Experimental and Theoretical Studies on the Reactivity of Titanium Chelidamate Complexes: The Significant Role of the Hydroxide Pyridine Moiety

Article abstract of DOI:10.1021/acs.organomet.8b00209

The reaction of [TiCp₂Cl₂] with dipotassium chelidamate yields the carboxylate derivative [TiCp₂{(OOC)₂PyOH}] (2), which is sparsely soluble in water at neutral pH but soluble in basic medium. [TiCp?Cl₃

Full text of DOI:10.1021/acs.organomet.8b00209

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Experimental and Theoretical Studies on the Reactivity of Titanium
Chelidamate Complexes: the Significant Role of the Hydroxide
Pyridine Moiety
,†
†
,‡
Rosa Fandos,* Carolina Hernandez, Anto†nio Otero,* Janira Pacheco,† Ana M. Rodríguez,§
́
†
́
́
María Jose Ruiz, and Juan Angel Organero
†Universidad de Castilla-La Mancha, Instituto de Nanociencia, Nanotecnología y Materiales Moleculares (INAMOL), Facultad de
Ciencias del Medio Ambiente, Avda. Carlos III, s/n, 45071 Toledo, Spain
‡
́
Universidad de Castilla-La Mancha, Centro de Innovacion en Química Avanzada (ORFEO-CINQA), Facultad de Ciencias y
́
Tecnologías Químicas Químicas, Avda. Camilo Jose Cela, 10, 13071 Ciudad Real, Spain
§
́
Universidad de Castilla-La Mancha, ETS Ingenieros Industriales, Campus de Ciudad Real, Avda. Camilo Jose Cela, 3, 13071
Ciudad Real, Spain
S
* Supporting Information
ABSTRACT: The reaction of [TiCp2Cl2] with dipotassium
chelidamate yields the carboxylate derivative [TiC-
p2{(OOC)2PyOH}] (2), which is sparsely soluble in water at
neutral pH but soluble in basic medium. [TiCp*Cl3] reacts with
chelidamic acid in the presence of NEt3 to yield the dimetallic
derivative [TiCp*{(OOC)2PyOH}]2O (5). The reaction of
[TiCp*Cl3] with potassium chelidamate renders the hydroxide
compound [TiCp*(OH){(OOC)2PyOH}] (6). Compound 6
evolves in DMSO solution to give the trimetallic derivative
[TiCp*(DMSO){(OOC)2PyO}]3 (7). Complex 6 reacts with
NaOH to render the heterometallic complex [TiCp*-
{(OOC)2PyONa(OH2)3}]2(O) (8). The structures of complexes
7 and 8b were determined by X-ray diffraction studies. The electrostatic potential surfaces, Mulliken population analysis, and
frontier molecular orbitals were determined for complex 6, allowing us to infer its reactivity.
Scheme 1
INTRODUCTION
■
The synthesis of water-soluble titanium complexes is attracting
considerable attention due to their potential in several fields;
for example, such complexes can be designed to be nontoxic
and environmentally safe precursors of titanium oxide and
other inorganic materials containing titanium.1 Moreover,
titanium complexes have been the subject of interest because
in some cases they show antitumor activity2 or have a strong
antibacterial effect.3 In both instances the stability of the
complexes in water solution is a central issue.
resulting from intra- or intermolecular proton transfer that can
affect the coordination properties of the ligand on account of
the sp3 character of the N in the keto form that can change the
planarity of the ONO coordinating moiety.6 In addition, this
multidentate ligand can be useful for the assembly of MOFs7 as
well as in the area of medicinal chemistry.8
It is worth pointing out that, in spite of the interesting
properties of chelidamic acid, its coordination chemistry with
titanium remains so far unexplored. Taking into consideration
the above comments, we have explored the behavior of two
In previous works we have found that dialkoxide or
dicarboxylate pincer ligands are suitable for the synthesis of
water-soluble tantalum complexes.4,5 Now, we seek to extend
this methodology to the synthesis of water-soluble titanium
complexes stabilized by carboxylate ligands. With this objective
in mind, we thought that it would be useful to draw an analogy
between the properties of titanium derivatives stabilized by
closely related ligands such as deprotonated dipicolinic or
chelidamic acids. The chelidamate anion can bond the metal in
a way similar to that for the dipicolinate group, but it
incorporates an OH moiety that could enhance the solubility
in water of the corresponding metal derivatives. Moreover,
chelidamic acid can adopt two tautomeric (Scheme 1) forms
Special Issue: In Honor of the Career of Ernesto Carmona
Received: April 10, 2018
© XXXX American Chemical Society
A
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organometallic titanium precursors toward the mentioned
chelidamic acid and its potassium salt. Herein, we report the
synthesis of several chelidamate titanium complexes and
compare their behavior in water solution with that of
analogous dipicolinate titanium derivatives. Computational
studies at B3LYP/6-31G**+LANL2DZ level of theory were
also performed to understand the reactivity of one of the
studied compound.
149.5, and 180.5 ppm. The carboxylate moieties give rise to a
singlet at 170.0 ppm.
Probably, the addition of a base produces the deprotonation
of the OH fragment to yield a proposed ionic derivative (3)
that because of its ionic charge and the possibility of hydrogen
bonding with the solvent is water soluble (Scheme 3). In
Scheme 3
RESULTS AND DISCUSSION
■
The reaction of group 4 metallocene complexes with 2,6-
pyridinedicarboxylate ligands has been previously studied;
more specifically, the complex [TiCp2(OOC)2Py] has been
synthesized by reaction of [TiCp2Me2] with dipicolinic acid.9
An alternative procedure is the reaction of [TiCp2Cl2] with the
corresponding sodium carboxylate.10
Following the last synthetic procedure, we have carried out
the synthesis of compound 2 by reaction of [TiCp2Cl2] with a
water solution of (KOOC)2PyOH (prepared in situ from
chelidamic acid with K2CO3) (Scheme 2). Complex 2 was
agreement with this proposal, we have seen that the reaction of
[TiCp2(OOC)2Py] (1) with a solution of NaOH in D2O
under the same experimental conditions does not increase the
solubility of the titanium derivative in D2O and no additional
signals in the NMR spectra appear.
Scheme 2
Complex 3 decomposes slowly in D2O with formation of
CpH; after 5 h in D2O solution, at room temperature, the ratio
of the integral of the signal corresponding to the Cp ligands in
compound 3 and those of the CpH is ca. 10/1. It is interesting
to highlight the solubility and relative stability of the complex
at high pH because conventional titanium compounds are
stable only in strong acidic media.1e
Given the interesting differences found in the reactivity of
[TiCp2(OOC)2Py] and [TiCp2(OOC)2PyOH] with NaOH,
we decided to extend the comparative study to electronically
and sterically more unsaturated titanium precursors. We have
previously reported the synthesis of a series of monocyclo-
pentadienyl titanium dipicolinate derivatives such as
[TiCp*Me(OOC)2Py], which reacts with water to yield the
dimetallic derivative [{TiCp*(OOC)2Py}2(μ2-O)] (4); com-
pound 4 is soluble in toluene or dichloromethane and
insoluble in water.11
isolated as an air-stable yellow solid, sparsely soluble in CH2Cl2
or DMSO and insoluble in toluene or pentane. It has been
spectroscopically and analytically characterized.
1
The H NMR spectrum of 2 in deuterated DMSO shows
two singlet signals at 6.18 and 7.25 ppm assigned to the Cp
ligand and to the aromatic protons of the carboxylate group,
respectively. In addition, the hydroxide proton appears in the
spectrum as a broad singlet at 12.43 ppm. The spectroscopic
data indicate that the coordination of the chelidamate ligand to
the titanium is analogous to that of the previously reported
dipicolinate group.
In order to study if the incorporation of the OH fragment
improves the solubility of the complex in water, we have
recorded the 1H NMR spectra of saturated solutions of
[TiCp2(OOC)2Py]9 (1) and [TiCp2(OOC)2PyOH] (2) in
D2O. The pattern of the 1H NMR spectrum of both complexes
is analogous to that found for each complex in DMSO-d6, and
the integral of the signal corresponding to the residual protons
of the solvent in comparison to that of the titanium-bonded Cp
ligands indicate that the solubility of both compounds in water
is poor.
An interesting issue when studying the solubility of
organometallic complexes in water is to understand how the
pH affects the complex properties; with this aim in mind, we
have compared the behavior of compounds 1 and 2 in basic
solutions.
In order to incorporate the chelidamate ligand into the
coordination sphere of the titanium center, we have followed
two different synthetic approaches. The reaction of
[TiCp*Cl3] with hydrated chelidamic acid in the presence of
triethylamine yields the dimetallic oxide [{TiCp*-
(OOC)2PyOH}2(μ2-O)] (5) (Scheme 4).
Complex 5 was isolated as an orange solid which is soluble
in dichloromethane, THF, or toluene, less soluble in Et2O or
pentane, and insoluble in water; it has been characterized
spectroscopically, and the purity of the sample was established
by elemental analysis. The dimetallic nature of the complex
was confirmed by mass spectrometry.
1
The H NMR spectrum of compound 5 in CDCl3 shows a
singlet signal at 1.99 ppm corresponding to the Cp* ligands
and two doublet signals due to the aromatic protons of the
carboxylate moiety at 7.64 and 7.74 ppm that indicate the lack
of a Cv symmetry plane in the ligand. The hydroxide protons
appear as a singlet at 12.53 ppm. The unsymmetrical
environment of the ligand is also confirmed by the 13C
NMR spectrum. The spectroscopic data along with the
structural data found for the analogous complex with the
dipicolinate ligand11 support the structure proposed in Scheme
4.
The addition of an NaOH solution in D2O (pH 12) to
compound 2 produces a yellow solution that has been studied
1
by NMR; the H NMR spectrum shows two singlet signals at
6.20 and 7.04 ppm corresponding respectively to the Cp
groups and the aromatic protons of the carboxylate ligands.
Additionally, the signal of the OH group does not appear. The
13C NMR spectrum shows the signal of the Cp at 119.1 ppm
and the resonances of the pyridinic ring carbon atoms at 117.6,
B
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Scheme 4
−H2O
As an alternative procedure for the incorporation of the
chelidamate group into the coordination sphere of the
titanium, we have carried out the reaction of [TiCp*Cl3]
with a water solution of (KOOC)2PyOH prepared in situ with
the expectation of obtaining compound 5 by a more direct
procedure (Scheme 5).
(2)
Ti−OH + ROH ⎯⎯⎯⎯⎯→ Ti−OR
Complex 6 is particularly interesting because it incorporates
both a Ti−OH and a ROH function and it evolves according
to reaction 1 or 2 depending on the experimental conditions.
In fact, complex 6 is unstable in DMSO-d6 solution and its
1
evolution has been monitored by H NMR. After 3 h at room
temperature the transformation is complete and the spectrum
shows a broad signal at 3.34 ppm that has a relative integral
corresponding to two protons assigned to the water molecule
eliminated in the condensation process. Moreover, the
spectrum shows a set of signals corresponding to the new
titanium compound [TiCp*(DMSO){(OOC)2PyO}]3 (7)
resulting from the condensation process shown in eq 2. The
most outstanding feature of the spectrum is the high field of
the signals attributed to the ring protons (6.08 ppm). The low
shift of the latter, in comparison to the values found for 5 or for
6, could indicate that in the new complex the chelidamate
fragment adopts the keto form (Scheme 6).
Scheme 5
However, the reaction takes place with formation of an
orange solid, [TiCp*(OH){(OOC)2PyOH}] (6), which, in
contrast with the properties found for compounds 4 and 5, is
insoluble in THF, CH2Cl2, or water but soluble in DMSO. It
has been analytically and spectroscopically characterized. The
1H NMR spectrum of 6 in DMSO-d6 shows two singlets at
1.81 and 7.19 ppm that can be assigned to the Cp* ligand and
to the aromatic protons of the carboxylate ligand. Moreover,
the spectrum shows two broad signals at 3.39 and 12.89 ppm
that can be assigned to the OH protons bonded to the titanium
and to the pyridine moiety, respectively. The structure
proposed is in agreement with the spectroscopic data as well
as with the structural features found for comparable
pentamethylcyclopentadienyl titanium complexes with the
dipicolinate ligand.11 It is noteworthy that under these
experimental conditions the condensation process to give the
dimetallic oxide species does not take place and a titanium
hydroxide can be isolated.
Metal hydroxide complexes are interesting precursors for the
synthesis of heterometallic derivatives,12 and their reactivity
can also serve as models of enzyme active sites.13 Moreover,
hydroxides are proposed to play a crucial role in the formation
of metal oxides.
Although discrete titanium hydroxide derivatives are not
common, a number have been described14 and are known to
easily suffer condensation reactions to yield the corresponding
oxides.15 In the case of compound 6 we can consider two
different intermolecular condensation reactions: (1) the
condensation of two Ti−OH units to yield a μ-oxide dimetallic
complex (eq 1) or (2) the condensation of the Ti−OH moiety
with the HOAr group of a second molecule that would render
an oligometallic titanium aryloxide derivative (eq 2).
Scheme 6
In order to unequivocally establish the coordination
environment of the titanium center in complex 7, we have
studied the structure by X-ray diffraction. Suitable crystals were
obtained by slow diffusion of Et2O into a saturated solution of
6 in wet DMSO. Figure 1 shows an ORTEP drawing of the
molecule, and some important bond distances and angles are
shown in Table 1. The crystals consist of discrete trimetallic
units with a triangular shape in which the titanium atoms are
placed at the vertex and the chelidamate ligands that bridge
two titanium atoms form the edge. Each titanium atom adopts
the geometry of a distorted octahedron with the Cp* ligand
and the pyridine nitrogen placed in apical positions while the
equatorial plane is formed by four oxygen atoms, three from
the chelidamate ligand and one from the DMSO that,
presumably, has been incorporated into the coordination
sphere of the metal in the crystallization process.
The Ti(1)−O(3) bond length of 1.919(5) Å is longer than
−H2O
those normally found in titanium aryloxide compounds,16
(1)
2Ti−OH ⎯⎯⎯⎯⎯→ Ti−O−Ti
C
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the Ti(1)−N(1) bond distance is somewhat shorter than those
found in titanium dipicolinate derivatives.11
Remarkably, the addition of pyridine to a suspension of
compound 6 in CDCl3 gives rise to the formation of 5
(Scheme 8). It is conceivable that, in deuterated chloroform,
Scheme 8
the addition of a base favors the condensation reaction of the
titanium-bonded hydroxide 6 (according to eq 1) to render
compound 5.
The deprotonation of a titanium-bonded hydroxide ligand to
yield an anionic derivative has been previously observed,18 but
in most cases the reaction of a terminal metal hydroxide with a
base results in condensation reactions that yield dimeric oxide-
bridged species.14
Figure 1. ORTEP drawing of the trimetallic chelidamate complex 7.
Hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 7 and 8b
As we have mentioned above, compound 6 is insoluble in
water; in order to increase its solubility and following the
procedure used for complex 2, we have carried out the reaction
of the titanium hydroxide 6 with a water solution of NaOH.
The reaction was carried in a 1:1 molar ratio by addition of a
solution of NaOH in water to a suspension of 6 in THF
(Scheme 9). The heterometallic derivative [TiCp*-
{(OOC)2PyONa(OH2)3}]2 (8) was isolated as an intense
orange solid that is soluble in water or DMSO and insoluble in
pentane, Et2O, or THF. Complex 8 is the result of both a
condensation process of two Ti−OH moieties to give the
expected Ti−O−Ti unit and the appropriate deprotonation of
pyridine-containing OH groups. Additionally, three H2O
molecules are proposed to be coordinated to both sodium
7
8b
Bond Lengths
1.919(5)
Ti(1)−O(3)
Ti(1)−N(1)
Ti(1)−O(1)
Ti(1)−C(9)
C(3)−C(4)#2
C(2)−C(3)
N(1)−C(2)
O(3)−C(4)
Ti(1)−O(4)
Ti(1)−O(11)
Ti(1)−N(1)
Ti(1)−O(1)
Ti(1)−O(3)
C(4)−C(5)
C(5)−C(6)
N(1)−C(6)
O(5)−C(4)
Na(1)−O(5)
Na(1)−O(14)
1.814(1)
2.085(2)
2.035(1)
2.036(1)
1.442(3)
1.364(3)
1.351(2)
1.258(2)
2.325(2)
2.276(2)
2.117(5)
2.018(4)
2.388(5)
1.408(6)
1.370(7)
1.310(5)
1.283(8)
2.10(1)
1
Bond Angles
151.6(4)
85.1(2)
centers (see Scheme 9). The H NMR of compound 8 in
C(4)−O(3)−Ti(1)
O(1)−Ti(1)−O(4)
C(4)−O(5)−Na(1)
Ti(2)−O(11)−Ti(1)
127.0(1)
175.01(9)
DMSO-d6 shows a broad singlet at 3.34 ppm with an integral
corresponding to 12 protons that can be assigned to the
proposed six water molecules in the molecule. Moreover, the
spectrum indicates that the complex in solution is Cs
symmetric.
Slow diffusion of Et2O into a saturated solution of 8 in
DMSO yields orange crystals of 8b that have been
characterized by X-ray diffraction. Figure 2 shows an
while the O(3)−C(4) bond distance (1.283(8) Å) is shorter
than the C−O bond lengths found in aryloxide ligands bonded
to titanium.17 These geometrical features point to an important
contribution of the keto form to the bonding of the
chelidamate fragment to the metal (Scheme 7b). Moreover,
Scheme 7
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Scheme 9
Figure 2. (a) ORTEP drawing of the asymmetric unit of complex 8b. Hydrogen atoms are omitted for clarity. (b) Representation of the polymeric
arrangement of 8b.
Scheme 10
ORTEP drawing of the molecules, and some selected bond
distances and angles can be found in Table 1.
terminal and one bridging DMSO molecule, and apical
positions occupied by the oxygens of one terminal and one
bridging DMSO ligands.
The titanium−O bond distances (Ti(1)−O(11) 1.814(1) Å
and Ti(2)−O(11) 1.806(1) Å) are in the range expected for
di-μ-oxo-dititanium bridging systems.19 In contrast, the
Ti(1)−O(11)−Ti(2) bond angle (175.01(9)°) is greater
than that found in complex 4 (129.6(1)°).11
The geometrical parameters within the pyridinic ring are
close to those in 7, indicating an important participation of the
keto form B (Scheme 10). In this way, the C(4)−C(5) bond
length (1.442(3) Å) is longer than the C(5)−C(6) distance
(1.364(3) Å): moreover, the O(5)−C(4) bond length
The structural study shows that complex 8b crystallizes in
DMSO to render a linear polymer in which Ti2Na2
tetrametallic units are brought together by DMSO bridging
ligands. The titanium centers display a pseudo-square-planar
pyramidal geometry in which the metal is bonded to the Cp*
ligand placed in an apical position to an oxide bridging ligand
and to the chelidamic fragment that bonds the metal as an
ONO pincer ligand. Moreover, the chelidamic group is bonded
to a sodium ion through the aryloxide moiety. The sodium
shows a trigonal-bipyramidal geometry with the equatorial
plane for the oxygen atoms of the chelidamic ligand, one
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(1.258(2) Å) is intermediate between those expected for C−O
single and double bonds. The Na(1)−O(5) and Na(2)−
O(10) bond distances (2.325(2) and 2.281(2) Å, respectively)
are long for terminal sodium aryloxides.20
electrostatic potential on a molecular surface. A deep blue
color indicates the highest electrostatic potential energy and
deep red the lowest. The color code of the obtained maps for 6
is in the range −0.031 to +0.011 au (Figure 4). Negative
THEORETICAL CALCULATIONS
■
Quantum chemical calculations at the DFT level were
performed to gain further insight into the reactivity of complex
6. The optimized structure of 6 in the gas phase shows a
distorted-trigonal-bipyramidal geometry of the titanium atom21
and reveals that the chelidamate moiety is not totally planar
(Figure 3). The orientation of the hydroxypyridine ring can be
Figure 4. Molecular electrostatic potential maps of 6 computed at the
B3LYP/6-31G**+LANL2DZ level in the gas phase and in the bulk
solvents CHCl3 and DMSO using the SMD model: (A) top view of 6;
(B) rotated 90° view. The deepest red color corresponds to the lowest
electrostatic potential energy value in atomic units (hartree) and blue
to the highest.
Figure 3. Geometry of the stationary point for 6 in the gas phase in
the electronic ground state computed at the B3LYP/6-
31g**+LANL2DZ level.
potential regions are related to reactions with electrophiles and
positive potential regions to reactions with nucleophiles. Figure
4A shows a top view of the MEP maps for 6, and they reveal
that the most negative potential regions are on the oxygen
atoms of the dicarboxylate moieties and on O(38) linked to a
titanium atom. Analyzing the maximum MEP values obtained
around the hydroxyl group atoms of 6, we observed an increase
in the nucleophilic character around O(38) as the polarity
increases (Table 2). On the other hand, no significant effect
defined by the torsion angles C(8)−C(7)−C(6)−O(3)
(−174.3°) and C(32)−C(31)−C(30)−O(28) (174.2°) that
reflect an slight deviation from the planarity due to the
coordination with the titanium atom. The calculated Ti(1)−
O(3), Ti−O(28), and Ti−N(2) bond lengths (2.018, 2.011,
and 2.155 Å, respectively) are in the range expected for
pyridine carboxylate titanium complexes11 and the Ti−O(38)
bond length (1.840 Å) is close to the values obtained
Table 2. MEP Values, in Atomic Units (hartree), Obtained
around Atoms of Hydroxyl Groups of 6
22
theoretically (1.812 Å) for Ti(OH)4 or experimentally
(1.810(2) Å) for terminal titanium hydroxide complexes.15,18
The bulk solvent effect of a low-polarity solvent (CHCl3)
and a polar solvent (DMSO) on the molecular geometry and
charge densities of 6 was investigated by the solvation model
based on density (SMD).23
atom
gas phase
CHCl3
DMSO
O(38)
H(39)
O(5)
−0.203
+0.054
−0.008
+0.090
−0.236
+0.059
−0.0006
+0.102
−0.311
+0.057
−0.004
+0.110
Geometrical parameters are close to those obtained in the
gas phase, but it is noticeable that, in DMSO, Ti(1)−O(28),
Ti(1)−O(3), O(5)−H(40), and O(38)−H(39) bond lengths
slightly increase (0.019, 0.014, 0.005, and 0.004 Å,
respectively) in comparison to those values in the gas phase
or using CHCl3 as solvent. The observed variations in O−H
bond lengths could be correlated with a diminishing of the pKa
values.24
The molecular electrostatic potential (MEP) is a good guide
for the purpose of assessing the reactivity related with the
molecular charge distribution and other related properties.
Molecular recognition between molecules can be also studied
through MEP, as it is through their potentials that the two
molecules first interact with each other.25 MEP maps show
different colors that represent the different values of the
H(40)
was observed around O(5), which possesses a less significant
nucleophilic character. In contrast, the most electrophilic
region was found on H(40) in DMSO, while H(39) has a
substantially less electrophilic character with no significant
solvent polarity effect (Table 2). In addition, a side view of the
MEP maps (Figure 4B) reveals that the size of the electrophilic
area around H(40) (deepest blue) increases as the polarity of
the solvent increases. Therefore, we can assert that that, in
DMSO, the titanium OH moiety is basic enough and the
pyridine OH is acidic enough to react with each other to give 7
plus water.
F
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used as references, and chemical shifts are reported in units of parts
per million relative to SiMe4.
Finally, it is also worth noting that the molecular moiety
with MEP values closer to zero (yellow-green) corresponds to
the pyridine ring (Figure 4B), indicating that this is the less
polar region of the molecule.
The electrostatic potential properties of 6 described above
can lead to intense antiparallel interactions between the most
electrophilic region (H(40)) of a chelidamate ligand and the
most nucleophilic region (O(38)) located in another ligand.
These interactions are expected to be more intense in polar
solvents such as DMSO, and they could be the driving force of
the molecular recognition that precedes the formation of
compound 7. Antiparallel interactions between some chelida-
mate derivatives have been previously reported.26
Synthesis of [TiCp2{(OOC)2PyOH}] (2). To a solution of
[TiCp2Cl2] (0.515 g, 2.07 mmol) in THF (5 mL) was added a
water solution of (HOOC)2PyOH (0.378 g, 2.06 mmol) and K2CO3
(0.285 g, 2.06 mmol), and the mixture was stirred, at room
temperature, for 1 h. Afterward, the solvent was removed under
vacuum and the residue was washed with water (2 × 5 mL) and dried
under vacuum. The yellow solid obtained was identified as
[TiCp2{(OOC)2PyOH}] (2). Yield: 0.565 g, 76%. 1H NMR
(DMSO-d6, room temperature): δ (ppm) 6.18 (s, 10H, Cp), 7.25
(s, 2H, Ar), 12.43 (br, 1H, OH). 1H NMR (D2O, room temperature):
δ (ppm) 6.25 (s, 10H, Cp), 7.09 (s, 2H, Ar). Anal. Calcd for
C17H13O5NTi: C, 56.84; H, 3.64; N, 3.89. Found: C, 56.57; H, 3.84;
N, 3.71.
Finally, the molecular orbital plots reveal that both HOMO
and LUMO orbitals have participation of d orbitals of the Ti
atom. They are mainly π type orbitals with a significant
contribution of the Cp* to the former and of the
hydroxypyridine moiety to the latter (Figure 5).
Reaction of [TiCp2{(OOC)2PyOH}] (2) with NaOH. To a sample
of compound 2 (10 mg) was added a solution of NaOH in D2O (0.6
mL, 0.01 M), and the yellow solution that formed was characterized
by NMR. 1H NMR (D2O, room temperature): δ (ppm) 6.20 (s, 10H,
Cp), 7.04 (s, 2H, Ar). 13C NMR (D2O, room temperature): δ (ppm)
117.6 (Cp), 119.1 (Ar), 149.5 (Ar), 170.0 (COO), 180.5 (Ar).
Synthesis of [{TiCp*(OOC)2PyOH}2(μ2-O)] (5). To a solution of
[TiCp*Cl3] (0.212 g, 0.73 mmol) in THF were added
[(HOOC)2PyOH].H2O (0.147 g, 0.73 mmol) and NEt3 (0.204
mL, 1.46 mmol), and the mixture was stirred, at room temperature,
for 72 h. The solvent was removed under vacuum, and the residue was
extracted with toluene. The orange solid obtained upon evaporation
of the solvent was washed with pentane and identified as 5 (0.164 g,
1
60%). H NMR (CDCl3, room temperature): δ (ppm) 1.99 (s, 30H,
Figure 5. Shapes of the HOMO and LUMO of compound 6
computed at the B3LYP/6-31G**+LANL2DZ level in the gas phase.
Cp*), 7.64 (d, J = 1.8 Hz, 2H, Ar), 7.74 (d, J = 1.8 Hz, 2H, Ar), 12.53
(s, 2H, OH).13C NMR (CDCl3, room temperature): δ (ppm) 12.1
(Cp*), 112.8 (Ar), 114.9 (Ar), 131.8 (Cp*), 149.9 (Ar), 152.4 (Ar),
167.3 (COO), 169.7 (COO), 173.7 (Ar). Anal. Calcd for
C34H36N2O11Ti2: C, 54.85; H, 4.87; N, 3.76. Found: C, 54.61; H,
5.04; N, 3.53. ESI-MS (electrospray ionization/mass spectrometry;
m/z): calculated for C34H36N2O11Ti2, 744.4; found, 744.9.
Synthesis of [TiCp*(OH){(OOC)2PyOH}] (6). To a solution of
[TiCp*Cl3] (0.248 g, 0.86 mmol) in THF (10 mL) was added a
solution of chelidamic acid (0.157 g, 0.86 mmol) and K2CO3 (0.118
g, 0.86 mmol) in 1 mL of H2O, and the mixture was stirred at room
temperature for 1 h. Afterward, the solvent was removed under
vacuum and the solid washed with water (3 × 5 mL). The orange
compound was identified as [TiCp*(OH){(OOC)2PyOH}] (6).
Yield: 0.281 g, 86%. 1H NMR (DMSO-d6, room temperature): δ
(ppm) 1.81 (s, 15H, Cp*), 3.39 (br, 1H, OH), 7.19 (s, 2H, Ar), 12.89
(br, 1H, OH). 13C NMR (DMSO-d6, room temperature): δ (ppm)
11.9 (Me-Cp*), 112.5 (Ar), 129.7 (Cp*), 151.4 (Ar), 166.0 (COO),
172.7 (Ar). Anal. Calcd for C17H19NO6Ti: C, 53.56; H, 5.02; N, 3.67.
Found: C, 54.07; H, 5.21; N, 3.43.
CONCLUSION
■
In this study we compare the properties of bis-
(cyclopentadienyl) and mono(cyclopentadienyl) titanium
chelidamate complexes with the behavior of previously
reported titanium dipicolinate derivatives. We have seen that
the OH moiety in the chelidamate ligand does not increase the
solubility of the titanocene complex in neutral D2O but allows
its solubilization in basic media. In the case of monocyclo-
pentadienyl complexes we have been able to isolate a titanium
hydroxide compound (6) that evolves in DMSO solution to
render a neutral trimetallic cyclic architecture in which the
chelidamate ligand behaves as an organic linker. Moreover, in
the presence of a base, in CDCl3, compound 6 reacts to yield a
dimetallic oxide. The hydroxide compound 6 can be
deprotonated with NaOH to render a water-soluble Ti−Na
heterometallic complex that crystallizes as a linear polymer.
The theoretical calculations reveal that the two hydroxyl
groups of 6 show significant differences in their nucleophilic
and electrophilic character that can justify the reactivity of 6 in
polar media.
Synthesis of [TiCp*(DMSO){(OOC)2PyO}]3 (7). The evolution
of a solution of 6 (20 mg) in DMSO-d6 was monitored by 1H and 13
C
NMR. After 3 h at room temperature the transformation was
complete and indicated the formation of the new compound 7 along
with the stoichiometric amount of water. 1H NMR (DMSO-d6, room
temperature): δ (ppm) 1.98 (s, 15H, Cp*), 3.34 (br, 2H, H2O), 6.08
(s, 2H, Ar). 13C NMR (DMSO-d6, room temperature): δ (ppm) 12.6
(Me-Cp*), 112.8 (Ar), 132.8 (Cp*), 148.1 (Ar), 168.2 (COO), 177.9
(Ar).
EXPERIMENTAL DETAILS
Crystals of 7 were obtained by slow diffusion of Et2O into a
saturated solution of 6 in DMSO.
■
General Procedures. The experimental procedures reported here
were carried out, unless otherwise mentioned, under an argon
atmosphere using standard vacuum line and Schlenk techniques. All
organic solvents were dried and distilled under an argon atmosphere.
Water was distilled and deoxygenated. [TiCp*Cl3] was prepared
according to literature procedures.27
Synthesis of [TiCp*{(OOC)2PyONa(OH2)3}]2(O) (8). To a
suspension of compound 6 (0.977 g, 2.56 mmol) in THF was
added NaOH (2.35 mL, 1.09 M in H2O), and the resulting
suspension was stirred at room temperature for 1 h. The solvent was
removed by filtration; the orange solid was dried under vacuum and
1
was characterized as 8. Yield: 1.003 g, 87%. H NMR (DMSO-d6,
The commercially available compounds dipicolinic acid and
chelidamic acid were used as received from Aldrich. 1H and 13C
NMR spectra were recorded on a 400 MHz Avance Bruker Fourier
transform spectrometer. Trace amounts of protonated solvents were
room temperature): δ (ppm) 1.68 (s, 30H, Cp*), 3.34 (s, 12H, H2O),
6.18 (s, 4H, Ar). 13C NMR (DMSO-d6, room temperature): δ (ppm)
11.6 (Me-Cp*), 115.2 (Ar), 126.9 (Cp*), 150.7 (Ar), 169.4 (COO),
G
DOI: 10.1021/acs.organomet.8b00209
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
181.6 (Ar). Anal. Calcd for C34H46N2O17Ti2Na2: C, 45.55; H, 5.17;
N, 3.12. Found: C, 45.66; H, 5.00; N, 3.01.
Ministerio de Economia y Competitividad (MINECO) of
Spain (Grants CTQ2014-52899-R and CTQ2014-51912-
REDC) and the Junta de Comunidades de Castilla-La Mancha
(Grant PPII-2014-011-P).
Crystals of 8b can be obtained by slow diffusion of Et2O into a
saturated solution of 8 in DMSO.
1H NMR (D2O, room temperature): δ (ppm) 1.74 (s, 30H, Cp*),
2.58 (DMSO), 6.68 (s, 4H, Ar). 13C NMR (D2O, room temperature):
δ (ppm) 11.1 (Me-Cp*), 116.2 (Ar), 131.3 (Cp*), 149.5 (Ar), 171.2
(COO), 182.4 (Ar).
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00209.
Crystallographic data for 7·4.5Me2SO and 8b (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for R.F.: Rosa.Fandos@uclm.es.
*E-mail for A.O.: Antonio.Otero@uclm.es.
■
ORCID
Antonio Otero: 0000-0002-2921-2184
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This paper is dedicated to Professor Ernesto Carmona,
excellent chemist and friend, on the occasion of his retirement.
We gratefully acknowledge financial support from the
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