Ring-slipped (2,2′-Bipyridine)(η3-cyclopentadienyl) (η5-cyclopentadienyl)-vanadium(II) and its oxidation to (2,2′-bipyridine)bis(η5-cyclopentadienyl)-vanadium(III) tetraphenylborate

Article abstract of DOI:10.1002/ejic.200700904

Reaction of vanadocene(II) (4) with 2,2′-bipyridine (5) leads to (2,2′-bipyridine)vanadocene(II) (6), exhibiting a η³, η⁵-metallocene unit, proved by X-ray diffraction. By selective one-electron oxidation employing [Cp₂Fe]BPh₄ salts, the cationic (2,2′-bipyridine)vanadocene(III) derivative 7 is formed. It is characterized by a classical bent bis(η⁵-cyclopentadienyl) vanadium(III) structure. Characteristic structural details comparing changes in the oxidation states are discussed. The ring-slip parameters are compared to those of tungsten and other vanadium complexes. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700904

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700904
Ring-Slipped (2,2Ј-Bipyridine)(η3-cyclopentadienyl)(η5-cyclopentadienyl)-
vanadium(II) and Its Oxidation to (2,2Ј-Bipyridine)bis(η5-cyclopentadienyl)-
vanadium(III) Tetraphenylborate
Markus Jordan,[a] Wolfgang Saak,[a] Detlev Haase,[a] and Rüdiger Beckhaus*[a]
Keywords: Coordination modes / Vanadium / Oxidation / Metallocenes / 2,2Ј-Bipyridine / Haptotropic rearrangement
Reaction of vanadocene(II) (4) with 2,2Ј-bipyridine (5) leads
to (2,2Ј-bipyridine)vanadocene(II) (6), exhibiting a η3,η5-me-
tallocene unit, proved by X-ray diffraction. By selective one-
electron oxidation employing [Cp2Fe]BPh4 salts, the cationic
(2,2Ј-bipyridine)vanadocene(III) derivative 7 is formed. It is
characterized by a classical bent bis(η5-cyclopentadienyl)va-
nadium(III) structure. Characteristic structural details com-
paring changes in the oxidation states are discussed. The
ring-slip parameters are compared to those of tungsten and
other vanadium complexes.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
Since the synthesis and characterization of ferrocene by
Fischer and Wilkinson in 1952 the cyclopentadienyl unit
has become one of the most important ligands in organo-
metallic chemistry.[1,2] In addition to the common η5-coor-
dination mode (a), η3- (b), η1- (c), ionic (d) and bridging
forms (e) are known (Figure 1).[3]
Figure 2. Prominent examples of stucturally characterized ring-
slipped complexes: dicarbonyl(η3-cyclopentadienyl)(η5-cyclopen-
tadienyl)tungsten (1),[6] dicarbonyl(η3-indenyl)(η5-indenyl)vanadi-
um(II) (2)[7a] and the oxidized “non-slipped” form dicarbon-
ylbis(η5-indenyl)vanadium(III) tetraphenylborate (3).[7b]
On the other hand, due to their electronic and steric ver-
Figure 1. The most common of about ten bonding modes of the satility, aromatic N-heterocycles play an important role as
Cp ligand: (a) η5, (b) η3, (c) η1, (d) ionic, (e) bridging.
classical ligands in coordination chemistry,[8] as bridging li-
gands in binuclear derivatives[9] and as building blocks for
supramolecular compounds.[10] Beyond their capability to
connect metal centres by forming ligand-to-metal bonds,
they provide the opportunity of π-backbonding and thereby
may affect delocalization and transport of electrons.[11] We
have investigated the behaviour of bis(azine)s in the coordi-
nation sphere of low-valent titanium complexes, leading to
bi-,[12] tri-,[12a,13] tetra-[12a,14] and up to octanuclear deriva-
tives,[12a] accompanied by spontaneous C–C coupling reac-
tions resulting from the strong reducing properties of titani-
um(II).[12a,13] Here we wish to report an example of the be-
haviour of 2,2Ј-bipyridine in the coordination sphere of va-
nadium(II) and vanadium(III). Floriani et al. described the
formation of a [Cp2V(2,2Ј-bipyridine)] derivative. However,
because of the lack of structural data the authors assumed
a monodentate coordination mode for the 2,2Ј-bipyridine
ligand (17e– complex) (Figure 3).[15]
Bonding modes of special interest are η3-Cp and η1-Cp
due to their interconversion chemistry, the so-called “ring
slippage”.[3] Whereas the first η1-Cp transition-metal com-
plex was reported only three years after the characterization
of ferrocene,[4] the η3-coordination mode was discussed for
several years. Generally, Cp-ligand isomerizations often
play a significant role in the chemistry of their metal com-
plexes.[5] Prominent examples of ring-slipped compounds
are given in the case of complexes exhibiting an electron
excess (Ͼ 18) like dicarbonyl(η3-cyclopentadienyl)(η5-cyclo-
pentadienyl)tungsten (1),[6] or particularly for indenyl com-
plexes such as those shown in Figure 2.[7]
[a] Institute of Pure and Applied Chemistry, Carl von Ossietzky
University,
P. O. Box 2503, 26111 Oldenburg, Germany
Fax: +49-441-798-3581
E-mail: ruediger.beckhaus@uni-oldenburg.de
5168
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5168–5172

(2,2Ј-Bipyridine)(η3-cyclopentadienyl)(η5-cyclopentadienyl)vanadium(II)
Figure 3. Assumed structure of [Cp2V(2,2-bipyridine)] by Floriani
et al.[15]
Results and Discussion
First at all, we reproduced the reaction of vanadocene(II)
(4) and 2,2Ј-bipyridine (5)[15] extended by the structural
characterization of the reaction product 6. In contrast to
the proposed monodentate structure, the 2,2Ј-bipyridine li-
gand was found to act as a typical chelating unit. Addition-
ally, one of the Cp rings exhibit an η3-coordination mode
(ring slippage) (Scheme 1). Upon oxidation of complex 6 by
ferrocenium tetraphenylborate, the Cp ring flips back to
η5-coordination to form (2,2Ј-bipyridyl)bis(η5-cyclopen-
tadienyl) vanadium(III) tetraphenylborate (7) (Scheme 2). A
number of slip parameters were applied to these structures,
and the results were compared with given examples of ring-
slipped complexes such as 1 and 2.
Figure 4. ORTEP plot of the solid-state molecular structure of
compound 6. Hydrogen atoms are omitted for clarity; thermal el-
lipsoids are drawn at the 50% probability level. Selected bond
lengths [Å] and angles [°]: V1–N1 2.036(1), V1–N2 2.053(1), C5–
C6 1.423(2), V1–Ct 2.01, N–V–N 76.89(5).
Furthermore the C11 atom slightly turns out of the plane
formed by C12–C15 to distort the otherwise rather aro-
matic planar Cp ring to an envelope-like conformation. The
angle Ω between plane C12–C13–C14–C15 and plane C11–
C12–C15 is 8.3° and is defined by Faller, Crabtree and
Habib as one of a number of slip parameters (Figure 5).[16]
Scheme 1. Formation and structure representation of compound 6.
Figure 5. Graphical definition of the dihedral angle Ω, the slip
Ǟ
vector S, whose norm is the slip distortion ∆, and the angle slip
Ψ, which is the angle between the plane normal and the centroid–
metal vector.
Synthesis and Characterization of (2,2Ј-Bipyridine)(η3-
cyclopentadienyl)(η5-cyclopentadienyl)vanadium(II) (6)
This distortion seems to be typical for η3-coordinated
cyclopentadienyl ligands, as can be seen in Table 1.[6,16] In
Vanadocene(II) (4) and 2,2Ј-bipyridine (5) were dissolved contrast to that, the η5-coordinated Cp ring shows an angle
in n-hexane, and the solution was stirred at room tempera- Ω of only 1.2° between plane C17–C18–C19–C20 and plane
ture overnight. The colour of the solution changed immedi- C16–C17–C20, leading to the expected planarity of a Cp
ately from violet to black. The solution was cooled to ring.
–70 °C, and complex 6 precipitated in form of black crystals
From the average of the M–C distances a further slip
(53%).
parameter, ∆M–C, can be calculated.[16] This parameter is
Suitable crystals for X-ray diffraction became available a direct verification for the η3-coordinated Cp ligand. While
by crystallization at –16 °C. Complex 6 crystallizes in the every carbon atom is almost equidistant from the metal in
space group R3; the trigonal unit cell contains 18 molecules η5-Cp, so that ∆M–C is very small, in η3-Cp two carbons
¯
of the compound. X-ray diffraction of these black flat have a significantly elongated distance to the metal centre.
prisms confirmed the bidentate chelate ligation of the 2,2Ј- This confirms the molecular structure of 6 in which ∆M–C
bipyridine (5). However, one of the Cp rings offered only of the η5-Cp is 0.04 Å, whereas the η3-Cp has a much
η3-coordination. This should be necessary to confirm the higher value of 0.56 Å. Also the slip distortion ∆ and the
18e– rule in 6. If both Cp rings were η5-coordinated, the angle slip Ψ (Figure 5) show clear indications for η3-coordi-
complex would contain 19 electrons. With one η3-coordi- nation modes.[16]
nated Cp ring, the preferred 17e– structure of 6 was formed.
The ORTEP plot (Figure 4) significantly shows the slippage atives (1,[6]2[7a]) selected structural ring-slip parameters are
of one Cp ring with respect to the other. summarized in Table 1.
For the 2,2Ј-bipyridine complex 6 and comparable deriv-
Eur. J. Inorg. Chem. 2007, 5168–5172
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5169

M. Jordan, W. Saak, D. Haase, R. Beckhaus
SHORT COMMUNICATION
Table 1. Slip parameters (see Figure 5) for compounds 6 and 7 and
similar complexes.
Compound
Slip parameters
Ref.
∆ = |ǞS| [Å]
Ψ [°] ∆M–C [Å] Ω [°]
[6,16]
[7a]
[c]
1
2
6
[η3]
[η5]
[η3]
[η5]
[η3]
[η5]
0.928
0.076
0.798
0.157
0.876
0.063
0.110
23.4 0.62
2.2 0.10
20.9 0.56
4.6 0.13
21.9 0.56
19.7
(4.9)[a]
–
Scheme 2. Formation of compound 7.
–
8.3
1.2
0.1
1.5
3.2
0.04
0.03
[c]
7[b] [η5]
[a] The uncertainty for this value is large. [b] Slip parameters of
only one Cp were calculated because of the disordering of the
others. [c] This work.
The V–N bonds of 6 are shortened relative to the only
known (2,2Ј-bipyridine)vanadocene complex (2,2Ј-bipyr-
idine)bis(η5-cyclopentadienyl)vanadium(IV) ditriflate (8) of
Ghosh et al.,[17] but the shortening of the C5–C6 bond of
the 2,2Ј-bipyridine (5) in the complex relative to the free
5[18] is more significant (Table 2). This shortening suggests
the reduced nature of the ligand in complex 6. In the neu-
tral vanadium(II) complex 6 and the cationic vanadium(III)
derivative 7 the bipyridine ligand is nearly located in the
equatorial plane (N–V–N) with a deviation of 5.35° (6) and
2.25° (7), respectively. These values are significantly lower
than the corresponding value of 23.09° in [(Cp)2Ti(2,2Ј-bi-
pyridine)].[19]
Figure 6. ORTEP plot of the solid-state molecular structure of 7
showing one ion pair of the asymmetric unit with idealized Cp
rings, which are disordered in two positions and refined with iso-
tropic coupled displacement parameters. Hydrogen atoms are omit-
ted for clarity; thermal ellipsoids are drawn at the 50% probability
level. Selected bond lengths [Å] and angles [°]: V1–N1 2.161(2), V1–
N2 2.165(2), C5–C6 1.466(4), V1–Ct 1.95, B1–C 1.66, Ct–V1–Ct
135.4, N1–V1–N2 74.2.
Table 2. Selected bond lengths [Å] for 5, 6, 7 and 8.
5
6
7
8
V1–N1
V1–N2
C5–C6
–
–
2.036(2)
2.053(1)
1.423(3)
2.161(2)
2.165(2)
1.466(4)
2.129(2)
2.128(2)
1.465(4)
Because of the disordering, the slip parameters for only
one Cp ring, which is not disordered, were calculated
(Table 1). The parameters are in good accordance with the
slip parameters of the η5-coordinated Cp rings of the other
compounds in Table 1.
1.490(3)
The V–Ct distances are insignificantly shorter than those
in complex 6 (1.95 Å), but the V–N bonds as well as the
connecting C5–C6 bond between the two heterocycles are
lengthened relative to those in complex 6 (Table 2). In this
case the heterocyclic ligand is not reduced by the metal
centre. The V–N bond of 7 is a little longer relative to that
in compound 8, but the C5–C6 bond is almost equal in
length (Table 2).[17]
Synthesis and Characterization of (2,2Ј-Bipyridine)bis(η5-
cyclopentadienyl)vanadium(III) Tetraphenylborate (7)
Direct oxidation of 6 with ferrocenium tetraphenylborate
leads to products that are insoluble in common solvents.
So recrystallization of red-violet ionic complex 7 was not
successful. In contrast to that, vanadocenium tetraphen-
ylborate (9), prepared by the reaction of 4 with ferrocenium
tetraphenylborate,[20] is passably soluble in thf. But when 5
was added directly to 9, an amorphous powder precipitated,
too.
Conclusions
A successful procedure is layering of an n-hexane solu-
The reaction of 4 with 5 formed the 17e– chelate complex
tion of 5 on a thf solution of 9 to form large red-violet (2,2Ј-bipyridine)(η3-cyclopentadienyl)(η5-cyclopentadienyl)-
needles of 7,which were suitable for X-ray diffraction, in vanadium(II) (6) with one η3-slipped Cp ring to avoid a
44% yield at the phase interface (Scheme 2). The ionic com- 19e– complex formation. Upon formal oxidation of 6, the
pound crystallizes in the monoclinic space group P21/c; the Cp ring flipped back to an η5-coordination mode, forming
unit cell contains eight ion pairs and there are two indepen- the ionic 18e– species (2,2Ј-bipyridine)bis(η5-cyclopen-
dent ion pairs in the asymmetric unit. Three of four Cp tadienyl)vanadium(III) tetraphenylborate (7). To the best of
rings in an asymmetric unit are disordered; however, both our knowledge this is one of the rare examples of structur-
rings in the cation are η5-coordinated as can be seen in the ally characterized (cyclopentadienyl)vanadium complexes
ORTEP plot in Figure 6.
with a structurally verified η3-ring-slippage.
5170
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 5168–5172

(2,2Ј-Bipyridine)(η3-cyclopentadienyl)(η5-cyclopentadienyl)vanadium(II)
(20 mL) was stirred at room temperature overnight. The colour of
the solution changed immediately from violet to black. Black crys-
tals of 6 were obtained by cooling the solution at –70 °C for several
days. They were isolated by decanting the solution, washing with
cold n-hexane and drying in vacuo. Crystals suitable for X-ray dif-
fraction were obtained by cooling the solution to –16 °C. Yield:
178 mg (53%). M.p.: 86 °C (dec.). MS (CI, isobutane): m/z (%) =
Experimental Section
Reagents and General Techniques: All operations were performed
in a nitrogen atmosphere with rigorous exclusion of oxygen and
moisture by using glovebox and Schlenk techniques. All chemicals
used were of reagent grade or higher purity; they were purified
according to standard protocols. Solvents were distilled from Na/
K alloy and benzophenone under nitrogen. Chemical ionization
(CI) mass spectra were taken with a Finnigan-MAT 95 spectrome-
ter. Isobutane was used as the ionization gas. Electrospray ioniza-
tion (ESI) mass spectra were recorded with a Finnigan-LCQ spec-
trometer. High-resolution electrospray ionization (HR-ESI) mass
spectra were measured with a Micromass Premier quadrupole-
time-of-flight (Q-TOF) instrument. IR spectra were recorded with
a Bruker VECTOR 22 spectrometer by using KBr pellets. Elemen-
tal analyses were carried out by the Analytische Laboratorien
GmbH, Lindlar (Germany). Melting points were determined by
using a “Mel-Temp” by Laboratory Devices, Cambridge. 2,2Ј-Bi-
pyridyl (5) was purchased from Merck. Vanadocene (4)[21] and van-
adocenium tetraphenylborate (9)[20] were prepared according to lit-
erature procedures.
337.2 (46) [M]+, 157.2 (100) [(2,2Ј-bpy-H)]+. IR (KBr): ν = 3044
˜
(w), 1571 (s), 1504 (s), 1454 (s), 1417 (m), 1354 (w), 1301 (m), 1254
(m), 1147 (m), 1124 (w), 1082 (w), 1015 (m), 955 (m), 818 (m), 806
(m), 784 (m), 764 (m), 741 (s) cm–1. C20H18N2V (337.30): calcd.
C 71.21, H 5.38, N 8.30; found C 70.97, H 5.30, N 8.34.
Synthesis of (2,2Ј-Bipyridine)bis(η5-cyclopentadienyl)vanadium(III)
Tetraphenylborate (7): A solution of vanadocenium tetraphenylbo-
rate (9) (61 mg, 0.116 mmol) in thf (40 mL) was layered with an n-
hexane solution (20 mL) of 2,2Ј-bipyridine (5) (20 mg,
0.127 mmol). After two weeks, complex 7 was obtained by de-
canting the solution as large red-violet needles, which were suitable
for X-ray diffraction. Yield: 35 mg (46%). M.p.: 220–222 °C (dec.).
MS (ESI, CH3CN): pos.: m/z (%) = 337.1 (100), 338.1 (18), 339.3
(2) [Cp2V(2,2Ј-bipy)]+, 168.7 (83) [Cp2V(2,2Ј-bipy)]2+; neg.: m/z (%)
= 318.5 (24), 319.3 (100), 320.3 (24) [BPh4]–. HR-MS (ESI-Q-TOF,
CH3CN): pos.: m/z (%) = 337.0906 (100) [Cp2V(2,2Ј-bipy)]+,
X-ray Diffraction: Single-crystal experiments were carried out with
a STOE IPDS diffractometer with graphite-monochromated Mo-
Kα radiation (λ = 0.71073 Å). The structures were solved by direct
phase determination and refined by full-matrix least-squares tech-
niques against F2 with the SHELXL-97 program system.[22] Crys-
tallographic details are given in Table 3. CCDC-649843 (6) and
-649844 (7) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
168.5456 (42) [Cp2V(2,2Ј-bipy)]2+. IR (KBr): ν = 3053 (m), 2963
˜
(m), 1601 (m), 1477 (m), 1442 (m), 1425 (m). 1261 (s), 1242 (s),
1155 (s), 1103 (br), 1017 (br), 945 (m), 802 (br), 769 (m), 733 (s),
703 (s), 657 (m), 610 (m) cm–1. C44H38BN2V (656.51): calcd. C
80.49, H 5.83, N 4.27; found C 79.86, H 5.85, N 4.83.
Acknowledgments
Table 3. Crystal structure data for compounds 6 and 7.
We would like to thank the Deutsche Forschungsgemeinschaft
(DFG, SPP-1118) for financial support.
6
7
Empirical formula
Formula weight
C20H18N2V
337.30
C44H38BN2V
656.51
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Colour
Crystal dimensions [mm]
Crystal system
Space group
A [Å]
B [Å]
C [Å]
α [°]
β [°]
black
blue-violet
0.42ϫ0.31ϫ0.08 0.45ϫ0.34ϫ0.13
trigonal
monoclinic
P21/c
19.1779(10)
18.4200(8)
19.2887(10)
90
95.883(8)
90
6778.0(6)
8
1.287
0.328
153
2.12 to 26.13
64 254
¯
R3
19.0200(7)
19.0200(7)
23.3811(11)
90
90
120
7325.1(5)
18
1.376
0.610
153
γ [°]
V [Å3]
Z
Dcalcd. [gcm–3
µ [mm–1]
T [K]
]
θ range [°]
No. of reflections collected
2.62 to 26.21
23 607
No. of independent reflections 3247
[R(int) = 0.0525]
2474
13 354
[R(int) = 0.0851]
7582
No. of reflections [IϾ2σ(I)]
Max., min. transmission
Final R indices [IϾ2σ (I)]
0.9529 and 0.7839 0.9902 and 0.9525
R1 = 0.0274
wR2 = 0.0633
R1 = 0.0429
wR2 = 0.0670
R1 = 0.0440
wR2 = 0.0871
R1 = 0.0934
wR2 = 0.0977
R indices (all data)
Synthesis of (2,2Ј-Bipyridine)(η3-cyclopentadienyl)(η5-cyclopen-
tadienyl)vanadium(II) (6): A solution of vanadocene (4) (181 mg,
1 mmol) and 2,2Ј-bipyridine (5) (190 mg, 1.2 mmol) in n-hexane
Eur. J. Inorg. Chem. 2007, 5168–5172
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5171

M. Jordan, W. Saak, D. Haase, R. Beckhaus
SHORT COMMUNICATION
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