Diarylmethyl substituted titanocenes: Promising anti-cancer drugs

Article abstract of DOI:10.1016/j.poly.2006.01.007

From the reaction of tert-butyl lithium with p-bromo-N,N-dimethylaniline (1a), p-bromoanisole (1b) or 1-bromo-3,5-dimethoxybenzene (1c), p-N,N-dimethylanilyl lithium (2a), p-anisyl lithium (2b) or (3,5-dimethoxyphenyl) lithium (2c), respectively, were obt

Full text of DOI:10.1016/j.poly.2006.01.007

Polyhedron 25 (2006) 2101–2108
www.elsevier.com/locate/poly
Diarylmethyl substituted titanocenes: Promising anti-cancer drugs
*
´
Clara Pampillon, Oscar Mendoza, Nigel J. Sweeney, Katja Strohfeldt, Matthias Tacke
Conway Institute of Biomolecular and Biomedical Research, The UCD School of Chemistry and Chemical Biology, Centre for Synthesis and
Chemical Biology (CSCB), University College Dublin, Belfield, Dublin 4, Ireland
Received 5 December 2005; accepted 11 January 2006
Available online 20 February 2006
Abstract
From the reaction of tert-butyl lithium with p-bromo-N,N-dimethylaniline (1a), p-bromoanisole (1b) or 1-bromo-3,5-dimethoxyben-
zene (1c), p-N,N-dimethylanilyl lithium (2a), p-anisyl lithium (2b) or (3,5-dimethoxyphenyl) lithium (2c), respectively, were obtained.
When reacted with 6-(p-N,N-dimethylanilinyl)fulvene (3a), 6-(p-methoxyphenyl)fulvene (3b) or 3,5-(dimethoxyphenyl)fulvene (3c), the
corresponding lithiated intermediates were formed (4a–c). Titanium tetrachloride was added ‘‘in situ’’, obtaining titanocenes 5a–c,
respectively. When these titanocenes were tested against pig kidney carcinoma (LLC-PK) cells, inhibitory concentrations (IC50) of
3.8 · 10ꢀ5 M, 4.5 · 10ꢀ5 M, and 7.8 · 10ꢀ5 M, respectively, were observed. These values represent improved cytotoxicity against
LLC-PK, compared to their ansa-analogues.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Anti-cancer drug; Cis-platin; Titanocene; Fulvene; LLC-PK; tert-Butyl lithium
1. Introduction
PK [21]. It was followed by reports about heteroaryl
[22] and methoxyphenyl [23,24] substituted ansa-titanoc-
enes, which show similar IC50 values. Our most cytotoxic
ansa-titanocene [1,2-di(cyclopentadienyl)-1,2-bis(m-dimeth-
oxyphenyl)ethanediyl] titanium dichloride (Titanocene Z)
shows an IC50 value of 2.1 · 10ꢀ4 M when tested on the
LLC-PK cell line [23].
Synthesising the analogous unbridged titanocenes by
establishing a completely new synthetic route, which has
been published recently, further increased the cytotoxic
effect. Bis-[(p-methoxybenzyl)cyclopentadienyl] titanium(IV)
dichloride (Titanocene Y), which has an IC50 value of
2.1 · 10ꢀ5 M when tested on the LLC-PK cell line, was
synthesised from fulvene and super hydride (LiBEt3H) fol-
lowed by transmetallation with titanium tetrachloride [25].
The anti-proliferative activity of Titanocene X, Y and Z
was studied in 36 human tumor cell lines [26] and in four
freshly explanted human tumors using Titanocene X [27].
These in vitro and ex vivo experiments showed that pros-
tate, cervix and renal cell cancers are prime targets for this
novel class of titanocenes.
Despite the resounding success of cis-platin and closely
related platinum antitumor agents, the movement of other
transition-metal anti-cancer drugs towards the clinic has
been exceptionally slow [1–3]. Metallocene dichlorides
(Cp2MCl2) with M = Ti, V, Nb and Mo show remarkable
antitumor activity [4,5]. The efficacy of Cp2TiCl2 in phase
II clinical trials in patients with metastatic renal-cell car-
cinoma [6] or metastatic breast cancer [7] was too low
to be pursued, leading to more synthetic effort in order
to increase the cytotoxicity of titanocene dichloride deriv-
atives [8–12]. A method starting from titanium dichloride
and fulvenes [13–16] allows direct access to highly
substituted ansa-titanocenes [17–20]. By using this
method, [1,2-di(cyclopentadienyl)-1,2-di-(4-N,N-dimethyl-
aminophenyl)-ethanediyl] titanium dichloride (Titanocene
X, Fig. 1) was synthesised in our workgroup, which has
an IC50 value of 2.7 · 10ꢀ4 M when tested for the cyto-
toxic effect on the pig kidney carcinoma cell line LLC-
*
Motivated by these very promising results, we want to
present in this paper a third method leading towards
Corresponding author.
E-mail address: matthias.tacke@ucd.ie (M. Tacke).
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.01.007

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C. Pampillo´n et al. / Polyhedron 25 (2006) 2101–2108
MeO
Me2N
OMe
Titanocene Y
Titanocene Z
Titanocene X
MeO
Cl
Ti
H
H
Cl
Ti
Cl
Ti
Cl
MeO
Cl
Cl
H
H
Me2N
MeO
MeO
Fig. 1. Structure of Titanocene X, Y and Z.
substituted titanocenes as possible anti-cancer drugs using
substituted benzyl lithium, benzyl substituted fulvenes
and titanium tetrachloride, resulting in the formation of
unbridged diarylmethyl substituted titanocenes. The
optimised synthesis, underlined by quantum chemical cal-
culations, and first results of very promising in vitro studies
will be shown.
was warmed up to 0 ꢁC for 20 min, resulting in the forma-
tion of the yellow lithium intermediate (2a).
In a second Schlenk flask, 1.00 g (5.07 mmol) of 6-
(p-N,N-dimethylanilinyl)fulvene (3a) was dissolved in
25 ml of THF and the resultant red solution was added
via a cannula at ꢀ78 ꢁC to the Schlenk flask containing
the lithiated intermediate (2a). The reaction mixture was
allowed to warm up to room temperature and left stirring
for 40 min. Afterwards, 2.53 ml (2.53 mmol) of titanium
tetrachloride was added at room temperature and the mix-
ture was refluxed for 24 h. The solvent was removed under
vacuum, resulting in a dark brown precipitate. This precip-
itate was dissolved in dichloromethane and filtered through
celite to remove the LiCl, followed by two gravity filtra-
tions. The solvent was removed under reduced pressure
forming a shiny black solid (5a), which was washed with
150 ml of pentane and then dried in vacuo (2.05 g,
2.82 mmol, 55.1% yield).
2. Experimental
2.1. General conditions
Titanium tetrachloride (1.0 M solution in toluene) and
tert-butyl lithium (1.7 M solution in cyclohexane) were
obtained commercially from Aldrich Chemical Co. THF
was dried over Na and benzophenone and it was freshly
distilled and collected under an atmosphere of argon prior
to use. Manipulations of air and moisture sensitive com-
pounds were done using standard Schlenk techniques,
under an argon atmosphere. NMR spectra were measured
on either a Varian 300 or a 500 MHz spectrometer. Chem-
ical shifts are reported in ppm and are referenced to TMS.
IR spectra were recorded on a Perkin–Elmer Paragon 1000
FT-IR Spectrometer employing a KBr disk. UV–Vis spec-
tra were recorded on a Unicam UV4 Spectrometer.
1H NMR (d ppm CDCl3, 300 MHz): 6.95 [d, 8H, J
8.68 Hz, C6H4N(CH3)2]; 6.74 [d, 8H,
J
7.53 Hz,
C6H4N(CH3)2]; 5.92–5.56 [m, 8H, C5H4]; 5.52 [s, 2H,
C5H4–CH–(C6H4N(CH3)2)2]; 2.92 [s, 24H, C6H4N(CH3)2].
13C NMR (d ppm CDCl3, 125 MHz): 148.2, 141.5,
133.2, 129.69, 129.68, 120.44, 113.4 [C5H4 and C6H4];
50.3 [C6H4N(CH3)2]; 41.3 [Cp–CH–(C6H4N(CH3)2)2].
IR absorptions (cmꢀ1 KBr): 3100, 3085, 2920, 2852,
1593, 1488, 1478, 1440, 1345, 1163, 1128, 802, 548, 527.
Anal. Calc. for C44H50N4Cl2Ti: Theory: C, 70.25; H,
6.94; N, 7.45; Cl, 4.71. Found: C, 70.00; H, 6.90; N, 7.40;
Cl, 4.71%.
2.2. Synthesis
6-(p-N,N-Dimethylanilinyl)fulvene (3a), 6-p-(methoxy-
phenyl)fulvene (3b) and 3,5-dimethoxyphenyl fulvene (3c)
were synthesised according to the already published proce-
dures [21,23].
UV–Vis (CH2Cl2/e:[cm2/mol]): k 230 nm (e 22770), k
402 nm (e 2020), k 509 nm (e 210), kmax 521 nm (weak).
2.3. Bis-[di-(p-N,N-dimethylaminophenyl)-
methylcyclopentadienyl] titanium (IV) dichloride,
{g5-C5H4–CH–[C6H4–N(CH3)2]2}2TiCl2 (5a)
2.4. Bis-[di-(p-methoxyphenyl)methylcyclopentadienyl]
titanium (IV) dichloride, {g5-C5H4–CH–
[C6H4–O–CH3]2}2TiCl2 (5b)
To a Schlenk flask with 1.00 g (5.18 mmol) of 4-bromo-
N,N-dimethylaniline (1a), 20 ml of THF was added until a
transparent solution was formed, while stirring. The solu-
tion was cooled down to ꢀ78 ꢁC for 15 min and 3.30 ml
(5.57 mmol) of tert-butyl lithium was added. The solution
To a Schlenk flask with 1.5 ml (5.43 lmol) of 4-bromo-
anisole (1b), 20 ml of THF was added until a transparent
solution was formed, while stirring at room temperature.
The solution was cooled down to ꢀ78 ꢁC for 15 min and
5.3 ml (85.7 lmol) of tert-butyl lithium was added. The

C. Pampillo´n et al. / Polyhedron 25 (2006) 2101–2108
2103
solution was allowed to warm up to 0 ꢁC for 20 min, result-
ing in the formation of the yellow lithium intermediate
(2b).
through celite to remove the LiCl. The black filtrate was fil-
tered additionally twice by gravity filtration. The solvent
was removed under reduced pressure forming a shiny black
solid (5c), which was washed with 20 ml of pentane and
then dried in vacuo (3.33 g, 4.05 mmol, 43.5% yield).
1H NMR (d ppm CDCl3, 300 MHz): 7.18 [t, 4H, J
2.24 Hz, C6H3(OCH3)2]; 6.61 [dd, 4H, J1 6.61 Hz, J2
2.23 Hz, C6H3(OCH3)2]; 6.49 [dd, 4H, J1 6.61 Hz, J2
2.42 Hz, C6H3(OCH3)2]; 6.53 [d, 4H, J 2.23 Hz, C5H4–
CH–(C6H3(OCH3)2)]; 6.52 [d, 4H, J 2.23 Hz, C5H4–CH–
(C6H3(OCH3)2)]; 5.29 [s, 2H, C5H4–CH–(C6H3(OCH3)2)];
3.72 [m, 24H, C6H3(OCH3)2].
In a second Schlenk flask, 1.00 g (5.43 mmol) of
p-(methoxyphenyl)fulvene (3b) was dissolved in 25 ml of
THF and the resultant red solution was added via a can-
nula at ꢀ78 ꢁC to the Schlenk flask containing the lithiated
intermediate. The reaction mixture was then allowed to
warm up to room temperature and left for stirring for
40 min. Titanium tetrachloride (2.7 ml, 2.715 mmol) was
added afterwards in situ at room temperature and the mix-
ture was refluxed for 24 h. Subsequently, the solvent was
removed under vacuum, resulting in the formation of a
dark brown to black precipitate. This precipitate was dis-
solved in dichloromethane and filtered through celite to
remove the LiCl, followed by two gravity filtrations. The
solvent was removed under reduced pressure forming a
shiny black solid (5b), which was washed with pentane
and then dried in vacuo (3.83 g, 5.45 mmol, 43.5% yield).
1H NMR (d ppm CDCl3, 300 MHz): 6.96 [d, 8H, J
8.68 Hz, C6H4OCH3]; 6.74 [d, 8H, J 7.53 Hz, C6H4OCH3];
5.92–5.56 [m, 8H, C5H4]; 5.52 [s, 2H, C5H4–CH–
(C6H4OCH3)2]; 2.92 [s, 12H, C6H4OCH3].
13C NMR (d ppm CDCl3, 500 MHz): 161.1, 160.7,
129.9, 122.9, 109.9, 109.6, 106.0, 100.2, 99.6 [C5H4 and
C6H3]; 56.33, 56.26 [C6H3(OCH3)2]; 33.9 [Cp–CH–
(C6H3(OCH3)2)2]. (The rotation of the 3,5-dimethoxyphe-
nyl group is hindered.)
IR absorptions (cmꢀ1 KBr): 2931, 2832, 1606, 1509,
1461, 1425, 1299, 1174, 1108, 1031, 831.
Anal. Calc. for C44H46O8Cl2Ti: Theory: C, 64.32; H,
5.64; Cl, 8.62. Found: C, 64.14; H, 5.60; Cl, 8.57%.
UV–Vis (CH2Cl2/e:[cm2/mol]): k 275 nm (e 79830), k
378 nm (e 17830), k 525 nm (e 3820), k 580 nm (weak), kmax
685 nm (weak).
13C NMR (d ppm CDCl3, 125 MHz): 158.5, 141.5,
135.9, 130.13, 130.12, 120.3, 114.0 [C5H4 and C6H4]; 55.4
[C6H4OCH3]; 50.7 [Cp–CH–(C6H4OCH3)2].
3. Results and discussion
IR absorptions (cmꢀ1 KBr): 3100, 3085, 2929, 2832,
1606, 1502, 1461, 1301, 1176, 1108, 1033, 827, 771, 527.
Anal. Calc. for C40H38O4Cl2Ti: Theory: C, 68.50; H,
5.46; Cl, 10.11. Found: C, 68.34; H, 5.46; Cl, 10.09%.
UV–Vis (CH2Cl2/e:[cm2/mol]): k 268 nm (e 21000), k
393 nm (e 1070) k 404 nm (e 1120), k 509 nm (e 210), kmax
523 nm (weak).
3.1. Synthesis
Fulvenes 3a, 3b, and 3c were synthesised by reacting the
corresponding benzaldehyde with freshly distilled cyclo-
pentadiene in the presence of pyrrolidine as a base [21,23]
and their structures are shown in Fig. 2.
The use of aryl lithium species for asymmetric addition
in the synthesis of other metallocenes has been previously
published [28–31]. This time, the method is used as a
straightforward approach for the synthesis of diarylmethyl
substituted metallocenes, as seen with titanocenes 5a–c
(Fig. 3).
The first step of the reaction consists of a bromine–lith-
ium exchange, in which the use of tert-butyl lithium
resulted in the formation of the functionalised lithium
intermediates 2a–c, obtaining better yields and no side-
reactions in comparison to n-butyl lithium. Side-reactions
were also avoided by cooling down the reaction to
ꢀ78 ꢁC during the addition of tert-butyl lithium and subse-
quent warming up to 0 ꢁC.
This step was followed by the nucleophilic addition of
the lithiated intermediate to the double bond of the ful-
venes 3a, 3b or 3c at ꢀ78 ꢁC. Then, the reaction mixture
was allowed to warm up to room temperature and it
resulted in the formation of the appropriate substituted
lithium cyclopentadienyl intermediates, 4a–c. After stirring
the reaction mixture for 40 min, two molar equivalents of
4a, 4b or 4c underwent a transmetallation reaction when
reacted with one molar equivalent of TiCl4 in THF under
reflux for 24 h, to give the appropriate unbridged
2.5. Bis-[di-(3,5-dimethoxyphenyl)methylcyclopentadienyl]
titanium (IV) dichloride, {g5-C5H4–CH–
[C6H4–(OCH3)2]2}2TiCl2 (5c)
To a Schlenk flask with 1.32 g (4.66 mmol) of 1-bromo-
3,5-dimethoxybenzene (1c), 20 ml of THF was added until
a transparent solution was formed, while stirring at room
temperature. The solution was cooled down to ꢀ78 ꢁC
for 15 min and 3.02 ml (5.13 mmol) of tert-butyl lithium
was added. The solution was allowed to warm up to 0 ꢁC
for 20 min, resulting in the formation of the yellow lithium
intermediate (2c).
In a second Schlenk flask, 0.92 g (4.66 mmol) of 3,5-
dimethoxyphenylfulvene (3c) was dissolved in THF, and
the resultant red solution was added via a cannula at
ꢀ78 ꢁC to the Schlenk flask containing the lithiated inter-
mediate. The reaction mixture was then allowed to warm
up to room temperature and left stirring for 40 min. Tita-
nium tetrachloride (2.33 ml, 2.33 mmol) was added after-
wards in situ at room temperature and the mixture was
refluxed for 24 h. Subsequently, the solvent was removed
under vacuum, resulting in the formation of a dark green
oil that was dissolved in dichloromethane and filtered

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C. Pampillo´n et al. / Polyhedron 25 (2006) 2101–2108
N(CH3)2
OCH3
H3CO
OCH3
H
H
H
3a
3b
3c
Fig. 2. Structure of fulvenes 3a–c.
N(CH3)2
N(CH3)2
Cl
Ti
Cl
(H3C)2N
OCH3
N(CH3)2
OCH3
5a
Cl
Cl
Ti
H3CO
H3CO
OCH3
Cl
OCH3
H3CO
OCH3
5b
Ti
Cl
OCH3
OCH3
H3CO
OCH3
5c
Fig. 3. Structure of Titanocenes 5a–c.
substituted titanocenes, 5a–c. The compounds obtained are
shiny dark red to dark brown solids (Scheme 1).
be observed (see Scheme 2), as the single methylic protons
were sitting on their respective carbons.
In order to overcome the problem of missing crystal
structures, density functional theory (DFT) calculations
were carried out for titanocenes 5a and 5b at the
B3LYP level using the 6-31G** basis set. Selected bond
lengths of the optimised structures of these titanocenes
are listed in Table 1 (for atom numbering, see Scheme
2). The molecular structures of 5a and 5b are presented
in Fig. 4.
3.2. Structural discussion
Despite our attempts to crystallise these three titanoc-
enes, only one crystal structure for 5a was obtained. Unfor-
tunately, this crystal structure was not publishable.
Nevertheless, the principal molecular structure of 5a could
be proven and even the pyramidal carbons 6 and 60 could

C. Pampillo´n et al. / Polyhedron 25 (2006) 2101–2108
2105
R1
R1
R2
R2
R2
R2
t-BuLi
t-BuBr
THF /
78 C
˚
Br
Li
1a-c
2a-c
R2
R1
R2
H
R1
R2
R2
THF
+
+
78 C
˚
Li
R2
Li
R2
R1
R1
R1
4a-c
R2
3a-c
2a-c
R1
R2
R2
R2
R2
R2
R1
R1
R2
R1
Cl
Ti
TiCl4
Cl
R2
2
2 LiCl
+
Li
R2
THF /
20 h reflux
R1
R1
R2
R2
R2
R1
4a-c
5a-c
1-5a
1-5b
1-5c
R1
R2
-N(CH3)2
H
-OCH3
H
H
-OCH3
Scheme 1. Synthesis of Titanocenes 5a–c.
The length of the bond between the metal centre and the
cyclopentadienyl carbons is similar for both titanocene
structures. They vary between 237.9–249.9 pm for 5a and
238.1–250.3 pm for 5b with slightly different values for
the different Cp rings. The same applies for the carbon–
carbon bonds of the cyclopentadienyl rings with bond
lengths between 141.0–141.9 pm for 5a and 141.0–
143.6 pm for 5b. These values suggest that the titanocenes
have a plane of symmetry bisecting the Cl–Ti–Cl plane
and the calculated structures exhibit C2 symmetry.
titanocenes within the range of 152.3–152.4 pm and the
same occurs for the methylic carbon–aryl bond (153.0–
153.7 pm).
The steric impediment of the aryls attached to the
methylic carbons causes a lengthening of the bond, in order
to relieve the resultant steric strain. The bond length
between the methylic carbons is in both cases too large to
suggest any bridge formation, 424.3 pm for 5a and
432.7 pm for 5b. As well the titanium–chlorine bond
lengths are almost identical in both cases.
The bond length between the methylic carbon centre
and the carbon centre of the Cp group is similar for both
The Cl–Ti–Cl angle was calculated for 5a to be 94.4ꢁ
and 96.9ꢁ for 5b. For 5a, the angle formed by the bonds

2106
C. Pampillo´n et al. / Polyhedron 25 (2006) 2101–2108
than the one formed by C(1) or C(10), the correspondent
methylic atom, and C(70) or (C700), respectively, by approx-
R
imately 10ꢁ in both 5a and 5b, due to the steric impediment.
7
3.3. Cytotoxicity studies
2
6
7'
3
R
1
Cl
4
The in vitro cytotoxicity of compounds 5a–c was deter-
mined by MTT-based assays involving a 48 h drug expo-
sure period, followed by 24 h of recovery time [32]. The
compounds were tested for their activity on pig kidney car-
cinoma (LLC-PK) cells and the results are shown in Fig. 5.
Compound 5a, which contains dimethyl amino groups, has
an IC50 value of 3.8 · 10ꢀ5 M, showing a higher cytotoxic-
ity than its ansa and mono-benzyl substituted analogues.
The ansa analogue of 5a, compound Titanocene X, has
an IC50 value of 4.5 · 10ꢀ4 M and the mono-benzyl substi-
tuted analogue has an IC50 value of 1.2 · 10ꢀ4 M. Com-
pound 5b, which contains p-methoxy groups, shows an
IC50 value of 4.5 · 10ꢀ5 M, very similar to the value
obtained for by the mono-benzyl substituted analogue
Titanocene Y, that showed the most significant IC50 value
of 2.1 · 10ꢀ5 M up to now. Compound 5c, which we
expected to have an increased aqueous solubility and con-
sequently increased cytotoxicity due to the higher number
of methoxy groups, has an IC50 value of 7.8 · 10ꢀ5 M,
which shows a slight decrease in magnitude in comparison
to 5a and 5b. It must also be noted that the cytotoxic action
of 5a, 5b and 5c differs from the mono-benzyl substituted
analogue Titanocene Y: at lower concentrations the com-
pounds show a less effective cell death induction than
Titanocene Y.
The best compound in this series, titanocene 5a, has an
over 10-fold decrease in magnitude in terms of IC50 values
when compared to the unsubstituted titanocene dichloride.
Compared to the value for cis-platin, the IC50 value for
these titanocenes shows an increase of approximately 6.4
in the order of magnitude when tested on the LLC-PK cell
line.
Titanocenes 5a–c presented in this paper do not have
stereocentres and therefore stereoisomers do not exist,
unlike their ansa analogues. But for 5c, the rotation of
the 3,5-dimethoxyphenyl groups was found to be hindered,
which explains the NMR-spectroscopic results. In terms of
in vivo and in vitro cell testing, this is advantageous. Previ-
ously, the presence of unseparated stereoisomers means
that the issue of whether the compounds’ cytotoxicities
are related to specific isomers was not addressed. This is
not of concern in the achiral bis-benzyl-substituted titanoc-
enes 5a–c.
5
Ti
3'
Cl
2'
1'
4'
R
7''
5'
6'
7'''
R
5a
-N(CH3)2
5b
-OCH3
R
Scheme 2. Numbering scheme for structural discussion of titanocenes 5a
and 5b.
Table 1
Selected bond lengths from the DFT-calculated structures of complexes 5a
and 5b
DFT structure
Bond length (pm) 5a
Bond length (pm) 5b
Ti–C(1)
Ti–C(2)
Ti–C(3)
Ti–C(4)
249.9
242.4
239.5
237.9
244.7
247.6
246.5
240.6
239.8
238.9
143.3
141.1
141.6
141.9
141.6
141.6
141.9
141.4
141.0
141.3
152.4
152.3
432.7
153.5
153.0
153.5
153.5
237.5
236.0
250.3
244.7
238.1
240.1
242.5
247.3
246.6
240.8
239.9
238.6
141.5
142.0
141.5
141.2
143.4
141.6
141.9
141.4
141.0
143.4
152.4
152.4
424.3
153.4
153.6
153.0
153.7
237.2
235.7
Ti–C(5)
Ti–C(10)
Ti–C(20)
Ti–C(30)
Ti–C(40)
Ti–C(50)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(1)
C(10)–C(20)
C(20)–C(30)
C(30)–C(40)
C(40)–C(50)
C(5)–C(10)
C(1)–C(6)
C(10)–C(60)
C(6)–C(60)
C(6)–C(7)
C(6)–C(70)
C(60)–C(700)
C(60)–C(7000)
Ti–Cl(1)
Ti–Cl(2)
4. Conclusions and outlook
between C(6), C(7) and C(70) is 112.7ꢁ, and the angle
formed between C(60), C(700) and C(7000) is 111.4ꢁ. The cor-
responding calculated values for 5b are 111.9ꢁ and 111.4ꢁ.
The angle between C(1) or C(10), the correspondent
methylic atom, and C(7) or C(7000), respectively, is smaller
The carbolithiation of 6-arylfulvenes with aryl lithium
species followed by transmetallation offers a general way
into the synthesis of achiral diarylmethyl substituted metal-
locenes. In the case of methoxy or dimethylamino substit-
uents and titanium(IV) as the central metal, these

C. Pampillo´n et al. / Polyhedron 25 (2006) 2101–2108
2107
Fig. 4. DFT calculated structures of 5a and 5b.
1.0
0.5
0.0
Titanocene 5a, IC50= 3.8 x 10-5M
Titanocene 5b, IC50= 4.5 x 10-5M
Titanocene 5c, IC50= 7.8 x 10-5M
1E-12 1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3
Log10Drug Concentration [mol/l]
Fig. 5. MTT-based cytotoxicity curves show the effect of compounds 5a–c on pig kidney carcinoma (LLC-PK) cells.
compounds exhibit significant cytotoxicity against kidney
cancer cells with IC50 values in the 10ꢀ5 M region, which
makes them already promising anti-cancer drugs. Never-
theless, it is intended to employ this carbolithiation method
for future synthesis of titanocenes exhibiting cytotoxicities
in the single digit lM region.
Conway Institute of Biomolecular and Biomedical Re-
search, UCD, for the use of tissue culture facilities for
the cell testing experiments and for his advice on the
subject.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.poly.
2006.01.007.
Acknowledgements
The authors thank Science Foundation Ireland (SFI) for
funding through grant (04/BRG/C0682). In addition,
funding from the Higher Education Authority (HEA)
and the Centre for Synthesis and Chemical Biology (CSCB)
through the HEA PRTLI cycle 3 as well as COST D20
(WG 0001) was provided. The authors also thank Dr.
W.M. Gallagher of the Department of Pharmacology,
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