Unexpected Competition between Vanadium and Zinc in the Synthesis of Sterically Bulky Metallocenes

Article abstract of DOI:10.1021/om980691w

Attempts to form (Cp³ⁱ)₂V and (Cp⁴ⁱ)₂V (Cpⁿⁱ = C₅(iPr)_nH_5-n) from the reaction of zinc-reduced VCl₃ and K[Cpⁿⁱ] in THF at room temperature produces the corresponding zincocenes (Cp³ⁱ)₂Zn and (Cp⁴ⁱ)₂Zn instead. In refluxing THF, the same reactions generate the expected (Cpⁿⁱ)₂V vanadocenes. The latter are also formed from the reaction of aluminum-reduced VCl₃ with K[Cpⁿⁱ] at room temperature. Reaction of 3 equiv of K[Cp³ⁱ] or K[C₅Ph₄H] with VCl₃ in THF produces (Cp³ⁱ)₂V and (C₅Ph₄H)₂V, respectively, whereas the equivalent reaction with K[Cp⁴ⁱ] generates (Cp⁴ⁱ)₂VCl. These experiments suggest that zincocenes are intermediates whenever vanadocenes are synthesized from zinc-reduced VCl₃ but that zinc complexes will be isolated only when certain cyclopentadienyl rings and temperature conditions are used. A single-crystal X-ray diffraction study of (Cp³ⁱ)₂V found the metal to reside on a crystallographically imposed inversion center, with an average V-C(ring) distance of 2.274-(9) A?; the isopropyl groups orient themselves to minimize unfavorable steric interactions between the rings.

Full text of DOI:10.1021/om980691w

Organometallics 1999, 18, 1663-1668
1663
Un exp ected Com p etition betw een Va n a d iu m a n d Zin c in
th e Syn th esis of Ster ica lly Bu lk y Meta llocen es
J ason S. Overby, Kumudini C. J ayaratne, Nathan J . Schoell, and
Timothy P. Hanusa*
Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37235
Received August 18, 1998
Attempts to form (Cp3i)2V and (Cp4i)2V (Cpni ) C5(iPr)nH5-n) from the reaction of zinc-
reduced VCl3 and K[Cpni] in THF at room temperature produces the corresponding zincocenes
(Cp3i)2Zn and (Cp4i)2Zn instead. In refluxing THF, the same reactions generate the expected
(Cpni)2V vanadocenes. The latter are also formed from the reaction of aluminum-reduced
VCl3 with K[Cpni] at room temperature. Reaction of 3 equiv of K[Cp3i] or K[C5Ph4H] with
VCl3 in THF produces (Cp3i)2V and (C5Ph4H)2V, respectively, whereas the equivalent reaction
with K[Cp4i] generates (Cp4i)2VCl. These experiments suggest that zincocenes are intermedi-
ates whenever vanadocenes are synthesized from zinc-reduced VCl3 but that zinc complexes
will be isolated only when certain cyclopentadienyl rings and temperature conditions are
used. A single-crystal X-ray diffraction study of (Cp3i)2V found the metal to reside on a
crystallographically imposed inversion center, with an average V-C(ring) distance of 2.274-
(9) Å; the isopropyl groups orient themselves to minimize unfavorable steric interactions
between the rings.
In tr od u ction
(Cp3i) and tetraisopropylcyclopentadienyl (Cp4i) rings,10,11
we have found that solutions of zinc-reduced VCl3 may
function as a source not only of V(II) but also of Zn(II)
alone. Our investigation into the conditions under which
the latter occurs suggest that zinc serves as more than
a reducing agent for trivalent vanadium in the produc-
tion of other vanadocenes, including the parent Cp2V.
Owing to its convenient physical properties and
commercial availability, vanadium trichloride serves as
a common entry point into low-valent vanadium chem-
istry. It is easily reduced by metallic zinc in THF to form
a material at one time thought to contain “VCl2(thf)2”1,2
but was later shown to consist of the vanadium-zinc
species [V2Cl3(thf)6]2(Zn2Cl6).3,4 The latter is a useful
source of the V(II) ion in organovanadium synthesis and
has been used to generate a number of vanadocenes and
related compounds (eq 1; M ) Li, Na, K; Cp′ ) Cp, Cp*,
C5Ph4H, Me2C5H5).1,3,5-7 With other ligands, the final
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations were per-
formed with the rigorous exclusion of air and moisture using
high-vacuum, Schlenk, or drybox techniques. Proton NMR
spectra were obtained on a Bruker NR-300 spectrometer at
300 MHz and were referenced to the residual proton reso-
nances of C6D6 (δ 7.15). Infrared data were measured with KBr
pellets as previously described.12 Solution magnetic suscepti-
bility data were obtained in C6D6 using the Evans NMR
method.13-16 Elemental analyses were performed by Oneida
Research Services, Whitesboro, NY, the University of Illinois
Microanalytical Laboratory, or Desert Analytics, Tucson, AZ.
Ma ter ia ls. Vanadium(III) chloride, iron(II) chloride, zinc
iodide, and zinc powder were purchased from Aldrich and were
used as received. Aluminum powder was used as received from
Acros Organics. K[Cp3i],12 K[Cp4i],12 K[C5Ph4H],6 and Cp2Zn17
were prepared by literature methods. Solvents for reactions
were distilled under nitrogen from sodium or potassium
2 VCl3 + Zn 4MCp′8 2Cp′2V + ZnCl2 + 4MCl (1)
products from reactions using VCl3/Zn have been shown
to contain both vanadium and zinc, such as [VZnH2-
8
(BH4)(PMePh2)2]2 and [VZnO(C6H5CO2)3(thf)]4‚2thf.9
In the present study of vanadium metallocenes con-
taining the “encapsulating” triisopropylcyclopentadienyl
(1) Ko¨hler, F. H.; Pro¨ssdorf, W. Z. Naturforsch., B: Anorg. Chem.,
Org. Chem. 1977, 32B, 1026-1029.
(2) Hall, V. M.; Schmulbach, C. D.; Soby, W. N. J . Organomet. Chem.
1981, 209, 69-76.
(3) Bouma, R. J .; Teuben, J . H.; Beukema, W. R.; Bansemer, R. L.;
Huffman, J . C.; Caulton, K. G. Inorg. Chem. 1984, 23, 2715-2718.
(4) Cotton, F. A.; Duraj, S. A.; Roth, W. J . Inorg. Chem. 1985, 24,
913-917.
(5) Kowaleski, R. M.; Basolo, F.; Trogler, W. C.; Gedridge, R. W.;
Newbound, T. D.; Ernst, R. D. J . Am. Chem. Soc. 1987, 109, 4860-
4869.
(10) Burkey, D. J .; Hays, M. L.; Duderstadt, R. E.; Hanusa, T. P.
Organometallics 1997, 16, 1465-1475.
(11) Overby, J . S.; Schoell, N. J .; Hanusa, T. P. J . Organomet. Chem.
1998, 560, 15-19.
(12) Williams, R. A.; Tesh, K. F.; Hanusa, T. P. J . Am. Chem. Soc.
1991, 113, 4843-4851.
(6) Castellani, M. P.; Geib, S. J .; Rheingold, A. L.; Trogler, W. C.
Organometallics 1987, 6, 1703-1712.
(13) Evans, D. F. J . Chem. Soc. 1959, 2003-2005.
(14) Grant, D. H. J . Chem. Educ. 1995, 72, 39-40.
(15) Sur, S. K. J . Magn. Reson. 1989, 82, 169-173.
(16) Shubert, E. M. J . Chem. Educ. 1992, 69, 62.
(17) Fischer, B.; Wijkens, P.; Boersma, J .; van Koten, G.; Smeets,
W. J . J .; Spek, A. L.; Budzelaar, P. H. M. J . Organomet. Chem. 1989,
376, 223-233.
(7) Wilson, D. R.; J u-Zheng, L.; Ernst, R. D. J . Am. Chem. Soc. 1982,
104, 1120-1122.
(8) Bansemer, R. L.; Huffman, J . C.; Caulton, K. C. J . Am. Chem.
Soc. 1983, 105, 6163-6164.
(9) Cotton, F. A.; Duraj, S. A.; Roth, W. J . Inorg. Chem. 1984, 23,
4042-4045.
10.1021/om980691w CCC: $18.00 © 1999 American Chemical Society
Publication on Web 03/31/1999

1664 Organometallics, Vol. 18, No. 9, 1999
Overby et al.
benzophenone ketyl. NMR solvents were vacuum-distilled from
Na/K (22/78) alloy and stored over 4A molecular sieves.
Attem p ted Syn th esis of Bis(tr iisop r op ylcyclop en ta -
d ien yl)va n a d iu m (II), (Cp 3i)2V. A THF solution of [V2Cl3-
(thf)6]2(Zn2Cl6) was prepared by refluxing VCl3 (0.10 g, 0.65
mmol) in THF followed by reduction with zinc powder (0.05 g,
0.76 mmol).3,4 To this solution was added K[Cp3i] (0.30 g, 1.3
mmol), whereupon the solution turned purple. After the
mixture was stirred overnight at room temperature, the THF
was removed by rotary evaporation. The residue was extracted
with hexanes (45 mL), and the resulting dark solution was
evaporated to a solid. A 1H NMR spectrum indicated that the
only identifiable species present was the zinc metallocene
(Cp3i)2Zn.18
Syn th esis of Bis(tr iisop r op ylcyclop en ta d ien yl)va n a -
d iu m (II), (Cp 3i)2V. Meth od A. In the dark, a 125 mL
Erlenmeyer flask was charged with a magnetic stirring bar,
VCl3 (0.23 g, 1.5 mmol), and K[Cp3i] (1.0 g, 4.3 mmol). To this
was added 25 mL of THF, at which time the resulting solution
turned dark brown-purple. This mixture was stirred for 48 h;
subsequently the solution was evaporated to dryness and
extracted with 30 mL of hexane. The hexane extract was
(Cp4i)2Zn;18 in addition, a crystal obtained from the solid was
examined with X-ray diffraction and found to have unit cell
dimensions matching those reported for (Cp4i)2Zn.18
Syn t h esis of Bis(t et r a isop r op ylcyclop en t a d ien yl)-
va n a d iu m (III) Ch lor id e, (Cp 4i)2VCl. As in the procedure
used for (Cp3i)2V (method A), a 125 mL Erlenmeyer flask was
equipped with a magnetic stirring bar and charged with VCl3
(0.10 g, 0.64 mmol) and K[Cp4i] (0.53 g, 1.9 mmol). THF (30
mL) was added, and the resulting solution was stirred in the
dark for 48 h. The mixture was evaporated to dryness and the
residue extracted with 30 mL of hexane. Filtration of the
hexane extract through a medium-porosity glass frit removed
precipitated KCl. The filtrate was evaporated, leaving a
brownish oily material. Fractional sublimation (140-160 °C,
10-6 Torr) of this using a Kugelrohr apparatus gave an
extremely viscous brown material. Upon prolonged standing
(>1 week), brown-black (Cp4i)2VCl solidified (0.21 g, 0.38 mmol)
in 60% yield. Anal. Calcd for C34H58ClV: C, 73.82; H, 10.57.
Found: C, 74.04; H, 10.84. Principal IR bands (KBr, cm-1):
2964 (vs), 2877 (s), 1457 (m), 1370 (m), 1323 (w), 1263 (m),
1175 (m), 1095 (m), 1029 (m), 801 (m). Magnetic susceptibil-
ity: µcorr ) 2.92 µB.
Syn t h esis of Bis(t et r a isop r op ylcyclop en t a d ien yl)-
va n a d iu m (II), (Cp 4i)2V. Meth od A. A solution of VCl3(thf)3
prepared by refluxing VCl3 (0.10 g, 0.64 mmol) in THF
overnight, was placed into a 125 mL Erlenmeyer flask
containing a magnetic stirring bar. Excess aluminum powder
(0.021 g, 0.78 mmol) was added, along with KH (0.01 g). After
it was stirred for several hours, the mixture turned blue-green,
indicating the presence of the [V2Cl3(thf)6]+ cation. To this was
added K[Cp4i] (0.35 g, 1.3 mmol), and a purple solution
developed within 10 min. After this mixture was stirred for
several hours, the solution was evaporated to dryness and the
residue extracted with hexanes (50 mL). Filtration through a
medium-porosity glass frit followed by removal of the solvent
produced a purple solid. This crude material was fractionally
sublimed using a Kugelrohr apparatus (140-160 °C/10-6 Torr)
to give purple (Cp4i)2V (0.24 g, 0.46 mmol) in 72% yield, mp
152-156 °C. Anal. Calcd for C34H58V: C, 78.87; H, 11.29.
Found: C, 77.71; H, 11.21. Principal IR bands (KBr, cm-1):
2967 (vs), 2875 (vs), 1460 (s), 1374 (s), 1176 (m), 1070 (s), 1015
(s), 915 (w), 862 (m), 808 (m), 637 (w), 538 (w). Magnetic
susceptibility: µcorr ) 3.91 µB.
Meth od B. A THF solution of [V2Cl3(thf)6]2(Zn2Cl6) was
prepared by refluxing VCl3 in THF followed by reduction with
zinc powder (0.05 g, 0.76 mmol). To this solution was added
K[Cp4i] (0.35 g, 1.5 mmol), whereupon the mixture turned
purple. After the mixture was refluxed and stirred overnight,
the THF was removed by rotary evaporation. The residue was
extracted with hexanes (45 mL), and the resulting dark
solution was reduced to dryness to give a dark purple solid.
This solid was identified as (Cp4i)2V (0.25 g, 0.48 mmol) (64%)
by comparison of its magnetic susceptibility measurement and
IR spectrum to those of previously prepared material.
Rea ction of Cp 2Zn w ith [V2Cl3(th f)6]2(Zn 2Cl6). A THF
solution of [V2Cl3(thf)6]2(Zn2Cl6), prepared as described above
(VCl3, 0.32 g, Zn, 0.065 g), was placed into a 125 mL
Erlenmeyer flask containing a magnetic stirring bar. To this
solution was added Cp2Zn (0.50 g, 2.6 mmol). The mixture
turned purple-gray and was stirred overnight. Removal of the
THF by rotary evaporation left a purple-gray solid. Fractional
sublimation as described in the literature19 yielded Cp2V (60%),
identified by its magnetic susceptibility20 and IR spectrum.21
Rea ction of K[Cp 3i] w ith F eCl2/Zn I2. To a 125 mL
Erlenmeyer flask containing a magnetic stirring bar was added
FeCl2 (0.082 g, 0.65 mmol) and ZnI2 (0.21 g, 0.66 mmol),
filtered through
a medium-porosity glass frit to remove
precipitated KCl. The filtrate was evaporated to dryness and
the remaining solid fractionally sublimed (100-120 °C, 10-6
Torr) to give purple (Cp3i)2V (0.51 g, 1.2 mmol) in 80% yield,
mp 85-87 °C. Anal. Calcd for C28H46V: C, 77.56; H, 10.69.
Found: C, 77.29; H, 10.28. Principal IR bands (KBr, cm-1):
2964 (vs), 2871 (s), 1460 (s), 1380 (s), 1276 (m), 1180 (m), 1102
(m), 1029 (s), 816 (s), 420 (m). Magnetic susceptibility: µcorr
)
3.83 µB. X-ray-quality crystals of (Cp3i)2V were grown by slow
evaporation of a hexane solution.
Meth od B. A solution of VCl3(thf)3 prepared by refluxing
VCl3 (0.10 g, 0.64 mmol) in THF overnight, was placed into a
125 mL Erlenmeyer flask containing a magnetic stirring bar.
Excess aluminum powder was added (0.021 g, 0.78 mmol)
along with KH (0.01 g). After it was stirred for several hours,
the mixture turned blue-green, indicating the presence of the
[V2Cl3(thf)6]+ cation. To this was added K[Cp3i] (0.30 g, 1.3
mmol), and a purple solution developed within 10 min. After
this mixture was stirred at room temperature for several
hours, the solution was evaporated to dryness and the residue
extracted with hexanes (50 mL). Filtration through a medium-
porosity glass frit, followed by removal of the solvent, produced
a purple solid. This crude material was fractionally sublimed
as described above to give (Cp3i)2V (0.25 g, 0.58 mmol) in 89%
yield.
Meth od C. A THF solution of [V2Cl3(thf)6]2(Zn2Cl6) was
prepared by refluxing VCl3 in THF followed by reduction with
zinc powder (0.05 g, 0.76 mmol). To this solution was added
K[Cp3i] (0.30 g, 1.3 mmol), whereupon the mixture turned
purple. After the mixture was refluxed and stirred overnight,
the THF was removed by rotary evaporation. The residue was
extracted with hexanes (45 mL), and the resulting dark
solution was reduced to dryness to give dark purple (Cp3i)2V
in 85% yield, identified by its magnetic susceptibility and IR
spectrum.
Attem p ted Syn th esis of Bis(tetr a isop r op ylcyclop en -
ta d ien yl)va n a d iu m (II), (Cp 4i)2V. A THF solution of [V2Cl3-
(thf)6]2(Zn2Cl6) was prepared by refluxing VCl3 (0.10 g, 0.64
mmol) in THF followed by reduction with zinc powder (0.05 g,
0.76 mmol). To this solution was added K[Cp4i] (0.35 g, 1.5
mmol), whereupon the solution turned purple. After the
mixture was stirred overnight at room temperature, the THF
was removed by rotary evaporation. The residue was extracted
with hexanes (45 mL), and the resulting solution was evapo-
rated to leave a pink solid. A 1H NMR spectrum indicated that
the only identifiable species present was the zinc metallocene,
(19) King, R. B. Organometallic Syntheses; Academic: New York,
1965; Vol. 1.
(20) Ko¨nig, E.; Desai, V. P.; Kanellakopulos, B.; Dornburger, E. J .
Organomet. Chem. 1980, 187, 61-67.
(21) Fritz, H. P. Chem. Ber. 1959, 92, 780-791.
(18) Burkey, D. J .; Hanusa, T. P. J . Organomet. Chem. 1996, 512,
165-173.

Synthesis of Sterically Bulky Metallocenes
Organometallics, Vol. 18, No. 9, 1999 1665
Ta ble 1. Cr ysta l Da ta a n d Su m m a r y of X-r a y Da ta
Collection for (Cp 3i)2V
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for (Cp 3i)2Va
formula
fw
color of cryst
cryst dimens, mm
space group
a, Å
C28H46
433.61
purple
V
V(1)-C(2)
2.284(4)
2.267(4)
2.279(4)
2.278(4)
2.262(4)
1.928
C(2)-C(3)
1.422(6)
1.405(6)
1.421(6)
1.426(5)
1.411(6)
1.51(1)
V(1)-C(3)
C(2)-C(6)
V(1)-C(4)
C(3)-C(4)
0.85 × 0.33 × 0.35
V(1)-C(5)
C(4)-C(5)
P1h
V(1)-C(6)
C(5)-C(6)
9.178(4)
9.429(8)
8.797(2)
99.26(6)
116.96(2)
77.22(7)
660.8(7)
1
V(1)-ring centroid
CH-CH3 (av)
C(ring)-CH (av)
b, Å
c, Å
1.50(2)
CH3-CH-CH3 (av)
ring centroid-V(1)-ring centroid
ring normal-V(1)-ring normal
111.4(9)
180
180
R, deg
â, deg
γ, deg
V, Å3
a
The rings are planar to within 0.002 Å. The average displace-
ment of the methine carbon from the ring plane is 0.063 Å.
Z
D(calcd), g/cm3
radiation type
abs coeff, cm-1
transmissn factors
scan speed, deg/min
scan width
1.089
Cu KR
32.10
0.57-1.00
4.0
1.63 + 0.30 tan θ
20 e 2θ e 120
2099
1958
1742
0.060
0.080
3.29
0.03
0.37, -0.61
provided in Scheme 1. In attempts to synthesize iso-
propylated vanadocenes, we initially used a zinc-reduced
solution of vanadium(III) chloride in THF, which has
previously been shown to yield the heterobimetallic
complex [V2Cl3(thf)6]2(Zn2Cl6).3,4 Subsequent reaction of
this solution with the [Cp′]- anion usually forms the
corresponding vanadocenes.1,3,5,6
Unexpectedly, upon workup of the room-temperature
reactions of VCl3/Zn with the [Cp3i]- and [Cp4i]- anions,
no vanadium-containing compounds were isolated, and
the corresponding zincocenes (Cp3i)2Zn and (Cp4i)2Zn18
were recovered instead. However, mildly forcing condi-
tions (THF reflux) generated the isopropylated vana-
docenes from the VCl3/Zn/[Cpni]- mixtures (eq 2). Hex-
limits of data collcn (deg)
tot. no. of rflns
no. of unique rflns
no. of rflns with I > 3.0σ(I)
R(F)
Rw(F)
goodness of fit
max ∆/σ in final cycle
max/min peak (final diff peak)
(e/Å3)
followed by THF (40 mL). The solution was stirred until the
solids had dissolved. K[Cp3i] (0.30 g, 1.3 mmol) was added,
and the mixture immediately turned deep yellow; within
minutes, the mixture had become orange-brown. After this
mixture was stirred for 2 h, the solvent was removed by rotary
evaporation and the solid extracted with hexanes (40 mL).
Rotary evaporation of this solution gave an orange-red solid
identified as (Cp3i)2Fe by 1H NMR spectroscopy;22 no (Cp3i)2-
Zn was detected.
X-r a y Cr ysta llogr a p h y of (Cp 3i)2V. A suitable crystal of
(Cp3i)2V was located and sealed in a glass capillary tube. All
measurements were performed on a Rigaku AFC6S diffracto-
meter with graphite-monochromated Cu KR (λ ) 1.541 78 Å)
radiation. Relevant crystal and data collection parameters for
the present study are given in Table 1.
Cell constants and an orientation matrix for data collection
were obtained from systematic searches of limited hemispheres
of reciprocal space; sets of diffraction maxima were located
whose setting angles were refined by least squares. The space
group P1h was chosen from a consideration of unit cell param-
eters and statistical analysis of intensity distributions. Sub-
sequent solution and refinement of the structure confirmed
the choice.
Data collection was performed using continuous ω-2θ scans
with stationary backgrounds (peak/background counting time
2/1). Data were reduced to a unique set of intensities and
associated σ values in the usual manner. The structure was
solved by direct methods (SHELXS-86, DIRDIF) and Fourier
techniques. All non-hydrogen atoms were refined anisotropi-
cally. The hydrogen atoms were inserted in calculated positions
on the basis of packing considerations and d(C-H) ) 0.95 Å.
The positions were fixed for the final cycles of refinement.
Final difference maps were featureless. Selected bond dis-
tances and angles are listed in Table 2.
“VCl2” + 2K[Cpni] THF 8 (Cpni)2V + 2KClV (2)
reflux
anes extraction of the reaction mixture yielded crude
(Cp3i)2V as a purple solid. Fractional sublimation of the
solid produced analytically pure material that is ex-
tremely air- and moisture-sensitive despite the presence
of bulky substituents on the rings. The ring substituents
are able to block interaction with larger reagents,
however; no reaction is observed between (Cp3i)2V and
diphenylacetylene, for example, unlike the adduct formed
with Cp2V.23 The corresponding purple (Cp4i)2V must
be handled with slightly greater care but, with a
Kugelrohr apparatus, can be purified by sublimation as
for (Cp3i)2V.
A series of reactions was conducted in order to
investigate the origin of the zinc complexes isolated from
the room-temperature VCl3/Zn/[Cpni]- experiments. The
possibility that (Cpni)2V species were in fact the initially
formed products but that they subsequently reacted in
the presence of Zn(II) to generate zincocenes is unlikely
from the failure of separately prepared (Cp3i)2V to react
with Zn(II) (supplied by soluble ZnI2) (i.e., eq 3).
THF
(Cp3i)2V + Zn(II) room temp, 1 h8 no reaction
(3)
Conversely, we find that it is possible for zincocene to
transfer its rings to V(II) (generated from VCl3/Zn), as
in eq 4.
“VCl2” + Cp2Zn f Cp2V + ZnCl2
(4)
Resu lts
Syn th esis of Isop r op yla ted Va n a d ocen es. An
overview of the reactions conducted in this study is
If aluminum powder and a small amount of KH are
used to reduce VCl3, both (Cpni)2V vanadocenes are
(22) Sitzmann, H. J . J . Organomet. Chem. 1988, 354, 203-214.
(23) Petersen, J . L.; Griffith, L. Inorg. Chem. 1980, 19, 1852-1858.

1666 Organometallics, Vol. 18, No. 9, 1999
Sch em e 1. Su m m a r y of th e Rea ction s Con d u cted in Th is Stu d y
Overby et al.
vanadium does not occur, and instead (Cp4i)2VCl is
isolated in 60% yield as a brown-black viscous oil that
solidifies after standing for 1 week.
Sch em e 2. P r op osed Rou te for th e F or m a tion of
Va n a d ocen es fr om VCl3/Zn Mixtu r esa
Ma gn etic P r op er ties. The magnetic moments of the
vanadium complexes were measured using the Evans
NMR method in C6D6 solution.13-16 The experimentally
determined values for (Cp3i)2V (3.83 µB) and (Cp4i)2V
(3.91 µB) are close to the spin-only value of 3.87 µB
expected for complexes with three unpaired electrons.
This value is typical of other vanadocenes, e.g.: Cp2V,
3.78 µB;20 Cp*2V, 3.78 µB;25 (C5Ph4H)2V, 3.74 µB.6 The
experimentally determined value for (Cp4i)2VCl of 2.82
µB corresponds well to the expected value of 2.83 µB for
a d2 configuration with two unpaired electrons; it
compares favorably with those found for Cp*2VCl (2.78
µB) and Cp*2VI (2.79 µB).25
a
Zincocenes are likely intermediates, although they are not
always detected.
formed at ambient temperature (eq 5).
2K[Cpni
VCl3 + Al + KH
]8 (Cpni)2V + 2KClV (5)
THF
Solid -Sta te Str u ctu r e of (Cp 3i)2V. Owing to the
initial difficulty in forming (Cp3i)2V, we were interested
in determining whether its structure was in any way
atypical for a vanadocene. Crystals of (Cp3i)2V were
grown by slow evaporation of a hexanes solution. The
molecule displays a classic sandwich geometry, with
rigorously parallel rings by virtue of a crystallographi-
cally imposed inversion center at the vanadium atom.
Crystals of (Cp3i)2V are isomorphous with all other
structurally characterized bis(triisopropyl)cyclopenta-
dienyl complexes of the first-row transition metals (Cr-
Co).10,11,26 An ORTEP view of the molecule displaying
the numbering scheme used in the tables is provided
in Figure 1.
The average vanadium-carbon distance of 2.274(8)
Å is indistinguishable from the 2.269(6) and 2.268(4) Å
V-C distances in Cp2V27 and (C5Ph4H)2V,6 respectively.
The range of V-C distances in (Cp3i)2V (2.262(4)-2.284-
(4) Å (∆ ) 0.022 Å)) is also similar to that in Cp2V
(2.260(1)-2.278(1) Å (∆ ) 0.018 Å)); (C5HPh4)2V dis-
As a further check on the intermediacy of organozinc
species in these reactions, a VCl3/Zn/K[Cp4i] reaction in
THF was stopped after the mixture was stirred over-
night, and an aliquot of the solution was removed and
dried and the residue extracted with hexanes. Only
(Cp4i)2Zn was present in the sample (1H NMR). When
the same reaction was then continued with refluxing
overnight, (Cp4i)2V was isolated upon hexanes extraction
of the reaction.
An alternate route to vanadocenes that requires the
addition of a third equivalent of [Cp′]- as the reducing
agent was also investigated; it has been previously
employed to prepare Cp2V,19 (MeCp)2V,24 and Cp*2V.25
We find that it works equally well with both [Cp3i]- and
the bulky [C5Ph4H]- (eq 6). If 3 equiv of K[Cp4i] is
3K[Cp3i,C5Ph4H] + VCl3 THF8
(Cp3i,C5Ph4H)2V + 3KClV (6)
allowed to react with VCl3, however, reduction of the
(26) Hays, M. L.; Burkey, D. J .; Overby, J . S.; Hanusa, T. P.; Yee,
G. T.; Sellers, S. P.; Young, V. G., J r. Organometallics 1998, 17, 5521-
5527.
(27) Antipin, M. Y.; Lyssenko, K. A.; Boese, R. J . Organomet. Chem.
1996, 508, 259-262.
(24) Rettig, M. F.; Drago, R. S. J . Am. Chem. Soc. 1969, 91, 1361-
1370.
(25) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Inorg.
Chem. 1984, 23, 1739-1748.

Synthesis of Sterically Bulky Metallocenes
Organometallics, Vol. 18, No. 9, 1999 1667
but more highly charged Zn(II) center, followed by a
shift to the V(II) center. The results of the experiments
in eqs 3 and 4 indicate that Cp2Zn is an efficient
cyclopentadienyl ring transfer agent to V(II) but that
the converse reaction with (Cp3i)2V and Zn(II) does not
occur. These results are consistent with previous studies
that have established the high thermodynamic30 and
kinetic31,32 stability of the V-Cp bond. It is reasonable
to expect that zincocenes are previously undetected
intermediates in the synthesis of other vanadocenes
(Scheme 2).
That heat is necessary to drive the reactions with
VCl3/Zn/Cpni to form the vanadocenes suggests that the
isopropylated zincocenes reside in a local thermody-
namic well along the potential energy surface of the
reaction profile, a minimum that is missing or is less
pronounced with other zincocene intermediates. Owing
to the stability of the (Cpni)2Zn complexes, the temper-
ature of the reaction mixtures serves as a “switch” that
determines which metal eventually appears in the
product. Presumably, however, if the syntheses of other
vandocenes were conducted at sufficiently low temper-
atures, zincocenes could be isolated (or at least detected)
from their reaction mixtures.
F igu r e 1. ORTEP diagram of the non-hydrogen atoms of
(Cp3i)2V, giving the numbering scheme used in the text.
Thermal ellipsoids are shown at the 30% level.
Despite the greater amount of slippage of the rings
in (Cp4i)2Zn relative to other structurally characterized
zincocenes,18 the fact that the substituted zincocenes are
isolated instead of the vanadocenes is consistent with
enhanced thermodynamic stability for the former. This
hypothesis is supported by the failure to detect any
organoaluminum products when reactions are con-
ducted using aluminum metal as the reductant. If these
reactions followed the same path proposed for the zinc-
reduced reactions, organoaluminum complexes (e.g.,
“(Cpni)2AlCl”) would serve as intermediates. Compounds
of this type are likely to have σ-bonded, labile ligands,33
and should readily react with the [V2Cl3]+ moiety to
generate the more covalent (Cpni)2V complexes.
The role of steric interactions in determining the
outcome of these reactions is not easily analyzed. The
reaction given in eq 6, which employs an extra equiva-
lent of the cyclopentadienyl salt as the reducing agent
for V(III), forms (Cp3i)2V and (C5Ph4H)2V but not
(Cp4i)2V; the bis(cyclopentadienyl) chloride (Cp4i)2VCl is
isolated instead. Electronically, it is not obvious why
[Cp3i]- and [C5Ph4H]- should reduce V(III) but [Cp4i]-
will not. Nevertheless, it is not entirely reasonable that
steric effects should hinder the formation of (Cp4i)2V but
that the generation of the comparably encumbered (C5-
Ph4H)2V is unimpeded.
plays a slightly larger range of 2.234(4)-2.282(4) Å (∆
) 0.048 Å). The large effective size of V(II) (0.93 Å)28
may contribute to the small differences among the V-C
lengths, despite the varying kinds and numbers of
substituents on the cyclopentadienyl rings.
The rings in (Cp3i)2V adopt a perfectly staggered
orientation, with the isopropyl groups exhibiting angles
of 15.2, 71.3, and 65.2° (involving C(2), C(4), and C(5),
respectively) with respect to the cyclopentadienyl ring
plane. The closest inter-ring contact involving the
isopropyl groups is 4.14 Å (between C(9) and C(15′)),
which is outside the sum of the van der Waals radii for
two methyl groups (4.0 Å).29 The isopropyl methine
carbons are bent out of the ring plane by an average
displacement of 0.063 Å. The long V-C bonds coupled
with judicious arrangement of the isopropyl substituents
serve to make the molecule essentially strain-free.
Discu ssion
Under sufficiently forcing conditions (refluxing THF),
vanadocenes are reliably formed from VCl3/Zn/Cp′ mix-
tures regardless of the Cp′ ring involved (Cp*,5 Cp3i,
Cp,4i C5Ph4H,6 etc.). When the temperature is lowered,
differences begin to appear, and at room temperature
(Cpni)2V species are not formed. Although zinc-contain-
ing products have been isolated from reactions with [V2-
Cl3(thf)6]2(Zn2Cl6),8,9 zincocenes have not been previ-
ously reported from VCl3/Zn/[Cp′]- mixtures. The
isolation of the (Cpni)2Zn complexes raises two questions
about the reactions. (1) Are Cp′2Zn species involved in
other vanadocene syntheses using the Zn/VCl3 route?
(2) If so, why do (Cp3i)2Zn and (Cp4i)2Zn not readily
transfer their rings to form vanadocenes?
It is worth noting that related difficulties are encoun-
tered in forming (Cp3i)2Cr and (Cp4i)2Cr using a zinc-
reduced solution of CrCl3; as with the vanadium reac-
tions, the zincocenes (Cpni)2Zn are the sole products
isolated.11 In contrast to the vanadocenes, however, no
difficulty is encountered in preparing (Cp3i)2Cr or (Cp4i)2-
Cr when starting from a solution of CrCl2 in THF. Steric
The answer to the first question requires an estima-
tion of the relative rates of transfer of the [Cp′] anion
from Na+/K+ to V(II) and Zn(II). Rapid [Cp′]- ring
transfer could occur from Na+/K+ to the similarly ionic
(30) Chipperfield, J . R.; Sneyd, J . C. R.; Webster, D. E. J . Organomet.
Chem. 1979, 178, 177-189.
(31) Switzer, M. E.; Rettig, M. F. J . Chem. Soc., Chem. Commun.
1972, 687-688.
(32) Borisov, Y. A. Zh. Strukt. Khim. 1977, 18, 29-32; Chem. Abstr.
1977, 87, 52580e.
(28) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751-767.
(29) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960.
(33) Fisher, J . D.; Budzelaar, P. H. M.; Shapiro, P. J .; Staples, R.
J .; Yap, G. P. A.; Rheingold, A. L. Organometallics 1997, 16, 871-
879.

1668 Organometallics, Vol. 18, No. 9, 1999
Overby et al.
effects apparently do not impede the use of the Cr(II)/
Cpni combination, even though metallocene Cr-Cp′
bonds are usually shorter than analogous V-Cp′ bonds,
and the crowding should be somewhat greater in the
chromocenes.
is not a consequence of some peculiarity of the VCl3/Zn
mixture. The isopropylated zincocenes evidently possess
a higher degree of stability than might be expected from
the lability of other zincocenes.
Ack n ow led gm en t is made to the donors of the
Petroleum Research Fund, administered by the Ameri-
can Chemical Society, for support of this research.
Funds for the X-ray diffraction facility at Vanderbilt
University were provided through NSF Grant CHE-
8908065.
Con clu sion s
It is now apparent that [V2Cl3(thf)6]2(Zn2Cl6), gener-
ated from the zinc reduction of VCl3, can function as a
source of V(II) alone, V(II) and Zn(II) together,8,9 or Zn-
(II) alone. The isolation of (Cpni)2Zn from VCl3/Zn
solutions is not simply a consequence of the steric bulk
of the ligands or of the encapsulation of the metal center
usually provided by [Cpni]- ([Cp4i]2Zn displays a highly
slipped structure).18 As the zincocenes are also formed
in the course of reactions with CrCl3/Zn, their formation
Su p p or tin g In for m a tion Ava ila ble: Listings of atom
fractional coordinates and U values, bond distances and angles,
and anisotropic thermal parameters. This material is available
free of charge via the Internet at http://pubs.acs.org.
OM980691W