ORIGINAL PAPER

Molybdenum Bisphosphonates with Cr(III) or Mn(III)

Ions

Ali Saad • Guillaume Rousseau • Hani El Moll • Olivier Oms •

Pierre Mialane • Jerome Marrot • Loıc Parent •

Israel-Martyr Mbomekalle • Remi Dessapt • Anne Dolbecq

Received: 11 September 2013

Ó Springer Science+Business Media New York 2013

Abstract The synthesis of MoVI bisphosphonates (BPs) complexes in the presence

of a heterometallic element has been studied. Two different BPs have been used, the

alendronate ligand, [O3PC(C3H6NH3)(O)PO3]4- (Ale) and a new BP derivative

with a pyridine ring linked to the amino group, [O3PC(C3H6NH2CH2C5H4N)

(O)PO3]4- (AlePy). Three compounds have been isolated, a tetranuclear MoVI

complex with CrIII ions, (NH4)5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Cr]�11H2O

(Mo4(Ale)2Cr), its MnIII analogue, (NH4)4.5Na0.5[(Mo2O6)2(O3PC(C3H6NH3)(O)-

PO3)2Mn]�9H2O (Mo4(Ale)2Mn), and a cocrystal of two polyoxomolybdates,

(NH4)10Na3[(Mo2O6)2(O3PC(C3H6NH2CH2C5H4N)(O)PO3)2Cr]2[CrMo6(OH)6O18]�37H2O ([Mo4(AlePy)2Cr]2[CrMo6]). In this latter compound an Anderson-type

POM [CrMo6(OH)6O18]3- is sandwiched between two tetranuclear MoVI com-

plexes with AlePy ligands. The protonated triply bridging oxygen atoms bound to

the central CrIII ion of the Anderson anion develop strong hydrogen bonding

interactions with the oxygen atoms of the bisphosphonate complexes. The UV–Vis

spectra confirm the coexistence in solution of both POMs. Cyclic voltammetry

experiments have been performed, showing the reduction of the Mo centers. In

strong contrast with the reported MoVI BP systems, the presence of trivalent cations

This paper is dedicated to the memory of Roland Contant and Louis Nadjo.

Electronic supplementary material The online version of this article (doi:10.1007/s10876-013-0655-3)

contains supplementary material, which is available to authorized users.

A. Saad � G. Rousseau � H. El Moll � O. Oms � P. Mialane � J. Marrot � L. Parent �I.-M. Mbomekalle � A. Dolbecq (&)

Institut Lavoisier de Versailles, UMR 8180, Universite de Versailles Saint-Quentin en Yvelines,

45 Avenue des Etats-Unis, 78035 Versailles Cedex, France

e-mail: anne.bastin@uvsq.fr

R. Dessapt

Institut des Materiaux Jean Rouxel, CNRS, Universite de Nantes, 2 rue de la Houssiniere, BP 32229,

44322 Nantes Cedex, France

123

J Clust Sci

DOI 10.1007/s10876-013-0655-3

in close proximity to the MoVI centers dramatically impact the potential solid-state

photochromic properties of these compounds.

Keywords Polyoxometalate � Chromium � Bisphosphonate � Molybdenum �Cocrystallization � Electrochemistry

Introduction

Polyoxometalates (POMs) are usually considered as metal–oxygen cluster anions

where the metal centers (usually Mo, W, V) are in their highest oxidation states.

They exhibit a great diversity of structures and properties. Since the pioneering

work of Contant in Paris and Nadjo in Nancy and then Orsay on the synthesis and

electrochemical characterizations of POMs in the seventies and eighties [1–8], great

synthetic efforts have been made which have led to a diversification of the structures

and properties of POMs [9–16]. Among them, hybrid POMs with bisphosphonate

(BP) ligands attract an increasing interest due to their potential applications in

medicine, photochromism or material science, as reported in two recent review

papers [17, 18]. The studied bisphosphonates (BPs) have the general formula

H2O3PC(R1)(R2)PO3H2 (R1 = H, OH), R2 being an organic group which allows

their functionalization and can bring additional properties to the functionalized

POM. This can be highlighted, considering that, for example, BPs belong to a class of

commercial drugs used to treat bone resorption diseases and have also potent activities

against tumor cells [19]. POMs have also been known for their biological properties

[20, 21]. The activity of MoVI complexes with BPs against tumor cell lines has been

studied, suggesting synergistic effect between both organic and inorganic components

[22]. On a synthetic point of view, a large number of molecular structures with MoVI

ions connected to BPs have been reported by various groups [23–26]. MoV

complexes, built from the connection of {Mo2VO4} dimers via BP ligands also exhibit

a large diversity of nuclearities and shapes [27–31].

Recently, polyoxomolybdate-based cage assemblies containing lanthanides and

BP ions have been reported, using the organic soluble MoVI precursor

[N(C4H9)4][Mo8O26] [32]. However, besides these complexes and the compounds

with general formula {(Mo2O6)2(O3PC(R)(O)PO3)2M}(R = (CH2)3NH3, M = VIV;

R = CH3, M = CrIII, FeIII, VIV, MnII) [33, 34], the introduction of an additional

transition metal has been rarely studied for the formation of MoVI/BPs molecular

compounds. Moreover, the few 3d-incorporating species have been synthesized by a

two-steps procedure involving the reoxidation of MoV complexes. We have thus

decided to explore the direct reactivity of MoVI ions in water with BP ligands in the

presence of additional transition metal ions. We have studied the influence of the

nature of the metal ion and of the BP ligand (Scheme 1) on the structure of the final

product and we report here the straightforward synthesis, the structure and the

electrochemical properties of the two isostructural polyoxomolybdobisphosphonates

(NH4)5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Cr]�11H2O (Mo4(Ale)2Cr), and (NH4)4.5Na0.5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Mn]�9H2O (Mo4(Ale)2Mn). In addition,

A. Saad et al.

123

the compound (NH4)10Na3[(Mo2O6)2(O3PC(C3H6NH2CH2C5H4N)(O)PO3)2Cr]2[Cr-

Mo6(OH)6O18]�37H2O ([Mo4(AlePy)2Cr]2[CrMo6]), which represents a rare example

of a cocrystal of two distinct polyoxomolybdates has also been characterized.

Experimental

Chemical and Reagents

The alendronic acid H2O3PC(C3H6NH2)(OH)PO3H2 [35] (H4Ale) and the sodium

salt of the Anderson ion Na3[CrMo6(OH)6O18]�8H2O [36] (CrMo6), used as a

reference in this work, have been synthesized according to reported procedures. All

other chemicals were used as purchased without purification.

Synthesis of Na2[HO3PC(C3H6NHCH2C5H4N)(OH)PO3H]�3H2O (Na2H2AlePy)

Alendronic acid (6 g, 25 mmol) was dissolved in a solution of triethylamine

(14.68 mL, 0.1 mmol) in 120 mL of MeOH. The solution was stirred for 15 min

then 2-pyridinecarboxaldehyde (2.28 mL, 25 mmol) was added. The solution was

refluxed for 90 min and then allowed to cool down to room temperature. After slow

addition of (TBA)BH4 (6.02 g, 25 mmol), the resulting mixture was refluxed

overnight. The solution was then allowed to cool down to room temperature and

NaPF6 (7.52 g, 50 mmol) was added in several portions. The solution was stirred for

10 min until the appearance of a precipitate. The brownish solid was filtered,

washed three times with MeOH and Et2O and isolated after centrifugation. The

ligand Na2H2AlePy (7.4 g, yield 70 %) was used without further purification. 31P

NMR (200 MHz, D2O): d ppm 17.73 (s); 1H NMR (200 MHz, D2O): d ppm 8.43 (s,

1Harom), 7.76 (m, 1Harom), 7.35 (m, 2Harom), 4.21 (s, 2H, NH–CH2–Carom), 3.02 (m,

2H, NH–CH2–CH2–), 1.90 (m, 4H, –CH2–CH2–CH2–C). Anal. calc. for

C10H22N2Na2O10P2 (found): C 27.41 (27.37), H 5.06 (4.39), N 6.39 (6.40), Na

10.49 (9.92), P 14.14 (14.91).

Synthesis of (NH4)5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Cr]�11H2O (Mo4(Ale)2Cr)

To a solution of (NH4)6Mo7O24�4H2O (0.495 g, 0.4 mmol) in 5 mL of water was

added H4Ale (0.303 g, 1.2 mmol) and CrCl3�6H2O (0.187 g, 0.7 mmol) and the pH

was adjusted to 6 using a 33 % solution of NH3 in water. The solution was stirred

for 1 h at 80 °C, cooled to room temperature and slowly evaporated after

centrifugation. Green crystals (0.350 g, yield 36 % based on Mo) were filtered after

Scheme 1 Representation of the protonated form of the two ligands used in this study

Molybdenum BPs with Cr(III) or Mn(III) Ions

123

2 days. Anal. calc. for C8H60Mo4N7O37P4Cr (found): C 6.83 (6.89), H 4.30 (4.16),

Mo 27.29 (27.46), N 6.97 (6.99), P 8.81 (8.63), Cr 3.70 (3.71). IR (m/cm-1):

1631(w, sh), 1602(m), 1521(w), 1428(s), 1119(s), 1029(s), 1006(s, sh), 904(m),

864(m), 814(m), 692(m), 645(m), 596(m), 522(m), 447(w), 365(w), 326(w).

Synthesis of (NH4)4.5Na0.5[(Mo2O6)2(O3PC(C3H6NH3)(O)PO3)2Mn]�9H2O

(Mo4(Ale)2Mn)

To a solution of Na2MoO4�2H2O (0.242 g, 1 mmol) in 10 mL of 1 M

CH3COONH4/CH3COOH buffer was added H4Ale (0.125 g, 0.5 mmol) and the

pH was adjusted to 7.5 with a 33 % solution of NH3 in water. The solution was

stirred for 30 min at room temperature. Mn(CH3COO)3�2H2O (0.070 g, 0.25 mmol)

was added and the pH readjusted at 7.5 using a 33 % solution of NH3 in water. The

solution was stirred for 1 h at 80 °C and cooled to room temperature. The solution

was slowly evaporated after centrifugation. Brown crystals (0.087 g, yield 25 %

based on Mo) were filtered after 4 days. Anal. calc. for C8H56Mo4Na0.5-N6.5O35P4Mn (found): C 6.97 (7.03), H 4.10 (4.10), Mo 27.86 (27.04), N 6.61

(6.82), Na 0.83 (0.80), P 8.99 (9.12), Mn 3.99 (3.99). IR (m/cm-1): 1636(m),

1598(m), 1526(w), 1119(s), 1060(s, sh), 1042(s), 1024(s), 1001(s), 894(s), 853(s),

705(m), 644(m), 482(m), 397(m).

Synthesis of (NH4)10Na3[(Mo2O6)2(O3PC(C3H6NH2

CH2C5H4N)(O)PO3)2Cr]2[CrMo6(OH)6O18]�37H2O ([Mo4(AlePy)2Cr]2[CrMo6])

To a solution of (NH4)6Mo7O24�4H2O (0.495 g, 0.4 mmol) in 5 mL of water was

added Na2H2AlePy (0.537 g, 1.2 mmol) and CrCl3�6H2O (0.187 g, 0.7 mmol) and

the pH was adjusted to 6 using a 33 % solution of NH3 in water. The solution was

stirred for 40 min at 80 °C, cooled to room temperature and slowly evaporated after

centrifugation. Pink crystals (0.100 g) of a compound, which was identified by

comparison of its infrared spectrum with that of CrMo612 to be an ammonium salt of

the [CrMo6(OH)6O18]3- Anderson-type anion, were removed by filtration after

2 days. Green crystals (0.350 g, yield 39 % based on Mo) were then filtered after

another 2 days period. The presence of sodium counter-ions coming from the AlePy

precursor was detected by single crystal X-ray structure analysis and confirmed by

elemental analysis. Anal. calc. for C40H179Mo14N18Na3O113P8Cr3 (found): C 10.59

(10.41), H 3.98 (3.42), Mo 29.61 (29.74), N 5.56 (5.53), P 5.46 (5.63), Cr 3.44

(3.49). IR (m/cm-1): 1628(w), 1426(m), 1121(m), 1045(m), 1023(m, sh), 904(s),

862(s), 813(s), 704(m), 638(s), 449(w), 414(w).

Infrared spectra were recorded on a Nicolet 6700 FT spectrometer.

X-ray Diffraction

Data collections were carried out by using a Siemens SMART three-circle

diffractometer equipped with a CCD bidimensional detector using the

A. Saad et al.

123

monochromatized wavelength k(Mo Ka) = 0.71073 A. Absorption correction

was based on multiple and symmetry-equivalent reflections in the data set using

the SADABS program [37] based on the method of Blessing [38]. The structure

was solved by direct methods and refined by full-matrix least-squares using the

SHELX-TL package [39]. H atoms on the triply bridging oxygen atoms of

the Anderson-type POM in [Mo4(AlePy)2Cr]2[CrMo6] have been located in the

Fourier difference map. All other H atoms have been placed at calculated

positions. In both structures there is a discrepancy between the formulae

determined by elemental analysis and that deduced from the crystallographic

atom list due to the difficulty in locating all the disordered water molecules.

These disordered water molecules, when located, were refined isotropically and

with partial occupancy factors. Crystallographic data are given in Table 1.

Crystallographic data for the structural analysis have been deposited with the

Cambridge Crystallographic Data Centre, CCDC No.959907 (Mo4(Ale)2Cr) and

959908([Mo4(AlePy)2Cr]2[CrMo6]). Copies of the data can be obtained free of

charge on application to the Director, CCDC, 12 Union Road, Cambridge CB2

1EZ, UK (Fax: int. code (1223)336-033; e-mail for deposit@ccdc.cam.ac.uk).

Powder diffraction data was obtained on a Bruker D5000 diffractometer using Cu

radiation (1.54059 A).

Table 1 Crystallographic data for Mo4(Ale)2Cr and [Mo4(AlePy)2Cr]2[CrMo6]

Mo4(Ale)2Cr [Mo4(AlePy)2Cr]2[CrMo6]

Empirical formula C8H60Mo4N7O37P4 C40H179Cr3Mo14N18Na3O113P8

Formula weight, g 1406.27 4536.90

Crystal system Monoclinic Monoclinic

Space group P21/n P21/c

a (A) 11.064 (5) 12.7063 (7)

b (A) 17.197 (7) 15.949 (1)

c (A) 11.603 (5) 32.818 (2)

b (°) 97.98 (1) 95.05 (1)

V (A3) 2186 (2) 6625 (1)

F 2 2

qcalc (g cm-3) 2.136 2.274

l (mm-1) 1.618 1.750

Data (parameters) 3,734 (261) 19,285 (817)

Rint 0.0536 0.0633

GOF 1.219 1.027

R ([2r(I)) R1a= 0.0904 R1

a= 0.0594

wR2b= 0.2861 wR2

b= 0.1462

aR1 ¼

P

Foj jÿ Fcj jP

Fcj j

bwR2 ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

P

wðF2oÿF2

c Þ2

P

wðF2o Þ

2

r

Molybdenum BPs with Cr(III) or Mn(III) Ions

123

UV–Vis Spectra

Electronic absorption spectra were recorded on a Perkin-Elmer Lambda 19

spectrometer on 2 9 10-3 M aqueous solutions.

Electrochemical Studies

Electrochemical data was obtained using an EG & G 273 A potentiostat driven by a

PC with the M270 software. A one-compartment cell with a standard three-electrode

configuration was used for cyclic voltammetry experiments. The reference electrode

was a saturated calomel electrode (SCE) and the counter electrode a platinum gauze

of large surface area; both electrodes were separated from the bulk electrolyte

solution via fritted compartments filled with the same electrolyte. The working

electrode was a 3 mm OD glassy carbon disc (GC, Le Carbone Lorraine, France).

The pre-treatment of this electrode before each experiment has been described

elsewhere [40]. The polyanion concentration was 2 9 10-4 M. Prior to each

experiment, solutions were de-aerated thoroughly for at least 30 min with pure Ar.

A positive pressure of this gas was maintained during subsequent work. All cyclic

voltammograms were recorded at a scan rate of 10 mV s-1 unless otherwise stated.

All experiments were performed at room temperature, which is controlled and fixed

for the lab at 20 °C. Results were very reproducible from one experiment to another

and slight variations observed over successive runs are attributed to the uncertainty

associated with the detection limit of our equipment (potentiostat, hardware and

software) rather than to the working electrode pre-treatment or to possible variations

in temperature.

Results and Discussion

Structures

The Mo4(Ale)2Cr complex is similar to the molybdenum etidronate complexes with

CrIII, FeIII, VIV, MnII ions, with alendronate replacing the etidronate ligands [33,

34]. It can be briefly described as a centrosymmetric POM in which two MoVI

dimeric units are linked to a central CrIII ion lying on an inversion center (Fig. 1a).

The two MoVI ions in each dimeric unit are connected to two oxygen atoms of the

phosphonate groups and to the deprotonated hydroxo group of an alendronate

ligand. The octahedral coordination sphere of the CrIII ion is constituted by the four

remaining available oxygen atoms of the two phosphonate ligands and of two

oxygen atoms bound to Mo ions of two distinct dimeric units. As usually observed

in MoVI/Ale complexes [10], the amino group of the alendronate ligand is

protonated. These amino groups are involved in N–H���O hydrogen bonds with

oxygen atoms of the POM and generate a 2D supramolecular network (Fig. 1b and

Table 2).

The structure of [Mo4(AlePy)2Cr]2[CrMo6] is made up of a 2:1 molecular

combination of the anticipated Mo4(AlePy)2Cr POM, analogous to the Mo4(Ale)2Cr

A. Saad et al.

123

complex described above, and an unexpected Anderson-B-type POM

[CrMo6(OH)6O18]3- (Fig. 2a). In the Anderson POM, six MoO6 octahedra share

edges with each other and are assembled around a central CrO6 octahedron with

which they also share edges. The POM has thus an overall wheel shape with planar

configuration. The triply bridging oxygen atoms are protonated, as observed for the

other structures of Anderson-B-type POMs [XMo6(OH)6O18]3- (X = Cr, [31, 41]

Fe [42], Ni[43]). It has been possible to locate these H atoms in the Fourier

difference map. Each H atom of the Anderson POM is engaged in a hydrogen-bond

with either a terminal or a bridging oxygen atom of an adjacent Mo/BP complex

(Fig. 3; Table 2), forming a sandwich-like supermolecule. The Mo/BP and

(a)

(b)

Fig. 1 a Mixed polyhedral and ball and stick representation of the Mo4(Ale)2Cr POM; H atoms havebeen omitted for clarity. b Hydrogen bond interactions (depicted as red dotted lines) in the structure ofMo4(Ale)2Cr. Grey octahedra = MoO6, purple octahedra = CrO6, grey spheres = Mo, purple

spheres = Cr, black spheres = C, blue spheres = N, green spheres = P (Color figure online)

Molybdenum BPs with Cr(III) or Mn(III) Ions

123

Anderson planes are almost perpendicular and the arrangement of these POMs in

the unit cell delimits rectangular grids (Fig. 2b). It should be noted that the

hydrogen atoms on the protonated amino groups do not form any H-bond with

oxygen atoms of the POMs and are only H-bonded to water molecules

(Table 2).The pyridyl rings interact in two different ways with adjacent POMs: i)

p–p interactions between the organic parts of Mo4(AlePy)2Cr POMs (Fig. SI1a)

allow their connection along the c direction, ii) interactions between the pyridyl ring

of a Mo4(AlePy)2Cr POM and the surface of a neighbouring Anderson ion can also

be identified (Fig. SI1b). The cooperative effects of these interactions might explain

the formation of a trapped Anderson anion when AlePy is used instead of Ale.

Structures with the cocrystallization of two distinct polyoxotungstates have been

sometimes encountered [44–46]; however, examples with polyoxomolybdates are

far rarer, possibly because the kinetics of formation of polyoxomolybdates is much

faster than that of polyoxotungstates, the crystallization of metastable species being

thus often encountered for polyoxotungstates but not for polyoxomolybdates [47]. A

nice example of a supermolecule with two different polyoxomolybdates, an

Anderson and two Lindquist POMs, can be cited [48]. However in this case, the

interaction between the Anderson and the Lindquist POMs is covalent.

Synthesis and Characterizations

The new ligand Na2[HO3PC(C3H6NHCH2C5H4N)(OH)PO3H] (Na2H2AlePy) was

prepared in high yield starting from the alendronic acid H2O3PC(C3H6NH2)(OH)-

PO3H2 (H4Ale). It is obtained in methanol by reaction of H4Ale with 2-pyridine-

carboxaldehyde, the intermediate imino species being reduced in situ by using

[N(C4H9)4]BH4, following a procedure already reported for alendronate derivatives

[26, 49]. The Mo4(Ale)2Cr and Mo4(Ale)2Mn complexes are synthesized in a one

step procedure by mixing the MoVI precursor, the ligand, and the additional metal

ions (CrIII or MnIII) (Scheme 2). The synthetic pH is equal to 7.5 for Mo4(Ale)2Mn

Table 2 Geometry of hydrogen-bonding interactions in Mo4(Ale)2Cr and [Mo4(AlePy)2Cr]2[CrMo6] for

which N���O\ 3.0 A

N–H���O H���O (A) N���O (A) N–H���O (°)

Mo4(Ale)2Cr

N1-H1A���O10 1.957 2.842 172.45

N1-H1B���O7 2.176 2.861 133.35

N1-H1C���O3 1.887 2.776 177.54

[Mo4(AlePy)2Cr]2[CrMo6]

N1-H1A���O8 W 2.099 2.993 172.30

N3-H3C���O14 W 1.994 2.874 165.57

N3-H3D���O13 W 1.857 2.728 162.47

O31-H31���O17 1.765 2.652 178.62

O32-H32���O16 1.829 2.724 164.84

O36-H36���O12 1.783 2.651 171.55

A. Saad et al.

123

and 6 for the chromium analogue. This synthetic protocol is thus far simpler than the

one used for the etidronate complexes which involves the oxidative Mo–Mo bond

cleavage [33, 34]. Although the experimental procedure is identical for Ale and

AlePy, with AlePy, the expected [(Mo2O6)2(AlePy)2Cr]5- anions cocrystallize with

[CrMo6(OH)6O18]3- Anderson-type POMs. The presence of these anions is quite

unpredictable. Indeed, their formation has been usually reported at more acidic pH

[36, 50]. It can also be noted that the nitrogen atoms of the amino group and of the

pyridine ring remain uncoordinated although the coordination of these atoms to

either MoVI or CrIII ions could have been anticipated.

It has not been possible to obtain single crystal of sufficient quality for X-ray

single crystal diffraction of Mo4(Ale)2Mn. However the comparison of the

experimental X-ray powder pattern of Mo4(Ale)2Mn with the powder pattern

calculated from the structure solved from single-crystal X-ray diffraction data of

Mo4(Ale)2Cr (Fig. SI2) has allowed to confirm that both compounds are

isostructural. It indicates also the crystalline homogeneity of each compound.

Fig. 2 a Polyhedral representation of the two anions that cocrystallize in [Mo4(AlePy)2Cr]2[CrMo6]; bViewof the unit-cell along the a axis. H atoms and water molecules have been omitted for clarity. Greyoctahedra = MoO6, purple octahedra = CrO6, black spheres = C, blue spheres = N, green spheres = P(Color figure online)

Molybdenum BPs with Cr(III) or Mn(III) Ions

123

The infrared spectra of the three compounds contain intense bands around

1,420 cm-1 which can be attributed to the NH4? cations. The P–O vibrations of the

organic ligand are encountered between 1,100 and 1,000 cm-1 while the Mo–O

vibrations are found below 920 cm-1. The IR spectra of Mo4(Ale)2Cr and

Mo4(Ale)2Mn are identical. In the range 1,200–500 cm-1, the spectrum of

[Mo4(AlePy)2Cr]2[CrMo6] can be seen as the superimposition of those of

Mo4(Ale)2Cr and CrMo6 (Fig. SI3).

The UV–Vis spectra of 2 9 10-3 M aqueous solutions of Mo4(Ale)2Mn,

Mo4(Ale)2Cr, [Mo4(AlePy)2Cr]2[CrMo6] and CrMo6 have been recorded (Fig. 4).

The Mo4(Ale)2Mn POM exhibits a broad absorption peak at 480 nm

(e = 40 L mol-1 cm-1), which can be attributed to d–d transitions related to the

central MIII ion, while for the Cr analogue Mo4(Ale)2Cr this absorption band

appears around 640 nm (e = 15 L mol-1 cm-1). The spectrum of a solution of

[Mo4(AlePy)2Cr]2[CrMo6] is almost the exact superimposition of two times that of

Fig. 3 Hydrogen-bondinginteractions (depicted as reddotted lines) between theAnderson POM and the oxygenatom of one dimeric unit of theMo/BP complex in[Mo4(AlePy)2Cr]2[CrMo6].Grey spheres = Mo, purplespheres = Cr, blackspheres = C, small blackspheres = H, blue spheres = N,green spheres = P (Color figureonline)

A. Saad et al.

123

Mo4(Ale)2Cr and one time that of the Anderson anion CrMo6, confirming the

coexistence of both POMs in the compound.

We have recently evidenced that hybrid organic–inorganic BP MoVI POMs

exhibit strong solid-state photochromic properties at room temperature [26, 51].

According to the reported mechanism [52, 53], the UV excitation induces an

electron transfer into the POM from an oxygen atom to the adjacent MoVI cations.

This is followed by the moving of a labile hydrogen atom from the ammonium

group of the BP ligand onto the POM, and the so-created hydroxyl group traps the

excited electron onto the molybdenum centre. The appearance of coloration is then

due to the photoreduction of MoVI (4d0) to MoV (4d1) cations and occurs via d–d

transitions and/or MoVI/MoV intervalence transfers. Concomitantly to the reduction

of the POM, the photogenerated hole moves onto the nitrogen atom of the BP group.

This electron transfer assisted by H atom displacement banishes the fast electron/

hole recombination into the POM and allows maintaining the coloration after

switching off the UV irradiation. Noticeably, the photoresponses of the BP MoVI

POMs are highly tuneable playing with the composition of the POM units, the

nature of the grafted BP groups, and the design of the H-bonding network at the

organic–inorganic interface. In marked contrast with the previous hybrid MoVI/BP

systems [26], Mo4(Ale)2Mn, Mo4(Ale)2Cr, and [Mo4(AlePy)2Cr]2[CrMo6] exposed

under UV light (365 nm, 6 W) do not develop any photochromic properties in the

crystalline state at room temperature. This is quite justified in the case of

[Mo4(AlePy)2Cr]2[CrMo6] considering that, as underlined above, the protonated

amino groups of the AlePy ligands are not implied in direct H-bonding interactions

with the POM units. However, it is much more surprising in the cases of

Mo4(Ale)2Mn and Mo4(Ale)2Cr, for which the ammonium group of the Ale ligands

develops short intermolecular N–H���O contacts with terminal and bridging O atoms

of adjacent MoVI dimeric units (see above). At first sight, and as already observed in

nonphotochromic hybrid POM systems containing a FeIII cation [42], we strongly

suspect that the presence of a trivalent transition metal ion in close proximity to the

MoO6 octahedra should dramatically reduce the life time of the photogenerated

MoV cations by capturing the excited electron on the molybdenum site.

Scheme 2 Synthetic protocols for the preparation of the three MoVI/BP complexes

Molybdenum BPs with Cr(III) or Mn(III) Ions

123

Electrochemistry

The electrochemical behavior of the compounds Mo4(Ale)2Cr and Mo4(Ale)2Mn

was investigated in 0.5 M Li2SO4 ? H2SO4/pH 3.0 and in 1.0 M LiCH3COO ?

CH3COOH/pH 6.0. Figure 5a shows the CVs ofMo4(Ale)2Mn at pH 3.0 (blue) and at

pH 6.0 (black). These stable patterns are obtained after the first complete cycle

between 0.0 and -1.0 V or -1.2 V versus SCE for pH 3.0 and pH 6.0 respectively.

Scanning towards the negative potentials results in the reduction of Mo centres as

expected [26]. The two compounds show the same behavior in these media but the Cr

containing compound is easier to reduce than the Mn one, whereas both compounds

are isostructural and carry the same formal charge (Table 3).

Scanning towards the positive potentials results in the oxidation of the MnIII

centre and the subsequent deposition of manganese oxides species on the working

electrode surface. The reduction of these oxides is observed in the reverse scan as a

more or less sharp cathodic peak located at ?0.78 and ?0.47 V versus SCE for

pH 3.0 and pH 6.0 respectively (Fig. 5b and Fig. SI4). On the other side, no

evidence of the oxidation of the CrIII centre in Mo4(Ale)2Cr could be detected by

cyclic voltammetry (Fig. SI4).

Conclusion

In conclusion, we present a one-step synthesis of hybrid organic inorganic

polyoxomolybdates functionalized by BP ligands. With alendronate, isostructural

tetranuclear MoVI/BP complexes with a central heterometal (CrIII or MnIII) in

octahedral coordination, have been isolated. With an alendronate ligand function-

alized by a pyridine group, a compound with a 2:1 molecular combination of the

Fig. 4 UV–Vis spectra of 2 9 10-3 M aqueous solutions of the [Mo4(AlePy)2Cr]2[CrMo6], CrMo6,Mo4(Ale)2Mn and Mo4(Ale)2Cr complexes (Color figure online)

A. Saad et al.

123

anticipated tetranuclear POM and an unexpected Anderson-B-type POM

[CrMo6(OH)6O18]3- crystallizes, showing that small variations on the organic

ligand can have a strong influence on the nature of the reaction product. The

presence of trivalent cations in close proximity to the MoVI centers dramatically

reduce the life time of the photogenerated MoV cations by capturing the excited

electron on the molybdenum site, preventing the observation of photochromic

properties unlike what has been observed previously for other MoVI/BP compounds.

We are currently exploring the reactivity of FeIII cations with MoVI ions and

biologically active BP ligands in order to develop a new family of complexes with

antitumoral activities.

Acknowledgments This work was supported by CNRS, UVSQ and the French ANR (grant ANR-11-

BS07-011-01-BIOOPOM). Clotilde Menet is gratefully acknowledged for her participation in the

synthesis of the compounds.

References

1. P. Souchay, R. Contant, and J.-M. Fruchart (1967). C. R. Hebd. Seances Acad. Sci., Ser. C 264, 976.

2. B. Keita, T. Lucas, and L. Nadjo (1986). J. Electroanal. Chem. 206, 343.

3. R. Massart, R. Contant, J.-M. Fruchart, J.-P. Ciabrini, and M. Fournier (1977). Inorg. Chem. 16,

2916.

0.4 0.8 1.2

0.0

6.0

12.0 (b)

M o4 (A l e)

2 M n

E / V v s . S C E

p H 3 . 0

-1.2 -0.8 -0.4 0.0

-12.0

-6.0

µ

0.0

(a)

M o4 (A l e)

2 M n

I /

A

µI

/

A

E / V v s . S C E

p H 3 . 0

p H 6 . 0

Fig. 5 aComparisonof theCVs obtainedwithMo4(Ale)2Mn in 0.5 MLi2SO4 ? H2SO4/pH 3.0 (blue) andin 1.0 M LiCH3COO ? CH3COOH/pH 6.0 (black). The CVs were limited to the waves attributed toreduction ofMocentreswhen scanning towards the negative potentials.bCVobtainedwithMo4(Ale)2Mn in0.5 M Li2SO4 ? H2SO4/pH 3.0 (blue). The CVs were limited to range ?0.4 to ?1.2 V when scanningtowards the oxidation potentials. POM concentration 0.2 mM, working electrode glassy carbon, referenceelectrode SCE, auxiliary electrode platinum, scan rate 10 mV s-1 (Color figure online)

Table 3 Peak potentials observed in the cyclic voltammograms of Mo4(Ale)2Cr and Mo4(Ale)2Mn

Epc (Mo) Epa (Mn)

pH 3.0 pH 6.0 pH 3.0 pH 6.0

Mo4(Ale)2Cr -0.69 -0.85 – –

Mo4(Ale)2Mn -0.80 -0.89 ?1.01 ?0.80

Molybdenum BPs with Cr(III) or Mn(III) Ions

123

4. R. Contant and A. Teze (1985). Inorg. Chem. 24, 4610.

5. B. Keita and L. Nadjo (1988). J. Electroanal. Chem. 243, 87.

6. B. Keita and L. Nadjo (1989). Mat. Chem. Phys 22, 77.

7. B. Keita and L. Nadjo (1987). J. Electroanal. Chem. 227, 77.

8. J.-P. Ciabrini, R. Contant, and J.-M. Fruchart (1983). Polyhedron 2, 1229.

9. M. T. Pope Heteropoly and Isopoly Oxometalates (Springer, New York, 1983).

10. P. Gouzerh and A. Proust (1998). Chem. Rev. 98, 77.

11. A. Muller and S. Roy (2003). Coord. Chem. Rev. 245, 153.

12. B. Keita and L. Nadjo (2007). J. Mol. Catal. A 262, 190.

13. U. Kortz, A. Muller, J. van Slageren, J. Schnack, N. S. Dalal, and M. Dressel (2009). Coord. Chem.

Rev. 253, 2315.

14. E. Cadot, M. N. Sokolov, V. P. Fedin, C. Simonnet-Jegat, S. Floquet, and F. Secheresse (2012).

Chem. Soc. Rev. 41, 7335.

15. H. Lv, Y. V. Geletii, C. Zhao, J. W. Vickers, G. Zhu, Z. Luo, J. Song, T. Lian, D. G. Musaev, and C.

L. Hill (2012). Chem. Soc. Rev. 41, 7572.

16. P. Yin, D. Li, and T. Liu (2012). Chem. Soc. Rev 41, 7368.

17. A. Banerjee, B. S. Bassil, G.-V. Roschenthaler, and U. Kortz (2012). Chem. Soc. Rev. 41, 7590.

18. A. Dolbecq, P. Mialane, F. Secheresse, B. Keita, and L. Nadjo (2012). Chem. Commun. 48, 8299.

19. Y. Song, F. Y. Lin, F. Yin, M. Hensler, C. A. Rodrigues Poveda, D. Mukkamala, R. Cao, H. Wang,

C. T. Morita, D. Gonzalez Pacanowska, V. Nizet, and E. Oldfield (2009). J. Med. Chem. 52, 976.

20. J. T. Ruhle, C. L. Hill, D. A. Judd, and R. F. Schinazi (1998). Chem. Rev. 98, 327.

21. B. Hasenknopf (2005). Front. Biosci. 10, 275.

22. H. El Moll, W. Zhu, E. Oldfield, L. Marleny-Rodriguez, P. Mialane, J. Marrot, N. Vila,

I. M. Mbomekalle, E. Riviere, C. Duboc, and A. Dolbecq (2012). Inorg. Chem. 51, 7921.

23. V. S. Sergienko (2001). Russian. J. Coord. Chem. 27, 723.

24. U. Kortz and M. T. Pope (1995). Inorg. Chem. 34, 2160.

25. H. Tan, W. Chen, D. Liu, X. Feng, Y. Li, A. Yan, and E. Wang (2011). Dalton Trans. 40, 8414.

26. H. El Moll, A. Dolbecq, I. M. Mbomekalle, J. Marrot, P. Deniard, R. Dessapt, and P. Mialane (2012).

Inorg. Chem. 51, 2291.

27. E. Dumas, C. Sassoye, K. D. Smith, and S. C. Sevov (2002). Inorg. Chem. 41, 4029.

28. C. du Peloux, A. Dolbecq, P. Mialane, J. Marrot, and F. Secheresse (2004). Dalton Trans. 1259.

29. H.-Q. Tan, W.-L. Chen, D. Liu, and E.-B. Wang (2010). J. Clust. Sci. 21, 147.

30. J.-D. Compain, P. Mialane, J. Marrot, F. Secheresse, W. Zhu, E. Oldfield, and A. Dolbecq (2010).

Chem. Eur. J. 16, 13741.

31. A. Banerjee, F. S. Raad, N. Vankova, B. S. Bassil, T. Heine, and U. Kortz (2011). Inorg. Chem. 50,

11667.

32. J. Niu, X. Zhang, D. Yang, J. Zhao, P. Ma, U. Kortz, and J. Wang (2012). Chem. Eur. J. 18, 6759.

33. H. Tan, W. Chen, D. Liu, Y. Li, and E. Wang (2010). Dalton Trans. 39, 1245.

34. L. Zhang, J. Sun, Y. Zhou, S. ul Hassan, E. Wang, and Z. Shi (2012). CrystEngComm 14, 4826.

35. V. Kubıcek, J. Kotek, P. Hermann, and I. Lukes (2007). Eur. J. Inorg. Chem. 333.

36. A. Perloff (1970). Inorg. Chem. 9, 2228.

37. G. M. Sheldrick SADABS; Program for Scaling and Correction of Area Detector Data (University of

Gottingen, Gottingen, 1997).

38. R. Blessing (1995). Acta Crystallogr. A51, 33.

39. G. M. Sheldrick SHELX-TL version 5.03, Software Package for the Crystal Structure Determination

(Siemens Analytical X-ray Instrument Division, Madison, 1994).

40. N. Vila, P. A. Aparicio, F. Secheresse, J. M. Poblet, X. Lopez, and I. M. Mbomekalle (2012). Inorg.

Chem. 51, 6129–6138.

41. M. Singh, and A. Ramanan (2011). Cryst. Growth. Des. 11, 3381 and references therein.

42. R. Dessapt, M. Gabard, M. Bujoli-Doeuff, P. Deniard, and S. Jobic (2011). Inorg. Chem. 50, 8790.

43. F.-X. Liu, C. Marchal-Roch, D. Dambournet, A. Acker, J. Marrot, and F. Secheresse (2008). Eur.

J. Inorg. Chem. 2191.

44. X. Fang, K. McCallum, H. D. Pratt III, T. M. Anderson, K. Dennis, and M. Luban (2012). Dalton

Trans. 41, 9867.

45. X. Fang and M. Luban (2011). Chem. Commun. 47, 3066.

46. S.-T. Zhang, J. Zhang, J. Modesto Clemente-Juan, D.-Q. Yuan, and G.-Y. Yang (2009). Angew.

Chem. Int. Ed. 48, 7176.

47. R. Contant and G. Herve (2002). Rev. Inorg. Chem. 22, 63.

A. Saad et al.

123

48. J. Zhang, J. Hao, Y. Wei, F. Xiao, P. Yin, and L. Wang (2010). J. Am. Chem. Soc. 132, 14.

49. H. El Moll, G. Rousseau, A. Dolbecq, O. Oms, J. Marrot, M. Haouas, F. Taulelle, E. Riviere, W.

Wernsdorfer, D. Lachkar, E. Lacote, B. Keita, and P. Mialane (2013). Chem. Eur. J. 19, 6753.

50. L. Yu, S.-Z. Li, and J.-P. Wang (2006). Acta Cryst E 62, il90.

51. J.-D. Compain, P. Deniard, R. Dessapt, A. Dolbecq, O. Oms, F. Secheresse, J. Marrot, and P. Mialane

(2010). Chem. Commun. 46, 7733.

52. T. Yamase (1998). Chem. Rev. 98, 307.

53. R. Dessapt, M. Collet, V. Coue, M. Bujoli-Doeuff, S. Jobic, C. Lee, and M.-H. Whangbo (2009).

Inorg. Chem. 48, 574.

Molybdenum BPs with Cr(III) or Mn(III) Ions

123