Formation of triniobocene cationic and neutral mononiobocene species as a function of solvent in the reaction of Cp2Nb{(μ-H)2BR2} (R2 = C4H8, C5H10, C8H14) with B(C6F5)3

Article abstract of DOI:10.1021/om0107744

Reactions of the niobocene cyclic organohydroborates Cp₂Nb{(μ-H)₂BR₂} (R₂ = C₄ H₈, C₅H₁₀, C₈H₁₄), with B(C₆F₅)₃ in the poorly coordinating solvent toluene and in the coordinating solvent diethyl ether give different products. In toluene, the salt [Cp₂Nb(μ-H)(η⁵-η¹-C₅ H₄)-Nb (η⁵-η¹-C₅H₄)₂ Nb{(μ-H)(η⁵-C₅H₄B (C₆F₅)₂)}]⁺[HB(C₆ F₅)₃]^-1, 1, which contains a triniobocene cation, is formed. On the other hand in diethyl ether, the reaction of Cp₂Nb{(μ-H)₂BR₂} with B(C₆F₅)₃ produces the covalent complex CpNb(C₆F₅){(μ-H) (η⁵-C₅H₄B(C₆ F₅)₂)}, 2. In the formation of these compounds Nb^III is oxidized to Nb^IV. Upon the basis of ESR spectra, 1 and 2 are paramagnetic, each with one unpaired electron. The structures of 1 and 2 were determined by single-crystal X-ray analysis. Complex 2 forms weak adducts in the solvents THF and pyridine. The Nb-H-B bridge is cleaved at the Nb-H site to form CpNb(C₆F₅)-(L) {η⁵-C₅H₄B(H)(C₆ F₅)₂} (L = THF, pyridine). In pyridine, a second adduct in much smaller yield is also formed, CpNb(C₆F₅)(H){η⁵-C₅H₄ B(Py)(C₆F₅)₂}, with the Nb-H-B bridge being cleaved at the H-B site.

Full text of DOI:10.1021/om0107744

Organometallics 2001, 20, 5717-5723
5717
F or m a tion of Tr in iobocen e Ca tion ic a n d Neu tr a l
Mon on iobocen e Sp ecies a s a F u n ction of Solven t in th e
Rea ction of Cp 2Nb{(µ-H)2BR2} (R2 ) C4H8, C5H10, C8H14)
w ith B(C6F 5)3
Shengming Liu, Fu-Chen Liu, Gordon Renkes, and Sheldon G. Shore*
Department of Chemistry, The Ohio State University, 100 West 18th Avenue,
Columbus, Ohio 43210
Received August 23, 2001
Reactions of the niobocene cyclic organohydroborates Cp2Nb{(µ-H)2BR2} (R2 ) C4H8, C5H10,
C8H14), with B(C6F5)3 in the poorly coordinating solvent toluene and in the coordinating
solvent diethyl ether give different products. In toluene, the salt [Cp2Nb(µ-H)(η5-η1-C5H4)-
Nb(η5-η1-C5H4)2Nb{(µ-H)(η5-C5H4B(C6F5)2)}]+[HB(C6F5)3]-, 1, which contains a triniobocene
cation, is formed. On the other hand in diethyl ether, the reaction of Cp2Nb{(µ-H)2BR2}
with B(C6F5)3 produces the covalent complex CpNb(C6F5){(µ-H)(η5-C5H4B(C6F5)2)}, 2. In the
formation of these compounds NbIII is oxidized to NbIV. Upon the basis of ESR spectra, 1
and 2 are paramagnetic, each with one unpaired electron. The structures of 1 and 2 were
determined by single-crystal X-ray analysis. Complex 2 forms weak adducts in the solvents
THF and pyridine. The Nb-H-B bridge is cleaved at the Nb-H site to form CpNb(C6F5)-
(L){η5-C5H4B(H)(C6F5)2} (L ) THF, pyridine). In pyridine, a second adduct in much smaller
yield is also formed, CpNb(C6F5)(H){η5-C5H4B(Py)(C6F5)2, with the Nb-H-B bridge being
cleaved at the H-B site.
Sch em e 1
In tr od u ction
The compound B(C6F5)3 and variations of this mol-
ecule have been the reagents of choice in the removal
of carbanions from group IV metallocene derivatives.1,2
One of the main goals has been the preparation of
cations that might have useful properties in catalytic
olefin polymerization.1,3-5 However, there has been
significantly less effort with respect to the investigation
of the basic chemistry of hydride ion abstraction from
metallocene derivatives. Previous studies of hydride
abstraction that have been reported involve removal of
a terminal hydride from the metal.3,6,7 We found that
B(C6F5)3 abstracts a hydride ion from the zirconocene
cyclic organohydroborate derivative Cp2ZrH{(µ-H)2-
BC4H8}.6 In the poorly coordinating solvent benzene, the
[HB(C6F5)3]- salt of the unsupported hydrogen-bridged
cation [(µ-H){Cp2Zr(µ-H)2BC4H8}2]+ (Scheme 1, a ) is
produced. On the other hand, in the coordinating solvent
(1) Chen, E. Y.-X.; Marks, T. J . Chem. Rev. 2000, 100, 1391, and
references therein.
(2) Chen, E. Y.-X.; Abboud, K. A. Organometallics 2000, 19, 5541.
(3) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994, 116,
10015.
(4) Guram, A. S.; J ordan, R. F. In Comprehensive Organometallic
Chemistry, II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Elsevier
Science, Ltd.: New York, 1995; Vol. 4, Chapter 12.
(5) Recent reviews of cationic metallocenes and olefin polymeriza-
tion: (a) A thematic issue on cationic metallecenes and homogeneous
olefin polymerization: Chem. Rev. 2000, 100. (b) Abbenhuis, H. C. L.
Angew. Chem., Int. Ed. 1999, 38, 1058. (c) Mcknibht, A. L.; Waymouth,
R. M. Chem. Rev. 1998, 98, 2587. (d) Kaminsky, W. J . Chem. Soc.,
Dalton Trans. 1998, 1413. (e) Bochmann, M. J . Chem. Soc., Dalton
Trans. 1996, 255.
(6) Liu, F.-C.; Liu, J .; Meyers, E. A.; Shore, S. G. J . Am. Chem. Soc.
2000, 122, 6106.
(7) Antinolo, A.; Carillo-Hermosilla, F.; Fernandez-Baeza, J .;
Garcia-Yuste, S.; Otero, A.; Sanchez-Prada, J .; Villasenor, E. J .
Organomet. Chem. 2000, 609, 123.
Et2O, the [HB(C6F5)3]- salt of the [Cp2Zr(OEt)(OEt2)]+
cation was produced (Scheme 1, b). Both of these
reactions appear to involve initial removal of a terminal
hydrogen from the zirconium atom.
More recently we examined hydride ion abstractions8
from the titanocene organohydroborates Cp2Ti{(µ-
H)2BR2} (R2 ) C4H8, C5H10, C8H14)9 (Chart 1), which
10.1021/om0107744 CCC: $20.00 © 2001 American Chemical Society
Publication on Web 11/21/2001

5718 Organometallics, Vol. 20, No. 26, 2001
Liu et al.
Ch a r t 1
F igu r e 1. Molecular structure of the cation in [Cp2Nb-
(µ-H )(η5-η1-C5H 4)Nb(η5-η1-C5H 4)2Nb{(µ-H )(η5-C5H 4B-
(C6F5)2)}]+[HB(C6F5)3]-, 1.
do not possess a terminal hydride-metal bond. These
reactions with B(C6F5)3 also give products that are
dependent upon the type of solvent employed (Scheme
1, c and d ). In a poorly coordinating solvent, the co-
valent metathesis product Cp2Ti{(µ-H)2B(C6F5)2} was
generated while the [HB(C6F5)3]- salt of the titanocene
cation, [Cp2Ti(OEt2)2]+, was isolated from the coordinat-
ing solvent diethyl ether. There is no change in the
oxidation state of titanium in these reactions.
The present study is concerned with the examina-
tion of reactions of niobocene cyclic organohydroborates
Cp2Nb{(µ-H)2BR2} (R2 ) C4H8, C5H10, C8H14) with
B(C6F5)3 in poorly coordinating and in coordinating sol-
vents. These niobocene complexes are comparable to the
titanocene systems9 in structure and in oxidation state
(III) of the metal. As reported here, although solvent
dependence was observed, remarkably different results
were obtained for the group V niobocene complexes than
for those produced by comparable reactions of group IV
titanocene derivatives.8
This compound contains the first example of a tri-
niobocene cation. The salt is soluble in CH2Cl2, THF,
and pyridine. It is slightly soluble in diethyl ether and
in toluene. There is no apparent reaction with any of
these solvents. Complex 1 is stable in the absence of
air. Interestingly, in reaction 1 hydrogen is evolved and
the niobiums are oxidized from the III oxidation state
to the IV oxidation state. Pentafluorobenzene, HC6F5,
is also formed (identified by GC-MS(EI): calcd for
HC6F5, m/z ) 168.1; obsd, m/z ) 168.1). One other
product of this reaction is also formed which is con-
firmed by 11B NMR. It depends on the cyclic organohy-
droborate ligand employed. The transannular hydrogen-
bridged 10-membered cyclic organodiborane B2(µ-H)2(µ-
C4H8)210 is formed when R2 ) C4H8 (reaction 1), and the
10
organodiboranes (µ-H)2(BC5H10)210 and (µ-H)2(BC8H14)2
are formed when R2 ) C5H10 and C8H14, respectively.
Yields of 1 fall in the range 45-50%.
The structure of 1 was determined by single-crystal
X-ray analysis. The structure of the cation [Cp2Nb-
(µ-H)(η5-η1-C5H4)Nb(η5-η1-C5H4)2Nb{(µ-H)(η5-C5H4B-
(C6F5)2)}]+ is shown in Figure 1. Crystal data are listed
in Table 1, and selected bond lengths and bond angles
are listed in Table 2.
Resu lts
F or m a tion of [Cp 2Nb(µ-H)(η5-η1-C5H4)Nb(η5-η1-
C5H4)2Nb{(µ-H)(η5-C5H4B(C6F 5)2)}]+[HB(C6F 5)3]-, 1,
in th e P oor ly Coor d in a tin g Solven t Tolu en e. The
niobocene cyclic organohydroborates Cp2Nb{(µ-H)2BR2}
(R2 ) C4H8, C5H10, C8H14) (Chart 1) react with B(C6F5)3
in the poorly coordinating solvent toluene to produce
the brown-colored salt [Cp2Nb(µ-H)(η5-η1-C5H4)Nb(η5-
η1-C5H4)2Nb{(µ-H)(η5-C5H4B(C6F5)2)}]+[HB(C6F5)3]-, 1,
as shown in reaction 1.
There are two types of hydrogen bridge bonds,
Nb1-Ha-Nb2 and Nb3-Hb-B1 in the cation. The
Nb-H distances, Nb1-Ha (1.86(9) Å), Nb2-Ha (2.06-
(9) Å), and Nb3-Hb (1.94(6) Å), in the cation are not
much different from those in the starting materials,
Cp2Nb{(µ-H)2BR2} (R2 ) C4H8, 1.82(3) and 1.76(3) Å; R
) C5H10, 1.85(3) Å, R ) C8H14, 1.80-1.89 Å).9,11 The
B-H distance (B1-Hb, 1.32(7) Å) is also close to
those in the starting materials. The cation contains a
covalent bond between Nb2 and Nb3, through spin
pairing of the valence electron on each of these NbIV
atoms. This Nb2-Nb3 distance, 3.1208(8) Å, is compa-
rable to the distance, 3.105(6) Å, between the NbIV
atoms in the covalent bond of the diamagnetic molecule
[(η5-C5H5)(η5-η1-C5H4)NbH]2 (Figure 2a) reported by
Guggenberger and Tebbe.12 With the exception of the
(9) Liu, F.-C.; Plecˇnik, C. E.; Liu, S.; Liu, J .; Meyers, E. A.; Shore,
S. G. J . Organomet. Chem. 2001, 627, 109.
(10) (a) Young, D. E.; Shore, S. G. J . Am. Chem. Soc. 1969, 91, 3497.
(b) Ko¨ster, R. Angew. Chem. 1960, 72, 626. (c) Brown, H. C.; Negishil,
E.-I. J . Organomet. Chem. 1971, 26, C67. (d) Brown, H. C.; Pai, G. G.
Heterocycles 1982, 17, 77.
(11) Hartwig, J . F.; De Gala, S. R. J . Am. Chem. Soc. 1994, 116,
3661.
(8) Plecˇnik, C. E.; Liu, F.-C.; Liu, S.; Liu, J .; Meyers, E. A.; Shore,
S. G. Organometallics 2001, 20, 3599.

Triniobocene Cationic Species
Organometallics, Vol. 20, No. 26, 2001 5719
Ta ble 1. Cr ysta llogp h ic Da ta for
distance between Nb1 and Nb2 in the Nb1-H-Nb2
bridge, 3.370 Å, accommodates the smaller tilt angle.
The cation of complex 1 is paramagnetic. It contains
one unpaired electron, believed to be located on Nb1.
The ESR spectrum consists of a well-resolved decet
signal (93Nb, I ) 9/2) in CH2Cl2. The 11B NMR spectrum
of complex 1 in CD2Cl2 consists of a doublet due to the
Cp 2Nb(µ-H)(η5-η1-C5H4)Nb(η5-η1-C5H4)2Nb-
{(µ-H)(η5-C5H4B(C6F 5)2)}][HB(C6F 5)3] (1) a n d
Cp Nb(C6F 5){(µ-H)(η5-C5H4B(C6F 5)2} (2)
1
2
empirical formula
fw
C
74 H45 B2F25Nb3
C28H10BF15Nb
735.08
-70
1709.45
-100
T, °C
-
[HB(C6F5)3] anion at -25.1 ppm J (11B-1H) ) 96 Hz
cryst size (mm)
radiation (λ, Å)
cryst syst
space group
a, Å
0.19 × 0.17 × 0.15 0.38 × 0.35 × 0.35
and a broad signal at -15.4 ppm that is assigned to the
boron atom of the cation.
MoKR (0.71073)
monoclinic
C2/c
MoKR (0.71073)
orthorhombic
Pbca
15.536(1)
16.083(1)
Boron-11 NMR spectra of reaction mixtures of
Cp2Nb{(µ-H)2BC4H8} with B(C6F5)3 in toluene were
studied over the temperature range -70 to 27 °C. No
reaction was indicated until the temperature reached
-10 °C. Intermediate species formed could not be
detected over the temperature range studied. Only the
starting materials and the final products could be
detected in the 11B NMR spectra.
19.536(1)
13.268(1)
51.378(1)
98.86(1)
13158(1)
8
1.726
0.630
4.60 to 50.12
40 509
11 582
b, Å
c, Å
19.639(1)
â, deg
V, Å3
4907.2(5)
8
1.990
0.626
4.90 to 50.04
20 429
Z
density (calcd, g/cm3)
µ, mm-1
2θ range (deg)
no. of reflns collected
no. of ind reflns
Rint
For m ation of CpNb(C6F5){(µ-H)(η5-C5H4B(C6F5)2)},
2, in th e Coor d in a tin g Solven t Dieth yl Eth er . In
diethyl ether, the reaction between Cp2Nb{(µ-H)2BR2}
(R2 ) C4H8, C5H10, C8H14) and B(C6F5)3 produces the
molecular complex CpNb(C6F5){(µ-H)(η5-C5H4B(C6F5)2)},
2 (reaction 2). NbIII is oxidized to NbIV with the evolution
4339
0.0326
0.0919
max/min. transmn
no. of data/restraints/
params
0.9114, 0.8896
11582/0/962
0.8108, 0.7970
4339/0/410
goodness-of-fit on F2
final R indices [I )
2σ(I)]a,b
1.048
R1 ) 0.0582,
wR2 ) 0.1205
1.032
R1 ) 0.0271,
wR2 ) 0.0641
R1 ) 0.0379,
wR2 ) 0.0683
final R indices (all data) R1 ) 0.1114,
wR2 ) 0.1402
largest diff peak and
0.633 and -0.720 0.319 and -0.329
hole (e Å-3
)
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) {∑wFo - Fc2)2/∑w(Fo2)2}1/2
.
a
b
2
metal-metal bond between them, Nb2 and Nb3 have
coordination environments similar to Nb1: two η5-
coordinated Cp groups, a bridge hydrogen, and one η1-
C5H4 unit in which the Nb-C σ bond distances (2.218-
(6)-2.294(6) Å) are in general slightly shorter than the
Nb-C σ bond distances in [(η5-C5H5)(η5-η1-C5H4)NbH]2
(2.297(3)-2.439(8) Å).12 The distance of the σ bond B1-
C30 of the BC5H4 unit, 1.579(2) Å, is slightly shorter
than that reported for B-C(aryl) bond distances (ca.
1.60 Å).13 However, the B-C distances in the BC6F5
units of the cation are slightly longer (1.625(10)-1.629-
(11) Å) and in the anion they are significantly longer
(1.635(9)-1.653(9) Å) than reported B-C(aryl) dis-
tances.14 These longer distances are believed to reflect
the electron-withdrawing effect of the pentafluorphenyl
ring on the electron pair in the covalent B-C bond. The
distances Nb-Centroid (η5-η1-Cp) (2.033-2.054 Å) are
shorter than Nb-Centroid (η5-Cp) distances (2.079,
2.084 Å), which is consistent with Guggenberger and
Tebbe’s observations of 2.034 Å for the Nb-Centroid (η5-
η1-Cp) and 2.075 and 2.082 Å for the Nb-Centroid (η5-
Cp) distances. The σ bonds Nb2-C31, Nb3-C29 asso-
ciated with η1-C5H4 and B1-C30 associated with BC5H4
constrain the C5H4 rings closer to the metal centers.
Each of these bonds is tilted with respect to the plane
of the C5 ring with a tilt angle of ∼30°, with the
exception of the angle, ∼24°, between the Nb1-C21
bond and the plane of the associated C5 ring. The long
of H2 in this reaction. The ESR spectrum in CD2Cl2
reveals a well-resolved decet. The other product depends
on the cyclic organohydroborate ligand employed. The
transannular hydrogen-bridged 10-membered cyclic or-
10
ganodiborane B2(µ-H)2(µ-C4H8)2 is formed when R2 )
C4H8 (reaction 2), and the organodiboranes (µ-H)2-
10
10
(BC5H10 2
)
and (µ-H)2(BC8H14 2
)
are formed when R2
) C5H10 and C8H14, respectively. Yields are in the range
70-75%. This compound is soluble in CH2Cl2. It is
slightly soluble in diethyl ether, and it forms weak
adducts in THF and pyridine. Solid 2 is green-brown in
color. It is stable at room temperature in the absence
of air.
The molecular structure of complex 2, shown in
Figure 3, was determined by single-crystal X-ray analy-
sis. Crystal data are given in Table 1, and selected bond
angles and distances are given in Table 3. A C5H4 ring
is η5-coordinated to Nb1 and η1-coordinated to a B(C6F5)2
group, while niobium and boron are linked through a
Nb1-H-B bridge. A Cp ring and a C6F5 ring are also
bound to Nb1 through η5 and η1 coordination, respec-
tively. The bond Nb1-C111, with a distance of 2.282-
(2) Å, is significantly longer than all of the σ Nb-C bond
distances of the Nb(η1-C5H4) units in 1 (2.218(6), 2.223-
(6), and 2.229(6) Å). The σ B1-C10 bond distance, 1.592-
(4) Å, in the B-C5H4 unit is comparable to that of the
B1-C30 distance in 1. The σ B-C bond distances of the
BC6F5 units, 1.621(4), 1.611(4) Å, are comparable to
(12) (a) Guggenberger, L. J .; Tebbe, F. N. J . Am. Chem. Soc. 1971,
93, 5924. (b) Guggenberger, L. J . Inorg. Chem. 1973, 12, 294.
(13) Allen, F. H.; Kennared, Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987, S1.
(14) Otwinowski, Z.; Minor, W. In Methods in Enzymology; Carter,
C. W., J r., Sweet, R. M., Eds.; Academic Press: New York, 1997; Vol.
276(A), p 307.

5720 Organometallics, Vol. 20, No. 26, 2001
Liu et al.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) for
[Cp 2Nb(µ-H)(η5-η1-C5H4)Nb(η5-η1-C5H4)2Nb{(µ-H)(η5-C5H4B(C6F 5)2)}][HB(C6F 5)3] (1)
Bond Lengths
Nb(1)-C(21)
2.218(6)
2.399
2.388
2.084
2.079
1.86(9)
2.223(6)
2.382
2.370
2.054
Nb(3)-Centroid(C36-C30)
Nb(3)-C(20)
Nb(3)- - -B(1)
Nb(3)-H(B)
2.047
av Nb(1)-C(11-15)
av Nb(1)-C(16-10)
Nb(1)-Centroid(C11-C15)
Nb(1)-Centroid(C16-C10)
Nb(1)-HA
2.229(6)
2.712(8)
1.94(6)
1.579(11)
1.625(10)
1.629(11)
1.32(7)
1.374(10)
1.398(10)
1.341(8)
1.653(9)
1.635(9)
1.638(9)
1.07(5)
C(30)-B(1)
B(1)-C(111)
B(1)-C(121)
B(1)-H(B)
C(111)-C(116)
C(111)-C(112)
C(112)-F(112)a
B(2)-C(211)a
B(2)-C(221)a
B(2)-C(231)a
B(2)-H(2)a
Nb(2)-C(31)
ave Nb(2)-C(21-25)
ave Nb(2)-C(26-20)
Nb(2)-Centroid(C21-C25)
Nb(2)-Centroid(C26-C20)
Nb(2)-Nb(3)
2.040
3.1208(8)
2.06(9)
2.365
2.375
2.033
Nb(2)-HA
ave Nb(3)-C(31-35)
ave Nb(3)-C(36-30)
Nb(3)-Centroid(C31-C35)
Bond Angles
134.3
Centroid(C11-C15)-Nb(1)-Centroid(C16-C10)
Centroid(C11-C15)-Nb(1)- C(21)
Centroid(C16-C10)-Nb(1)- C(21)
Centroid(C11-C15)-Nb(1)-H(A)
Centroid(C16-C10)-Nb(1)-H(A)
C(21)-Nb(1)-H(A)
Centroid(C31-C35)-Nb(3)-Nb(2)
Centroid(C36-C30)-Nb(3)-Nb(2)
Centroid(C31-C35)-Nb(3)-H(B)
Centroid(C36-C30)-Nb(3)-H(B)
C(20)-Nb(3)-H(B)
78.6
120.1
104.6
94.3
108.7
110.8
106.9
104.2
77(3)
136.3
110.6
108.9
101.8
100.7
117.1
78.4
83(2)
47.0(2)
137.6
109.3
110.4
83(2)
C(39)-C(30)-B(1)
124.7(7)
118.8(6)
118.1(6)
113.1(7)
113.1(6)
99(3)
102(3)
109(3)
113.9(7)
124.2(7)
121.8(6)
118.0(8)
119.3(6)
Centroid(C21-C25)-Nb(2)-Centroid(C26-C20)
Centroid(C21-C25)-Nb(2)-C(31)
Centroid(C26-C20)-Nb(2)-C(31)
Centroid(C21-C25)-Nb(2)-H(A)
Centroid(C26-C20)-Nb(2)-H(A)
Centroid(C21-C25)-Nb(2)-Nb(3)
Centroid(C26-C20)-Nb(2)-Nb(3)
C(31)-Nb(2)-H(A)
C(36)-C(30)-B(1)
C(30)-B(1)-C(111)
C(30)-B(1)-C(121)
C(111)-B(1)-C(121)
C(30)-B(1)-H(B)
C(111)-B(1)-H(B)
C(121)-B(1)-H(B)
C(116)-C(111)-C(112)
C(116)-C(111)-B(1)
C(112)-C(111)-B(1)
F(112)-C(112)-C(113)
F(112)-C(112)-C(111)
C(31)-Nb(2)-Nb(3)
Centroid(C31-C35)-Nb(3)-Centroid(C36-C30)
Centroid(C31-C35)-Nb(3)-C(20)
Centroid(C36-C30)-Nb(3)-C(20)
a
Bond lengths in the[HB(C6F5)3]- anion.
F igu r e 3. Molecular structure of CpNb(C6F5){(µ-H)(η5-
C5H4B(C6F5)2)}, 2.
The 11B NMR spectrum of complex 2 in CD2Cl2 con-
sists of a sharp resonance at -15.4 ppm, which is simi-
lar to the 11B signal of the cation in 1. The bridge hy-
F igu r e 2. Comparison of structural features of complex
1 (b) with those of [(η5-C5H5)(η5-η1-C5H4)NbH]2 (a) and
those of complex 2 (c).
1
drogen could not be detected in the H NMR spectrum.
In THF, complex 2 forms a weak adduct through
addition of the THF to niobium with cleavage of the
Nb-H-B hydrogen bridge bond at the Nb-H site. The
resulting bridge cleavage product contains a terminal
B-H bond, as indicated by a doublet in the 11B NMR
spectrum (δ ) -24.9 ppm; J (d,11B-1H) ) 89 Hz). This
adduct, (CpNb(C6F5)(THF){η5-C5H4B(H)(C6F5)2}, is zwit-
terionic. It is represented in reaction 3 as an A type
those in 1. Because of the constraints imposed by
Nb1-H1-B1 and B1-C10 of the C5H4 unit, the
NB1-Centroid(C16-C10) distance, 2.038 Å, is shorter
than the Nb1-Centroid(C11-C15) distance, 2.072 Å,
which results in the B1-C10 bond being bent ∼30° out
of the C5H4 plane.

Triniobocene Cationic Species
Organometallics, Vol. 20, No. 26, 2001 5721
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles (d eg) for Cp Nb(C6F 5){(µ-H)(η5-C5H4B(C6F 5)2)} (2)
Bond Lengths
Nb(1)-C(111)
2.282(2)
2.390
2.366
2.072
2.038
2.742(3)
1.94(2)
1.592(4)
B(1)-H(1)
1.32(2)
av Nb(1)-C(11-15)
av Nb(1)-C(16-10)
Nb(1)-Centroid(C11-C15)
Nb(2)-Centroid(C16-C10)
Nb(1)- - -B(1)
C(121)-C(126)
C(121)-C(122)
C(122)-F(122)
C(111)-C(112)
C(112)-F(112)
B(1)-C(121)
B(1)-C(131)
1.378(3)
1.384(3)
1.354(3)
1.384(3)
1.360(3)
1.621(4)
1.611(4)
Nb(1)-H(1)
C(10)-B(1)
Bond Angles
40.3
106.3
107.8
112.3
92.0
123.1(2)
105(1)
82.7(7)
97(1)
113.1(2)
122.0(2)
119.8(2)
116.0(2)
124.2(2)
107.8(2)
Centroid(C11-C15)-Nb(1)-Centroid(C16-C10)
Centroid(C11-C15)-Nb(1)-C(111)
Centroid(C16-C10)-Nb(1)-C(111)
Centroid(C11-C15)-Nb(1)-H(1)
Centroid(C16-C10)-Nb(1)-H(1)
C(10)-B(1)-C(131)
C(131)-B(1)-H(1)
104(1)
C(10)-B(1)-C(121)
C(131)-B(1)-C(121)
C(132)-C(131)-B(1)
C(136)-C(131)-B(1)
F(132)-C(132)-C(133)
F(132)-C(132)-C(131)
C(133)-C(132)-C(131)
C(112)-C(111)-Nb(1)
C(16)-C(17)-Nb(1)
C(18)-C(17)-Nb(1)
C(16)-C(17)-H(17)
C(16)-C(10)-C(19)
C(16)-C(10)-B(1)
112.9(2)
112.0(2)
127.1(2)
119.0(2)
116.6(2)
120.4(2)
123.0(2)
124.6(2)
70.5(1)
72.7(2)
126.1
105.1(2)
127.3(2)
117.3(2)
C(121)-B(1)-H(1)
C(111)-Nb(1)-H(1)
C(10)-B(1)-H(1)
C(116)-C(111)-C(112)
C(116)-C(111)-Nb(1)
F(112)-C(112)-C(111)
F(112)-C(112)-C(113)
C(111)-C(112)-C(113)
C(16)-C(17)-C(18)
C(19)-C(10)-B(1)
cleavage product. Complex 2 also forms weak adducts
The 11B NMR spectra of the reaction mixture of
Cp2Nb{(µ-H)2BC4H8} with B(C6F5)3 in diethyl ether
were studied over the temperature range -70 to 27 °C.
The reaction proceeds slowly at -70 °C. Starting ma-
terials Cp2Nb{(µ-H)2BC4H8} (δ ) 61.4 ppm), Et2OB-
(C6F5)3 (δ ) 3.8 ppm), and an intermediate species with
a very sharp signal at -15.2 ppm are observed. A
weaker signal, a poorly resolved doublet, is seen at
-24.9 ppm. This signal is consistent with the formation
of a diethyl ether adduct, CpNb(C6F5)(Et2O){η5-C5H4B-
(H)(C6F5)2, like the THF and pyridine adducts of A type
represented by reaction 3. The very sharp signal at
-15.2 ppm (19 Hz at half-height) is close to that of 2
(-15.4 ppm) but is believed to be due to an intermediate
species since 2 is insoluble in diethyl ether and its signal
is broader (43 Hz at half-height in CD2Cl2). The signal
for the intermediate product shifts to lower field with
higher temperature, -14.4 ppm. Above 20 °C, the
reaction proceeds sufficiently rapidly that the interme-
diate is no longer detectable, but the signal assigned to
the diethyl ether adduct of 2 is the predominant one.
Upon standing at room temperature, complex 2 slowly
precipitates from the solution. Cooling the solution to
-90 °C in 10 °C increments did not cause the reappear-
ance of the signal assigned to the intermediate species.
Su m m a r y a n d Discu ssion
with pyridine. In pyridine, the predominant adduct of
2 is CpNb(C6F5)(Py){η5-C5H4B(H)(C6F5)2}, also of the A
type, producing a 11B NMR spectrum of a well-resolved
doublet (δ -24.9 J (11B-1H) ) 90 Hz). To a lesser extent,
an additional product is observed in the 11B NMR
spectrum. This product produces a broad 11B NMR
singlet (δ ) -4.4 ppm) that is assigned to a B-N bond.
This suggests that this complex is formed through
cleavage of the Nb-H-B bond at the H-B site to yield
the complex CpNb(C6F5)(H){η5-C5H4B(Py)(C6F5)2}, rep-
resented in reaction 3 as compound B. The THF and
pyridine adducts of complex 2 could not be isolated.
They dissociate upon pumping to dryness the THF and
pyridine solutions to yield solid 2.
Results from the present work (summarized in reac-
tions 1 and 2) are in sharp contrast with those from the
reactions of the related complexes Cp2Ti{(µ-H)2BR2} (R2
) C4H8, C5H10, C8H14)8 with B(C6F5)3 (summarized in
Scheme 1, c and d ). The only differences between these
complexes are the atomic number of Ti and Nb and the
ease of oxidation from the III to the IV state. It is of
interest to summarize the differences in results. In the
poorly coordinating solvent toluene, Cp2Ti{(µ-H)2B-
(C6F5)2} is produced, but in the coordinating solvent
diethyl ether, [Cp2Ti(OEt2)2]?[HB(C6F5)3]- is produced.
On the other hand Cp2Nb{(µ-H)2BR2} (R2 ) C4H8,
C5H10, C8H14) produces [Cp2Nb(µ-H)(η5-η1-C5H4)Nb-
(η5-η1-C5H4)2Nb{(µ-H)(η5-C5H4B(C6F5)2)}]+[HB-

5722 Organometallics, Vol. 20, No. 26, 2001
Liu et al.
by MaXus software.16 The structures were solved by direct
methods and refined using the SHELXTL-97 (difference
electron density calculation, full matrix least-squares refine-
ments) structure solution package.17 Data merging was per-
formed using the data preparation program supplied by
SHELXTL-97. After all non-hydrogen atoms were located and
refined anisotropically, H atoms of Cp rings were calculated
assuming standard -CH geometries. All other hydrogen atoms
were located and refined isotropically.
(C6F5)3]-, 1, in toluene and CpNb(C6F5){(µ-H)(η5-C5H4B-
(C6F5)2)}, 2, in diethyl ether. Furthermore, while the
oxidation state of TiIII is unchanged in its reactions,
niobium is oxidized from NbIII to NbIV.
Some structural features of the triniobocene cation
of complex 1 and the covalent complex 2 are in common
with the covalent, diamagnetic, NbIV complex [(η5-C5H5)-
(η5-η1-C5H4)NbH]2 (Figure 2a) and with each other
12
P r ep a r a tion of [Cp 2Nb(µ-H)(η5-η1-C5H4)Nb(η5-η1-C5H4)2-
Nb{(µ-H)(η5-C5H4B(C6F 5)2)}]+[HB(C6F 5)3]-, 1. Complex 1
was prepared from reactions of Cp2Nb{(µ-H)2BC4H8}, Cp2Nb-
{(µ-H)2BC5H10}, and Cp2Nb{(µ-H)2BC8H14} with B(C6F5)3 in
toluene under similar conditions and produced products in
comparable yields. A typical synthetic procedure is described.
In a drybox a 129 mg (0.42 mmol) quantity of Cp2Nb{(µ-
H)2BC4H8} and 144 mg (0.28 mmol) of B(C6F5)3 were put in a
flask. After degassing, about 10 mL of toluene was transferred
to the flask at -78 °C under vacuum. The system was warmed
to room temperature. After stirring for 24 h, the brown-green
solution was filtered and concentrated. During the reaction
H2 evolved and, in addition to complex 1, the products C6F5H
(identified by GC-MS(EI): calcd for C6HF5, m/z ) 168.1; obsd,
m/z ) 168.1) and B2(µ-H)2(µ-C4H8)2 (identified by its 11B NMR
spectrum10) were generated. Crystallization of the solution at
-18 °C produced brown crystals (116 mg, 48%), which were
suitable for X-ray analysis. 11B NMR (250 MHz, CD2Cl2, 25
°C): δ -25.1 (d, J (11B-1H) ) 96 Hz, [HB(C6F5)3]-), -15.4 (s,
(µ-H)(η5-C5H4B(C6F5)2). IR (KBr): 3124w, 2953m, 2924s, 2853s,
2360w, 1749w, 1646s, 1516s, 1477vs, 1443s, 1412s, 1384s,
1341m, 1290s, 1195w, 1100s, 1045m, 1011m, 974s, 893m, 826s,
794m, 762m, 729m, 680m, 632w, 606w, 576w, 567w, 542w,
505w. Anal. Calcd for C74H45B2F25Nb3: C, 47.25; H, 1.92.
Found: C, 46.99; H, 1.94.
P r ep a r a tion of Cp Nb(C6F 5){(µ-H)(η5-C5H4B(C6F 5)2)}, 2.
Complex 2 was prepared from reactions of Cp2Nb{(µ-H)2-
BC4H8}, Cp2Nb{(µ-H)2BC5H10}, and Cp2Nb{(µ-H)2BC8H14} with
B(C6F5)3 in diethyl ether under similar conditions and pro-
duced products in comparable yields. A typical synthetic proce-
dure is described. In a drybox a 207 mg (0.71 mol) quantity of
Cp2Nb{(µ-H)2BC4H8} and 363 mg (0.71 mmol) of B(C6F5)3 were
put in a flask. The flask was evacuated, and about 5 mL of
diethyl ether was condensed into the flask at -78 °C. The
system was warmed to room temperature. After stirring for
48 h, the brown-green solution was filtered and concentrated.
During the reaction H2 evolved, and in addition to complex 2,
the product B2(µ-H)2(µ-C4H8)2 (identified by its 11B NMR
spectrum10) formed. Brown crystals for X-ray and chemical
analysis were obtained at room temperature and washed with
hexane (381 mg, 73%). To minimize contamination by unre-
acted B(C6F5)3, toluene is a better agent for washing 2 free of
contaminants. 11B NMR (250 MHz, CD2Cl2, 25 °C): δ -15.4
(s, 43 Hz at half-height). IR (KBr): 3125s, 2977m, 2928m
2872m, 2603w, 2362m, 2090w, 1847w, 1646s, 1600m, 1520vs,
1455vs, 1382s, 1357s, 1290s, 1270s, 1255m, 1214m, 1187m,
1100s, 1060s, 1011s, 972s, 846s, 827s, 793s, 767s, 745m, 730m,
695m, 617w, 576w, 542m. Anal. Calcd for C28H10BF15Nb: C,
45.75; H, 1.37. Found: C, 46.37; H, 1.16.
(Figure 2, b and c). In general, distances and angles
involving corresponding atoms are in accord with those
reported for [(η5-C5H5)(η5-η1-C5H4)NbH]2.12
The reactions by which compounds 1 and 2 are formed
do not lend themselves to the isolation and or identifica-
tion of intermediate species. In the case of compound
1, the presence of intermediate species could not be
detected. Although 11B NMR spectroscopy did indicate
the presence of a low-temperature intermediate in the
formation of compound 2, isolation and identification
of the intermediate was not achieved. However, it is
possible to comment on the formation of the covalent
complex 2 in that it appears that its immediate precur-
sor is the A type zwitterionic complex CpNb(C6F5)-
(Et2O){η5-C5H4B(H)(C6F5)2}. At room temperature com-
plex 2 slowly precipitates from solution. Presumably
diethyl ether is slowly eliminated from the type A
complex, resulting in the formation and precipitation
of 2.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations were carried out
on a standard high-vacuum line or in a drybox under an
atmosphere of nitrogen. Diethyl ether, THF, toluene, and
pyridine were dried over sodium benzophenone and freshly
distilled prior to use. Methylene chloride was dried over CaH2
and distilled before use. Reactions were carried out in the dry
solvents under nitrogen. B(C6F5)3 (Aldrich) was used as
received. The complexes Cp2Nb{(µ-H)2BR2} (R2 ) C4H8, C5H10
,
C8H14) were prepared by procedures described in the litera-
ture.9 Elemental analyses were performed by Galbraith Labo-
ratories, Knoxville, TN. Proton NMR spectra (δ(TMS) 0.00
ppm) were recorded on a Bruker AM-250 NMR spectrometer
operating at 250.11 MHz and on a Bruker AM-400 NMR
spectrometer operating at 400.13 MHz. 11B spectra (externally
referenced to BF3‚OEt2 (δ0 ppm)) were collected at 128.38 or
80.25 MHz as noted. Infrared spectra were recorded on a
Mattson Polaris Fourier transform spectrometer with 2 cm-1
resolution. ESR spectra for complexes 1 and 2 were observed
at 25 °C in CH2Cl2 on a Bruker ESP300 ESR spectrometer at
9.75 GHz and 1.0 mW power. Both complexes produce similar
well-resolved 10-line spectra, as expected for d1, 93Nb (I ) 9/2)
with the centers of the fields at 3515 G, a ) 115 G and g )
1.98.
X-r a y Str u ctu r e Deter m in a tion . Single-crystal X-ray
diffraction data for 1 and 2 were collected on a Nonius
KappaCCD diffraction system which employs graphite-mono-
chromated Mo KR radiation (λ ) 0.71073 Å). A single crystal
was mounted on the tip of a glass fiber coated with Fomblin
oil (a perfluoropolyether). Crystallographic data were collected
at -100 °C for 1 and -70 °C for 2. Unit cell parameters were
obtained by indexing the peaks in the first 10 frames and
refined employing the whole data set. All frames were inte-
grated and corrected for Lorentz and polarization effects using
the Denzo-SMN package (Nonius BV, 1999).14 Absorption
correction was applied using the SORTAV program15 provided
Br id ge Hyd r id e Clea va ge Rea ctivity of Com p ou n d 2.
Reaction of complex 2 with THF: A 34 mg sample of complex
2 was placed in 0.4 mL of d8-THF in a quartz NMR tube that
was then sealed. 11B NMR (128.38 MHz, d8-THF): δ -25.6,
(d, J (11B-1H) ) 90 Hz (d, η5-C5H4B(H)(C6F5)2-). Reaction of
complex 2 with pyridine: A 32 mg sample of complex 2 was
(16) 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: Scotland, Nonius BV: Delft, The Netherlands, and Mac-
Science Co. Ltd.: Yokohama, J apan, 1998.
(15) (a) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33. (b)
Blessing, R. H. J . Appl. Crystallogr. 1997, 30, 421-426.
(17) Sheldrick, G. M. SHELXTL-97: A Structure Solution and
Refinement Program; University of Go¨ttingen: Germany, 1998.

Triniobocene Cationic Species
Organometallics, Vol. 20, No. 26, 2001 5723
placed in 0.4 mL of d5-pyridine in a quartz NMR tube that
was then sealed. 11B NMR (128.38 MHz, d5-pyridine): δ -4.4
(br, η5-C5H4B(Py)(C6F5)2), -25.1(d, J (11B-1H) ) 90 Hz, η5-
C5H4B(H)(C6F5)2-).
The NMR tubes were opened under vacuum, and solvent
was pumped away to dryness from pyridine and THF solu-
tions. The remaining solids were dissolved in CD2Cl2. The 11B
NMR spectra of the solution were that of complex 2.
sharp, width at half-height ) 19 Hz, unidentified), ca. -24.9
ppm (br, very weak, CpNb(C6F5)(Et2O){η5-C5H4B(H)(C6F5)2}).
With an increase in temperature the signals at -15.2 and
-24.9 ppm move to slightly lower field. At 10 °C these signals
shift to -14.4 and -23.9 ppm, respectively, and a new signal
at 29.0 ppm (br, B2(µ-H)2(µ-C4H8)2) appears. Above 20 °C, the
reaction proceeds sufficiently rapidly that the signal at -14.4
ppm is no longer detectable, but the signal at -23.9 ppm is
the predominant signal. Upon standing at room temperature,
complex 2 slowly precipitates from the solution. The unit cell
and space group of this crystalline precipitate are in excellent
agreement with those parameters of 2. Cooling the solution
to -90 °C produces no evidence for the emergence of the signal
attributed to the intermediate product which appeared at low
temperature when the reaction was initiated.
Low -Tem p er a tu r e 11B NMR Stu d y of Rea ction Solu -
tion s. Samples of Cp2Nb{(µ-H)2BC4H8} (20 mg, 0.067 mmol)
and 23 mg(0.045 mmol) of B(C6F5)3 (in 3:2 ratio) were placed
in a quartz NMR tube with a graded seal. The tube was
evacuated, and about 0.4 mL of toluene was condensed into
the tube at -78 °C and was then sealed. Boron-11 NMR
spectra were taken at every 10 °C starting from -70 to 27 °C
at 128.38 MHz. From -70 to -20 °C the signals observed were
-3.4 ppm (B(C6F5)3) and 59.2 ppm (Cp2Nb{(µ-H)2BC4H8}). The
reaction proceeds slowly above -10 °C. As the temperature is
raised, new signals appear at 29.0 ppm (br, B2(µ-H)2(µ-C4H8)2)
and -24.8 ppm (d, J (11B-1H) ) 90 Hz, [HB(C6F5)3]-). There
is no indication of the formation of an intermediate species
formed over the temperature range studied.
Ack n ow led gm en t. This work was supported by the
National Science Foundation through Grants CHE 97-
00394 and CHE 99-01115. We thank Dr. J ohnie Brown
of the Campus Chemical Instrument Center for obtain-
ing the GC-MS spectra.
Samples of Cp2Nb{(µ-H)2BC4H8} (23 mg, 0.075 mmol) and
38 mg (0.074 mmol) of B(C6F5)3 (in 1:1 ratio) were placed in a
quartz NMR tube with a graded seal. The tube was evacuated,
and about 0.4 mL of diethyl ether was condensed into the tube
at -78 °C which was then sealed. The Boron-11 NMR spectra
were taken at every 10 °C interval starting from -70 to 27 °C
and from 27 to -90 °C at 128.38 MHz. The reaction proceeds
slowly at -70 °C. Signals appear at 61.4 ppm (br, Cp2Nb(µ-
H)2BC4H8), 3.8 ppm (br, Et2OB(C6F5)3), and -15.2 ppm (very
Su p p or tin g In for m a tion Ava ila ble: Molecular structure
of compound 1; molecular structure of compound 2; tables of
crystallographic data, positional and thermal parameters, and
interatomic distances and angles for 1, 2; ESR spectra of
compound 1 and 2. Two X-ray crystallographic files in CIF
format. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM0107744