Niobium complexes containing a new chiral heteroscorpionate ligand and the reactivity of such a complex with O2 to give the first gem-diolate niobium complex

Article abstract of DOI:10.1039/b300497j

The synthesis of a novel unsymmetrical bis(pyrazol-1-yl) ligand, (3,5-diphenylpyrazol-1-yl-3′,5′-dimethylpyrazol-1-yl)methane (dpmpzm), 1, has been studied. Deprotonation at the methylene group of 1 with BuⁿLi, followed by reaction with carbon dioxide yielded a racemic mixture of [{Li(dpmpza)(H₂O)}₄], 2 [dpmpza = (3,5-diphenylpyrazol-1-yl-3′,5′-dimethylpyrazol-1-yl)acetate], which is a novel chiral monoanionic NNO tripod ligand. This compound is an excellent precursor for the introduction of this scorpionate ligand into transition metal complexes. The complexes [NbCl₃(dme)(RC≡CR′)] (dme = 1,2-dimethoxyethane) reacted with 2, to give the corresponding [NbCl ₂(dpmpza)(RC≡CR′)] complexes (R = R′ = Me 3; R = R′ = SiMe₃, 4; R = Ph, R′ = Me, 5; R = Ph, R′ = Et, 6). The structures of these complexes have been determined by spectroscopic methods. Variable-temperature NMR studies of these complexes were carried out in order to study their dynamic behaviour in solution. The barriers to alkyne rotation have also been calculated. The reactivity of [NbCl₂(dpmpza) (Me₃SiOC≡CSiMe₃)], 4, toward molecular oxygen is particularly noteworthy since it led to the formation of the first gem-diolate niobium species, [(NbCl₂O)₂(μ-η¹O, O′tpzpdo)], 7 [tpzpdo = 1,3-bis(3,5-diphenylpyrazol-1-yl)-1,3- bis(3′,5′-dimethylpyrazol-1-yl)-2,2′-propanediolate], in a process that has no precedent in the literature. The molecular structure of this gem-diolate has been determined crystallographically. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b300497j

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Niobium complexes containing a new chiral heteroscorpionate
ligand and the reactivity of such a complex with O2 to give the
first gem-diolate niobium complex
A. Otero,* J. Fernández-Baeza, A. Antiñolo, J. Tejeda, A. Lara-Sánchez, L. Sánchez-Barba,
M. T. Expósito and A. M. Rodríguez
Departamento de Química Inorgánica, Orgánica y Bioquímica, Universidad de Castilla-La
Mancha, 13071-Ciudad Real, Spain. E-mail: antonio.otero@uclm.es
Received 13th January 2003, Accepted 25th February 2003
First published as an Advance Article on the web 10th March 2003
The synthesis of a novel unsymmetrical bis(pyrazol-1-yl) ligand, (3,5-diphenylpyrazol-1-yl-3Ј,5Ј-dimethylpyrazol-1-
yl)methane (dpmpzm), 1, has been studied. Deprotonation at the methylene group of 1 with BunLi, followed by
reaction with carbon dioxide yielded a racemic mixture of [{Li(dpmpza)(H2O)}4], 2 [dpmpza = (3,5-diphenylpyrazol-
1-yl-3Ј,5Ј-dimethylpyrazol-1-yl)acetate], which is a novel chiral monoanionic NNO tripod ligand. This compound is
an excellent precursor for the introduction of this scorpionate ligand into transition metal complexes. The complexes
᎐
[NbCl (dme)(RC᎐CRЈ)] (dme = 1,2-dimethoxyethane) reacted with 2, to give the corresponding [NbCl (dpmpza)-
᎐
3
2
᎐
(RC᎐CRЈ)] complexes (R = RЈ = Me, 3; R = RЈ = SiMe , 4; R = Ph, RЈ = Me, 5; R = Ph, RЈ = Et, 6). The structures
᎐
3
of these complexes have been determined by spectroscopic methods. Variable-temperature NMR studies of these
complexes were carried out in order to study their dynamic behaviour in solution. The barriers to alkyne rotation
᎐
have also been calculated. The reactivity of [NbCl (dpmpza)(Me SiC᎐CSiMe )], 4, toward molecular oxygen is
᎐
2
3
3
particularly noteworthy since it led to the formation of the first gem-diolate niobium species, [(NbCl2O)2(µ-η1-O,OЈ-
tpzpdo)], 7 [tpzpdo = 1,3-bis(3,5-diphenylpyrazol-1-yl)-1,3-bis(3Ј,5Ј-dimethylpyrazol-1-yl)-2,2Ј-propanediolate], in a
process that has no precedent in the literature. The molecular structure of this gem-diolate has been determined
crystallographically.
possesses two different pyrazolyl rings. The unsymmetrical
(3,5-diphenylpyrazol-1-yl-3Ј,5Ј-dimethylpyrazol-1-yl)methane
(dpmpzm), 1, was obtained by reaction of 3,5-dimethylpyrazole
with 3,5-diphenylpyrazole, dichloromethane and base (Scheme
1). The compounds were separated by exploiting the differences
in solubility and 1 could be isolated in up to 25% yield from the
Introduction
We have recently been interested in the synthesis of new
“heteroscorpionate” ligands1 with pyrazole rings. The target
compounds are related to the tris(pyrazol-1-yl)methane
system,2 where one of the pyrazole groups has been replaced
by a carboxylate, dithiocarboxylate or ethoxide group, namely
bis(3,5-dimethylpyrazol-1-yl)acetate (bdmpza), bis(3,5-di-
methylpyrazol-1-yl)dithioacetate (bdmpzdta) and 2,2-bis(3,5-
reaction mixture containing the three possible products (bdm-
pzm ϩ bdphpzm ϩ 1). The 1H and 13C{1H} NMR spectra of 1
exhibit two set of signals for the two different pyrazole rings
dimethylpyrazol-1-yl)ethoxide (bdmpze). The aim of these
(see Experimental).
changes is to provide a small degree of steric hindrance and
considerable coordinative flexibility. These compounds were
found to be excellent reagents for the introduction of scorpion-
ate ligands into niobium complexes and a series of alkyne-
containing niobium complexes was isolated and characterized.3
These compounds have an interesting dynamic behaviour in
solution and this was studied by variable-temperature NMR
techniques. More recently, we extended the range of complexes
to include group 4 metals.4 We describe here the synthesis
and characterization of niobium complexes with a new chiral
heteroscorpionate ligand and the reactivity of one such com-
plex with molecular oxygen to give the first gem-diolate
niobium compound. Gem-diolate organometallic species are
very rare and, to date, only three classes of complex in this area
have been reported in the literature. One such example is a
dioxomethylene species, Cp2Zr(Cl)–OCH2O–Zr(Cl)Cp2, which
was spectroscopically characterized as a reactive intermediate
in the reaction between Schwartz’s reagent [Cp2Zr(Cl)H] and
CO2.5 The second class is derived from the gem-diol (C5H4N)2-
C(OH)2 and a number of mononuclear6 and polynuclear7
complexes of late transition metals are known. Finally, a
third type of late transition metal complex incorporating the
Scheme
1
Synthesis of unsymmetrical (3,5-diphenylpyrazol-1-yl-
3Ј,5Ј-dimethylpyrazol-1-yl)methane, 1.
Deprotonation at the methylene group of 1 with BunLi,
followed by reaction with carbon dioxide yielded a racemic
mixture of [{Li(dpmpza)(H2O)}4], 2 [dpmpza = (3,5-diphenyl-
pyrazol-1-yl-3Ј,5Ј-dimethylpyrazol-1-yl)acetate] (Scheme 2).
Although the reaction was carried out under rigorously
anhydrous experimental conditions, the presence of adventi-
tious moisture in the reaction mixture during the work-up
procedure was probably responsible for the presence of the
coordinated water molecule. The mass spectrum (FAB) of 2
indicates a tetranuclear formulation (see Experimental) and,
in accordance with the structure found for [{Li(bdmpza)-
(H2O)}4],3 we propose in solid state, an analogous arrangement
consisting of tetrameric units. Their IR spectrum shows two
8
dianion of hexafluoropropane-2,2-diol, (CF3)2C(OH)2 has
been described.
Results and discussion
Based on the synthesis of bdmpzm published by Elguero et al.,9
we have prepared a new bis(pyrazol-1-yl)methane, which
D a l t o n T r a n s . , 2 0 0 3 , 1 6 1 4 – 1 6 1 9
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
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Scheme 4 Proposed structures for the three diasteroisomers of
complexes 3–7.
Scheme 2 Synthesis of complex [{Li(dpmpza)(H2O)}4], 2.
strong bands at 1649 and 1456 cmϪ1, which are assigned to
cis disposition with respect to the 3,5-diphenylpyrazol ring
(isomer B) or the trans position with respect to the oxygen atom
(isomer C).
νas(CO2Ϫ) and νs(CO2Ϫ), respectively. In the lithium compound
[{Li(bdmpza)(H2O)}4],3 the νas(CO2Ϫ) and νs(CO2Ϫ) (∆νas–s
)
bands differ by 180 cmϪ1 and in this complex the carboxylate is
involved in a coordination bridge at two lithium atoms. For
compound 2 the value of ∆νas–s (193 cmϪ1) is also in accordance
with an O-coordination bridge.10 A structurally analogous
cubane-like array, in which the vertices are alternately occupied
by four Li atoms and four bridging carboxylate groups can be
also proposed for 2.
For complexes 3 and 4 which contain symmetrical alkynes,
two sets of NMR signals for both pyrazol and alkyne ligands
are observed at room temperature a situation in accordance
with the presence of two isomers. In the 1H NMR spectra, each
set of signals shows a 1 : 3 intensity pattern for the scorpionate
and the alkyne ligands. NOE experiments were carried out in
order to know the disposition of the two isomers. Thus, irradi-
ation in major signals of the alkyne Me (3) or SiMe3 (4) groups
enhances only the Me3 signal, which is clearly the Me group of
the 3,5-dimethylpyrazole ring as it is in closer spatial proximity
to the alkyne ligand in the cis position (isomer A). When the
same irradiation were carried out on the minor signals, a
response was not observed in the NOE experiments. This result
suggests a trans disposition of alkyne ligands with respect to
oxygen atom in the minor isomer (isomer C). Only one set of
NMR signals is observed for the unsymmetrical alkyne com-
plexes 5 and 6, a fact that is consistent with the presence of only
one isomer. Isomer (A) is proposed to be present in these
complexes on basis of NOE experiments. The 13C-{1H} NMR
spectra of complexes 3–6 agree with these proposed structures.
The 13C-NMR resonances for the carbon atoms of the alkyne
ligands appear at ca. δ = 250, indicating that the alkyne ligand
behaves as a four-electron donor (see Experimental). An empir-
ical correlation between the alkyne π donation and 13C chemical
shift for the bound alkyne carbons has previously been
1
The H NMR spectrum contains six singlets (CH, H4, H4Ј,
Me3 and Me5 of the pyrazole rings and H2O) and one multiplet
(Ph3 and Ph5 of the one pyrazole ring). Homonuclear NOE
(nuclear Overhauser enhancement) difference spectroscopy was
also performed in order to confirm the assignment of the sig-
nals for the Me3, Me5, H4 and H4Ј groups. The 13C–{1H} NMR
spectrum exhibits two set of resonances, as one would expect
for the presence of two different pyrazole rings. A 1H–13C
heteronuclear correlation (HETCOR) experiment was carried
out and allowed us to assign the resonances corresponding to C4,
Me3 and Me5 of the pyrazole ring (see Experimental). Finally,
the 7Li NMR spectrum exhibits a singlet at δ 1.59 for the
lithium atom. In this compound, the carbon atom bridge is a
chiral centre and we have confirmed the presence in solution of
the corresponding two enantiomers by addition of a chiral shift
reagent, namely (R)-(–)-(9-anthryl)-2,2,2-trifluoroethanol, to a
solution of complex 2. This process gives rise to the appearance
1
in the H NMR spectrum of two signals for each proton as
1
observed.12 The H- and 13C-NMR spectra of these complexes
a result of the presence of the two diastereoisomers from the
corresponding two enantiomers. Compound 2 was used in
the formation of some niobium complexes in order to test
its coordinative capacity as scorpionate ligand. Thus, the
lithium compound 2 reacted at room temperature in a 1/4 : 1
molar ratio (Scheme 3) with the complexes [NbCl3(dme)-
with symmetrical alkynes exhibit only one set of signals for the
substituted groups. This observation is due to rotation of the
alkyne ligand, as previously observed for other alkyne-contain-
ing niobium complexes.2,3 We assume that a six-coordinate
description of the complexes (see Fig. 1) in which the alkyne
occupies a single site is preferable to the alternative seven-
coordinate model in which each alkyne carbon is considered to
occupy a separate coordination position. Based on this assump-
tion we propose a single rotation of the alkyne ligand around
the bisector of the metal–alkyne isosceles triangle. Free energy
values, ∆G‡, for the complexes 3–6 were calculated13 from vari-
able-temperature NMR studies. The values allow us to establish
a relationship between the steric demand of the alkyne and the
rotation phenomenon. In fact, as can be seen in Fig. 2, the
higher ∆G‡ values and coalescence temperatures were found in
the cases of the bulkier alkyne substituents.
11
᎐
(RC᎐CRЈ)] (dme = 1,2-dimethoxyethane) in THF to give,
᎐
after the appropriate work-up, the complexes [NbCl2(dpmpza)-
᎐
(RC᎐CRЈ)] (R = RЈ = Me, 3; R = RЈ = SiMe , 4; R = Ph, RЈ =
᎐
3
Me, 5; R = Ph, RЈ = Et, 6), which were isolated as red, blue,
orange and brown solids, respectively.
Scheme 3 Synthesis of complexes 3–6
The mass spectra of complexes 3–6 indicate a mononuclear
formulation (see Experimental). In the IR spectra the value
∆νas–s is ca. 230 cmϪ1, which is in accordance with a single
oxygen coordination. Two strong bands are also observed at ca.
330 and 370 cmϪ1 and these have been assigned to ν(Nb–Cl).
These results agree with a proposed octahedral disposition in
which the metal atom is surrounded by an NNO-coordinated
“scorpionate” ligand, an alkyne and two chloride ligands (see
Scheme 4). The alkyne ligand can be located in the cis position
with respect to the 3,5-dimethylpyrazole ring (isomer A), in a
Fig. 1 Proposed structure for complexes 3, 4 (major isomer), 5 and 6.
D a l t o n T r a n s . , 2 0 0 3 , 1 6 1 4 – 1 6 1 9
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Fig. 2 Graphic representation of free energy values, ∆G‡, in the
rotation of the alkyne ligands for the complexes 3–6. Coalescence
temperature values for these complexes are: 180, 3 (major isomer); 263,
4 (major isomer); 193, 5; 203 K, 6.
The variable-temperature NMR study for complex 6 illus-
trates a good example of a system with an unsymmetrical
alkyne. In Fig. 3 it can be seen that a triplet appears for the CH3
groups in PhCCEt at 293 K. When the temperature was lowered
below 170 K two signals (relative intensities 3 : 1) were observed
for each of the diastereoisomers present (see Scheme 5). By
NOE experiments, we have established that, the major isomer is
the one in which the Et group lies between the pyrazole ring and
carboxylate group, and the phenyl group lies between two chlor-
ine atoms (see Fig. 1). The addition of (R)-(–)-(9-anthryl)-2,2,2-
trifluoroethanol to a solution of 6 gave rise to the appearance
of four triplets for the Et group of the alkyne ligand in the
1H-NMR spectrum at 170 K. These four signals correspond to
the four diastereoisomers from the corresponding four stereo-
isomers (see Scheme 5).
Scheme 5 Proposed structures for the four enantiomers of complexes
with an unsymmetrical alkyne ligand.
Scheme 6 Synthesis of [(NbCl2O)2(µ-η1-O,OЈ-tpzpdo)], 7.
ligand behaves as a tripod in its coordination to each niobium
atom. An oxo and two chloride ligands complete the coordin-
ation sphere of each metal.
The molecular structure of complex 7 was determined by
X-ray diffraction and it was found that this complex crystallizes
¯
in the trigonal space group R3c with 18 molecules per unit cell.
The asymmetric unit contains only half a molecule of complex
gem-diolate. As illustrated in Fig. 4, the structure consists of a
dimeric unit possessing a crystallographic 2-fold axis passing
through the C2 atom. As a result of the symmetry of the space
group, the crystal shows a racemic mixture (aR,R,R ϩ
aS,S,S)14 for this compound (the aS,S,S form refers to the
complex shown in Fig. 4).
Fig. 3 Variable-temperature 1H NMR spectra in the region of the Et,
᎐
alkyne group of the complex [NbCl2(dpmpza)(PhC᎐CEt)], 6. ᭹ =
᎐
Central signal of the triplet.
Each niobium centre has a distorted octahedral environment
with the “tpzpdo” ligand coordinated in a tridentate fashion
through the oxygen atom and the two pyridinic nitrogen atoms
of the pyrazole rings. The major distortion appears in the Cl2–
Nb1–O1 angle, which has a value of 156.0(2)Њ. The Nb1–N1
bond distance of 2.243(7) Å is as one would expect for an Nb
pyrazolyl complex15 but the elongated Nb1–N3 distance of
2.568(8) Å indicates that the oxygen atom O2 exerts a strong
trans influence. The Nb1–O2 distance of 1.711(5) Å is as
expected for a double bond.16
᎐
The reactivity of [NbCl (dpmpza)(Me SiC᎐CSiMe )], 4,
᎐
2
3
3
toward molecular oxygen is particularly noteworthy since it
led to the formation of the first gem-diolate niobium
species, [(NbCl2O)2(µ-η1-O,OЈ-tpzpdo)],
7 [tpzpdo = 1,3-
bis(3,5-diphenylpyrazol-1-yl)-1,3-bis(3Ј,5Ј-dimethylpyrazol-1-
yl)-2,2Ј-propanediolate], in a process that has no precedent in
the literature. In fact, the reaction of complex 4 with molecular
oxygen in CH2Cl2 at room temperature renders complex 7,
which was isolated as a white solid in good yield (85%) after the
appropriate work-up (Scheme 6). It is thought that an initial
oxidation process from SNbIII to NbV occurs and, after the loss
of CO2, a novel anionic gem-diolate bridging two metal centres
is present in complex 7. In addition, this unusual ligand con-
tains two chiral centres as well as two bis(pyrazol-1-yl) moieties,
which are also coordinated to both metals in such a way that the
1
The H and 13C{1H} NMR spectra of 7 show two sets of
signals, one for each of the two different pyrazolyl groups
of the 1,3-bis(3,5-diphenylpyrazol-1-yl)-1,3-bis(3Ј,5Ј-dimethyl-
pyrazol-1-yl)-2,2Ј-propanediolate ligand. This observation
indicates the equivalence of the same type of pyrazole
ring and the existence in solution of either a single dia-
stereoisomer [rac(aR,R,R ϩ aS,S,S)] (found in the X-ray
D a l t o n T r a n s . , 2 0 0 3 , 1 6 1 4 – 1 6 1 9
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Fig. 4 Structure of complex 7 (30% probability ellipsoids). The phenyl
rings are omitted for clarity. Selected bond lengths [Å] and angles [Њ]:
Nb1–O2 1.711(5); Nb1–O1 1.906(5); Nb1–N1 2.243(7); Nb1–Cl1
2.327(3); Nb1–Cl2 2.350(3); Nb1–N3 2.568(8); O1–C2 1.387(8); N2–C1
1.44(1); N4–C1 1.44(1); C1–C2 1.56(1); N1–Nb1–Cl1 167.0(2); O1–
Nb1–Cl2 156.0(2); O2–Nb1–N3 170.1(3); C2–O1–Nb1 137.0(4); O1Aa–
C2–O1 112(1); O1Aa–C2–C1Aa 110.7(4); O1–C2–C1Aa 105.9(4);
O1Aa–C2–C1 105.9(4); O1–C2–C1 110.7(4); C1Aa–C2–C1 111(1).
Symmetry transformations used to generate equivalent atoms: a x Ϫ y
ϩ 1/3, Ϫy ϩ 2/3, Ϫz ϩ 1/6.
crystal study in the solid state) or a rapid isomerization equi-
librium between the two diastereoisomers [rac(aR,R,R ϩ
aS,S,S) and rac(aS,R,R ϩ aR,S,S)] (see Scheme 7). A third
diastereoisomer is possible [rac(aS,R,S ϩ aR,R,S)] but this
1
would give rise to four sets of signals in the H and 13C{1H}
NMR spectra, one for each of the pyrazolyl groups. The reson-
ance in the 13C NMR spectrum attributed to the gem-diolate
group (CO22Ϫ) is observed at δ = 101.0, which is similar to the
value found for the gem-diolate-containing zirconium com-
Scheme 7 In complex 7 it is possible to obtain three diastereoisomers:
[rac(aR,R,R ϩ aS,S,S)] (as corroborated by X-ray diffraction in the
solid state), [rac(aS,R,R ϩ aR,S,S)] and [rac(aS,R,S ϩ aR,R,S)].
1
plex.5 A homonuclear NOE study and a H–13C correlation
experiment (HETCOR) allowed us to assign the resonances
corresponding to the different types of protons and carbon
atoms.
Scheme 8 shows our proposed reaction pathway leading to
complex 7. We assume that complex 4 (NbIII) is initially oxid-
ized by molecular oxygen to give a new and unstable NbV
complex, [NbCl2(O)(κ3-dpmpza)] (a), with the loss of an alkyne
and the incorporation of an oxo ligand (step 1). Similar
complexes have previously been prepared and isolated as very
stable species; for example, the reaction between [NbCl2-
3
᎐
(κ -bdmpze)(PhC᎐CMe)] [bdmpze = bis(3,5-dimethylpyrazol-
᎐
1-yl)ethoxide]3 and molecular oxygen leads to the formation of
[NbCl2(O)(κ3-bdmpze)], which was characterizated by an X-ray
diffraction study.17
One point that must be addressed is the reason for the differ-
ent behaviour of (a) in comparison to other similar complexes.
On considering our proposed route the answer to this question
lies in the different basic strengths of the pyrazole rings. Indeed,
the pKa values18 for aqueous 3,5-diphenylpyrazole and 3,5-di-
methylpyrazole are 1.74 and 4.12, respectively, and this suggests
that the coordination ability of the first pyrazole at the niobium
centre is less than for the latter. Therefore, we propose that in
the second step the oxygen atom of a carboxylate group from a
neighbouring molecule can displace the 3,5-diphenylpyrazole
from the metal centre of the other molecule and give rise to a
binuclear intermediate such as (b). Finally, a concerted decarb-
oxylation through a six-membered cyclic transition state (c)19
would give complex 7 (step 3). Attempts to isolate and charac-
terize some of the different proposed intermediates on a pre-
parative scale were unsuccessful, although the formation of
CO2 was confirmed. A study of the course of the reaction on a
small scale by means of NMR spectroscopy was also carried
Scheme 8 Different steps proposed for the synthesis of complex 7.
out. However, informative results were not obtained and the
only species detected were complexes 4 and 7. It should be
noted that other possible mechanisms based on a decarboxyl-
ation process can not be excluded.
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In conclusion, we describe the synthesis of a novel unsym-
metrical bis(pyrazol-1-yl) ligand, which by reaction with BunLi
and carbon dioxide yielded a lithium compound containing is a
novel chiral monoanionic NNO tripod. This compound is
excellent precursor for the introduction of this heteroscopion-
ate ligand into transition metal complexes. This fact has been
confirmed by reactions with different niobium complexes. New
mononuclear niobium species with a new scorpionate ligand
have been isolated. These alkyne complexes present a dynamic
behaviour in solution, which corresponds to alkyne rotation.
This behaviour has been studied by variable-temperature NMR
spectroscopy. The reaction of one alkyne complex with molecu-
lar oxygen to give the first gem-diolate niobium complex in a
process that has no precedent in the literature, has been also
carried out.
gaseous CO2, after which the solution was colourless. The sol-
vent was removed and on addition of hexane (70 cm3) a white
solid was obtained. This solid was crystallised from a mixture
of THF–hexane. Yield 96%. (Found: C, 66.58; H, 5.21; N,
14.10. C22H21Li N4O3 requires C, 66.58; H, 5.34; N, 14.14%.) 1H
NMR (DMSO, 297 K), δ 6.51 (s, 1 H, CH), 6.77 (s, 1 H, H4),
5.74 (s, 1H, H4Ј), 1.96 (s, 3 H, Me3), 2.57 (s, 3 H, Me5), 7.26–7.77
(m, 10H, Ph), 3.36 (s, 2H, H2O). 13C–{1H}-NMR (DMSO),
δ 75.4 (CH), 142.4, 145.5, 145.9, 149.3 (C3 or 5), 103.0 (C4), 106.7
(C4Ј), 13.8 (Me3), 13.4 (Me5), 134.2–125.6 (Ph), 165.3 (CO2Ϫ).
7Li NMR (DMSO), δ 1.59 (s). IR (Nujol) 1557 ν(C᎐N), 1649
᎐
νas(CO2Ϫ), 1456 cmϪ1 νs(CO2Ϫ). Mass spectrum (m/z assign-
ment, % intensity, D = Daltons): 1141 D [Li4(bdmpza)3], 20; 763
D [Li3(bdmpza)2], 19; 385 D [Li2(bdmpza)], 100.
3
3
᎐
[NbCl (ꢀ -dpmpza)(MeC᎐CMe)], 3. To a THF (100 cm )
᎐
2
᎐
solution of NbCl (dme)(MeC᎐CMe) (1.00 g, 2.91 mmol) was
᎐
3
Experimental
added an equimolar quantity of [{Li(dpmpza)(H2O)}4], 2 (1.10
g, 0.73 mmol). The solution was stirred for 18 h at room tem-
perature. The solvent was removed under vacuum and the solid
extracted with CH2Cl2. A red solid was obtained after removal
of the solvents. Yield 65%. (Found: C, 52.68; H, 4.32; N, 9.31.
All reactions were performed using standard Schlenk-tube
techniques under an atmosphere of dry nitrogen. Solvents were
distilled from appropiate drying agents and degassed before use.
Microanalyses were carried out with a Perkin-Elmer 2400 CHN
analyser. Mass spectra were recorded on a VG Autospec
instrument using the FAB technique and 3-nitrobenzyl alcohol
as matrix. Infrared spectra were recorded in the region 4000–
1
C26H25Cl2N4NbO2 requires C, 52.99; H, 4.27; N, 9.50%.) H
NMR (CD2Cl2, 297 K), major isomer δ 6.91 (s, 1 H, CH), 6.58
(s, 1 H, H4), 6.09 (s, 1 H, H4Ј), 2.04 (s, 3 H, Me3), 1.77 (s, 3 H,
5
᎐
200 cmϪ1 using a Perkin-Elmer 883 spectrophotometer. 1H, 13
C
Me ), 7.86–7.41 (m, 10 H, Ph), 2.85 (s, 6 H, MeC᎐); minor
᎐
isomer δ 6.86 (s, 1 H, CH), 6.60 (s, 1 H, H4), 6.05 (s, 1 H, H4Ј),
1.87 (s, 3 H, Me3), 1.73 (s, 3 H, Me5), 7.86–7.41 (m, 10 H, Ph),
7
and Li NMR spectra were registered on a Varian Unity FT-
300 spectrometer referenced to the residual deuteriated solvent.
The NOE difference spectra were recorded with the following
acquisition parameters: spectrum width 5000 Hz, acquisition
time 3.27 s, pulse width 90Њ, relaxation delay 4 s, irradiation
power 5–10 dB, number of scans 120. Two-dimensional NMR
spectra were acquired using standard VARIANT-FT software
and processed using an IPC-Sun computer. The NMR probe
temperatures were varied using an Oxford Instruments VTC 4
2.78 (s, 6 H, MeC᎐). 13C–{1H}-NMR (CD2Cl2), major isomer
᎐
᎐
δ 67.9 (CH), 159.0, 158.6, 156.3, 154.2, 147.2, 146.9, 143.9,
or
5
142.0 (C3
for both isomers), 109.0 (C4), 110.6 (C4Ј), 15.2
(Me3), 10.2 (Me5), 131.0–126.4 (Ph for both isomers), 161.3,
Ϫ
᎐
᎐
161.4 (CO2 for both isomers), 13.5 (MeC᎐), 247.6, 243.5 (C᎐C
᎐
᎐
for both isomers); minor isomer δ 66.9 (CH), 109.2 (C4), 110.0
4Ј
3
5
᎐
(C ), 15.3 (Me ), 10.9 (Me ), 15.2 (MeC᎐). IR (Nujol) 1554
᎐
ν(C᎐N), 1692 ν (CO Ϫ), 1462 νs(CO2Ϫ), 370, 329 cmϪ1 ν(Nb–
᎐
as
2
unit, measured by a thermocouple and calibrated with CH3OH.
Cl). Mass spectrum (m/z assignment, % intensity, D = Daltons):
590 D [M ϩ 1], 10; 554 D [M Ϫ Cl], 100.
᎐
The complexes [NbCl (dme)(RC᎐CRЈ)] were prepared as
᎐
3
reported previously.11
3
᎐
[NbCl (ꢀ -dpmpza)(Me SiC᎐CSiMe )], 4. The synthetic pro-
᎐
2
3
3
Preparations
cedure was the same as for complex 3, using NbCl3(dme)-
Dpmpzm, 1. In a 1000 cm3 three neck flask provided with a
reflux condenser 3,5-diphenylpyrazole (5.00 g, 22.69 mmol),
3,5-dimethylpyrazole(10.90g,113.45mmol),potassiumcarbonate
(18.93 g, 137.00 mmol), finely ground potassium hydroxide
(7.68 g, 137 mmol), TBAB (1.30 g, 4.04 mmol) and 500 cm3
of CH2Cl2 were added. The reaction mixture was stirred for
12 h at reflux temperature. Salts were removed by filtration
and the filtrate was concentrated under vacuum to dryness
yielding a crude mixture of bis(3,5-dimethylpyrazol-1-yl)-
methane (bdmpzm), bis(3,5-diphenylpyrazol-1-yl)methane
᎐
(Me SiC᎐CSiMe ) (1.00 g, 2.21 mmol) and [{Li(dpmpza)-
᎐
3
3
(H2O)}4], 2 (0.87 g, 0.55 mmol), to give 4 as a blue solid. Yield
60%. (Found: C, 51.04; H, 5.28; N, 7.94. C30H37Cl2N4NbO2Si2
requires C, 51.10; H, 5.31; N, 7.82.) 1H NMR (CD2Cl2, 297 K),
major isomer δ 6.87 (s, 1 H, CH), 6.70 (s, 1 H, H4), 5.86 (s, 1 H,
H4Ј), 2.24 (s, 3 H, Me3), 1.69 (s, 3 H, Me5), 7.94–7.32 (m, 10 H,
᎐
Ph), 0.16 (s, 18 H, Me SiC᎐); minor isomer δ 6.82 (s, 1 H, CH),
᎐
3
6.67 (s, 1 H, H4), 5.98 (s, 1 H, H4Ј), 2.21 (s, 3 H, Me3), 1.68 (s,
5
᎐
3 H, Me ), 7.94–7.32 (m, 10 H, Ph), 0.15 (s, 18 H, Me SiC᎐).
᎐
3
13C–{1H}-NMR (CD2Cl2), major isomer δ 67.6 (CH), 158.5,
154.6, 152.7, 149.3, 147.2, 146.7, 142.2, 141.7 (C3
for both
or
5
(bdphpzm)
and
(3,5-diphenylpyrazol-1-yl-3Ј,5Ј-dimethyl-
isomers), 109.8 (C4), 110.7 (C4Ј), 13.3 (Me3), 10.3 (Me5), 131.0–
pyrazol-1-yl)methane (dpmpzm). Extraction with cold Et2O
yielded a mixture of compounds bdmpzm and dpmpzm, 1. This
mixture was washed with hexane and a white residue of pure
dpmpzm, 1, was isolated. Yield 25%. (Found: C, 76.23; H, 6.10;
N, 17.33. C21H20N4 requires C, 76.80; H, 6.10; N, 17.06%.)
1H-NMR (CDCl3, 297 K): δ 6.15 (s, 2 H, CH2), 6.62 (s, 1 H, H4),
5.83 (s, 1H, H4Ј), 2.19 (s, 3 H, Me3), 2.52 (s, 3 H, Me5), 7.32–7.85
(m, 10H, Ph). 13C–{1H}-NMR (CDCl3), δ 60.1 (CH2), 141.0,
145.9, 148.6, 151.5 (C3 or 5), 103.8 (C4), 106.2 (C4Ј), 13.6 (Me3),
Ϫ
126.1 (Ph for both isomers), 165.4, 160.6 (CO2 for both
᎐
᎐
isomers), 1.1 (Me SiC᎐), 259.1, 250.4 (C᎐C for both isomers);
᎐
᎐
3
minor isomer δ 68.0 (CH), 108.9 (C4), 110.3 (C4Ј), 13.6 (Me3),
5
᎐
10.1 (Me ), Ϫ0.4 (Me SiC᎐). IR (Nujol) 1558 ν(C᎐N), 1692
᎐
᎐
3
νas(CO2Ϫ), 1463 νs(CO2Ϫ), 379, 330 cmϪ1 ν(Nb–Cl). Mass
spectrum (m/z assignment, % intensity, D = Daltons): 705 D
[M ϩ 1], 18; 670 D [M Ϫ Cl], 100.
11.1 (Me5), 125.6–133.1 (Ph). IR (Nujol) 1557 cmϪ1 ν(C᎐N).
3
᎐
[NbCl (ꢀ -dpmpza)(PhC᎐CMe)], 5. The synthetic procedure
᎐
᎐
2
᎐
was the same as for complex 3, using NbCl (dme)(PhC᎐CMe)
᎐
3
[{Li(dpmpza)(H2O)}4], 2. In a 250 cm3 Schlenk tube, dpm-
pzm, [dpmpzm = (3,5-diphenylpyrazol-1-yl- 3Ј,5Ј-dimethyl-
pyrazol-1-yl)methane, 1] (4.00 g, 12.20 mmol) was dissolved in
dry THF (70 cm3) and cooled to Ϫ70 ЊC. A 1.6  solution of
BunLi (7.80 cm3, 12.20 mmol) in hexane was added and the
solution was stirred for 1 h. The reaction mixture was warmed
to 0 oC and the resulting yellow solution was then treated with
(1.00 g, 2.47 mmol) and [{Li(dpmpza)(H2O)}4], 2 (0.93 g, 0.62
mmol), to give 5 as an orange solid. Yield 85%. (Found: C,
57.10; H, 4.32; N, 8.49. C31H27Cl2N4NbO2 requires C, 57.16; H,
4.17; N, 8.60.) 1H NMR (CD2Cl2, 297 K), δ 7.01 (s, 1 H, CH),
6.59 (s, 1 H, H4), 5.92 (s, 1 H, H4Ј), 1.86 (s, 3 H, Me3), 1.78 (s,
5
᎐
3 H, Me ), 7.97–7.37 (m, 15 H, Ph), 3.23 (s, 3 H, ᎐CMe).
᎐
13C–{1H}-NMR (CD2Cl2), δ 67.7 (CH), 158.8, 154.4, 146.5,
D a l t o n T r a n s . , 2 0 0 3 , 1 6 1 4 – 1 6 1 9
1618

View Article Online
or
141.3 (C3 5), 110.1 (C4), 110.3 (C4Ј), 15.2 (Me3), 10.2 (Me5),
M. Lanfranchi and M. A. Pellinghelli, J. Chem. Soc., Dalton Trans.,
Ϫ
1999, 3537; T. C. Higgs and C. J. Carrano, Inorg. Chem., 1997, 36,
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2 J. Fernández-Baeza, F. A. Jalón, A. Otero and M. E. Rodrigo,
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Hermosilla, E. Diez-Barra, J. Fernández-Baeza, M. Fernández-
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and J. Tejeda, J. Chem. Soc., Dalton Trans., 1998, 3737.
3 A. Otero, J. Fernández-Baeza, J. Tejeda, A. Antiñolo, F. Carrillo-
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Lopez, J. Chem. Soc., Dalton Trans., 2000, 2367.
4 A. Otero, J. Fernández-Baeza, A. Antiñolo, F. Carrillo-Hermosilla,
J. Tejeda, E. Díez-Barra, A. Lara-Sánchez, L. Sánchez-Barba,
I. López-Solera, M. R. Ribeiro and J. M. Campos, Organometallics,
2001, 20, 2428; A. Otero, J. Fernández-Baeza, A. Antiñolo,
F. Carrillo-Hermosilla, J. Tejeda, A. Lara-Sánchez, L. Sánchez-
Barba, M. Fernández-López, A. M. Rodríguez and I. López-Solera,
Inorg. Chem., 2002, 41, 5193.
5 N. E. Schlörer and S. Berger, Organometallics, 2001, 20, 1703;
N. E. Schlörer, E. J. Cabrita and S. Berger, Angew. Chem., Int. Ed.,
2002, 41, 107.
6 G. Annibale, L. Canovese, L. Cattalini, G. Natile, M. Biagini-Cingi,
A. Manotti-Lanfredi and A. Tiripicchio, J. Chem. Soc., Dalton
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Jacobson and W. P. Jensen, Inorg. Chim. Acta, 1986, 111, 67;
S. Sommerer, W. P. Jensen and R. A. Jacobson, Inorg. Chim. Acta,
1990, 172, 3.
7 V. Tangoulis, S. Paschalidou, E. G. Bakalbassis, S. P. Perlepes,
C. P. Raptopoulou and A. Terzis, J. Chem. Soc., Chem. Commun.,
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Soc., 1990, 112, 2647.
9 J. Sebastián, P. Sala, J. Del Mazo, M. Sancho, C. Ocho, J. Elguero,
J. P. Fayet and M. C. Vertut, J. Heterocycl. Chem., 1982, 19, 1141;
E. Díez-Barra, A. De la Hoz, A. Sánchez-Migallon and J. Tejeda,
J. Chem. Soc., Perkin Trans., 1993, 1, 1079.
᎐
130.6–127.6 (Ph), 161.5 (CO2 ), 22.9 (᎐CMe) 248.5, 232.4
᎐
(C᎐C); IR (Nujol) 1559 ν(C᎐N), 1693 ν (CO Ϫ), 1462 νs(CO2Ϫ),
᎐
᎐
᎐
as
2
367, 332 cmϪ1 ν(Nb–Cl). Mass spectrum (m/z assignment, %
intensity, D = Daltons): 652 D [M ϩ 1], 21; 616 D [M Ϫ Cl],
100.
3
᎐
[NbCl (ꢀ -dpmpza)(PhC᎐CEt)], 6. The synthetic procedure
᎐
2
᎐
was the same as for complex 3, using NbCl (dme)(PhC᎐CEt)
᎐
3
(1.00 g, 2.38 mmol) and [{Li(dpmpza)(H2O)}4], 2 (0.90 g,
0.60 mmol), to give 6 as a brown solid. Yield 75%. (Found: C,
57.50; H, 4.32; N, 8.49. C32H29Cl2N4NbO2 requires C, 57.76; H,
4.39; N, 8.41.) 1H NMR (CD2Cl2, 297 K), δ 6.99 (s, 1 H, CH),
6.61 (s, 1 H, H4), 5.97 (s, 1 H, H4Ј), 1.86 (s, 3 H, Me3), 1.79 (s,
3 H, Me5), 7.96–7.35 (m, 15 H, Ph), A 3.58, B 3.40, X 1.33
᎐
[ABX3, JAB = 17.0 Hz, JAX = JBX = 7.5 Hz, (᎐CCH CH )].
᎐
2
3
13C–{1H}-NMR (CD2Cl2), δ 67.9 (CH), 158.8, 154.8, 147.0,
or
142.3 (C3 5), 109.3 (C4), 110.7 (C4Ј), 15.5 (Me3), 10.4 (Me5),
Ϫ
᎐
132.5–126.0 (Ph), 161.5 (CO2 ), 31.6 (᎐CCH CH ), 12.6
᎐
2
3
᎐
᎐
(᎐CCH CH ) 251.5, 232.7 (C᎐C); IR (Nujol) 1555 ν(C᎐N),
᎐
᎐
᎐
2
3
1687 νas(CO2Ϫ), 1457 νs(CO2Ϫ), 368, 329 cmϪ1 ν(Nb–Cl). Mass
spectrum (m/z assignment, % intensity, D = Daltons): 665 D [M
ϩ 1], 15; 630 D [M Ϫ Cl], 100.
[(NbCl2O)2(ꢁ-ꢂ1-O,OЈ-tpzpdo)], 7. A CH2Cl2 (100 cm3) solu-
tion of NbCl (κ -dpmpza)(Me SiC᎐CSiMe ), 4 (0.15 g, 0.20
3
᎐
᎐
2
3
3
mmol), was treated with dry O2. The solution was stirred for 4 h
at room temperature. The solvent was removed under vacuum
and a white solid was obtained. Yield 85%. Crystals were grown
from a solution in toluene by slow evaporation of the sol-
vent. (Found: C, 48.82; H, 3.71; N, 10.21. C43H38Cl4N8Nb2O4
requires C, 48.79; H, 3.61; N, 10.59.) 1H NMR (CD2Cl2, 297 K),
δ 7.21 (s, 2 H, CH), 6.78 (s, 2 H, H4), 5.94 (s, 2 H, H4Ј), 2.51
(s, 6 H, Me3), 1.69 (s, 6 H, Me5), 6.64–7.94 (m, 20 H, Ph).
13C–{1H}-NMR (CD2Cl2), δ 69.2 (CH), 149.3, 148.4, 143.8,
142.5 (C3 or 5), 108.6 (C4), 110.8 (C4Ј), 15.6 (Me3), 10.0 (s, Me5),
139.1–126.5 (Ph), 101.0 (s, CO22Ϫ). IR (Nujol) 1554 ν(C᎐N),
᎐
348, 290 cmϪ1 ν(Nb–Cl).
Crystal data for 7. C43H38N8O4Cl4Nb2 (1058.44); crystal size
¯
0.4 × 0.3 × 0.1 mm; trigonal, space group R3c, a = 36.442(5),
b = 36.442(8), c = 19.618(3) Å, Z = 18, V = 22563(7) Å3, ρcalc
=
1.402 g cmϪ3, T = 293 K, µ = 7.16 cmϪ1, 17979 reflections
measured, 5954 were unique (Rint = 0.1485) and 1184 observed
[I > 2σ(I )]; Nonius-Mach3 diffractometer, MoKα radiation
(λ = 0.71073 Å) graphite monochromated; ω scans technique to
a maximum value of 2θ = 56Њ. The structure was solved by
direct methods and refined with the full-matrix, least-squares
method; R1 = 0.0531, wR2 = 0.0912 for [I > 2σ(I )], GOF = 0.831;
data/parameters: 5954/265; largest diff. peak and hole, 0.294
10 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Wiley-VCH, New York, 1997.
11 E. J. Roskamp and S. F. Pedersen, J. Am. Chem. Soc., 1987, 109,
6551.
12 See J. L. Templeton, Adv. Organomet. Chem., 1989, 29, 1.
13 J. Sandström, Dynamic NMR Spectroscopy, Academic Press, New
York, 1982.
14 For axial chirality see: E. L. Eliel and S. H. Wilen, Stereochemistry
of Organic Compounds, Wiley-Interscience, New York, 1994, 1119–
1190.
and Ϫ0.359 e ÅϪ3
CCDC reference number 188238.
See http://www.rsc.org/suppdata/dt/b3/b300497j/ for crystal-
lographic data in CIF or other electronic format.
.
15 A. Antiñolo, F. Carrillo-Hermosilla, J. Fernández-Baeza, M.
Lanfranchi, A. Lara-Sanchez, A. Otero, E. Palomares, M. A.
Pellinghelli and A. M. Rodríguez, Organometallics, 1998, 17,
3015.
16 S. Minhas, A. Devlin, D. T. Richens, A. C. Benyei and P. Lightfoot,
J. Chem. Soc., Dalton Trans., 1998, 953.
Acknowledgements
We gratefully acknowledge financial support from the
Dirección General de Enseñanza Superior e Investigación,
Spain (Grant No. PB98–0159-C02–01).
17 A. Otero, J. Fernández-Baeza, A. Lara-Sánchez, M. Fernández-
López and L. Sánchez-Barba, unpublished results.
18 J. Elguero, E. González and R. Jacquier, Bull. Soc. Chim. Fr., 1968,
707, 5009.
19 F. A. Carey, Organic Chemistry, McGraw-Hill, New York, 1987,
732–773.
References
1 A. Otero, J. Fernández-Baeza, J. Tejeda, A. Antiñolo, F. Carrillo-
Hermosilla, E. Diez-Barra, A. Lara-Sánchez, M. Fernández-López,
D a l t o n T r a n s . , 2 0 0 3 , 1 6 1 4 – 1 6 1 9
1619