Synthesis and crystal structure of hexacerium(IV) cluster-containing Keggin polyoxometalate trimer

Accepted Manuscript Synthesis and crystal structure of hexacerium(IV) clustercontaining Keggin polyoxometalate trimer

Satoshi Matsunaga, Yusuke Inoue, Kohei Mihara, Kenji Nomiya PII: DOI: Reference:

S1387-7003(17)30247-2 doi: 10.1016/j.inoche.2017.04.008 INOCHE 6613

To appear in:

Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

29 March 2017 12 April 2017 14 April 2017

Please cite this article as: Satoshi Matsunaga, Yusuke Inoue, Kohei Mihara, Kenji Nomiya , Synthesis and crystal structure of hexacerium(IV) cluster-containing Keggin polyoxometalate trimer. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Inoche(2017), doi: 10.1016/ j.inoche.2017.04.008

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ACCEPTED MANUSCRIPT Synthesis and crystal structure of hexacerium(IV) cluster-containing Keggin polyoxometalate trimer Satoshi Matsunagaa, Yusuke Inouea, Kohei Miharaa, Kenji Nomiyaa* a

Department of Chemistry, Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa

*

Corresponding author. Tel: +81463594111; fax: +81463589684

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E-mail address: [email protected] (K. Nomiya)

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259-1293, Japan

ABSTRACT novel

hexacerium(IV)

cluster-containing

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A

polyoxometalate

K18[{Ce6(μ3-O)4(μ3-OH)4(OAc)2}(A-α-SiW9O34)2(α-SiW11O39)]·37H2O

(K18Ce6-POM),

(POM), was

unexpectedly obtained by a reaction of the potassium salt of open-Dawson POM,

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K13[{K(H2O)3}2{K(H2O)2}α,α-Si2W18O66]·19H2O (K-open POM), with (NH4)2[Ce(NO3)6], in a 0.5 M KOAc/HOAc buffer solution (pH 4.8) at 80 °C, and unequivocally characterized by X-ray crystallography, 1H,

13

C, and

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Si NMR spectroscopy, FTIR spectroscopy, complete elemental

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analysis, and TG/DTA. X-ray crystallography revealed that the monolacunary Keggin unit,

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[α-SiW11O39]8-, and the two trilacunary Keggin units, [A-α-SiW9O34]10-, are connected by a central Ce6-oxo/hydroxide cluster moiety, [Ce6(μ3-O)4(μ3-OH)4(OAc)2]10+, to form a trimeric Keggin polyanion, [{Ce6(μ3-O)4(μ3-OH)4(OAc)2}(A-α-SiW9O34)2(α-SiW11O39)]18- (Ce6-POM). 1H, 13C, and 29

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Si NMR spectra in D2O suggested that the Ce6-POM was obtained as a single species and its

Keywords:

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molecular structure was maintained in solution.

Polyoxometalate/ Cerium(IV)/ Cluster / Oligomer / 29Si NMR / X-ray structure analysis

Polyoxometalates (POMs) are discrete metal oxide clusters that are of current interest as soluble metal oxides with applications in catalysis, medicine, and materials science[1]. The preparation of POM-based materials is therefore an active field of research. Lacunary POMs are vacant derivatives of the saturated POM structures. They can be viewed as multidentate inorganic ligands, which can coordinate to transition metal ions. The lacunary sites of these POMs can encapsulate a variety of transition metal ions, resulting in the formation of multi-transition metal aggregates, and unstable 1

ACCEPTED MANUSCRIPT multinuclear-metal oxo/hydroxide clusters in solution can be isolated as stable forms using the lacunary POMs[2]. Therefore, it is possible to develop new multifunctional materials that find a number of applications ranging from catalysis to molecular magnetism. On the other hand, tetravalent cerium compounds such as cerium ammonium nitrate (NH4)2[CeIV(NO3)6] (CAN) are widely used as versatile oxidants in various applications, including organic synthesis[3], as a water oxidation catalyst[4], and in DNA hydrolysis[5]. Until recently, it was accepted that CAN in solution adopted a monomeric [Ce(NO3)6]2- ion structure, which acted as a

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simple one-electron oxidant. However, in 2015, Ellis et al. revealed the actual solution structure of CAN using a combination of synchrotron X-ray absorption and Raman spectroscopies[6]. Their results suggested that CAN in acidic nitrate solution exists as an oxo-bridged dinuclear complex, i.e.,

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[(H2O)6–7CeIV-O-CeIV(OH2)6–7]6+. This dimeric structure allows CAN to involve two of its electrons

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during oxidation reactions, and not just one electron, as assumed previously. Synthetic and structural studies of multinuclear-Ce4+-oxo/hydroxide complexes would further develop our understanding of the tetravalentcerium chemistry.

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In this context, multinuclear-Ce4+-oxo/hydroxide clusters, stabilized by lacunary POMs, are intriguing target compounds from both synthetic and structural points of view. Although many POMs POM

architectures

are

still

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containing trivalent-cerium clusters have been reported, examples of tetravalent cerium-containing limited:

H7K10[Ce2(PW10O38)(PW11O39)2]·28H2O[7a],

(ala)4(OH)2(H2O)10]·∼40H2O[7b],

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Cs10[Ce(PMo11O39)2]·20H2O[7c],

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K9Na7[{Ce2O(H2O)5}{WO(H2O)}{AsW9O33}2]2·∼48H2O[7b],

K2Na10[Ce4As4W44O151

Mn0.5K5Na18[Ce4As4W41O149]·∼50H2O[7b], Cs2Na8[Ce4(μ3-O)2(SiW9O34)2(OAc)2]·40H2O[7d],

Na13H7[Ce(SeW17O59)2]·31H2O[7e],

Na13[Ce3(μ2-OH)3(μ3-O)(SiW9O34)2]·25H2O[7f],

K14[Ce3(μ2-O)3(SiW9O34)2]·23H2O[7f],

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Na32{[Ce6(μ3-O)4(μ3-OH)4]2(W8O32)(SiW9O34)4}·57H2O[7f]. In this study, we unexpectedly obtained a novel hexacerium(IV) cluster-containing K18[{Ce6(μ3-O)4(μ3-OH)4(OAc)2}(A-α-SiW9O34)2(α-SiW11O39)]·37H2O

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polyoxometalate,

(K18Ce6-POM), and unequivocally characterized it by X-ray crystallography, solution (1H, 13C, and 29

Si) NMR spectroscopy, FTIR spectroscopy, complete elemental analysis, and TG/DTA. The

potassium

salt

of

the

Keggin

POM

trimer

K18[{Ce6(μ3-O)4(μ3-OH)4(OAc)2}(A-α-SiW9O34)2(α-SiW11O39)]·37H2O

containing

Ce6

(K18Ce6-POM),

cluster, as

a

crystalline sample, was obtained in 10.3% yield based on W. K18Ce6-POM was synthesized by a 1:2 molar ratio reaction of the separately prepared K13[{K(H2O)3}2{K(H2O)2}α,α-Si2W18O66]·19H2O (K-open POM) [8] with (NH4)2[Ce(NO3)6] in a 0.5 M KOAc/HOAc buffer solution (pH 4.8), followed by stirring at 80 °C for 20 min. Because we initially aimed to synthesize the Ce4+-containing open-Dawson POM, K-open POM was used as a precursor. However, the target open-Dawson POM was not obtained. Instead, the Ce4+6 cluster-containing Keggin POM tetramer was unexpectedly 2

ACCEPTED MANUSCRIPT obtained despite using K-open POM (see the molecular structure). Recently, Duval et al. reported the Ce-containing POM [Ce4(μ3-O)2(SiW9O34)2(OAc)2]10- (Ce4-sandwich), in which the Ce4 cluster was sandwiched by two trilacunary Keggin POMs and coordinated by two acetate anions[7d]. Ce4-sandwich was synthesized by the reaction of trilacunary Keggin [α-SiW9O34]10- with (NH4)2[Ce(NO3)6] in 1 M NaOAc/HOAc buffer solution (pH 4.7) at 50 °C for 30 min, followed by the addition of Ce(NO3)3. Therefore, the two Ce-containing POMs, i.e., Ce6-POM and Ce4-sandwich, are synthesized under similar conditions. However, there are major differences between

the

synthesis

of

Ce6-POM

and

Ce4-sandwich:

1)

the

precursor

POM,

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[{K(H2O)3}2{K(H2O)2}α,α-Si2W18O66]13- (K-open POM) vs. [A--SiW9O34]10-; and 2) the countercations of the precursor and the buffer solution, i.e., K+ vs. Na+. In particular, the K+ ions,

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which stabilize the structure of Ce6-POM by the interactions between the K+ ions and the oxygen

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atoms of the POM moiety, are observed in the crystal structure of Ce6-POM (see the molecular structure); therefore, K+ ions play a critical role in the synthesis of Ce6-POM.

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The K18Ce6-POM was characterized via complete elemental analysis, i.e., H, C, Ce, K, O, Si, and W analyses. Prior to analysis, the sample was dried overnight at room temperature under a vacuum of 10-3 to 10-4 Torr. All elements (H, C, Ce, K, O, Si, and W) were observed for a total

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analysis of 99.96%, indicating the high purity of the compound. The recorded data were in good accordance with the calculated values for the formula constituting two water molecules, K18[{Ce6(μ3-O)4(μ3-OH)4(OAc)2}(A-α-SiW9O34)2(α-SiW11O39)]·2H2O

(see

Supplementary

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Information). Before analysis, the weight loss observed during drying was 6.66%, corresponding to

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approximately 35 water molecules (calcd. 6.58 %); therefore, the sample contained a total of 37 crystalline water molecules. On the other hand, during the TG/DTA measurements carried out under an air atmosphere (Figure S2), a weight loss of 6.83% was observed at temperatures below 200 °C,

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corresponding to around 36 water molecules (calcd. 6.78 %). Thus, the complete elemental analysis and TG/DTA indicated the presence of 36–37 water molecules for the sample under atmospheric The

formula

for

K18Ce6-POM

presented

herein

was

determined

to

be

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conditions.

K18[{Ce6(μ3-O)4(μ3-OH)4(OAc)2}(A-α-SiW9O34)2(α-SiW11O39)]·37H2O, based on the results of the complete elemental analysis. X-ray crystallographic data of Ce6-POM reveals that the monolacunary Keggin unit, [α-SiW11O39]8-, and the two trilacunary Keggin units, [A-α-SiW9O34]10-, are connected by a central Ce6-oxo/hydroxide cluster cation, [Ce6(μ3-O)4(μ3-OH)4(OAc)2]10+, to form a trimeric Keggin polyanion, [{Ce6(μ3-O)4(μ3-OH)4(OAc)2}(A-α-SiW9O34)2(α-SiW11O39)]18- (Figure 1). The potassium salt of open-Dawson structural POM (K-open POM) was used as a precursor. However, the open-Dawson structure was not maintained, and the trilacunary and monolacunary Keggin polyanions were formed by the partial decomposition of K-open POM during the reaction. Four crystallographically independent cerium atoms were present in the Ce6 cluster unit. The six 3

ACCEPTED MANUSCRIPT cerium atoms were linked by μ3-oxygen atoms, forming the Ce6 oxo/hydroxide cluster. The three cerium atoms (Ce1, Ce2, and Ce3) were bonded to the oxygen atoms of the lacunary sites (O25, O26, O27, O28, O29, and O30; average bond length 2.3198 Å) and the inner SiO44- anions (O52, O53; average bond length 2.6417 Å) of the trilacunary Keggin units. The residual cerium atom (Ce4) was coordinated by the monolacunary Keggin unit via the four oxygen atoms of the lacunary site (independent O31, O32; average 2.3379 Å). Ce1 and Ce4 were located on a special position (x, -y+1/2, z). Two acetate ligands bridged the two cerium atoms (Ce2-Ce2 and Ce3-Ce3). Thus, all

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cerium atoms can be considered as 8-coordinate, and the whole symmetry of the molecule is shown with approximate Cs symmetry.

Many interactions between the oxygen atoms of the polyanion moiety and the K + cations were

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observed in the crystal structure. It should be noted that the K3 and K4 atoms interacted with the

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oxygen atoms of the Ce6-POM (O17, O28, O29, O33, O34, O61, O63) [K3-O17, 3.399(13); K3-O29, 2.633(11); K3-O33, 2.733(21); K3-O63, 3.055(14); K4-O28, 2.777(11); K4-O34, 2.844(16); K4-O61, 2.912(12) Å] (Figure 2). The K+ cations could stabilize the trimeric Keggin

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structure to a better extent than other cations such as Na+ and Cs+. Ce4-sandwich was isolated as a Na+/Cs+ mixed salt, and the interactions between the polyoxoanion moiety and countercations were

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different from those of our K18Ce6-POM. These characteristics suggest that the different countercations have a significant influence on the products and K+ ions are critical for the formation of Ce6-POM.

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The values of the bond valence sum (BVS) calculations[9] (Table S1), which were calculated

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based on the observed bond lengths, were in the range 3.480−4.068 (average 3.853) for four cerium atoms. These values are in reasonable agreement with the tetravalent oxidation state of cerium atoms. BVS calculations of the eight oxygen atoms that are linked to the cerium atoms in the Ce6 cluster unit

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suggest that four oxygen atoms (independent O59, O60, O62) are protonated, i.e., they are ascribed to the OH- anions. The residual four oxygen atoms (independent O58, O64, O65) are ascribed to the O2-

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anions (Table S1). These results are consistent with the complete elemental analysis of K18Ce6-POM. Very recently, Duval et al. reported a POM containing a similar hexanuclear cerium cluster, {[Ce6((μ3-O)4(μ3-OH)4)]2(W8O32)(SiW9O34)4}32- (Ce12-W8)[7f]. Unfortunately, it is difficult to discuss their structural details due to the poor data quality and poor crystal quality. This compound was composed of two Ce6 clusters, each of them stabilized by two trilacunary POMs, and these two subunits were linked by a W8O32 unit. Despite the structural similarity, the stability in solution of the two Ce6-containing POMs, i.e., our Ce6-POM and Duvals’ Ce12-W8, were different (see solution NMR). The solution

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Si NMR spectrum of Ce6-POM in D2O showed two peaks at -85.71 ppm and

-85.78 ppm in approximately 2:1 ratio (Figure 3). The two peaks are due to the Si atoms of inner SiO44- ions, of trilacunary and monolacunary Keggin units, respectively. The peaks due to the 4

ACCEPTED MANUSCRIPT coordinated acetate were observed at 1.94 ppm for 1H NMR and 183.66, 25.76 ppm for 13C NMR in D2O (Figure S3 and S4, respectively). These spectra are consistent with the molecular structure of Ce6-POM from X-ray crystallographic analysis, suggesting that the Ce6-POM was obtained as single species and its molecular structure was maintained in solution. The 183W NMR spectrum could not be obtained due to the low solubility of K18Ce6-POM. Ce4-sandwich, reported by Duval et al., showed a singlet signal at -84 ppm for 29Si NMR[7d]. The POM containing the similar Ce6 cluster, Ce12-W8, reported by Duval et al., showed two broad peaks at -83.44 and -84.44 for

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Si NMR, suggesting that Ce12-W8 was unstable in solution[7f].

Additionally, 1H,

13

C, and

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Therefore, Ce6-POM was clearly distinguishable from Ce4-sandwich and Ce12-W8 by 29Si NMR. Si NMR spectra of our Ce6-POM showed sharp peaks that were

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consistent with the solid-state structure, and therefore, Ce6-POM was stable even in solution.

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Interestingly, the solution stability of these two similar Ce6 cluster-containing POMs, i.e., Ce6-POM and Ce12-W8, were quite different from each other.

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In summary, we prepared a novel hexacerium(IV) cluster-containing POM, K18Ce6-POM, from K-open POM as precursor, and unequivocally characterized it. Single-crystal X-ray analyses revealed that the monolacunary Keggin unit, [α-SiW11O39]8-, and the two trilacunary Keggin units, are

connected

by

a

central

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[A-α-SiW9O34]10-,

Ce6-oxo/hydroxide

cluster

moiety,

[Ce6(μ3-O)4(μ3-OH)4(OAc)2]10+, to form a trimeric Keggin polyanion, Ce6-POM. The solution NMR spectra in D2O indicated that Ce6-POM was obtained as a single species and its structure was

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maintained even in solution. Two related compounds, Ce4-sandwich and Ce12-W8, were recently

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reported by Duval [7d, e]. However, our Ce6-POM could be selectively synthesized using K-open POM as precursor and K+ ions as countercations, and was found by

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Si NMR to be clearly

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distinguishable from the two related Ce4+-containing POMs.

Acknowledgments

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This work was supported by the Strategic Research Base Development Program for Private Universities of the Ministry of Education, Culture, Sports, Science and Technology of Japan.

Appendix A. Supplementary data CCDC-1522597 contains the supplementary crystallographic data for this paper. This data can be obtained free of charge from the Cambridge Crystallographic Data Center via the internet at http://www.ccdc.cam.ac.uk/data_request/cif,

or

by

e-mailing

[email protected].

Supplementary data to this article can be found online at http://dx.doi.org/xxxxxxxxxxxx.

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Figure Captions Figure 1. (a) Molecular structure of Ce6-POM, (b) its polyhedral representation, and (c) the partial structure around the Ce6 cluster moiety. Color code: C, gray; Ce, yellow polyhedron; O, red; Si, blue polyhedron; W, gray polyhedron. Symmetry code, A: x, -y+1/2, z.

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Figure 2. Interactions between K+ cations and the oxygen atoms of Ce6-POM; front view (left), side

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Figure 3. Solution 29Si NMR spectrum of Ce6-POM in D2O.

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view (right). Symmetry code, A: x, -y+1/2, z.

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Figures and Schemes

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ACCEPTED MANUSCRIPT Table of Contents A novel hexacerium(IV) cluster-containing polyoxometalate, K18Ce6-POM, was unexpectedly obtained, and unequivocally characterized by X-ray crystallography, 1H, 13C, 29Si, and

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FTIR, complete elemental analysis, and TG/DTA.

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Graphical abstract

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W NMR,

ACCEPTED MANUSCRIPT Highlights Ver. 0.01 (20170223)

· A novel hexacerium(IV) cluster-containing POM (Ce6-POM) was synthesized. · One monolacunary and two trilacunary Keggin units are connected by the Ce6 cluster.

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· Ce6-POM maintained its molecular structure in solution.

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