Tuning the Dimensionality of Polyoxometalate-Based Materials by

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Tuning the Dimensionality of PolyoxometalateBased Hybrid Materials Using a Mixture of Ligands Guillaume Rousseau, L. Marleny Rodriguez-Albelo, William Salomon, Pierre Mialane, Jerome Marrot, Floriant Doungmene, Israël Martyr Mbomekallé, Pedro de Oliveira, and Anne Dolbecq Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg501524a • Publication Date (Web): 26 Nov 2014 Downloaded from http://pubs.acs.org on December 1, 2014

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Crystal Growth & Design

Tuning the Dimensionality of PolyoxometalateBased Materials by Using a Mixture of Ligands Guillaume Rousseau,† Luisa Marleny Rodriguez-Albelo,‡ William Salomon,† Pierre Mialane,† Jérôme Marrot,† Floriant Doungmene,§ Israël-Martyr Mbomekallé,§ Pedro de Oliveira,*§ and Anne Dolbecq*,†

†

Institut Lavoisier de Versailles, UMR 8180 CNRS, Université de Versailles Saint-Quentin

en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles cedex, France ‡

Institute of Materials Science and Technology (IMRE), University of Havana, Havana,

10400, Cuba §

Laboratoire de Chimie Physique, UMR 8000 CNRS, Université Paris-Sud, 91405 Orsay

cedex, France

1 ACS Paragon Plus Environment


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ABSTRACT: Five molecular, 1D, 2D or 3D organic-inorganic hybrid polyoxometalates (POMs) based on the {ε-PMoV8MoVI4O40Zn4} (εZn) Keggin unit have been synthesized under hydrothermal

conditions

using

a

mixture

of

O-

and

N-donor

ligands.

(TBA)6[PMoV8MoVI4O37(OH)3Zn4]2(C14H8O4)3·6H2O (εε2(biphen)3) is a 3D material with two interpenetrated

networks

built

biphenyldicarboxylate

from

dimeric

(εZn)2

POMs

linked

by

(biphen)

(TBA)2[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)3(C14H8O4)1/2·H2O

ligands. (εε(bim)3(biphen)1/2),

(TBA)3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)2(C8H4O4)·6H2O (TBA)7/3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)8/3(C8H4O4)2/3

4,4’-

(εε(bim)2(isop)), (εε(bim)8/3(bdc)2/3)

and

(TBA)3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)2(C9H3O6)2/3·6H2O (εε(bim)2(trim)2/3) all consist of monomeric εZn units bound to two types of organic ligands: benzimidazole (bim) and one of the following carboxylate ligands: biphen, 1,3-benzenedicarboxylate (isop), 1,4benzenedicarboxylate

(bdc)

or

1,3,5-benzenetricarboxylate

(trim)

ligands.

While

ε(bim)3(biphen)1/2 is a molecular complex, ε(bim)2(isop) and ε(bim)8/3(bdc)2/3 adopt a chain arrangement and ε(bim)2(trim)2/3 is a 2D compound. In these materials, the limitation of the dimensionality is a direct consequence of the protonation of the nitrogen atom of the bim ligands. The electrocatalytic activity for the hydrogen evolution reaction (HER) of these five new POM-based coordination polymers has been studied, showing that their performance depends mainly on the presence of the εZn Keggin units but also on their structure. Modified electrodes fabricated with ε(bim)2(trim)2/3 entrapped in a carbon paste revealed that this hybrid is the most efficient electrocatalyst of the series, being stable and catalysing the HER in the pH 1-5 range.


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INTRODUCTION Polyoxometalates (POMs) may be seen as soluble anionic metal oxide clusters of d-block transition metals in high oxidation states (WVI, MoV,VI, VIV,V).1 Remarkably, it has been shown that almost all of the elements of the periodic table can be incorporated into a POM matrix, forming a family of molecular materials with a tremendous structural variety and exhibiting properties in fields ranging from magnetism to catalysis.2 These molecular POMs can be used as building units for the elaboration of purely inorganic3 or hybrid organicinorganic insoluble extended materials.4 Among the latter species, hybrid materials where the POM building units are linked via transition metal complexes through direct M-O-M’ bonds (M = W, Mo,…; M’ = Cu, Co…) are frequently encountered,5 but POM complexes with metallic anchoring centres can also be connected by bridging organic ligands,6 leading to polyoxometalate-based coordination polymers sometimes called POMOFs.7 We have been working for a few years on the synthesis of POMOFs based on a ε-Keggin unit, {εPMoV8MoVI4O40Zn4} (εZn), which presents the advantage of having four ZnII capping ions available for coordinating O- or N-donor ligands. The charge of this unit can be modulated by the presence of protons (from 0 to 5) on some of the bridging oxygen atoms. Several redox active compounds have already been isolated. The frameworks of the POMOFs built with this POM unit are anionic and this anionic charge is compensated in almost all the POMOFs isolated

so

far

by

tetrabutylammonium

cations.

The

3D

material

with

1,4-

benzenedicarboxylate (bdc) linkers, (TBA)3[PMo12O36(OH)4Zn4](bdc)2 (Z-POMOF1) shows a high electrocatalytic activity for the reduction of bromates.8 Replacing bdc by 1,3benzenedicarboxylate

(isop)

has

led

to

the

2D

compound

(TBA)3[PMoV8MoVI4O36(OH)4Zn4](isop)2 (εε(isop)2) which has been successfully used as a reducing agent of graphite oxide to obtain graphene.9 The most remarkable result for this family of compounds is the very good activity as heterogeneous electrocatalysts for the HER 3 ACS Paragon Plus Environment


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observed

for

a

POMOF

with

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1,3,5-benzenetricarboxylate

(trim)

linkers,

(TBA)3[PMo12O36(OH)4Zn4](trim)4/3 (εε(trim)4/3).10 We have extended this family of POMOFs materials by using a mixture of two ligands, following a strategy developed in Metal Organic Frameworks (MOFs),11 and also in POM-based materials.12 We thus report herein the results obtained when mixtures of a carboxylate and a N-donor blocking ligand were used as reactants. The use of a mixture of di- and tri- carboxylate ligands has also been considered. Benzimidazole was chosen as the N-donor ligand in order to tune the dimensionality of the POMOF as it can act as a terminal ligand when monoprotonated. Molecular as well as 1D, 2D and 3D materials have been obtained. We also report a study on the electrocatalytic hydrogen evolution reaction on these compounds in aqueous media.

EXPERIMENTAL SECTION Synthesis and Characterizations: All reagents were purchased from commercial sources and

used

as

received.

Hydrothermal

syntheses

were

carried

out

in

23

mL

polytetrafluoroethylene lined stainless steel containers under autogenous pressure. Commercially available reagents were used as received, without further purification. All reactants were briefly stirred before heating. The mixture was heated to 200°C over a period of 1h, kept at 200°C for 70h and cooled down to room temperature over a period of 80h, except for ε2(biphen)3 which was heated at 180°C using the same heating and cooling program. The pH mixture was measured before (pHi) and after the reaction (pHf). The products were isolated by filtration after sonication which allows removing a brown powder always present after the reaction and washed with ethanol. For the five POM-based coordination polymers, the nature and amount of reactants is quasi-identical, except for the Mo(VI) precursor, which can be either Na2MoO4·2H2O or (NH4)6Mo7O24·4H2O. The


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replacement of one by the other does not change the nature of the product but influences the yield and the crystallinity. Preparation of (TBA)6[PMoV8MoVI4O37(OH)3Zn4]2(C14H8O4)3·6H2O (εε2(biphen)3): A mixture of Na2MoO4·2H2O (0.847 g, 3.50 mmol), molybdenum powder 99.99% (0.060 g, 0.62 mmol), H3PO3

(0.040 g,

0.50

mmol),

Zn(CH3COO)2·2H2O

(0.219 g,

1

mmol),

4,4’-

biphenyldicarboxylic acid (0.242 g, 1 mmol), tetrabutylammonium hydroxide 40 w.t. % solution in water (200 µL, 0.30 mmol) and H2O (9 mL) was stirred and the pH was adjusted to 5 with 4M HCl (pHf = 5.5). Dark red cubic crystals suitable for X-ray diffraction study were collected after filtration (0.130 g, 16% based on Zn). Anal. calc. for C138H258Mo24N6O98P2Zn8 (found): C 25.67 (26.04), H 4.02 (3.48), Mo 35.66 (36.04), N 1.30 (1.32), P 0.96 (0.86), Zn 8.10 (7.90). IR (ν/cm-1): 2961 (s), 2931 (s), 2873 (s), 2623 (sh), 2348 (m), 1962 (w), 1595 (m), 1545 (w), 1480 (m), 1459 (sh), 1482 (m), 1374 (s), 1174 (w), 1146 (sh), 1104 (w), 1010 (sh), 994 (sh), 982 (m), 962 (m), 937 (s), 817 (s), 783(s), 769 (sh), 711 (m), 681 (w), 590 (m), 541 (w), 521 (w), 485 (w). Preparation

of

(TBA)2[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)3(C14H8O4)1/2·H2O

(εε(bim)3(biphen)1/2): A mixture of (NH4)6Mo7O24·4H2O (0.618 g, 0.50 mmol), molybdenum powder 99.99% (0.060 g, 0.62 mmol), H3PO3 (0.040 g, 0.50 mmol), Zn(CH3COO)2·2H2O (0.219 g, 1 mmol), 4,4’-biphenyldicarboxylic acid (0.121 g, 0.5 mmol), benzimidazole (0.059 g, 0.5 mmol), tetrabutylammonium hydroxide 40 w.t. % solution in water (200 µL, 0.30 mmol) and H2O (9 mL) was stirred and the pH was adjusted to 5 (pHi) with 2M HCl (pHf = 5.3). Dark red blocks suitable for X-ray diffraction study were collected after filtration (0.080 g, 11% based on Zn). Anal. calc. for C60H98Mo12N8O43PZn4 (found): C 23.52 (23.12), H 3.22 (3.42), Mo 37.58 (37.64), N 3.66 (3.66), P 1.01 (1.03), Zn 8.54 (8.60). IR (ν/cm-1): 2961 (m), 2873 (m), 1622 (w), 1597 (m), 1555 (w), 1495 (w), 1461 (w), 1438 (w), 1378(m), 1305 (w),



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1278 (w), 1258 (m), 1160 (w), 1116 (w), 1004 (sh), 978 (sh), 967 (sh), 926 (s), 908 (sh), 809 (s), 783 (s), 760(s), 743 (sh), 707 (s), 634 (m), 591 (s), 546 (m), 476 (m), 433 (m). Preparation of (TBA)3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)2(C8H4O4)·6H2O (εε(bim)2(isop)): A mixture of (NH4)6Mo7O24·4H2O (0.618 g, 0.50 mmol), molybdenum powder 99.99% (0.060 g, 0.62 mmol), H3PO3 (0.040 g, 0.50 mmol), Zn(CH3COO)2·2H2O (0.219 g, 1 mmol), benzene-1,3-dicarboxylic acid (0.083 g, 0.5 mmol), benzimidazole (0.059 g, 0.5 mmol), tetrabutylammonium hydroxide 40 w.t. % solution in water (200 µL, 0.30 mmol) and H2O (9 mL) was stirred and the pH was adjusted to 5 (pHi) with 2M HCl (pHf = 4.9). Dark red parallelepipedic blocks suitable for X-ray diffraction study were collected after filtration (0.130 g, 15% based on Zn). Anal. calc. for C70H138Mo12N7O50PZn4 (found): C 25.31 (24.53), H 4.18 (3.79), Mo 34.66 (35.24), N 2.95 (2.85), P 0.93 (0.97), Zn 7.87 (7.29). IR (ν/cm-1): 2957 (m), 2864 (w), 1618 (m), 1574 (sh), 1507 (w), 1453 (sh), 1437 (m), 1344 (s), 1305 (m), 1274 (w), 1256 (w), 1149 (w), 1063 (w), 961 (sh), 925 (s), 777 (s), 764 (sh), 702 (m), 583 (s), 546 (m). Preparation of (TBA)7/3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)8/3(C8H4O4)2/3 (εε(bim)8/3(bdc)2/3): A mixture of Na2MoO4·2H2O (0.847 g, 0.50 mmol), molybdenum powder 99.99% (0.060 g, 0.62 mmol), H3PO3 (0.040 g, 0.50 mmol), Zn(CH3COO)2·2H2O (0.219 g, 1 mmol), benzene1,4-dicarboxylic acid (0.083 g, 0.5 mmol), benzimidazole (0.059 g, 0.5 mmol), tetrabutylammonium hydroxide 40 w.t. % solution in water (200 µL, 0.30 mmol) and H2O (9 mL) was stirred and the pH was adjusted to 5 (pHi) with 2M HCl (pHf = 4.7). Dark red parallelepipedic blocks suitable for X-ray diffraction study were collected after filtration (0.186 g, 24% based on Zn). Anal. calc. for C61.3H104.7Mo12N7.7O42.7PZn4 (found): C 23.95 (24.52), H 3.43 (3.79), Mo 37.43 (37.34), N 3.49 (3.07), P 1.00 (0.98), Zn 8.50 (8.00). IR (ν/cm-1): 2957 (m), 2864 (w), 1589 (m), 1501 (w), 1465 (sh), 1379 (sh), 1356 (m), 1304 (w),


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1248 (w), 1150 (w), 1105 (w), 1062 (w), 1002 (w), 966 (sh), 957 (sh), 928 (s), 910 (sh), 809 (m), 777 (s), 745 (m), 700 (m), 647 (w), 585 (s), 548 (m), 480 (m), 433 (m). Preparation

of

(TBA)3[PMoV8MoVI4O38(OH)2Zn4](C7H6N2)2(C9H3O6)2/3·6H2O

(εε(bim)2(trim)2/3): A mixture of (NH4)6Mo7O24·4H2O (0.618 g, 0.50 mmol), molybdenum powder 99.99% (0.060 g, 0.62 mmol), H3PO3 (0.040 g, 0.50 mmol), Zn(CH3COO)2·2H2O (0.219 g, 1 mmol), benzene-1,3,5-tricarboxylic acid (0.105 g, 0.5 mmol), benzimidazole (0.059 g, 0.5 mmol), tetrabutylammonium hydroxide 40 w.t. % solution in water (200 µL, 0.30 mmol) and H2O (9 mL) was stirred and the pH was adjusted to 5 (pHi) with 2M HCl (pHf = 4.7). Dark red parallelepipedic blocks suitable for X-ray diffraction study were collected after filtration (0.110 g, 13% based on Zn). Anal. calc. for C68H156Mo12N7O60PZn4 (found): C 24.78 (23.29), H 4.15 (3.74), Mo 34.94 (36.93), N 2.97 (2.88), P 0.94 (1.01), Zn 7.93 (6.40). IR (ν/cm-1): 2960 (m), 2868 (m), 1618 (w), 1574 (w), 1505 (w), 1471 (w), 1436 (w), 1380(w), 1344 (m), 1305 (w), 1278 (w), 1256 (w), 1152 (w), 1104 (w), 1002 (sh), 972 (sh), 959 (sh), 930 (s), 809 (m), 776(s), 762 (s), 704 (s), 647 (w), 587 (s), 546 (m), 480 (m), 424 (m). Infrared spectra were recorded on a Nicolet 6700 FT-IR spectrophotometer. Crystal Structure Determination. Single crystal X-ray diffraction data collections were carried out by using a Siemens SMART three-circle diffractometer equipped with a CCD bidimensional detector using the monochromatised wavelength λ(Mo Kα) = 0.71073 Å. Absorption correction was based on multiple and symmetry-equivalent reflections in the data set using the SADABS program13 based on the method of Blessing.14 The structure was solved by direct methods and refined by full-matrix least-squares using the SHELX-TL package.15 For the structure of ε(bim)2(trim)2/3, the TBA cations could not be located properly in the structure due to severe disorder and the data set was corrected with the program SQUEEZE,16 a part of the PLATON package of crystallographic software used to calculate the solvent or counter-ion disorder area and to remove its contribution to the overall 7 ACS Paragon Plus Environment


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intensity data. A phenyl ring of a benzimidazole ligand has also been found disordered and a disordered scheme proposed. The H atoms of the disordered C atoms were not included in the models and these atoms were not refined anisotropically. Crystallographic data are given in Table 1 and the complete data can be found in the cif file as Supporting Information.. Powder X-ray diffraction data was obtained on a Brüker D5000 diffractometer using Cu radiation (1.54059 Å). Stability Tests. 50 mg of ε(bim)2(trim)2/3 were stirred in 25 mL of an electrolyte solution at pH = 1, 3 and 5 for 48h. The solid was filtered, washed with water, dried with EtOH before recording the IR spectra and powder X-ray diffraction patterns. Experimental section for the electrochemical studies. Chemicals. Pure water obtained with a Milli-Q Intregral 5 purification set was used throughout. All reagents were of high-purity grade and were used as purchased without further purification. HCl (Prolabo), LiCl, Li2SO4.H2O, LiCH3COO.2H2O (Acros Organics), H2SO4 (Aldrich) and CH3COOH (Sigma-Aldrich) were commercial products. The compositions of the media used are: 1.0 M LiCl + HCl, pH 1.0, selected according to previous results;8,9 0.5 M Li2SO4 + H2SO4, pH 2.0 and 3.0; 1.0 M LiCH3COO + CH3COOH, pH 4.0 and 5.0. Electrochemistry. 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 threeelectrode 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 Carbon Paste Electrode (CPE), prepared as previously described.8,9 Prior to each experiment, solutions were thoroughly de-aerated for at least 30 min with pure Ar. A positive pressure of this gas was


Page 9 of 26

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kept during subsequent work. All experiments were performed at room temperature, which is controlled and fixed for the lab at 20°C.

Table 1. Crystallographic data for ε2(biphen)3, ε(bim)3(biphen)1/2, ε(bim)2(isop) and ε(bim)2(trim)2/3. ε2(biphen)3

ε(bim)3(biphen)1/2

ε(bim)2(isop)

ε(bim)2(trim)2/3

Empirical formula

C138H258Mo24N6O98P2Zn8

C60H98Mo12N8O43PZn4

C70H138Mo12N7O50PZn4

C68H136Mo12N7O50PZn4

Formula weight, g

6456.96

3063.19

3321.60

3295.57

Crystal system

monoclinic

orthorhombic

monoclinic

triclinic

Space group

P21/n

Pccn

P21/c

P-1

a/Å

21.685(1)

37.903(3)

26.362(15)

26.480(3)

b/Å

29.728(1)

20.749(2)

15.478(10)

26.845(3)

c /Å

34.829(1)

24.389(2)

27.341(18)

27.075(3)

α/°

90

90

90

71.282(2)

β/°

96.342(1)

90

102.221(16)

68.629(3)

γ/°

90

90

90

63.698(2)

V / Å3

22315.0(15)

19181(3)

10903(12)

15772(3)

Ζ

4

4

4

6

ρcalc / g cm-3

1.922

2.122

2.023

2.082

µ / mm-1

2.236

2.592

2.293

2.377

Data / Parameters

65876 / 2261

22040 / 1094

81849 / 1159

71382 / 2178

Rint

0.0611

0.1583

0.0857

0.0483

GOF

1.067

1.001

1.027

0.915

a

R (>2σ(I))

a

R = 1

R1 = 0.0637

R1a

wR2 b= 0.1097

wR2 b= 0.1598

∑ Fo − Fc ∑ Fc

b

wR

2

=

= 0.0695

a

R1 = 0.0572

R1a = 0.0610

wR2 b= 0.1165

wR2 b= 0.1485

2 2 2 ∑ w( Fo − Fc ) 2 2 ∑ w( Fo )



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RESULTS AND DISCUSSION Structures. The structures of the four novel POM-based coordination polymers materials, one with biphen and the three others with mixed linkers will be described in this section. The inorganic building block of the structures of ε(bim)3(biphen)1/2, ε(bim)2(trim)2/3 and ε(bim)2(isop) is the well-known {ε-PMoV8MoVI4O40Zn4} POM (noted εZn, Figure 1a). In this POM, eight electrons of eight MoV ions are localized in four MoV-MoV bonds. Accordingly, the MoV···MoV distances are equal to ~2.6 Å while MoVI···MoVI distances are longer (~3.2 Å) and the compound is red contrary to reduced POMs with delocalized electrons, the socalled “molybdenum blue species”.17 Four capping ZnII ions in tetrahedral coordination are bound to three oxygen atoms of the POMs and either to an oxygen atom of a carboxylate linker or to a nitrogen atom of a N-donor ligand. The overall symmetry of εZn is thus tetrahedral. In all the structures, the valence of the Mo ions has been determined by bond valence sum calculations and confirmed by an examination of the Mo-Mo distances (Figure SI1-SI4). Bond valence sum calculations indicate also the localization of the protons on the bridging oxygen atoms.

Figure 1. Ball and stick representations of a) the monomeric {ε-PMoV8MoVI4O40Zn4} Keggin building unit common to ε(bim)3(biphen)1/2, ε(bim)2(isop) and ε(bim)2(trim)2/3 and b) of the dimeric unit in ε2(biphen)3. 10 ACS Paragon Plus Environment

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In ε2(biphen)3, the inorganic building unit is not εZn but a dimerised form, resulting from the connection of two entities via Zn-O-Mo linkages (Figure 1b), as previously observed for ε2(trim)2, ε2(im)4 and ε2(pazo)4.7b Each of the six Zn ions of the dimeric (εZn)2 units, which remain available for coordination, is linked to a monodentate carboxylate group of the biphen linker (Figure SI5a). The connection of the POMs via the biphen linkers affords a 3D material (Figure 2a). The schematic view of the structure (Figure 2b) clearly shows the presence of two interpenetrated networks which, together with the presence of TBA counterions, prevents the generation of any porosity, as observed for all the compounds of this family of POMOFs.7b There are three independent biphen molecules, which are more or less twisted (Figure SI5b). a c

a)

b)

Figure 2. a) View along the b axis of the 3D framework built from the connection of the dimeric units by biphenyldicarboxylate linkers in ε2(biphen)3. b) Schematic representation of the two interpenetrated networks; the POM is schematised by the central P atom. TBA cations have been omitted for clarity.

In ε(bim)3(biphen)1/2, two εZn POMs are linked by a bidentate biphen molecule to form a dumbbell-shaped molecule (Figure 3). The three remaining ZnII ions on each POM are bound 11 ACS Paragon Plus Environment


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to a bim ligand. The bim ligands are protonated, a probable consequence of the low synthetic pH (pH = 5). Consequently the three bim molecules act as monodentate ligands and the compound is molecular. Contrary to what is observed for ε2(biphen)3, the biphen molecules are flat (Figure SI6a) and the structure is quite compact (Figure SI6b).

Figure 3. Representation of the dimer formed by two εZn POMs linked by a biphenyldicarboxylate ligand in ε(bim)3(biphen)1/2. In ε(bim)2(isop), two ZnII ions among the four of the tetrahedral εZn POM unit are linked to an oxygen atom of a bidentate isop linker, while the other two are bound to terminal protonated bim ligands to generate zig-zag chains (Figure 4a). These chains stack on top of each other along the b axis (Figure SI7a). Strong N-HO interactions between bim ligands of one plane and bridging oxygen atoms of POMs of the neighbouring ones ensure the cohesion of the structure (Figure 4b and SI7b). When bim ligands are mixed with trim linkers, the 2D POMOF ε(bim)2(trim)2/3 is obtained (Figure 5a). One trim linker is bound to three POMs, each POM being bound to two different trim linkers via Zn-O-C bonds, the other two ZnII ions being connected to terminal protonated bim molecules like in ε(bim)2(isop). The adjacent plane is shifted (Figure 5b) and interacts with the planes below and above via N-HO interactions (Figure SI8).


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b c

a)

b)

Figure 4. a) View along the a axis of a 1D zig-zag chain in ε(bim)2(isop). b) H-bond interactions (dotted lines) between the benzimidazole groups of one POM and a bridging oxygen atom of a POM in an adjacent chain; dNHO = 1.807(5) Å.

b a

a) b a

b)

Figure 5. a) View along the c axis of a 2D plane in ε(bim)2(trim)2/3. b) Schematic representation of a plane (in black) and of the adjacent planes (in yellow); the POM is schematised by its central P atom. 13 ACS Paragon Plus Environment


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While crystals of sufficient quality for single-crystal X-ray diffraction analysis have been isolated for the four compounds ε2(biphen)3, ε(bim)3(biphen)1/2, ε(bim)2(trim)2/3 and ε(bim)2(isop), this has not been the case for ε(bim)8/3(bdc)2/3, for which only a structural model has been obtained despite numerous attempts.18 Nevertheless, this has allowed to determine that a 1D chain similar to that observed in ε(bim)2(isop) coexists with a dimer close to the one found in ε(bim)3(biphen)1/2.

Synthesis and characterisations. The synthesis of the POMOF materials was performed by the reaction of a MoVI precursor, Mo as reducing agent, H3PO3, zinc acetate and a mixture of two ligands, either two O-donors or an O-donor and a N-donor ligand, and TBAOH in water at 200°C. The pure phase with biphenyldicarboxylic acid was not previously reported and was thus also characterised to serve as a reference. These synthetic conditions, which are very close for the five phases (see experimental section), are known to generate in situ the {εPMoV8MoVI4O40Zn4} core.7b As observed for all the POM-based coordination polymers of this family,7b the value of the initial pH (around 5.0) is critical, as is the presence of the TBA cations. The number of TBA counter-ions varies from 2 to 3. Their role is not only a chargebalancing one, which is provided also by protons on the POM, but also a structure directing and a space filling role.10 The five compounds are isolated as dark red insoluble crystals in moderate yield. The detailed formula of the POMOFs has been deduced from the results of the structural determination and from the elemental analysis. Table 2 gathers the nature of the compounds obtained when two linkers are used as a mixture of reactants. It must be noticed that in all the experiments where two carboxylate linkers have been mixed, a POMOF with only one kind of linker is isolated, while when bim is mixed with an O-donor ligand, a mixedlinker phase is obtained. The stability order of the POMOFs with carboxylate linkers can be deduced from Table 2: ε(trim)4/3 > ε(isop)2 > Z-POMOF1 > ε2(biphen)3. 14 ACS Paragon Plus Environment

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For all the phases with mixed linkers, the ratio of both reactants was equal. As a representative study, a few sets of experiments were performed on ε(bim)2(trim)2/3 in order to apprehend the influence of the bim/trim initial ratio on the formation of the final product. Even for low bim/trim ratios (up to 1:3), ε(bim)2(trim)2/3 is still obtained, albeit with a lower yield, although the bim/trim ratio in the product is 3:1, i.e. 9 times larger.

Table 2. Abbreviated formula and dimensionality of the phases obtained by mixing two ligands, all the experimental conditions (nature and initial amount of the reactants, heating and cooling program) being otherwise identical. Ligand

bim

biphenyl

bdc

isop

trim HO

N

HO

N H

O

HOOC

bim

biphenyl

bdc isop

O

O COOH

HO

OH

OH O

O

O

OH OH

O

ε(bim)4

ε(bim)3(biphen)1/2

ε(bim)8/3(bdc)2/3

ε(bim)2(isop)

ε(bim)2(trim)2/3

0D

0D

1D

1D

2D

ε2(biphen)3

Z-POMOF1

ε(isop)2

ε(trim)4/3

3D

3D

2D

3D

Z-POMOF1

ε(isop)2

ε(trim)4/3

ε(isop)2

ε(trim)4/3

trim

ε(trim)4/3

The crystalline homogeneity of all the phases has been checked by comparison of the experimental X-ray powder pattern with the powder pattern calculated from the structure solved from single-crystal X-ray diffraction data (Figure SI9). Furthermore, in the infrared spectrum of the five phases, several regions can be distinguished (Figures 6 and SI10): the νC-O vibrations of the carboxylate linker are identified around 1620 and 1570 (νasym) and 1380 and 1350 cm-1 (νsym) and the νC-N vibrations of the bim ligand around 1300, 1275 and 1250 cm-1. The signature of the TBA cations is found around 1480 cm-1 while the P-O and Mo=O



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vibrations of the inorganic skeleton of the POM are encountered around 1060 cm-1 and 930 cm-1 respectively. The Mo-O-Mo vibrations are found below 930 cm-1. As the vibration regions of the carboxylate and N-donor ligands are clearly distinct, the infrared spectroscopy is thus an easy tool to unveil the formation of mixed-linker phases. The comparison of the IR spectrum of ε(bim)8/3(bdc)2/3 with that of the phases with only one kind of ligand thus allows to confirm the presence of both ligands (Figure 6) even if no X-ray structure of sufficient quality has been obtained.

ε(bim)4 ε(bim)8/3(bdc)2/3)

Z-POMOF1

νC-O carboxylate 2000

1800

δC-H TBA+

1600

1400

νC-N bim 1200 ν (cm-1)

νMo-O εZn

νMo=O ε Zn 1000

800

600

Figure 6. A representative example of the comparison of the infrared spectra of the phases with only one kind of carboxylate linker, here Z-POMOF1,8 with only the bim ligand, ε(bim)4,9 and with both ligands, here ε(bim)8/3(bdc)2/3, showing the vibration domains of the εZn POMs, of the organic ligands and of the counter-ions; the domain of the νC-N vibrations of the bim groups and the νC-O vibrations of the biphen ligand are highlighted in yellow and grey, respectively.

Electrochemistry. The POM-based coordination polymers reported herein are insoluble in common solvents and their electrochemical properties have been studied in the solid state using modified carbon paste electrodes (CPE). Cyclic voltammetry experiments were first performed at pH 1 (Figures 7 and SI11). The five compounds exhibit the same electrochemical trend in the potential range between +0.20 V and −0.22 V vs. SCE. Two reversible waves are attributed to the reduction of the MoVI centres in fragment εPMo128 (Figure SI12) and denoted herein 16 ACS Paragon Plus Environment

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(MoVI/V)2 and (MoVI/V)3. The linear dependence of the peak currents on the scan rate confirms that the electroactive species is immobilised on the electrode (Figure SI13). Upon scanning to higher potentials, and up to +0.50 V, all compounds give rise to a wave corresponding to the oxidation of MoV, whose midpoint potential, E0’, falls between +0.24 and +0.30 V vs. SCE and which is assigned to the Mo centres (MoVI/V)1 (see Table SI1). These three waves are bielectronic.8

3.0

0.0

I / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Mo

-3.0 (Mo

VI / V

VI / V

)2 ε(bim)

-6.0 (Mo -0.3

)1

VI / V

2

(trim)

2/3

)3 0.0

0.3

0.6

E / V vs.SCE

Figure 7. Cyclic voltammogram of ε(bim)2(trim)2/3/CPE obtained in 1.0 M LiCl + HCl / pH 1.0. Scan rate: 100 mV s-1; reference electrode: SCE.

When experiments are carried out at lower scan rates, the most negative wave progressively loses its reversibility and becomes electro-catalytic for scan rates below 10 mV s-1. This process corresponds to the hydrogen evolution reaction (HER), as shown previously for ε(trim)4/3,10 and can be verified visually by the formation of gas bubbles on the electrode surface. The CVs of the five compounds recorded at 2 mV s-1 are represented in Figure 8. The onset potential values, Eonset, of the HER at pH 1 for the five compounds range from -0.05 up to 0.02 V vs SCE and are close to the onset potential determined for ε(trim)4/310 (Table SI2). Compared to a bare Pt electrode this represents an anodic shift ranging from 0.26 to 0.32 V.



This value seems difficult to apprehend considering that the thermodynamic potential for the H2 evolution reaction is equal to -0.30 V vs. SCE at pH 1, but such a high value has already been encountered.19 An unexpectedly high proton concentration within the POMOF, leading to an even more acidic local pH, might account for these results. Phenomena of marked proton uptake by POMs have been reported before.20 Another explanation could be that the onset potential is set by the H2POMOF/H2 redox potential which might be different from that of the H+/H2 couple, as suggested by Morozan and Jaouen.21 Indeed, a mechanism has been proposed for the HER.10 First, the POMOF is reduced and protonated: POMOF + 2e- + 2H+ ⇆ H2POMOF This is followed by the two-step catalytic process: H2POMOF + 2e- + 2{Li(H2O)n}+⇆ [Li2(H2O)2nPOMOF]+ H2 [Li2(H2O)2nPOMOF] + 2H+ ⇆ H2POMOF + 2{LiH2O)n}+

0.0

I / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ε(bim)

-1.5 ε(bim)

2

(isop)

8/3

( b d c) 2/3

ε ( b i p h e n)

2

ε(bim)

-3.0

-0.3

ε(bim)

0.0

3 2

3

( b i p h e n) (trim)

1/2

2/3

0.3

0.6

E / V vs. SCE

Figure 8. Cyclic voltammograms of the five POMOFs/CPE obtained in 1.0 M LiCl + HCl / pH 1.0. Scan rate: 2 mV s-1; reference electrode: SCE.


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As expected, considering the HER mechanisms, the electrochemical parameters of the HER depend on the pH, the process occurring at more negative potentials as the pH increases. The ε(bim)2(trim)2/3 compound has been selected to illustrate this phenomenon (Figure 9 and SI14). The slopes of the plots of the dependence of the midpoint redox potential, E0’, on the pH are close to −0.06V/pH, indicating that there is a proton coupled to each electron transferred (Figure SI15). Interestingly, the efficiency of the HER is kept at pH 5, the current being close to that measured at pH 1.

It should be noticed that the values of the electrocatalytic currents vary significantly from one compound to the other (Figure 8). These results are reproducible, as shown by the small error bars on the average currents obtained over several tests with the same POMOF (Figure SI16). ε(bim)2(trim)2/3 is thus the most efficient electrocatalyst of the series and its performance is close to that of ε(trim)4/3.10 Obviously, the redox behaviour of this family of compounds, and the related electro-catalytic properties, depend mainly on the presence of the molybdic entity εPMo12.

0.0

I / µA

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.2 ε ( b i m )2( t r i m )2/3

pH1 pH2 pH3 pH4 pH5

-0.4

-0.6 -0.3

0.0

0.3

0.6

E / V v s. S C E

Figure 10. Cyclic voltammograms of ε(bim)2(trim)2/3/CPE obtained in media at different pH values ranging from 1 to 5. Scan rate: 2 mV s-1; reference electrode: SCE.



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Finally, we have checked the stability of the compounds in the electrolyte solutions. The absence of significant changes on the IR spectra and powder X-ray diffraction patterns of ε(bim)2(trim)2/3 (taken as a representative example of the family) stirred for 48h in the electrolyte solutions, confirms the absence of decomposition of the POMOFs in these aqueous media (Figure SI17).

CONCLUSION In conclusion, the reactivity of the Zn(II)-capped ε-Keggin polyoxomolybdate {εPMoV8MoVI4O40Zn4} towards a mixture of two different ligands has been explored. While the mixture of two different carboxylate ligands leads systematically to a phase with only one kind of ligand, it has been possible to isolate four new polyoxometalate-based coordination polymers with a mixture of benzimidazole and a carboxylate ligand L. Their structure is either molecular (L = biphenyldicarboxylate), 1D (L = 1,3-benzenedicarboxylate or 1,4benzenedicarboxylate) or 2D (L = 1,3,5-benzenetricarboxylate). In these compounds, protonated benzimidazoles act as terminal ligands, preventing the formation of frameworks with higher dimensionality. In addition, the 3D compound ε2(biphen)3, containing only 4,4’biphenyldicarboxylate as ligand, has also been characterised. The electrocatalytic activity for the hydrogen evolution reaction of these materials entrapped in carbon paste electrodes has been investigated in aqueous solutions between pH 1 and pH 5. The POMOF with benzimidazole and biphenyldicarboxylate ε(bim)2(trim)2/3 exhibits the largest catalytic current of the reported series of compounds and has the second most favourable onset potential (-3 mV vs. SCE). Importantly, it has therefore been found that, for such systems, not only high-dimensionality materials can exhibit high catalytic activity but that even molecular complexes can act as efficient heterogeneous HER catalysts. These results confirm the great potentialities of these materials as green HER catalysts considering that: i) they are easily 20 ACS Paragon Plus Environment

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synthesised in water through one-pot procedures, ii) they contain abundant, cheap (platinum free) and non-toxic metals, iii) they are stable and operate in water with very low overpotentials.

ASSOCIATED CONTENT Supporting Information: X-ray crystallographic data in CIF format, selected bond distances, bond

valence

sums

and

additional

figures

for

the

structures

of

ε2(biphen)3,

ε(bim)3(biphen)1/2, ε(bim)2(isop) and ε(bim)2(trim)2/3 (Figures SI1-SI8), comparison of the experimental X-ray powder patterns and of the powder patterns calculated from the structure solved from single crystal X-ray diffraction (Figure SI9) and infrared spectra of the mixed linker

phases

(Figure

SI10),

cyclic

voltammograms

of

ε(bim)8/3(bcd)2/3/CPE;

ε(bim)3(biphen)1/2/CPE; ε2(biphen)3/CPE and ε(bim)2(isop)/CPE at a scan rate 100 mV s-1 (Figure SI11), cyclic voltammograms of ε(bim)2(trim)2/3/CPE at a scan rate 100 mV s-1 showing the domains of electroactivity of the various Mo centres (Figure SI12), cyclic voltammograms of ε(bim)2(trim)2/3/CPE as a function of the scan rate (Figure SI13), oxidation, Epa, and reduction, Epc, peak potentials for the five compounds (Table SI1), potential onset (Table SI2), cyclic voltammograms of ε(bim)2(trim)2/3/CPE obtained in media at different pH values ranging from 1 to 5 (Figure SI14), average cathodic currents (HER) at −0.2 V vs. SCE of four essays for the five POMOFs (Figure SI15), plots of E0’=f(pH) in ε(bim)2(trim)2/3 (Figure SI16), X-ray powder patterns and IR spectra of ε(bim)2(trim)2/3 stirred for 48h in various electrolytes (Figure SI17). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors 21 ACS Paragon Plus Environment


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*E-mail for A. D.: [email protected] *E-mail for P. de O.: [email protected]

ACKNOWLEDGMENTS This work was supported by the Ministère de l’Enseignement Supérieur et de la Recherche, the CNRS, the Université de Versailles Saint Quentin en Yvelines, the Université Paris-Sud and a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program n°ANR-11-IDEX-0003-02 and CHARMMMAT ANR11-LABX-0039. Bérengère Leynaud is gratefully acknowledged for her participation in the synthesis of the POMOF materials. A. D. thanks Caroline Mellot-Draznieks and Emmanuel Cadot for fruitful discussions.

REFERENCES

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(3) (a) du Peloux, C. ; Dolbecq, A. ; Mialane, P. ; Marrot, J. ; Rivière, E. ; Sécheresse, F. Angew. Chem. Int. Ed. 2001, 40, 2455. (b) Streb, C.; Ritchie, C.; Long, D.-L.; Kögerler, P.; Cronin, L. Angew. Chem. Int. 2007, 119, 7723. (c) Ritchie, C.; Streb, C.; Thiel, J.; Mitchell, S. G.; Miras, H. N.; Long, D.-L.; Boyd, T.; Peacock, R. D.; McGlone, T.; Cronin, L. Angew. Chem. Int. Ed. 2008, 47, 6881. (d) Compain, J.-D.; Nakabayashi, K.; Ohkoshi, S.-I. Inorg. Chem. 2012, 51, 4897. (e) Zhang, Z.; Sadakane, M.; Murayama, T.; Izumi, S.; Yasuda, N.; Sakaguchi, N.; Ueda, W. Inorg. Chem. 2014, 53, 903. (4) (a) Du, D.-Y.; Qin, J.-S.; Li, S.-L. ; Su, Z.-M. ; Lan, Y.-Q. Chem. Soc. Rev. 2014, 43, 4615. (b) He, W. W.; Li, S.-L; Zhang, H.-Y.; Yang, G.-S.; Zhang, S.-R.; Su, Z.-M. ; Lan, Y.Q. Coord. Chem. Rev. 2014, 279, 141. (5) (a) Yao, S.; Yan, J.-H.; Duan, H.; Zhang, Z.-M.; Li, Y.-G.; Han, X.-B.; Shen, J.-Q.; Fu, H.; Wang, E.-B. Eur. J. Inorg. Chem. 2013, 4770. (b) An, H.-Y.; Wang, E.-B.; Xiao, D.-R.; Li, Y.-G.; Su, Z.-M.; Xu, L. Angew. Chem. Int. Ed. 2006, 45, 904. (c) Liu, C.-M.; Zhang, D.Q.; Zhu, D.-B. Cryst. Growth Des. 2006, 6, 524. (6) (a) Lei, C.; Mao, J.-G.; Sun, Y.-Q.; Song, J.-L. Inorg. Chem. 2004, 43, 1964. (b) Mandic, S.; Heakey, M. R.; Gotthardt, J. M.; Alley, K. G.; Gable, R. W.; Ritchie, C.; Boskovic, C. Eur. J. Inorg. Chem. 2013, 1631. (c) Han, J. W.; Hill, C. L. J. Am. Chem. Soc. 2007, 129, 15095. (d) Yu, K.; Zhou, B.-B.; Yu, Y.; Su, Z.-H.; Yang, G.-Y. Inorg. Chem. 2011, 50, 1862. (7) (a) Zheng, S.-T.; Zhang, J.; Yang, G.-Y. Angew. Chem. Int. Ed. 2008, 47, 3909. (b) Dolbecq, A.; Mialane, P.; Sécheresse, F.; Keita, B.; Nadjo, L. Chem. Commun. 2012, 48, 8299. (c) Miras, H. N.; Vilà-Nadal, L.; Cronin, L. Chem. Soc. Rev. 2014, 43, 5679. (8) Rodriguez-Albelo, L. M.; Ruiz-Salvador, A. R.; Sampieri, A.; Lewis, D. W.; Gómez, A.; Nohra, B.; Mialane, P.; Marrot, J.; Sécheresse, F.; Mellot-Draznieks, C.; Ngo Biboum, R.; Keita, B.; Nadjo, L.; Dolbecq, A. J. Am. Chem. Soc. 2009, 131, 16078.



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(9) Rodriguez Albelo, L. M.; Rousseau, G.; Mialane, P.; Marrot, J.; Mellot-Draznieks, C.; Ruiz-Salvador, A. R.; Li, S.; Liu, R.; Zhang, G.; Keita, B.; Dolbecq, A. Dalton Trans. 2012, 41, 9989. (10) Nohra, B.; El Moll, H.; Rodriguez Albelo, L. M.; Mialane, P.; Marrot, J.; MellotDraznieks, C.; O’Keeffe, M.; Ngo Biboum, R.; Lemaire, J.; Keita, B.; Nadjo, L.; Dolbecq, A. J. Am. Chem. Soc. 2011, 133, 13363. (11) See for example (a) Dybtsev, D. N.; Chun, H.; Kim, K. Angew. Chem. Int. Ed. Engl. 2004, 43, 5033. (b) Klein, N.; Senkovska, I.; Gedrich, K.; Stoeck, U.; Henschel, A.; Mueller, U.; Kaskel, S. Angew. Chem., Int. Ed. Engl. 2009, 48, 9954. (c) Furukawa, H.; Ko, N.; Go, Y. B.; Aratani, N.; Choi, S. B.; Choi, E.; Yazaydin, A. O.; Snurr, R.; O’Keeffe, M.; Kim, J.; Yaghi, O. M. Science 2010, 329, 424. (d) Chevreau, H.; Devic, T.; Salles, F.; Maurin, G.; Stock, N.; Serre, C. Angew. Chem. Int. Ed. 2013, 52, 5056. (12) (a) Sha, J.-Q.; Sun, J.-W.; Li, M.-T.; Wang, C.; Li, G.-M.; Yan, P.-F.; Sun, L.-J. Dalton Trans. 2013, 42, 1667. (b) Huang, L.; Zhang, J.; Chen, L.; Yang, G.-Y. Chem. Commun. 2012, 48, 9658. (13) Sheldrick, G. M. SADABS, program for scaling and correction of area detector data, University of Göttingen, Germany, 1997. (14) Blessing, R. Acta Crystallogr. 1995, A51, 33. (15) Sheldrick, G. M. SHELX-TL version 5.03, Software Package for the Crystal Structure Determination, Siemens Analytical X-ray Instrument Division : Madison, WI USA, 1994. (16) P. van der Sluis, and A. L. Spek, Acta Crystallogr., Sect. A. 1990, 46, 194. (17) Müller, A.; Meyer, J.; Krickemeyer, E. Diemann, E. Angew. Chem. Int. Ed. Engl. 1996, 35, 1206.


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(18) The unit-cell parameters are: a = 45.804(9), b = 26.261(5), c = 31.526(6) Å, β = 119.12(1)°, V = 33254.2(1) Å3, space group Cc. (19) Chen, J. ; Huang, J. ; Swiegers, G. F.; Too, C. O.; Wallace, G. G. Chem. Commun. 2004, 308. (20) Keita, B.; de Oliveira, P.; Nadjo, L. Kortz, U. Chem. Eur. J. 2007, 13, 5480. (21) Morozan, A.; Jaouen, F. Energy Environ. Sci. 2012, 5, 9269.



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For Table of Contents Use Only

Tuning the Dimensionality of Polyoxometalate-Based Materials by Using a Mixture of Ligands

Guillaume Rousseau,† Luisa Marleny Rodriguez-Albelo,‡ William Salomon,† Pierre Mialane,† Jérôme Marrot,† Floriant Doungmene,§ Israël-Martyr Mbomekallé,§ Pedro de Oliveira,*§ and Anne Dolbecq*,†

H2

pH1 pH 2 pH3 pH4 pH5 -0.3

0.0

0.3

E / V v s. S C E

2H+

Hybrid organic-inorganic polyoxometalates built from the connection of Keggin type anions by O and N-donor linkers have been synthesized under hydrothermal conditions. These materials exhibit remarkable electrocatalytic activity for the HER reaction, even the molecular ones; they can be seen as green heterogeneous catalysts as they contain abundant and nontoxic metals and operate in water.


Tuning the Dimensionality of Polyoxometalate-Based Materials by

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