Rational synthesis and X-ray structure of[MnII

4 (H2O)2(AsVW9O34)2]10� from [AsIII4 W40O140]28�, MnO4

� andMn2�

Cristina Rosu a,*, Debbie C. Crans b,*, Timothy J.R. Weakley c

a Department of Inorganic Chemistry, Babes-Bolyai University, Cluj-Napoca, Romaniab Department of Chemistry, Colorado State University, Fort Collins, CO 80523-1872, USA

c Department of Chemistry, University of Oregon, Eugene, OR 97403, USA

Received 15 October 2001; accepted 25 January 2002

Abstract

The rational synthesis of an arsenic(V) oxopolymetalate cluster from an arsenic(III) oxopolymetalate cluster has been

accomplished by a redox reaction. The sandwich Mn-complex of AsVW9O349�(Na9K[MnII

4 (H2O)2(AsVW9O34)2] �/35H2O (1) was

prepared from a tetrameric complex [As4W40O140]28� containing AsIIIW9O339� units using both MnVII and MnII salts as reagents.

Structures of 1 and Na27K[As4W40O140] �/56H2O (2), are reported. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Heteropolyoxometalate; Arsenic; Tungstate; Lacunary; Synthesis; Redox chemistry

1. Introduction

The synthesis of large and giant oxomolybdates and

oxotungstates requires access to good starting materials

and synthetic routes.[1,2] Specifically, a large heteroox-

otungstate of composition AsIII12 CeIII

16 (H2O)36W148-

O524]76� has been reported [2] and can be described as

originating from building blocks of lacunary anions

such as AsIIIW9O339� [2�/9]. Building blocks such as

AsVW9O349� [10�/12] has been used when forming

oxometalates containing As in oxidation state V. In

this paper we show that AsIII�/AsV oxometalates can be

linked through redox chemistry and synthetic strategy.

The first precursor, AsW9O339� was initially prepared

by Tourne et al. from As2O3, Na2WO4 and HCl [6]. At

this time dimeric [3,4,10], trimeric [5,6] and tetrameric

structures [2,7�/9] have all been reported. Tetrameric

structures, containing four units of AsW9O339�, are

linked through WO22� groups and are also bonded to

Na [2], Co [8], or Mn [9]. The sandwich anions consist of

two AsIIIW9O339� units linked by three metal atoms and

have been reported with Cu, Mn, Co, and V [3,4]. The

AsV sandwich compounds are linked by four metal ions,

and those containing Zn, Co, Cu and Mn have been

structurally characterized [10�/12].This paper describes the synthesis and structure of the

mixed sodium-potassium salts of the Mn-sandwich

AsVW9O349�complex (Na9K[MnII

4 (H2O)2(AsVW9O34)2] �/35H2O) and the tetrameric condensation product of

AsIIIW9O339� (Fig. 1, Na27K[As4W40O140] �/56H2O)).

This work supplements structural findings [7,12] that

were published while this work was in progress. More

importantly, the synthetic approaches to these materials

were previously limited by the starting materials het-

eroatom oxidation state. The concept of using a redox

agent as part of the synthetic strategy is illustrated and

thus allows application of a greater number of pre-

cursors for oxometalate synthesis. In this work we show

by employing redox chemistry in the synthetic strategy,

that an oxotungstate in the AsIII class of compounds can

be converted to an oxometalate in the AsV class of

compounds.

* Corresponding author. Tel.: �/1-970-491-7635; fax: �/1-970-491-

1801.

E-mail addresses: crisrosu@yahoo.com (C. Rosu),

crans@lamar.colostate.edu (D.C. Crans).

Polyhedron 21 (2002) 959�/962

www.elsevier.com/locate/poly

0277-5387/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 0 9 1 5 - 4

2. Experimental

2.1. Reagents, solvents and procedures

The reagents were obtained from Merck and used

without further purification. Elemental analyses were

carried out by the microanalysis laboratories of ‘Babes-Bolyai’ University, Cluj-Napoca, Romania. Tungsten,

manganese, sodium, potassium and arsenic were deter-

minated by ICP. Water was determined by thermogravi-

metric analyses (TGA). Thermogravimetric studies (20�/

300 8C) were carried out with a MOM-OD 102 Paulik

Erday thermoanalyser, at a heating rate of 10 8C min�1

and 250 mg sample.

2.2. Preparation of Na27K[AsIII4 W40O140] �/56H2O

The synthesis described in the literature was modified

slightly [7]. An aqueous solution of Na2WO4 �/2H2O (33

g, 100 mmol) in 100 ml water was adjusted to pH 4.5

with glacial acetic acid and maintained at this pH for 5

min. The solution was heated to the boiling point andAs2O3 (0.99 g, 5 mmol) in 11 M HNO3 (1.3 ml) was

added slowly under vigorous stirring. For stabilization

and crystallization of the compound 0.211 g (2.68 mmol)

KCl was added. The resultant clear, pale-yellow solution

was heated under reflux for 1 h. In the event insoluble

impurities formed, these were removed by filtration

while the solution was hot. After 4�/5 days of slow

evaporation at ambient temperature prismatic colorlesscrystals were formed. The yield was 24.6 g (2.13 mmol,

85%). Recrystallization from hot water generated more

uniform crystals. IR(cm�1): 1640(m), 950(sh), 862(w),

750(sh), 690(m), 580(m), 510(w). 183W NMR (D2O�/

H2O; Na2WO4 (0 ppm) �/100, �/110, �/119, �/138,

�/191 and �/200 ppm. Elemental Analyses (%): Found:

W, 63.53; As, 2.31; Na, 5.35; K, 0.35; H2O, 8.75. Calc.:W, 63.60; As, 2.59; Na, 5.37; K, 0.34; H2O, 8.72%.

2.3. Preparation of Na9K[MnII4 (H2O)2(AsVW9O34)2] �/

35H2O

Na7KAs4W40O140 �/56H2O (2.89 g, 0.251 mmol) was

dissolved in 25 ml of water. Solutions of 0.143 g (0.633

mmol) of KMnO4 in 5 ml of water and 0.431 g (1.76

mmol) of Mn(OAc)2 �/4H2O in 5 ml of water were addeddrop-wise in an alternating fashion. The reaction

mixture was then stirred for 30 min at 70 8C. Insoluble

impurities were filtered off while the solution was hot,

and the solution was left at ambient temperature.

Yellow�/orange crystalline plates of Na9K[MnII4 (H2O)2-

(AsVW9O34)2] �/35H2O were obtained after 10 days. The

yield was 2.1 g (0.37 mmol, 74%). IR(cm�1): 1642(m),

945(sh), 904(m), 858(w), 748(sh), 575(m), 510(w). Ele-mental Analyses(%): Found: W, 58.30; As, 2.55; Mn,

3.88; Na, 3.70; K, 0.65; H2O, 11.75. Calc.: W, 58.27; As,

2.64; Mn, 3.87; Na, 3.64, K, 0.68; H2O, 11.72%.

2.4. Crystal structure analyses

Data were collected by use of a Nonius CAD4 serial

diffractometer. Table 1 contains a summary of crystaldata and the final residuals for each compound. A more

extensive table including particulars of data collection

and structure refinement is provided in the supplemen-

tary material. The orientation parameters and cell

dimensions were obtained from the diffractometer

setting angles for 25 centered reflections. Data were

corrected for absorption on the basis of azimuthal (c )

scans. A SIR-92 E-map [13] showed all non-oxygenatoms of each polyoxoanion and many oxygen and

sodium atoms. The TEXSAN program suite [14] was used

in all calculations. Hydrogen atoms were not located or

included in the refined model.

2.4.1. Na27K[AsIII4 W40O140] �/56H2O

A crystal of dimensions 0.11�/0.14�/0.25 mm was

mounted on a fiber and protected with a coating ofepoxy. The 4/m Laue symmetry, lack of systematic

absences other than those due to the I-centering, and

acentric distribution of intensities indicated the space-

group I/4: An atom clearly present at the 4 site at the

center of the tetramer was identified as K on the basis of

distances from O atoms, though its rather high refined

thermal parameter may imply that the site is also partly

occupied by a water molecule. Four of the sevenindependent Na atoms appeared to require fractional

occupancy factors [Na(4�/7), identified from bond

lengths to oxygen]; altogether 22 Na were located per

Fig. 1. The structure of the anion in Na27K[As4W40O140] �/56H2O.

C. Rosu et al. / Polyhedron 21 (2002) 959�/962960

tetramer, it is likely that others are present but

disordered. The As and W atoms and Na(1,2,3) were

refined anisotropically. Anisotropic refinement of anion

oxygen atoms led to many non-positive-definite ellip-

soids, and the results given here are for isotropically-refined oxygens and illustrated in Fig. 1.

2.4.2. Na9K[MnII4 (H2O)2(AsVW9O34)2] �/35H2O

A yellow plate of dimensions 0.07�/0.22�/0.41 mm

was secured by a trace of glue in a capillary containing

mother-liquor. The triclinic symmetry together with the

centric distribution of intensities indicated the space-

group P/1: A total of five independent Na atoms were

located, but one required a site occupancy factor of 0.5

for a satisfactory refinement. Five of the 20 independent

water oxygens also appeared to be on half-occupiedsites. The K atom whose presence was implied by the

elemental analysis could not be located, and may be

disordered over the sites of non-ligand water molecules.

All atoms except oxygen atoms of water molecules were

refined anisotropically.

3. Results and discussion

3.1. Synthesis of AsIII�/oxotungstate tetramer

The synthetic strategy for formation of the tetrameric

condensation product illustrated in 1 was modifiedslightly from that reported by Leyrie and Herve [9].

The precursor sodium arsenite was replaced by As2O3,

which was dissolved in 11 M HNO3. Also the HCl was

replaced by glacial acetic acid and HNO3. This proce-

dure was found to generate X-ray quality crystals

directly from the reaction solution, although recrystalli-

zation from water resulted in a preparation with uni-

form crystal size.

40Na2WO4�2As2O3�52HOAc�KCl

0 Na27K[As4W40O140]�NaCl�52NaOAc

�26H2O (1)

3.2. Synthesis of AsV�/oxotungstate sandwich compound

Attempts to modify the AsIII in the tetrameric

condensation product to form the AsV series compoundswere explored using an oxidation reagent that is mild

but yet sufficient to accomplish the oxidation. MnO4� is

commonly used to oxidize AsIII to AsV and was tried

initially, but proved to be an unsuitable reagent.

Furthermore, when treating solutions of tetramer with

MnO4�, no change in color could be observed over

several days. In the synthesis of complex oxometalates

in molybdenum chemistry it is common to use reagentswhich involved two different oxidation states of the

metal ion in order to achieve the desired redox potential

in the reaction solutions. This approach was adapted in

this system. Thus, the addition of both MnO4� and

Mn(OAc)2 did prove to be successful and the resulting

reaction is shown in Eq. (2). The colorless solution turns

violet upon addition of MnO4� which followed by

addition of Mn(OAc)2 changes the violet solutionbrown; the color continues to change for 5 h after

which time the solution is deep orange. Although some

flexibility exists with regard to the exact ratio of

components, best yields were obtained when 1 equiv.

of tetramer was treated with a threefold excess of

MnO4� and a sevenfold excess of Mn2� salt. When

Mn(OAc)2 was added to solutions of tetramer the color

changed to yellow indicating a replacement of cations,however, only very small light yellow crystals were

obtained. We conclude, that both redox states of the

reagent were necessary to achieve the conversion of the

Table 1

Crystallographic data for compounds Na27K[As4W40O140] �/56H2O and

Na9K[MnII4 (H2O)2(AsVW9O34)2] �/35H2O

Composition Na27K[As4W40O140] �/56H2O

Formula weight 11562.3

Space-group I/4/

a , b (A) 19.491(2)

c (A) 25.783(3)

V (A3) 9795(2)

Z 2

Dcalc (g cm�3) 3.920

T (8C) 23

l (A) 0.71073

m (cm�1) 242

Relative transmission coefficient 0.571�/1.000 (c )

Observed reflections 3831 [I ]/s (I )]

Total independent reflections 4687

R (F ), wR (F ) (observed) 0.050, 0.052

R (F 2), wR (F 2) (total) 0.091, 0.108

Composition Na9K[Mn4As2W18O70H4] �/35H2O

Formula weight 5679.4

Space-group P/1/

a (A) 11.6368(12)

b (A) 14.2113(16)

c (A) 17.6343(20)

a (A) 98.876(9)

b (8) 105.682(9)

g (8) 113.290(9)

V (A3) 2463.9(7)

Z 1

Dcalc 3.824

T (8C) 22

l 0.71073

m (cm�1) 223.0

Relative transmission coefficient 0.322�/1.000 (c )

Observed reflections 5812 [I ]/s (I )]

Total independent reflections 7160

R (F ), wR (F ) (observed) 0.038, 0.042

R (F 2), wR (F 2) (total) 0.059, 0.081

R (F )�/SjjFoj�/jFcjj/SjFoj, wR (F 2)�/[Sw (jFoj2�/jFcj2)2/Sw jFoj4]1/2.

C. Rosu et al. / Polyhedron 21 (2002) 959�/962 961

AsIII oxometalate to the AsV oxometalate. In addition to

providing an example of linking these two classes of

compounds, this method significantly improves the yield

of the synthesis of this type of material [7].

2Na27K[As4W40O140]�4KMnO4�14Mn(OAc)2

�12H2O

0 4Na9K[Mn4(H2O)2(AsW9O34)2]�8WO3�2MnO2

�2KOAc�18NaOAc�8HOAc (2)

3.3. Structure of Na27K[AsIII4 W40O140] �/56H2O

The tetrameric anion [As4W40O140]28� consists of

four a-B-AsIIIW9O33 units sharing octahedron corners

with four linking WO6 groups [W(10)]. It generallyresembles the anion in Na28[(a-B-AsO3W9O30)4(WO2)4] �/2NaCl �/55H2O that was reported while this work was in

progress [7]. The structure differs from that of the anion

in (NH4)28Co2As4W40O140 �/20�/22H2O [8] in the follow-

ing details. The point symmetry of the anion is exactly,

rather than approximately 4: The site designated [8] S1

at the center of the tetramer is occupied by K� instead

of by NH4�, bonded to the two cis O atoms of each

bridging WO6 group that are not involved in W�/O�/W

linkages. The four S2 sites [8] are all occupied by Na�

cations [Na(1)] instead of by two Co2� and two NH4�.

The four oxygens of the anion bonded to Na(1) define

an approximate square which is planar to within one

standard deviation (0.03 A), and Na(1) lies 0.63(1) A

from the plane on the side away from the As atom with

a possible weak bond [2.82(2) A] to a water oxygen atom[O(43)] in an apical position. The As� � �Na(1) distance

[3.26(1) A] is to be contrasted with the short (2.65 A)

As� � �Co distance [8] in the other anion, where the Co is

effectively six-coordinate. The differences from the

anion in Na28[(a-B-AsO3W9O30)4(WO2)4] �/2NaCl �/55H2O include the potassium, rather than sodium,

central cation and the higher (/4) crystallographic point

symmetry.

3.4. Structure of Na9K[MnII4 (H2O)2(AsVW9O34)2] �/

35H2O

The product of reaction (2) is [MnII4 (H2O)2-

(AsVW9O34)2]10�. The assumed degree of hydration of

the salt is based on the X-ray analysis. The anion lies on

a crystallographic center of symmetry, with approximatepoint symmetry 2/m (C2h ) and contains a planar set of

four Mn2� ions separating two a-B-AsW9O349� groups.

Two Mn atoms each carry one water ligand. The anion

is isostructural with the [CoII4 (H2O)2(PVW9O34)2]10�

[15], [MnII4 (H2O)2(PVW9O34)2]10� [15], and [CoII

4 (H2O)2-

(AsVW9O34)2]10� anions [10,11], with similar corre-

sponding dimensions. The all-potassium salt

K10[MnII4 (H2O)2(AsVW9O34)2] �/18H2O, was reported

while this work was in progress [12].

4. Supplemental material

Supplementary data are available from Fachinforma-

tionszentium Karlsruhe (FIZ) for Na27K[AsIII4 W40O140] �/

56H2O the number is 412184 and for Na9K[MnII4 -

(H2O)2(AsVW9O34)2] �/35H2O the number is 412183.

Acknowledgements

C.R. and D.C.C. thank the Humboldt Foundationfor a Fellowship and Research Award, respectively.

D.C.C. also thanks The Institute of General Medicine of

the National Institutes of Health for funding. We thank

Dr. Tiberiu Frentiu for the microanalyses at the

Analytical Laboratory of the Department of Chemistry

of ‘Babes-Bolyai’ University, Cluj-Napoca, Romania.

We also thank Dr. Christopher D. Rithner and Dr.

Jason Smee for recording the 183W NMR spectra for us.

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C. Rosu et al. / Polyhedron 21 (2002) 959�/962962