Cyclopentadienyl molybdenum(II) compounds bearing carboxylic acid functional group

Article abstract of DOI:10.1016/j.ica.2018.04.032

This work describes a procedure giving cyclopentadienyl molybdenum(II) compounds bearing carboxylic acid function group. It involves synthesis of carboxylic acid ester functionalized cyclopentadienides, their coordination to molybdenum(II) precursor and saponification of ester function groups. The method is not limited only to compounds with the function group directly attached in the cyclopentadienyl ring but also to those functionalized in the side chain. The attempts to synthesize the indenyl analogues were only partially successful due to low stability in the saponification step. All reported structure types were elucidated from spectroscopic measurements and verified by X-ray crystallography. The second part of the work describes an effect of the outer-coordination sphere on cytotoxicity of the cationic molybdenum(II) compounds bearing N,N-chelating ligands. The cytotoxicity of the modified species bearing phenanthroline ligand toward human leukemia cells MOLT-4 (IC₅₀ = 10.5 ± 0.5 μmol l^?1) is higher than reported for cisplatin (IC₅₀ = 15.8 ± 1.9 μmol l^?1).

Full text of DOI:10.1016/j.ica.2018.04.032

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Inorganica Chimica Acta 479 (2018) 66–73
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Research paper
Cyclopentadienyl molybdenum(II) compounds bearing carboxylic acid
functional group
a
b
a
c
a
ˇ
ˇ
ˇ
ˇ
ˇ ˇ
Jirí Schejbal , Lucie Melounková , Jaromír Vinklárek , Martina Rezácová , Zdenka Ru˚ zicková ,
d
e,
⇑
ˇ
ˇ
Ivana Císarová , Jan Honzícek
a Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
b Department of Analytical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
c Department of Medical Biochemistry, Faculty of Medicine in Hradec Králové, Charles University in Prague, Šimkova 870, 500 01 Hradec Králové, Czech Republic
d Department of Inorganic Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030/8, 128 43 Prague 2, Czech Republic
e Institute of Chemistry and Technology of Macromolecular Materials, Faculty of Chemical Technology, University of Pardubice, Studentská 573, 532 10 Pardubice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
This work describes a procedure giving cyclopentadienyl molybdenum(II) compounds bearing carboxylic
acid function group. It involves synthesis of carboxylic acid ester functionalized cyclopentadienides, their
coordination to molybdenum(II) precursor and saponification of ester function groups. The method is not
limited only to compounds with the function group directly attached in the cyclopentadienyl ring but
also to those functionalized in the side chain. The attempts to synthesize the indenyl analogues were only
partially successful due to low stability in the saponification step. All reported structure types were elu-
cidated from spectroscopic measurements and verified by X-ray crystallography. The second part of the
work describes an effect of the outer-coordination sphere on cytotoxicity of the cationic molybdenum(II)
compounds bearing N,N-chelating ligands. The cytotoxicity of the modified species bearing phenanthro-
Received 31 December 2017
Received in revised form 11 April 2018
Accepted 18 April 2018
Available online 20 April 2018
Keywords:
Molybdenum
Cyclopentadienyl
Cytotoxicity
Leukemia
line ligand toward human leukemia cells MOLT-4 (IC50 = 10.5 0.5
l
mol lꢀ1) is higher than reported for
cisplatin (IC50 = 15.8 1.9
l
mol lꢀ1).
Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction
ing various substituents in the cyclopentadienyl ring and another
bidentate ligands [21,22,28]. Enhanced in vitro cytotoxicity toward
leukemia cells MOLT-4 was observed in case of water-soluble
derivatives obtained by functionalization of cyclopentadienyl ring
with amine function groups [24]. Another approach to reach
water-soluble derivatives of [(g5-Cp0)Mo(CO)2L2][BF4] involves
attachment of carboxylic acid group that is subject of present
study.
The first notes about the modification of cyclopentadienyl
ligand with carboxylic acid functional group are dated shortly after
discovery ferrocene [29,30]. Nevertheless, a quest for such modifi-
cation has emerged much later within a development of water sol-
uble organometallic compounds designed for homogenous
catalysis [31] and medicinal applications [32]. Strong hydrogen
bond systems in the carboxylic acid functionalized organometallic
compounds have been utilized for crystal engineering as well [33–
35]. Nevertheless, the current research is focused mainly on the
assembly of biomolecule conjugates suitable for target drug deliv-
ery [36–38], where acid functionalized cyclopentadienyl com-
pounds form convenient building blocks.
Various transition metal complexes and organometallic com-
pounds attract attention of biochemists and farmacochemists since
cytostatic properties of cisplatin (cis-[PtCl2(NH3)2]) were discov-
ered by Rosenberg et al. [1,2]. In past decades, a large variety of
transition metal compounds was scrutinized and many structural
patterns with promising cytostatic properties were recognized.
Nevertheless, the quest for new species with enhanced activity
toward cisplatin resistant tumor cells and reduced side-effects is
still ongoing [3–20].
Our ongoing development of molybdenum-based cytostatic
drug [21–24] follows the fundamental work of Romão et al., who
established cyclopentadienyl and indenyl molybdenum com-
pounds [(g5-Cp0)Mo(CO)2L2][BF4] (Cp0 = C5H5, C9H7, L2 = neutral
chelating ligand) as a new class of highly cytotoxic species active
against several tumor cell lines [25]. In following studies, an early
insight into mechanism of their action was reported [26,27] and
number of active compounds was extended with congeners bear-
Four distinct synthetic strategies are available for transition
metal compounds bearing carboxylic acid functional group in the
⇑
Corresponding author.
E-mail address: jan.honzicek@upce.cz (J. Honzícek).
ˇ
https://doi.org/10.1016/j.ica.2018.04.032
0020-1693/Ó 2018 Elsevier B.V. All rights reserved.

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J. Schejbal et al. / Inorganica Chimica Acta 479 (2018) 66–73
67
g
5-bonded cyclopentadienyl ligand. The direct coordination of
(4) and dimethyl terephthalate (5), respectively (Scheme 2).
Although such stoichiometry leads to appearance of ꢁ25% side
products (8-Na, 9-Na), it prevent contamination with starting die-
sters (4, 5). We note that 1: 1 stoichiometry does not give pure
monocyclopentadienide (6-Na, 7-Na) but a mixture with appropri-
ate bis(cyclopentadienide) and unreacted diester. As both contam-
inants are hardly removable are by simple purification processes,
we decided to use an excess of NaCp in order to prevent the con-
tamination with diester and not separate the products in this reac-
tion step but after coordination to molybdenum and saponification
process (Scheme 2).
Thiele’s acid [(C5H5COOH)2] is only described for technetium or
rhenium carbonyls [39,40]. More convenient pathway consists of
metalation/carbonation procedure [30,41,42], which is commonly
used for a direct functionalization of electron-rich middle and late
transition metal compounds resistant to reduction under condi-
tions of the metalation [e.g.
(g (g
5-C5H5)2Fe, 5-C5H5)Mn(CO)3,
(g
5-C5H5)( 7-C7H7)V]. Carboxylic acid group could be also gener-
g
ated from ester group by hydrolysis [43]. Starting ester substituted
derivatives are usually accessible from functionalized alkali metal
cyclopentadienides [44,45]. Such strategy has been successfully
used for compounds with one or more functionalities in the
cyclopentadienyl ring [43,46] as well as in the side chain [37,47].
Another approach covers oxidation of aldehyde (CHO),
alcohol (CH2OH), ketone (COR) and alkyne (CH„CR) functionalities
[48–50].
The aim of this study is to modify the outer coordination sphere
of molybdenum(II) compounds with the carboxylic acid function-
ality in the attempt to improve their cytotoxic properties. As nat-
ure of ligand L2 plays important role in a drug efficiency [21,25],
Cyclopentadienides 6a and 7a react with [(g
3-C3H5)Mo(CO)2
(NCMe)2Cl] to give complexes with ester group in side chain 10
and 11, respectively. After standard work up, these products con-
tain about 25% of appropriate bridged compound (12 and 13) as
evidenced by 1H NMR spectroscopy. Such dinuclear complexes
were isolated by long term stirring with sodium hydroxide solu-
tion in wet methanol. The mononuclear species are dissolved as
the ester groups hydrolyze. Pure species 12 and 13 are then
obtained after recrystallization from toluene. Infrared spectra of
the compounds 12 and 13 show two CO stretching bands of the
four N,N-chelators of different
toxic properties of these derivatives were examined in vitro on
human leukemia cell line MOLT-4.
p-system size were chosen. Cyto-
carbonyl ligand at ꢁ1936 cmꢀ1
(
m
a) and ꢁ1845 cmꢀ1
(ms). Stretch-
ing band of the keto group appears at ꢁ1635 cmꢀ1. 1H NMR spectra
of the compounds 12 and 13 show two apparent triplets at ꢁ5.8
and ꢁ5.5 ppm (3J = 4J = 2.4 Hz) typical for monosubstituted Cp
ligand. Allyl ligands give signals at 3.89, 2.80 and 1.54 ppm. The
dinuclear character of the species 12 and 13 is apparent from pat-
tern of the bridge that is, in both cases, typical for symmetrical dis-
ubstituted benzene. 1,3-disubstituted ring of the compound 12
gives two triplets at 8.08 ppm (H2) and 7.58 ppm (H5) and doublet
of doublets at 7.93 ppm (H4,6). In case of 13, four equivalent pro-
tons of 1,4-disubstituted benzene give one singlet at 7.80 ppm.
Single crystals of the compound 13, suitable for X-ray analysis,
were obtained by vacuum sublimation. The molecules consist of
2. Results and discussion
2.1. Synthesis of allyl molybdenum precursors
The target molybdenum compounds [(g5-Cp0)Mo(CO)2L2][BF4]
are commonly synthesized from allyl precursors by protonation
with strong acid followed with addition of appropriate chelating
ligand. The starting derivative with carboxylic group directly
attached in the cyclopentadienyl ring [(g g
3-C3H5)( 5-C5H4COOH)
Mo(CO)2] (3) was already reported [51,52]. Nevertheless, we
decided to develop alternative synthetic pathway, using hydrolysis
of ester group, which could be further applicable for congeners
bearing the carboxylic group in the side chain.
Ethyl ester functionalized compound [(
Mo(CO)2] (2), readily available from Na[C5H4COOEt] (1-Na) and
[(
3-C3H5)Mo(CO)2(NCMe)2Cl], undergoes saponification of ester
group in the mixture NaOH/MeOH/water. The desired species bear-
ing carboxylic group (3) is then obtained in high yield after acidi-
fication (Scheme 1).
two [(g a bridging
3-C3H5)Mo(CO)2] fragments connected via
bis(cyclopentadienyl) ligand with rigorous overall Ci symmetry
(Fig. 1). The coordination sphere of both molybdenum atoms could
be taken as pseudo-tetrahedral as centroids of
g -
3-allyl and g5
g g
3-C3H5)( 5-C5H4COOEt)
cyclopentadienyl are considered to occupy one coordination site
each. The geometric parameters, related with central metal, are
given Table 1. Small dihedral angle between the Cp ring and a
plane the ketone, defined by C1, C6, C7 and O1, [Pl1–Pl2 = 6.4(2)°]
suggests more effective conjugation than between bridging ben-
zene ring and the keto group [Pl2–Pl3 = 34.09(18)°]. The conjuga-
tion is also apparent from a shorter C1–C6 bond [1.471(5) Å]
compared to neighboring C6–C7 bond [1.503(5) Å].
The complexes with the carboxylic group in the side chain of
the cyclopentadienyl ligand 14 and 15 were prepared by saponifi-
cation of aforementioned crude compounds 10 and 11, respectively
(Scheme 2). Treatment with sodium hydroxide solution in aqueous
methanol leads to selective hydrolysis of the ester groups to give
sodium salts of desired products. The pure species with carboxylic
group (14 and 15) are then precipitated upon acidification.
CO stretching bands of the carboxylic group were observed in
the infrared spectra of the compounds 14 and 15 at 1688 cmꢀ1
and 1683 cmꢀ1, respectively. 1H NMR spectra show, beside the sig-
nals of allyl and cyclopentadienyl ligand, a typical pattern of
unsymmetrical 1,3- and 1,4-disubstituted benzene, respectively.
Crystal structure of the compound 14, determined by X-ray diffrac-
tion analysis, proves a similar coordination sphere of molybdenum
as aforementioned for compound 13. A strong hydrogen bonding
connects the carboxylic groups of a pair of molecules 14 to give a
cyclic dimer as evident from short O2ꢂ ꢂ ꢂO30 intermolecular dis-
tances [2.630(2) Å], see Fig. 2.
g
Deprotonation of 3 with sodium methanolate gives stable
sodium salt Na[(g g
3-C3H5)( 5-C5H4COO)Mo(CO)2] (3-Na) in high
yield. Delocalization of the negative charge over both oxygen
atoms of the carboxylate is evident from low wavenumber of the
antisymmetric COO stretching mode (1584 cmꢀ1). Lower
wavenumbers of the carbonyl stretching modes (
s = 1839 cmꢀ1) reflect a higher electron density on molybdenum
available for -backbonding than observed for 3 (
a = 1927 cmꢀ1
s = 1861 cmꢀ1).
Cyclopentadienides 6-Na and 7-Na, necessary for synthesis of
m ,
a = 1913 cmꢀ1
m
p
m
,
m
molybdenum compounds with the carboxylic group in the side
chain of the Cp ligand, were prepared by reaction of sodium
cyclopentadienide with 1.5 equivalents of dimethyl isophthalate
Scheme 1. Synthesis of molybdenum compound 3. Reagents: a) [(g
3-C3H5)Mo
(CO)2(NCMe)2Cl]/THF, b) NaOH/MeOH/water, c) HCl (aq.).
The assembly of the ester-functionalized indenyl molybdenum
compound [(g g
3-C3H5)( 5-C9H6COOMe-2)Mo(CO)2] (18) is summa-

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68
J. Schejbal et al. / Inorganica Chimica Acta 479 (2018) 66–73
Scheme 2. Synthesis of molybdenum compounds with carboxylic group in the side chain of cyclopentadienyl. Reagents: a) NaCp/THF, b) [(g
3-C3H5)Mo(CO)2(NCMe)2Cl], c)
NaOH/MeOH/water, d) HCl (aq.).
Fig. 2. ORTEP drawing of [(g g
3-C3H5)( 5-C5H4COC6H4COOH-3)Mo(CO)2] (14). The
labeling scheme for all non-hydrogen atoms is shown. Thermal ellipsoids are drawn
at the 30% probability level.
Fig. 1. ORTEP drawing of [1,4-{(g g
3-C3H5)Mo(CO)2( 5-C5H4CO)2}2C6H4] (13). The
labeling scheme for all non-hydrogen atoms is shown. Thermal ellipsoids are drawn
at the 30% probability level. One position of disordered atoms is omitted for clarity.
Table 1
Scheme 3. Synthesis of indenyl molybdenum compound 18. Reagents: a) (COOBr)2/
CH2Cl2, b) water, c) SOCl2/CH2Cl2, d) EtOH, e) n-BuLi/THF, f) [(g
3-C3H5)Mo
Selected bond lengths (Å) and bond angles (°) of molybdenum complexes.
(CO)2(NCMe)2Cl].
13
14
18
Mo–CO
1.961(5), 2.075(5) 1.948(2), 1.962(2) 1.945(3), 1.951(3)
Mo–Cg(C5)[a]
Mo–Cg(C3)[a]
OC–Mo–CO
2.0078(18)
1.997(7)
2.0032(9)
2.0057(2)
78.67(10)
125.53(10)
2.0270(11)
2.034(4)
77.48(19)
78.46(11)
125.89(15)
Cg(C3)–Mo–Cg(C5)a,b 123.5(2)
a
Cg(C5) is centroid of cyclopentadienyl ring.
Cg(C3) is centroid of allyl ligand.
b
Fig. 3. ORTEP drawing of C9H7COOH-2 (16). The labeling scheme for all non-
hydrogen atoms is shown. Thermal ellipsoids are drawn at the 30% probability level.
rized in the Scheme 3. In the first step, electrophilic substitution of
[BrCOCO]+ cation, generated from oxalyl bromide, on polarized
double bond of indene gives indene-2-carbonyl bromide that read-
ily hydrolyzes to appropriate carboxylic acid 16 [53]. X-ray diffrac-
tion analysis of 16 evidences the appearance of a cyclic dimer, in
the solid state, with carboxylic groups connected through strong
hydrogen bonds [O1ꢂ ꢂ ꢂO30 = 2.612(2) Å)] (Fig. 3).
complex [(g g
3-C3H5)( 5-C9H6COOEt-2)Mo(CO)2] (18); see Scheme 3.
1H NMR spectrum of the compound 18 reveals appearance of two
isomers in solution generated by orientation of allyl ligand. The
equimolar mixture of exo- and endo-isomer, observed at room tem-
Esterification followed by deprotonation and a reaction with
perature, is rather unusual as exo-isomers of parent [(
(g
g
3-C3H5)
[(g
3-C3H5)Mo(CO)2(NCMe)2Cl] gives ester-substituted indenyl
5-C9H7)Mo(CO)2] is about 1 kcal/mol more stable [54] and

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J. Schejbal et al. / Inorganica Chimica Acta 479 (2018) 66–73
69
becomes predominant in CDCl3 solution at room temperature
([endo]/[exo] = 2.55) [55]. The stabilization of exo-isomer has been
also reported for various ring substituted congeners, including
those with ester group in 1-position ([endo]/[exo] = 3–4) [56–58].
X-ray diffraction analysis verifies the proposed molecular struc-
ture of 18. Fig. 4 depicts the solid-state structure of the exo-isomer,
in which carbon atoms of the allyl ligand are in eclipsing configu-
ration with OC–Mo–CO moiety. The molecule reaches a conforma-
tion with benzene ring opposite to carbonyl ligands similarly as its
unsubstituted parent [54].
Unfortunately, our attempts to hydrolyze the ester group selec-
tively were unsuccessful due to lability of the bond between
molybdenum and substituted indenyl ligand. Only intractable tarry
solid was obtained at conditions necessary for saponification.
2.2. Synthesis of complexes with N,N-chelating ligands
Compounds 3, 14 and 15 were used for synthesis of cationic
compounds bearing chelating ligands. According to established
procedure [21,59], the complexes were treated with tetrafluoro-
boric acid in order to protonate allyl ligand and exchange it with
N,N-chelators. Nevertheless, the desired cationic cyclopentadienyl
compounds (19–22) were obtained only from precursor with the
carboxylic group directly attached in the cyclopentadienyl ring
(Scheme 4). In other cases (14 and 15), a more powerful elec-
tron-withdrawing character of the substituent led to protonation
and subsequent exchange of the cyclopentadienyl ring to give triv-
Scheme 4. Synthesis of cationic molybdenum compounds bearing N,N-chelating
ligands. Reagents: a) HBF4∙Et2O/MeCN, b) N,NL/CH2Cl2. Abbreviations: 1,10-phenan-
throline (phen), dipyrido[3,2-a:20,30-c]phenazine (dppz), 4,5-diazafluoren-9-one
(dafone) and 2,20-biquinoline (bq).
crystalizes as a solvate 19ꢂ0.5Et2O. The coordination sphere of
molybdenum forms a square pyramid with centroid of cyclopenta-
dienyl ring in the apical position. The basal plane is occupied
with cis-coordinated carbonyl ligands and nitrogen atoms of the
phenanthroline. The solid-state structure is stabilized with strong
hydrogen bond between carboxylic group of the cyclopentadienyl
ligand and tetrafluoroborate anion [O1ꢂ ꢂ ꢂF1 = 2.685(4) Å].
ial cationic allyl compounds of type [(
g
3-C3H5)Mo(CO)2L2(MeCN)]
3-C3H5)( 5-C5H5)Mo(CO)2]
5-C5H4COOH)Mo(CO)2L2]
[BF4]. Similar behavior was previously observed for some other
ring-substituted congeners of [(
[60,61].
g
g
The cyclopentadienyl complexes [(
g
[BF4] (19: L2 = phen, 20: L2 = dppz, 21: L2 = dafone, 22: L2 = bq)
2.3. Cytotoxicity study
show characteristic CO stretching bands of carbonyl ligands at
higher wavenumbers (
than observed for starting complex 3. In the cationic complexes,
a lower electron density on molybdenum results in weaker back-
m
a: 1993–1974 cmꢀ1
;
m
s: 1906–1863 cmꢀ1
)
Cytotoxic activity of the cationic compounds 19–22 was evalu-
ated on human T-lymphocytic leukemia cells MOLT-4 in exponen-
tial grow phase using standard WST-1 viability assays. The effect of
the molybdenum compounds was estimated after 24 h exposition
as half-maximal inhibitory concentration (IC50) calculated from
cytotoxicity curves; Table 3. The modification of the cationic
compounds with the carboxylic acid group have only a minor effect
on solubility in water that led us to use previously described
protocol involving predissolution of the compounds in dimethyl
sulfoxide [62].
donation into the
p-antibonding orbitals of the carbonyl ligands.
A strong band at 1723–1717 cmꢀ1 was assigned to CO stretching
of carboxylic group. In spectrum of compound 21, keto-group of
dafone ligand gives another CO stretching band at 1740 cmꢀ1
.
1H NMR spectra of the compounds 19–22 show a pattern
consistent with expected Cs symmetry. The substituted cyclopen-
tadienyl ligand gives two pseudo triplets at 5.85–6.65 ppm
(3J = 4J = 2.4 Hz). Low-fielded signals in region of 7.9–10.0 ppm
prove coordination of given N,N-chelating ligands.
The obtained IC50 values suggest rather minor effect of carboxylic
acid function group on cytotoxicity of the molybdenum(II) com-
pounds. The considerably improvement was observed only in case
The structure of phenanthroline complex 19 was determined by
X-ray diffraction analysis (Fig. 5; Table 2). The compound
of phenanthroline complex 19 (IC50 = 10.5 0.5 l
mol lꢀ1) when
Fig. 4. ORTEP drawing of [(
g
3-C3H5)(
g
5-C9H6COOMe-2)Mo(CO)2] (18). The labeling
Fig. 5. ORTEP drawing of [(g
5-C5H4COOH)Mo(CO)2(phen)][BF4] present in crystal
scheme for all non-hydrogen atoms is shown. Thermal ellipsoids are drawn at the
20% probability level. One position of disordered atoms is omitted for clarity.
structure of 19ꢂ0.5Et2O. The labeling scheme for all non-hydrogen atoms is shown.
Thermal ellipsoids are drawn at the 30% probability level.