The effect of solvent in the reaction of Cp2ZrH {μ-H)2BC4H8} with B(C6F5)3: Formation of [HB(C6F5)3- salts of two unsupported hydrogen

Full text of DOI:10.1021/ja001056w

6106
J. Am. Chem. Soc. 2000, 122, 6106-6107
Communications to the Editor
The Effect of Solvent in the Reaction of
Cp2ZrH{(µ-H)2BC4H8} with B(C6F5)3: Formation of
[HB(C6F5)3]- Salts of the Unsupported
Hydrogen-Bridged Cation
[(µ-H){Cp2Zr(µ-H)2BC4H8}2]+ and the Cation
[Cp2Zr(OEt2)(OEt)]+
Fu-Chen Liu, Jianping Liu, Edward A. Meyers, and
Sheldon G. Shore*
Department of Chemistry
The Ohio State UniVersity
Columbus, Ohio 43210
ReceiVed March 27, 2000
Boron Lewis acids BF3, BCl3, and B(C6F5)3 can serve as
hydride ion abstracting agents.1,2 A seminal contribution2 on
cationic zirconocene polymerization catalysts reported hydride
ion abstraction from (C5Me5)2ZrH2 by B(C6F5)3 in benzene and
in toluene to produce the salt [(C5Me5)2ZrH][HB(C6F5)3]. While
there is significant interest in generating metallocene cations,3
with particular interest in alkyl carbanion abstraction,4 relatively
little effort to produce them through hydride ion abstraction by
B(C6F5)3 has been reported. Described here are hydride ion
abstraction reactions by B(C6F5)3 that are influenced by the nature
of the solvent to produce new zirconocene cations, one of which
is possibly the first example of a bimetallic hydrogen-bridged
cation in which there is no metal-metal bonding between the
bridged metals.5 The common unsupported hydrogen-bridged
systems are either neutral or anionic.
Figure 1. (a) Molecular structure of [(µ-H)(Cp2Zr(µ-H)2BC4H8)2]+ and
the selected bond distances (Å) and angle (deg), Zr(1)-B(1): 2.567(5);
Zr(2)-B(2): 2.561(5); Zr(1)-H: 1.99(4); Zr(2)-H: 1.94(4); Zr(1)-
H(1): 1.97(4); Zr(1)-H(2): 1.96(4); Zr(2)-H(3): 1.97(4); Zr(2)-
H(4): 2.00(4); B(1)-H(1): 1.19(4); B(1)-H(2): 1.27(4); B(2)-H(3):
1.26(4); B(2)-H(4): 1.15(4); Zr(1)-H-Zr(2): 163.3; Zr(1)- - -Zr(2)
3.898(4). (b) Molecular structure of [(Cp2Zr(OEt)(OEt2)]+ and the selected
bond distances (Å) and angle (deg), Zr(1)-O(1): 2.209(8); Zr-O(2):
1.884(8). O(1)-Zr(1)-O(2): 91.9(3); Zr(1)-O(2)-C(16): 159.6(9).
Reactions of Cp2ZrH{(µ-H)2BC4H8}, 1,6 with B(C6F5)3 in
benzene and in diethyl ether were investigated. Complex 1
contains a terminal Zr-H bond and two Zr-H-B bonds. Since
bridge hydrogens tend to be abstracted as protons rather than
hydride ion,7 it was of interest to determine if the reaction with
B(C6F5)3 would remove only H- from the Zr-H bond and
generate a zirconocene cation that has retained the {(µ-H)2BC4H8}
moiety. It was observed that solvent plays a key role in
determining the course of this reaction. In benzene, a poorly
coordinating solvent, the unusual unsupported single hydrogen-
Scheme 1
(1) Toft, M. A.; Leach, J. B.; Himpsl, F. L.; Shore, S. G. Inorg. Chem.
1982, 21, 1952.
(2) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015.
(3) Guram, A. S.; Jordan, R. F. ComprehensiVe Organometallic Chemistry,
II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier Science, Ltd.:
New York, 1995; Vol 4. Chapter 12,
(4) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391 and references
bridged cation [(µ-H){Cp2Zr(µ-H)2BC4H8}2]+, 2 (Figure 1a), was
isolated as the [HB(C6F5)3]- salt (78% yield). However, from the
coordinating solvent, diethyl ether, the cation [Cp2Zr(OEt2)-
(OEt)]+, 3 (Figure 1b), was isolated as the [HB(C6F5)3]- salt (68%
yield).8-10
The double hydrogen bridged unit {(µ-H)2BC4H8} from reactant
1 is retained in the unsupported hydrogen bridge cation 2, thereby
suggesting that hydride ion is abstracted from the Zr-H bond in
the initial phase of the reaction. Proton and 11B NMR spectra are
in accord with the solid-state structure.8 The reaction between
complex 1 and B(C6F5)3 occurs in a 2:1 molar ratio even when
the reactants are employed in a 1:1 molar ratio. In the earlier
reported reaction2 between B(C6F5)3 and (C5Me5)2ZrH2 to produce
[(C5Me5)2ZrH][HB(C6F5)3] the formation of a dinuclear unsup-
ported hydrogen bridged cation analogous to complex 2 was not
therein.
(5) Albinati, A.; Chaloupka, S.; Eckert, J.; Venanzi, L. M.; Wolfer, M. K.
Inorg. Chem. Acta 1997, 259, 305. (b) Bau, R.; Drabnis, M. H. Inorg. Chem.
Acta, 1997, 259, 27.
(6) Liu, F.-C., Ph.D. Dissertation, Ohio State University, Columbus, Ohio,
1998.
(7) (a) Shore, S. G. Pure Appl. Chem. 1977, 49, 717. (b) Inkrott, K. E.;
Shore, S. G. J. Am. Chem. Soc. 1978, 100, 3953.
(8) Preparation of [(µ-H)(Cp2Zr(µ-H)2BC4H8)2][HB(C6F5)3]: In the drybox
a 147 mg (0.50 mmol) of Cp2ZrH{(µ-H)2BC4H8} and 128 mg (0.25 mmol)
of B(C6F5)3 were put in a flask. After degassing, 5 mL of benzene was
transferred to the flask. After stirring for 10 min, hexane was transferred to
the system to produce a white ppt that was washed with hexane twice and a
215 mg (mmol), 78% yield of white product was isolated. 11B NMR (d8-
toluene) δ ) -13.12 ((B-H-Zr)2, t, JB-H ) 69 Hz), -24.85 ppm
([HB(C6,F5]-, d, JB-H ) 72 Hz). 1H NMR (d8-toluene) δ ) 5.68 (s, Cp), 1.66
(br, 8H, â-H), 0.88 (br, 8H, R-H), -2.02 (br, 2 µ-H), -3.89 (br, 2 µ-H), and
1
-5.56 ppm (br, 1 µ-H). H{11B} NMR (d8-toluene) 4.27 (br, HB). Calcd C,
50.47; H, 3.87. Found. C, 50.36; H, 4.01.
10.1021/ja001056w CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/09/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 25, 2000 6107
Scheme 2
observed. Interestingly, the reaction of (1,2-Me2Cp)2ZrMe2 with
B(C6F5)3 requires a 2:1 molar ratio of reactants to produce the
related methyl-bridged cation [(1,2-Me2Cp)2ZrMe]2(µ-Me)+.11
When the reactants are employed in a 1:1 molar ratio, the ion-
paired complex [(1,2-Me2Cp)2(Me)Zr]+- - -[MeB(C6F5)3]- is pro-
duced.12
reacts with a second molecule of 1 to form 2. A recent report
showed that 1 can function as a hydride donor to form an
unsupported hydrogen bridge in the compound {Cp2Zr{(µ-H)B-
(C4H8)CH2Ph}}{(µ-H)2BC4H8}.13
In diethyl ether, the overall reaction of 1 with B(C6F5)3 (Scheme
2) was established from single crystal and NMR analysis of the
crystalline product, [Cp2Zr(OEt2)(OEt)][HB(C6F5)3], by NMR
spectra of the known organodiborane (B2(µ-H)2(C4H8)2)14 formed,
and the GC-MS spectrum of the ethane evolved. This is a rapid
reaction that precluded observation of intermediates. However,
it is reasonable to assume that ethane is formed through the
intermediate species shown in this Scheme 2. The formation of
the ethoxyl group on the cation is reminiscent of the conversion
of coordinated THF to a butoxyl group in the formation of [(C5H4-
Me)2Zr(O-n-Bu)(THF)]+.15
Scheme 1 suggests a pathway for the formation of 2. Hydride
ion abstraction from 1 produces a 16-electron Zr cation which
(9) Preparation of [Cp2Zr(OEt)(OEt2)][HB(C6F5)3]: In the drybox, 294 mg
(1 mmol) of Cp2ZrH{(µ-H)2BC5H10}, 512 mg (1 mmol) of B(C6F5)3, and 15
mL of ether were put in a flask. The reaction was monitored by the 11B NMR.
The anion [HB(C6F5)3]- (-26.9 ppm, d, JB-H ) 88 Hz) and organodiborane
B2(µ-H)2(C4H8)2 (28.5 ppm) were formed in <5 min. But with increasing
time B2(µ-H)2(C4H8)2 diminished in concentration while a new signal at 58.0
ppm (singlet) appeared, indicative of a trialkyl borane. After stirring this system
for 1 h, the ether solution was removed by filtration, and 583 mg (68.3%
yield) of white solid was isolated. Complex 3 has limited solubility in Et2O.
For the NMR spectra in THF the Et2O in 3 is displaced by THF solvent. 11
B
Acknowledgment. This work was supported by the National Science
Foundation through Grant CHE 99-01115. NMR spectra and GC-MS
were obtained at The Ohio State University Campus Chemical Instru-
mentation Center.
NMR (THF): δ ) -26.43 ppm (d, JB-H ) 93 Hz). 1H NMR (d8-THF): δ )
6.59 (s, Cp), 4.30 (2H, q, J ) 7 Hz), 3.38 (4H, q, J ) 7 Hz), 1.21 (3H, t, J
1
) 7 Hz), and 1.11 ppm (6H, J ) 7 Hz). H{11B} NMR (d8-THF): 3.80 (m,
HB, JH-B ) 4.6 Hz). Calcd C, 47.84; H, 3.07. Found: C, 47.01; H, 3.09.
(10) Crystal data for [(µ-H)(Cp2Zr(µ-H)2BC4H8)2][HB(C6F5)3], 2: mono-
clinic, space group P21/n, a ) 11.924(2) Å, b ) 17.218(3) Å, c ) 21.714(4)
Å, â ) 94.79(3)°, V ) 4442.6(15) Å3, Z ) 4, F(calcd) ) 1.637 mg/m3, fw )
1094.67, µ ) 0.566 mm-1. R1 (6293 independent reflections with I > 2σ(I)
and 675 parameters) ) 0.0479 and wR2 (29957 reflections measured) ) 0.1242.
Crystal data for [Cp2Zr(OEt2)(OEt)][HB(C6F5)3], 3: monoclinic, space group
C2/c, a ) 20.028(1) Å, b ) 15.467(1) Å, c ) 22.608(1) Å, â ) 107.26(1)°,
V ) 6687.5(7) Å3, Z ) 8, F(calcd) ) 1.696 mg/m3, fw ) 853.58, µ ) 0.445
mm-1. R1 (3018 independent reflections with I > 2σ(I) and 526 parameters)
) 0.0775 and wR2 (12937 reflections measured) ) 0.2114.
Supporting Information Available: Tables of crystallographic data,
positional and thermal parameters, and bond distances and angles for
[(µ-H){Cp2Zr(µ-H)2BC4H8}2][HB(C6F5)3] and [Cp2Zr(OEt2)(OEt)][HB-
(C6F5)3] (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA001056W
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