Synthesis, characterization and reactivity of tetracyclopentadienylniobium(IV), a precursor to organometallic derivatives of niobium(IV)

Article abstract of DOI:10.1016/S0022-328X(01)01014-2

In toluene as medium, tetra(cyclopentadienyl)niobium(IV), NbCp₄, has been prepared in satisfactory yields from the reaction of NaCp with: (a) Nb₂C1₁₀, (b) NbCl₄(THF)₂, or (c) NbCp₂Cl₂. Tetracyclopentadienylniobium(IV) has been characterized by X-ray diffraction. Crystal data: C₂₀H₂₀Nb, M=353.29 g mol^-1, hexagonal, space group P6₅ (no. 170), a=b=9.396(2), c=31.23(3) ?, V=2388(2) ?³, Z=6, d_calc=1.48 g cm^-3, λ(Cu-K_α)=1.54184 ?, T=291 K, μ=62.04 cm^-1, F(000)=1686. Two of the four cyclopentadienyl ligands are bonded to niobium in a pentahapto fashion, the other two being monohapto. NbCp₄ undergoes cyclopentadiene elimination in the presence of species containing active protons such as Ph₃SiOH or strong acids, the products being tris- or biscyclopentadienyl compounds depending on the molar ratio of the reagents.

Full text of DOI:10.1016/S0022-328X(01)01014-2

Journal of Organometallic Chemistry 630 (2001) 275–280
www.elsevier.com/locate/jorganchem
Synthesis, characterization and reactivity of
tetracyclopentadienylniobium(IV), a precursor to organometallic
derivatives of niobium(IV)
Fausto Calderazzo a, Cecilia Giardi a, Ulli Englert b, Guido Pampaloni a,*
a Uni6ersita` di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, I-56126 Pisa, Italy
b Institut fu¨r Anorganische Chemie der RWTH Aachen, Professor-Pirlet-Strasse 1, D-52074 Aachen, Germany
Received 3 April 2001; accepted 18 May 2001
Abstract
In toluene as medium, tetra(cyclopentadienyl)niobium(IV), NbCp4, has been prepared in satisfactory yields from the reaction
of NaCp with: (a) Nb2C110, (b) NbCl4(THF)2, or (c) NbCp2Cl2. Tetracyclopentadienylniobium(IV) has been characterized by
X-ray diffraction. Crystal data: C20H20Nb, M=353.29 g mol−1, hexagonal, space group P65 (no. 170), a=b=9.396(2),
3
c=31.23(3) A, V=2388(2) A , Z=6, dcalc=1.48 g cm−3, u(Cu–Ka)=1.54184 A, T=291 K, v=62.04 cm−1, F(000)=1686.
Two of the four cyclopentadienyl ligands are bonded to niobium in a pentahapto fashion, the other two being monohapto. NbCp4
undergoes cyclopentadiene elimination in the presence of species containing active protons such as Ph3SiOH or strong acids, the
products being tris- or biscyclopentadienyl compounds depending on the molar ratio of the reagents. © 2001 Elsevier Science B.V.
All rights reserved.
,
,
,
Keywords: Niobium; Cyclopentadienyl; Structures; Complexes
reactivity of this system with respect to substances
containing active protons has been carried out.
1. Introduction
By reaction of the pentachlorides with excess NaCp
in benzene, Fischer and coworkers obtained low yields
of the tetracyclopentadienyl derivatives of niobium(IV)
and tantalum(IV) (9.3 and 10.9% for niobium and
tantalum, respectively) [1]. With reduced reaction times,
the yield of the niobium compound was increased to ca.
60% [2]. On the basis of IR and hydrogenation data, a
structure consisting of two pentahapto and two mono-
hapto cyclopentadienyl ligands was proposed [1]. As for
the reactivity of these systems, little is known [3] except
the reaction with hydrogen chloride yielding NbCp2Cl2.
In the framework of our studies concerning the syn-
thesis and the reactivity of polycyclopentadienyl deriva-
tives of early transition elements, we wish to report an
improved synthetic route to and the crystal and molec-
ular structure of NbCp4. In addition, a study of the
2. Results and discussion
We started our approach to the chemistry of tetracy-
clopentadienylniobium(IV) with a reinvestigation of the
known synthetic procedures, see Scheme 1. Although
preferable from the point of view of the availability of
the starting material, the reaction involving Nb2Cl10
requires some care because when the reaction times are
longer than 2–3 h, the yield decreases to ca. 10–20%.
* Corresponding author. Tel.: +39-050-91-8221219; fax: +39-
050-20-237.
E-mail address: pampa@dcci.unipi.it (G. Pampaloni).
Scheme 1.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01014-2

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F. Calderazzo et al. / Journal of Organometallic Chemistry 630 (2001) 275–280
On the other hand, when using niobium(IV) derivatives
[NbCl4(THF)2 or NbCp2Cl2] reactions are slower but
the required longer reaction times are not detrimental
for the yield. This suggests that the higher Lewis acidity
of niobium(V) intermediates affects their stabilities in
some unknown way.
Similar to what is observed in the case of the tetracy-
clopentadienyls of Group 4, NbCp4 is affected by Lewis
bases; red–violet solutions of NbCp4 in THF or Et2O
turn deep blue–green after some hours. Moreover, a
reaction with deposition of brown solids occurs in the
presence of chlorinated hydrocarbons.
Fig. 1. View of the molecular structure of NbCp4.
Due to the paramagnetism of the compound both in
the solid state [1] and in solution [4], an NMR study
was precluded and we decided to solve the structure of
NbCp4 by X-ray diffraction. Due to its reactivity to-
wards oxygen and/or moisture both in solution or in
the solid state, we had difficulty in obtaining single
crystals of the compound; after several attempts, small
single crystals were obtained by layering heptane on a
toluene solution of NbCp4. Although the quality of the
crystals did not allow an accurate solution of the
structure, the crystallographic data have established
that NbCp4 is mononuclear and, in agreement with
earlier suggestions [1], it presents two pentahapto and
two monohapto cyclopentadienyl ligands, Fig. 1. It is
rather surprising that the structure of niobium is similar
to that of TiCp4 and HfCp4 [5] and substantially differ-
ent from that of ZrCp4, the latter presenting three
pentahapto- and one monohaptocyclopentadienyl lig-
ands [6]. Table 1 reports the bond distances and angles.
The cyclopentadienyl ligands are coordinated to nio-
bium in a distorted tetrahedral geometry; the angle at
niobium between the two monohapto ligands is 83.4(6)°
and that between the centroids of the two h5-cyclopen-
tadienyls is 131.1°, values similar to those observed in
NbCp2(CH2Ph)2 (79.3 and 133°) [7], NbCp2-
(CH2SiMe2)2 (83.0 and 135.4°) [8], TiCp4 (86.3 and
Table 1
,
Bond distances (A) and angles (°) in NbCp4. Estimated standard
deviations in parentheses refer to the least significant digit
Nb–C(21)
Nb–C(22)
Nb–C(24)
Nb–C(25)
Nb–C(13)
Nb–C(12)
2.29(3)
2.33(3)
2.35(2)
2.36(3)
2.36(3)
2.37(3)
Nb–C(11)
Nb–C(15)
Nb–C(23)
Nb–C(14)
Nb–C(41)
Nb–C(31)
2.39(3)
2.40(3)
2.42(3)
2.44(3)
2.46(3)
2.48(2)
C(41)–Nb–C(31)
C(31)–Nb–Cpcent(1) 105.4(5)
C(31)–Nb–Cpcent(2) 109.1(4)
83.4(6)
C(41)–Nb–Cpcent(1) 114.3(6)
C(41)–Nb–Cpcent(2) 103.2(6)
Cpcent(1) and Cpcent(2) refer to the computed h5-C5H5 centroids for
atoms C(11)–C(15) and C(21)–C(25), respectively.
Scheme 2.
129.9°) and HfCp4 (88.0 and 130.0°) [5]. The centroid–
5
,
Nb distances are 2.082 and 2.019 A. The two h -cy-
ucts allowed isolation of the tricyclopentadienyl deriva-
tives, NbCp3L (L=CF3COOH, Ph3SiOH) and of the
mixed-ligand compound NbCp2(OOCCF3)(OSiPh3), see
Scheme 3.
Similarly, NbCp3Cl can be isolated from the reaction
of NbCp4 with PyHCl in toluene as medium. As in the
case of LH, the use of a PyHCl/Nb molar ratio of 2
slowly produces the dichloride NbCp2Cl2. On the other
hand, no tricyclopentadienyl has been obtained in a
pure state by using pyridinium bromide or iodide. In
this case, independent of the molar ratio, the dihalides
NbCp2X2 have been obtained, see Scheme 4.
clopentadienyls are almost planar with a maximum
deviation from the least-square plane of 0.008 and
0.026 A with C–C bond distances in the range 1.37(2)–
,
,
1.41(2) A. According to the data observed for MCp4
(M=Ti, Hf) [5] and for the niobium(V) derivative
Nb(h5-C5H5)2(h1-C5H5)(NtBu) [9], we could expect two
different types of C–C bonds; however, the high stan-
dard deviations of the C–C bond distances do not
allow any significant trend to be observed.
The availability of NbCp4 allowed its reactivity to be
studied, mainly towards substances containing active
protons. The niobium compound NbCp4 reacts with
CF3COOH or Ph3SiOH to give 1:1 or 1:2 derivatives
depending on the molar ratio, see Scheme 2. The large
difference in the reaction times necessary to convert
quantitatively the tetracyclopentadienyl into the prod-
NbCp4 reacts with 9,10-dihydroxyphenanthrene with
protonolysis of two cyclopentadienyl rings and forma-
tion of NbCp2(C14H8O2), see Eq. (1).
NbCp4+C14H8O2“NbCp2(C14H8O2) +2C5H6
(1)

F. Calderazzo et al. / Journal of Organometallic Chemistry 630 (2001) 275–280
277
Scheme 3.
NbCp2(C14H8O2) shows an IR spectrum in the solid
state characterized by the absorptions typical of the
phenanthrenediolato ligand, similar to what is observed
for the analogous compounds of zirconium(IV) and
hafnium(IV) [10] thus suggesting that we are dealing
with a niobium(IV) derivative containing the 9,10-
phenanthrenediolato O-O% ligand.
sponding to a formal 20-e counting. The lower
reactivity of the NbCp3L systems may be tentatively
assigned to the fact that they are in fact 17-e com-
pounds containing two h5- and one h1-cyclopentadienyl
groups.
4. Experimental
Unless otherwise stated, all of the operations were
carried out under an atmosphere of prepurified argon.
Solvents were dried by conventional methods prior to
use.
3. Conclusions
This paper has filled a gap in our knowledge of solid
state data for tetracyclopentadienyls of early transition
elements, showing that the titanium, hafnium, niobium
and probably tantalum (based on the Debye diagrams
already reported in the literature [1]) have similar
molecular structures, i.e. two of the four cyclopentadi-
enyl ligands are bonded to the metal in a pentahapto
fashion, the other two being monohapto bonded. At
variance with these compounds, three cyclopentadienyl
ligands are pentahapto bonded, and one is monohapto
bonded in the zirconium derivative, ZrCp4.
IR spectra were recorded on a FT-1725X instrument
on solutions or Nujol or polychlorotrifluoroethylene
(PCTFE) mulls prepared under rigorous exclusion of
moisture and air. Magnetic susceptibilities were mea-
sured with a magnetic balance (Sartorius 4104, electro-
magnet Varian V 2900) according to the Faraday
method using CuSO4·5H2O for calibration at three
different field strengths (9400, 10 200, 10 800 G) and
averaging the observed values. Diamagnetic corrections
were calculated using Pascal contributions [14].
Moreover, the study of the reactivity of NbCp4 to-
wards substances containing active protons has allowed
the isolation of the triscyclopentadienyl compounds
NbCp3L. It is noteworthy that the corresponding com-
pounds of Group 4 elements appear to be transient
species: the biscyclopentadienyl derivatives MCp2L2 are
obtained independent of the LH/M molar ratio. Al-
though much work is necessary in order to clarify the
solid-state structure of the paramagnetic NbCp3L
derivatives, we may tentatively assume that the differ-
ent reactivity observed on going from ZrCp4 to NbCp4
depends on a different hapticity of the cyclopentadienyl
ligands in MCp3L. As a matter of fact, the known
examples of structurally characterized [ZrcCp3L]n+ sys-
tems (L=H···AlEt3, n=0 [11]; L=Me, n=0 [11]; CO,
acetonitrile, ButNC, n=1 [12,13]), present a pentahap-
tocoordination of the cyclopentadienyl ligands corre-
Toluene suspensions of NaCp were obtained in the
following way: a suspension of NaH in THF was
treated with freshly distilled cyclopentadiene at room
temperature (r.t.). The solution was dried in vacuo and
the colourless residue was suspended in toluene.
Trifluoroacetic acid, CF3COOH, was used in admix-
ture with the corresponding anhydride. The total
amount of trifluoroacetate groups was determined by
acid–base titration in aqueous solution, while the
CF3COOH/(CF3CO)2 molar ratio was obtained from
the 19F-NMR spectrum. Triphenylsilanol, Ph3SiOH
(Aldrich) was dried over P4O10 in vacuo for 24 h.
Pyridinium halides, PyHX, X=Cl, Br, I, were prepared
by reaction of pyridine with the appropriate hydrogen
halide in heptane. 9,10-dihydroxyphenanthrene was
prepared by reduction of 9,10-phenanthrenequinone
with dihydrogen in the presence of Pd/C in toluene at
r.t. Large scale preparations of NbCp2Cl2 [15] and
NbC14(THF)2 [16] were carried out according to the
literature. Nb2Cl10 (Fluka) was sublimed prior to use.
4.1. Preparation of NbCp4
From Nb2Cl10. A suspension of NaCp (obtained
from 6.10 g, 254 mmol of NaH) in toluene (500 ml) was
Scheme 4.

278
F. Calderazzo et al. / Journal of Organometallic Chemistry 630 (2001) 275–280
Table 2
NbCp2Cl2 (1.0 g, 3.4 mmol). The reaction mixture was
stirred for 15 h at r.t. obtaining a violet suspension. By
a procedure similar to the preceding one, 0.63 g (53%
yield) of NbCp4 were obtained.
Crystallographic data and details of the structure refinement for
NbCp4
Empirical formula
Formula weight
Crystal size (mm)
Temperature (K)
C20H20Nb
353.29
0.30×0.30×0.10
291
4.2. NbCp4: crystal structure solution and refinement
,
Wavelength (A)
1.54184
Hexagonal
P65 (no. 170)
Crystals of NbCp4 (NbCp2Cl2 as starting material)
were obtained from toluene–heptane solution. Crystal-
lographic data and details of the structure refinement
for NbCp4 are in Table 2. Data were collected at 291 K
on an Enraf-Nonius CAD4 diffractometer equipped
with a graphite monochromator. The crystals are thin,
weakly diffracting platelets; hence intensity data were
collected with Cu–Ka radiation. A numerical absorp-
tion correction (minimum transmission 0.128, maxi-
mum transmission 0.354) was applied before averaging
symmetry-equivalent data. After merging, 2691 inde-
pendent reflections remained for structure solution by
direct methods [17a]. The structure model was com-
pleted by Fourier difference syntheses. It was kept as
simple as possible with isotropic displacement parame-
ters for the C atoms and refined with full-matrix least-
squares on F2 [17b]. Convergence was reached for 2691
reflections and 90 variables at agreement factors of
wR2=0.1749 (all data), R1=0.1093 [I\2|(I)].
Crystal system
Space group
Unit cell dimensions
,
a=b (A)
9.396(2)
31.23(3)
2388(2)
6.00
1.48
62.04
1086
1.4–65.0
−115h59, −95k511,
−365l536
10445
2691 [Rint=0.289]
99.9
Numerical
0.354 and 0.128
Full-matrix least-squares on
F2
2691/21/90
0.946
,
c (A)
3
,
Volume (A )
Z
Dcalc (g cm−3
)
Absorption coefficient (cm−1
F(000)
)
q Range for data collection (°)
Index ranges
Reflections collected
Independent reflections
Completeness to q=64.95° (%)
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I\2|(I)]
R indices (all data)
R1=0.1093, wR2=0.1517
R1=0.2882, wR2=0.1749
0.00(9)
4.3. Reaction of NbCp4 with CF3COOH
Absolute structure parameter
Largest difference peak and hole 0.657 and −0.632
(e A−3
)
CF3COOH/Nb molar ratio 1.1. A 10:1 (v/v) mixture
of CF3COOH and (CF3CO)2O (0.07 ml, 0.82 mmol of
CF3COOH) was added at r.t. to a red–violet suspen-
sion of NbCp4 (0.30 g, 0.78 mmol). Immediate reaction
with formation of a violet solution was observed. After
3 h stirring at r.t., the volume of the solution was
reduced to ca. 5 ml and heptane (25 ml) was added.
Upon cooling at ca. −30 °C, a blue–black microcrys-
talline solid separated out which was recovered by
filtration and dried in vacuo affording 0.15 g (46%
yield) of NbCp3(OOCCF3) as a moisture- and oxygen-
sensitive product. Anal. Found: C, 50.3; H, 4.0; Nb,
24.1. Calc. for C17H15F3NbO2: C, 50.8; H, 3.7; Nb,
23.2%. IR (Nujol, cm−1): 3092m, 1695s (was COO),
1576w, 1410s (ws COO), 1262w, 1196s (was CF3), 1143 5
(ws CF3), 1016m, 978w, 823s, 786m-s, 721s, 669m,
614m, 523m, 489w, 419m. Magnetic measurement: dia-
treated at r.t. with Nb2Cl10 (6.62 g, 12.2 mmol). The
suspension turned brown on mixing the reagents and
violet after stirring for a few minutes. The suspension
was stirred for 2 h, filtered, and the solid washed with
toluene until the washings were almost colourless (ca.
6×25 ml). The red–violet solution was evaporated to
dryness in vacuo at r.t. affording a dark brown residue
which was treated with heptane (50 ml). The black
microcrystalline solid was collected by filtration and
dried in vacuo at r.t. affording 5.12 g (52% yield) of
NbCp4 in the form of an oxygen- and moisture-sensi-
tive solid. Anal. Found: C, 68.5; H, 4.7; Nb, 25.9. Calc.
for C20H20Nb: C, 68.1; H, 5.7; Nb, 26.3%. IR (Nujol,
cm−1): 3076m, 1591w, 1433w, 1123w, 1009w, 818m,
749m, 720s, 669w, 619m-w, 601m-w.
From NbCl4(THF)2. A suspension of NaCp (5.0 g,
51 mmol) in toluene (250 ml) was treated at r.t. with
NbCl4(THF)2 (3.15 g, 8.3 mmol). The suspension
turned brown on mixing the reagents. The reaction
mixture was stirred for 15 h at r.t. obtaining a violet
suspension. Isolation of the product was similarly car-
ried out yielding 1.45 g (49% yield) of NbCp4.
magnetic correction −187×10−6 cgsu,  =1.8×
corr
mol
10−3 cgsu, veff (298 K) 2.08 BM.
CF3COOH/Nb molar ratio 2:1. A 10:1 (v/v) mixture
of CF3COOH and (CF3CO)2O (0.5 ml, 5.8 mmol of
CF3COOH) was added at r.t. to a red–violet suspen-
sion of NbCp4 (1.19 g, 2.9 mmol) in toluene (20 ml).
Immediate reaction with formation of a violet solution
was observed which turned dark brown. After 15 h
stirring at r.t., the volume of the suspension was re-
duced to ca. 5 ml and heptane (25 ml) was added. The
From NbCp2Cl2. A suspension of NaCp (1.96 g, 20
mmol) in toluene (100 ml) was treated at r.t. with

F. Calderazzo et al. / Journal of Organometallic Chemistry 630 (2001) 275–280
279
microcrystalline solid which separated out was recov-
ered by filtration and dried in vacuo affording 1.14 g
(88% yield) of NbCp2(OOCCF3)2 as a moisture- and
oxygen-sensitive product. Anal. Found: C, 38.5; H, 2.7;
Nb, 20.3. Calc. for C14H10F6NbO4: C, 37.4; H, 2.2; Nb,
20.7%. IR (Nujol, cm−1): 3120m, 1699vs (was COO),
1578w, 1405s (ws COO), 1188s (was CF3), 1143 5 (ws
CF3), 1016m, 954w, 827s, 790m, 723s, 669m, 614m,
521w, 493w, 413m. Magnetic measurement: diamag-
tion of NbCp3(OOCCF3) with Ph3SiOH or from
NbCp3(OSiPh3) and CF3COOH. Only the procedure
relative to the former reaction is described in detail, the
latter being performed in a similar way. A solution of
NbCp3(OOCCF3) (0.46 g, 1.2 mmol) in toluene (20 ml)
was treated with Ph3SiOH (0.34 g, 1.2 mmol). After 14
h stirring at r.t., the solution was partially evaporated
in vacuo and heptane (20 ml) was added. The suspen-
sion was filtered and the solid was dried in vacuo at r.t.
affording 0.55 g of NbCp2(OOCCF3)(OSiPh3) as a mi-
crocrystalline solid sensitive to moisture and oxygen.
Anal. Found: C, 58.6; H, 4.5; Nb, 15.0; Si, 4.8. Calc.
for C30H25NbO3Si: C, 58.9; H, 4.1; Nb, 15.2; Si, 4.6%.
IR (Nujol, cm−1): 3113w, 1695s, 1428s, 1410m, 1191vs,
1157w, 1138m-s, 1111m-s, 967s, 512s. Magnetic mea-
netic correction −180×10−6 cgsu,  =1.9×10−3
corr
mol
cgsu, veff (298 K) 2.16 BM.
The same compound (IR and analytical data) was
obtained after
a 14 h reaction at r.t. between
NbCp3(OOCCF3) and trifluoroacetic acid in toluene.
surement: diamagnetic correction −329×10−6 cgsu,
4.4. Reaction of NbCp4 with Ph3SiOH

=1.18×10−3 cgsu, veff (298 K) 1.68 BM.
corr
mol
Ph3SiOH/Nb molar ratio 1.1. A red–violet suspen-
sion of NbCp4 (0.53 g, 1.3 mmol) in toluene (20 ml)
was treated with solid Ph3SiOH (0.36 g, 1.3 mmol). A
slow reaction was observed. After 15 h stirring at r.t.
the deep brown solution was partially evaporated in
vacuo and heptane (20 ml) was added. The resulting
suspension was filtered and the solid was dried in vacuo
affording 0.39 g (54% yield) of NbCp3(OSiPh3) as a
moisture- and oxygen-sensitive product. Anal. Found:
C, 69.9; H, 5.1; Nb, 16.8; Si, 4.6. Calc. for
C33H30NbOSi: C, 70.3; H, 5.0; Nb, 16.5; Si, 4.9%. IR
(Nujol, cm−1): 3082m, 1463s, 1428m, 1377s, 1113m,
958m, 812mw, 759w, 741w, 720m, 699m, 512m. Mag-
netic measurement: diamagnetic correction −319×
4.6. Reaction of NbCp4 with pyridinium halides
(a) PyHCl/Nb molar ratio 1:1. A suspension of
NbCp4 (0.63 g, 1.6 mmol) in toluene (30 ml) was
treated at r.t. with PyHCl (0.19 g, 1.6 mmol). Forma-
tion of a deep red solution was observed upon mixing
the reagents. After 15 h stirring at r.t., the suspension
was filtered, the volume of the solution was reduced to
ca. 10 ml and heptane (20 ml) was added. The suspen-
sion was filtered and the solid was dried in vacuo at r.t.
affording 0.32 g (62% yield) of NbCp3Cl as a dark
brown microcrystalline solid sensitive to moisture and
oxygen. Anal. Found: C, 55.0; H, 4.9; Cl, 11.0; Nb,
26.6. Calc. for C15H15ClNb: C, 55.6; H, 4.6; Cl, 10.9;
Nb, 28.7%. IR (Nujol, cm−1): 3079m, 1418w, 1305w,
1017mw, 1009mw, 817m-s, 798w, 729s.
(b) PyHCl/Nb molar ratio 2:1. A suspension of
NbCp4 (0.46 g, 1.2 mmol) in toluene (25 ml) was
treated at r.t. with PyHCl (0.28 g, 2.4 mmol). The
resulting brown suspension was stirred for 60 h at r.t.
The suspension was filtered and the pale brown solid
was washed with toluene (3×10 ml) and dried in vacuo
at r.t. affording 0.22 g (63% yield) of NbCp2Cl2 [15,18]
which was identified by analytical and spectroscopic
methods.
(c) PyHX (X=Br, I)/Nb molar ratio 2:1. Only the
reaction of PyHBr with NbCp4 is described in detail,
the others being performed in a similar way. A suspen-
sion of NbCp4 (0.44 g, 1.15 mmol) in toluene (25 ml)
was treated at r.t. with PyHBr (0.37 g, 2.3 mmol). The
resulting red–brown suspension was stirred for 14 h at
r.t. After filtration the brown solid was washed with
toluene (3×10 ml) and dried in vacuo at r.t. affording
0.37 g (76% yield) of NbCp2Br2 [15,18] which was
identified by analytical and spectroscopic methods. The
iodo derivative, NbCp2I2 [15,18], was obtained as a
red–brown solid (77% yield).
10−6 cgsu,  =1.8×10−3 cgsu, veff (298 K) 2.08
corr
mol
BM.
Ph3SiOH/Nb molar ratio 1:2. A red–violet suspen-
sion of NbCp4 (0.43 g, 1.1 mmol) in toluene (20 ml)
was treated with solid Ph3SiOH (0.59 g, 2.2 mmol). A
slow change of colour to deep brown and deep green
was observed. After 6 days stirring at r.t. the suspen-
sion was dried in vacuo at r.t. and heptane (20 ml) was
added. The resulting suspension was filtered and the
solid was dried in vacuo affording 0.39 g (70% yield) of
NbCp2(OSiPh3)2 as a moisture- and oxygen-sensitive
product. Anal. Found: C, 68.2; H, 5.4; Nb, 12.4; Si, 7.1.
Calc. for C46H40NbO2Si2: C, 71.5; H, 5.0; Nb, 12.0; Si,
7.2.%. IR (Nujol, cm−1): 3080m, 1464s, 1427m-s,
1378s, 11 12m, 910m-w, 809m-w, 699s, 508m-s. Mag-
netic measurement: diamagnetic correction −444×
10−6 cgsu,  =1.9×10−3 cgsu, veff (298 K) 2.14
corr
mol
BM.
The same compound (IR and analytical data) was
obtained after
a 15 h reaction at r.t. between
NbCp3(OSiPh3) and triphenylsilanol in toluene.
4.5. Preparation of NbCp2(OOCCF3)(OSiPh3)
This compound was obtained either from the reac-

280
F. Calderazzo et al. / Journal of Organometallic Chemistry 630 (2001) 275–280
[2] F.W. Siegert, H.J. De Liefde-Mijer, Rec. Trav. Chim. Pays-Bas
88 (1969) 1445.
4.7. Reaction of NbCp4 with 9,10-dihydroxy-
phenanthrene: preparation of NbCp2(C14H802)
[3] R. Mercier, J. Douglade, J. Amaudrut, J. Sala-Pala, J.E. Guer-
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Strukt. Khim. 14 (1973) 813; Chem. Abstr. 80 (1974) 21174b.
[5] TiCp4: J.L. Calderon, F.A. Cotton, B.G. DeBoer, J.J. Takats, J
Am. Chem. Soc. 93 (1971) 3592. HfCp4: R.D. Rogers, R.V.
Bynum, J.L. Atwood, J. Am. Chem. Soc. 103 (1981) 692. V.I.
Kulishov, E.M. Brainina, N.G. Bokiy, Yu.T. Struchkov, J.
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[7] P.B. Hitchcock, M.F. Lappert, C.R.C. Mime, J. Chem. Soc.
Dalton Trans. (1981) 180.
A suspension of NbCp4 (0.355 g, 1.0 mmol) in tolu-
ene (25 ml) was treated at r.t. with 9,10-dihydrox-
yphenanthrene (0.205 g, 1.0 mmol). Immediate reaction
with formation of a deep red solution was observed.
After 15 h at r.t., the volume of the solution was
reduced to ca. 10 ml and heptane (20 ml) was added.
The suspension was filtered and the solid was dried in
vacuo at r.t. affording 0.32
g (75% yield) of
NbCp2(C14H802) as a red–brown microcrystalline solid
sensitive to moisture and oxygen. Anal. Found: C, 66.0;
H, 4.4; Nb, 21.0. Calc. for C24H18NbO2: C, 66.8; H, 4.2;
Nb, 21.5%. IR (Nujol; cm−1): 3102m-w, 1600m-w,
1580m, 1511w, 1487m, 1338s, 1262w, 1223w, 1112w,
1055s, 1029s, 929w, 808s, 790vs, 754s, 723s, 687m,
574m, 544mw. Magnetic measurement: diamagnetic
correction −222×10−6 cgsu,
cgsu, veff (298 K) 1.70 BM.

=1.31×10−3
corr
mol
[8] J. Liu, J.L. Petersen, Chem. J. Chin. Univ. 6 (1985)103; Chem.
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G. Tripepi, J. Organomet. Chem. 543 (1997) 201.
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[12] T. Brackemeyer, G. Erker, R. Fro¨hlich, Organometallics 16
(1997) 531.
5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC no. 160280 for compound NbCp4.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (Fax: +44-1223-336-033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
[13] T. Brackemeyer, G. Erker, R. Fro¨hlich, J. Prigge, U. Peuchert,
Chem. Ber. Rec. 130 (1997) 899.
[14] E. Ko¨nig, Magnetische Eigenschaften der Koordinations- und
8
Metallorganischen Verbindungen der Ubergangselemente. In:
Landolt-Bo¨rnstein, Zahlenwerte und Funktionen aus Naturwis-
senschaften und Technik, vol. 2, Springer Verlag, Berlin, 6th
edn., 1966, p.16.
Acknowledgements
[15] J.A. Labinger, Niobium and Tantalum. In: G. Wilkinson,
F.G.A. Stone, E.W. Abel (Eds.), Comprehensive Organometallic
Chemistry, vol. 3, Pergamon, Oxford, 1982, p. 766.
[16] L.E. Manzer, Inorg. Synth. 21 (1982) 135.
[17] (a) G.M. Sheldrick, SHELXS-97, Program for Crystal Structure
Solution, University of Go¨ttingen, Germany, 1997;
(b) G.M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Go¨ttingen, Germany, 1997.
[18] D.E. Wigley, S.D. Gray, Niobium and Tantalum. In: E.W.
Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehensive
Organometallic Chemistry II, vol. 4, Pergamon, Oxford, 1995, p.
121.
The authors wish to thank the Ministero dell’Univer-
sita` e della Ricerca Scientifica e Tecnologica (MURST,
Roma), Programma di Ricerca Scientifica di Notevole
Interesse Nazionale, Cofinanziamento MURST 1999–
2000 for financial support.
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