

N3–B3–N11

N4–Sb2–Cl3

N4–Sb2–Cl4

Cl3–Sb2–Cl4

Sb2–N4–B4

B4–N4–B6

Sb2–N4–B6

B4–N5–B5

B4–N6–B6

N4–B4–N5

N5–B5–N6

N6–B6–N4

Br1–P1–Br1A

P1–N1–B1

B1–N1–B3

P1–N1–B3

B1–N2–B2

B2–N3–B3

N1–B1–N2

N2–B2–N3

N3–B3–N1

99.86(8)

113.2(4)

119.8(6)

125.5(6)

126.5(6)

117.0(6)

115.0(6)

116.1(6)

Br2–P2–Br2A

P2–N4–B4

B4–N4–B6

P2–N4–B6

B4–N5–B5

B4–N6–B6

N4–B4–N5

N5–B5–N6

N6–B6–N4

98.99(8)

118.8(6)

112.7(5)

126.7(6)

126.6(6)

115.8(7)

115.0(6)

117.0(6)

The packing diagram of the molecules is depicted in part

Figure 3 depicts the molecular structure of the bis(boraz-

b of Figure 2, which reveals that the molecules are stacked inyl)chlorophosphane 4. This compound crystallizes with

on top of each other with the SbCl₂groups oriented in op- two molecules of toluene,^[18]which are site disordered. The

¯

posite directions. The planar borazine rings are twisted triclinic space group is P1 with Z = 2. Relevant structural

against each other by 7° and are arranged perpendicularly parameters are summarized in Table 4 together with the

to the respective SbCl₂planes. Although the orientation of data for the corresponding bromide 6 (Figure 4). The con-

the Sb atoms with respect to the neighboring molecules sug- figuration at the P atom is pyramidal (sum of bond angles:

gests a higher coordination number than 3, the shortest 303.5°). This represents partial sp³hybridization similar to

intermolecular Sb–N distance is 3.693 Å, which is too long that found in triaminophosphanes.^[19]The P–Cl bond

to be considered as a weak bond (sum of van der Waals length [2.307(1) Å] of 4 is longer than that in PCl₃

radii: 3.8 Å). Even the intermolecular Sb–Cl interaction is [2.02(2) Å]^[20]and is also significantly longer than in most

weak because the respective Sb–Cl distance (3.310 Å) is aminochlorophosphanes (2.109–2.197 Å).^[21]The largest

only 0.05 Å shorter than the sum of the van der Waals radii. angle at the P1 atom is N1–P1–N4 (106.7°), while the angles

N1–P1–Cl1 and N4–P1–Cl1 are only 100.3(1)° and

96.4(1)°, respectively. The opening of the N1–P1–N4 bond

angle is most likely due to the steric demand of the trimeth-

ylborazinyl groups. The rings of these groups are not

planar, as shown by the position of the atoms N1 and N4,

which are bent out of the ring plane by 17° and 7°, respec-

tively. The consequence of this ring distortion is that the

methyl groups are also bent away from the ring plane in the

opposite direction of the N1(4) atoms. Atoms C1 and C3

are more strongly affected, as shown by the value of 10° for

Figure 2. (a) The molecular structure of the two independent mole-

cules in (Me₃B₃N₃H₂)SbCl₂(9). Selected bonding parameters are

listed in Table 3. CH hydrogen atoms have been omitted. (b) Inter-

action of one of the two independent molecules of 9. Dotted lines

represent the closests intermolecular interactions.

Figure 3. The molecular structure of compound (Me₃P₃N₃H₂)₂PCl

(4). Hydrogen atoms and the disordered toluene molecule have

been omitted. See Table 4 for selected molecular parameters.

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H. Nöth, B. Gemünd, R. T. Paine

FULL PAPER

C1 and 13° for C3 (C4 and C6 are displaced only by 5° and

The two borazine rings are twisted against each other by

9°, respectively). Nevertheless, the sum of the bond angles 51°. One of the borazine rings (N1 to B3) is planar, and the

at atoms N1 and N4 is 360°. It is also worthy to note that P1 atom is coplanar with this ring. The other borazine ring

the B1–N1–P1 bond angle is 114.5(3)°, which contrasts (N4 to B6) shows a half-chair conformation, with the plane

with the value of 126.2° for the angle B3–N1–P1 [similarly B3–N4–B6 forming an angle of 15.9° with the B3–N5–B5–

B4–N3–P1 = 113.6(3)° and B6–N4–P1 = 125.7(3)°].

N6–B6 plane. Atoms C4 and C6 are placed below the plane

of these five atoms. As observed in 4, the N5–B4 and N6–

B6 bonds are no longer parallel to one another, as would

be expected for a planar borazine moiety, but are twisted

against each other by 5°. This results in torsion angles of

7° for C6–B6–N5–P1 and 14° for C4–B4–N4–P1.

Once again, the B–N bonds to atoms N1 and N4 are

longer [1.475(5) and 1.469(5)°] than all other B–N bonds in

the rings. The shortest B–N bonds are B1–N2, B3–N3, B4–

N5, and B6–N6, while those to the p-B atoms correspond

best with those of symmetrically substituted borazines

[1.418(6) Å].^[17]The N–B–N bond angles vary from 113.9°

to 115.8(3)°. While the bond angles B–N(1,4)–B are close

to 120°, all other B–N–B bond angles are on average 126.7°.

The P–Br bond length is 2.333(1) Å, which is 0.07 Å longer

than in the dibromophosphane 5.

Table 4. Selected bonding parameters for the bis(trimethylboraz-

inyl)halogenophosphanes 5 (X = Cl) and 6 (X = Br).

4

6

4

6

P1–X

P1–N1

B1–N1

B1–N2

B2–N2

B2–N3

B3–N3

B3–N1

X–P1–N1

N1–P1–N4

P1–N1–B1

P1–N1–B3

B1–N1–B3

B1–N2–B2

B2–N3–B3

N1–B1–N2

N3–B3–N1

2.307(2) 2.334(1)

1.702(4) 1.704(3)

1.474(7) 1.475(5)

1.423(6) 1.410(6)

1.405(7) 1.418(5)

1.425(8) 1.416(6)

1.437(6) 1.414(5)

1.481(7) 1.469(5)

100.3(1) 99.2(1)

106.7(2) 106.2(1)

114.5(3) 126.5(2)

126.2(3) 112.9(2)

119.1(4) 120.5(3)

125.9(5) 126.9(4)

125.6(5) 126.5(4)

116.6(5) 115.8(3)

115.2(5) 116.3(3)

P1–N4

B4–N4

Be4–N5

B5–N5

B5–N6

B6–N6

B6–N4

X–P1–N4

1.698(4) 1.717(3)

1.463(6) 1.475(5)

1.422(7) 1.419(5)

1.422(7) 1.420(6)

1.424(7) 1.425(6)

1.425(7) 1.415(5)

1.481(6) 1.469(5)

96.4(1)

101.1(1)

N1–P1–N4 108.7(4) 106.2(2)

P1–N4–B4 125.7(3) 126.6(2)

P1–N4–B6 120.6(4) 114.0(2)

B4–N4–B6 126.1(4) 118.9(3)

B4–N5–B5 125.8(4) 115.6(3)

B5–N6–B6 126.1(4) 126.3(4)

N5–B5–N6 114.5(4) 113.0(4)

N6–B6–N4 116.0(4) 116.6(3)

Reactions of the Bromo(trimethylborazinyl)phosphanes with

Lithium Tetrahydridoaluminate

The reaction of the bromides 5 and 6 with LiAlH₄should

allow the synthesis of the corresponding phosphanes,

(Me₃B₃N₃H₂)PH₂and (Me₃B₃N₃H₂)₂PH, as shown in

Equations (3) and (4), although it is known that amino-

phosphanes (R₂N)_2–nPH_n+1are prone to undergo amine eli-

mination.^[22]

As shown in Equation (4), the reaction of 6 with LiAlH₄

in diethyl ether/hexane gives the compound bis(trimethyl-

borazinyl)phosphane (13) in 75% yield. In contrast, the ex-

pected reaction [Equation (3)] between the dibromophos-

phane 5 and LiAlH₄in the same solvents takes a different

course. Thus, after removing the solid formed, the filtrate

Figure 4. ORTEP plot of the molecular structure of (Me₃B₃N₃H₂)₂- was found to be free of bromide. The crystals that separated

PBr (6). Selected bonding parameters are listed in Table 4. CH hy-

from the concentrated filtrate within a week were found to

drogen atoms have been omitted.

exhibit three ¹¹B NMR signals at δ = 34.1, 35.3, and

36.8 ppm (in a ratio of about 1:1:1) but only a single ³¹P

Amongst the B–N–B bond angles, those at atoms N1 and NMR signal as a triplet at δ = 6.5 ppm (¹J

= 430 Hz)

³¹P,¹H

N4 are almost identical [119.1(4)° and 120.6(4)°, respec- when dissolved in C₆D₆(Figure 5, a). These data give evi-

+

tively]. The N–B–N bond angles to the B atoms opposite dence for the presence of a phosphonium R(X)PH₂unit,

1

to N1 and N4 are only 114.7(4)° and 114.5(4)°, respectively, and this was confirmed by the H NMR spectrum, which

and are thus the smallest N–B–N bond angles of the two shows two doublets (see Figure 5, b).

borazine moieties. In contrast, the B1–N2–B2 and B4–N5–

B5 angles are quite wide [125.9(4)°]. The two borazine rings triplet, while the other doublet is split into a quartet (see

are twisted against each other by 56.6°. Figure 5, b). This gives convincing evidence that an ethyl

The signal of one of the doublets is further split into a

Compound 6 crystallizes free of solvent molecules in the group is attached to the P atom and that the solid is defi-

orthorhombic system, space group Pbca, with Z = 8. It is nitely not compound 12, as would be expected from Equa-

thus not isomorphous with 4 although the structural pa- tion (3). At this stage we assumed that the compound was

rameters of its trimethylborazinyl units are similar to those most likely the zwitterionic phosphonium borazinate 14.

1

³¹P,¹H

of 4. This can readily be seen by comparing the data in The J

coupling constant for the doublet of quartets is

1

³¹P,¹H

Table 4. The sum of bond angles at the P1 atom is 306.4°, 380 Hz, while the J

coupling constant of the doublet

and, in analogy to 4, the largest angle at the P atom is of triplets is 370 Hz. Each of these resonances displays a

3

106.2(1)° for N1–P1–N4.

further coupling of 7 Hz corresponding to J₁_H,₁_H. Neither

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Borazinyl-Substituted Phosphanes, Arsanes and Stibanes

to compound 14. This reaction sequence is unique as no

zwitterionic borazine is yet known and nor is a noncoordi-

nated singly deprotonated borazine ring.

X-ray Structures

The monophosphane 13 crystallizes from hexane as col-

orless needles (orthorhombic, space group Pbca with Z =

8). Its molecular structure is shown in Figure 6 and struc-

tural parameters are listed in Table 5. Its P–N bonds are

significantly longer than those in compounds 4, 5, and 6.

The sum of bond angles at the P1 atom is 311°, which is a

few degrees larger than in any of the borazinylphosphanes

described previously. This results primarily from an N1–

P1–H bond angle of 102.5(9)°. On the other hand, the N1–

P1–N4 bond angle is 105.7(1)°, which corresponds with

those found in 4 and 6. The angles at the P1 atom are, in

general, smaller than those found for aminophosphanes.^[19]

1

Figure 5. (a) ³¹P NMR spectrum of 14. (b) H NMR spectrum of

14.

of the signals shows a roof effect. This assignment was veri-

fied by recording a two dimensional ¹H-³¹P spectrum.^[5]

The structure of 14 was finally confirmed by an X-ray

structure determination. The ¹¹B NMR spectrum, with its

three ¹¹B NMR signals, is also in line with these arguments.

Figure 6. Molecular structure of the phosphane derivative 13. Se-

lected bonding parameters are listed in Table 5. CH hydrogen

atoms have been omitted.

The two borazine units of 13 are almost planar and are

twisted against each other by 84°, an angle that is signifi-

It is clear that the formation of 14 requires a number of cantly larger than in 4 or 6, where the angle between the

steps. One hypothesis for its formation is summarized in planes B1–N1–B3 and B1–B2–B3 is 5.2° and 8.0°, respec-

Equations (5) to (8). The crucial step here is the formation tively. The exocyclic angles at N1 are 119.0(2)° for B1–N1–

of EtBr by an ether cleavage with AlBr₃, which is a well P1 and 120.0(2)° for B3–N1–P1, while the P1–N1–B2–C2

known process. EtBr is needed to form the phosphonium torsion angle is 177°. The B4–N4–P1 and B6–N4–P1 angles

salt shown in Equation (7), which is the logical precursor are 118.2(2)° and 121.7(2)°, respectively, whereas the P1–

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4287

H. Nöth, B. Gemünd, R. T. Paine

FULL PAPER

Table 5. Selected bonding parameters for (Me₃B₃N₃H₂)₂PH (13) and (Me₃B₃N₃H)PH₂Et (14).

13

14

P1–N1

P1–H1

N1–B1

B1–N2

N2–B2

B2–N3

N3–B3

B3–N1

N1–P1–N4

N4–P1–H1

B1–N1–P1

B3–N1–P1

B1–N1–B3

B1–N2–B2

B2–N3–B3

N1–B1–N2

N2–B2–N3

N3–B3–N1

1.742(2)

1.42(1)

P1–N4

1.734(2)

P1–N1

P1–C4

P1–H1

P1–H2

N1–B1

B1–N3

N2–B2

B2–N3

1.694(2)

1.930(3)

1.23(4)

1.435(4)

1.429(4)

1.427(4)

1.407(5)

1.433(4)

1.464(4)

105.7(1)

102.8(9)

119.0(2)

120.0(2)

120.4(2)

124.8(3)

125.8(2)

117.3(3)

115.0(3)

116.4(3)

N4–B4

B4–N5

N5–B5

B5–N6

N6–B6

B6–N4

N1–P1–H1

1.454(4)

1.413(4)

1.415(4)

1.418(4)

1.419(4)

1.460(4)

102.5(9)

1.32(4)

1.462(3)

1.413(3)

1.439(3)

11.428(3)

1.421(3)

1.439(3)

119.5(2)

119.0(2)

121.3(2)

125.0(2)

125.3(2)

116.9(2)

114.9(2)

116.5(2)

N3–B3

B3–N1

B4–N4–P1

N6–N4–P1

B4–N4–B6

B4–N5–B5

B5–N6–B6

N4–B4–N5

N5–B5–N6

N6–B6–N4

118.2(2)

121.8(2)

120.0(2)

125.5(2)

125.4(2)

117.0(2)

114.7(3)

116.9(2)

B1–N1–P1

B2–N1–P1

B1–N1–B3

B1–N2–B2

N2–N3–B3

N1–B1–N2

N2–B2–N3

N3–B3–N1

H1–P1–H2

H1–P1–C4

H2–P1–C4

H1–P1–N1

H2–P1–N1

N1–P1–C4

P1–C4–C5

103.4(9)

114.1(9)

113.6(4)

107.3(4)

102.6(4)

114.53(9)

112.2(3)

N4–B5–C5 torsion angle is only 33°, which deviates signifi- [B1–N2 = 1.413(3), B3–N3 = 1.421(3) Å] and intermediate

cantly from the corresponding P1–N1–B2–C2 torsion an- bond lengths [N2–B2 = 1.439(3), N3–B2 = 1.428(3) Å].

gle. Once again, the B–N bonds to N1 and N4 are longer

[1.452(4), 1.464(4), 1.454(4), and 1.460(4) Å] than all other revealed weak peaks on atoms N2 and N3. They refined

B–N bonds (av. 1.43 Å). best with a site occupation factor (SOF) of 0.5 as the U_i

The difference Fourier analysis for NH hydrogen atoms

Compound 14 crystallizes in the orthorhombic space values were not compatible with an SOF of 1. Figure 7

group Pca2₁with Z = 8. Its molecular structure, depicted shows only one occupied NH position. The X-ray structure

in Figure 7, demonstrates the presence of a phosphonium analysis excludes an Et/Me exchange between the P atom

group. The bond angles at the P1 atom range from 102.6(4)° and a B atom.

(N1–P1–H2) to 114.53(9)° (N1–P1–C4). The two P–H

bonds seem to be significantly different [1.32(4) and

1.23(4) Å] and the bond angle between them is 103.4°. The

P–C bond [1.930(3) Å] is much longer than in other organ-

ophosphonium salts [d(PC) = 1.674 to 1.849 Å],^[23]and this

bond is still 0.1 Å longer than the “normal” P–C single

bond length when an electronegativity correction is ap-

plied.^[24]

Trimethylborazinyldiphosphanes and Cyclotetraphosphanes

The trimethylborazinyl-halogeno-phosphanes appeared

to be precursors for borazinyldiphosphanes and cyclophos-

phanes because it is well known that the dehalogenation of

di- and monoorgano-halogeno-phosphanes leads to diphos-

phosphanes P₂R₄and cyclophosphanes (RP)_n, respectively.

Diphosphanes can also be prepared by the reaction of

R₂PX with Me₃SiPR₂. However, no reaction was observed

on treating the bromide 6 with Me₃SiPR₂[Equation (9)]

even at reflux in toluene.

After replacement of Me₃SiPR₂with LiPmes₂, however,

good yields of the diphosphane 17 were obtained [Equa-

tion (10)]. The analogous reaction with LiPPh₂generated

16, although it could not be completely separated from ac-

companying Ph₂PPPh₂.

As the reaction shown in Equation (11) proceeded with-

out problems, we also decided to treat the dibromide 5 with

sodium in toluene. This reaction gave exclusively tetrakis-

(trimethylborazinyl)cyclotetraphosphane (19), as shown in

Figure 7. ORTEP plot of the molecular structure of the zwitterion

14. Selected bonding parameters are listed in Table 5.

The borazine ring of 14 is planar and large B–N–B bond Equation (12). This compound crystallized as strongly in-

angles at atoms N2 and N3 [125.0(2)° and 125.3(2)°, respec- tergrown needles, which were also obtained by treating 5

tively] are once again observed; the B1–N1–B2 bond angle with activated Mg or sodium naphthalide in thf.

is 121.3(2)°. The B1–N1–P1 and B2–N1–P1 bond angles

While the crystal quality of 16 was insufficient for an X-

are smaller than 120° [119.5(2)° and 119.0(2)°, respectively], ray structure determination, single crystals of 17 and 18

and the N–B–N bond angles are also acute [114.9(2)– were obtained. However, those of 17 were quite small and

116.5(2)°]. Equivalent pairs of B–N bond lengths range did not diffract too well. Nevertheless, the refinement con-

from 1.462(3) and 1.459(3) Å (to N1) and include short verged at R₁= 6.5%.

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Borazinyl-Substituted Phosphanes, Arsanes and Stibanes

We expected that the reaction of 5 with Li₂Pmes might gle but very broad ¹¹B resonance, which contrasts with the

yield the 2,4-bis(mesityl)-1,3-bis(trimethylborazinyl)cyclo- borazinyl-halogeno-phosphanes. As expected, 18 exhibits a

tetraphosphane D. However, the ³¹P NMR spectroscopic single ³¹P resonance signal at δ = 108.7 ppm, while there

data of the solution showed that this ideal composition is are two doublets at δ = 113.9 and –20.6 ppm for 17, with a

not achieved. Rather, a mixture of the cyclotetraphosphanes ¹J

coupling constant of 223 Hz. Thus, the mesityl-

³¹P,³¹

P

A–F (Scheme 1) is formed, with no evidence for cyclotri- bonded P nuclei are much better shielded than the P nuclei

phosphanes or cyclopentaphosphanes. Isolated single crys- bearing borazine rings. Two sets of proton signals are ob-

tals turned out to be the isomer 2,3-bis(mesityl)-1,4-bis(tri- served for the BMe groups in 17 and 18, with those of the

methylborazinyl)cyclotetraphosphane (E).

o-BMe protons being better shielded than the p-BMe pro-

4

³¹P,¹H

tons. The former appear as doublets due to J

coup-

ling, whereas no such coupling is observed for the o-Me

groups of the mesityl substituents.

The NMR spectra of compound 19 are rather simple,

with only one signal appearing in each of the ¹¹B and ³¹P

NMR spectra. The ¹¹B NMR signal is rather broad. Since

1

there are also single sets of H NMR signals for the o-BMe

and p-BMe groups, this suggests that the compound has

an all-trans conformation. The o-BMe protons appear as

4

³¹P,¹H

doublets with a J

coupling constant of 5.2 Hz. These

data also demonstrate that no cyclotriphosphane or cyclo-

pentaphosphane is generated upon dehalogenation of 5.

In contrast, the NMR spectra of the solution obtained

by treating 5 with Li₂Pmes are surprisingly complex. There

are many ³¹P NMR signals in the range from δ = –60 to

–300 ppm, which indicates the presence of cyclotetraphos-

phanes. Most of these signals could be assigned. Thus,

the weak triplets at δ = –171.2 and –299.0 ppm are due to

the presence of cis-bis(mesityl)bicyclo[1.1.0]tetraphos-

Scheme 1. Isomers of dimesitylbis(trimethylborazinyl)cyclotetra-

phosphane.

NMR Spectra

Table 6 lists the chemical shifts and coupling constants of phane,^[24]which is also present as its trans-isomer (δ =

the nuclei ¹H, ¹¹B, and ³¹P for the diphosphane derivatives. –292.2 ppm (dd) and two triplets at δ = –155.7 and

Surprisingly, both diphosphanes 17 and 18 show only a sin- –161.2 ppm).^[24]

Table 6. NMR spectroscopic data for compounds 17, 18, 19 and E. Half width (Hz) in parentheses.

Compound

δ¹¹B

δ³¹P

δ¹H

o-BMe

δ¹H

p-BMe

δ¹H

NH

δ¹H

p-CMe

δ¹H

o-CMe

δ¹H

CH

17

18^[c]

19

E

36.4 (1250)

36.3 (1260)

37.1 (1225)

36.8 (1300)

113.9,^[a]–20.6^[a]0.87^[b]

0.13

0.15

0.05

0.04

4.39

4.49

4.48

2.03

–

2.56

–

6.67

–

108.7

63.0

0.84

0.87^[d]

0.99^[e]

60.9, –49.2

2.80

2.71^[f]

1

4

31

1

31

1

31

1

31

1

³¹P,³¹

P

P, H

[a] J

= 223 Hz. [b] J

= 6.8 Hz. [c] bor = Me₃B₃N₃H₂. [d] J

= 5.2 Hz. [e] J

= 5.75 Hz. [f] J

= 3.57 Hz.

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4289

H. Nöth, B. Gemünd, R. T. Paine

FULL PAPER

Fractional crystallization provided crystals of the isomer pected for an A₂X₂spin system. This is indeed observed,

E. A solution of these crystals shows a single but rather with two triplets at δ = 36.9 and –7.8 ppm (¹J

=

³¹P,³¹

P

broad ¹¹B NMR signal with h_1/2= 1300 Hz. This may well 170 Hz). The P2 atom of ring C should give an A₂X type

result from two overlapping ¹¹B signals because the ¹H spectrum, in other words a triplet, which is found at δ =

NMR spectra clearly show the presence of two types of 77.3 ppm. Another triplet at δ = 32.5 ppm, with a coupling

BMe, NH, CMe, and CH signals in the expected intensity constant of 232 Hz, can be assigned to the atom P4. Finally,

ratios. Both the o-BMe and o-CMe protons appear as dou- a doublet of doublets is observed at δ = –26.4 ppm repre-

4

³¹P,¹H

blets due to J

coupling.

senting atoms P1 and P3. The same signal types should

A two-dimensional ³¹P NMR spectrum^[5]and high-reso- result from the presence of isomer F. The respective chemi-

lution ³¹P NMR spectra of the reaction solution give evi- cal shifts are: a triplet at δ = –30.5 ppm for P2, a doublet

dence for the presence of all isomers of the cyclotetraphos- of doublets at δ = 41.4 ppm for P1 and P3, and a triplet

phanes A–F. The rings A and B should lead to an A₄spin at δ = 74.9 ppm for P4. The coupling constant is 168 Hz.

system (³¹P singlet for the all-trans conformer). The singlet Examples of the observed and calculated ³¹P NMR spectra

at δ = 63.0 ppm demonstrates the presence of isomer A, and are displayed in Figure 8. The program NUTS^[36]was used

another singlet at δ = –43.2 ppm stems from compound B. to obtain the calculated spectra.

Ring E should be represented by an AAЈXXЈ spin system

The mes and borazinyl substituents (or PX groups)

and should therefore show a higher order spectrum, which, therefore appear to have exchanged during the course of

in a first approximation, could appear as a doublet of dou- reaction to give a more or less random distribution of the

blets. This doublet of doublets can indeed be observed at δ cyclotetraphosphanes.

= 60.9 and –49.2 ppm and these signals show additional

signals at higher resolution to give a spectrum of higher

X-ray Structures

The tetrakis(trimethylborazinyl)diphosphane 18 crys-

tallizes from toluene as 18(C₆H₅Me)₂in the triclinic space

group P1; its molecular structure is depicted in Figure 9

order. Ring D should give a first-order spectrum, as ex-

¯

and relevant bonding parameters are listed in Table 7. The

borazinyl units in this compound are present in a staggered

conformation. The dihedral angles of opposite rings are

164° for both ring pairs while the dihedral angles of adja-

cent rings differ by 30°. The N4–P1–P2 angle is 108.4(2)°

while the N1–P1–P2 angle exceeds this angle by 7.8°. A

twist of 32° is found for the borazinyl rings containing N1

and N7, and this value is even larger for N4 and N10 (97°).

While the borazine rings containing N1 and N7 are planar,

those including atoms N4 and N10 are slightly bent into a

half-chair conformation, with N4 bent out of the plane by

11° and N10 by 9.2°. The BMe groups at the ortho positions

are bent backwards, with the respective NBC angles rang-

ing from 124.3(7)° to 127.4(7)°. Additional bonding param-

eters are listed in Table 7.

Figure 8. (a) Observed (bottom) and calculated (top) ³¹P NMR

spectra for the isomer 1,3-diborazinyl-2,4-dimesitylcyclotetraphos-

phane. (b) Observed (bottom) and calculated (top) ³¹P NMR spec-

trum of the isomer 1,2-diborazinyl-3,4-dimesitylcyclotetraphos-

phane (E).

Figure 9. The molecular structure of tetrakis(trimethylborazinyl)di-

phosphane (18). The H atoms of the methyl groups are not shown.

Selected bonding parameters are listed in Table 7.

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Borazinyl-Substituted Phosphanes, Arsanes and Stibanes

Table 7. Selected bonding parameters for P₂(H₂N₃B₃Me₃)₄(18).

P1–P2

P1–N1

B1–N1

B1–N2

N2–B2

B2–N3

N3–B3

B3–N1

P2–P1–N1

N1–P1–N4

P1–N1–B1

P1–N1–B3

B1–N1–B3

B1–N2–B2

B2–N3–B3

N1–B1–N2

N2–B2–N3

N3–B3–N1

2.287(2)

1.735(6)

1.44(1)

1.42(1)

1.45(1)

1.41(1)

1.49(1)

94.3(2)

101.8(5)

126.6(5)

115.4(5)

118.0(7)

126.21(7)

124.4(7)

117.5(7)

114.3(8)

119.4(7)

P1–N4

B4–N4

B4–N5

N5–B5

B5–N6

N6–B6

B6–N4

P2–P1–N4

1.748(5)

1.462(9)

1.418(9)

1.430(9)

1.43(1)

1.430(9)

1.468(9)

108.5(2)

P2–N7

B7–N7

B7–N8

N8–B8

B8–N9

N9–B9

B9–N7

P1–P2–N7

N7–P2–N10

P2–N7–B7

P2–N7–B9

B7–N7–B9

B7–N8–B8

B8–N9–B9

N7–B7–N8

N8–B8–N9

N9–B9–N7

1.755(6)

1.472(9)

1.42(1)

1–40(1)

1.42(1)

P2–N10

1.740(5)

1.461(9)

1.45(1)

B10–N10

B10–N11

N11–B11

B11–N12

N12–B12

B12–N10

P1–P2–N10

1.40(1)

1.443(9)

1.433(9)

1.459(9)

109.1(2)

1.452(9)

94.8(2)

102.7(3)

113.6(5)

126.7(5)

119.6(5)

126.1(4)

125.6(5)

116.6(6)

114.4(6)

117.7(5)

P1–N4–B4

P1–N4–B7

B4–N4–B6

B4–N5–B5

B5–N6–B6

N4–B4–N5

N5–B5–N6

N6–B6–N4

130.1(5)

111.3(5)

118.6(6)

126.4(7)

117.6(6)

113.3(7)

117.6(6)

P2–N10–B10

P2–N10–B12

B10–N10–B12

B10–N11–B11

B11–N12–B12

N10–B10–N11

N11–B11–N12

N12–B12–N10

111.8(5)

130.6(5)

117.5(5)

124.7(5)

123.5(6)

118.6(5)

115.5(5)

119.5(5)

Figure 10 shows the molecular structure of the “mixed” [2.268(1) Å], which may be a steric effect because the P–N

diphosphane 17, and Table 8 contains its bonding param- bond in 18 is about 0.03 Å shorter than in 17.

eters. The P–P bond in 18 [2.287(1) Å] is longer than in 17

The first fraction to crystallize from a hexane solution

obtained by treating Me₃B₃N₃H₂PBr₂with Li₂Pmes proved

to be uniform when inspected by microscopy. The crystals

¯

are monoclinic, space group P1, with Z = 2. There is also

one molecule of hexane in the unit cell. In a first attempt

we solved the structure by assuming that the crystal might

have the composition P₄(H₂N₃B₃Me₃)₄. The structure solu-

tion indeed revealed a cyclotetraphosphane ring, with each

of the four P atoms carrying a six-membered ring. The

structure refined to R = 6.6% with attached B₃N₃rings.

However, the thermal parameters and bond lengths clearly

showed that this assumption was wrong as there are two

long P–N bonds and two shorter ones, and the B–C bond

lengths are also shorter for those rings with “long” P–N

distances. This result suggests that these latter rings are

mesityl rings rather than borazines. In this case the R value

improved to R₁= 5.99%. The result of this latter refinement

Figure 10. Molecular structure of P,P-dimesityl-PЈ,PЈ-bis(tri- is shown in Figure 11, where it is clear that the crystal con-

methylborazinyl)diphosphane (17). Hydrogen atoms, with the ex-

tains species E. Selected bonding parameters are summa-

ception of the NH hydrogens, have been omitted. Selected bonding

rized in Table 9.

parameters are listed in Table 8.

Table 8. Selected bonding parameters for (Me₃B₃N₃H₂)₂P-P(C₆H₂Me₃)₂(17).

P1–P2

P1–N1

N1–B1

B1–N2

N2–B2

B2–N3

N3–B3

B3–N1

P2–P1–N1

N1–P1–N4

B1–N1–B3

B1–N2–B2

B2–N3–B3

N1–B1–N2

N2–B2–N3

N3–B3–N1

2.268(1)

1.778(2)

1.451(5)

1.415(4)

1.416(5)

1.413(5)

1.424(4)

1.454(4)

108.7(1)

102.4(1)

118.4(3)

124.1(3)

125.8(3)

119.0(3)

114.5(3)

117.1(3)

P1–N4

N4–B4

B4–N5

N5–B5

B5–N6

N6–B6

B6–N4

P2–P1–N4

1.771(3)

1.451(4)

1.416t(5)

1.412(5)

1.420(5)

1.409(5)

1.456(4)

102.4(1)

P2–C7

C7–C8

C8–C9

C9–C10

1.831(3)

1.422(4)

1.391(4)

1.401(5)

1.393(4)

1.402(4)

1.429(4)

96.3(1)

P2–C16

1.829(3)

1.420(4)

1.388(5)

1.391(4)

1.394(5)

1.393(5)

1.426(4)

110.9(9)

C16–C17

C17–C18

C18–C19

C19–C20

C20 C21

C21–C16

P1–P2–C16

C10–C11

C11–C12

C12–C7

P1–P2–C7

C7–P2–C16

C8–C7–C16

C8–C9–C10

C10–C11–C12 123.3(3)

C7–C8–C9 119.9(3)

C9–C10–C11 116.6(3)

C11–C12–C7 119.0(3)

102.1(1)

117.2(3)

122.9(3)

B4–N4–B6

B4–N5–B5

B5–N6–B6

N4–B4–N5

N5–B5–N6

N6–B6–N4

119.(3)

C17–C16–C21

C17–C18–C19

C19–C20–C21

C16–C17–C18

C18–C19–C20

C19–C20–C21

117.2(3)

123.1(3)

123.3(3)

120.0(3)

116.4(3)

123.3(3)

125.3(3)

124.9(3)

117.7(3)

114.8(3)

118.0(3)

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H. Nöth, B. Gemünd, R. T. Paine

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Discussion and Conclusion

The chemistry of borazines is well documented,^[29–31]al-

though neither B- nor N-phosphanylborazines have been

reported. We now show that (2,4,6-trimethylborazinyl)-

halogeno-phosphanes are readily available and may serve as

starting materials for N-(diorganophosphanyl)borazines. B-

Phosphanylborazines should also be accessible by treating

B-haloborazines with alkali metal organophosphides.

Because halophosphanes do not react with borazines of

the type R₃B₃N₃H₃, even in the presence of an HX ac-

ceptor, we have used the recently characterized N-mono-

lithio-trimethylborazine (2) as a stronger nucleophile to

bind the borazinyl unit to P^III, As^III, and Sb^IIIcenters. This

gives access to all members of the series (Me₃B₃N₃H₂)_n-

EX_3–n(E = P, As, Sb), although the compounds with n = 3

were not obtained in pure state. We have also found that

the borazinyl-halogeno-phosphanes are suitable reagents

for preparing the corresponding bis(borazinyl)phosphane

(13) as well as borazinyldiphosphanes or borazinylcyclo-

tetraphosphanes. However, the reaction of LiAlH₄with

Me₃B₃N₃H₂PBr₂in hexane/diethyl ether does not provide

Figure 11. The molecular structure of 3,4-dimesityl-1,2-bis(trime-

thylborazinyl)cyclotetraphosphane (E). Hydrogen atoms at the

methyl groups have been omitted. Selected bonding parameters are

listed in Table 9.

The cyclotetraphosphane ring is folded, as expected, and Me₃B₃N₃H₂PH₂; rather, this provides the zwitterionic phos-

it substituents are present in the trans orientation that is phonium borazinate 14, which shows that the P atom of

typical for other P₄R₄rings.^[25–28]The rings at atoms P1 (Me₃B₃N₃H₂)PHEt is a stronger basic center than the ring

and P3 are twisted against each other by 20° while those at N atoms because one of the NH atoms migrates to the P

atoms P2 and P4 make an angle of 93°. Amongst the P–P atom. This generates a new type of borazine deriva-

bonds, those to atom P4 are significantly longer than those tive.^[29–31]We are convinced that the reaction of LiAlH₄

to atom P2. The longer bonds match with those found for with Me₃B₃N₃H₂PBr₂in the absence of diethyl ether will

tetrakis(2,2,6,6-tetramethylpiperidino)cyclotetraphosphane

lead to Me₃B₃N₃H₂PH₂, whose P atom should be less basic

or diphenylbis(tetramethylpiperidino)cyclotetraphosphane than that of the PHEt group.

(2.245 Å).^[26]

The dehalogenation of organohalophosphanes with Na

The endocyclic P–P–P angles span the range from or other metals is often employed as a route to organo-

78.99(5)° (P4) to 84.35(5)° (P3) and are compatible with diphosphanes or organocyclophosphanes.^[32]While the de-

those of other cyclotetraphosphanes.^[25–28]The four- halogenation of 6 with Na or Mg produced the correspond-

membered ring is not uniformly bent, however, as the non- ing diphosphane derivative 18, its reaction with LiPmes₂

bonding P–P distances are 3.6 Å for P2–P4 and 2.8 Å for gives access to the mixed substituted diphosphane 17. How-

P1–P3.

ever, in the case of LiPPh₂the formation of P₂Ph₄is also

Table 9. Selected bonding parameters for (Me₃B₃N₃H₂)₂P₄mes₂(E).^[a]

P1–P4

P4–C20

C20–C21

C21–C22

C23–C24

C22–C23

C21–C22

C20–C21

2.215(2)

1.804(4)

1.428(7)

1.379(7)

1.395(7)

1.407(7)

1.379(7)

1.428(7)

84.34(6)

104.8(1)

114.4(2)

128.3(3)

113.7(4)

118.0(5)

118.5(5)

124.1(4)

115.8(5)

122.9(4)

P1–P2

P1–N1

N1–B1

B1–N2

N2–B2

B2–N3

N3–B3

B3–N1

P1–P2–P3

P2–P3–N1

P4–P2–N1

P2–N1–B1

P2–N1–B3

B1–N1–B3

N1–B1–N2

B1–N2–B2

N2–B2–N3

B2–N3–B3

2.207(2)

1.754(4)

1.449(7)

1.434(7)

1.420(7)

1.409(8)

1.441(7)

1.468(6)

84.34(6)

107.0(1)

109.5(1)

127.2(3)

122.2(4)

120.6(4)

117.4(5)

124.8(6)

115.1(5)

126.0(5)

P2–P3

P2–N4

N4–B4

B4–N5

N5–B5

B5–N6

N6–B6

B6–N4

P2–P1–P4

P4–P3–N4

P1–P2–N4

P2–N4–B4

P2–N4–B6

B4–N4–B6

N4–B4–N5

B4–N5–B5

N5–B5–N6

B5–N6–B6

2.251(2)

1.769(4)

1.451(6)

1.402(7)

1.411(8)

1.396(7)

1.410(8)

1.450(7)

80.95(6)

111.27(6)

106.8(1)

113.3(1)

127.6(4)

119.0(4)

118.0(5)

124.8(5)

115.0(4)

125.5(5)

P3–P4

P3–C10

C10–C11

C11–C12

C13–C14

C12–C13

C11–C10

C10–C15

2.256(2)

1.801(4)

1.433(6)

1.406(6)

1.404(7)

1.396(7)

1.433(7)

1.441(7)

79.11(6)

104.8(1)

114.4(21)

113.6(3)

128.3(3)

117.8(4)

119.1(4)

123.5(4)

115.9(4)

123.9(4)

P1–P4–P3

P2–P3–P4

P1–P4–C20

P3–P4–C20

P4–C20–C25

P4–C20–C21

C25–C20–C21

C24–C25–C20

C23–C24–C25

C24–C23–C22

C21–C22–C23

P1–P4–C20

P3–P4–C20

P4–C20–C21

P4–C20–C25

C15–C10–C11

C14–C15–C10

C13–C14–C15

C14–C13–C12

C11–C12–C13

[a] Average B–C bond length: 1.557 Å; average C–C(exo) bond length: 1.529 Å.

4292

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Eur. J. Inorg. Chem. 2007, 4282–4297

Borazinyl-Substituted Phosphanes, Arsanes and Stibanes

tain aryl groups. This is due to incomplete combustion due to bo-

ron carbide or nitride formation or to partial hydrolysis.

observed along with (Me₃B₃N₃H₂)₂PPPh₂(16). This indi-

cates that a Li/Br exchange occurs during the reaction, and

this also occurs during the reaction of Li₂Pmes with 4, as

shown by the formation of compounds A–F, which is also

accompanied by the formation of small amounts of bi-

cylo[1.1.0]tetraphosphanes. The tetra(borazinyl)cyclotetra-

Dichloro(2,4,6-trimethylborazinyl)phosphane (3): A solution of 2

(0.60 g, 2.96 mmol) in a mixture of hexane (10 mL) and diethyl

ether (5 mL) was slowly added to a solution of PCl₃(0.26 mL,

2.96 mmol) in hexane (30 mL). LiCl precipitated rapidly. After stir-

ring overnight the solid was removed by filtration (yield 95% of

LiCl) and the solvents were evaporated from the filtrate in vacuo.

The oil which remained was distilled at 55 °C/1 Torr to give pure

3. Yield 0.56 g (85%). C₃H₁₁B₃Cl₂N₃P (223.45): calcd. C 16.12, H

phosphane (19) shows only one ³¹P signal (δ

=

63 ppm),which fits nicely with the other ³¹P data recorded

for cyclotetraphosphanes of the type (R₂NP)₄.^[26]As far as

we are aware the exchange of organo substituents in or-

ganophosphorus chemistry is not common, although the

borazinyl group, with its P–N bond, is a species with higher

bond polarity and is therefore more likely to be more reac-

tive, similar to the aminophosphanes.

4.96, N 18.80; found C 16.40, H 5.03, N 18.40. IR: ν = 3428 s,

˜

3377 m, 3243 w, 2960 s, 2491 w, 2293 w, 2227 w, 2190 w, 2000 w,

1955 w, 1898 w, 1499 s, 1402 s, 1321 s, 1295 s, 1179 m, 1147 m,

1083 m, 1040 w, 1006 m, 982 s, 891 s, 811 m, 729 m, 626 w, 587 m,

532 w, 500 m. 459 m, 426 w, 362 w cm^–1.

This higher reactivity of the borazinylphosphanes with

respect to the N-unsubstituted borazines seems to be a con-

sequence of the changes in the structures of the new com-

pounds. As shown by X-ray structure determinations, the P

atom induces asymmetry in the borazine ring as shown by

the comparatively long B–N bonds to the P-carrying N

atom. This weakens the B1–N2 and B3–N4 bonds to the

neighboring boron atoms. Atoms B1(3), on the other hand,

usually form the shortest B–N bonds to atoms N2(5), while

the B–N bonds to B2(5) (para-position) show B–N bond

lengths that correspond to those found in symmetrically

substituted borazines (RB=NH)₃.^[30,31]In contrast, the lith-

ium borazine 2·OEt₂, which is dimeric, shows a different

distortion. Here, the bonds N1–B1(3) are the shortest

bonds in the borazine ring while the B1–N2 and N3–B3

bonds are the longest.^[3]This demonstrates that the P sub-

stituents have an opposite effect on the B1(3)–N1 bonds,

and therefore on the electron distribution in the P-substi-

tuted borazine rings, to the Li atom. Moreover, although

most of the borazine rings in the new compounds are

planar, several show a semi-chair distortion. The distortion

is only weak but is nevertheless significant. We assume that

this might be a packing effect. We will report shortly on

new boron-, aluminum-, and titanium-substituted boraz-

ines, the latter of which show the strongest BN bond differ-

ences to date, and we are continuing to explore further as-

pects of this new field of borazine chemistry.

Chlorobis(2,4,6-trimethylborazinyl)phosphane (4): Prepared in anal-

ogy to 3 from PCl₃(0.18 mL, 2.02 mmol) in hexane (30 mL) and 2

(0.82 g, 4.05 mmol) in hexane/diethyl ether (10:5 mL). The filtrate

was reduced to a fifth of its original volume in vacuo and then kept

at –25 °C. Yellowish crystals of 4 separated within 5 d. After drying

they melted at 110 °C (b.p. 140 °C/1 Torr). Yield 0.50 g (80%).

C₆H₂₂B₆ClN₆P (309.57): calcd. C 23.28, H 7.16, N 27.15; found C

22.95, H 6.96, N 26.43. IR: ν = 3441 s, 3411 s, 3402 s, 2988 m,

˜

2956 m, 2941 m, 2911 m, 1487 s, 1435 s, 1396 s, 1302 s, 1285 s,

1193 m, 1176 m, 1159 s, 1145 s, 1080 m, 1010 w, 973 s, 960 s, 891

s, 808 m, 751 m, 735 m, 721 m, 704 m, 640 w, 591 m, 578 m, 566 m,

539 w, 503 w, 493 w, 468 w, 446 w, 431 w, 405 w, 366 w, 320 w, 296

w cm^–1.

Dibromo(2,4,6-trimethylborazinyl)phosphane (5): Prepared in anal-

ogy to 3 from PBr₃(0.17 mL, 1.78 mmol) in hexane (30 mL) and 2

(360 mg, 1.78 mmol) in hexane/diethyl ether (10 mL each). LiBr

precipitated rapidly and the solution turned yellow. After stirring

overnight, filtration and evaporation of the solvents from the fil-

trate left a yellow oil (b.p. 95 °C/1 Torr). Within a month the liquid

turned into yellow needles (m.p. 50 °C). Yield 0.5 g (85%).

C₃H₁₁B₃Br₂N₃P (312.35): calcd. C 11.54, H 3.55, N 13.45; found

C 11.50, H 3.62, N 13.47. IR: ν = 3423 s, 3414 s, 2950 m, 2856 w,

˜

2284 w, 2218 w, 1993 w, 1947 w, 1859 w, 1867 w, 1487 s, 1432 s,

1394 s, 1369 m, 1355 m, 1320 m, 1290 s, 1191 s, 1176 s, 1144 s, 1079

s, 975 s, 940 m, 889 s, 809 s, 731 s, 687 s, 569 s, 480 w, 465 w, 436 m,

389 s, 351 s, 286 w, 258 m cm^–1.

Bromobis(2,4,6-trimethylborazinyl)phosphane (6): Prepared in anal-

ogy to 4 from PBr₃(0.18 mL, 1.63 mmol) dissolved in hexane

(30 mL) and 2 (0.66 g, 3.26 mmol) in hexane/diethyl ether (10 mL

each). After stirring overnight the precipitate (LiBr, 90%) was re-

moved by filtration and two thirds of the solvent was removed in

vacuo. Colorless crystals of 6 separated from the slightly yellow

solution at 5 °C within a week. Yield 0.40 g (80%), m.p. 145 °C.

C₆H₂₂B₆BrN₆P (354.03): calcd. C 20.36, H 6.26, N 23.74; found C

Experimental Section

All experiments were performed using inert gas techniques. Sol-

vents were dried by conventional methods and distilled under an

N₂atmosphere. Compounds 1 and 2 were prepared as described in

the literature,^[2]as were LiPmes₂, LiPmes, and LiPPh₂.^[32]All other

reagents were commercially available. NMR spectroscopic data

were recorded in C₆D₆solutions with TMS (int.), 85% H₃PO₄, or

BF₃·OEt₂(external) as standards with Bruker ACP 200 or Jeol FX

400 instruments. IR spectra were recorded as Nujol/Hostaflon

mulls. Mass spectroscopy was performed with a Varian Atlas CH-

7 instrument. A Siemens P4 four-circle diffractometer equipped

with an area detector and a low temperature device was used for

recording X-ray reflection data. Elemental analyses were performed

by the analytical laboratory of the department. C/N values often

deviate by more than 1%, particularly when the compounds con-

19.27, H 6.19, N 23.23. IR: ν = 3430 s, 3401 s, 3391 s, 3314 m,

˜

3142 m, 2989 s, 2859 m, 2808 m, 2459 w, 2296 w, 2224 w, 1993 w,

1945 w, 1747 w, 1682 w, 1483 s, 1395 s, 1282 s, 1224 m, 1192 m,

1174 m, 1158 s, 1142 s, 1037 w, 1006 w, 962 s, 889 s, 806 m, 759 m,

741 m, 704 m, 624 m, 596 m, 689 m, 567 m, 493 w, 454 w, 439 w,

423 w, 384 w, 362 w, 309 w, 284 w cm^–1. MS (70 eV): m/z (%) 354

(20) [M⁺], 339 (15) [M⁺– Me], 309 (10) [M⁺– 3Me], 274 (100)

[M⁺– Br], 259 (55), [M⁺– Br – Me], 244 (15) [M⁺– Br – 2Me],

121 (20) [B₃N₃Me₃H₂⁺], 111 (65) [PBr⁺].

Dichloro(2,4,6-trimethylborazinyl)arsane (7): A toluene solution of

2 (0.58 g, 2.86 mmol in 20 mL) was added dropwise to a solution of

AsCl₃(0.24 mL, 2.86 mmol) in hexane (30 mL). A white precipitate

formed immediately. After stirring overnight the solid was removed

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4293

H. Nöth, B. Gemünd, R. T. Paine

FULL PAPER

by filtration and the solvents evaporated in vacuo. A slightly yellow

oil (0.5 g) remained as residue which decomposed on attempted

distillation. Yield 500 mg (65%). ¹H NMR: δ = 0.15 (s, 3 H, BMe),

0.56 (s, 6 H, BMe), 4.42 ppm (br. s, 2 H, NH). ¹¹B NMR: δ = 35.6,

37.9 ppm (1:2).

repeated crystallization from toluene/hexane at –20 °C. ¹H NMR

(C₆D₆): δ = 0.18 (s, 3 H, BMe), 0.64 (d, 6 H, BMe), 4.55 ppm (br.

s, NH) ppm. ¹¹B NMR (C₆D₆): δ = 35.7, 39.5 ppm (2:1). ³¹P NMR

(C₆D₆): δ = 120.3 ppm.

Tris(2,4,6-trimethylborazinyl)stibane (impure) was obtained as a

white powder upon treatment of a suspension of SbCl₃(0.488 g,

2.13 mmol) in hexane (20 mL) with a solution of 2 (1.30 g,

6.24 mmol) in hexane (30 mL). After heating the mixture overnight

to reflux and removing the insoluble material a white powder re-

mained after evaporation of the solvent. The product was charac-

terized only by NMR spectroscopy. ¹H NMR: δ = 0.19 (s, 9 H,

Me), 0.52 (s, 18 H, Me), 4.31 ppm (s, 6 H, Me) and some additional

weak signals. ¹¹B NMR: δ = 35.1, 37.3 ppm (1:2).

Chlorobis(2,4,6-trimethylborazinyl)arsane (8): Prepared in analogy

to 4 from AsCl₃(0.30 mL, 3.58 mmol) in hexane (30 mL) and 2

(1.45 g, 7.16 mmol) in toluene (20 mL). LiCl was recovered in 95%

yield. The filtrate was concentrated and the white amorphous pre-

cipitate that formed within a week was isolated. Yield 0.9 g (75%),

m.p. 64 °C. No single crystals were obtained from toluene, diethyl

ether, or thf solutions. C₆H₂₂AsB₆ClN₆(353.49): calcd. C 20.38, H

6.27, N 23.37; found C 20.81, H 6.09, N 22.96. IR: ν = 3445 m,

˜

3419 s, 3411 s, 2942 s, 2951 s, 2909 s, 2858 m, 1478 s, 1397 s, 1303

s, 1285 s, 1193 m, 1177 m, 1161 m, 1154 m, 1118 w, 1083 m, 1045

w, 1026 w, 920 s, 889 s, 805 m, 753 w, 734 m, 717 s, 573 s, 539 m,

505 w, 485 w, 458 w, 430 w, 416 w, 344 m, 322 w, 288 w, 282 w, 274

w cm^–1.

Bis(2,4,6-trimethylborazinyl)phosphane (13): Compound 6 (1.65 g,

4.67 mmol) was dissolved in hexane (30 mL) and a solution of Li-

AlH₄in diethyl ether (0.83 mL of a 1.3  solution) was added to

this stirred solution to give a white precipitate. After stirring over-

night the solid was removed by filtration and three quarters of the

solvents were evaporated from the filtrate in vacuo. Storing the

solution at +5 °C gave platelets of 13 within two weeks (m.p.

82 °C). Yield 0.95 g (75%). C₆H₂₃B₆N₆P (275.12): calcd. C 26.19,

Dichloro(2,4,6-trimethylborazinyl)stibane (9): Freshly sublimed

SbCl₃(0.60 g, 2.62 mmol) was suspended in hexane (30 mL) and a

solution of 2 (0.53 g, 1.57 mmol) in hexane (20 mL) was added.

After keeping the suspension at reflux overnight the insoluble white

material was removed by filtration. The solvent was evaporated

from the filtrate in vacuo and the residue dissolved in a small quan-

tity of hexane. Colorless crystals separated from this solution at

–5 °C within a week. Yield 0.62 g (75%), m.p. 148–149 °C.

C₃H₁₁B₃Cl₂N₃Sb (314.23): calcd. C 11.47, H 3.53, Cl 22.56, N

H 8.43, N 30.54; found C 26.69, H 9.01, N 29.96. IR: ν = 3424 s,

˜

3402 s, 2958 s, 2942 s, 2932 s, 2846 w, 2489 w. 2293 s, 2254 s, 2175

w, 2122 w, 2080 w, 1980 w, 1915 w, 1901 w, 1879 w, 1483 s, 1397 s,

1371 m, 1316 s, 1291 s, 1206 m, 1196 m, 1186 w, 1163 s, 1143 w,

1105 w, 1093 w, 1065 m, 1040 w, 1021 w, 991 m, 962 s, 895 s, 834

w, 912 m, 785 w, 748 s, 725 w, 636 w. 605 m, 592 m, 572 m, 492 w,

479 w, 440 w, 394 w. 371 m, 356 w cm^–1.

13.37; found C 12.86, H 3.84, Cl 22.34, N 12.48. IR: ν = 3448 w,

˜

3421 s, 3406 s, 2964 s, 2964 m, 2946 m, 2910 w, 1474 s, 1407 s,

1332 m, 1317 s, 1303 s, 1292 s, 1262 s, 1191 m, 1172 m, 1153 m,

1082 s, 1020 m, 889 s, 802 s, 757 m, 740 m, 715 s, 661 w, 569 s,

531 m, 400 w, 392 w, 342 s, 331 s, 318 st cm^–1. MS (50 eV): m/z (%)

314 (5) [M⁺], 299 (45) [M⁺– Me], 207 (20) [MeSbCl₂⁺], 192 (100)

[SbCl₂⁺], 156 (65) [SbCl⁺], 122 (80) [Me₃B₃N₃H₂⁺].

Zwitterionic Ethylphosphonium 2,4,6-Trimethylborazinate 14: A di-

ethyl ether solution of LiAlH₄(0.29 mmol, 1.4 ) was added to a

hexane solution (30 mL) of 5 (0.52 g, 1.66 mmol) with stirring. The

solid formed was removed by filtration after stirring the mixture

overnight. The volume of the filtrate was then reduced to one fifth.

Crystals separated within a week on storing this solution at –5 °C.

Yield 0.20 g (65%). C₅H₁₇B₃N₃P (182.59): calcd. C 32.89, H 9.37,

Tris(2,4,6-trimethylborazinyl)arsane was obtained from 2 (0.96 g,

4.69 mmol) in a solution of hexane (30 mL) and AsCl₃(0.11 mL,

1.56 mmol) in 10 mL of hexane. After keeping the mixture at reflux

for 3 d the solid was filtered off and the filtrate reduced to one

third of its original volume. The solution still contained small

amounts of chloride. The ¹¹B NMR spectrum showed the forma-

tion of (Me₃B₃N₃H₂)₃As. ¹H NMR (C₆D₆): δ = 0.07 (s, 3 H, BMe),

0.61 (s, 6 H, BMe), 4.42 ppm (br. s, 2 H, NH) with some weak

signals from impurities. ¹¹B NMR (C₆D₆): δ = 35.5, 36.9 ppm.

N 23.01; found C 32.19, H 9.12, N 22.54. IR: ν = 3443 s, 3407 s,

˜

2950 s, 2911 s, 2900 s, 2853 s, 2332 s, 1490 s, 1407 s, 1370 s, 1360

s, 1262 s, 1199 s, 1183 s, 1166 s, 1144 s, 1098 s, 1059 s, 1021 s, 997

s, 943 s, 888 s, 806 s, 748 s, 730 m, 715 m, 676 m, 667 m, 649 m,

642 m, 603 m, 567 m, 546 w, 397 w, 378 m cm^–1. ¹H NMR: δ =

0.14 (s, 3 H, BMe), 0.30 (m, 5 H, Et). 0.35–0.47 (m, 5 H, PMe,

PCH₂), 0.54 (d, ⁴J

= 1.9 Hz, 6 H, BMe), 5.52 (dt, ¹J

³¹P,¹H

=

370, ³J₃₁_P,₁_H= 7 Hz, 2 H, PH₂), 5.59 ppm (dq, ¹J₃₁_P,₁_H= 379, ³J₁_H,₁_H

Chlorobis(2,4,6-trimethylborazinyl)stibane (10): A suspension of

SbCl₃(0.37 g, 1.62 mmol) in hexane (30 mL) was allowed to react

with a solution of 2 (0.657 g, 3.24 mmol) in hexane (20 mL). After

refluxing overnight the solid was removed by filtration and the fil-

trate reduced in volume by 3/4. From the solution settled a white

powder at –25 °C. Yield 0.50 g of 10 (75%), dec. Ͼ 200 °C.

C₆H₂₂B₆ClN₆Sb (400.36): calcd. C 17.99, H 5.53, N 20.99; found

= 7 Hz, 2 H, PH₂). ¹¹B NMR: δ = 34.1, 35.3 ppm. ³¹P NMR: δ =

1

³¹P,¹H

5.5 ppm (t, J

= 430 Hz).

Bis(mesityl)bis(2,4,6-trimethylborazinyl)diphosphane (17): A solu-

tion of 6 (0.88 g, 25 mmol) in hexane (20 mL) was added slowly to

a stirred suspension of LiPmes₂(0.69 g, 2.5 mmol) in hexane

(50 mL) at –78 °C. The mixture was then kept at reflux for 14 h.

Insoluble material was removed by filtration and about half of the

solvent was removed in vacuo. The filtrate was kept at –25 °C and

the microcrystalline material recrystallized from toluene at –30 °C

as colorless prisms. Two crystals were selected for the X-ray struc-

ture determination and proved to contain toluene. The crystals be-

came opaque in vacuo. After keeping them in vacuo for 3 h the

yield was 1.15 g (85%), m.p. 182 °C. C₂₄H₄₄B₆N₆P₂(542.47): calcd.

C 20.47, H 5.38, N 21.79. IR: ν = 3438 s, 3418 s, 2963 s, 2932 m,

˜

2861 m, 2791 w, 1473 s, 1449 m, 1429 m, 1300 s, 1370 m, 1360 w,

1321 m, 1282 w, 1261 s, 1193 w, 1170 w, 1095 s, 1021 w, 956 w, 893

w, 833 w, 803 w, 755 w, 700 m, 670 m, 658 m, 614 s, 533 w, 528 w,

477 w, 404 w cm^–1.

Tris(2,4,6-trimethylborazinyl)phosphane (11): PBr₃(1.64 mmol) dis-

solved in toluene (10 mL) was added to a solution of 2 (1.00 g,

4.93 mmol) in toluene (30 mL). A white precipitate formed rapidly. 53.04, H 8.16, N 15.46; found C 51.71, H 8.66, N 14.74. IR: ν =

˜

After keeping the mixture at reflux for one day the solid was re-

moved by filtration. A ³¹P NMR spectrum showed the formation

of 11 (δ = 120.3 ppm) along with minor amounts of 6 (δ =

191 ppm). The latter could not totally be removed from 6 despite

3424 m, 3414 m, 3403 w, 3100 w, 3089 m, 3025 m, 2965 s, 2924 s,

2864 m, 2731 w, 1667 w, 1601 m, 1549 w, 1478 s, 1432 s, 1396 m,

1298 s, 1277 s, 1188 m, 1178 m, 1152 m, 1083 m, 1031 w, 1016 w,

930 m, 896 m, 953 m, 861 m, 807 m, 787 w, 734 m, 699 w, 602 w,

4294

www.eurjic.org

Eur. J. Inorg. Chem. 2007, 4282–4297

Borazinyl-Substituted Phosphanes, Arsanes and Stibanes

581 s, 568 m, 559 m, 546 w, 521 w, 499 w, 453 w, 375 m, 365 m

cm^–1.

3437 w, 3422 w, 2969 m, 2890 w, 1483 s, 1397 s, 1380 m, 1364 w,

1282 s, 1261 m, 1193 m, 1178 m, 1150 m, 1131 w, 1082 m, 939 s,

878 s, 850 m, 808 m, 701 wm, 613 w, 574 w, 351 w, 343 w cm^–1.

Tetrakis(2,4,6-trimethylborazinyl)diphosphane (18): Freshly cut

pieces of elemental Na (300 mg) were added to a solution of 6

(1.61 g, 5.0 mmol) in toluene (50 mL). There was no reaction at

ambient temperature. The mixture was then kept at reflux (about

3 d) until the ³¹P NMR signal of the 6 could no longer be observed.

The solid material was then removed by filtration. The filtrate was

free of bromide. Two thirds of the toluene was then evaporated in

vacuo and the remaining solution kept at –25 °C. Crystals of 18

separated within 10 d. Yield 1.02 g (70%). C₁₂H₄₄B₁₂N₁₂P₂

(548.24): calcd. C 26.29, H 8.09, N 30.66; found C 28.34, H 8.36,

Dimesitylbis(2,4,6-trimethylborazinyl)cyclotetraphosphane (A–F): A

solution of 4 (0.33 g, 1.04 mmol) in toluene (20 mL) was slowly

added at –78 °C to a suspension of mesPLi₂(0.17 g, 1.04 mmol) in

the same solvent (20 mL). The stirred mixture was allowed to reach

room temperature and stirring was continued overnight. The insol-

uble material was then removed by filtration. The filtrate contained

no bromide and was investigated by NMR spectroscopy. The vol-

ume of the filtrate was further reduced to one fifth. Colorless, pris-

matic crystals were isolated after two weeks which proved to be

compound E. Yield 0.27 g (45%), m.p. 189 °C. Further fractional

crystallizations yielded no other pure product. C₂₄H₄₄B₆N₆P₄

N 29.86. IR: ν = 3432 s, 3413 s, 2956 m, 2943 m, 2855 m, 1483 s,

˜

1399 s, 1293 s, 1280 s, 1200 m, 1190 s, 1177 s, 1153 s, 1084 m, 1041

w, 1030 w, 1019 w, 933 s, 889 s, 808 m, 736 s, 727 s, 715 m, 649 m, (605.35): calcd. C 42.55, H 7.37, N 14.61; found C 44.46, H 7.63,

1

683 s, 563 m, 518 w, 491 w 431 w, 374 m, 358 m cm^–1. H NMR: δ

N 13. 84. IR: ν = 3437 w, 3428 w, 3409 w, 3400 m, 3025 w, 2959 m,

˜

= 0.15 (s, 12 H, BMe), 0.84 (s, 24 H, BMe), 4.49 ppm (s, 8 H, NH). 2860 w, 1602 m, 1481 s, 1399 s, 1370 m, 1361 w, 1285 s, 1263 m,

¹¹B NMR: = 36.3 ppm (br., h_1/2= 1260 Hz). ³¹P NMR: =

108.7 ppm (s).

1201 m, 1191 m, 1178 m, 1150 m, 1130 w, 1084 m, 1028 w, 939 s,

1

888 s, 850 m, 733 m, 701 m, 607 w, 571 m, 467 w, 353 w cm^–1. H

4

³¹P,¹H

NMR: δ = 0.043 (s, 6 H, BMe), 0.99 (d, J

= 5.75 Hz, 12 H,

Tetrakis(2,4,6-trimethylborazinyl)cyclotetraphosphane (19): A solu-

tion of 5 (0.33 g, 1.04 mmol) in hexane (50 mL) was treated with a

suspension of finely cut sodium chips (ca. 0.5 g). The reaction

started after heating the mixture to reflux. After 3 d the ³¹P NMR

signal of 5 could no longer be detected. The insoluble material was

removed by filtration. No bromide was found in the filtrate. After

reducing the volume of the filtrate by about 60% slightly yellowish

crystals separated within two weeks at –50 °C. Attempts to obtain

single crystals from hexane, toluene, or thf were unsuccessful. Yield

0.18 g (60%), m.p. 200 °C. C₁₂H₄₄B₁₂N₁₂P₄(610.18): calcd. C

BMe), 2.80 (d, ⁴J

= 3.57 Hz, 12 H, CMe), 2.82 (s, 6 H, CMe),

³¹P,¹H

4.47 (s, 4 H, NH), 6.68 ppm (s, 4 H, CMe). ³¹P NMR: δ = 74.9 (t,

¹J

_P= 168.3 Hz), –30.5 (t, ¹J

_P= 168.4 Hz), 48.6 (dd, ¹J

³¹P,³¹

³¹P,^{31 31}P,³¹

P

= 168.2 Hz), 63.4 (s), 60.7 (dd, ¹J

= 57.0 Hz), –49.5 (dd,

P

³¹P,³¹

¹J

= 57.0 Hz), 36.2 (t, J

=

1

³¹P,³¹

P

³¹P,³¹

P

³¹P,³¹

P

= 171.7 Hz), –8.1 (t, J

168.4 Hz), 77.3 (t, ¹J

= 170.2 Hz), –26.4 (dd, ¹J

³¹P,³¹

1

P

³¹P,³¹

P

³¹P,³¹

P

170.2 Hz), –32 ppm (t, J

= 170.2 Hz).

X-ray Structure Determinations: Single crystals were covered with

perfluoro ether oil under dinitrogen and a specimen selected. The

selected crystal was mounted on the top of a glass fiber and placed

23.62, H 7.26, N 27.55; found C 24.12, H 7.34, N 24.89. IR: ν =

˜

Table 10. Crystallographic data and data collection and structure solution parameters.

Compound

4

5

6

9

Empirical formula

C₁₃H₂₆B₆ClN₆P

398.17

0.10ϫ0.20ϫ0.20

triclinic

¯

P1

8.0317(6)

11.4558(9)

13.658(1)

83.692(2)

74.337(2)

74.062(2)

1162.6(2)

2

C₃H₁₁B₃Br₂N₃P

312.37

C₆H₂₂B₆BrN₆P

354.04

0.20ϫ0.20ϫ0.30

orthorhombic

Pbca

16.0349(4)

8.2488(2)

27.1422(5)

90

3590.1(1)

8

1.310

C₃H₁₁B₃Cl₂N₃Sb

314.23

0.25ϫ0.298ϫ0.42

monoclinic

P2₁/n

8.1660(2)

9.7796(3)

28.1685(8)

90

90.5608(2)

90

2249.4(1)

8

1.856

2.879

1200

Formula mass

Crystal size [mm]

Crystal system

Space group

a [Å]

b [Å]

c [Å]

α [°]

β [°]

0.35ϫ0.26ϫ0.19

orthorhombic

Pnma (62)

19.963(1)

7.4737(3)

15.2362(8)

90

2273.1(2)

8

1.825

γ [°]

V [Å³]

Z

ρ(calcd.) [Mgm^–3

]

1.135

0.243

415

µ [mm^–1

F(000)

]

7.225

1200

2.373

1440

Index ranges

–10 Յ h Յ 8, 14 Յ k Յ 14, –26 Յ h Յ 26, 8 Յ k

–19 Յ h Յ 19, 10 Յ k Յ 10, –10 Յ h Յ 10, 12 Յ k Յ 12,

–17 Յ l Յ 17

57.54

Յ 9, –20 Յ l Յ 17

56.00

–32 Յ l Յ 32

52.74

–32 Յ l Յ 35

57.84

2θ [°]

Temp. [K]

193(2)

6838

3581

2646

0.0195

270

200(2)

12290

2933

1696

0.0540

139

0.0758/0.00

0.926

183

18337

3515

2337

0.0624

203

0.0297/3.5073

1.104

193

12381

4446

4047

0.0186

233

0.0339/3.4530

1.140

Reflections collected

Reflections unique

Reflections observed (4σ)

R (int.)

Number of variables

Weighting scheme^[a]x/y

GOOF

0.1907/0.612

1.118

Final R(4σ)

0.0881

0.2748

1.270

0.0462

0.1184

1.277

0.0519

0.0904

0.319

0.0304

0.0719

0.696

Final wR2

Largest residual peak [e/Å³]

[a] w^–1= σ²F_o+ (xP)²+ yP; P = (F_o+ 2F_c)/3.

2

Eur. J. Inorg. Chem. 2007, 4282–4297

www.eurjic.org

4295

H. Nöth, B. Gemünd, R. T. Paine

FULL PAPER

Table 11. Crystallographic data and data collection and structure solution parameters.

Compound

13

14

17·2C₆H₈

E

Empirical formula

C₆H₂₃B₆N₆P

275.13

0.05ϫ0.4ϫ0.5

orthorhombic

Pbca

15.860(1)

8.0674(6)

25.886(2)

90

3312.2(4)

8

1.104

C₅H₁₇B₃N₃P

182.62

0.2ϫ0.3ϫ0.3

orthorhombic

Pca2₁

16.997(1)

4.5201(3)

14.537(1)

90

1116.9(1)

4

1.086

C₃₈H₅₆B₆N₆P₂

723.69

0.2ϫ0.3ϫ0.4

triclinic

¯

P1

11.173(2)

11.495(3)

16.176(4)

77.833(9)

80.90(1)

84.69(1)

2001.6(8)

2

C₂₄H₄₄B₆N₆P₄

605.80

0.2ϫ0.23ϫ0.3

triclinic

¯

P1

8.9312(7)

11.2962(9)

19.757(2)

81.619(2)

89.161(2)

70.801(2)

1861.1(3)

2

Formula mass

Crystal size [mm]

Crystal system

Space group

a [Å]

b [Å]

c [Å]

α [°]

β [°]

γ [°]

V [Å³]

Z

ρ(calcd.) [Mgm^–3

]

1.131

0.141

730

1.129

0.14

706

µ [mm^–1

F(000)

]

0.157

1168

0.199

392

Index ranges

–16 Յ h Յ 19, 10 Յ k Յ 10, –21 Յ h Յ 21, 5 Յ k Յ 5, –11 Յ h Յ 15, 14 Յ k Յ 14, –12 Յ h Յ 10, 13 Յ k Յ 13,

–31 Յ l Յ 32

58.58

–18 Յ l Յ 18

58.48

–20 Յ l Յ 20

58.46

–25 Յ l Յ 25

57.64

2θ [°]

Temp. [K]

193(2)

17739

3361

2060

0.0621

183

193(2)

5956

2381

2085

0.0244

181

193(2)

11732

6286

4531

0.0251

423

183(2)

10765

5627

3103

0.0521

407

Reflections collected

Reflections unique

Reflections observed (4σ)

R (int.)

Number of variables

Weighting scheme^[a]x/y

GOOF

0.0772/1.4325

1.029

0.0806/0.0039

0.926

0.200/0.000

1.034

0.3621//0.000

0.960

Final R(4σ)

0.0601

0.1431

0.0393

0.1100

0.214

0.0650

0.1728

0.604

0 054

0.1144

0.245

Final wR2

Largest residual peak [e/Å³] 0.229

[a] w^–1= σ²F_o+ (xP)²+ yP; P = (F_o+ 2F_c)/3.

2

on the goniometer head flushed with a cold stream of N₂. Data for

the determination of the unit cell were colleted at –80 °C for a

total of 75 frames. The unit cell was determined with the program

SMART.^[33]Data collection was performed in the hemisphere

mode. After data reduction the model structure was determined

with the program SAINT.^[34]The structure determination package

SHELXTL^[35]was used to refine the data. Non-hydrogen atoms

were refined anisotropically. Hydrogen atom positions were calcu-

lated and refined using a riding model. The NH hydrogen atoms

were located in the difference Fourier map and their positions were

refined with isotropic thermal parameters. The thermal ellipsoids

in the figures are shown at the 25% probability level. Distances are

given in Ångstroms and bond angles in degrees. Crystallographic

data and data related to data collection and refinement are summa-

rized in Tables 10 and 11.

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Acknowledgments

This work was supported by the Fond der Chemischen Industrie

and Chemetall GmbH, Frankfurt/Main. We thank Mr. P. Mayer

and Mrs. D. Ewald for the recording many of the NMR, IR, and

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structure determination. Finally, we would also like to thank Prof.

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Received: October 24, 2006

Published Online: August 8, 2007

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N-(2,4,6-trimethylborazinyl)-substituted phosphanes, arsanes, and stibanes

DOI: 10.1002/ejic.200601001

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