Formation of metallaboranes from the group IV transition metals and pentaborane(9): Crystal and molecular structure of [(Cp2Zr)2B5H8] [B11H14]

Article abstract of DOI:10.1021/ic010665h

The reactions between [(C₅H₅)₂MCl₂] (where M = Ti, Zr, Hf) and Li[B₅H₈] in a variety of solvents have been investigated. In the case of Zr, a pale orange solid, μ-(Cp₂ClZr)B₅H₈ (1), is formed in 70% yield. Compound 1 exists as a B₅H₉ cage with a Cp₂ClZr moiety replacing a bridging H atom. The variable temperature NMR spectra of 1 reveal two fluxional processes, one (ΔG? = 54 kJ mol^-1) which renders a plane of symmetry in the molecule and a higher temperature one (ΔG? = 48 kJ mol^-1) which renders all the basal B atoms equivalent. Dynamic processes are suggested to account for these observations. Passage of a CH₂Cl₂ solution of 1 through silica gel affords 2, [(Cp₂Zr)₂B₅H₈] [B₁₁H₁₄], a yellow, air-stable, crystalline solid, in 14% yield. The cation in 2, [(Cp₂Zr)₂B₅H₈]⁺, consists of a distorted spiro[2.2]pentane-like B₅ moiety comprising two B₃ triangles sharing a naked boron vertex. The two triangles are twisted 73° with respect to each other, and the two [Cp₂Zr] groups bond in a trihapto arrangement to the two opposite B-B-B edges. Each exterior B-Zr edge is H-bridged, and the B atoms possess terminal hydrogens. Reactions of Cp₂HfCl₂ with Li[B₅H₈ lead to the formation of the analogue of 2, [(Cp₂Hf)₂B₅H₈] [B₁₁H₁₄] (3). The precursor to 3, that is, the Hf analogue of 1, is not observed. Reaction between Li[B₅H₈] and Cp₂TiCl₂ afforded no identifiable products, but reaction with CpTiCl₃ resulted in cage coupling and the formation of B_1OH₁₄.

Full text of DOI:10.1021/ic010665h

Inorg. Chem. 2002, 41, 67−75
Formation of Metallaboranes from the Group IV Transition Metals and
Pentaborane(9): Crystal and Molecular Structure of
[(Cp2Zr)2B5H8][B11H14]
Rhodri Ll. Thomas, Nigam P. Rath, and Lawrence Barton*
Department of Chemistry, UniVersity of MissourisSt. Louis, St. Louis, Missouri 63121
Received June 21, 2001
The reactions between [(C H ) MCl2] (where M ) Ti, Zr, Hf) and Li[B H ] in a variety of solvents have been
5
5 2
5 8
investigated. In the case of Zr, a pale orange solid, µ-(Cp2ClZr)B H (1), is formed in 70% yield. Compound 1
5
8
exists as a B H cage with a Cp2ClZr moiety replacing a bridging H atom. The variable temperature NMR spectra
5
9
‡
of 1 reveal two fluxional processes, one (∆G ) 54 kJ mol-1) which renders a plane of symmetry in the molecule
‡
and a higher temperature one (∆G ) 48 kJ mol-1) which renders all the basal B atoms equivalent. Dynamic
processes are suggested to account for these observations. Passage of a CH Cl2 solution of 1 through silica gel
2
affords 2, [(Cp2Zr)2B H ][B H ], a yellow, air-stable, crystalline solid, in 14% yield. The cation in 2, [(Cp2Zr)2B H ]+,
5
8
11 14
5 8
consists of a distorted spiro[2.2]pentane-like B moiety comprising two B triangles sharing a naked boron vertex.
5
3
The two triangles are twisted 73° with respect to each other, and the two [Cp2Zr] groups bond in a trihapto
arrangement to the two opposite B−B−B edges. Each exterior B−Zr edge is H-bridged, and the B atoms possess
terminal hydrogens. Reactions of Cp2HfCl2 with Li[B H ] lead to the formation of the analogue of 2, [(Cp2Hf)2B H ]-
5
8
5 8
[B H ] (3). The precursor to 3, that is, the Hf analogue of 1, is not observed. Reaction between Li[B H ] and
11 14
5 8
Cp2TiCl2 afforded no identifiable products, but reaction with CpTiCl3 resulted in cage coupling and the formation of
B H .
10 14
Introduction
and others4 has identified many examples of small metal-
lapolyboranes containing group 6 metals and several contain-
ing group 7 metals,3m,o-s,5 there are very few examples of
systems containing metals from groups 4 and 5. Except for
tetrahydroborate complexes, only one titanaborane,6 one
niobaborane,7 and two tantalaboranes8 have been fully
characterized, and of the latter, only one is a conventional
cluster based on a standard borane framework.8b Thus, we
set out first to prepare examples of metallapolyboranes from
groups 4 and 5. This report describes our initial efforts to
We have been interested, recently, in the preparation of
bimetallapolyboranes based on smaller borane cages contain-
ing 3-7 boron atoms, especially those containing different
metals.1 A continuing goal of this work was to prepare
species containing one “early” and one “late” transition metal.
Much of the prior work in this area identified “late” transition
metal metallapolyboranes,2 but there are no examples of
systems containing metals from both ends of the d block
and many fewer examples containing “early” transition
metals. Although the seminal work of the Fehlner group3
(2) (a) Kennedy, J. D. Prog. Inorg. Chem. 1984, 32, 519. (b) Kennedy, J.
D. Prog. Inorg. Chem. 1986, 36, 519. (c) Gilbert, K. B.; Boocock, S.
K.; Shore, S. G. In ComprehensiVe Organometallic Chemistry;
Wilkinson, G., Abel, E. W., Stone, F. G. A., Eds.; Pergamon:
Elmsford, NY, 1982; Part 6, Chapter 41, pp 879-945. (d) Grimes, R.
N. In ComprehensiVe Organometallic Chemistry; Wilkinson, G., Abel,
E. W., Stone, F. G. A., Eds.; Pergamon: Elmsford, NY, 1982; Part 1,
Chapter 5.5, pp 459-453. (e) Barton, L.; Srivastava, S. K. In
ComprehensiVe Organometallic Chemistry; Wilkinson, G., Abel, E.
W., Stone, F. G. A., Eds.; Pergamon: Elmsford, NY, 1995; Vol. 1,
Chapter 8, pp 275-373. (f) Grimes, R. N. In ComprehensiVe
Organometallic Chemistry, II; Wilkinson, G., Abel, E. W., Stone, F.
G. A., Eds.; Pergamon: Elmsford, NY, 1995; Vol. 1, Chapter 9, pp
374-431. (g) Metal Interactions with Boron Clusters; Grimes, R. N.,
Ed.; Plenum Press: New York, 1982.
* Author to whom correspondence should be addressed. E-mail:
lbarton@umsl.edu.
(1) (a) Barton, L.; Srivastava, D. K. Organometallics 1991, 10, 2982. (b)
Bould, J.; Rath, N. P.; Barton, L. Organometallics 1995, 14, 2119.
(c) Bould, J.; Rath, N. P.; Barton, L. J. Chem Soc., Chem. Commun.
1995, 1285. (d) Bould, J.; Rath, N. P.; Barton, L. Angew. Chem. Int.
Ed. Engl. 1995, 34, 1641. (e) Bould, J.; Rath, N. P.; Barton, L. Inorg.
Chem. 1996, 35, 2062. (f) Bould, J.; Rath, N. P.; Barton, L. Inorg.
Chem. 1996, 35, 35. (g) Bould, J.; Pasieka, M.; Braddock-Wilking,
J.; Rath, N. P.; Barton, L.; Gloeckner, C. Organometallics 1995, 14,
5138. (h) Mac´ıas, R.; Rath, N. P.; Barton, L. Chem. Commun. 1998,
1081. (i) Thomas, R. Ll.; Barton, L. Inorg. Chim. Acta 1999, 289,
134.
10.1021/ic010665h CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/30/2001
Inorganic Chemistry, Vol. 41, No. 1, 2002 67

Thomas et al.
Aldrich, and the former was washed, before use, in pentane until
it was a free flowing powder. The purity of KH was checked by
measuring the H2 evolved on reaction with CH3OH at -78 °C.
NMR spectral studies were carried out using a Varian Unity plus
300 spectrometer operating at 96.2 MHz for 11B, 299.9 MHz for
1H, and 121.4 MHz for 31P. Chemical shifts (δ) are given in ppm
prepare group 4 and 5 metallapolyboranes using the reactions
of LiB5H8 with various group 4 cyclopentadienyl metal
halides. Herein, we describe the formation and characteriza-
tion of novel zircona- and hafnapolyboranes and also
attempts to prepare titanapolyboranes which result in cage
fusion reactions. A preliminary report of the zirconium
system has appeared.9
1
to high frequency (low field) of 0.00 ppm (SiMe4) for H, 0.00
ppm for 31P (85% H3PO4), and 0.0 ppm for 11B (F3BOEt2 in CDCl3).
Selective decoupling experiments were performed on a Bruker ARX
500 spectrometer at 500.1 MHz for 1H and 160.5 MHz for 11B and
at 202.5 MHz for 31P. Low resolution mass spectra were measured
on a VG ZAB-E in the FAB mode using 3-NBA, and the high-
resolution data were obtained on a Kratos MS-50, in the FAB mode
using Ar gas.
Experimental Section
General. Solvents used were reagent grade and were dried before
use. Reactions were carried out on a standard vacuum line, and
some reagents were introduced into vessels in a drybox.10 Thin-
layer chromatography (TLC) was performed using 20 × 20 cm2
glass plates coated with 0.1 cm of silica gel (Aldrich standard grade
with gypsum binder and fluorescent indicator) made from aqueous
slurries followed by drying in air at 80 °C. Of the starting materials,
B5H9 was obtained from laboratory stock and distilled on the
vacuum line before use; [Cp2ZrCl2], [Cp2TiCl2], and [CpTiCl3] were
obtained from Alfa Aesar and used as received, as was [Cp2HfCl2],
which was obtained from Strem. KH and LiMe were obtained from
Synthesis of (Cp2ZrCl)B5H8 (1). B5H9 (0.42 mL, 4.2 mmol)
was condensed onto a solution of LiMe in diethyl ether (3.35 mL,
1.4 M, 4.7 mmol) at -78 °C in an extractor on the vacuum line.
The mixture was stirred for 1 h and then frozen at -196 °C, and
the methane generated was removed under vacuum. To the frozen
mixture was added solid Cp2ZrCl2 (0.60 g, 2.05 mmol), and this
was followed by the condensation of CH2Cl2 (1 mL) into the
reaction vessel. The reaction mixture was warmed to -78 °C and
stirred for 15 h. The solvents were removed from the resultant
yellow solution under reduced pressure at this temperature. CH2-
Cl2 (5 mL) was condensed onto the solids at -78 °C and the
resultant suspension filtered at this temperature. Removal of the
solvent at -78 °C gave 1 as a pale orange solid (455 mg, 1.44
mmol, 70%). NMR spectral data (δ in ppm, CDCl3 solution)
(assignment [δ(11B) (1H in parentheses)]). 4 °C: B(2-5) [-9.5
(1.83, br s, 4H)], B(1) [-41.6, d, J ) 169 Hz (0.85)]; δ(1H) C5H5
[6.35], µH (-2.4, br s, 3H)]. -44 °C: B(2,3) [-6.2 (1.91, 0.86)],
B(4,5) [-11.6 (2.40, 2.26)], B(1) [-41.6 (0.45)]; δ(1H) C5H5 [6.35],
µH4,5 [-1.81], µH3,4;2,5 [-2.73, 2.93]. -93 °C (CD2Cl2 solution):
B(2,3) [ca. -6.2 (2.4, 2.26)], B(4,5) [splits into 2 br unresolved
resonances (1.90, 0.88)], B(1) [-41.6 (0.78)]. LRMS in the FAB
mode on a VG ZAB-E using 3-NBA gave the parent ion at m/z )
318 with the isotopic distribution for the envelope of C10H18B5-
ClZr matching quite well. Calcd for C10H18B5ClZr: (m/q, rel int)
315, 8.59; 316, 36.52; 317, 83.37; 318 100.00; 319, 71.93; 320,
62.02. Found: 315, 67.50; 316, 77.36; 317, 100.00; 318 99.75;
319, 90.00; 320, 73.33.
(3) (a) Deck, K. J.; Nishihara, Y.; Shang, M.; Fehlner, T. P. J. Am. Chem.
Soc. 1994, 116, 8408. (b) Nishihara, Y.; Deck, K. J.; Shang, M.;
Fehlner, T. P.; Haggerty, B. S.; Rheingold, A. L. Organometallics
1994, 13, 4510. (c) Ho, J.; Deck, K. J.; Nishihara, Y.; Shang, M.;
Fehlner, T. P. J. Am. Chem. Soc. 1995, 117, 10292. (d) Hashimoto,
H.; Shang, M.; Fehlner, T. P. Organometallics 1996, 15, 1963. (e)
Hashimoto, H.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1996,
118, 8164. (f) Aldridge, S.; Fehlner, T. P.; Shang, M. J. Am. Chem.
Soc. 1997, 119, 2339. (g) Aldridge, S.; Shang, M.; Fehlner, T. P. J.
Am. Chem. Soc. 1997, 119, 11120. (h) Fehlner, T. P. J. Organomet.
Chem. 1998, 550, 21. (i) Aldridge, S.; Hashimoto, H.; Kawamura,
K.; Shang, M.; Fehlner, T. P. Inorg. Chem. 1998, 37, 928. (j) Aldridge,
S.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1998, 120, 2586. (k)
Kawamura, K.; Fehlner, T. P. Organometallics 1998, 17, 1904. (l)
Weller, A. S.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1998,
120, 8283. (m) Weller, A. S.; Shang, M.; Fehlner, T. P. J. Chem.
Soc., Chem. Commun. 1998, 1787. (n) Weller, A. S.; Shang, M.;
Fehlner, T. P. Organometallics 1999, 18, 53. (o) Weller, A. S.; Shang,
M.; Fehlner, T. P. Organometallics 1999, 18, 853. (p) Lei, X.; Shang,
M.; Fehlner, T. P. J. Am. Chem. Soc. 1999, 121, 7451. (q) Ghosh, S.;
Lei, X.; Cahill, C. L.; Fehlner, T. P. Angew. Chem., Int. Ed. 2000, 39,
2900. (r) Ghosh, S.; Lei, X.; Shang, M.; Fehlner, T. P. Inorg. Chem.
2000, 39, 5373. (s) Ghosh, S.; Shang, M.; Fehlner, T. P. J. Organomet.
Chem. 2000, 614-615, 92. (t) Ghosh, S.; Rheingold, A. L.; Fehlner,
T. P. Chem. Commun. 2001, 895.
(4) (a) Gaines, D. F.; Hildebrandt, S. J. Inorg. Chem. 1978, 17, 794. (b)
Shimoi, M.; Katoh, K.; Ogino, H. J. Chem. Soc., Chem. Commun.
1990, 811. (c) Katoh, K.; Shimoi, M.; Ogino, H. Inorg. Chem. 1992,
31, 670. (d) Shimoi, M.; Katoh, K.; Uruichi, M.; Nagai, S.; Ogino, H.
Spec. Publ.sR. Soc. Chem. 1994, 143, 293. (e) Hata, M.; Kawano,
Y.; Shimoi, M. Inorg. Chem. 1998, 37, 4482. (f) Shimoi, M.; Nagai,
S.; Ichikawa, M.; Kawano, Y.; Katoh, K.; Uruichi, M.; Ogino, H. J.
Am. Chem. Soc. 1999, 121, 11744.
(5) (a) Gaines, D. F.; Hildebrandt, S. J. J. Am. Chem. Soc. 1974, 96, 5574.
(b) Calabrese, J. C.; Fischer, M. B.; Gaines, D. F.; Lott, J. W. J. Am.
Chem. Soc. 1974, 96, 6318. (c) Gaines, D. F.; Fischer, M. B.;
Hildebrandt, S. J.; Ulman, J. A.; Lott, J. W. AdV. Chem. Ser. 1976,
150, 311. (d) Hildebrandt, S. J.; Gaines, D. F.; Calabrese, J. C. Inorg.
Chem. 1978, 17, 790. (e) Fischer, M. B.; Gaines, D. F. Inorg. Chem.
1979, 18, 3200. (f) Chen, M. W.; Gaines, D. F.; Hoard, L. G. Inorg.
Chem. 1980, 19, 2989. (g) Chen, M. W.; Calabrese, J. C.; Gaines, D.
F.; Hillenbrandt, D. F. J. Am. Chem. Soc. 1980, 102, 4928.
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1996, 35, 791.
Reaction of 1 with HCl. Diethyl ether (5 mL) was condensed
at -78° onto pale orange solid 1 (120 mg, 0.37 mmol). The
suspension was cooled to -196 °C, and HCl gas (0.39 mmol) was
condensed onto the frozen mass. The mixture was warmed to -55
°C and stirred for 90 min, at which point the orange solid had
become colorless. The volatiles from the reaction were removed
in vacuo at -55 °C and collected at -196 °C over 18 h. The white
solid was identified as Cp2ZrCl2 (102 mg, 95%), and the volatiles
1
contained B5H9 as the only boron-containing product, by H, 13C,
and 11B NMR. This is characteristic of a metal atom in a bridging
position of a pyramidal borane cluster.11
Synthesis of [(Cp2Zr)2B5H8][B11H14] (2). A solution of 1 (300
mg, 0.94 mmol) in CH2Cl2 (10 mL) was warmed to room
temperature and filtered through a plug of silica in air. The solvent
was removed from the resultant yellow solution in vacuo, giving a
yellow solid, which was separated by TLC (75% CH2Cl2, 25%
pentane eluant) giving two bands, decaborane(14) (colorless, Rf )
0.9) and 2 (yellow, Rf ) 0.2) isolated as a yellow solid (43 mg,
14% based on Zr). NMR spectral data for 2. 11B (96.2 MHz) and
(7) Grebenik, P. D.; Leach, J. B.; Pounds, J. M.; Green, M. L. H.;
Mountford, P. J. Organomet. Chem. 1990, C1, 382.
(8) (a) Ting, C.; Messerle, L. J. Am. Chem. Soc. 1989, 111, 3449. (b)
Aldridge, S.; Hashimoto, H.; Shang, M.; Fehlner, T. P. Chem.
Commun. 1998, 207.
(9) Thomas, R. Ll.; Rath, N. P.; Barton, L. J. Am. Chem. Soc. 1997, 119,
12358.
(11) (a) Brice, V. T.; Shore, S. G. J. Chem. Soc., Dalton Trans. 1975, 334.
(b) Greenwood, N. N.; Staves, J. J. Chem. Soc., Dalton Trans. 1977,
1786.
(10) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air-SensitiVe
Compounds; John Wiley: New York, 1986.
68 Inorganic Chemistry, Vol. 41, No. 1, 2002

Formation of Metallaboranes
Table 1. Crystal Data and Structure Refinement for 2
Table 2. Bond Lengths (Å) and Angles (deg) for the Cation in
[(Cp2Zr)2B5H8][B11H14] 2a
identification code
empirical formula
fw
lb5397s/lt/ccd
C20H42B16Zr2
637.94
Bond Lengths
Zr(1)-B(1)
Zr(1)-B(4)
Zr(2)-B(1)
Zr(2)-B(2)
B(1)-B(2)
2.498(4)
2.525(4)
2.473(4)
2.512(4)
1.712(6)
B(1)-B(5)
B(1)-B(4)
B(1)-B(3)
B(2)-B(3)
B(4)-B(5)
1.719(6)
1.730(5)
1.731(5)
1.865(6)
1.870(6)
T
223(2) K
wavelength
cryst syst
0.710 73 Å
monoclinic
space group
unit cell dimensions
Cc
a ) 16.21690(10) Å, R ) 90°
b ) 23.46760(2) Å, â ) 90.7450(10)°
c ) 16.3222(2) Å, γ ) 90°
6211.24(9) Å3, Z ) 8
1.364 Mg/m3
Bond Angles
B(2)-B(1)-B(5)
B(2)-B(1)-B(4)
B(5)-B(1)-B(4)
B(2)-B(1)-B(3)
B(5)-B(1)-B(3)
B(4)-B(1)-B(3)
B(2)-B(1)-Zr(2)
B(5)-B(1)-Zr(2)
B(4)-B(1)-Zr(2)
B(3)-B(1)-Zr(2)
B(2)-B(1)-Zr(1)
B(5)-B(1)-Zr(1)
B(4)-B(1)-Zr(1)
B(3)-B(1)-Zr(1)
Zr(2)-B(1)-Zr(1)
B(1)-B(2)-B(3)
B(1)-B(2)-Zr(2)
B(3)-B(2)-Zr(2)
142.8(3)
128.8(3)
65.7(2)
B(1)-B(3)-B(2)
B(1)-B(3)-Zr(1)
B(2)-B(3)-Zr(1)
B(1)-B(4)-B(5)
B(1)-B(4)-Zr(1)
B(5)-B(4)-Zr(1)
B(1)-B(5)-B(4)
B(1)-B(5)-Zr(2)
B(4)-B(5)-Zr(2)
B(1)-Zr(1)-Ca
B(1)-Zr(1)-Cb
B(4)-Zr(1)-Ca
B(3)-Zr(1)-Ca
B(1)-Zr(2)-Cc
B(1)-Zr(2)-Cd
B(5)-Zr(2)-Cc
B(2)-Zr(2)-Cd
Zr(1)-B(1)-Zr(2)
56.7(2)
68.58(19)
101.5(2)
56.9(2)
69.03(19)
103.1(2)
57.4(2)
68.1(2)
102.0(2)
114.2
114.1
114.1
102.0
114.9
113.7
102.8
100.8
178.6
V
density (calcd)
abs coeff
0.683 mm-1
65.6(2)
127.9(3)
141.9(3)
71.16(19)
71.7(2)
108.8(2)
109.3(2)
108.0(2)
109.1(2)
70.7(2)
71.2(2)
178.55(19)
57.7(2)
68.68(18)
103.3(2)
F(000)
cryst size
θ range for data collection
index ranges
2576
0.4 × 0.2 × 0.2 mm3
1.74-27.50°
-21 e h e 21, 0 e k e 31,
-21 e l e 21
reflns collected
14 192
independent reflns
abs correction
max and min transmission
refinement method
data/restraints/parameters
GOF on F2
14 192 [R(int) ) 0.0000]
semiempirical, sadabs
0.98 and 0.85
full-matrix least-squares on F2
14 180/2/753
1.014
final R indices [I > 2σ(I)]
R indices (all data)
absolute structure parameter
largest diffraction
peak and hole
R1 ) 0.0386, wR2 ) 0.0867
R1 ) 0.0514, wR2 ) 0.0980
0.02(3)
a Ca, Cb, Cc, and Cd are the centroids of the four C5H5 rings, starting
with the one above Zr(1) in Figure 3 and moving clockwise.
0.617 and -0.596 e Å-3
be located from the difference Fourier map were included in the
final refinement in fixed positions. One of the [B11H14]- anions is
disordered. The disorder was resolved by including B(10) in two
position (B10B and B10C) with 50% occupancy. The structure
refinement parameters are given in Table 1, and selected bond
angles and distances are given in Table 2. A complete list of
positional and isotropic displacement coefficients for the hydrogen
atoms, anisotropic displacement coefficients for the non-hydrogen
atoms, and complete listings of bond distances and angles are
available as Supporting Information.
1H (299.9 MHz) (ppm, CDCl3 solution, 25 °C) (relative intensity,
assignment [δ(11B) (1H in parentheses)]): δ(11B) B(2,3,4,5) [+2.7
(+3.05)], B(1A) [-13.4 (2.13)], B (2A-6A) [-15.1 (1.96)],
B(7A-11A) [-16.1 (1.51)], B(1) [-28.2 (no H)]; δ(1H) C5H5
[6.07] 20H, endo-Hanion [-3.46] 3H, µHB,Zr [-5.15] 4H. A refers
to the anion [B11H14]-. HRMS (Kratos MS-50, FAB, Ar gas) gave
1
m/q for cation 2 of 12C20 H2811B590Zr2 503.0802 (obsd) and 503.0791
(calcd).
X-ray Diffraction Analysis. A colorless crystal of compound 2
was mounted on a glass fiber in a random orientation. Preliminary
examination and data collection were performed using a Siemens
SMART charge coupled device (CCD) detector system single-
crystal X-ray diffractometer equipped with a sealed-tube X-ray
source using graphite-monochromated Mo KR radiation (λ )
0.710 73 Å). Preliminary unit-cell constants were determined with
a set of 45 narrow-frame scans (0.3° in $). A total of 4026 frames
of intensity data were collected with a frame width of 0.3° in $ at
a crystal-to-detector distance of 4.91 cm. The double-pass method
of scanning was used to reduce noise. The collected frames were
integrated using an orientation matrix determined from the narrow-
frame scans. The SMART software package was used for data
collection, and SAINT12a was used for frame integration. Analysis
of the integrated data did not show any decay. Final cell constants
were determined by a global refinement of the x,y,z centroids of
8192 reflections (θ < 26.0°). Absorption corrections were applied
to the data using equivalent reflections (SADABS).12b Crystal data
and parameters for intensity data collection are listed in Table 1.
The SHELXTL-PLUS software package12c was used for structure
solutions (by direct methods) and refinement. Full-matrix least-
Synthesis of [(Cp2Hf)2B5H8][B11H14], (3). A solution of MeLi
in (C2H5)2O (1.0 mL, 1.4 M, 1.4 mmol) was cooled to -78 °C,
and B5H9 (0.12 mL, 1.2 mmol) was added. The mixture was stirred
for 1 h at -50 °C and then frozen at -196 °C before Cp2HfCl2
(0.23 g, 0.6 mmol) was added, followed by the condensation of
CH2Cl2 (5 mL). The reaction mixture was warmed to -35° and
stirred for 18 h. The solvents were removed from the resultant
yellow solution in vacuo at -35 °C. CH2Cl2 (25 mL) was condensed
onto the residue at -78 °C and the resultant suspension filtered
through SiO2 at room temperature. Removal of the solvent gave a
pale yellow solid, which was separated by preparative TLC to give
a bright yellow solid 3, Rf ) 0.2. (44 mg, 0.054 mmol, 18%). 11
B
(96.2 MHz) and 1H (299.9 MHz) NMR spectral data in ppm (CDCl3
solution, 25 °C) for 3 (assignment [δ(11B) (1H in parentheses)]):
δ(11B) B(2,3,4,5) [+2.8 (+3.20)], B(1A) [-13.4 (2.15)], B(2A-
6A) [-15.2 (1.95)], B(7A-11A) [-16.1 (1.54)], B(1) [-33.2 (no
H)]; δ(1H) C5H5 [5.98], µHanion [-3.46], µHB,Hf [-4.51]. Mi-
croanalysis [found(calculated)%]: C, 29.39 (29.57) %; H, 5.14
(5.21) %. Several attempts to isolate the intermediate species,
analogous to 1, were made but were unsuccessful. At temperatures
high enough to effect reaction, species 3 is the major, and only
isolable, product.
2
squares refinement was carried out by minimizing Σω(Fo -Fc2)2.
The non-hydrogen atoms were refined anisotropically to conver-
gence. All the hydrogen atoms, except the cage hydrogens, were
included in their calculated positions. Cage hydrogens which could
Reactions of Pentaborane(9) with Cyclopentadienyltitanium
Halides. Stirring solutions of Cp2TiCl2 or Cp2TiMe2 with LiB5H8
at various temperatures and using different solvents did not afford
identifiable products; however, reactions of LiB5H8 with CpTiCl3
(12) (a) Bruker Analytical X-ray Division, Madison, WI, 1997. (b) Blessing,
R. H. Acta Crystallogr. 1995, A51, 33. (c) Sheldrick, G. M. SHELXTL-
PLUS; Bruker Analytical X-ray Division: Madison, WI, 1997.
Inorganic Chemistry, Vol. 41, No. 1, 2002 69

Thomas et al.
afforded B10H14 in relatively high yields, so several different sets
of conditions were used to explore this reaction.
Initial Reaction of [B5H8]- with CpTiCl3. MeLi (2.0 mL ether
solution, 2.8 mmol) was placed in a round-bottom flask under N2
and placed on the vacuum line, and the flask was cooled to -196
°C and evacuated. B5H9 (2.7 mmol) was condensed into the flask.
The flask was warmed to ambient for a few minutes with shaking
and it was filled with N2. A stirbar and addition tube containing
CpTiCl3 (0.6 g 2.7 mmol) were added. After freezing at -196 °C
and evacuating, the CpTiCl3 was added, and 10 mL of THF was
condensed into the flask. Stirring for 3 h at -78 °C afforded a
brown/black slurry. Filtration afforded a brown residue and a dark
yellow filtrate. Attempts to isolate Ti-containing identifiable
products from both fractions were unsuccessful. Several procedures
failed, so the remaining material was evacuated, and sublimation
afforded pure B10H14 (45 mg, 0.36 mmol, 26%), identified by its
11B NMR spectra. The yield was based on the amount of B5H9 in
the starting mixture. Measurement of the H2 produced by destruction
of the residual B5H9 gives an estimate of the amount of unreacted
B5H9. This was ∼1.1 mmol, suggesting a 54% yield based on B5H9
consumed. During this process, it was clear that B10H14 had been
lost during workup.
Reaction of B5H9 with CpTiCl3. The previous procedure was
used without deprotonation of the B5H9. A 0.73 mmol portion of
CpTiCl3 and 0.75 mmol of B5H9 were used in this experiment. After
stirring for 3 days in THF, the solution color was unchanged, but
some solid had been deposited. Removal of the solvent in vacuo
led to the isolation of a mustard-colored solid, similar in appearance
to the Ti starting material. Exposure to air during weighing resulted
in the solid slowly turning black. The mass of the solid was the
same as the starting material, and only very small traces of B10H14
and no other boron-containing species were observed in the NMR
spectrum of the resultant black solution.
Use of a Nonstoichiometric Amount of CpTiCl3. A nonsto-
ichiometric amount of CpTiCl3 was used in this case to see if the
reaction can act catalytically. CpTiCl3 (0.9 mmol) was allowed to
react similarly with LiB5H8 prepared from 0.4 mmol B5H9, and
the products were worked up so as to optimize isolation of B10H14.
0.16 mmol (20 mg) B10H14 resulted, representing 8% yield based
on B5H9 in the starting mixture.
Small Scale “Stoichiometric” Reaction of CpTiCl3 with
LiB5H8. In this case, the same procedure as for the first reaction
was used when 0.5 mmol of CpTiCl3 was allowed to react with
LiB5H8 prepared from 0.5 mmol of B5H9. B10H14 was isolated by
CH2Cl2 extraction of the crude reaction mixture followed by
sublimation to give 10 mg, 0.08 mmol, of B10H14 (32% based on
[B5H8]- in the starting mixture). The amount of unreacted B5H9
was ∼0.36 mmol, suggesting a 65% yield of B10H14 based on B5H9
consumed.
Figure 1. Proposed structure of [2,3-µ-(Cp2ClZr)B5H8] (1).
NMR spectral data for 1, which are given in the Experi-
mental Section, also provide support for this formulation.
The ambient temperature 11B NMR spectrum exhibits a broad
overlapping peak at -9.54 ppm of area 4 and a doublet,
J(11B-1H) ) 169 Hz, of area 1 at -41.6 ppm. This suggests
a pyramidal B5 cage, which the proton spectra confirm. The
1H{11B} spectra show overlapping broad resonances of area
4 at 1.88 ppm which couple to the 11B peak of area 4 and a
singlet at 0.85 ppm associated with the 11B doublet of area
1. Also, the proton spectrum shows a broad single resonance
at -2.45 ppm, area 3, suggesting the presence of three
bridging H atoms. The C5H5 proton resonances were
observed at 6.35 ppm. Treatment of 1 with HCl affords Cp2-
ZrCl2 and B5H9, essentially quantitatively, suggesting that
the Cp2ZrCl moiety occupies a bridging position in a B5H8
cage. This has been used previously to indicate the presence
of bridging moieties in pyramidal borane cages.11 Thus, we
presume that the structure of 1 is that given in Figure 1.
At lower temperature, -44 °C, the 11B NMR spectrum of
1 affords three resonances in area ratio 2:2:1 at -6.2, -11.6,
and -46.1 ppm which couple, respectively, to three sets of
1H resonances at 1.90 and 0.85, 2.40 and 2.25, and 0.75 ppm.
In addition, the 1H spectrum exhibits two resonances in the
bridging region, one at -1.8 ppm, area 1, which couples to
the 11B resonance at -11.6, and one at -2.83 of area 2,
which couples strongly to the 11B resonance at -6.2 and
weakly to the one at -11.6 ppm. This again tends to confirm
a structure involving a B5H8 cage in which a bridging
position is occupied by the Zr moiety, as indicated in Figure
1. At even lower temperatures, in the -93 °C region, the
11B resonance at -11.6 has broadened substantially into two
very broad resonances of approximately equal area centered
at approximately -11.5 ppm. Although the 11B resonance
at -6.2 ppm broadens such that it is barely visible at -93
°C, it appears that all four basal boron atoms and their
associated terminal H atoms occupy unique environments
at very low temperatures, consistent with the structure
depicted in Figure 1. Thus, our results suggest that 1 is
fluxional and that there appear to be two processes occur-
ring: one which renders the two sets of basal B atoms, B(2)-
(3) and B(4)(5), equivalent and a higher energy process
which renders all the basal boron atoms equivalent, on the
NMR time scale.
Results and Discussion
The reaction of Cp2ZrCl2 with 2 equiv of LiB5H8 at
temperatures below approximately -35 °C, in THF, meth-
ylene chloride, or diethyl ether, affords a major product that
is insoluble in ether and only slightly soluble in chloroform
and methylene chloride. Filtration of the initial reaction
mixture and extraction using CH2Cl2 allows the product to
be isolated and purified as a pale orange solid, 1. Chemical
and spectroscopic methods identify 1 as [2,3-µ-(Cp2ClZr)-
B5H8] in 70% yield. Although 1 was not stable enough for
satisfactory elemental analysis, mass spectral analysis gives
a parent ion and the correct isotope envelope around m/q )
318, consistent with the formula [Cp2ZrCl(B5H8)].
70 Inorganic Chemistry, Vol. 41, No. 1, 2002

Formation of Metallaboranes
1
Figure 2. Variable temperature H NMR spectrum of the B-H-B bridging region in 1. / indicates B5H9.
Our proposals about the fluxional processes are based on
a detailed examination of the variable temperature proton
NMR spectra of 1. The behavior of the terminal basal proton
resonances mirrors that of the basal boron atoms, with a
single broad resonance at temperatures around ambient.
Similarly, a single broad resonance is seen for the bridging
H atoms. This suggests that the fluxional process, which
renders the basal terminal B atoms equivalent, also renders
the terminal H atoms equivalent and the bridging H atoms
equivalent. This requires that the Cp2Zr moiety be involved
in the fluxional process. As the temperature is lowered, the
first change observed is the collapse of the bridging H atom
resonance at -2.4 ppm and the appearance of two new ones
in 2:1 area ratio. As the two bridging resonances begin to
sharpen, the basal terminal resonance also broadens and splits
into two peaks at ∼2.3 and 0.86 ppm in 3:1 ratio. As the
temperature is lowered further to -30 °C, the basal terminal
H atoms further split into peaks of relative area 2:1:1, at
2.34, 1.93, and 0.84. At the lowest temperature recorded,
-93 °C, four resonances (2.39, 2.26, 1.90, and 0.88 ppm)
are observed for the basal terminal H atoms, and the three
bridging H atoms are observed at -1.82, -2.74, and -2.29
ppm. At this temperature, the apical H atom is observed at
0.78 ppm, distinct from the closest basal terminal H atom,
which is seen at 0.88 ppm. Measurement of the resonances
at small temperature differences allowed estimation of the
activation energy barriers for the two processes. The higher
temperature process has a barrier of ∼48 kJ mol-1, and the
lower temperature one is 54 kJ mol-1. The variable temper-
H atoms move in such cases.14 At the lowest temperatures,
we can assume that 1 is static on the NMR time scale because
all the atoms are observed as separate resonances, both in
1
the H and 11B NMR spectra. This would result from an
asymmetric arrangement of the two Cp and the Cl ligands
on Zr. The first change observed in the spectra of 1 on
warming, collapse of the basal boron resonances into two
of equal area, concomitantly with collapse of the bridging
H atom resonances and the emergence of two resonances in
area ratio 2:1, suggests that B(2) and B(3) are equivalent, as
are B(4) and B(5), with µH3,4 and µH2,5 being equivalent
but different from µH4,5. Thus, at this temperature, 1 has a
plane of symmetry through the apical boron and bisecting
the bonds B(2)-B(3) and B(4)-B(5). This can be explained
by a rotation about the three-center two-electron bond
between the Cp2ZrCl moiety and the adjacent B(2)-B(3)
atom pair as illustrated in Scheme 1. Such rotation would
generate the required plane of symmetry in the time-averaged
molecular structure. Thus, the experimental results are in
accord with our hypothesis for the low-temperature fluxional
process.
The process which renders all the basal boron atoms and
their associated terminal H atoms equivalent, and also the
bridging H atoms equivalent, must involve motion of the
bridging H atoms around the basal region in some way, or
motion of the Cp2ZrCl moiety, or both. If the bridging H
atoms are in motion at higher temperatures, then they must
occupy sites on the Cp2ZrCl moiety because there are no
available B-B sites. Whether the H atom resides in a
bridging B-Zr site, or on the Zr atoms as a terminal H atom
at points during the process, cannot be determined; however,
motion around the square base, such as the process shown
1
ature H NMR spectrum of the bridging region is given in
Figure 2, and it highlights the two processes.
It is believed that pyramidal boranes are only fluxional
when there is a basal B-B bond into which the bridging H
atoms, or substituent replacing a bridging H atom, can
migrate,13 as it has been demonstrated that only the bridging
(13) Rush, P. K.; Barton, L. Polyhedron 1985, 4, 1741.
(14) Brice, V. T.; Johnson, H. D.; Shore, S. G. J. Am. Chem. Soc. 1973,
95, 6629.
Inorganic Chemistry, Vol. 41, No. 1, 2002 71

Thomas et al.
Scheme 1
Scheme 2
Scheme 3
Scheme 4
process for this molecule. As temperature is raised, the
bridging hydrogen region of the proton NMR spectrum
changes from two peaks at -1.8 (integral (int) 1) and -2.8
(int 2) to a single peak at -2.4 (int 3). Before the
coalescence, these peaks broaden, and it is seen that the
integral 1 peak broadens significantly at a higher temperature
than the integral 2 peak. The temperature difference between
the broadening of these two peaks is ∼5 °. A comparison of
the broadness of the peaks at half-height shows that the
integral 2 peak is half as broad as the integral 1 peak as
they approach coalescence (72 Hz vs 130 Hz). Further, below
the coalescence point, both peaks are equally broad (∼60
Hz). This pattern suggests that, during the fluxional process,
the environment of the protons causing the integral 2 peak
changes more slowly than that causing the integral 1 peak.
This is consistent with a 1,2 exchange of the Zr with an
adjacent proton. If we assume that Zr and Ha in Scheme 4
are interchanged, then the environment of Hb is changed
from trans to Zr to adjacent to Zr, and Hc changes from
adjacent to Zr to trans to Zr. However, Ha is still adjacent
to Zr, and so, its environment is unchanged. Therefore, the
environment of the trans protons, which cause the integral 1
peak, changes more rapidly than that of the adjacent protons,
only one of which is affected by the 1,2 shift. This will cause
the broadening of the integral 1 peak to occur at a more
rapid rate than that of the integral 2 peak. Overall, the
repetition of this mechanism will render all bridging hydro-
in Scheme 2, is feasible. Scheme 2 indicates four possible
tautomers, varying in the placement of the bridging hydro-
gens. In fact, there would be 20 possible tautomers, if one
considered the two Zr-B bridging and the Zr terminal
positions as sites for occupancy by the fluxional hydrogens.
There is a growing number of examples of cyclopentadienyl
zirconium hydride complexes for which there is hydrogen
exchange between bridging and terminal H atoms on
zirconium.15 Indeed, some of the processes for which that
type of fluxional motion is suggested involve borane
species.15e,f,h-j Thus, the H atom can hop over the Zr moiety
so that all the Hµ atoms become equivalent on the NMR
time scale. An alternative or accompanying motion could
be a rocking-type 1,3 shift in the Hµ positions as indicated
in Scheme 3. Such processes have not previously been
suggested for square pyramidal boranes, but there certainly
are enough examples of octahedral clusters in borane
chemistry to render this proposal feasible.16 Indeed, we
invoked a similar mechanism to account for the fluxional
processes in the anion [Fe(CO)4B6H9]-.17
Support for the 1,2-shift mechanism illustrated in Scheme
2 arises from our observation of the lower energy fluxional
(15) (a) Kot, W. K.; Edelstein, N. M.; Zalkin, A. Inorg. Chem. 1987, 26,
1339. (b) Gozum, J. E.; Girolami, G. S. J. Am. Chem. Soc. 1991, 113,
3829. (c) Gozum, J. E.; Wilson, S. R.; Girolami, G. S. J. Am. Chem.
Soc. 1992, 114, 9483. (d) Bakhmutov, V. I.; Visseaux, M.; Baudry,
D.; Dormond, A.; Richard, P. Inorg. Chem. 1996, 35, 7316. (e) Spence,
R. E. v. H.; Parks, D. J.; Piers, W. E.; MacDonald, M.-A.; Zaworotko,
M. J.; Rettig, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1230. (f)
Sun, Y.; Spence, R. E. v. H.; Piers, W. E.; Parvez, M.; Yap, G. P. A.
J. Am. Chem. Soc. 1997, 119, 5132. (g) Etkin, N.; Hoskin, A. J.;
Stephan, D. W. J. Am. Chem. Soc. 1997, 119, 11420. (h) Chow, A.;
Liu, F.-C.; Fraenkel, G.; Shore, S. Magn. Reson. Chem. 1998, 36,
S145. (i) Liu, F.-C.; Liu, J.; Meyers, E. A.; Shore, S. G. Inorg. Chem.
1998, 37, 3293. (j) Liu, F.-C.; Liu, J.; Meyers, E. A.; Shore, S. G.
Inorg. Chem. 1999, 38, 2169.
(16) See for example: (a) Boone, J. L. J. Am. Chem. Soc. 1964, 86, 5036.
(b) Pipal, J. R.; Grimes, R. N. Inorg. Chem. 1977, 16, 3255. (c) Crook,
J. E.; Greenwood, N. N.; Kennedy, J. D.; McDonald, W. S. J. Chem.
Soc., Chem. Commun. 1982, 383. (d) Jiang, F.; Fehlner, T. P.;
Rheingold, A. L. J. Am. Chem. Soc. 1987, 109, 1860. (e) Bould, J.;
Rath, N. P.; Barton, L. Chem. Commun. 1995, 1285.
(17) Srivastava, D. K.; Rath, N. P.; Barton, L.; Ragaini, J. D.; Hollander,
O.; Godfroid, R.; Shore, S. G. Organometallics 1993, 12, 2017.
72 Inorganic Chemistry, Vol. 41, No. 1, 2002

Formation of Metallaboranes
gen atoms and all basal boron atoms equivalent, which is
observed in the proton and boron NMR spectra. Because
we expect that the H atom can “hop over” the Zr moiety, as
seen in Scheme 2, this represents effective exchange of the
Zr moiety and an adjacent H atom which is consistent with
our NMR observations. Comparisons with recent work of
Shore et al.18a and also Bercaw et al.18b suggest that such
motion could involve exchange with the cyclopentadienyl
hydrogen atoms as they observed in related systems, although
we were not able to demonstrate such exchange. Our
conclusions are that the 1,3 shift “tipping” motion is less
favored.
Figure 3. Structure of the cation in [(Cp2Zr)2B5H8][B11H14] (2), with 50%
probability thermal ellipsoids and the cyclopentadienyl groups abbreviated
for clarity. The terminal H on B(4) is obscured.
Species 1 is not very stable at room temperature, and we
were unable to obtain satisfactory elemental analysis, but if
a methylene chloride solution is filtered through a plug of
silica in air, a yellow solution is obtained from which an
air-stable, yellow crystalline solid, 2, is isolated. Compound
2 is identified unambiguously, from NMR spectra, elemental
analysis, and high-resolution mass spectra, and also from
an X-ray crystallographic study, as [(Cp2Zr)2B5H8]+[B11H14]-.
Obtained in 14% yield, 2 is air stable both as a solid and in
solution but decomposes slowly (over a period of days) in
solution in air to give decaborane(10) as the only easily
identifiable product.
NMR spectra of the crystalline product, 2, indicate the
presence of the [B11H14]- anion,19 suggesting that the
zirconium-containing species is cationic, which is consistent
with the low solubility of the material. In addition to
resonances due to [B11H14]-, there are two resonances in the
area ratio 4:1 in the 11B{1H} NMR spectrum suggesting a
B5 species. The coupled spectrum shows some broadening
of the resonance of area 4 but no change in the area 1
resonance. Proton spectra are also consistent with the
presence of the [B11H14]- anion.19 The remaining resonances,
not ascribed to solvent nor to anion, are observed in the ratio
5:1:1, that is, 20:4:4. The resonance of area 20 is assigned
to the cyclopentadienyl moieties, and the other hydrogens
are identified as boron-bonded H atoms from their broaden-
ing in the coupled 1H spectrum. Selective decoupling
confirms that all the cage H atoms are bonded only to the
boron atoms producing the integral 4 peak above. These
resonances, falling at 3.05 and -5.05 ppm, are assigned to
terminal BH groups and bridging B-H-Zr groups, respec-
tively. The high-resolution mass spectrum confirms that the
cation consists of a species containing two Cp2Zr moieties
and a B5H8 group.
Figure 4. Alternative view of the cationic cluster of 2, presented as a
drawing.
resembles that of a distorted spiro[2.2]pentane molecule; the
angle between the two planes is 73° rather than 90°. A stick
drawing of the structure is given in Figure 4. Each boron
atom bears a terminal hydrogen, and there are four Zr-H-B
bridge bonds. A unique feature of the structure is that the
central B atom does not bear a hydrogen atom. Such “naked”
B atoms are normally only observed in macropolyhedral
boranes at points of cage fusion and typically have high
connectivity.20 The latter is the case for B(1) which we
consider to have connectivity of six. The B-B connections
have normal B-B bonding distances: those between the
naked boron and the others, ranging from 1.686(7) to 1.731-
(5) Å, and those on the periphery of the B5 cluster, B(2)-
B(3) and B(4)-B(5), which are not bridged by hydrogen
atoms and are longer, ranging from 1.842(9) to 1.870(6) Å.
The planes described by connecting the centroids of the Cp
rings and the Zr are effectively orthogonal (99.7° and 100.9°)
to the adjacent ZrB3 planes which would be expected from
orbital considerations.21 The cation in compound 2 is thus
similar to [Cp2Zr{CH2(BR2)2}]22 in which the Zr coordinates
similarly to a {CH2(BR2)2} moiety. The nonbridged Zr-B
distances in 2, ranging from 2.473(4) to 2.508(5) Å, are
slightly longer than would be expected for a single Zr-B
Crystals suitable for an X-ray diffraction study were grown
by diffusion of pentane into a CDCl3 solution of 2. The
structure of the cation is shown in Figure 3 and is consistent
with the NMR data. Two crystallographically independent
ion pairs are found in the unit cell. The cations are not
significantly different from one another. All parameters listed
are for cation A. The arrangement of the boron atoms
(20) (a) Barton, L. Top. Curr. Chem. 1982, 100, 169. (b) Bould, J.; Clegg,
W.; Teat, S. J.; Barton, L.; Rath, N. P.; Thornton-Pett, M.; Kennedy,
J. D. Inorg. Chim. Acta 1999, 289, 95. (c) Yao, H.-J.; Hu, C.-H.; Sun,
J.; Jin, R.-S.; Zheng, P.-J.; Bould, J.; Greatrex, R.; Kennedy, J. D.;
Ormsby, D. L.; Thornton-Pett, M. Collect. Czech. Chem. Commun.
1999, 64, 927. (d) Bould, J.; Ormsby, D. L.; Yao, H.-J.; Hu, C.-H.;
Sun, J.; Jin, R.-S.; Shea, S. L.; Clegg, W.; Jelinek, T.; Rath, N. P.;
Thornton-Pett, M.; Greatrex, R.; Zheng, P.-J.; Barton, L.; Stibr, B.;
Kennedy, J. D. Spec. Publ.sR. Soc. Chem. 2000, 253, 171.
(21) (a) Lauher, J.; Hoffman, R. J. Am. Chem. Soc. 1976, 98, 1729. (b)
Bethon, R. A.; Rounsley, R. J.; Burns, R. C. Polyhedron 1993, 12,
2157.
(18) (a) Shore, S. G.; Liu, F.-C. Spec. Publ.sR. Soc. Chem. 2000, 253,
28. (b) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999,
18, 1873.
(19) Hosmane, N. S.; Wermer, J. R.; Hong, Z.; Getman, T. D.; Shore, S.
G.; Inorg. Chem. 1987, 26, 3638.
(22) Spence, R. E.; Parks, D. J.; Piers, W. E.; MacDonald, M. A.;
Zaworotko, M. J.; Rettig, S. J. Angew. Chem., Int. Ed. Engl. 1995,
34, 1230.
Inorganic Chemistry, Vol. 41, No. 1, 2002 73

Thomas et al.
bond (2.45 Å)23 but are shorter than typical Zr-B bonds
found in (η5-C2B3)Zr moieties,24 the only nonbridged Zr-B
bonds available for comparison.25 On the other hand, the
Zr-B distances for the Zr-H-B bridge bonds, ranging from
2.512(4) to 2.526(4) Å, are shorter than those observed
elsewhere,26 suggesting a strong bonding interaction between
the Cp2Zr moiety and the borane cage. Thus, each Zr appears
to be bonded in a trihapto mode to the boron cluster, the
Zr-B central connection being shorter than the peripheral
ones. The atoms Zr(1)-B(2)-B(1)-B(5) lie in a plane, as
do Zr(2)-B(3)-B(1)-B(4), with an angle of 54.8° between
the two planes. The central boron atom, B(1), lies almost
exactly on a vector between the two Zr atoms. The angle
Zr(1)-B(1)-Zr(2) is 178.6°.
An alternative description of the cationic cluster is that it
is a metal-stabilized hypho cluster.27 The hypho-pentaborane
cluster27 would be [B5H8],5- and thus, the metal moieties
would have to be formulated as [Cp2Zr3+]. Each would
contribute three electrons to the borane cage. This appears
to be an unreasonable assumption, because the formulation
Cp2Zr2+ has a d0 configuration, so 2 must be added to the
growing list of metallapolyboranes3c-t,28 which contain fewer
than the requisite number of electrons required by electron
counting rules.29 What is clear, however, for the structure
of 2 and for the other known group 4 metallapolyborane,
[Cp2Ti(B6H9)]2,6 is that unusual structures derive from
metallaboranes involving the early transition metals as has
been observed by Fehlner et al.3,8b,28
atoms in the open face as a pair of B-H-B bridges and a
normal endo-hydrogen atom. Because not all the cage
hydrogen atoms were located in our study, we are unable to
confirm this finding. We observed the three endo-hydrogens
as a single resonance in the NMR spectra, as did Shore et
al.,19 but our structure determination was unable to satisfac-
torily distinguish the bridging from the endo-terminal H
atoms in the open face of the anion. Our data suggest that
our structure is a good one, and because of the nature of the
two cations, it belongs to a different space group, Cc rather
than group P1h, reported for the [P(CH3)3H]+ salt.30 The
undecaborate anion is formed as solutions of [B5H8]- are
allowed to warm to room temperature in the presence of
B5H9.31 In our case, passage through silica or the presence
of the “Cp2ZrCl” moiety may catalyze the formation of
higher boranes. Such organometallic species are known to
catalyze polymerization of unsaturated hydrocarbons32a and
also the dehydrogenative coupling of silanes.32b
The reaction between cyclopentadienyltitanium halides and
[B5H8]- proceeds differently from those of their Zr and Hf
homologues. Stirring Cp2TiCl2 and [B5H8]- under a variety
of conditions afforded no identifiable products, but reactions
of [B5H8]- and CpTiCl3 afforded relatively good yields of
B10H14. Although attempts were not made to maximize the
yields of the latter, there appears to be promise in this process
for the conversion of B5H9 to B10H14. Varying the amount
of CpTiCl3 indicated that the reaction was stoichiometric
rather than catalytic. A catalytic route may be possible if a
cocatalyst is used to reoxidize the Ti(III) to Ti(IV), but it is
important that the cocatalyst does not react further with the
titanium complex. It is known that PbCl2 will readily oxidize
Ti(III) to Ti(IV), so we carried out some reactions of
CpTiCl3/PbCl2 with [B5H8]-. The results of this study show
the pentaborane to be converted to unidentifiable boron-
containing products, with only traces of B10H14 detected by
NMR spectroscopy. The reaction of PbCl2 with B5H8- gives
small traces of B10H14. Given these findings, attempts to
devise catalytic conversion of B5H9 to B10H14 were aban-
doned. Our results indicate that the yields of B10H14, based
on CpTiCl3, are the most consistent, suggesting that it is the
limiting reagent in these reactions. In the CpTiCl3-promoted
conversion of B5H9 to B10H14, prior deprotonation of the B5H9
was necessary because only traces of B10H14 were obtained
when B5H9 was used. If the reaction is carried out by first
treating CpTiCl3 with MeLi followed by addition of B5H9,
most of the B5H9 remains unreacted in the reaction vessel.
Apparently, the Ti-Me groups do not appear to be strong
enough bases to deprotonate the pentaborane.
The identification of the [B11H14]- anion in this reaction
requires some comment. It was clearly identified by NMR
spectroscopy, and this was confirmed by the X-ray diffraction
study. The structure observed, an icosahedron of BH units
with one removed, supports the recent results of Shore et
al.30 who identified this cluster with three non-exo-hydrogen
(23) Ma¨nnig, D.; No¨th, H.; Schwartz, M.; Weber, S.; Wietelmann, U.
Angew. Chem., Int. Ed. Engl. 1985, 24, 998.
(24) For example see: (a) Thomas, C. J.; Jia, L.; Zhang, H.; Siriwardane,
U.; Maguire, J. A.; Wang, Y.; Brooks, K. A.; Weiss, V. P.; Hosmane,
N. S. Organometallics 1995, 14, 1365. (b) Bowen, D. E.; Jordan, R.
F.; Rogers, R. D. Organometallics 1995, 14, 3630. (c) Mao, S. S. H.;
Tilley, T. D.; Rheingold, A. L.; Hosmane, N. S. J. Organomet. Chem.
1997, 533, 257.
(25) There is a class of zirconium borides in which a B atom is located at
the center of a Zr octahedron. The Zr-B distances are ∼2.3 Å, but
these are not really comparable to normal Zr-B bonds. Xie, X.;
Reibenspies, J. H.; Hughbanks, T. J. Am. Chem. Soc. 1998, 120, 11391.
(26) A structure search identified 22 compounds containing Zr-B con-
nections with one or two Zr-H-B bridges. The Zr-B distances range
from 2.56 Å in [µHCp2Zr(µH)2BC4H8}2]+ (Liu, F.-C.; Liu, J.; Meyers,
E. A.; Shore, S. G. J. Am. Chem. Soc. 2000, 122, 6106) to 3.23 Å in
the cation [Cp*2Zr(Me)(C2B9H12)] (Hlatky, G. G.; Turner, H. W.;
Eckman, R. R. J. Am. Chem. Soc. 1989, 111, 2728); the mean value
is 2.62 Å.
(27) (a) Lipscomb, W. N. Boron Hydrides; W. A. Benjamin: New York,
1963, p 221. (b) Fratini, A. V.; Sullivan, G. W.; Denniston, M. L.;
Hertz, R. K.; Shore, S. G. J. Am. Chem. Soc. 1974, 96, 3013. (c)
Alcock, N. W.; Colquhoun, H. G.; Sawyer, J. F.; Wallbridge, M. G.
H. J. Chem. Soc., Dalton. Trans. 1982, 2243.
(28) (a) Fehlner, T. P. Spec. Publ.-R. Soc. Chem. 1997, 201, 463. (b)
Fehlner, T. P. J. Chem. Soc., Dalton Trans. 1998, 1525. (c) Weller,
A. S.; Aldridge, S.; Fehlner, T. P. Inorg. Chim. Acta 1999, 289, 85.
(d) Lei, X.; Shang, M.; Fehlner, T. P. Organometallics 2000, 19, 4429.
(e) Lei, X.; Shang, M.; Fehlner, T. P. Organometallics 2001, 20, 1479.
(29) (a) Williams, R. E. AdV. Inorg. Chem. Radiochem. 1976, 18, 67. (b)
Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 60. (c) Rudolph,
R. W. Acc. Chem. Res. 1976, 9, 446.
The experimental results suggested to us that a stoichi-
ometry of 4 Ti(IV) for each mole of B10H14 produced was
required. Assuming the reaction involved is described by the
(30) Getman, T. D.; Krause, J. K.; Shore, S. G. Inorg. Chem. 1988, 27,
2399.
(31) Wermer, J. R.; Shore, S. G. Mol. Struct. Energ. 1986, 5, 13 and
references therein.
(32) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University
Science Books, Mill Valley, CA, 1987, Chapter 11. (b) Corey, J. Y.
In AdVances in Silicon Chemistry; Larson, G., Ed.; JAI Press:
Greenwich, CT, 1991; Vol. 1, pp 327-387.
74 Inorganic Chemistry, Vol. 41, No. 1, 2002

Formation of Metallaboranes
equation 4Ti(IV) + 4[B5H8]- f 4Ti(III) + B10H14 + 2B5H9,
we obtained yields in the range 54-65% based on the
equation and 26-32% based on the initial amount of B5H9
used. Pentaborane(9) is recovered in the reaction, so taking
this into account, yields based on consumption of B5H9 or
on the amount of Ti used are in the range 54-65%.
borane cages. Our contribution to this area continues, and
we will report soon on this aspect of our work.
Conclusions
We have prepared and characterized some novel group
IV metallaboranes. Compound 1, µ-(Cp2ClZr)B5H8, consists
of a B5H8 cage with a [Cp2ZrCl]+ moiety substituting for a
bridging proton. Compound 1 exhibits some novel dynamic
behavior for which we propose explanations based on NMR
data. Passage of 1 through silica affords [(Cp2Zr)2B5H8]-
[B11H14] (2), a rare cationic metallaborane with a novel
structure. A hafnium analogue of 2, [(Cp2Hf)2B5H8][B11H14]
(3), is also obtained, but the presumed precursor, the analogue
of 1, although probably formed, was not observed. The use
of the analogous Ti starting materials reagents leads only to
coupling of B5H9 to B10H14. These results portend well for
study of other group 4 and 5 derivatives of the polyboranes.
This latter chemistry is clearly similar to the metal-
promoted fusion reactions of Grimes33 and Sneddon.34 The
former work involves oxidative fusion which requires
exposure of the intermediate to air, something which our
system cannot tolerate, although our yields of B10H14 are
about 1.5 times those reported for the RuCl3-promoted
conversion of [B5H8]- to B10H14.33 The latter work by
Sneddon et al. involves the catalytic conversion of B5H9 to
1:2′-(B5H8)2, in the presence of PtBr2, in 92% yield.35
Treatment of 1:2′-(B5H8)2 with Li(BEt3H) affords [B9H14]-
in 94% yield. Conversion of [B9H14]- salts to B10H14
proceeds in 40% yield,31,36 and thus, extension of the Sneddon
method to the preparation of B10H14 from B5H9 represents
∼34% yield. Although our method competes well with those
just described, the “one pot” synthesis of B10H14 from B5H9
described by Shore et al.36 represents the most convenient
available route. These coupling or fusion methods have not
led to better methods for B10H14 formation, but they all
represent good general approaches to the formation of higher
Acknowledgment. This work was supported by the
Petroleum Research Fund of the American Chemical Society
(ACS-PRF Grant 31001-AC3) and the National Science
Foundation (Grant CHE-9727570). We also acknowledge
instrumentation grants from the NSF (Grants CHE-9309690
and CHE-9318696), the DOE (Grant DE-FG02-92CH10499),
and the UMsSt. Louis Center for Molecular Electronics.
We thank Professor Michael Gross of the Washington
University Mass Spectrometry NIH Research Resource
(Grant P41RR094) for the mass spectra.
(33) (a) Brewer, C. T.; Grimes, R. N. J. Am. Chem. Soc. 1984, 106, 2722.
(b) Brewer, C. T.; Grimes, R. N. J. Am. Chem. Soc. 1985, 107, 3552.
(c) Brewer, C. T.; Swisher, R. G.; Sinn, E.; Grimes, R. N. J. Am.
Chem. Soc. 1985, 107, 3558.
(34) (a) Briguglio, J. J.; Carroll, P. J.; Corcoran, E. W., Jr.; Sneddon, L.
G. Inorg. Chem. 1986, 25, 4618. (b) Sneddon, L. G. Pure Appl. Chem.
1987, 59, 837. (c) Corcoran, E. W., Jr.; Sneddon, L. G. Mol. Struct.
Energ. 1988, 5, 71.
(35) Corcoran, E. W., Jr.; Sneddon, L. G. Inorg. Chem. 1983, 22, 182.
(36) Toft, M. A.; Leach, J. B.; Himpsl, F. L.; Shore, S. G. Inorg. Chem.
1982, 21, 1952.
Supporting Information Available: Crystallographic data,
positional parameters, anisotropic displacement parameters, and
bond lengths and angles for 2 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
IC010665H
Inorganic Chemistry, Vol. 41, No. 1, 2002 75