ORI GIN AL PA PER

Self-Assembly, Crystal Structure and Analysisof Intermolecular Interactions of the SupramolecularCompound Based on Hexamolybdochromate(III),Sulfate and Piperazine

Feng Yao • Ya-Guang Chen • Ali R. Salimi •

Masoud Mirzaei

Received: 28 February 2011 / Published online: 21 May 2011

� Springer Science+Business Media, LLC 2011

Abstract A new supramolecular compound based on Anderson-B hexamolybdo-

chromate, (H2Pz)3[Cr(OH)6Mo6O18H](SO4)2�12H2O (1) (Pz = piperazine) was

synthesized and characterized by elemental analysis, IR spectroscopy, and single-

crystal X-ray diffraction (Mo Ka). The compound crystallizes in monoclinic system,

P21/c space group with a = 13.5708(6) A, b = 17.3711(8) A, c = 22.2387(9) A,

b = 110.631(2)�; V = 4906.3(4) A3, Z = 4, Dc = 2.290 g/cm3, F(000) = 3364.0;

l = 1.905, S = 1.033. The final R = 0.0398 and wR = 0.0971. The H2pz2? ions and

sulfate anions in 1 are arranged through hydrogen bonds into a hexagonal network in

[202] plane and hexamolybdochromates anions (CrMo6) fill in the hexagonal

vacancies. The networks stack in such a way that each anion links two sulfate ions

from adjacent networks via hydrogen bonds with short (CrMo6)O���OSO3 distances of

2.637–2.697 A. A lot of hydrogen bonds are formed between water molecules, sul-

fate, H2pz2? ions and CrMo6 anions, which are the dominating force constructing the

supramolecular structure. Hirshfeld surface analysis of 1 gives us the details of

intermolecular interactions in the crystals of 1 in a visual manner and shows that the

CrMo6 anion acts as a stronger hydrogen bond donor than as an acceptor.

Electronic supplementary material The online version of this article

(doi:10.1007/s10876-011-0382-6) contains supplementary material, which is available to authorized

users.

F. Yao � Y.-G. Chen (&)

Key Laboratory of Polyoxometalates Science of Ministry of Education, College of Chemistry,

Northeast Normal University, Changchun 130024, People’s Republic of China

e-mail: chenyg146@nenu.edu.cn

A. R. Salimi � M. Mirzaei

Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad,

P.O. Box 917791436, Mashhad, Iran

123

J Clust Sci (2011) 22:309–318

DOI 10.1007/s10876-011-0382-6

Keywords Anderson-B hexamolybdochromate � Crystal structure �Intermolecular interaction � Piperazine � Sulfate � Hirshfeld surface analysis

Introduction

Supramolecular compounds based on polyoxometalates (POMs) construct a small

subset of the large family of POMs which have attracted considerable attention

worldwide due to variety of their structures and potential application [1–6]. The

self-assembly of supramolecular structure performs via ligand–metal coordination

bonds, hydrogen bonds, electrostatic force and p–p stacking interaction. Among the

non-covalent motifs, hydrogen bonding and p–p stacking interactions have been

employed as synthetic paradigms to rationally design superstructures [7, 8]. There

are a lot of reports on syntheses, characterization and studies of properties of the

supramolecular compounds based on POMs [9–17] but Hirshfeld surfaces analyses

on these compounds are rare [18]. Hirshfeld surface analysis is the method

developed by Spackman et al. [19–22] for elucidating and comparing intermolecular

interactions as well as for spotting common features and trends in specific classes of

compounds. After our first report [18], we obtained another analogue

(H2Pz)3[Cr(OH)6Mo6O18H](SO4)2�12H2O (1) (Pz = piperazine) from pre-synthe-

sized hexamolybdochromate (CrMo6) and in a high concentration of sulfate through

self-assembly in solution and found that the components of this compound

undertake a such different arranging pattern that a hexagonal network in [202] plane

with CrMo6 filling forms with hydrogen bonds. Herein, we report the self-assembly,

crystal structure and the Hirshfeld surface analysis of the intermolecular interactions

of 1.

Experimental

Materials and Methods

All chemicals of reagent grade were purchased from commercial sources and used

as received. Elemental analyses (C, H, N) were performed on a Perkins-Elmer 2400

CHN Elemental Analyzer. Mo and Cr were determined on a Prodigy inductively

coupled plasma spectrometer. IR spectrum was recorded in the range of

400–4000 cm-1 on a Magna-560 FT-IR spectrophotometer.

Synthesis

An aqueous solution (20 mL) containing 0.1231 g (0.1 mmol) Na3[CrMo6H6

O24]�H2O [23, 24] was heated at approximately 60 �C. Then, into it a solution of

CuSO4�5H2O (0.0500 g, 0.2 mmol) in 5 mL water and 10 mL aqueous solution of

Pz (0.12 g, 0.6 mmol) were added dropwise. The pH of the resulting mixture was

adjusted with dilute H2SO4 solution to about 3.15 and the final mixture was stirred

310 F. Yao et al.

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for 40 min at 60 �C, then filtered. Slow evaporation of this filtrate resulted in a

light-red block crystalline product within 2 weeks (Yield: ca. 35% based on Cr).

Anal. calcd for C12H67CrMo6N6O44S2: C 8.55, H 3.65, N 4.99, S 3.80, Cr 3.09, Mo

34.15(%); found C 8.41, H 3.45, N 4.90, S 3.69, Cr 2.91, Mo 35.01(%). IR (KBr)m:

3422.35(s), 3026(w), 2960(w), 1615.53(ms), 1458.47(m), 1314.83(w), 1083.25(m),

918.5 (s), 643.87(s), 564.85 (w), 443.1(m) cm-1.

X-ray Crystallography

X-ray diffraction data were collected on a SMART APEX II CCD diffractometer

with graphite monochromated MoKa radiation at 293 K with x scans. Empirical

absorption correction was applied. The structure was solved by direct methods and

refined by full-matrix least-squares on F2 with the SHELXL-97 program package

[25, 26]. The positions of the hydrogen atoms attached to carbon and nitrogen atoms

were fixed according to riding mode. All the non-hydrogen atoms except C2 atom

were refined anisotropically. A summary of crystallographic and structural

refinement data for 1 is provided in Table 1.

Table 1 Crystallographic and structural refinement data of 1

Empirical formula C12H67CrMo6N6O44S2

Mr 1691.43

Crystal dimension (mm3) 0.10 9 0.11 9 0.18

Range of h (�) 1.98–28.34

Crystal system Monoclinic

Space group P2(1)/c

a (A) 13.5708(6)

b (A) 17.3711(8)

c (A) 22.2387(9)

b (deg) 110.631(2)

V (A3) 4906.3(4)

Z 4

Dcalcd (g cm-3) 2.290

F(000) (e) 3364.0

l (mm-1) 1.905

Refl. measured 29824

Refl. Unique (Rint) 12237(0.0333)

R1/wR2a [I [ 2r(I)] 0.0398, 0.0971

GoF (F2)a 1.033

(D/r)max 0.001

±(Dq) (eA3) 2.030, -0.962

a w = 1/[r2(Fo2) ? (0.0485P)2 ? 6.9690P] where P = (Fo

2 ? 2Fc2)/3

Synthesis and Characterization of a Supramolecular Compound 311

123

Results and Discussions

Structural Description

Compound 1 consists of one discrete anion [Cr(OH)6Mo6O18H]2- (CrMo6), three

piperazinium cations H2Pz2?, two sulfate anions and twelve water molecules

(Fig. 1). The CrMo6 anion has a B-type Anderson structure [23, 24] and consists of

one Cr(OH)6 octahedron and six MoO4(OH)2 octahedra which are in a plane and

share the octahedral edges each other. The oxygen atoms are divided into three

types, twelve terminal (Ot), six l2-bridging (Ob2) oxygen atoms and six l3-bridging

(Ob3) oxygen atoms of hydroxyl groups. The bond lengths of three types of Mo–O

bonds are in the sequence of Ob3 (2.260(3)–2.310(3) A) [ Ob2 (1.926 (3)–

2.055(3) A) [ Ot (1.693(3)–1.712(3) A) (Table 2). CrO6 octahedron is in the

centre of the CrMo6 anion. The coexistence of terminal, l2-bridging and l3-bridging

hydroxyl oxygen atoms makes the anion become both acceptor and donor of

hydrogen bond. M–O bond lengths and O–M–O bond angles (Table 2) show that

MoO6 octahedra are more severely distorted than CrO6 octahedron and these data

are all consistent with that in Ref [23, 24].

Piperazine as an organic base is easily protonized in acidic solution, forming

H2Pz2? ion. H2Pz2? ions, in a chair configuration, are excellent hydrogen-bond

donors [27–29] and at the same time provide counter-charge. The hydrogen bonds

between H2Pz2? ions and sulfate anions (Table 3) [27] lead to a hexagonal network

(Fig. 2 left) in [202] plane, CrMo6 anions are located in the hexagons (Fig. 2 right)

and water molecules fill in the vacancies left by space stacking of H2Pz2? ions and

sulfate anions CrMo6 anions. Between these networks there exist electrostatic force,

van der Waals force and a lot of hydrogen bonds. These networks stack in such a

way that each CrMo6 anion links two sulfate ions from two adjacent networks with

short {Mo6O18Cr(OH)6}O���O{SO4} distances (Table 3), shorter than general

Fig. 1 ORTEP drawing of 1 with thermal ellipsoids at 50% probability. Hydrogen atoms are omitted forclarity

312 F. Yao et al.

123

hydrogen bonds, showing a strong hydrogen bonding interaction (Fig. 3). Such a

network stacking gives shorter distances between CrMo6 anions belonging to two

networks of 3.295–3.515 A for terminal oxygen atoms than that in Ref [18].

Another obvious difference is the role of sulfate: in 1 the sulfates are the component

of the networks and in Ref [18] the sulfates control the distance between the layers.

A careful survey of the structure indicates that water molecules link the CrMo6

anions, sulfate ions and H2Pz2? ions with hydrogen bonds, that is, the water

molecules act as a ‘‘glue’’ to assemble the supramolecular structure.

The bond valence sum calculation (Electronic Supplementary Table S1) for

oxygen atoms of CrMo6 anion and for nitrogen atoms of piperazines indicates that

all l3-oxygen atoms link with a hydrogen atom and one l2-oxygen atom is

protonized and all nitrogen atoms are protonized. According to these results the

formula (H2Pz)3[Cr(OH)6Mo6O18H](SO4)2�12H2O is given.

Hirshfeld Surface Analysis

The Hirshfeld surfaces are defined implicitly by the simple equation as the ratio

between the sum of spherical atom electron densities for a molecule and the same

Table 2 Selected M–O bond lengths (A) and bond angles (�) in [Cr(OH)6Mo6O18H]2-

Mo1–O12t 1.693(3) Mo1–O1b3 2.298(3) Mo1–O23b2 1.934(3)

Mo1–O11t 1.712(3) Mo1–O2b3 2.304(3) Mo1–O10b2 1.952(3)

Mo2–O19 1.693(3) Mo2–O1 2.286(3) Mo2–O23 1.941(3)

Mo2–O7 1.700(3) Mo2–O3 2.310(3) Mo2–O9 1.959(3)

Mo3–O13 1.693(3) Mo3–O3 2.270(3) Mo3–O9 1.919(3)

Mo3–O18 1.702(3) Mo3–O4 2.307(3) Mo3–O21 1.993(3)

Mo4–O17 1.694(3) Mo4–O5 2.260(3) Mo4–O21 1.926(3)

Mo4–O14 1.699(3) Mo4–O4 2.271(3) Mo4–O22 2.032(3)

Mo5–O20 1.691(3) Mo5–O5 2.261(3) Mo5–O15 1.891(3)

Mo5–O8 1.702(3) Mo5–O6 2.282(3) Mo5–O22 2.055(3)

Mo6–O16 1.693(3) Mo6–O2 2.286(3) Mo6–O10 1.929(3)

Mo6–O24 1.704(3) Mo6–O6 2.310(3) Mo6–O15 1.986(3)

O2–Cr1 1.968(3) O3–Cr1 1.970(3) O5–Cr1 1.990(3)

O1–Cr1 1.963(3) O4–Cr1 1.971(3) O6–Cr1 1.966(3)

O12–Mo1–O11 105.27(18) O12–Mo1–O23 100.89(16) O11–Mo1–O23 98.37(16)

O12–Mo1–O10 96.86(16) O11–Mo1–O10 100.26(16) O23–Mo1–O10 149.82(13)

O12–Mo1–O1 93.85(14) O11–Mo1–O1 160.07(15) O23–Mo1–O1 72.06(12)

O10–Mo1–O1 82.62(12) O12–Mo1–O2 161.07(15) O11–Mo1–O2 92.11(15)

O23–Mo1–O2 83.58(12) O10–Mo1–O2 72.23(12) O1–Mo1–O2 69.83(10)

O1–Cr1–O6 96.16(12) O1–Cr1–O2 84.05(13) O6–Cr1–O2 83.79(12)

O1–Cr1–O3 84.15(12) O6–Cr1–O3 179.59(13) O2–Cr1–O3 96.50(12)

O1–Cr1–O4 96.45(12) O6–Cr1–O4 96.44(12) O2–Cr1–O4 179.41(14)

O3–Cr1–O4 83.26(12) O1–Cr1–O5 179.15(12) O6–Cr1–O5 83.13(12)

O2–Cr1–O5 96.33(12) O3–Cr1–O5 96.56(12) O4–Cr1–O5 83.17(12)

Synthesis and Characterization of a Supramolecular Compound 313

123

sum for the entire crystal [19–22]. The molecular Hirshfeld surfaces reflect

intermolecular interactions in a novel visual manner as triangulation surfaces. The

surface shape relies on the interaction between molecules in the crystal as well as

between atoms in the molecule. Derived from the Hirshfeld surface, the 2D-

fingerprint plots, in which the points on the surface are a function of the closest

distances from the point to nuclei inside (di) and outside (de) the surface, provide a

visual summary of the frequency of each combination of de and di across the surface

of a molecule, so they not only indicate which intermolecular interactions are

Table 3 Selected hydrogen bonds

D–H d(H���A) d(D���A) (A) \DHA (�) A

N1–H13A 2.020 2.875 158.38 O25 [x, -y ? 1/2, z ? 1/2]

N2–H1C 1.902 2.831 159.39 O26 [-1 ? x, -y ? 1/2, z ? 1/2]

N3–H15B 2.032 2.900 161.43 O30 [x, -y ? 1/2, z - 1/2]

N4–H16B 2.091 2.895 148.19 O29

N5–H17A 2.000 2.866 161.07 O28

N6–H18A 1.950 2.849 177.62 O31 [x, -y ? 1/2, z - 1/2]

O1–H11D 1.710 2.678 168.93 O30

O2–H11C 1.700 2.668 168.63 O26 [x - 1, y, z]

O3–H11E 1.712 2.680 168.79 O28 [x - 1, y, z]

O4–H11F 1.719 2.695 173.56 O31

O5–H11G 1.651 2.631 178.47 O25 [x - 1, y, z]

O6–H11H 1.707 2.686 176.19 O29

Fig. 2 Hydrogen bond network formed by H2Pz2? ions and sulfate anions (left) and filling of CrMo6 inthe network (right) in [202] plane of 1. All hydrogen atoms and water molecules are omitted for clarity

314 F. Yao et al.

123

present, but also give the relative area of the surface corresponding to each kind of

interaction. Figure 4 demonstrates the Hirshfeld surface and 2D fingerprint plot of

CrMo6 on which the molecular electrostatic potential has been color-coded as Min

(de, di) surface, the blue points for a small contribution and through green to red

points for the greatest contribution. In comparison with the 2D fingerprint plot of

AlMo6 anion [18], one can found out that the plate structure of these clusters

provide different electrostatic potentials and in effect, the different hydrogen

bonding and van der Waals interactions patterns. In fact, the 2D fingerprint plot

containing two sharp features shows the pattern characteristic of hydrogen bonding

interactions. The upper feature corresponds to the hydrogen bond donor and the

lower one to the acceptor. The asymmetric pattern of the 2D fingerprint of CrMo6

anion reveals that this moiety acts as a stronger hydrogen bond donor than an

acceptor, forming stronger hydrogen bonds with sulfate anions from two sides of the

plane of CrMo6 cluster, very different from that in Ref [18].

The 2D fingerprint plot of sulfate anion (Fig. 5 right-down) show that the SO42-

anion acts only as an H-bond acceptor in the crystals. The short distances of

de = 0.7 A should be assigned to the close interaction of sulfate anion with O–H of

Anderson anion.

The hydrogen bonding patterns and chemical environment of piperazinium ions

can be compared based on the 2D fingerprint plots shown in Fig. 5. These plots

indicate that these piperazinium ions with the same structure have different

interaction patterns. In the plot of pz-1 molecule, the short contact in H-bond donor

part of plot (de ? di = 1.876) is assigned to C1H1c���O1W interaction. The sharp

features of the pz-2, 3, 4, 5 plots are assigned to amine groups, which provide an

Fig. 3 The network stacking of 1 showing hydrogen bonds with dashed lines viewed from b axis (left)and the hydrogen bond interaction between sulfate anions and CrMo6 anion (right). All hydrogen atomsand water molecules are omitted for clarity

Synthesis and Characterization of a Supramolecular Compound 315

123

H-bonding capacity in many ways. The pz-2 plot shows the very close contact with

de = 0.6 A and di = 0.6 A distances which should be assigned to the close

interaction of N1H13B���H37BO8W.

Conclusion

A new supramolecular compound based on Anderson-B hexamolybdochromate was

successfully obtained in a high molar ratio of SO42-/Cr(OH)6Mo6O18

3- and with

pre-synthesized Cr(OH)6Mo6O183- as starting material. H2Pz2? ions as excellent

hydrogen-bond donors and sulfate anions form to a hexagonal network through

hydrogen bonds. The hydroxyls of Anderson-B structure anion make it synchro-

nously act as donor and acceptor of hydrogen bonds, different from other POMs.

Water molecules act as a ‘‘glue’’ to assemble the overall structure through hydrogen

bonds. The Hirshfeld surfaces and the 2D breakdown fingerprint plots have

indicated visually the nature of intermolecular interactions in this supramolecular

compound and shown that the CrMo6 anion acts as a stronger hydrogen bond donor

than an acceptor.

Supplementary material

Copies of the data (CCDC No. 684232) can be obtained free of charge on

application to CCDC, 12 Union Road, Cambridge CB2 2EZ, UK (Fax: (?44)

1223-336033; e-mail: deposit@ccdc.cam.ac.uk. Supporting information of Table S1

is included.

Fig. 4 Hirshfeld surfaces mapped with curvedness (left) and 2D fingerprint plot of CrMo6 (right) in 1

316 F. Yao et al.

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Fig. 5 2D fingerprint plots of five organic moieties and SO42- anion in the crystals of 1

Synthesis and Characterization of a Supramolecular Compound 317

123

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