The first structurally characterised homoleptic organovanadium(III) compound

Article abstract of DOI:10.1039/b106364m

The homoleptic, tetrahedral, paramagnetic d² species [NBu₄][V^III(C₆Cl₅)₄] 1 has been obtained and characterised by analytical, spectroscopic and X-ray diffraction methods.

Full text of DOI:10.1039/b106364m

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The first structurally characterised homoleptic organovanadium(III
)
compound†
Pablo J. Alonso, Juan Forniés,* M. Angeles García-Monforte, Antonio Martín and Babil Menjón
Instituto de Ciencia de Materiales de Aragón, Facultad de Ciencias, Universidad de Zaragoza–C.S.I.C.,
Pza. S. Francisco s/n, E-50009 Zaragoza, Spain. E-mail: forniesj@posta.unizar.es
Received (in Cambridge, UK) 17th July 2001, Accepted 4th September 2001
First published as an Advance Article on the web 3rd October 2001
The homoleptic, tetrahedral, paramagnetic d2 species
[NBu4][VIII(C6Cl5)4] 1 has been obtained and characterised
by analytical, spectroscopic and X-ray diffraction meth-
ods.
The organometallic chemistry of vanadium is chiefly dominated
by species containing carbonyl and cyclopentadienyl ligands.1
In contrast, simple, homoleptic s-organovanadium compounds
unsupported by cyclopentadienyl or other donor ligands are still
rare. The chemistry of these derivatives has, however, received
renewed attention especially because some organovanadium(II
)
or -(III) species appear as active intermediates in chemical
fixation processes of molecular nitrogen.2 It is thus thought that
the formation of [(VR3)2(m-N2)] is likely to occur via the
homoleptic intermediates VR3 [R = C6H2Me3-2,4,6 (mes)3 or
CH2But 4]. To the best of our knowledge, the only homoleptic
organovanadium(III) compounds that have been isolated to date
are: anionic [VR4]2 [R = mes5 or C6H3(OMe)2-2,66] and
neutral [VR3] [R = mes,3 C(CN)37 or CH(SiMe3)2 ]. However,
8
Fig. 1 Thermal ellipsoid diagram of the anion of 1. Selected distances (pm)
and angles (°): V–C(1) 214.4(5), V–C(7) 215.8(6), V–C(13) 215.2(5), V–
C(19) 214.2(5), V–Cl(2) 364.6(2), V–Cl(6) 311.6(2), V–Cl(8) 311.5(2), V–
Cl(12) 366.3(2), V–Cl(14) 366.3(2), V–Cl(18) 313.6(2), V–Cl(20)
304.9(2), V–Cl(24) 368.3(2); C(1)–V–C(7) 112.1(2), C(1)–V–C(13)
117.6(2), C(1)–V–C(19) 101.2(2), C(7)–V–C(13) 98.1(2), C(7)–V–C(19)
116.1(2), C(13)–V–C(19) 112.6(2).
unequivocal structural information is not available for any of
them. We now report the synthesis and structural character-
isation of [NBu4][V(C6Cl5)4] 1, a remarkably stable homoleptic
organovanadium(III) compound.
The reaction of [VCl3(thf)3]9 with LiC6Cl510 in a 1+8 molar
ratio followed by the appropriate treatment‡ allows the isolation
of [NBu4][V(C6Cl5)4] 1 as a deep green solid. Once isolated,
complex 1 is air- and moisture-stable in the solid state. This
behaviour is in contrast with the reported observation of the ease
with which the related compound [Li(thf)4][V(mes)4] is air-
oxidised to give neutral [V(mes)4].12 Complex 1 behaves as a
1+1 electrolyte in dilute acetone solution (3.1 3 1021
mol dm23).13
rings is only moderate; (ii) the ortho-Cl–V distances are too
long to be considered as the result of a bonding interaction; and
(iii) the central core is well described as a slightly distorted
tetrahedron with no evidence for the need of a higher
coordination polyhedron.
Complex 1 has also been studied by EPR spectroscopy both
in the X- and Q-bands.¶ Frozen solutions of 1 in THF/CH2Cl2
mixtures gave poor-quality spectra with very low signal-to-
noise ratios. However, the X-band EPR spectrum of a powder
sample of 1 shows four clearly defined features at room
temperature, located at ca. 120, 280, 670 and 840 mT (Fig. 2).
At lower temperatures, the two outer signals shift downfield,
whereas the central ones remain almost unchanged.
The solid-state structure of 1 has been established by single-
crystal X-ray diffraction analysis.§ The structure of the anion
[V(C6Cl5)4]2 is depicted in Fig. 1. It can be seen that all four s
V–C bonds are virtually identical, while the C–V–C angles take
values between 98.1(2) and 117.6(2)°. Hence, the local
geometry around the vanadium centre can be described as
slightly distorted tetrahedral (cf. ideal C–V–C angle: 109.5°).
The V–C distances in 1 [ranging from 214.2(5) to 215.8(5) pm]
are similar to those found in other heteroleptic s-arylvana-
dium(III) compounds, such as [V{C6H2(CF3)-2,4,6}2Cl(thf)]
[V–C 214.5(4) and 215.9(4) pm],15 [PF(NEt2)3][V(mes)3F] [V–
C
210.7(4)–213.2(4) pm]16 and [V(mes)3(thf)] [V–C
209.5(6)–211.7(7) pm].17 The V–Cipso–Cortho angles within
each pentachlorophenyl ring are dissimilar with values amount-
ing to ca. 115 and ca. 130°, respectively. The swing of the aryl
rings also implies the existence of two sets of ortho-Cl–V
distances each approaching 310 and 366 pm, respectively.
These structural features have been previously used as criteria
which help to determine the existence of ortho-Cl…M inter-
actions in other pentachlorophenyl metal derivatives.18 Bearing
in mind these criteria we suggest the non-existence of additional
ortho-Cl…V interactions in 1 since: (i) the swing of the C6Cl5
† Dedicated to Professor Dr Rafael Uso´n on the occasion of his 75th
birthday.
Fig. 2 X-band EPR spectrum of a powder sample of 1 at room temperature
(microwave frequency 9.545 GHz): experimental (a) and calculated (b).
2138
Chem. Commun., 2001, 2138–2139
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106364m

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Vanadium(III) is a non-Kramers paramagnetic system (d2),
which is usually ‘EPR silent’ unless located in a high-symmetry
site such as cubic, octahedral or tetrahedral. When located in a
§ Crystal data for 1: C40H36Cl20NV, M = 1290.64, orthorhombic, space
group Pbca (no. 61), a = 1938.00(10), b = 2125.86(13), c = 2590.52(14)
pm, U = 10.6727(10) nm3, T = 293(2) K, graphite-monochromated Mo-
Ka radiation, l = 71.073 pm, Z = 8, Dc = 1.606 g cm23, F(000) = 5168,
measured absorption correction based on y scans, Enraf-Nonius CAD4
diffractometer, 9361 intensity data collected, 9358 unique (Rint = 0.0031)
which were used in all calculations. The structure was solved by Patterson
and Fourier methods and refined anisotropically by full-matrix, least
squares on F2 (program SHELXL 93; ref 14) to final values of R1 = 0.0589
[for 5228 data with I > 2s(I)] and wR2 = 0.1398 (all data) for 559
parameters.
CCDC reference number 167901. See http://www.rsc.org/suppdata/cc/
b1/b106364m/ for crystallographic data in CIF or other electronic format.
¶ EPR spectra were measured using a Bruker ESP388E spectrometer
equipped with an ERV411T variable temperature device. The microwave
frequency was determined with a Hewlett/Packard HP5350B frequency
counter and the magnetic field strength was measured with a Bruker
ER035M gaussmeter.
3
tetrahedral site, the F ground term in the free V(III) ion splits
3
into two triplets (3T1 and T2) and one singlet (3A2), the latter
being the lowest energy level. Since the V(III) centre in 1 is in
a nearly tetrahedral environment, the ground state orbital wave
function is expected to be close to a 3A2 orbital with an isotropic
g-factor and the spin-Hamiltonian given in eqn. (1).∑
r r




H = gmBB⋅S + D SZ2 − 1 S(S +1) + E(SX2 − SY2 )
(1)
3


The analysis of the EPR spectrum of 1 using eqn. (1) gives a
satisfactory agreement between the calculated [Fig. 2(b)] and
experimental [Fig. 2(a)] spectra with the following values of the
spin-Hamiltonian parameters: g = 1.98(1), D = 13.59(5) GHz
and h = 0.16(1). It is interesting to note that these values also
allow the reproduction of the Q-band spectrum.
∑ S = 1, mB is the Bohr magneton, (X, Y, Z) are the principal axes of the
second rank tensor describing the zero-field splitting (ZFS) effects due to
the admixture of the triplet orbitals to the ground state singlet. This mixture
is a consequence of both the spin-orbit and the non-cubic crystal field
contributions and is usually described by the D and E parameters, being 0
@ h = E/D @ 1/3.
In contrast to the profusion of EPR reports involving V(IV
)
species (d1), there are very few EPR data available for V(III).
These data have been obtained on samples of V(III) doped into
ionic lattices (e.g. CaF2:V, g = 1.93319 and CdF2+V, g =
1.93720) and into semiconductors (g = 1.92–1.99).21 Recently,
a detailed EPR study on samples of V(III) doped into
CsGa(SO4)2·12H2O has been reported using high-field, multi-
frequency techniques. The spectra thus obtained for the
[V(H2O)6]3+ cation have been analysed with an axial spin-
Hamiltonian with g∑ = 1.955, g4 = 1.869 and D = 143
GHz.22
1 N. G. Connelly, in Comprehensive Organometallic Chemistry, ed. G.
Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon Press, Oxford,
1982, vol. 3, ch. 24, pp. 647–704; P. Berno, S. Gambarotta and D.
Richeson, in Comprehensive Organometallic Chemistry II, ed. E. W.
Abel, F. G. A. Stone and G. Wilkinson, Elsevier Science Ltd., Oxford,
1995, vol. 5, ch. 1, pp. 1–55.
2 G. J. Leigh and J. S. de Souza, Coord. Chem. Rev., 1996, 154, 71.
3 R. Ferguson, E. Solari, C. Floriani, D. Osella, M. Ravera, N. Re, A.
Chiesi-Villa and C. Rizzoli, J. Am. Chem. Soc., 1997, 119, 10104.
4 J.-K. F. Buijink, A. Meetsma and J. H. Teuben, Organometallics, 1993,
12, 2004.
5 W. Seidel and G. Kreisel, Z. Anorg. Allg. Chem., 1976, 426, 150.
6 W. Seidel, P. Scholz and G. Kreisel, Z. Anorg. Allg. Chem., 1979, 458,
263.
7 H. Koehler, P. Laub and A. Frischkorn, J. Less-Common Met., 1971, 23,
171.
The moderate departure of the g value obtained for 1 from the
free electron value can be attributed to some degree of
delocalisation of the unpaired electrons over the aryl ligands in
the [V(C6Cl5)4]2 anion. This assumption, however, cannot yet
be confirmed, since the hyperfine structure expected to appear
in 1 (51V, I = 7/2, 100% natural abundance) is unresolved due
to the broadness of the features in the EPR spectrum.
The remarkable stability of 1 has allowed the first structural
characterisation of a homoleptic organovanadium(III) derivative
to be obtained. Moreover, this is, to the best of our knowledge,
the first V(III) compound which has a defined EPR spectrum and
in which the vanadium centre is not a doping but rather a
constituent element of the sample. On the other hand, the
inertness of 1 could well act as a drawback for its chemical
reactivity which is currently under study.
We thank Professor P. H. Rieger (Brown University,
Providence) for helpful EPR discussions and suggestions. We
also thank the Dirección General de Enseñanza Superior
(Projects PB98-1595-C02-01 and PB98-1593) for financial
support and the Gobierno de Aragón for a grant to
M. A. G.-M.
8 G. K. Barker, M. F. Lappert and J. A. K. Howard, J. Chem. Soc., Dalton
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9 B. Heyn, B. Hipler, G. Kreisel, H. Schreer and D. Walther, Anorga-
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Notes and references
16 M. R. Kopp and B. Neumüller, Z. Anorg. Allg. Chem., 1999, 625,
‡ Experimental procedure: to a solution of LiC6Cl5 (ca. 55 mmol) in Et2O
(60 cm3) at 278 °C was added [VCl3(thf)3] (2.54 g, 6.80 mmol). The
suspension was allowed to warm up to room temperature and, after 15 h of
stirring, the by then deep green solid was filtered, washed with Et2O (3 3
5 cm3) and extracted in CH2Cl2 (50 cm3). The extract was evaporated to
dryness and the resulting residue was redissolved in PriOH (80 cm3) and
filtered. The addition of NBu4Br (4.4 g, 13.6 mmol) to the filtrate caused the
precipitation of a first fraction of 1 as a deep green solid. By standing the
mother liquor at 230 °C overnight, a second crop of 1 was obtained (45%
overall yield). Anal. found for C40H36Cl20NV: C 37.28, H 2.80, N 0.65.
Calc.: C 37.22, H 2.80, N 1.08%. IR(KBr)/cm21: 1482m, 1458w, 1379w,
1321s, 1312s, 1283vs, 1224m, 1145w, 1063s, 1025w, 883w (NBu4+), 828s
(C6Cl5: X-sensitive vibr.),11 740w (NBu4+), 669s and 346w. FAB-MS: m/z
1039 [V(C6Cl5)4]2, 827 [V(C6Cl5)3Cl]2, 792 [V(C6Cl5)3]2, 615
363.
17 M. Vivanco, J. Ruiz, C. Floriani, A. Chiesi-Villa and C. Rizzoli,
Organometallics, 1993, 12, 1794.
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Güdel, Inorg. Chem., 1999, 38, 5928.
[V(C6Cl5)2Cl2]2 and 545 [V(C6Cl5)2]2. LM(acetone)
mol21
=
108.1 S cm2
.
Chem. Commun., 2001, 2138–2139
2139