Synthesis and x-ray and neutron structures of Zr{(μ-H) 2BC8H14}4

Article abstract of DOI:10.1021/ic048629g

The complex Zr(9-BBN)₄ [9-BBN = (μ-H)₂BC ₈H₁₄] has been synthesized via the reaction of K(9-BBN) with ZrCl₄ in diethyl ether. The structure of the title compound has been determined by X-ray and neutron single-crystal diffraction techniques. Each 9-BBN ligand is coordinated to the Zr atom via two B-H-Zr bridges, and these metal-ligand bonding interactions are further augmented by three prominent C-H...Zr agostic interactions. Average molecular parameters derived from the neutron analysis: Zr-H = 2.051(8) A, B-H = 1.286(7) A, Zr...B = 2.409(6) A, Zr-H-B = 87.7(4)°, H-Zr-H = 58.9(3)°. The Zr...H distances corresponding to the three C-H...Zr agostic interactions are 2.424(7), 2.663(8), and 2.551(7) A. The fourth potential C-H...Zr interaction has a Zr...H distance [3.146(7) A] that is too long to be considered in the agostic range. Single-crystal X-ray diffraction data were collected on an Enraf-Nonius Kappa CCD diffraction system, and neutron diffraction data were collected on the quasi-Laue diffractometer VIVALDI at the Institut Laue-Langevin; the final agreement factor for the neutron analysis is 6.52% for 2557 reflections with I > 2σ(I).

Full text of DOI:10.1021/ic048629g

Inorg. Chem. 2005, 44, 2459−2464
Synthesis and X-ray and Neutron Structures of Zr{(µ-H)2BC8H14}
4
Errun Ding, Bin Du, Edward A. Meyers, and Sheldon G. Shore*
Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210
Muhammed Yousufuddin and Robert Bau*
Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089
Garry J. McIntyre
Institut Laue LangeVin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France
Received September 30, 2004
The complex Zr(9-BBN)4 [9-BBN
in diethyl ether. The structure of the title compound has been determined by X-ray and neutron single-crystal
diffraction techniques. Each 9-BBN ligand is coordinated to the Zr atom via two B Zr bridges, and these metal
ligand bonding interactions are further augmented by three prominent C ‚‚‚Zr agostic interactions. Average molecular
parameters derived from the neutron analysis: Zr 2.051(8) Å, B 1.286(7) Å, Zr‚‚‚ 2.409(6) Å,
Zr 87.7(4) , H Zr 58.9(3) . The Zr‚‚‚H distances corresponding to the three C ‚‚‚Zr agostic
interactions are 2.424(7), 2.663(8), and 2.551(7) Å. The fourth potential C ‚‚‚Zr interaction has a Zr‚‚‚H distance
) (µ-H)2BC8H14] has been synthesized via the reaction of K(9-BBN) with ZrCl4
−H−
−
−H
−H
)
−H
)
B )
−H−B
)
°
−
−H
)
°
−H
−H
[3.146(7) Å] that is too long to be considered in the agostic range. Single-crystal X-ray diffraction data were collected
on an Enraf-Nonius Kappa CCD diffraction system, and neutron diffraction data were collected on the quasi-Laue
diffractometer VIVALDI at the Institut Laue-Langevin; the final agreement factor for the neutron analysis is 6.52%
for 2557 reflections with I > 2σ(I).
Introduction
diffraction investigation of the same molecule in the gas
phase in 1971.3c Those structural determinations were in
substantial agreement with each other: the molecule was
found to have essentially tetrahedral symmetry with a single
terminal B-H bond collinear with each Zr‚‚‚B vector and
the remaining three hydrogens at bridging positions between
Coordination complexes containing the simplest hydro-
borate anion BH4 as a ligand have received considerable
-
attention for quite some time.1-3 These molecules are of
interest because they may serve as reducing agents or
precursors for the synthesis of molecules with bridging or
terminal hydrides or other inorganic materials such as
borides.2f,g They could also serve as catalysts for certain
organic reactions.2a
(2) Selected papers for tetrahydroborate complexes: (a) Knizek, J.; No¨th,
H. J. Organomet. Chem. 2000, 614-615, 168. (b) Fischer, P. J.;
Young, J. V. G.; Ellis, J. E. Angew. Chem., Int. Ed. 2000, 39, 189. (c)
Hafid, A.; Nyassi, A.; Sitzmann, H.; Visseaux, M. Eur. J. Inorg. Chem.
2000, 2333. (d) Ashworth, N. J.; Conway, S. L. J.; Green, J. C.; Green,
M. L. H. J. Organomet. Chem. 2000, 609, 83. (e) Conway, S. L. J.;
Doerrer, L. H.; Green, M. L. H.; Leech, M. A. Organometallics 2000,
19, 630. (f) Choukroun, R.; Douziech, B.; Donnadieu, B. Organo-
metallics 1997, 16, 5517. (g) Corazza, F.; Floriani, C.; Chiesi-Villa,
A.; Guastini, C. Inorg. Chem. 1991, 30, 145. (h) Csa´sza´r, A.; Hedberg,
L.; Hedberg, K.; Burns, R. C.; Wen, A. T.; McGlinchey, M. J. Inorg.
Chem. 1991, 30, 1371. (i) Broach, R. W.; Chuang, I.; Marks, T. J.;
Williams, J. M. Inorg. Chem. 1983, 22, 1081.
The first structure of a d0 tetrahydroborate complex, Zr-
{(µ-H)3BH}4 (Chart 1), was determined by an X-ray dif-
fraction study at -160 °C in 19673d and by an electron
* To whom correspondence should be addressed. E-mail: bau@usc.edu.
(1) For reviews on the chemistry and structures of tetrahydroborate
complexes (a, c, e, and f) and on metal hydrides in general (b and d)
see: (a) Markhaev, V. D. Russ. Chem. ReV. 2000, 69, 727. (b) Bau,
R.; Drabnis, M. H. Inorg. Chim. Acta 1997, 259, 27. (c) Xu, Z.; Lin,
Z. Coord. Chem. ReV. 1996, 156, 139. (d) Teller, R. G.; Bau, R. Struct.
Bonding 1981, 44, 1. (e) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977,
77, 263. (f) James, B. D.; Wallbridge, M. G. H. Prog. Inorg. Chem.
1970, 11, 99.
(3) Structures of Zr(BH4)4 and Ti(BH4)3(PCH3)2: (a) Jensen, J. A.; Wilson,
S. R.; Girolami, G. S. J. Am. Chem. Soc. 1988, 110, 4977. (b) Marks,
T. J.; Shimp, L. A. Inorg. Chem. 1972, 11, 1542. (c) Plato, V.;
Hedberg, K. Inorg. Chem. 1971, 10, 590. (d) Bird, P. H.; Churchill,
M. R. Chem. Commun. 1967, 403.
10.1021/ic048629g CCC: $30.25
Published on Web 03/09/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 7, 2005 2459

Ding et al.
Chart 1
Experimental Section
General Procedures. All manipulations were carried out on a
standard high vacuum line or in a drybox under a nitrogen
atmosphere. Diethyl ether and hexane were distilled under nitrogen
from Na/benzophenone. All solvents for vacuum line manipulations
were stored in a vacuum over a Na/K alloy. Deuterated solvents
were obtained from Cambridge Isotope Laboratories and were
vacuum transferred from the Na/K alloy. ZrCl4 and (C8H14)B(µ-
H)2(BC8H14) (9-BBN dimer) were purchased from Aldrich and used
as received. Potassium hydride (35 wt % dispersion in mineral oil)
was purchased from Aldrich, washed with hexane, and dried under
vacuum prior to use. K(H2BC8H14)10 was prepared by literature
procedures. NMR spectra were recorded on a Bruker AM-250 NMR
spectrometer operating at 250.11 MHz at 303 K, and boron 11
spectra were externally referenced to BF3OEt2 (δ 0.00). Infrared
spectra were recorded on a Mattson Polaris Fourier transform
spectrometer with 2 cm-1 resolution.
the zirconium and boron atoms. This basic structure was
subsequently confirmed by a single-crystal neutron diffrac-
tion study on the analogous hafnium complex, Hf{(µ-
H)3BH)}4, in 1983.4
Some of us5 have reported the preparation and chemistry
of organohydroborate derivatives that contain, in addition,
the cyclopentadiene (Cp) ligand: (a) Cp2MX{(µ-H)2BR2}
[M ) Zr, Hf; X ) Cl, H, D, CH3, CH2Ph, OSiPh3; BR2 )
BC4H8, BC5H10, BC8H14], (b) Cp2M{(µ-H)2BR2} [M ) Ti,
Nb; R2 ) C4H8, C5H10, C8H14], (c) CpZr{(µ-H)2BR2}3 [BR2
) BC5H10, BC8H14], (d) CpZrCl{(µ-H)2BC8H14}2. We have
also reported a few other organohydroborate derivatives that
do not contain the Cp ligand.6 In the present paper, we
describe the preparation and characterization of a new 9-BBN
hydroborate complex of zirconium as well as the X-ray and
neutron diffraction analyses of its structure. Zr{(µ-H)2BC8H14}4
is the first example of an organohydroborate derivative from
group 4 without Cp. Unlike the M[(µ-H)3BH]4 (M ) Zr,
Hf) complexes, all of the B-H groups in this compound
are involved in bridging interactions to the metal atom.
Interestingly, three agostic interactions were formed between
the R C-H of three BC8H14 units and zirconium metal in
this molecule. Agostic interactions are of general conse-
quence in organometallic chemistry since they often lead to
C-H activation and may serve to stabilize various catalytic
species.7-9
Preparation of Zr{(µ-H)2BC8H14}4. A solution of K(H2BC8H14)
(324.2 mg, 2.0 mmol) in 50 mL of diethyl ether was added dropwise
to a solution of ZrCl4 (116.5 mg, 0.5 mmol) in 100 mL of diethyl
ether. After the mixture was stirred at room temperature for 12 h,
the white solid (KCl) was removed by filtration, and after the
solvent was removed under vacuum, a white product was obtained.
This white solid was redissolved in ether and kept at -30 °C for
crystallization. White crystalline Zr{(µ-H)2BC8H14}4 was obtained
1
in a 78% yield. 11B NMR (ether): δ 17.47. H NMR (methylene
chloride-d2): δ 1.96-1.85 (m, â-H), 1.76-1.67 (m, â-H and γ-H),
1.60 (br s, µ-H), 1.56-1.52 (m, s, R-H). IR (KBr): 2979 (m), 2947
(m), 2918 (s), 2899 (s), 2873 (s), 2833 (s), 2654 (w), 1967 (m),
1923 (m), 1446 (m), 1387 (s), 1341 (s), 1284 (m), 1203 (m), 1111
(m), 923 (m), 829 (m). Anal. Calcd for C32H64B4Zr: C, 65.89; H,
11.06. Found: C, 66.12; H, 11.08.
X-ray Structural Determination. Single-crystal X-ray diffrac-
tion data were collected using graphite monochromated Mo KR
radiation (λ ) 0.71073 Å) on an Enraf-Nonius Kappa CCD
diffraction system. A single crystal of Zr{(µ-H)2BC8H14}4 was
mounted on the tip of a glass fiber coated with Fomblin oil (a
pentafluoropolyether), and crystallographic data were collected at
150 K. Unit cell parameters were obtained by indexing the peaks
from the first 10 frames and refined employing the whole data set.
All frames were integrated and corrected for Lorentz and polariza-
(4) Broach, R. W.; Chung, I. S.; Marks, T. J.; Williams, J. M. Inorg.
Chem. 1983, 22, 1081.
(8) For recent molecular structures of group 4 complexes with hydrogen
interactions see: (a) Tanski, J. M.; Parkin, G. Organometallics 2002,
21, 587. (b) Jia, L.; Zhao, J.; Ding, E.; Brennessel, W. W. J. Chem.
Soc., Dalton Trans. 2002, 2608. (c) Sun, Y.; Metz, M.; Stern, C. L.;
Marks, T. J. Organometallics 2000, 19, 1625. (d) Keaton, R. J.;
Jayaratne, K. C.; Fettinger, J. C.; Sita, L. R. J. Am. Chem. Soc. 2000,
122, 12909. (e) Chen, Y. X.; Metz, M. V.; Li, L.; Stern, C. L.; Marks,
T. J. J. Am. Chem. Soc. 1998, 120, 6287. (f) Procopio, L. J.; Carroll,
P. J.; Berry, D. H. J. Am. Chem. Soc. 1994, 116, 177. (g) Fryzuk, M.
D.; Mao, S. S. H. J. Am. Chem. Soc. 1993, 115, 5336. (h) Yang, X.;
Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623. (i)
Hyla-Kryspin, I.; Gleiter, R.; Kru¨ger, C.; Zwettler, R.; Erker, G.
Organometallics 1990, 9, 517.
(9) Selected papers for chemistry reactions of group 4 complexes with
hydrogen agostic interaction: (a) Beswick, C. L.; Marks, T. J. J. Am.
Chem. Soc. 2000, 122, 10358. (b) Thorshaug, K.; Strvneng, J. A.;
Rytter, E. Macromolecules 2000, 33, 8136. (c) Beswick, C. L.; Marks,
T. J. Organometallics 1999, 18, 2410. (d) Moscardi, G.; Piemontesi,
F.; Resconi, L. Organometallics 1999, 18, 5264. (e) Thorshaug, K.;
Strvneng, J. A.; Rytter, E.; Ystenes, M. Macromolecules 1998, 31,
7149. (f) Gleiter, R.; Hyla-Kryspin, I.; Niu, S. Q.; Erker, G.
Organometallics 1993, 12, 3828. (g) Prosenc, M. H.; Janiak, C.;
Brintzinger, H. H. Organometallics 1992, 11, 4036. (h) Erker, G.;
Fro¨mberg, W.; Angermund, K.; Schlund, R.; Kru¨ger, C. J. Chem. Soc.,
Chem. Commun. 1986, 372.
(5) (a) Lacroix, F.; Plecnik, C. E.; Liu, S.; Liu, F. C.; Meyers, E. A.;
Shore, S. G. J. Organomet. Chem. 2003, 687, 69. (b) Chen, X.; Liu,
S.; Plecnik, C. E.; Liu, F. C.; Shore, S. G. Organometallics 2003, 22,
275. (c) Ding, E.; Liu, F. C.; Liu, S.; Meyers, E. A.; Shore, S. G.
Inorg. Chem. 2002, 41, 5335. (d) Liu, S.; Liu, F. C.; Renkes, G.; Shore,
S. G. Organometallics 2001, 20, 5717. (e) Plecnik, C. E.; Liu, F. C.;
Liu, J.; Liu, S.; Meyers, E. A.; Shore, S. G. Organometallics 2001,
20, 3599. (f) Liu, F. C.; Plecnik, C. E.; Liu, J.; Liu, S.; Meyers, E. A.;
Shore, S. G. J. Organomet. Chem. 2001, 627, 109. (g) Ho, N. N.;
Bau, R.; Plecnik, C. E.; Shore, S. G.; Wang, X.; Schultz, A. J. J.
Organomet. Chem. 2002, 654, 216. (h) Liu, F. C.; Liu, J.; Meyers, E.
A.; Shore, S. G. J. Am. Chem. Soc. 2000, 122, 6106. (i) Liu, F. C.;
Du, B.; Liu, J.; Meyers, E. A.; Shore, S. G. Inorg. Chem. 1999, 38,
3228. (j) Liu, F. C.; Liu, J.; Meyers, E. A.; Shore, S. G. Inorg. Chem.
1999, 38, 2169. (k) Liu, F. C.; Liu, J.; Meyers, E. A.; Shore, S. G.
Inorg. Chem. 1998, 37, 3293. (l) Liu, J.; Meyers, E. A.; Shore, S. G.
Inorg. Chem. 1998, 37, 496. (m) Jordan, G. T., IV; Liu, F. C.; Shore,
S. G. Inorg. Chem. 1997, 36, 5597. (n) Jordan, G. T., IV; Shore, S.
G. Inorg. Chem. 1996, 35, 1088.
(6) Chen, X.; Lim, S.; Plecnik, C. E.; Liu, S.; Du, B.; Meyers, E. A.;
Shore, S. G. Inorg. Chem. 2004, 43, 692-698.
(7) For reviews of agostic interactions see: (a) Grubbs, R. H.; Coates, G.
W. Acc. Chem. Res. 1996, 29, 85. (b) Brookhart, M.; Green, M. L.
H.; Wong, L. L. Prog. Inorg. Chem. 1988, 36, 1. (c) Ginzhurg, A. G.
Russ. Chem. ReV. 1988, 57, 1175.
(10) Ko¨ster, R.; Seidel, G. Inorg. Synth. 1983, 22, 198.
2460 Inorganic Chemistry, Vol. 44, No. 7, 2005

Synthesis and Structure of Zr(9-BBN)4
tion effects using the Denzo-SMN package (Nonius BV, 1999).11
An absorption correction was applied using the SORTAV program12
provided by MaXus software.13 The structure was solved by direct
methods and refined using SHELXTL-9714 (difference electron
density calculation, full matrix least-squares refinements). All non-
hydrogen atoms were located and refined anisotropically.
Table 1. Crystallographic Data for Zr{(µ-H)2BC8H14}4
X-ray Analysis
empirical formula
fw
C32H64B4Zr
583.29
cryst syst, space group
unit cell dimensions
triclinic, P1h
a ) 11.246(2) Å
b ) 11.388(2) Å
c ) 12.898(2) Å
1638.46(2) Å3
2, 1.182 g/cm3
150 (2) K
R ) 92.65(2)°
â ) 96.55(2)°
γ ) 91.80(2)°
Neutron Structure Determination. Crystals of the title com-
pound are stable at room temperature but are extremely air-sensitive.
A suitable “neutron-sized” crystal with approximate dimensions of
0.9 × 0.9 × 0.9 mm3 was mounted in a 2 mm diameter quartz
capillary, wedged between two plugs of quartz wool, inside an
argon-filled glovebox. The crystal was cooled to 150 K, and data
were collected on the Very-Intense Vertical-Axis Laue Diffracto-
meter (VIVALDI) at the Institut Laue-Langevin.15 Eleven Laue
diffraction patterns, each accumulated over 4 h, were collected at
(mostly) 20° intervals in a rotation of the crystal perpendicular to
the incident beam. A total of 14 032 reflections were recorded of
which 4642 were independent, corresponding to 64% of the
complete unique set to d ) 0.78 Å, the average minimum value of
d observed over all patterns. The intensities were indexed and
processed using the program LAUEGEN,16a,b and the reflections
were integrated and the background was removed using the program
INTEGRATE+.16c The reflections were normalized to a constant
incident wavelength using the program LAUENORM.16d This
normalization would also have corrected for the absorption variation
in this nearly spherical sample. Subsequent calculations for structure
determination were carried out using the SHELXTL package.14
Initial H positions were obtained from the results of the earlier X-ray
structure determination. Least-squares refinement of all atomic
coordinates and anisotropic temperature factors resulted in a final
agreement factor of R(F) ) 6.52% for 2557 independent reflections
with I > 2σ(I). Relevant crystallographic data are summarized in
Table 1. Since only the ratios between unit cell dimensions can be
determined in the white-beam Laue technique, the dimensions found
by X-ray diffraction were used in the neutron refinement; the
observed ratios and angles were, however, in accord with the X-ray
values.
V
Z, calcd density
temperature
cryst size
absorption coefficient
wavelength
0.40 × 0.31 × 0.23 mm3
0.355 mm-1
0.71073 Å
GOF
1.181
no. of reflns collected
no. of independent reflns
no. of reflns with
I > 2σ(I)
R1 [I > 2σ(I)]
wR2 (all data)
7509
7509
6691
0.0292
0.0971
Neutron Analysisa
150(1) K
temperature
wavelength range of
reflns accepted
cryst size
0.9-2.7 Å
0.9 × 0.9 × 0.9 mm3
min d spacing observed
θ range for data collcn
no. of reflns collected
no. of independent reflns
no. of reflns with
I > 2σ(I)
0.72 Å
4-72°
14032
4642
2557
no. of params refined
refinement method
860
full matrix, least-
squares on F2
1.137
GOFb
final R indices [I > 2σ(I)]
R1 ) 0.0652,
wR2 ) 0.0985
R1 ) 0.1637,
wR2 ) 0.1130
R indices (all data)
a Unit cell parameters for the neutron analysis (and their estimated
standard deviations) were assumed to be the same as those for the X-ray
analysis since both data sets were collected at the same temperature. b With
w ) 1/[σ2(Fo ) + (0.03P)2], where P ) [max(Fo , 0) + 2Fc ]/3
2
2
2
Results and Discussion
This complex is composed of four 9-BBN hydroborate
ligands, each functioning as a bidentate ligand. The white
solid complex is stable at room temperature in the absence
of air, and it is soluble in THF and diethyl ether as well as
benzene. In diethyl ether, it slowly decomposes to the 9-BBN
dimer, (C8H14B)(µ-H)2(BC8H14), and an unknown material.
Upon decomposition of the title zirconium/9-BBN hydro-
borate complex, the color of the ether solution becomes dark
(from colorless to yellow to brown), a process which can be
monitored in solution by 11B NMR spectroscopy at room
temperature. Initially, only one resonance (17.47 ppm), that
of the pure complex of Zr{(µ-H)2BC8H14}4, was present in
the spectrum. After two days, the signal at 17.47 ppm had
noticeably diminished and two new peaks became observ-
able: one at 27.28 ppm corresponding to the 9-BBN
hydroborate dimer [(C8H14B)(µ-H)2(BC8H14)] and another
unidentified signal at 15.50 ppm. After two weeks at room
Synthesis and Characterization. The complex Zr{(µ-
H)2BC8H14}4 was obtained in 78% yield from the reaction
of ZrCl4 and 4 mol of K(H2BC8H14) in diethyl ether or THF
(reaction 1)
(11) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter, C.
W., Jr., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol.
276(A), p 307.
(12) (a) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33. (b) Blessing,
R. H. J. Appl. Crystallogr. 1997, 30, 421.
(13) Mackay, S.; Gilmore, C. J.; Edwards, C.; Tremayne, M.; Stuart, N.;
Shankland, K. MaXus: A computer program for the solution and
refinement of crystal structures from diffraction data; University of
Glasgow: Glasgow, Scotland; Nonius BV: Delft, The Netherlands;
and Mac-Science Co. Ltd.: Yokohama, Japan, 1998.
(14) SHELXTL, version 5.10; Bruker Analytical X-ray systems, 1997.
(15) Wilkinson, C.; Cowan, J. A.; Myles, D. A. A.; Cipriani, F.; McIntyre,
G. J. Neutron News 2002, 13, 37.
(16) (a) Campbell, J. W. J. Appl. Crystallogr. 1995, 28, 228. (b) Campbell,
J. W.; Hao, Q.; Harding, M. M.; Nguti, N. D.; Wilkinson, C. J. J.
Appl. Crystallogr. 1998, 31, 23. (c) Wilkinson, C. J.; Khamis, H. W.;
Stansfield, R. F. D.; McIntyre, G. J. J. Appl. Crystallogr. 1988, 21,
471. (d) Campbell, J. W.; Habash, J.; Helliwell, J. R.; Moffat, K.
Information Quarterly for Protein Crystallography, No. 18; SERC
Daresbury Laboratory: Warrington, England, 1986.
Inorganic Chemistry, Vol. 44, No. 7, 2005 2461

Ding et al.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Zr(9-BBN)4
Bond Lengths
X-ray
neutron
Zr‚‚‚B(1)
Zr‚‚‚B(2)
Zr‚‚‚B(3)
Zr‚‚‚B(4)
average
2.352(2)
2.421(2)
2.388(2)
2.501(2)
2.416(2)
2.344(5)
2.412(7)
2.383(6)
2.496(5)
2.409(6)
Zr-H(1A)
Zr-H(1B)
Zr-H(2A)
Zr-H(2B)
Zr-H(3A)
Zr-H(3B)
Zr-H(4A)
Zr-H(4B)
average
2.01(2)
1.94(2)
1.98(2)
1.92(2)
2.01(2)
1.95(2)
2.01(2)
1.95(2)
1.971(13)
2.036(9)
2.016(7)
2.084(7)
2.061(9)
2.107(9)
2.035(7)
2.052(7)
2.014(9)
2.051(8)
B(1)-H(1A)
B(1)-H(1B)
B(2)-H(2A)
B(2)-H(2B)
B(3)-H(3A)
B(3)-H(3B)
B(4)-H(4A)
B(4)-H(4B)
average
1.19(2)
1.16(2)
1.22(2)
1.19(2)
1.18(2)
1.11(2)
1.20(2)
1.16(2)
1.18(2)
1.295(8)
1.303(7)
1.269(15)
1.284(7)
1.302(7)
1.248(15)
1.301(10)
1.286(9)
1.286(7)
Figure 1. Molecular structure of Zr{(µ-H)2BC8H14}4 from the X-ray
analysis; the ellipsoids are shown at 50% probability.
temperature, the original signal at 17.47 ppm had completely
disappeared indicating that the complex Zr{(µ-H)2BC8H14}4
had completely decomposed to the 9-BBN hydroborate dimer
[(C8H14B)(µ-H)2(BC8H14)] and an unknown material in
diethyl ether. Unfortunately, we could not isolate and
Zr‚‚‚H(11)
2.449(21)
2.663(22)
2.579(25)
3.222(23)
0.953(22)
0.976(23)
0.874(26)
1.013(24)
2.424(7)
2.663(8)
2.551(7)
3.146(7)
1.124(9)
1.119(9)
1.131(8)
1.111(8)
Zr‚‚‚H(25)
Zr‚‚‚H(31)
Zr‚‚‚H(45)
C(11)-H(11)
C(25)-H(25)
C(31)-H(31)
C(45)-H(45)
1
characterize the latter species. In the H NMR spectrum of
the title compound, Zr{(µ-H)2BC8H14}4, the hydrogen signals
of the 9-BBN hydroborate ligand appeared at δ ) 1.96-
1.84, 1.76-1.66, and 1.56-1.52 ppm, consistent with those
in the complex CpZr{(µ-H)2BC8H14}3.5c The signals at 1.60
ppm are assigned to the bridging hydrogen atoms. We
attempted to abstract a hydride ion from Zr{(µ-H)2BC8H14}4
by employing B(C6F5)3 in toluene and in diethyl ether, but
the product was too unstable to be isolated. The initial 11B
NMR spectrum at room temperature consists of a doublet at
-24.68 ppm in toluene and -23.98 ppm in ether which
indicates the formation of an anion with a B-H bond.
However, this signal disappeared very quickly at room
temperature. A 13C NMR spectrum was also obtained, but
the proximity of the boron atom made agostic interactions
nonobservable.
Solid-state infrared spectra of Zr{(µ-H)2BC8H14}4 show
that most of the C-H stretches occur in the normal region
from 2979 to 2833 cm-1. However, weak absorption also
appeared at a lower frequency, 2655 cm-1, which might be
evidence of agostic M-H-C interactions in this molecule.7b
Bands assigned to M-H-B stretches appear from 1967 to
1923 cm-1 in this compound.
X-ray Structural Results. The solid-state structure of
Zr{(µ-H)2BC8H14}4 was initially determined by a single-
crystal X-ray diffraction analysis (Figure 1). Crystallographic
data and selected bond distances and angles are given in
Tables 1 and 2.
Suitable single crystals of Zr{(µ-H)2BC8H14}4 were ob-
tained by crystallization from diethyl ether at -30 °C. Each
of the four 9-BBN hydroborate units functions as a bidentate
ligand bonded to the zirconium atom through two hydrogen
bridges. Unlike the arrangement of hydroborate ligands in
Bond Angles
X-ray
neutron
B(1)-Zr-B(2)
B(1)-Zr-B(3)
B(1)-Zr-B(4)
B(2)-Zr-B(3)
B(2)-Zr-B(4)
B(3)-Zr-B(4)
average
106.73(7)
114.06(7)
111.50(7)
110.24(7)
112.45(7)
101.98(7)
109.9(7)
106.6(2)
114.4(2)
111.5(2)
109.9(2)
112.8(2)
101.9(2)
109.5(2)
H(1A)-Zr-H(1B)
H(2A)-Zr-H(2B)
H(3A)-Zr-H(3B)
H(4A)-Zr-H(4B)
average
52.4(9)
52.2(9)
50.2(9)
52.9(8)
51.9(9)
59.4(3)
58.3(3)
58.0(4)
59.9(3)
58.9(3)
Zr-H(1A)-B(1)
Zr-H(1B)-B(1)
Zr-H(2A)-B(2)
Zr-H(2B)-B(2)
Zr-H(3A)-B(3)
Zr-H(3B)-B(3)
average
90.7(11)
94.2(11)
95.0(11)
99.9(11)
94.3(13)
96.6(13)
95.1(13)
86.4(5)
87.1(3)
88.5(4)
89.1(5)
85.2(5)
89.7(4)
87.7(4)
Zr-H(4A)-B(4)
97.5(6)
102.6(7)
93.5(5)
95.8(5)
86.9(2)
93.2(3)
89.7(2)
Zr-H(4B)-B(4)
Zr‚‚‚B(1)-C(11)
86.51(11)
92.99(11)
86.52(10)
107.29(12)
118.0(13)
104.8(13)
105.5(13)
115.8(13)
105.8(13)
106.3(13)
117.7(17)
106.2(17)
104.1(16)
119.2(13)
102.5(13)
106.1(13)
Zr‚‚‚B(2)-C(25)
Zr‚‚‚B(3)-C(31)
Zr‚‚‚B(4)-C(45)
107.4(2)
118.2(4)
104.4(4)
104.6(5)
116.5(5)
105.3(5)
105.2(4)
117.5(5)
105.2(5)
104.4(5)
115.0(4)
107.6(5)
105.3(6)
B(1)-C(11)-H(11)
C(12)-C(11)-H(11)
C(18)-C(11)-H(11)
B(2)-C(25)-H(25)
C(24)-C(25)-H(25)
C(26)-C(25)-H(25)
B(3)-C(31)-H(31)
C(32)-C(31)-H(31)
C(38)-C(31)-H(31)
B(4)-C(45)-H(45)
C44-C45-H45
C46-C45-H45
2462 Inorganic Chemistry, Vol. 44, No. 7, 2005

Synthesis and Structure of Zr(9-BBN)4
Zr{(µ-H)2BH}4, 3c,3d a symmetrical tetrahedral configuration,
the four boron atoms that surround Zr in the title com-
pound are in a slightly distorted tetrahedral arrangement
[B‚‚‚Zr‚‚‚B angles ) 114.06(7)°, 106.73(7)°, 110.24(7)°,
111.50(7)°, 101.98(7)°, 112.45(7)°]. The Zr‚‚‚B distances in
this complex are also slightly unequal [2.352(2), 2.388(2),
2.421(2), 2.501(2) Å] but are still comparable to the Zr‚‚‚B
distance of 2.34(3) Å in Zr(BH4)4.3c,3d In contrast, the
Zr‚‚‚B distances in the complex CpZr{(µ-H)2BC8H14}3 are
significantly longer [2.541(2), 2.544(2), 2.556(2) Å].5c The
average Zr-bridge-hydrogen bond length in the title com-
pound is 1.97 Å, similar to those in two other known
complexes, CpZr(Cl){(µ-H)2BC8H14}2 (1.98 Å)5c and Cp*Zr-
(Cl){(µ-H)2BC8H14}2 (1.99 Å).5c In the complex reported
here, Zr{(µ-H)2BC8H14)}4, zirconium is formally associated
with 16 valence electrons, but interestingly, the “electron
deficiency” of zirconium appears to be partially compensated
by three agostic interactions involving the R C-H bonds of
three of the BC8H14 units, Zr‚‚‚H11 ) 2.449(21) Å,
Zr‚‚‚H25 ) 2.663(22) Å, Zr‚‚‚H31 ) 2.579(25) Å. These
estimated distances were later measured more accurately by
the subsequent neutron diffraction analysis (vide infra). All
three Zr‚‚‚H distances are less than the sum of the zirconium
covalent radius and the hydrogen van der Waals radius, 2.70
Å (Zr,17 1.50 Å; H,18 1.20 Å). Because of these agostic
interactions, three of the Zr‚‚‚B-C angles are highly distorted
Figure 2. Neutron structure of Zr{(µ-H)2BC8H14}4 showing the eight
hydride positions with 50% probability ellipsoids.
Zr‚‚‚B-C angle average of 161.2(3)° (involving Zr‚‚‚B1-
C15, Zr‚‚‚B2-C21, and Zr‚‚‚B3-C35). In fact, the fourth
BBN ligand may show evidence of being weakly agostic as
demonstrated by the inequality of the two Zr‚‚‚B-C angles
associated with it [107.4(2)° and 143.5(2)°]. However, the
associated Zr‚‚‚H45 bond distance of 3.146(7) Å involving
the fourth BBN ligand is noticeably longer than the other
three interactions and does not support the notion of a fourth
agostic interaction. The average bond distance of the three
R C-H bonds that interact with Zr (C11-H11, C25-H25,
and C31-H31) is 1.125(9) Å, which is very slightly longer
than the average bond distance of the other five R C-H
bonds with no or very weak interaction with Zr metal
[1.109(8) Å]. A summary of selected bond distances and
angles from the neutron study is given in Table 2.
[Zr‚‚‚B(1)-C(11)
) 86.51(11)°, Zr‚‚‚B(2)-C(25) )
92.99(11)°, Zr‚‚‚B(3)-C(31) ) 89.40(10)°] as compared to
the fourth angle [Zr‚‚‚B(4)-C(45) ) 107.29(12)°] whose
BBN ligand has a much weaker agostic interaction with the
zirconium atom. This might be the first example of three
dominant agostic interactions between hydrogen and metal
in one molecule. If these agostic interactions are included,
22 electrons are associated with zirconium. In contrast, in
the case of zirconium tetrahydroborate, Zr[(µ-H)3BH)]4, there
are 24 electrons associated with zirconium.1c,3c,d
Neutron Structural Results. Essentially, the results from
the earlier X-ray analysis were confirmed, but the neutron
analysis provided more precise H atom parameters. Notably,
the neutron-determined B-H bond lengths are longer, as
expected, and more nearly equal. As anticipated, each BBN
ligand was found to coordinate to the metal atom in a
bidentate fashion. It was mentioned earlier that the overall
geometry of the title complex (especially the Zr‚‚‚B-C
angles) is severely strained due to the presence of agostic
interactions from three of the BBN ligands. Analysis of the
neutron data resulted in a more accurate characterization of
these three agostic bond distances [Zr‚‚‚H11 ) 2.424(7) Å,
Zr‚‚‚H25 ) 2.663(8) Å, Zr‚‚‚H31 ) 2.551(7) Å] which are
in good agreement with the values calculated from the X-ray
data. As a result of these interactions, the compound displays
a pronounced tilting of the three agostically related BBN
ligands relative to the metal center as revealed by the average
Zr‚‚‚B-C angle of 89.9(2)° (involving Zr‚‚‚B1-C11,
Zr‚‚‚B2-C25, and Zr‚‚‚B3-C31) as opposed to the opposite
Our neutron data analysis has provided the first accurate
neutron-diffraction measurement of a Zr-H-B bridge bond.
The Zr-hydride distances range from a minimum of
2.014(9) Å to a maximum of 2.107(9) Å, giving an average
Zr-H distance of 2.051(8) Å. Other average molecular
parameters are as follows: B-H ) 1.286(7) Å, Zr‚‚‚B )
2.409(6) Å, Zr-H-B ) 87.7(4)°, and H-Zr-H )
58.9(3)°. One other feature worth mentioning is that this
complex demonstrates a slight distortion in the bidentate
bridges: the strained character arising from the bicyclic
framework of the BBN ligand is apparent in the low
H-Zr-H “bite angle” [average ) 58.9(3)°]. In addition, the
Zr-H-B angles [average Zr-H-B ) 87.7(4)°] are much
smaller for the three agostically related BBN ligands than
that for the fourth ligand (with boron atom B4) where no
interaction is apparent [average Zr-H-B ) 94.7(5)°].
Neutron studies of other group IV metal-hydride-boron
complexes that involve bidentate ligands have been reported.
In an earlier paper on Hf(BH4)2(η5-CH3C5H4)2,19 the average
(17) Howard, W. A.; Tina, M. T.; Parkin, G. Inorg. Chem. 1995, 34, 5900.
(18) Huheey, J.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, 4th ed.;
Harper Collins: New York, 1993; Table 8.1, p 292.
(19) Johnson, P. L.; Cohen, S. A.; Marks, T. J.; Williams, J. M. J. Am.
Chem. Soc. 1978, 100, 2709
Inorganic Chemistry, Vol. 44, No. 7, 2005 2463

Ding et al.
Figure 4. ORTEP plot of the neutron structure of Zr{(µ-H)2BC8H14}4
with 50% probability ellipsoids showing the three major agostic interactions.
All H atoms in the molecule except those involved in the agostic interactions
have been removed for clarity. Note highly distorted geometry caused by
these agostic interactions (e.g., compare the Zr-B2-C21 and Zr-B2-
C25 angles).
Figure 3. ORTEP plot of the core structure of Zr{(µ-H)2BC8H14}4 derived
from neutron data, showing the Zr-hydride-boron bridge bonds. The
carbon atoms and the hydrogen atoms of the C-H bonds have been removed
for clarity. Atoms are shown as 50% probability ellipsoids.
Hf-H distance was reported to be 2.095(8) Å. In a very
recent paper we published containing a Ti-H-B bridge,5e
the average Ti-H distance was found to be 1.919(16) Å.
The Zr-H distances from the present work [average )
2.051(3) Å] are significantly longer than the average Ti-H
distance but slightly shorter than the Hf-H distance which
is in keeping with the expected trend in covalent radii of Ti
< Zr < Hf. Furthermore, the Zr-H-B bridge bond angles
[average ) 87.7(4)°] are consistently smaller, except for the
agostic free BBN ligand, compared to the other group IV
complexes that have been analyzed by neutron diffraction
[average for Ti-H-B ) 95.4(12)°, average for Hf-H-B
) 96.8(7)°]. However, strictly speaking, a comparison
between the M-H-B angles of the three compounds may
not really be meaningful as a consequence of the strain of
the BBN ligands in the present work. Figures 2-4 show
ORTEP plots of the neutron structure of Zr{(µ-H)2BC8H14}4.
Acknowledgment. This work was supported by the
National Science Foundation through Grant CHE 02-13491
(to S.G.S.) and the American Chemical Society through
Grant PRF-40715-AC3 (to R.B.).
Supporting Information Available: X-ray and neutron crystal-
lographic files for structural analysis of the title compound
Zr{(µ-H)2BC8H14}4 in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC048629G
2464 Inorganic Chemistry, Vol. 44, No. 7, 2005