Isolation of a New Nonaborane Cluster Form: Arachno-B9H11·(PPh3)2

Article abstract of DOI:10.1021/ic010810r

The novel cluster arachno-B₉H₁₁(PPh₃)₂ (1) is formed in relatively high yield in the reaction between B₅H₉ and Cp₂ZrCl₂/n-BuLi in the presence of PPh₃ and air. The structure is based on that of iso-arachno-nonaborane, with the two phosphines in exposed positions in the cluster where three of the five endo-hydrogen atoms reside in [B₉H₁₄]^-. Compound 1 is the second member of the unique class of borane clusters to which the versatile species B₁₀H₁₂L₂ also belongs.

Full text of DOI:10.1021/ic010810r

6852
Inorg. Chem. 2001, 40, 6852-6854
structural studies11 suggest that the structure shown as V is
correct.
Isolation of a New Nonaborane Cluster Form:
arachno-B9H11‚(PPh3)2
Mitsuhiro Hata, Nigam P. Rath, and
Lawrence Barton*
Department of Chemistry, University of Missouri-St. Louis,
8001 Natural Bridge Road, St. Louis, Missouri 63121
ReceiVed July 30, 2001
Introduction
The nine vertex, nonclosed polyhedral boranes have been the
subject of much interest and activity for many years.1 The
structures of the arachno-nonaboranes provide interesting, and
consistent, deviations from the coordination number pattern
recognition theory for boranes and carbaboranes, as described
by Williams in 1976.2 The theory requires that, to generate
arachno clusters from nido systems, a highest connectivity
vertex, adjacent to the open face, is removed. The predicted
cluster framework for an arachno nine-vertex system is illu-
strated by structure I and recognized as n-arachno-B9H15.3 It is
obtained by the removal of a five-connected vertex from nido-
B10H14 (structure II). In contrast, the more commonly encoun-
tered nine-vertex arachno species are derived by removal of a
low connectivity vertex from nido-B10H14 to generate derivatives
of what are referred to as i-B9H15 (structure III)4 and i-C2B7H13.5
Herein, we add to the series of known nonaborane species by
describing the new species B9H11‚(PPh3)2, which is formed in
the oxidative fusion of B5H9 in the presence of Cp2ZrCl2/
n-BuLi and PPh3.
Experimental Section
Reactions were carried out under high vacuum or a dry nitrogen
atmosphere. Solvents were reagent grade. THF and CH2Cl2 were dried
and distilled from sodium/benzophenone and CaH2, respectively, prior
to use. PPh3 and n-BuLi 1.6 M hexane solutions were commercially
purchased from Aldrich and used as received. Cp2ZrCl2 (Alfa Aesar)
was used as received. Column chromatography was done on a 2 cm2
x 12 cm column using 200-400 mesh silica gel (Natland International).
NMR spectra were recorded on a Varian Unity Plus 300 spectrometer
and a Bruker ARX 500 spectrometer. 1H, 31P, and 11B chemical shifts
are referenced to SiMe4 (0.0 ppm), BF3‚Et2O (0.0 ppm), and 85%
H3PO4 (0.0 ppm), respectively, and reported in ppm, with a negative
sign indicating an upfield shift. The IR spectrum was recorded on a
Perkin-Elmer 1600 FTIR spectrometer. The mass spectrum was
obtained in the FAB mode on a JEOL MStation JMS-700 spectrometer
using 3-nitrobenzyl alcohol (3-NBA). Atlantic Microlabs Inc., Norcross,
GA performed elemental analysis.
Synthesis of arachno-B9H13(PPh3)2 (1). To a 50 mL three-neck flask
equipped with a stir bar, stoppers, and a high-vacuum stopcock, was
added Cp2ZrCl2 (293 mg, 1.0 mmol). The flask was evacuated on the
vacuum line, and THF (3 mL) was condensed in at -196 °C. After
nitrogen was introduced, a stopper was replaced with a septum, and
n-BuLi (1.6 M hexane solution, 1.3 mL, 2.0 mmol) was added dropwise
at -78 °C with vigorous stirring. The reaction mixture became a light
yellow solution, and stirring was continued at -78 °C for 1h. The
septum was replaced with the stopper, the flask was degassed at -196
°C, and B5H9 (0.94 mmol) was introduced. The reaction mixture was
allowed to warm to room temperature and stirred for 1h, affording a
red solution. Evaporation of all the volatiles at room temperature under
high vacuum gave a deep red solid. PPh3 (262 mg, 1.0 mmol) and
CH2Cl2 (6 mL) were added to the open flask. Then air was bubbled
into the reaction mixture with stirring at 0 °C for 1h. Evaporation of
the solvent afforded an orange solid which contained B9H11‚(PPh3)2
(1), along with B9H14-, BH3‚PPh3, and unidentified zirconocene species.
Nonaborane adducts and anions in this class also show the
same tendency. Thus, the well-known arachno anion [B9H14]-
(IV),6 and the adducts 4-L-arachno-B9H13 (VI)7 and 5-L-
arachno-B9H13 (VIII),8 all adopt the i-B9H15 structure, as does
a reported borane VII,9 which is an isomer of VIII. It appears
that the positions of the endo and bridging H atoms in [B9H14]-
are not satisfactorily resolved in the literature. The result of the
original structural study reported by Greenwood6 is shown as
structure IV, but recent calculations10,11 and two unpublished
(1) (a) Shore, S. G. In Boron Hydride Chemistry; Muetterties, E. L., Ed.;
Academic Press, New York, 1975; Chapter 3, pp 144-146. (b) Barton,
L. Top. Curr. Chem. 1982, 100, 169.
(2) Williams, R. E. AdV. Inorg. Chem. Radiochem. 1976, 18, 66.
(3) Huffman, J. C., Ph.D. Dissertation, Indiana University, 1974; p 771.
(b) Beaudet, R. A. Mol. Struct. Energ. 1986, 5, 417.
(4) (a) Dobson, J.; Keller, P. C.; Schaeffer, R. J. Am. Chem. Soc. 1965,
87, 3522. (b) Dobson, J.; Keller, P. C.; Schaeffer, R. Inorg. Chem.
1968, 7, 399.
(5) Voet, D.; Lipscomb, W. N. Inorg. Chem. 1967, 6, 113.
(6) (a) Greenwood, N. N.; Gysling, H. J.; McGinnety, J. A.; Owen, J. D.
J. Chem. Soc., Chem. Comm., 1970, 505. (b) Greenwood, N. N.;
Gysling, H. J.; McGinnety, J. A.; Owen, J. D. J. Chem. Soc., Dalton
Trans. 1972, 986.
(7) Wang, F.; E. Simpson, P. G.; Lipscomb, W. N. J. Chem. Phys. 1961,
8, 464.
(11) (a) Fang, H., Ph.D. Dissertation, University of Missouri, St. Louis,
1996; [Diss. Abstr. Intl. 1995, 56, B 4306]. (b) Huffman, J. D. Report
No. 82210, Indiana University Department of Chemistry, Molecular
Structure Center, 1982; Cited in: Getman, T. D., Krause, J. A.,
Niedenzu, P. M, Shore, S. G. Inorg. Chem. 1989, 28, 1507. (c) Bould,
J.; Greatrex, R.; Kennedy, J. D.; Ormsby, D.; Londesborough, M. G.
S.; Callahan, K. F.; Thornton-Pett, M.; Teat, S. J.; Clegg, W.; Fang,
H.; Rath, N. P.; Barton, L.; Spalding, T. R., submitted for publication.
(8) Callaghan, K. L. F.; Dorfler, U.; McGrath, T. D.; Thornton-Pett, M.;
Kennedy, J. D. J. Organomet. Chem. 1998, 550, 441.
(9) Andrews, S. J.; Welch, A. J. Acta Cryst. 1985, C41, 1208.
(10) Hofmann, M.; Schleyer, P. von R. Inorg. Chem. 1999, 38, 652.
10.1021/ic010810r CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/15/2001

Notes
Inorganic Chemistry, Vol. 40, No. 26, 2001 6853
Column chromatography using CH2Cl2/hexane ) 1:1 as elute was
carried out to afford 1 (Rf 0.5) as white solid in up to 50% yield based
on B5H9. However, because of partial decomposition on silica gel, it
also included BH3‚PPh3. Further purification was effected by recrys-
tallization from CH2Cl2/ether to afford a white crystalline solid (103
mg, 0.163 mmol, 35% based on B5H9). 1H{11B} NMR (300 MHz,
CDCl3): δ 7.8-7.3 (m. 30H, Ph), 2.85 [br, s, 2H, BH(7,9)], 2.64 [br,
s, 1H, BH(5)], 2.43 [br, s, 1H, BH(8)], 1.76 [br, s, 2H, BH(1,2)], -0.04
[br, s, 2H, BH(4,6)], -0.59 [br, s, 1H, BH(3)], -1.88 [br, s, 2H, Hµ7,8
,
Hµ8,9] ppm. 11B{1H} NMR (96.2 MHz, CDCl3): 0 [br, 2B, B(7,9)],
-2.25 [br, 1B, B(5)], -9.72 [br, 2B, B(1,2)], -15.43 [br, 1B, B(8)],
-41.42 [br, 2B, B(4,6)], -51.67 [br, 1B, B(3)] ppm. 31P{1H} NMR
(121 MHz, CDCl3): 18.5 (br, m) ppm. IR (KBr): ν (BH) ) 2546 (m),
2420 (m), 2471 (w) cm-1. Mass spectral data for 1: LRMS, FAB with
3-NBA. Calc. for [M + H]+ C36H41B9P2 max: 634, obs. 634. The mass
envelope in this region for the measured spectrum matches that
calculated from the known isotopic abundances of the constituent
elements. The observed m/q values (relative intensity) for the protonated
molecular ion cluster were 629 (5.9); 630 (12.0); 631 (27.5); 632 (58.0),
633 (91.9); 634 (100.00); 635 (64.00); 636 (20.2); 637 (4.6). The
calculated m/q data are 629 (0.63); 630 (3.92); 631 (16.32); 632 (45.97);
633 (85.77); 634 (100.00); 635 (64.68); 636 (20.2); 637 (3.25).
Elemental Anal. Calcd for C36H41B9P2: C, 68.31; H, 6.53. Found: C,
68.08; H, 6.54.
Crystal Structure Determination.12 Data collection was carried out
using a Bruker CCD diffractometer. The structure was solved at -50
°C using direct methods and refined by full matrix least squares
refinement to convergence. The hydrogen atoms on boron were located
and refined freely. All other hydrogen atoms were treated using an
appropriate riding model.
Figure 1. Two views of the molecular structure of arachno-B9H13-
(PPh3)2 with 50% thermal ellipsoids. One has the phenyl groups omitted
for clarity.
Table 1. Crystal Data and Structure Refinement for Compound 1
chem
formula
temp/K
D(calcd)
Mg/m3
a/Å
B9H11(PPh3)2 empirical
formula
C36H41B9P2
Results and Discussion
223(2) K
1.182
fw
632.92
Treatment of Cp2ZrCl2 with n-BuLi at dry ice temperature
and stirring for 1 h followed by addition of B5H9 afforded a
red solid which contained boron but decomposed before we
could identify it. These experiments were done in order to try
to obtain Cp2Zr derivatives of B5H9. Cp2ZrCl2/n-BuLi combina-
tions are commonly used in organic synthesis.13 When a
suspension of Cp2ZrCl2 in THF is treated with 2 equiv of n-BuLi
at -78 °C, metathesis occurs to give Cp2ZrBu2. This complex
affords Cp2Zr(CH2dCHEt) and BuH, via â-hydride elimination
on warming to 0 °C, and serves as a convenient precursor to
zirconocene “Cp2Zr” due to the lability of the π ligand. Thus,
we thought that “Cp2Zr” might be incorporated into the B5 cage
as a vertex, since as a 14e metal fragment, it should supply
two skeletal electrons, as does the BH fragment. To stabilize
the metallaborane product, we added PPh3. This also afforded
no identifiable product, but accidental exposure to air formed a
white solid soluble in CH2Cl2, CHCl3, and only sparingly soluble
in ethers. These observations led to a modified preparative
procedure in which air was bubbled through the solution. This
procedure is related to the well known oxidative fusion used in
many laboratories but pioneered in polyhedral borane chemistry
by Grimes.14 This allowed for convenient isolation of the white
solid which, on recrystallization from CH2Cl2/ether, afforded a
pure white microcrystalline solid B9H11(PPh3)2 (1) in 35% yield.
The 11B NMR spectrum of 1 exhibited six resonances in a
2:1:2:1:2:1 area ratio suggesting a symmetrical cluster containing
radiation (λ, Å) Mo KR (0.71073)a
9.3251(3)
28.3923(9)
3557.3(2)
4
space group
b/Å
P21/c
c/Å
13.5243(4)
96.556(2)°
0.148
V/Å-3
Z
â/°
abs coeff
mm-1
final R indices R1b ) 0.0464 R indices
[I > 2σ(I)]
wR2c ) 0.1265
(all data)
a Graphite monochromator. b R1 ) ∑||Fo| - |Fc|| /∑|Fo|. c wR2 )
[∑[w(Fo - Fc2)2] /∑[w(Fo )2 ]]1/2
.
2
2
nine boron atoms. The observation of the broad phosphine
resonance at 18.5 ppm confirmed the presence of a phosphine-
borane type moiety.15 The H NMR spectrum afforded reso-
1
nances in a 2:1:1:2:2:1 ratio assigned to terminal H atoms on
boron and a resonance at -1.82 ppm of area 2 assigned to the
two bridging H atoms. The individual assignments of the NMR
spectra, obtained from 2D (1H-11B) COSY experiments run
on the Bruker 500 MHz spectrometer, are given in the
Experimental Section. These results suggested that the cluster
contained 9 boron atoms, 11 hydrogen atoms, and 2 phosphines
associated with 2 of the boron atoms.
Crystals suitable for X-ray diffraction were grown very slowly
from CH2Cl2/ether solutions at 23 °C, and the resultant structure
is given in two views in Figure 1; crystal data and structure
refinement for 1 are given in Table 1. The structure is that
derived from iso-arachno-nonaborane(15), with the phosphine
ligands symmetrically disposed in the prominent 6,8 positions.
This is quite novel for this class of species, as is clear from an
examination of the known arachno-nonaborane cluster motifs
illustrated as structures IV-IX. The bond distances and angles
(12) (a) SHELXTL Software Package; Bruker Analytical X-ray Division:
Madison, WI, 2001. (b) For further details see Experimental Section
in: McQuade, P.; Hupp, K.; Bould, J.; Fang, H.; Rath, N. P.; Thomas,
R. Ll.; Barton, L. Inorg. Chem. 1999, 38, 5415.
(13) (a) Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron Lett.
1986, 27, 2829. (b) Negishi, E.; Montchamp. J.-L. In Metallocenes;
Togni, A., Halterman, R. I., Eds.: VCH: Weinheim, 1998; Chapter
5, pp 241-319.
(14) (a) Wong, K.-S.; Bowser, J. R.; Pipal, J. R.; Grimes, R. N. J. Am.
Chem. Soc. 1978, 100, 5045. (b) Grimes, R. N. Pure Appl. Chem.
1987, 59, 847.
(15) (a) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spec-
troscopy of Boron Compounds; Springer-Verlag: Heidelberg, 1978;
pp 340-344. (b) Nainan, K. C.; Ryschewitsch, G. E. Inorg. Chem.
1969, 8, 2671.

6854 Inorganic Chemistry, Vol. 40, No. 26, 2001
Notes
correlate very well. Note the numbering scheme for 1 is
modified in order to facilitate the comparison. Since the
numbering is detemined by the placement of substituents, those
for illustrations VI and XI are necessarily different.
Compound 1 is only the second member of the important
class of arachno-borane clusters to which belongs the species
arachno-B10H12L2, where L is a Lewis base such as CH3CN or
Me2S. Such species, which are described by the formula
L2BnHn+2, are rare and are not known for any other borane
clusters.16 Probably no boron hydride cluster has seen more
applications than the series of compounds described by the
general formula B10H12L2 and the species for which they are
precursors.17 It is reasonable to expect similar chemistry for 1.
The species B10H12L2 is the precursor to the best known of the
carboranes, C2B10H12,18 and also may be converted to closo-
[B10H10]2- 19
. Thus, B9H11L2 could be a very useful synthon in
polyhedral borane chemistry, perhaps not as the stable PPh3
adduct, but involving a weaker donor base, such as Me2S or
Me2NC6H5.
Figure 2. Correlation diagram comparing the 11B NMR spectra of
[B9H14]-, [B9H12Br2]-, and B9H11(PPh3)2. Note that the numbering
scheme for 1 has been modified to correlate with those for [B9H14]-
and [B9H12Br2]- and is given.
Acknowledgment. We thank the National Science Founda-
tion (Grant # CHE 9727570) and the Missouri Research Board
for support of this work. We also acknowledge the National
Science Foundation, the Department of Energy, and the UM-
St. Louis Center for Molecular Electronics for providing funds
used to purchase the NMR spectrometers, X-ray diffractometer,
and mass spectrometer. We acknowledge the assistance of
Professors J. B. Wilking with the NMR spectra and R. E. K.
Winter with the mass spectra.
Table 2. Selected Interatomic Distances [Å] and Angles [deg] for 1
B(4)-B(5)
B(4)-B(9)
B(5)-B(6)
1.817(3) B(6)-B(7)
1.864(3) B(7)-B(8)
1.818(3) B(8)-B(9)
1.883(3)
1.826(4)
1.829(4)
119.8(2)
117.1(1)
B(5)-B(4)-B(9) 106.9(2)
B(5)-B(6)-P(2)
B(7)-B(6)-P(2)
B(5)-B(4)-P(1)
B(9)-B(4)-P(1)
125.4(1)
112.9(1)
B(8)-B(7)-B(6) 116.7(2)
B(9)-B(8)-B(7) 104.1(2)
B(8)-B(9)-B(4) 117.9(2)
B(4)-B(5)-B(6) 105.1(2)
Supporting Information Available: Crystallographic file (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
B(5)-B(6)-B(7) 106.2(2)
do not differ much from those of related arachno-nonaboranes,
IC010810R
3b
Table 2. The long distances in B10H14 and in [B9H14]-,11
corresponding to B(6)-B(7) and B(4)-B(9) in 1, are ca. 0.1 Å
longer in 1 than the analogous distances in [B9H14]-, reflecting
the increased electron density on B(4) and B(6). Also, the angle
B(4)-B(5)-B(6) is about 10 ° less in 1, clearly suggesting that
electronic rather than steric factors influence the structure.
It is pertinent to compare the 11B NMR spectra of 1 with the
other arachno-nonaboranes, and we have chosen to include only
[B9H14]-, the unpublished [B9H12Br2]-,11c and none of the
monoadducts of the type illustrated by structures VI and VII.
A correlation diagram is given as Figure 2. The effects of the
shielding of B(6,8) by donation of electron density by the
phosphine moieties and the opposite effect on B(7) are the major
features noted. Otherwise, the spectra of the three related species
(16) Of course there is another borane species which belongs to this class,
B2H4L2, which, however, we do not define as a cluster. See (a) Hertz,
R. K.; Denniston, M. L.; Shore, S. G. Inorg. Chem. 1978, 17, 2673.
(b) DePoy, R.; Kodama, G. Inorg. Chem. 1985, 24, 2871.
(17) For example see: (a) Hawthorne, M. F. Spec. Publ. R. Soc. Chem.
1997, 201, 261. (b) Hawthorne, M. F. Spec. Publ. R. Soc. Chem. 2000,
253, 197.
(18) (a) Heying, T. J.; Ager, J. W.; Clark, S. L.; Mangold, D. J.; Goldstein,
H. L.; Hillman, M.; Polak R. J.; Szymanski, J. W. Inorg. Chem. 1969,
2, 1089. (b) Fein, M. M.; Bobinski, J.; Mayes, N.; Schwartz, N.; Cohen,
M. S. Inorg. Chem. 1963, 2, 1111. (c) Fein, M. M.; Paustian, J. E.;
Bobinski, J.; Lichstein, B. M.; Mayes, N.; Schwartz, N.; Cohen, M.
S. Inorg. Chem. 1963, 2, 1115.
(19) Muetterties, E. L,; Knoth, W. H. Chem. Eng. News, May 9 1966, p
88. (b) Muetterties, E. L.; Knoth, W. H. Polyhedral Boranes; Marcel
Dekker: New York, 1968.