Synthesis of halogenated polyhedral phosphaboranes. Crystal structure of closo-1,7-P2B10Cl10

Article abstract of DOI:10.1021/ic990112v

The 12-vertex closo-phosphaborane 1,7-P₂B₁₀Cl₁₀ (1) has been prepared in low yield from the pyrolysis reaction of B₂Cl₄ with PCl₃ at temperatures above 400 °C. A single-crystal X-ray structure determination of 1 (monoclinic space group P2₁/n with a = 9.239(2) A, b = 16.786(3) A, c = 15.739(3) A, β = 93.25(3)°, and Z = 4) confirmed that, consistent with its 26 skeletal electron count, the phosphaborane adopts a distorted icosahedral structure with the phosphorus atoms in the 1,7-positions. Crystals of 1 contain toluene in a 1:1 molar ratio embedded between each P atom of neighboring cluster molecules. Alteration of the pyrolytic conditions resulted in the formation of the phosphaboranes P₄B₈Cl₆ (2) and P₂B₈Cl₈ (3), which were characterized spectroscopically. Copyrolysis of B₂Cl₄ with a mixture of PCl₃ and AsCl₃ at 450 °C generated the six-vertex arsaphosphaborane AsPB₄Cl₄ (4) and traces of the icosahedral arsaphosphaborane AsPB₁₀Cl₁₀. These compounds are examples of heteroboranes which contain two different group-15 atoms within a single molecule.

Full text of DOI:10.1021/ic990112v

1282
Inorg. Chem. 2000, 39, 1282-1287
Synthesis of Halogenated Polyhedral Phosphaboranes. Crystal Structure of
closo-1,7-P2B10Cl10
Willi Keller, Gisela Sawitzki, and Wolfgang Haubold*
Institut fu¨r Chemie, Universita¨t Hohenheim, Garbenstrasse 30, D-70599 Stuttgart, Germany
ReceiVed January 26, 1999
The 12-vertex closo-phosphaborane 1,7-P2B10Cl10 (1) has been prepared in low yield from the pyrolysis reaction
of B2Cl4 with PCl3 at temperatures above 400 °C. A single-crystal X-ray structure determination of 1 (monoclinic
space group P21/n with a ) 9.239(2) Å, b ) 16.786(3) Å, c ) 15.739(3) Å, â ) 93.25(3)°, and Z ) 4) confirmed
that, consistent with its 26 skeletal electron count, the phosphaborane adopts a distorted icosahedral structure
with the phosphorus atoms in the 1,7-positions. Crystals of 1 contain toluene in a 1:1 molar ratio embedded
between each P atom of neighboring cluster molecules. Alteration of the pyrolytic conditions resulted in the
formation of the phosphaboranes P4B8Cl6 (2) and P2B8Cl8 (3), which were characterized spectroscopically.
Copyrolysis of B2Cl4 with a mixture of PCl3 and AsCl3 at 450 °C generated the six-vertex arsaphosphaborane
AsPB4Cl4 (4) and traces of the icosahedral arsaphosphaborane AsPB10Cl10. These compounds are examples of
heteroboranes which contain two different group-15 atoms within a single molecule.
Introduction
[7,8-P2B9H10]-.10b In this laboratory we have synthesized the
perhalogenated closo species 1,2-P2B4X4 (X ) Cl or Br) by
copyrolysis of B2X4 and PX3 at 330 °C11a,b and have established
the structure of the chloro compound by single-crystal X-ray
diffraction analysis.11a This synthetic approach has also led to
the synthesis of new polyhedral boranes with carbon,12 arsenic,13a,b
antimony,13b sulfur,13b or selenium14 occupying the heterover-
tices. We now report the extension of this method to the
synthesis of several new perchlorinated phosphaboranes and
mixed arsaphosphaboranes.
Since the discovery of the first carboranes in 1962,1 electron-
counting rules2 and isoelectronic/isolobal principles3 have guided
the syntheses of a wide range of boranes incorporating not only
carbon but also other main group elements into their cluster
frameworks.4 Phosphorus-containing polyhedral boranes have
been reported in quite a number, and the scope in structural
diversity found for these compounds ranks the phosphaboranes
among the most extensively developed classes of heteroboranes.
Common examples are phosphaboranes5 and -carboranes6 in
which the cage-inserted phosphorus atoms are attached to an
exopolyhedral group and thereby donate four electrons to the
cage-bonding. In contrast, closo-diphosphaboranes of the general
formula P2BnRn are rare in which each phosphorus atom
possesses an exopolyhedral lone pair of electrons and thus
contributes three skeletal electrons.7 They are of special interest
since they can be seen as the phosphorus analogues of the closo-
dicarboranes, and, in fact, the majority of the reports covering
this type of compound deal with theoretical aspects related to
their bonding8 rather than with their reactivity. Examples include
the trigonal bipyramidal type cage compounds P2(BNR2)3 with
the phosphorus atoms occupying the axial positions.9 The 12-
vertex parent closo-diphosphaborane 1,2-P2B10H10 is known10a
and can be converted by thermal isomerization to the meta
isomer, 1,7-P2B10H1010a or by degradation reaction to the anion
Experimental Section
Instrumentation. 11B NMR (80.25 or 160.46 MHz) and 31P NMR
(101.26 or 202.46 MHz) spectra were recorded on a Bruker WM-250
and a Bruker AM 500. All 11B chemical shifts were referenced to
external BF3‚O(C2H5)2, 31P chemical shifts to external 85% H3PO4 with
(8) (a) Solouki, B.; Bock, H.; Haubold, W.; Keller, W. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 1044-1046. (b) Glass, J. A.; Whelan, T. A.;
Spencer, J. T. Organometallics 1991, 10, 1148-1161. (c) McKee,
M. L. J. Phys. Chem. 1991, 95, 9273-9278. (d) Jemmis, E. D.;
Subramanian, G. J. Phys. Chem. 1994, 98, 9222-9226. (e) Burdett,
J.; Eisenstein, O. J. Am. Chem. Soc. 1995, 117, 11939-11945. (f)
Schleyer, P. v. R.; Subramanian, G.; Dransfeld, A. J. Am. Chem. Soc.
1996, 118, 9988-9989. (g) Jemmis, E. D.; Kiran, B.; Coffey, D., Jr.
Chem. Ber./Recl. 1997, 130, 1147-1150. (h) Lorenzen, V.; Preetz,
W.; Keller, W.; Haubold, W. Z. Naturforsch. 1999, 54b, 1229-1234.
(9) (a) Wood, G. L.; Duesler, E. N.; Narula, C. K.; Paine, R. T.; No¨th, H.
J. Chem. Soc., Chem. Commun. 1987, 496-498. (b) Dou, D.; Wood,
G. L.; Duesler, E.; Paine, R. T.; No¨th, H. Inorg. Chem. 1992, 31,
3756-3762. Review: Paine, R. T.; No¨th, H. Chem. ReV. 1995, 95,
343-379.
(10) (a) Little, J. L.; Kester, J. G.; Huffman, J. C.; Todd, L. J. Inorg. Chem.
1989, 28, 1087-1091. (b) Little, J. L.; Whitesell, M. A.; Chapman,
R. W.; Kester, J. G.; Huffman, J. C.; Todd, L. J. Inorg. Chem. 1993,
32, 3369-3372.
(11) (a) Haubold, W.; Keller, W.; Sawitzki, G. Angew. Chem., Int. Ed.
Engl. 1988, 27, 925-926. (b) Keller, W.; Sneddon, L. G.; Einholz,
W.; Gemmler, A. Chem. Ber. 1992, 125, 2343-2346.
(1) Shapiro, I.; Good, C. D.; Williams, R. E. J. Am. Chem. Soc. 1962,
84, 3837-3840.
(2) (a) Williams, R. E. Inorg. Chem. 1971, 10, 210-214. (b) Wade, K.
AdV. Inorg. Chem. Radiochem. 1976, 18, 1-66. (c) Williams, R. E.
AdV. Inorg. Chem. Radiochem. 1976, 18, 67-142. (d) Rudolph, R.
W. Acc. Chem. Res. 1976, 9, 446-452.
(3) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711-725.
(4) Review: Saxena, A. K.; Maguire, J. A.; Hosmane, N. S. Chem. ReV.
1997, 97, 2421-2462.
(5) Getman, T. D.; Deng, H. B.; Hsu, L. Y.; Shore, S. G. Inorg. Chem.
1989, 28, 3612-3616 and references therein.
(12) Haubold, W.; Keller, W. J. Organomet. Chem. 1989, 361, C54-C56.
(13) (a) Scha¨fer, R.; Einholz, W.; Keller, W.; Eulenberger, G.; Haubold,
W. Chem. Ber. 1995, 128, 735-736. (b) Einholz, W.; Scha¨fer, R.;
Keller, W.; Vogler, B. Z. Naturforsch. 1997, 52b, 221-226.
(14) Keller, W.; Sowa, M.; Haubold, W. In Abstracts of the IMEBORON-X
Meeting; Durham, July 11-15, 1999; p 133.
(6) Keller, W.; Barnum, B. A.; Bausch, J. W.; Sneddon, L. G. Inorg. Chem.
1993, 32, 5058-5066 and references therein.
(7) Review: Todd, L. J. In ComprehensiVe Organometallic Chemistry-
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier Science
Ltd.: Oxford, 1995; Vol. 1, Chapter 7, pp 257-273.
10.1021/ic990112v CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/02/2000

Crystal Structure of closo-1,7-P2B10Cl10
Inorganic Chemistry, Vol. 39, No. 6, 2000 1283
Table 1. NMR Data for Phosphaboranes 1-4
a negative sign indicating an upfield shift. High- and low-resolution
mass spectra were obtained on a Finnigan MAT 8230 (70 eV). GC-
MS spectra were obtained on a Hewlett-Packard 5971. Each compound
exhibited a strong parent envelope whose intensity pattern was
consistent with the calculated spectrum based on natural isotopic
abundances.
General Procedures and Materials. All manipulations were
conducted using standard high-vacuum or inert atmosphere techniques
as described by Shriver.15 The solvents were reagent grade, dried and
purified according to standard procedures. Phosphorus trichloride,
arsenic trichloride, and boron trichloride were freshly vacuum-distilled
before use. Diboron tetrachloride was prepared by cocondensation of
BCl3 with copper vapor onto cooled walls (-196 °C) of a reactor similar
to that described by Timms.16 After repeated fractionation, B2Cl4
exhibited a vapor pressure at 0 °C of 44 Torr.
compd
11B, ppm (assgn or rel areas)
31P, ppm
1,7-P2B10Cl10 (1)
7.6 (B5,12), 1.9 (B4,6,8,11),
0.7 (B9,10), -2.0 (B2,3)
24.8 (3B), 15.9 (1B)
8.7
-123
P4B8Cl6 (2)
1,10-P2B8Cl8 (3)
1,2-PAsB4Cl4 (4)
-182
-195
-169
24.8 (B4,6), 7.0 (B3,5)
Table 2. Crystallographic Data for closo-1,7-P2B10Cl10‚C6H5CH3
empirical formula
fw
C7H8B10Cl10P2
616.67
cryst size (mm)
cryst syst
space group
a, Å
b, Å
c, Å
0.38, 0.42, 0.20
monoclinic
P21/n (No. 14)
9.239(2)
16.786(3)
15.739(3)
93.25(3)
2437.0(8)
4
Synthesis of closo-1,7-P2B10Cl10 (1), P4B8Cl6 (2), and P2B8Cl8 (3).
In a typical reaction, 4.9 g (30 mmol) of B2Cl4 and 1.03 g (7,5 mmol)
of PCl3 were condensed in vacuo into a 500 mL round-bottomed flask
equipped with a seal constriction and a break-seal joint. After sealing,
the flask was removed from the vacuum line and heated in an oven at
400 °C for 1 h. The flask was allowed to cool to room temperature
over a period of 20 h, resulting in an ochre solid deposited on the walls
and partly suspended in colorless liquid. After opening to the vacuum
line, all volatile compounds were vacuum-evaporated at 0 °C and the
remaining residue was extracted five times with 10 mL of BCl3 into a
separate 50 mL flask. The extractant BCl3 was evaporated at 0 °C and
the residue fractionated by sublimation in vacuo to give at Tsubl < 45
°C 70 mg of P2B4Cl4, at Tsubl 45-85 °C 110 mg of a mixture containing
P2B4Cl4, P4B8Cl6 (2), and P2B8Cl8 (3), and after repeated fractionation
at Tsubl > 85 °C 60 mg of pure P2B10Cl10 (1), each fraction as white
solid. Data for 1 are as follows. Mp > 230 °C; MS, m/z (rel int): 524
(100, M+); 407 (21, [M - BCl3]+); 290 (26, [M - 2BCl3]+). Exact
mass calcd for 31P211B1035Cl10: 521.72911. Found: 521.73376. 11B NMR
(ppm, CDCl3, 160.46 MHz): 7.6 (B5,12), 1.9 (B4,6,8,11), 0.7 (B9,10),
-2.0 (B2,3). 31P NMR (ppm, CDCl3, 101.26 MHz): -123 ppm. Data
for 2 are as follows. MS, m/z (rel int): 423 (100, M+); 306 (17, [M -
BCl3]+); 258 (27, [M - P2B3Cl2]+). Exact mass calcd for 31P411B8-
35Cl6: 421.78261. Found: 421.77611. 11B NMR (ppm, rel int, CDCl3,
160.46 MHz): 24.8 (3B), 15.9 (1B). 31P NMR (ppm, CDCl3, 101.26
MHz): -182 ppm. Data for 3 are as follows. MS, m/z (rel int): 432
(100, M+); 350 (42, [M - BCl2]+); 315 (76, [M - BCl3]+). Exact
mass calcd for 31P211B835Cl8: 429.77279. Found: 429.77935. 11B NMR
(ppm, CDCl3, 160.46 MHz): 8.7. 31P NMR (ppm, CDCl3, 101.26
MHz): -195 ppm.
Formation of closo-1,2-AsPB4Cl4 (4). B2Cl4 (1.11 g, 6.8 mmol)
and a mixture of 0.12 g of PCl3 (0.9 mmol) and 0.16 g (0.9 mmol) of
AsCl3 were condensed in vacuo into two separate 200 mL round-
bottomed flasks connected by a tube. After sealing, the apparatus was
held for 1 h at 450 °C and allowed to cool to room temperature over
a period of 20 h. Workup was done as described above and gave a
total of 50 mg of raw sublimate. Crystallization in an evacuated, sealed
tube at 300 °C afforded, after slow cooling, orange crystals of B9Cl9
(15 mg) which could be separated from the amorphous, white solid
containing P2B4Cl4, AsPB4Cl4 (4), and As2B4Cl4. Equal amounts of 15
mg each of P2B4Cl4 and 4 could be separated by slow sublimation at
0 °C in vacuo from 5 mg of As2B4Cl4 remaining as residue, but P2B4-
Cl4 and 4 could not further be fractionated by sublimation at lower
temperatures: -20 °C was too cold for sublimation, and very slow
sublimation at -13 °C still afforded only both components in mixture.
Appropriate conditions for thermal crystallization yielded crystals of
P2B4Cl4 and 4, but the latter were not suitable for X-ray diffraction.
Data for 4 are as follows. MS, m/z (rel int) 292 (100, M+); 209 (5, [M
- BCl2]+); 174 (85, [M - BCl3]+). Exact mass calcd for 75As31P11B4-
35Cl4: 289.80799. Found: 289.81265. 11B NMR (ppm, CDCl3, 160.46
MHz): 24.8 (2B), 7.0 (2B). 31P NMR (ppm, CDCl3, 202.46 MHz):
-169 ppm. The NMR data for the new phosphaboranes 1-4 are given
in Table 1.
â, deg
V, Å3
Z
D
calcd, g cm-3
1.681
T, °C
20
1.272
0.71073
56
5893
3431
0.0670
µ(Mo KR), cm-1
λ, Å
2θ max, deg
no. of indep reflns
no. of obsd reflns (I > 2σ(I))
R (obs)a
R (all)
Rw (obs)
0.1276
0.1567
Rw (all)
0.1812
0.12
largest diff peak and hole, e/Å3
a R ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑[w(Fo - Fc2)2]/∑[w(Fo )2]]1/2
.
2
2
X-ray Structure Determinaton of 1,7-P2B10Cl10 (1). Crystals
suitable for X-ray diffraction were obtained from a 30 mg sample of
1, which initially was dissolved in 2 mL of toluene. After removal of
the toluene in vacuo, 2 mL of n-pentane was added to the solid, which
only scarcely dissolved. By slow evaporation of n-pentane at -10 °C
under an argon stream, colorless crystals of 1‚C6H5CH3 precipitated
within several days. Apparently, toluene exhibits specific lattice
(electronic and/or steric) affinity toward 1, leading to an appropriate
molecular pattern suitable for crystal packing. The colorless crystals
were sealed in thin-wall glass capillaries under an argon atmosphere.
The unit cell parameters were acquired by least-squares refinement of
the angles 2θ (18-24°) for 27 well-centered reflections collected at
20 °C on a Siemens P4 diffractometer using graphite-monochromatized
Mo KR radiation (λ ) 0.71073 Å; ω scan technique). The Siemens
SHELXS-86 and SHELXL-93 software packages were used for solution
by direct methods, refinement, and diagrams of the structure. Details
of the data collection and structure determination are listed in Table 2.
Results and Discussion
New Phosphaboranes P2B10Cl10, P4B8Cl6, and P2B8Cl8. Our
initial study involved the thermal cage closure reaction of an
approximately equimolar mixture of B2Cl4 and PCl3 at 330 °C
to the six-vertex phosphaborane P2B4Cl4.11a The presence of
11a
ions in the mass spectrum indicative of P2B5Cl5 suggested
that this synthetic approach might be applicable to the synthesis
of larger phosphaborane cage systems. Following the thermal
conversion of neat B2Cl4 to the n-vertex boron monochlorides
BnCln (n ) 8-12) in which the size n and the distribution of
products has been found to be sensitive to the temperature and
time of reaction under thermolytic conditions, we have reex-
amined the copyrolysis of B2Cl4 with PCl3 in a molar ratio of
5:1 at temperatures above 400 °C, i.e., by employing reaction
conditions that should augment the amount of active BCl
moieties and thus increase the probability of forming larger
(15) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-SensitiVe
Compounds, 2nd ed.; John Wiley & Sons: New York, 1986.
(16) Timms, P. L. AdV. Inorg. Chem. Radiochem. 1972, 14, 121-171.

1284 Inorganic Chemistry, Vol. 39, No. 6, 2000
Keller et al.
and AsCl3 separately into the two flasks of a two-flask reactor,
and copyrolysis at 330 °C produces the six-vertex diarsaborane
closo-1,2-As2B4Cl4.13a Elevating the temperature to 450 °C
results in the formation of the icosahedral diarsaborane 1,2-
As2B10Cl10 and a molecule with elemental composition As4B8-
Cl6.13b The formulation of this compound is postulated as the
conjuncto-arsaborane (As2B4Cl3)2.13b
By adopting similar conditions to a copyrolysis of B2Cl4 with
a mixture of AsCl3 and PCl3 we have now obtained the new
mixed 6-vertex arsaphosphaboranes AsPB4Cl4 (4) andsonly in
tracessthe 12-vertex AsPB10Cl10. Based on the idealized eq 5,
the yield of 4 was approximately 5%.
AsCl3 + PCl3 + 7B2Cl4 f AsPB4Cl4 + 10BCl3 (5)
Figure 1. Molecular structure of 1,7-P2B10Cl10 (1) showing the atom-
numbering scheme. The numbering of the boron atoms has been omitted
for clarity. Selected bond distances (Å) and bond angles (deg) are as
follows (estimated standard deviations in parentheses): P(1)-B(2)
2.060(6), B(2)-B(3) 1.926(7), B(10)-B(12) 1.775(9), B(11)-B(12)
1.854(8), B-Paverage 2.0601 (total 10), B-Baverage 1.8214 (total 20),
B-Claverage 1.7622 (total 10), C-C(cycl)average 1.373 (total 6), C(1)-
C(7) 1.521(10), P(1)-P(7) 3.538, B(2)-B(9) 3.497, Cl(2)-Cl′(10′)
3.508; B(2)-P(1)-B(6) 53.2(2), B(2)-P(1)-B(3) 55.8(2), B(10)-
B(11)-B(12) 58.1(3), B(4)-B(3)-B(8) 58.3(3), P(1)-B(2)-B(3) 62.0-
(3), B(4)-B(9)-B(5) 62.3(3), B(4)-P(1)-B(6) 93.3(2), B(2)-P(1)-
B(4) 94.7(2), B(6)-B(10)-B(9) 109.1(4), P(1)-B(6)-B(10) 113.9(3),
B(4)-B(8)-B(7) 115.2(4), P(1)-B(2)-P(7) 119.0(3).
The elemental composition of 4 was confirmed by exact-
mass determination, and the 1,2-positions of the heteroelements
could be clearly assigned by 11B NMR spectroscopy.
NMR Studies. The 11B NMR spectrum of compound 1
consists of four resonances in a 2:4:2:2 pattern. This is consistent
with the C2V symmetry of an icosahedral molecule with two
equivalent heteroatoms. 11B-11B COSY NMR spectroscopy was
applied in order to assign 11B resonances to specific boron nuclei
but also to differentiate between positional isomers. In contrast
to the 1,2-isomer, the 1,7-isomer (see Figure 1 for the number-
ing) has a boron environment (2,3) that is adjacent to only one
other boron type (4,6,8,11). The signal at -2.0 ppm which
exhibits a cross peak only with the unique area 4 resonance at
1.9 ppm can therefore be assigned to B(2,3) and the latter, also
indicative from its integration, to B(4,6,8,11). The remaining
resonances at 7.6 and 0.7 ppm must come from either B(5,12)
or B(9,10). Both positions exhibit the same interboron linkages,
and therefore 11B-11B COSY NMR spectroscopy does not allow
one to assign these 11B resonances unequivocally. On the other
hand, the boron-phosphorus connectivities are not equal at these
positions and several diagnostics, such as selective B-P
coupling,10 broadening of the corresponding line width,11b or
weakening of 11B-11B correlations by neighboring hetero-
atoms,18 have been used to distinguish such signals unambigu-
ously. However, none of these features are found applicable
for 1, and therefore both equivocal signals are tentatively
assigned as follows: Comparison with the data of the previously
reported parent phosphaborane 1,7-P2B10H10,10a where the lowest
field signal could be reliably assigned to B(5,12) due to its
splitting into a doublet, suggests that the resonance at 7.6 ppm,
which appears at the lowest field in 1, should correspond also
to boron atoms B(5,12), and consequently the signal at 0.7 ppm
will correspond to B(9,10). These assignments are also similar
to those19 determined for 1,7-C2B10H10 and give further evidence
for the influence of an antipodal deshielding effect of hetero-
atoms to cross-cage boron atoms which has been observed in
the 11B spectra of a series of heteroborane clusters.20
phosphaborane cages. In agreement with our initial report, this
reaction again yielded P2B4Cl4 as the major product. However,
we were also able to isolate the 12-vertex closo-phosphaborane
P2B10Cl10 (1) and prepare phosphaboranes with the molecular
formula P4B8Cl6 (2) and P2B8Cl8 (3), as suggested in eqs 1-3.
2PCl3 + 13B2Cl4 f P2B10Cl10 + 16BCl3
4PCl3 + 15B2Cl4 f P4B8Cl6 + 22BCl3
2PCl3 + 11B2Cl4 f P2B8Cl8 + 14BCl3
(1)
(2)
(3)
After repeated fractional sublimation, closo-1,7-P2B10Cl10 (1)
(Figure 1) could be obtained in pure form, and it represents the
first structurally characterized icosahedral closo-diheteroborane
with the heteroatoms in nonadjacent positions. Compounds 2
and 3 were always obtained in a mixture with P2B4Cl4. High-
resolution mass determinations confirmed the molecular formula,
and analysis of the NMR signals for each compound led to clear
assignments and elucidation of the structures of these com-
pounds: boron-boron linked conjuncto-3,3′-(1,2-P2B4Cl3)2
geometry for 2 (Figure 3) and a 10-vertex closo-1,10-P2B8Cl8
geometry for 3 (Figure 4) with the P atoms in 1,10-positions.
Based on the idealized eq 1, the yield of 1 was approximately
5%. 1 does not melt until 230 °C in a sealed capillary. The
new phosphaboranes are air-sensitive but thermally stable, not
decomposing or rearranging until temperatures above 500 °C
in an evacuated glass tube. They are soluble in common aprotic
solvents as, e.g., benzene, toluene, chloroform, and methylene
chloride and slightly soluble in pentane.
The 31P NMR spectrum of 1 shows one resonance at -123
ppm which is remarkably upfield when compared to the 31P
(17) Morrison, J. A. In AdVances in Boron and the Boranes; Liebman, J.
F., Greenberg, A., Williams, R. E., Eds.; VCH Publishers: New York,
1988; pp 151-189.
New Mixed 6- and 12-Vertex Arsaphosphaboranes
AsPB4Cl4 and AsPB10Cl10. Parallel to the present study, first
attempts to prepare arsaboranes by the same type of reaction in
a single flask reactor failed. Thereby, AsCl3 is reduced almost
quantitatively by B2Cl4 to elemental arsenic and BCl3, as shown
in eq 4. This redox reaction is suppressed by condensing B2Cl4
(18) Fontaine, X. L. R.; Kennedy, J. D.; McGrath, M.; Spalding, T. R.
Magn. Reson. Chem. 1991, 29, 711-720 and references therein.
(19) (a) Garber, A. R.; Bodner, G. M.; Todd, L. J.; Siedle, A. R. J. Magn.
Reson. 1977, 28, 383. (b) Siedle, A. R.; Bodner, G. M.; Garber, A.
R.; Beer, D. C.; Todd, L. J. Inorg. Chem. 1974, 13, 2321-2324.
(20) (a) Hermanek, S. Chem. ReV. 1992, 92, 325-362. (b) Teixidor, F.;
Vinas, C.; Rudolph, R. W. Inorg. Chem. 1986, 25, 3339-3345. (c)
Fehlner, T. P.; Czech, P. T.; Fenske, R. F. Inorg. Chem. 1990, 29,
3103-3109.
2AsCl3 + 3B2Cl4 f 2As + 6BCl3
(4)

Crystal Structure of closo-1,7-P2B10Cl10
Inorganic Chemistry, Vol. 39, No. 6, 2000 1285
Figure 2. Perspective view of crystal packing at 1,7-P2B10Cl10‚C6H5CH3.
NMR signal of the reported10a all-hydrogen phosphaborane, the
ortho-isomeric 1,2-P2B10H10 (-19.3 ppm). Apparently, unlike
the antipodal deshielding effect caused by heteroatoms, chlorine
as a +M-type substituent exhibits a shielding influence from
positions B 5, 12 to the opposite P 1, 7. Antipodal shielding
toward P, C, and B nuclei has also been observed in substituted
icosahedral boranes such as 9,12-Br2-1,2-CHPB10H8 and
2- 19,21
1-ClB12H11
.
Figure 3. Proposed structure for P4B8Cl6 (2).
Attempts to separate the compounds P4B8Cl6 (2) and P2B8-
Cl8 (3) by fractionation or crystallization processes from their
mixtures with 1,2-P2B4Cl4 failed. However, the assignment of
the NMR signals to compounds 2 and 3 was possible by
comparison of NMR and GC/mass spectral data of different
fractions containing different amounts of either compound. Two
11B NMR signals at 24.8 and 15.9 ppm which show cross-peak
correlation in 11B-11B COSY NMR spectroscopy are present
in a ratio of 3:1 in all mixtures containing 2, 3, and 1,2-P2B4-
Cl4 and could be clearly assigned to P4B8Cl6 (2). The corre-
sponding signal at 31P NMR consists of one resonance at -182
ppm, not much different from that of the previously reported
closo-1,2-P2B4Cl4 at -187 ppm.11a Exact mass measurements
on the parent ion support the formulation of the compound 2
as P4B8Cl6, and application of Wade’s rules suggests that two
P2B4Cl3 units are symmetrically boron-boron linked to the
phosphaborane (P2B4Cl3)2, where three 11B NMR signals in a
ratio of 2:1:1 are expected. However, only two 11B NMR signals
are observed in 2, which suggests that the area 3 resonance is
due to accidental overlap of an area 2 and an area 1 signal.
Similar superposition of 11B signals has been observed for the
coupled-cage carborane 2,2′-(1,6-C2B4H5)2, where the chemical
shifts of the linked boron atoms B(2,2′) and the adjacent
B(3,3′,5,5′) differ only by 0.3 ppm.22 Since 2 shows only one
signal in the 31P NMR spectrum, the isomer with the P atoms
on positions having different boron environments P(1,1′,4,4′)
is excluded. On this basis, the remaining possible structures for
2 are 3,3′-(1,2-P2B4Cl3)2 and 2,2′-[1,6-P2B4Cl3]2. The NMR data
tend to favor the former isomer, shown in Figure 3, due to the
small difference in 31P NMR chemical shifts between 2 and
the cis-diphosphaborane closo-1,2-P2B4Cl4.
Figure 4. Proposed structure for P2B8Cl8 (3).
Figure 5. Proposed structure for AsPB4Cl4 (4).
molecule, which fits only for a bicapped square antiprism with
the P atoms in apical 1,10-positions, as represented in Figure
4. Resolution enhancement of the 11B NMR spectrum resulted
in a splitting of the resonance at 8.7 ppm into a pseudo-triplet
with a splitting of the outer lines corresponding to Σ[1J(31P,11B)
1
3
+ J(11B,11B) + J(31P,11B)] ) 42.6 Hz.
The 11B NMR spectrum of 1,2-AsPB4Cl4 (4) consists of two
signals of equal intensities at 24.8 and 7.0 ppm, indicating a
structure analogous to those of 1,2-P2B4Cl4 and 1,2-As2B4Cl4
with adjacent heteroatoms as given in Figure 5. Surprisingly,
unlike other previously reported mixed diheteroboranes as 1,2-
PEB10H10 (E ) As, Sb),10b where the signal of the boron atoms
B(9,12) is split due to the unsymmetrical substitution, in 4 no
splitting of the 11B NMR signal at 24.8 ppm, which derives
from the boron positions B(4,6) lying opposite to the hetero-
atoms P and As, could be observed. The 31P NMR spectrum
The 11B NMR and the 31P NMR spectra of P2B8Cl8 (3) consist
of one signal each, at 8.7 ppm and -195 ppm, respectively,
indicating equivalent boron and phosphorus sites in this
(21) Srebny, H. G.; Preetz, W.; Marsmann, H. C. Z. Naturforsch. 1983,
39b, 189-196.
(22) Corcoran, E. W.; Sneddon, L. G. J. Am. Chem. Soc. 1984, 106, 7793-
7800.

1286 Inorganic Chemistry, Vol. 39, No. 6, 2000
Keller et al.
ref
Table 3. 11B Chemical Shifts for Icosahedral Boranes
shows one signal at -169 ppm which is downfield when
compared to the signal for 1,2-P2B4Cl4, found at -187 ppm.
This downfield trend of the 11B and 31P chemical shifts in
compounds in which one phosphorus atom is substituted by
arsenic is in agreement with the NMR data reported for the 12-
vertex diheteroboranes 1,2-P2B10H10 and 1,2-PAsB10H10.10
The signal assignments of 1, 4, and other halogenated
heteroboranes have been further confirmed using homonuclear
1D 11B{11B} decoupling experiments which are discussed in a
separate paper.23 These studies also provide insights into
1J(31P,11B), 1J(31P,31P), and 1J(11B,11B) and allow comparisons
with previously reported values.
δ ppm
compd
2-
B12H12
B12Cl12
-15.3 -15.3
-12.9 -12.9
-15.3
-12.9
-15.3 20a
-12.9 17
2-
E(1,7)
P2B10H10
P2B10Cl10
B(5,12) B(9,10) B(4,6,8,11) B(2,3)
11.2
7.6
3.4
0.7
-0.4
1.9
-3.3
-2.0
10a
this work
E(1,2)
As2B10H10
As2B10Cl10
B(9,12) B(3,6)
B(4,5,7,11) B(8,10)
15.5
12.9
3.8
6.0
0.9
3.6
-0.8
0.3
25
13b
26
24
(CH)2B10H10 -1.2
(CH)2B10Cl10 1.2
-14.3
-13.0
-12.9
-10.3
-8.2
-7.3
E(1)
SB11H11
SB11Cl11
B(12)
18.4
13.3
B(7-11) B(2-6)
Chemical Shifts of Halogenated Icosahedral Heterobo-
ranes. In recent years, there have been several reports on factors
describing characteristic 11B NMR chemical shift changes
caused by skeletal heteroatom or ligand substitution in parent
boranes.20 Thus, replacement of an icosahedral cage-boron
vertex BH- by an isolobal heterovertex E ) CH < P < As <
Sb < S+ results in a progressive deshielding of the skeletal boron
atoms which is most significant at the boron atom opposite the
heteroatom even though it is the farthest away. This observation
has been labeled the antipodal effect or A-effect.
-4.2
-6.7
-3.7
27
13b
1.0
deshielding effect of heteroatoms is gradually diminished by
chlorine ligands, which may mainly result from the antipodal
shielding of chlorine ligands bound to boron atoms cross-cage
to the heteroatoms. The strongest diminution of the antipodal
deshielding is found at SB11Cl11 with an increment of 5.1 ppm
on position B(12). In 1,7-P2B10Cl10 (1) not only the “antipodal”
boron atoms B(5,12) but also B(9,10) are deshielded compared
to the all-hydrogen compound which gives rise to a conversion
of the 2:2:4:2 signal ratio into 2:4:2:2.
Substitution of hydrogen ligands by halogen atoms has been
found to result in a superposition of several, partly counteracting
effects. Primarily, halogen ligands cause an antipodal effect
which is reverse in sign compared to the antipodal effect caused
by heteroatoms. In addition, due to both the inductive attraction
and the π-donation exhibited by electronegative substituents
bearing free electron pairs, there is a progressive -I type
deshielding mainly of the substituted R-boron atoms in the order
Br < Cl < F and, in turn, a +M type shielding of all neighboring
â-, γ-, and δ-skeletal atoms which again is the most effective
at the cross-cage positions. The order of the described effects
depends on the nature of the substituents and cluster geometry.
Experience has shown that heteroatoms (except Al, Si) exhibit
a stronger antipodal effect than do +M substituents at the same
positions. In halogenated heteroboranes it is therefore difficult
to assess the NMR spectra as substitution increases. The most
common examples for the exo-cage substituent effects are partial
halogenation, either mono- or dihalogenated carboranes. How-
ever, until recently there was a lack of experimental NMR data
on fully halogenated heteroboranes with the exception of
B-perchloro-o-carborane.24 Supplementation with the present
NMR data and that of the previously reported perchloroarsa-
and thiaboranes13b (Table 3) gives a number of such compounds
which still is quite small, but allows one to qualitatively assess
some shielding trends in perchlorinated icosahedral closo-
heteroboranes and also helps to quantify the changes in 11B
NMR chemical shifts introduced by the substitution of het-
erovertices and chlorine ligands.
Crystal Structure Analysis of 1. As expected, the single-
crystal X-ray analysis confirms the general structural features
proposed for 1 based upon spectral data. The structure consists
of a distorted icosahedron of 10 boron atoms and two phos-
phorus atoms in the 1,7-positions whereby the symmetry is
reduced from Ih to C2V. Within the limits of error there are two
mirror planes passing through atoms B(2,3,9,10) and through
B(5,12),P(1,7). An ORTEP drawing with atom labeling, selected
bond lengths, and bond angles is given in Figure 1. Deviations
from regular bond distances and angles of the ideal icosahedral
structure are obviously a consequence of the substitution of the
larger-sized phosphorus atoms. Within the pentagonal belt
defined by P1-B2-P7-B8-B4 the bond angles of P1-B2-
P7 and B4-B8-B7 have been increased to 119.0(3)° and 115.2-
(4)° compared with the ideal pentagonal angle of 108° while
the bond angle of B2-P1-B4 has been reduced to 94.7(2)°.
Similar, but less distinct, trends have been observed at the
icosahedral monophosphorus phosphaborane CH3PB11H11, in
which the average pentagonal B-B-B and B-P-B bond
angles have been found to be 110.2° and 101.5°, respectively.5
The boron-boron distance between the equivalent B2 and B3
atoms, adjacent to both phosphorus atoms, is 1.926(7) Å while
the average boron-boron distance is 1.827 Å, which is slightly
2-
longer than those found in B12H12 (1.77 Å).28 The shortest
boron-boron distances (average 1.786 Å) are located between
B(5,12) and B(9,10), which are the boron atoms opposite and
farthest from the phosphorus atoms. The average B-P distance
of 1 (2.060 Å) compares well with the B-P bond lengths
reported for other icosahedral phosphaboranes (2.038-1.953
Å).5,10b The intramolecular P-P distance is 3.538 Å while that
of the cross-cage boron atoms B5-B12 is markedly shorter
(2.910 Å). The intermolecular Cl-Cl contacts are 3.508 Å or
longer.
As can be observed in Table 3, perchlorination of closo-
B12H122- shifts the 11B NMR signal 2.4 ppm to the lower field,
indicating that the -I-type electron attraction of chlorine in
2-
closo-B12Cl12 exceeds its electron donation brought by +M
type shielding, which leads to a net deshielding of the substituted
boron atoms. In analogy, compared to the unsubstituted
compounds, the boron atoms of the chlorinated carborane are
deshielded at all positions and those of the heteroboranes E2B10-
Cl10 (E ) P, As) and SB11Cl11 at most positions, but not at
those antipodal to the heteroatoms. Obviously, the antipodal
(25) Little, J. L.; Pao, S. S.; Sugathan, K. K. Inorg. Chem. 1974, 13, 1752-
1756.
(26) Venable, T. L.; Hutton, W. C.; Grimes, R. N. J. Am. Chem. Soc. 1984,
106, 29-37.
(23) Keller, W.; Haubold, W.; Wrackmeyer, B. Magn. Reson. Chem. 1999,
37, 545-550.
(24) Bregadse, W. I.; Usjatinski, A. J.; Antonowitsch, W. A.; Godowikow,
N. N. IzV. Akad. Nauk SSSR, Ser. Khim. 1988, 670-674.
(27) Hnyk, D.; Vajda, E.; Bu¨hl, M.; Schleyer, P. v. R. Inorg. Chem. 1992,
31, 2464-2467.
(28) Wunderlich, J. A.; Lipscomb, W. N. J. Am. Chem. Soc. 1960, 82,
4427-4428.

Crystal Structure of closo-1,7-P2B10Cl10
Inorganic Chemistry, Vol. 39, No. 6, 2000 1287
Crystals of P2B10Cl10 contain toluene in a 1:1 molar ratio as
illustrated in Figure 2. The toluene rings are exactly perpen-
dicular to the P atoms of two neighboring icosahedrons at a
distance between 3.04 and 3.08 Å with a mean P-C distance
of ∼3.1 Å, which is well below the sum of the van der Waals
radii of ∼3.5 Å. The proximity and the linear orientation of
toluene between two cluster molecules suggest a significant
electronic interaction between the lone pairs of the P atoms and
the aromate and cannot be compared with the bonding in
metallaborane sandwiches where the aromatic ligands function
as isoelectronic and isolobal electron donors to the metal.
Cluster Size and Isomer Preferences. The preferred forma-
tion of phosphaborane clusters of sizes 6, 10, and 12 by a
thermodynamically controlled synthetic route is in agreement
with recent computional studies dealing with the relative stability
and “three-dimensional aromaticity” of closo-borane dianions
BnHn2-, closo-monocarborane anions CBn-1Hn-, and closo-
dicarboranes C2Bn-2Hn.29 It was found that on the basis of
average BH-bond energies for each member’s most stable
positional isomer, the 12- and 6-vertex polyhedra are more stable
than the other members of their families, followed by the 10-
and 7-vertex systems.
P2B8Cl8 (3) obviously prefer the apical position, i.e., where the
connectivity is the lowest and the charge density is the
largest29a,30 within the reference compound B10H102-. In contrast,
the coordination number of the cluster vertices at 1 and 2 is
equal for all positions. Calculation8g of the relative stabilities
of icosahedral diphosphaboranes P2B10H10 suggests that the
isomer stability should follow the order 1,12 > 1,7 > 1,2, which
is confirmed in part by the previously described thermal
conversion of 1,2-P2B10H10 to the 1,7-isomer at 560 °C10a and
the present synthesis of 1,7-P2B10Cl10 (1) via pyrolytic conditions
at temperatures above 400 °C. The para isomer is not yet
amenable to synthesis, which suggests that the formation of this
compound may require temperatures beyond its thermal sta-
bility.
Acknowledgment. Support of this work by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen Indus-
trie is gratefully acknowledged. We thank Drs. Ho¨nle and Nuss
at the Stuttgart Max-Planck-Institut for mounting the crystalline
sample in capillaries and Hans-Ju¨rgen Gutke for his assistance
in obtaining the high-resolution mass spectral data. Prof. William
Quintana at the New Mexico State University and Prof. Robert
Greatrex from the Leeds group are also thanked for helpful
comments.
The sites of the heteroatom placements in the new phos-
phaboranes follow closely trends already observed in closo-
dicarba- and parent phosphaboranes. Analogous to the 10-vertex
C2B8H10 carboranes with 1,10-C2B8H10 as the thermodynami-
cally most stable positional isomer,30 the phosphorus atoms in
Supporting Information Available: Interatomic distances, selected
bond angles, and selected dihedral angles between least-squares planes.
X-ray crystallographic files for the structure determination of 1‚toluene,
in CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
(29) (a) Schleyer, P. v. R.; Najafian, K. Inorg. Chem. 1998, 37, 3454-
3470. (b) Schleyer, P. v. R.; Najafian, K.; Mebel, A. M. Inorg. Chem.
1998, 37, 6765-6772.
(30) Ott, J. J.; Gimarc, B. M. J. Am. Chem. Soc. 1986, 108, 4303-4308.
IC990112V