The synthesis and characterization of [IMesH] +[(η3-C5H5)V(N)Cl 2]-: An anionic vanadium(v) complex with a terminal nitrido ligand

Article abstract of DOI:10.1039/b922744j

A series of new half-sandwich vanadium complexes have been prepared. The structures of two new anionic vanadium half-sandwich complexes, [CpVCl ₃]^- and [CpV(N)Cl₂]^- are presented. ¹⁵N isotopic labelling studies have been conducted to unambiguously assign the VN infra red stretching frequencies of both a neutral and an anionic (cyclopentadienyl)vanadium nitrido complex. The influence of strongly π-basic coligands on the hapticity of the cyclopentadienyl ligands in half-sandwich complexes of vanadium is discussed.

Full text of DOI:10.1039/b922744j

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The synthesis and characterization of [IMesH]+[(g3-C5H5)V(N)Cl2]-: An
anionic vanadium(V) complex with a terminal nitrido ligand†
Carl E. Johnson,a Everett A. Kysor,a Michael Findlater,b Jerry P. Jasinski,a Andrew S. Metell,a
Jonathan W. Queena and Colin D. Abernethy*a
Received 30th October 2009, Accepted 10th February 2010
First published as an Advance Article on the web 1st March 2010
DOI: 10.1039/b922744j
A series of new half-sandwich vanadium complexes have been prepared. The structures of two new
anionic vanadium half-sandwich complexes, [CpVCl3]- and [CpV(N)Cl2]- are presented. 15N isotopic
≡
labelling studies have been conducted to unambiguously assign the V N infra red stretching
frequencies of both a neutral and an anionic (cyclopentadienyl)vanadium nitrido complex. The
influence of strongly p-basic coligands on the hapticity of the cyclopentadienyl ligands in half-sandwich
complexes of vanadium is discussed.
activity of metal-nitrogen multiple bonds; in particular, such
Introduction
d-
highly polar Md+
N
bonds may show useful and unusual
≡
The many interesting and useful physical properties displayed by
transition-metal nitride compounds have provided the motivation
for ongoing efforts to prepare and characterize new examples of
such complexes.1 However, while many nitride complexes of the
mid transition-metals (Groups 6, 7 and 8) have been prepared,
relatively few examples of early transition-metal (Groups 4 and
5) nitride complexes have been studied. The principal reason for
this disparity is that, while mid transition-metal nitride complexes
are typically mononuclear and feature terminal nitrido ligands, in
≡
reactivity towards unsaturated organic substrate molecules
analogous to that observed for early transition-metal imido
systems.7
At the present time there are no reported structures of a
Group 4 metal complex with a truly terminal nitrido ligand,
although a borane-capped nitrido complex of titanium has
recently been reported.8 In addition, only three terminal nitrido
complexes of Group 5 metals are currently included in the
Cambridge Structural Database (CSD): two nitrido vanadium
complexes: V(N)Cl2(pyridine)2;9 and V(N)Cl2(quinuclidine)2;10
together with one nitrido niobium complex, [Nb(N)(N[iPr]Ar)3]-
(N[iPr]Ar = isopropyl(3,5-dimethylphenyl)amide).11 The paucity
of structurally characterized Group 5 metal complexes with
terminal nitrido ligands prompted our interest in the synthe-
sis of new vanadium complexes which display this unusual
bondng motif. Furthermore, in view of the almost ubiquitous
role that the cyclopentadienyl ligand (Cp) and its substituted
derivatives play in early transition-metal chemistry, (cyclopen-
tadienyl)vanadium nitride complexes are conspicuously absent
from the literature. In fact, only three vanadium nitride species
containing the pentamethylcyclopentadienyl ligand (Cp*) have
been reported to date, namely: [Cp*V(m2-N)Cl]2;12 the struc-
turally related [Cp*V(m2-N)(N3)]2;13 and [Cp*V(m3-N)]4,14 in all
of which, the nitride ligands symmetrically bridge vanadium
centres.
early transition metal systems, the highly polar Md+
N
d- terminal
bonding mode combines a very nucleophilic nitrido group with
an electron-deficient metal centre. There is, therefore, a strong ten-
dency to form dimeric or oligomeric species with nitrides bridging
two or three metal centres. For example, in the case of vanadium,
several anionic vanadium nitride dimers have been reported. In
these species the bridging unit may be a V2(m2-N)2 linkage; for
instance, [{[(Me3Si)2N]2V}2(m2-N)2]-,2 prepared by Gambarotta
et al. and [V((Me3Si)N{CH2CH2N(SiMe3)}2)(m2-N)]2 ,3 prepared
-
by Cloke et al. Alternately, dimerization may occur through the
interaction of two nucleophilic nitrido ligands with a bridging
t
-
+
=
s-block metal cation; for instance, [Ar(Bu )N]3V N Na ]2 (Ar =
3,5-Me2C6H3),4 prepared by Cummins et al., which is presumed to
be dimeric with N ◊ ◊ ◊ Na+ ◊ ◊ ◊ N bridging interactions by analogy to
its structurally characterized niobium analogue.5 The complexity
of such dimeric and oligomeric species can make their characteri-
zation extremely difficult.6
However, complexes of early transition-metals with termi-
nal nitrido ligands are very attractive synthetic targets. Crys-
tallographic, vibrational, and theoretical studies of these sys-
tems will allow much greater insight into the nature and re-
In order to prepare the target monomeric terminal nitrido
complexes our strategy was to employ sterically demanding
and strongly electron-donating ancillary N-heterocyclic carbene
ligands to prevent the formation of dimers or oligomers. In
this paper, we report the details of the syntheses of several
new mono(cyclopentadienyl)vanadium complexes, two of which
contain the CpV N moiety (Scheme 1). 15N isotopic labelling
≡
aDepartment of Chemistry, Keene State College, 229 Main St., Keene, New
Hampshire, 03435-2001, USA. E-mail: cabernethy@keene.edu; Fax: +1 603
358 2897
of these compounds has allowed the unambiguous assignment
≡
of the V N infra red stretching frequencies of these complexes.
bDepartment of Chemistry, Campus Box 3290, Caudill and Kenan Labora-
tories, The University of North Carolina at Chapel Hill, Chapel Hill, NC,
27599-3290, USA
Also presented is a discussion of the effects that strong p-electron
donor coligands, such as nitrido or oxo, have on the hapticity
of the cyclopentadienyl ligand in half-sandwich complexes of
vanadium.
† CCDC reference numbers 752793 and 752794. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b922744j
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Cp2M + IMes·HCl → CpMCl(IMes) + CpH
(2)
The synthesis of 1 was prepared by an analogous reaction from
Cp2VCl and IMes·HCl in refluxing THF (Scheme 1).
Attempts to grow crystals of 1 suitable for X-ray diffraction
studies, by layering the THF solution with hexanes, all resulted in
the formation of an extremely air-sensitive olive brown powder.
The 1H NMR spectrum of this material (CDCl3) shows a
number of broad resonances consistent with the spectrum of
a paramagnetic complex. The assignments of these resonances
were made on the basis of both their integrated intensities, and
by comparison with the assignments made by Tilset et al. in
1
the H NMR spectra of the paramagnetic chromium complexes:
CpCrCl(IMes); CpCrPh(IMes); and CpCrCl2(IMes).17 Notable
features of the 1H NMR spectrum of 1 include a significant upfield
shift of the protons of the imidazol-2-ylidene ring (d = -2.5), and
the far-downfield and very broad resonance of the Cp protons (d =
135, Dn1/2 = 140 Hz). The paramagnetic nature of 1 was confirmed
by an Evans method determination of its magnetic susceptibility
(meff = 2.71 mB, 291 K, CDCl3 solution),19 which indicated the
presence of two unpaired electrons in the complex.
Unfortunately, reproducible elemental analyses of this material
were unobtainable due to its extreme sensitivity to both air
and moisture. The identity of 1 was confirmed, however, by the
addition of stoichiometric amounts of 4.0 M HCl(dioxane) to
THF solutions at -35 ◦C, which resulted in the formation of
[IMesH]+[CpVCl3]-, 2, in approximately 60% yield (Scheme 1).
Layering of THF solutions of 2 with toluene resulted in the
formation of pale yellow crystals, which were suitable for analysis
by single crystal X-ray diffraction experiments. In addition to the
cations and anions of 2, these crystals also contained molecules of
toluene within the lattice. The crystal structures of the constituent
ions of 2 are shown in Fig. 1. The 1H NMR spectrum of 2 displays
broad resonances consistent with the expected paramagnetic
nature of its vanadium(III) complex anion; the resonances of the
protons belonging to the [IMesH]+ are both broadened and shifted
from their expected positions, while the much broader resonance
for the Cp protons of the [CpVCl3]- is located far downfield at d 150
(Dn1/2 = 120 Hz). The magnetic susceptibility of 2 was determined
Scheme 1 Preparation of complexes 1-4.
Results and discussion
The work of Doherty et al. provides a convenient entry point for
Cp*VN chemistry; trimethylsilylazide oxidizes the vanadium(III)
cluster, [Cp*V(m2-Cl)2]3, to form the vanadium(V) nitride dimer,
[Cp*V(m2-N)Cl]2, in high yield according to eqn (1):12
2[Cp*V(m2 -Cl)2 ]3 +6Me3SiN3
(1)
↓
3[Cp*V( m2 -N)Cl]2 +6Me3SiCl+6N2
Later, Abernethy showed that sodium azide could also be
similarly employed for the synthesis of this compound.15
Unfortunately, this synthetic method cannot be directly applied
to CpV systems as the required ‘CpVCl2’ starting material is un-
suitable, being both oligomeric, with very low solubility in organic
solvents, and unstable with respect to disproportionation.16 The
stability of the CpVCl2 moiety can be increased, however, by
the coordination of an additional ligand that is both strongly
electron-donating and sterically demanding. In this study, the
N-heterocyclic carbene, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene (IMes), has been employed to form the synthetically
useful CpVCl2(IMes) (1) complex, which has been further reacted
to produce several other vanadium half-sandwich complexes
including two containing the CpVN moiety.
Fig. 1 ORTEP drawing of 2 with 50% probability ellipsoids for non-hy-
An elegant method of preparing CpMCl(IMes) com-
plexes by the reaction of a metallocene with 1,3-bis(2,4,6-
trimethylphenyl)imidazolium chloride (IMes·HCl) (eqn (2)), was
developed independently by both Tilset et al. (M = Cr)17 and
Cowley et al. (M = Ni):18
◦
˚
drogen atoms. Important distances (A) and angles ( ): V(1)–Cl(1) 2.352
(1); V(1)–Cl(2) 2.329 (1); V(1)–Cl(3) 2.318 (1); V(1)–Cpcent 1.949 (7);
Cl(1)–V(1)–Cl(2) 95.25 (4); Cl(1)–V(1)–Cl(3) 95.25 (4); Cl(2)–V(1)–Cl(3)
103.19 (4); Cpcent–V(1)–Cl(1) 120.1 (2); Cpcent–V(1)–Cl(2) 119.2 (2);
Cpcent–V(1)–Cl(3) 117.4 (2).
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by the Evans method to be 2.87 mB (CDCl3 solution at 291 K),19
consistent with the expected presence of two unpaired electrons
in the complex anion. Elemental analyses of crystals of 2 were in
accord with its empirical formula: C26H30N2Cl3V.
stretching vibration based on the results of the 15N labelling study
described below.
≡
V N stretching frequencies of 3 and 4
THF solutions of 1 were reacted with equimolar amounts of
NaN3 at reflux for 1.5 h, during which time bubbles of N2 were
evolved, the olive solution turned dark blue/green and the solid
NaN3 was seen to dissolve. The proposed reaction, shown in
Scheme 1, comprises of a metathesis reaction replacing a chloride
ligand with azide, followed by a two-electron oxidation of the
vanadium(III) centre to vanadium(V) with the concomitant two-
electron reduction of azide to nitride and dinitrogen gas. The THF
was removed in vacuo and the solid residue extracted into toluene.
Layering of these toluene solutions with hexanes resulted in the
precipitation of an extremely air-sensitive blue/black powder 3,
CpV(N)Cl(IMes). This compound decomposed when dissolved in
CDCl3; NMR studies were, therefore, performed on solutions in
THF-d8. The 1H and 13C NMR spectrum of 3 (THF-d8) indicate
the presence of both an IMes and a Cp ligand in a 1 : 1 ratio; the
resonances assigned to the IMes protons are very similar to those
observed in the NMR spectra of other diamagnetic complexes
featuring this ligand, while the Cp protons resonate at d 4.53
and the Cp carbon nuclei at d 123.0. The IR spectrum of freshly
≡
Infra red stretching frequencies of terminal V N bonds have
been reported from 1033 cm-1, in [V(N)Cl3]-,20 to 960 cm-1, in
V(N)Cl2(pyridine)2.9 The IR spectra of 3 and 4 display character-
istic strong absorptions at 945 and 955 cm-1, respectively. These fre-
quencies are somewhat lower than those previously reported, but
≡
are in accord with the longer than expected V N terminal bond
observed in 4. In order to determine unambiguously whether these
absorbances are indeed V N stretching frequencies, 15N isotopic
≡
labelling studies were performed. The syntheses of 3 and 4 were
15
15
15
≡
≡
repeated using sodium azide-1- N (Na N NN & NaN N N).
The products of these reactions were expected to contain an
approximately 1 : 1 mixture of: 3 and CpV(15N)Cl(IMes) (3a); and
4 and [IMesH]+[CpV(15N)Cl2]- (4a), respectively. The IR spectra
of the products of these reactions are shown in Fig. 3, together
with the IR spectra of pure samples of 3 and 4.
prepared samples of 3 in Nujol mull display a strong absorption
-1
≡
at 945 cm , which we assign to the V N stretching vibration
based on the results of an 15N labelling study (vide infra). Samples
of 3 immediately hydrolyzed on exposure to air; perhaps, as a
consequence of this reactivity, the elemental analyses of 3 poorly
reflect the expected values.
Repeated attempts to grow X-ray quality crystals of 3 resulted
in the formation of the blue/black powder described above. These
failures led us to add an equimolar amount of 4.0 M HCl(dioxane)
at -35 ◦C to THF solutions of 3 prepared in situ (Scheme 1).
On warming to room temperature, the dark blue/green solution
turned royal blue. After concentration in vacuo and layering with
hexanes X-ray quality crystals of [IMes.H]+[CpV(N)Cl2]-, 4, were
formed in 56% yield based on the amount of Cp2VCl used. The X-
ray crystal structure of 4 is shown in Fig. 2. The 1H and 13C NMR
spectra of this compound show both resonances typical of an
[IMesH]+ cation and resonances due to the Cp ligand of the anion
(1H: d 5.93, 13C: d 125.9). Elemental analyses are in accord with
the structure as determined by single-crystal X-ray diffraction. The
Fig. 3 IR spectra of: (a) 3; (b) 3 and 3a; (c) 4; and (d) 4 and 4a.
The IR spectrum of the sample containing a mixture of 3 and 3a
was almost identical to that containing 3 alone, the only observable
difference being an additional sharp absorption at 915 cm-1.
Similarly, compared to the IR spectrum of 4, the spectrum of the
sample containing 4 and 4a displayed an additional single sharp
absorption at 930 cm-1.
IR spectrum of a freshly made Nujol mull of crystalline 4 shows
-1
≡
a strong absorbance at 955 cm , which is assigned to the V N
The magnitude of the differences between the wavenumbers
≡
of the original absorptions assigned to the V N stretching
frequencies of 3 and 4 and those of the new absorptions appearing
in the spectra of the samples containing 3a and 4a are consistent
with the difference in the reduced masses of the two atom pairs
14
(i.e. V N and V 15N). These results thus serve to confirm
≡
≡
-1
≡
the V N stretching frequencies in 3 and 4 as 945 and 955 cm ,
respectively. The similarlty between the stretching V N vibrations
of 3 and 4 indicates that the VN moiety must have a similar
bonding mode in both complexes. These vibrational data, there-
fore, support the postulated monomeric structure of 3 shown in
Scheme 1.
≡
Fig. 2 ORTEP drawing of 4 with 50% probability ellipsoids for non-hy-
◦
˚
drogen atoms. Important distances (A) and angles ( ): V(1)–N(3) 1.620(1);
V(1)–Cl(1) 2.3375(7); V(1)–Cl(2) 2.3557(9); N(3)–V(1)–Cl(1) 103.33(4);
N(3)–V(1)–Cl(2) 103.99(4); Cl(1)–V(1)–Cl(2) 95.87(1).
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X-ray structures
indicating that the three carbon atoms that interact closely with the
vanadium centre retain their delocalized C–C p-bonding (allyl),
while the two more distant C atoms have developed a more
The ionic 2 is the first structurally characterized compound
containing the complex anion, [CpVCl3]-, which has a three-
legged piano stool structure (Fig. 1). The bond distances and
angles within the [IMesH]+cation in 2 are unremarkable being
very similar to those observed in other ionic compounds featuring
the [IMesH]+ cation, such as [IMesH]+Cl-.21 The important
bond distances and angles for the [CpVCl3]- anion are similar
to those reported for the [(C5H4Me)VCl3]- anion as deter-
mined in the compound, tetramethyltetrathiafulvalenium (methyl-
cyclopentadienyl)vanadium trichloride.22
=
localized C C bonding character (alkene), as expected for an
3
(h -C5H5)M system.23 Finally, the Cp ligand is non-planar with an
angle of 3.48◦ between the C(23)-C(24)-C(25) and C(25)-C(26)-
C(22)-C(23) planes. Accordingly, [CpV(N)Cl2]- is best described
as [(h -C5H5)V(N)Cl2]-.
3
Ring-slippage in half-sandwich complexes of vanadium containing
a strongly p-basic ligand
The ionic compound 4, illustrated in Fig. 2, is the first struc-
turally characterized compound containing the complex anion,
[CpV(N)Cl2]-. The monomeric nature of the anionic complex
in 4 contrasts with many of the other structurally characterized
vanadium nitride anions, which are dimeric.2,3,4 In the case of 4, the
pseudo-octahedral coordination of the anion’s vanadium centre
and large steric bulk of the cation combine to prevent dimerization
and so preserve the terminal nature of the anion’s nitrido ligand.
At first sight, the eta-3 coordination of the Cp ligand in
[CpV(N)Cl2]- is counterintuitive as the complex with the expected
5
eta-5 Cp coordination, [(h -C5H5)V(N)Cl2]-, would be a 16-
3
electron species, while the observed [(h -C5H5)V(N)Cl2]- is a
14-electron species. However, apart from the carbyne ligand,
the nitrido ligand is the strongest known p-electron donor.24
Accordingly, the nitrido ligand has a very strong trans influence.
In the case of [CpV(N)Cl2]-, there is competition for the same
vanadium acceptor orbitals between the p-donor orbitals of both
the nitrido ligand and those of the carbon atoms of the Cp ligand,
which are trans to it. This competition accounts for both the
≡
A detailed view of this anion is given in Fig. 4. The V N bond in
˚
4 (1.620(1) A) is longer than those observed in the two previously
˚
10
≡
characterized complexes with terminal V N bonds: 1.598(7) A in
9
˚
V(N)Cl2(pyridine)2; and 1.57(2) A in V(N)Cl2(quinuclidine)2.
≡
slightly longer than expected V N bond in this complex as well
as the loss of metal-carbon bonding between the vanadium centre
and the two carbon atoms of the Cp ligand trans to the nitrido
ligand. The eta-3 Cp coordination in [CpV(N)Cl2]- is, therefore,
due to the presence of the strong trans influence of the nitrido
coligand.
If, as we propose, it is the presence of the strongly p-basic
nitrido ligand that causes the ring-slippage in [CpV(N)Cl2]- then
other p-basic ligands with a strong trans influence should also
induce similar effects in closely related (C5R5)V(A)Cl2 complexes
(A = strongly p-basic ligand). This is, in fact, the case. Although
not commented upon by the authors of the original papers, the
structures of both CpV(O)Cl2,25 and Cp*V(O)Cl2,26 reveal similar
elongation of the V–C distances for the carbon atoms trans
to the oxo ligand, the concomitant shortening of the distance
between these two atoms, and non-planar ring systems (Table 1). In
addition, DFT geometry optimizations without constraints using
Gausian 03, Revision C.02,27 employing B3LYP functionals28
with 6-311G** basis sets29 not only show ring-slippage in these
complexes but actually overstates this phenomenon (both the V–
C distances for the carbon atons trans to the p-donor ligand and
the bend in the Cp ligand) particularly for the nitrido complex
of 4 (Table 1); this discrepancy may be due to the absence of
any intermolecular interactions in the model systems. In view
of their structural features, the (cyclopentadienyl)vanadium oxo
Fig. 4 Crystallographically determined structure of [CpV(N)Cl2]- an-
ion in 4, showing the asymmetrically bonded cyclopentadienyl ligand
(hydrogen atoms have been omitted for clarity). Vanadium–carbon and
˚
carbon–carbon distances (A): V(1)–C(22) 2.400(2); V(1)–C(23) 2.318(2);
V(1)–C(24) 2.305(2); V(1)–C(25) 2.301(1); V(1)–C26 2.395(2); C(22)–C(23)
1.421(3); C(23)–C(24) 1.422(3); C(24)–C(25) 1.412(2); C(25)–C(26)
1.424(2); C(26)–C(22) 1.385(2).
3
complexes should also, properly, be regarded as (h -C5H5)V(O)Cl2
3
and (h -C5Me5)V(O)Cl2.
The cyclopentadienyl ligand of the anionic complex in 4 does
not adopt the usual eta-5 coordination mode. This is evidenced
by several distinguishing structural features of the ligand. Firstly,
the long V–C distances observed for the carbon atoms in the Cp
Interestingly, both the 1H and 13C NMR spectra of 4, recorded
under ambient conditions, have singlet resonances attributable to
the Cp ligand in the anion (1H: 5.71 ppm; 13C: 129.1 ppm). The
simplicity of the spectra is most likely due to rapid rotation of the
cyclopentadienyl in solution at room temperature. It is worthy to
note that both CpV(O)Cl2 and Cp*V(O)Cl2 also display simple 1H
and 13C NMR spectra under ambient conditions due, presumably,
to similar ◦fluxional behavior of the Cp and Cp* ligands in these
formally d systems.25,26
˚
ring located trans to the nitrido ligand (2.400(2) and 2.395(2) A).
Secondly, the C–C distances within the Cp ligand are asymmetric,
the distance between the two carbon atoms positioned trans to
˚
the nitrido ligand, C(22)-C(26) 1.3852(2) A, is shorter than the
˚
other observed C–C distances (all in the range 1.412–1.424 A)
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˚
Table 1 Experimental and calculated bond lengths (A) in half-sandwich complexes of vanadium with p-donor coligands (e.s.d.’s are given in parentheses)
and bend in cyclopentadienyl ligand (◦). Carbon atoms are numbered with respect to those in 4. Unusual distances are highlighted in bold
[CpV(N)Cl2]-
Experimental
CpV(O)Cl2
Cp*V(O)Cl2
Calculated
Experimental18
Calculated
Experimental19
Calculated
V(1)-C(22)
2.400 (2)
2.318 (2)
2.305 (2)
2.301 (2)
2.395 (2)
1.421 (2)
1.422 (3)
1.412 (2)
1.424 (2)
1.385 (3)
3.48
2.631
2.400
2.326
2.399
2.631
1.431
1.415
1.415
1.431
1.394
5.26
2.412 (2)
2.286 (2)
2.243 (2)
2.264 (2)
2.402 (2)
1.420 (3)
1.409 (3)
1.400 (3)
1.429 (3)
1.382 (3)
3.58
2.497
2.314
2.279
2.314
2.497
1.447
1.424
1.424
1.448
1.401
4.65
2.44 (1)
2.31 (1)
2.28 (1)
2.28 (1)
2.42 (1)
1.46 (2)
1.43 (2)
1.43 (1)
1.42 (1)
1.38 (1)
3.24
2.465
2.311
2.281
2.311
2.464
1.434
1.414
1.414
1.434
1.392
5.44
V(1)-C(23)
V(1)-C(24)
V(1)-C(25)
V(1)-C(26)
C(22)–C(23)
C(23)–C(24)
C(24)–C(25)
C(25)–C(26)
C(26)–C(22)
Bend in Cp ligand
Synthesis of CpVCl2(IMes) (1)
Conclusions
A mixture of (C5H5)2VCl (0.45 g, 2.1 mmol) and IMes·HCl (0.71 g,
2.1 mmol) was dissolved in THF (100 mL) and heated to reflux
for 1.5 h. The resultant olive green solution was allowed to cool
then filtered to remove dark brown solid impurities. The volume
was then reduced to 15 mL in vacuo. This solution was layered
with hexane (50 mL) and set aside for 7 days, after which time an
olive brown powder had formed. This was collected by filtration
N-heterocyclic carbenes have been used as coligands to sta-
bilize highly reactive vanadium containing moieties. Further-
more, the addition of HCl(dioxane) has been shown to be a
convenient method for the removal of N-heterocyclic carbene
ligands from these vanadium complexes, resulting in the their
replacement with chloride and the concomitant formation of
ionic compounds comprised of imidazolium cations and anionic
vanadium complexes. Ionic compounds containing the previously
unknown complex anions [CpVCl3]- and [CpV(N)Cl2]- have
been prepared by this method and structurally characterized. In
addition, 15N isotopic labelling studies of the nitrido vanadium
complexes prepared has allowed the unambiguous assignment
and washed with cold (0 ◦C) hexane. Yield 0.75 g, 73% based
1
on vanadium. H NMR (270 MHz, THF-d8): d–2.5 (br, Dn1/2
=
=
100 Hz, 2H, C CH), 2.3 (br, Dn1/2 = 60 Hz, 6H, p-CH3), 4.7 (br,
Dn1/2 = 50 Hz, 12H, o-CH3), 8.3 (br, Dn1/2 = 50 Hz, 4H, Ar-H),
135 (br, Dn1/2 = 180 Hz, 5H, C5H5). This material is extremely
air-sensitive; samples of 1 left for several days in Schlenk flasks
under an argon atmosphere acquired a bright green patina, while
those exposed directly to air immediately turned dark green. Mp:
125 ◦C (dec). Reproducible elemental analyses were unobtainable,
presumably due to decomposition of the samples during shipping
due to its extreme air-sensitivity. The effective magnetic moment of
1 was determined to be 2.71 BM (based on a molecular formula of
C26H29N2Cl2V) by the Evans NMR method on a CDCl3 solution
at 291 K (using a 270 MHz instrument with a field strength of
6.34 Tesla).19
≡
of the V N stretching frequencies in the infra red spectra of
these complexes. Finally, the strong trans influence of p-donor
ligands, such as nitrido and oxo, have been shown to induce
ring-slippage in half-sandwich complexes of vanadium; the com-
plexes [CpV(N)Cl2]-, CpV(O)Cl2 and Cp*V(O)Cl2 should properly
3
3
be referred to as [(h -C5H5)V(N)Cl2]-, (h -C5H5)V(O)Cl2 and
(h -C5Me5)V(O)Cl2.
3
Experimental
General methods
Synthesis of [IMesH]+[CpVCl3]- (2)
All synthetic procedures and manipulations were performed
under dry argon or nitrogen using standard Schlenk line or
dry box techniques. All solvents were degassed and dried using
A mixture of (C5H5)2VCl (0.45 g, 2.1 mmol) and IMes·HCl (0.71 g,
2.1 mmol) was dissolved in THF (100 mL) and heated to reflux for
1.5 h. The resultant olive green solution was filtered to remove solid
impurities, and cooled to -35 ◦C. 0.55 mL of 4.0 M HCl(dioxane)
(2.2 mmol) were added drop-wise via a micro syringe. On warming
to room temperature the solution turned yellow. This solution was
reduced in volume to 10 mL in vacuo, layered with 50 mL of toluene
and left to stand for 7 days. After this time large quantities of pale
yellow crystals had formed. These were collected by filtration and
washed with ice-cold THF (5 mL). Yield 0.50 g, 45% based on
vanadium. 1H NMR (270 MHz, CDCl3): d 2.0 (br, Dn1/2 = 50 Hz,
5
an MBraun Manual Solvent Purification System. Chlorobis(h -
cyclopentadienyl)vanadium(III) was prepared by the reaction
of vanadium trichlorode with (cyclopentadienyl)thallium as de-
scribed by Manzer;30 the (cyclopentadienyl)thallium used in this
reaction was synthesized by the method of Nielson et al.31
Bis(2,4,6-trimethylphenyl)imidazium chloride was prepared as
described by Tilset et al.17 All other chemicals were obtained
commercially and used as received. 1H and 13C spectra were
obtained using a JEOL 270 MHz spectrometer, operating at 270.0
and 67.5 MHz, respectively. Infrared spectra were recorded as
Nujol mulls on a Perkin Elmer Spectrum RX1 FT-IR spectrom-
eter. Elemental analyses were performed by Robertson Microlit
Laboratories, Madison, NJ, USA.
12H, o-CH3), 2.8 (br, Dn1/2 = 50 Hz, 6H, p-CH3), 6.6 (Dn1/2
=
=
55 Hz, 4H, Ar-H), 7.4 (Dn1/2 = 65 Hz, 2H, C CH), 150 (br,
Dn1/2 = 160 Hz, 5H, C5H5), NCHCN resonance not observed.
Mp: 150 ◦C (dec). Elemental analysis calculated (%) for
C26H30N2Cl3V: C, 59.16; H, 5.73; N, 5.31; Cl, 20.15. Found: C,
3486 | Dalton Trans., 2010, 39, 3482–3488
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Table 2 Crystallographic data for 2 and 4
59.43; H, 6.00; N 5.15; Cl 20.85. The effective magnetic moment
of 2 was determined to be 2.78 BM (based on a unit formula of
C26H30N2Cl3V) by the Evans NMR method on a CDCl3 solution
at 291 K (using a 270 MHz instrument with a field strength of
6.34 Tesla).19
2
4
Chemical Formula
Crystal habit
C21H25N2·C5H5Cl3V C21H25N2·C5H5Cl2NV1
Block
Prism
P1
¯
Crystal system
Space group
C2/c
Monoclinic
29.542 (5)
14.710 (5)
14.792 (5)
—
Triclinic
8.3520 (17)
8.7980 (18)
18.550 (4)
79.78 (3)
89.34 (3)
72.16 (3)
1275.6 (5)
2
1.318
506.39
528
0.62
30.0
7456
7433
0.023
0.035
0.100
0.94
290
1.06, -0.39
˚
a/A
˚
b/A
Synthesis of CpV(N)Cl(IMes) (3)
˚
c/A
a (◦)
b (◦)
g (◦)
A mixture of (C5H5)2VCl (0.45 g, 2.1 mmol) and IMes·HCl (0.71 g,
2.1 mmol) was dissolved in THF (100 mL) and heated to reflux
for 1.5 h. The resultant olive green solution was filtered to remove
dark solid impurities, and NaN3 (0.14 g, 2.1 mmol) added. The
mixture was then stirred for 3 h at 60 ◦C, during which time
the solid NaN3 dissolved, bubbles of N2 were evolved, and the
solution turned dark blue. The solution was reduced in volume
to 25 mL in vacuo, filtered to remove solid NaCl, and layered
with 50 mL of hexane. A dark blue/black powder precipitated on
standing for 3 days. This was collected via filtration and washed
with ice-cold THF (5 mL). Yield 0.55 g, 58% based on vanadium.
1H NMR (270 MHz, THF-d8): d 2.30 (s, 12H, o-CH3), 2.40 (s,
94.190 (5)
—
6411 (3)
8
1.094
527.81
2192
3
˚
V/A
Z
D
calcd/g cm-1
M
F(000)
m Mo-Ka/mm-1
0.573
27.49
◦
Hmax
/
No. reflections measured 12353
No. unique reflections
Rint
R
wR2
S
No. parameters
7233
0.047
0.069
0.183
1.08
6H, p-CH3), 4.53 (s, 5H, C5H5), 7.03 (s, 4H, Ar-H), 6.31 (s, 2H,
295
1
C CH). 13C{ H} NMR (67.5 MHz, THF-d8): d 16.2 (s, o-CH3),
=
Largest diffraction peak 0.51, -0.36
-3
˚
and hole/e A
19.5 (s, p-CH3), 105.1 (s, C5H5), 123.0 (s, NC=CN), 124.0 (s,
Ar-C3,5), 130.4 (s, Ar-C4), 133.8 (s, Ar-C2,6), 139.2 (s, Ar-C1),
NCN resonance not observed. Mp: 110 ◦C (dec). IR (Nujol) n =
945 cm-1 (s sharp) (V N). Elemental analysis calculated (%) for
≡
Crystallography
C26H29N3ClV: C, 66.45; H, 6.22; N, 8.94; Cl, 7.54. Found: C, 64.90;
H, 6.08; N 8.62; Cl 8.75.
Diffraction data for 2 were collected at 153 K with a Nonius Kappa
CCD diffractometer, those for 4 were collected at 120 K with
a Bruker SMART diffractometer, both using Mo-Ka radiation
˚
(l = 0.71073 A). The structures were solved by direct methods
(2 SHELX-97;32 4 SIR9233) and refined with full-matrix least-
squares technique on F2 (2 SHELXL-97;32 4 CRYSTALS34).
The hydrogen atoms were included in geometrically calculated
positions riding on the concerned atoms and refined with thermal
parameters tied to that of the riding atom. The crystal lattice
of 2 contained a badly disordered solvent molecule which could
not be unambiguosly identified. The SQUEEZE35 subroutine
of PLATON36 software was therefore applied to create new
reflection data where contributions from the disordered solvent
were removed from the original data but were included in the
structure factor calculations. The details of crystallographic data
and structure refinement parameters for 2 and 4 are summarized
in Table 2.
Synthesis of [IMesH]+[CpV(N)Cl2]- (4)
A mixture of (C5H5)2VCl (0.45 g, 2.1 mmol) and IMes·HCl (0.71 g,
2.1 mmol) was dissolved in THF (100 mL) and heated to reflux
for 1.5 h. The resultant olive green solution was filtered to remove
dark insoluble impurities, and NaN3 (0.14 g, 2.1 mmol) added.
◦
The mixture was then stirred for 3 h at 60 C during which the
solid NaN3 was seen to dissolve, bubbles of N2 were evolved, and
the solution turned dark blue. The resultant dark blue solution
was filtered and then cooled to -35 ◦C. 4.0 M HCl in 1,4-dioxane
(0.53 mL, 2.1 mmol) was added drop-wise via a micro syringe.
The solution quickly turned green and was allowed to warm to
room temperature then stirred for a further 1 h. The solution was
reduced in volume to 20 mL in vacuo then layered with hexanes
(20 mL) and set aside at -25 ◦C for 7 days. During this time, green
crystals formed. These were collected by filtration, washed with
ice-cold THF (1.0 mL) and dried under a stream of argon. Yield:
Acknowledgements
1
0.81 g, 76% based on vanadium. H NMR (270 MHz, CDCl3):
We thank Research Corporation for providing generous sup-
port through Cottrell College Science Award No. CC6614.
Acknowledgement is also made to the donors of the American
Chemical Society Petroleum Research Fund for support of
this research. This material is also based, in part, upon work
supported by the National Science Foundation under Grant
No. CHE-0521047 (Brandeis University). CDA also wishes to
thank the reviewers for many helpful comments, which greatly
improved our understanding of the compounds presented in this
paper.
d 2.16 (s, 12H, o-CH3), 2.34 (s, 6H, p-CH3), 5.93 (s, 5H, C5H5),
=
7.01 (s, 4H, Ar-H), 7.14 (s, 2H, C CH), 9.95 (s, 1H, NCHCN).
1
13C{ H} NMR (67.5 MHz, CDCl3): d 18.2 (s, o-CH3), 21.5 (s, p-
CH3), 125.9 (s, NC=CN), 128.1 (s, C5H5), 130.2 (s, Ar-C3,5), 134.4
(s, Ar-C2,6), 135.4 (s, Ar-C4), 141.9 (s, Ar-C1), NCHN resonance
not observed. Mp: 270 ◦C (dec). IR (Nujol) n = 955 cm-1 (s sharp)
≡
(V N). Elemental analysis calculated (%) for C26H30N3Cl2V: C,
61.67; H, 5.97; N, 8.30; Cl, 14.00. Found: C, 61.05; H, 5.80; N,
8.15; Cl 14.50.
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The Royal Society of Chemistry 2010
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©

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3488 | Dalton Trans., 2010, 39, 3482–3488
This journal is
The Royal Society of Chemistry 2010
©