Structural characterization of dinuclear Ti(IV) complexes of rigid tetradentate dianionic diamine bis(phenolato) ligands; effect of steric bulk on coordination features

Article abstract of DOI:10.1016/j.jorganchem.2008.09.056

A small difference in diamine bis(phenolato) ligands, namely an additional single methylene unit, directs formation of dinuclear Ti(IV) complexes rather than mononuclear ones as characterized by X-ray crystallography. Varying steric bulk of the ligand affects the coordination number in the dinuclear complexes and the ligand to metal ratio. A ligand with reduced steric bulk leads to a L₂Ti₂(OiPr)₄ type complex featuring two octahedral metal centers bridged only by the two phenolato ligands, whereas a bulky ligand leads to a Ti₂(μ-L¹)(μ-OiPr)₂(OiPr)₄ type complex with a single chelating ligand, two bridging isopropoxo ligands, and two terminal isopropoxo groups on each of the two metal centers, which are of trigonal bi-pyramidal geometry.

Full text of DOI:10.1016/j.jorganchem.2008.09.056

Journal of Organometallic Chemistry 693 (2008) 3947–3950
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Structural characterization of dinuclear Ti(IV) complexes of rigid tetradentate
dianionic diamine bis(phenolato) ligands; effect of steric bulk on coordination
features
Cesar M. Manna, Michal Shavit, Edit Y. Tshuva *
Institute of Chemistry, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2008
Received in revised form 23 September
A small difference in diamine bis(phenolato) ligands, namely an additional single methylene unit, directs
formation of dinuclear Ti(IV) complexes rather than mononuclear ones as characterized by X-ray crystal-
lography. Varying steric bulk of the ligand affects the coordination number in the dinuclear complexes
2
008
and the ligand to metal ratio. A ligand with reduced steric bulk leads to a L
2 2 4
Ti (OiPr) type complex fea-
Accepted 29 September 2008
Available online 4 October 2008
turing two octahedral metal centers bridged only by the two phenolato ligands, whereas a bulky ligand
1
2 2 4
leads to a Ti (l-L )(l-OiPr) (OiPr) type complex with a single chelating ligand, two bridging isopropoxo
ligands, and two terminal isopropoxo groups on each of the two metal centers, which are of trigonal bi-
pyramidal geometry.
Keywords:
Titanium(IV)
O ligands
Ó 2008 Elsevier B.V. All rights reserved.
Dinuclear compounds
Coordination modes
1
. Introduction
Ti(IV) complexes of salan-type diamine bis(phenolato) ligands
ary diamines conveniently give flexible ligands with a single ethyl-
ene linker between the N-atoms, the additional NH group in the
corresponding products derived from the primary diamines is able
to further react with formaldehyde to give a stable five-membered
have been reported to be useful for different applications, ranging
from catalysis to biological activity [1–3]. In these complexes, the
diamine bis(phenolato) ligands of various R groups including ortho
ring. Notably, this reaction is possible both with a flexible ethylene
1
bridge (H
2
L ) as well as with a more constrained trans-cyclohexyl-
0
0
2
1
2
Me and tBu and various R groups including H and –(CH
2
)
4
–, were
ene bridge (H
2 2 2
L ). Thus, H L and H L were obtained in 88% and
shown to lead to well-defined mononuclear C -symmetrical com-
2
54% yields, respectively, as colorless powders. The diastereotopic
2
plexes upon reaction with a Ti(IV) precursor (Scheme 1) [1,4–7].
We reported several such complexes that were studied for biolog-
ical applications, and some were found to be highly cytotoxic
against colon and ovarian cells [3]. Herein, we report the synthesis
of rigid ligands of this family that include an additional single
methylene bridge between the two N-donors (Scheme 2) [8], as
well as the synthesis and structural characterization of their Ti(IV)
complexes. The small structural difference adds significantly to the
ligand rigidity and thus has a major effect on the complexes
formed.
nature of the four benzylic hydrogen atoms in H
by two doublets in its H NMR spectrum of an AX system, while
2
L
is manifested
1
1
these protons appear as a single singlet in the spectrum of H
2
L .
1
Reacting H L with one equivalent of Ti(OiPr) in THF at RT gave
2
4
a yellow solution. Single crystals suitable for X-ray crystallography
were obtained from dichloromethane at ꢀ35°. An ORTEP presenta-
tion of the structure is presented in Fig. 1, and a list of selected
bond lengths and angles is given in Table 1.
2
The structure features a dinuclear species of the formula Ti (l-
L )(l-OiPr) (OiPr) where a single chelating ligand bridges the two
2 4
1
Ti(IV) centers in a total of Ti:L ratio of 2:1 providing one covalent O
donor and one coordinative N-donor to each metal center. Two
additional bridging isopropoxo groups give a Tiꢁ ꢁ ꢁTi distance of
2
. Results and discussion
The synthesis of H
L1–2 is based on a single-step Mannich con-
3
.28 Å. Each Ti(IV) center is also bound to two terminal isopropoxo
2
groups that complete an octahedral geometry. Interestingly, an al-
most planar –Ti–O–Ti–O– array is observed, with a Ti(1)–O(3)–
Ti(2)–O(4) torsion angle of 0.6°. This complex exhibits an approx-
densation between the primary diamine, two equivalents of the
substituted phenol, and formaldehyde (Scheme 3) [8,9]. As second-
imate C
It is thus obvious that the large tBu groups in H
binding of a second dianionic bis(phenolato) ligand in replacement
2
-symmetrical dinuclear core.
L1 preclude
2
*
Corresponding author. Fax: +972 2 6584282.
E-mail address: tshuva@chem.ch.huji.ac.il (E.Y. Tshuva).
0
022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.09.056

3
948
C.M. Manna et al. / Journal of Organometallic Chemistry 693 (2008) 3947–3950
of two additional isopropoxo groups, even when a 1:1 ligand to
metal ratio was employed. Since binding of a second tetradentate
ligand should enhance the complex hydrolytic stability, it was
in Fig. 2, and a list of selected bond lengths and angles is given in
Table 2.
The structure features another dinuclear complex, of the for-
2
interesting to explore the complex obtained with the less sterically
mula Ti
2
(l
-L )
2
(OiPr)
4
, exhibiting a C
i
-symmetry. In this structure,
2
crowded ligand H
substituents.
2
L , featuring Me groups instead of the tBu
two chelating ligands are able to bridge the two Ti(IV) centers due
to their reduced steric bulk, giving a much larger Tiꢁ ꢁ ꢁTi distance of
7.77 Å, since no additional single-atom bridging isopropoxo li-
gands are present. The second bridging ligand may be regarded
2 4
Reacting H in THF at RT gave
L2 with one equivalent of Ti(OiPr)
another yellow solution. Trace amounts of single crystals suitable
for X-ray crystallography were obtained from dichloromethane/
hexane at RT. An ORTEP presentation of the structure is presented
as a replacement for the two bridging isopropoxo groups observed
1
in Ti
2
(l
-L )(
2
l-OiPr) (OiPr)
4
, however, since only one N-donor in
each chelating ligands provides a coordinative bond due to the lar-
ger Tiꢁ ꢁ ꢁTi distance, each Ti(IV) center is penta-coordinate with a
trigonal bi-pyramidal geometry (Tiꢁ ꢁ ꢁN(2): 4.35, 5.18 Å). The two
isopropoxo groups are in a cis configuration on each Ti(IV) center,
and the cyclohexyl rings adopt a chair-like conformation. The
planes of the two central phenolato rings of two separate ligands
are parallel to one another with <2° deviation. It appears that over-
all, despite the incomplete coordination sphere, binding of the sec-
ond chelating ligand is achieved on the expense of binding to two
bridging isopropoxo groups.
OiPr OiPr
R
R
R
OH
HO
Ti
R'
O
O
N
N
R
N
N
R'
R''
R'
R''
R''
R'
R''
Scheme 1.
Cytotoxicity was only measured of the coordinativelly saturated
1
2
2 2 4 2 2 4
Ti (l-L )(l-OiPr) (OiPr) , as Ti (l-L ) (OiPr) of the penta-coordi-
nate metal centers demonstrated significant hydrolytic instability
in air. The measurements were performed on colon HT-29 and
ovarian OVCAR-1 cells based on the MTT (methylthiazolyldiphe-
nyl-tetrazolium bromide) assay as previously described [3]. No
substantial activity was observed for this complex towards both
cell types. Given the activity we observed for some mononuclear
counterparts towards these particular cells and its dependence
on steric bulk [3], we may conclude that a combination of coordi-
native saturation, strong ligand binding, and small steric groups
are required for obtaining fruitful biological interactions and cyto-
toxic activity [3].
H2L1
t
Bu
tBu
OH
HO
N
N
t
Bu
t
Bu
H2L2
OH
HO
N
N
3
. Experimental
Scheme 2.
The synthesis of ligands was generally based on a published
procedure [9]. Paraformaldehyde (95%), aqueous formaldehyde
30–38%), ethylenediamine (99%), trans-1,2-diaminocyclohexane
(
R
(99%), 2,4-di-tert-butylphenol (99%), 2,4-dimethylphenol (97%),
and titanium tetra(isopropoxide) (97%) were purchased and used
as is. All solvents for Ti(IV) complex preparation were distilled
from K or K/benzophenone under nitrogen. All experiments requir-
ing dry atmosphere were performed in a M. Braun dry-box or un-
der nitrogen atmosphere using Schlenck line techniques. NMR data
were recorded using AMX-400 MHz Bruker spectrometer. X-ray
diffraction data were obtained with Bruker Smart Apex diffractom-
eter. Elemental analyses were performed in the microanalytical
laboratory in our institute. Cytotoxicity was measured on HT-29
colon and OVCAR-1 ovarian cells obtained from ATCC Inc. using
the methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay as
OH
H N
NH2
O
2
+
+
R'
R'
H
H
R
R
R
O
OH
HO
H
N
H
N
H
H
R
R
R'
R'
previously described [3,10].
1
H
2
L was synthesized as previously described [8] in 88% yield.
1
3
C NMR (400 MHz; CCl
20.8, 74.4, 59.2, 51.8, 34.9, 34.2, 31.7, 29.6. Anal. Calc. for
: C, 77.90; H, 10.30; N, 5.51. Found: C, 78.11; H,
0.31; N, 5.59%.
3
D): d 154.1, 140.7, 135.7, 123.3, 123.1,
R
R
1
C
1
OH
HO
33
52 2 2
H N O
N
N
2
R
R
H
2
L
was synthesized similarly by reacting paraformaldehyde
8 mmol) with trans-1,2-diaminocyclohexane (4 mmol) and 2,4-
dimethylphenol (9 mmol) in refluxing methanol. A colorless pow-
(
R'
R'
1
der precipitated in 54% yield. H NMR (400 MHz; CCl
(s, 2H, OH), 6.83 (s, 2H, Ar), 6.57 (s, 2H, Ar), 4.15 (d, J = 14 Hz, 2H,
ArCH ), 3.51 (s, 2H, NCH N), 3.36 (d, J = 14 Hz, 2H, ArCH ), 2.32
m, 2H, cy), 2.18 (s, 6H, CH ), 2.17 (s, 6H, CH ), 2.08 (m, 2H, Cy),
1.84(m, 2H, cy), 1.29 (m, 4H, cy). C NMR (400 MHz; CCl D): d
3
D): d 10.48
1
H L : R = tBu; R' = H
2
2
H L : R = Me; R' = -CH CH CH CH -
2
2
2
2
2
2
2
2
(
3
3
1
3
Scheme 3.
3

C.M. Manna et al. / Journal of Organometallic Chemistry 693 (2008) 3947–3950
3949
1
Fig. 1. ORTEP drawing of Ti
2
(
l
-L )(
l
-OiPr)
2
(OiPr)
4
in 50% probability ellipsoids; CH
2
Cl
2
solvent and H atoms were omitted for clarity.
1
2
53.1, 130.8, 127.8, 126.2, 124.6, 120.6, 75.8, 69.1, 56.3, 29.0, 24.1,
0.4, 15.6. Anal. Calc. for C25 : C, 76.10; H, 8.69; N, 7.10.
34 2 2
H N O
Found: C, 75.83; H, 8.84; N, 7.03%.
1
Ti was synthesized by reacting Ti(OiPr)
2
(
l
-L )(
l
-OiPr)
2
(OiPr)
4
4
1
(
0.67 mmol) with H L (0.69 mmol) in THF at RT under nitrogen
2
atmosphere. Following removal of the isopropanol with the sol-
vent, yellow single crystals were obtained from dichloromethane
1
at ꢀ35 °C (10%). H NMR (400 MHz; CCl
3
D): d 7.22 (d, J = 2.4 Hz,
2
H, Ar), 6.91 (d, J = 2.4 Hz, 2H, Ar), 4.88 (m, 6H, CH), 4.23 (s, 4H,
CH ), 4.20 (s, 2H, CH ), 3.05 (s, 4H, CH ), 1.47 (s, 18H, C(CH ),
.29 (s, 18H, C(CH ), 1.27 (d, J = 6.2 Hz, 36H, CH ). C NMR
400 MHz; CCl D): d 158.9, 139.8, 135.7, 124.7, 124.4, 123.1, 76.2,
9.0, 34.9, 34.1, 31.7, 29.7, 26.7. Anal. Calc. for C51 Ti : C,
4.14; H, 9.50; N, 2.93. Found: C, 63.77; H, 9.84; N, 2.92%.
2
2
2
3 3
)
1
3
1
3
)
3
3
(
3
5
6
90
H N
2
O
8
2
Table 1
1
2 2 4
Selected bond lengths (Å) and angles (°) for Ti (l-L )(l-OiPr) (OiPr) .
Lengths
Ti(1)–O(1)
Ti(1)–O(3)
Ti(1)–O(4)
Ti(1)–O(5)
Ti(1)–O(6)
Ti(1)–N(1)
1.9190(17)
2.0914(17)
2.0206(17)
1.8144(18)
1.7888(18)
2.438(2)
Ti(2)–O(2)
Ti(2)–O(3)
Ti(2)–O(4)
Ti(2)–O(7)
Ti(2)–O(8)
Ti(2)–N(2)
1.9121(17)
2.0116(17)
2.0966(17)
1.8182(18)
1.7936(19)
2.430(2)
2
Angles
Fig. 2. ORTEP drawing of Ti
2
(l
-L )
2
(OiPr)
4
in 50% probability ellipsoids; H atoms
O(6)–Ti(1)–O(5)
O(6)–Ti(1)–O(1)
O(5)–Ti(1)–O(1)
O(6)–Ti(1)–O(4)
O(5)–Ti(1)–O(4)
O(1)–Ti(1)–O(4)
O(6)–Ti(1)–O(3)
O(5)–Ti(1)–O(3)
O(1)–Ti(1)–O(3)
O(4)–Ti(1)–O(3)
O(6)–Ti(1)–N(1)
O(5)–T(1)–N(1)
O(1)–Ti(1)–N(1)
O(4)–Ti(1)–N(1)
O(3)–Ti(1)–N(1)
101.40(9)
O(8)–Ti(2)–O(7)
O(8)–Ti(2)–O(2)
O(7)–Ti(2)–O(2)
O(8)–Ti(2)–O(3)
O(7)–Ti(2)–O(3)
O(2)–Ti(2)–O(3)
O(8)–Ti(2)–O(4)
O(7)–Ti(2)–O(4)
O(2)–Ti(2)–O(4)
O(3)–Ti(2)–O(4)
O(8)–Ti(2)–N(2)
O(7)–Ti(2)–N(2)
O(2)–Ti(2)–N(2)
O(3)–Ti(2)–N(2)
O(4)–Ti(2)–N(2)
101.37(9)
and disorder were omitted for clarity.
92.61(8)
96.61(8)
101.03(8)
93.16(8)
161.33(7)
96.64(8)
159.64(8)
91.87(7)
74.04(7)
174.11(8)
83.41(8)
83.41(7)
81.94(7)
79.21(7)
96.35(8)
94.16(8)
103.29(8)
92.70(8)
157.47(8)
92.35(8)
162.93(8)
94.37(7)
74.11(7)
168.62(8)
87.74(8)
76.01(7)
82.85(7)
80.02(7)
Table 2
2
Selected bond lengths (Å) and angles (°) for Ti
2
(l
-L )
2
(OiPr)
4
.
Lengths
Ti(1)–O(1)
Ti(1)–O(2)
Ti(1)–O(3)
1.855(2)
1.8667(19)
1.775(2)
Ti(1)–O(4)
Ti(1)–N(1)
1.801(2)
2.369(2)
Angles
O(3)–Ti(1)–O(4)
O(3)–Ti(1)–O(1)
O(4)–Ti(1)–O(1)
O(3)–Ti(1)–O(2)
O(4)–Ti(1)–O(2)
100.85(10)
96.37(10)
115.19(10)
96.11(10)
114.11(10)
O(1)–Ti(1)–O(2)
O(3)–Ti(1)–N(1)
O(4)–T(1)–N(1)
O(1)–Ti(1)–N(1)
O(2)–Ti(1)–N(1)
125.40(9)
176.26(10)
82.33(9)
80.43(8)
84.28(8)

3
950
C.M. Manna et al. / Journal of Organometallic Chemistry 693 (2008) 3947–3950
Crystal data: C51
0.9072(11), b = 13.5112(13), c = 19.7812(19) Å,
H
92
N
2
O
8
Ti
2
ꢁ CH
2
Cl
2
, M = 1041.99, triclinic, a =
= 85.605(2)°,
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.jorganchem.
2008.09.056.
1
a
3
b = 82.960(2)°,
group P1, Z = 2,
c
= 89.553(2)°, U = 2884.7(5) Å , T = 173(1) K, space
ꢀ
ꢀ1
2
l(Mo K
a
) = 0.418 mm , Reflections measured
3
1637, 12440 unique (Rint = 0.0336), R(F ) for [I > 2r(I)] = 0.0625,
Rw for [I > 2
r
(I)] = 0.1451.
2
Ti
2
(l-L )
2
(OiPr) was synthesized by reacting Ti(OiPr)
4
4
References
2
(
2
0.17 mmol) with H L (0.17 mmol) in THF at RT under nitrogen
[
[
1] A. Yeori, S. Groysman, I. Goldberg, M. Kol, Inorg. Chem. 44 (2005) 4466–4468.
2] Y. Sawada, K. Matsumoto, S. Kondo, H. Watanabe, T. Ozawa, K. Suzuki, B. Saito,
T. Katsuki, Angew. Chem., Int. Ed. 45 (2006) 3478–3480.
atmosphere. Following removal of the isopropanol with the sol-
vent, yellow single crystals were obtained from dichloromethane/
hexane at RT (10%). Further characterization of this compound
was impeded by its hydrolytic instability.
[3] M. Shavit, D. Peri, C.M. Manna, J.S. Alexander, E.Y. Tshuva, J. Am. Chem. Soc. 129
2007) 12098–12099.
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480–2485.
(
[
Crystal data:
62 92 4 8 2
C H N O Ti , M = 1117.20, monoclinic, a =
2
1
2.2048(9), b = 17.7609(13), c = 14.7463(11) Å, b = 106.2130(10)°,
[5] A.J. Chmura, M.G. Davidson, M.D. Jones, M.D. Lunn, M.F. Mahon, A.F. Johnson, P.
Khunkamchoo, S.L. Roberts, S.S.F. Wong, Macromolecules 39 (2006) 7250–
7257.
3
U = 3069.4(4) Å , T = 173(1) K, space group P2
K
1
/n, Z = 2,
l(Mo
ꢀ1
a) = 0.315 mm , Reflections measured 33876, 6706 unique
[
6] J. Balsells, P.J. Carroll, P.J. Walsh, Inorg. Chem. 40 (2001) 5568–5574.
2
(R
int = 0.0396), R(F ) for [I > 2
r
(I)] = 0.0757, Rw for [I > 2
r
(I)] =
[7] S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt, M. Kol, Inorg. Chem. 45 (2006)
4783–4790.
[
[
10] M. Shavit, D. Peri, A. Melman, E.Y. Tshuva, J. Biol. Inorg. Chem. 12 (2007) 825–
30.
0
.1741.
8] X. Xu, Y. Yan, Y. Zhang, Q. Shen, Inorg. Chem. 46 (2007) 3743–3751.
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Acknowledgment
[
8
We thank Dr. Shmuel Cohen for solution of the X-ray structures.
Appendix A. Supplementary material
CCDC 693366 and 693367 contains the supplementary crystal-
2
1
2 2 4 2 2 4
lographic data for Ti (l-L ) (OiPr) and Ti (l-L )(l-OiPr) (OiPr) .