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Journal of the Chilean Chemical Society

versão On-line ISSN 0717-9707

J. Chil. Chem. Soc. vol.59 no.3 Concepción set. 2014

http://dx.doi.org/10.4067/S0717-97072014000300014 

 

CRYSTAL STRUCTURE OF HEMIKIS(TETRAKIS(μ2-2-BUTENOATO)-BIS(2-BUTENOATO)-TETRA-AQUA-DI-SAMARIUM) HEMIKIS(BIS(μ2-BUTENOATO)-TETRAKIS(2-BUTENOATO) TETRA AQUA-DI-SAMARIUM) TRIHYDRATE

 

ANA MARÍA ATRIA,a MARIA TERESA GARLANDb* AND RICARDO BAGGIOc

a Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Casilla 233, Santiago, Chile, b Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago de Chile, Chile, and c Departamento de Física, Centro Atómico Constituyentes, Comisión Nacional de Energía Atómica, Buenos Aires, Argentina.
* e-mail: aatria@ciq.uchile.cl


ABSTRACT

The structure of the title compound [Sm2(crot)6(H2O)4]2-3(H2O) (I), (crot= crotonate = butenoate = O2CCHCHCH3)] consists of two independent, centrosymmetric dimeric units, of similar composition but diverse coordination, viz., one of them doubly bridged (2×[η2:η1:μ2]) and the remaining one quadruply bridged (2×[η1:η1:μ2], 2×[η2:η1:μ2]). The asymmetric unit is completed by three solvato water molecules, one of them depleted (occupation: 0.735 (14)). The difference in bridging strengths is readily evidenced in the two quite dissimilar Sm···Sm distances observed: 4.1402 (8)Å,(doubly bridged), 3.9439 (8)Å (quadruply bridged). SmO9 polyhedra suyvey similar coordination distances which span the range 2.388 (3)-2.611 (3)Å for one of the Sm cations and 2.357 (3)-2.588 (3)Å for the other. All water hydrogens are involved in H-bonding, leading to the formation of a strongly linked 2D structure parallel to (010). These planar arrays are in turn transversally linked by one single bridging water molecule. There are in addition C= C···C=C p interactions providing both to intra as well as intra planar cohesion. The compound is isomorphous to its Gd isologue1, but the present description unveils a number of molecular and supramolecular details not discussed therein.


1. INTRODUCTION

The structures of metallic carboxylates have been profusely analyzed as model materials for the study of physico-chemical properties of more complex materials, and among them carboxylate-bridged lanthanides are of relevance due to their outstanding role in molecular magnetism, etc. As a continuation of our interest on the structural and magnetic properties of carboxylate-bridged lanthanide complexes we report herein the structural study of [Sm2(crot)6(H2O)4]2·3(H2O) (I), (crot= crotonate = butenoate = O2CCHCHCH3) an interesting complex containing two independent dimers in the unit cell, one of them doubly bridged (2×[η2:η1:μ2]) and the remaining one quadruply bridged (2×[η1:η1:μ2], 2×[η2:η1:μ2]). Even if the compound is isomorphous to its already reported Gd isologue1, the present description if of relevance since it unveils a number of molecular and supramolecular details not discussed therein. In addition, it provides the second example of an homo-dinuclear Sm-crotonate reported so far, the first one being reported in Atria et al.2

2. Experimental

2.1. Synthesis and crystallization

Reported compound was one of the products resulting from the synthesis with crotonic acid and ( R )-(+)-α- methylbenzylamine. The procedure used was as follows:

A mixture of Sm2O3 (0,3487g, 1 mmol) and crotonic acid (0,5165g, 6 mmol) was dissolved in water (100 mmol), and then ( R )-(+)-α- methylbenzylamine (0,1211g, 1 mmol) dissolved in ethanol (10 ml) was added. The resulting mixture was refluxed for 24 h, filtered while hot. The filtrate was left at room temperature. On standing, a two fase crystalline system was obtained consisting on a major fraction of ill crystallized material, useless for x-ray diffraction (where probably the methylbenzylamine resided) and a few well developed, colourless crystals suitable for single crystal diffraction, which correspond to the Sm-crotonate herein reported. We are working at present in the crystallization of the remaining fase.

All chemicals and solvents were reagent grade, and used without further purification.

2.2. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1. All the H atoms included in the model were originally found in a difference Fourier, but treated differently in refinement: C—H's were repositioned in their expected positions and thereafter allowed to ride [d(C—Haromatic) = 0.95 Å, d(C—Hmethyl) = 0.98 Å], while O—H's were refined with restrained d(O—H) = 0.85 (1) Å, d(H···H) = 1.35 (2) Å. In all cases, [Uiso(H) = 1.2× (1.5× for methyl) Ueq (host). Water molecule O7W appeared depleted, and the oxygen occupation factor refined to 0.735 (14); since the corresponding H's could not be confidently located they were not included in the model. One of the butenoate anions (trailing number 6) appeared split into two sites sharing the same carboxylate group (occupation factors: 0.645 (6),0.355 (6) respectively). Both groups were refined with similarity restraints in distances and displacement factors.

Table 1. Experimental details.
 
 

Computer programs: SMART 9, SAINT 10 SAINT, SHELXS97 (Sheldrick 2008), SHELXL97 Sheldrick, 2008), SHELXTL (Sheldrick, 2008) 11, SHELXTL; PLATON12

The final difference map showed rather large peaks and holes (Extreme values: 1.13, -1.80 eÅ-3 at 0.74, 0.67Å from Sm2.

3. RESULTS AND DISCUSSION

As ditto, the structure of [Sm2(crot)6(H2O)4]2-3(H2O) (I) consists of two independent, centrosymmetric dimeric units, of similar composition but diverse coordination (Fig. 1). The asymmetry unit is completed by three water solvates, one of them (O7W) appearing depleted with a refined occupation of 0.735 (14).

 
Figure 1. Molecular diagram of (I), with ellipsoids drawn at a 40% probability level. In open bonds, the minor part of the split butenoate anion. In broken lines, the intradimeric H-bond.

Insets: schematic representation of both SmO9 coordination polyhedra, where SmO5 basal planes (oxygen atoms coloured in red) are capped from the top by oxygens in cyan and from the bottom by oxygens in grey.

Symmetry codes: i: -x, 1-y, 1-z; ii: 1-x, 1-y, -z

Both cation environments are of the SmO9 type, and the coordination polyhedra are similar in their being based on doubly capped (above and below) SmO5 pentagons (In red in the insets of Figure 1). Even if both SmO5 basal planes are rather similar, the way in which they are capped is not, being in a 2+2 fashion around Sm1 (Fig 1, upper inset), and 1+3 around Sm2.

Sm1 has its SmO9 environment defined by six oxygens from three chelating crotonates (atoms O1n,O2n from crotonates n=1,2,3), one extra bond from the centrosymmetric image of one of these, O21i, i: -x, 1-y, 1-z ), which acting as a short Sm—O—Smi bridge between neigbouring centrosymmetry related Sm centers defines one of the dimers, and finally two aqua molecules, O1W and O2W.

The second samarium cation Sm2 is also nine coordinated, this time to four carboxylate oxygens from two chelate crotonato anions (O1n, O2n from crotonates n=4,5) and one extra bond from the centrosymmetric image of one of them, O14ii, ii: 1-x, 1-y, -z ), which also here acts as a short Sm—O—Smii bridge between neigbouring centrosymmetry related Sm centers constituting one of the links defining the second dimer. The second link is provided by crotonato n=6 which via O16ii and O26 acts as a long -O—C—O- bridge between adjacent cations. Thus, this second dimer is held together by two pairs of centrosymmetric bridges, two short and two long, which define two closed, almost perpendicular loops (87.5 (2)°) around the symmetry center. The coordination is completed by two aqua molecules, O3W and O4W.

The difference in bridging strengths in both dimers is readily evidenced by the two quite dissimilar Sm-Sm distances: Sm1-Sm1i (single bridge): 4.1402 (8)Å, Sm2···Sm2ii(double bridge): 3.9439 (8)Å. Sm—O Coordination distances span the range 2.388 (3)-2.611 (3)Å for Sm1 and 2.357 (3)-2.588 (3)Å for Sm2. The three independent oxygens involved in the formation of the Sm2 dimer present the shortest Sm—O distances. The one subtending the single bridge in the Sm1 dimer is in turn the second shortest, at a distance very similar to these of the monodentate ligands O1W and O2W.

These two types of coordination, doubly and quadruply bridged (type 1 and type 2, respectively) are usual in dimeric Ln carboxylates in general and crotonates in particular. A search in the CSD3 revealed that among these latter dimers, those bridged solely through "type 1" links have only been reported for Nd4 and Dy, Sm2, all of them surveying two water molecules as ancillary ligands. Quadruply bridged ("type 2") dimers seem to be more often found in compounds with organic ancillary ligands, as phenanthroline (Gd)1; Dy5 or Eu,Tb6) and bispyridine (Gd, Ho)7 even though there are also examples with water (Gd)7. The concurrent appearance of both bridging types in the same structure, as found in the present Sm structure is much more rare and has only been found before in the Gd isologue1.

Regarding packing interactions, all the hydrogen atoms attached to fully occupied water molecules (see refinement section for details) take part in H-bonding (Table 3), linking dimers together into tight 2D structures parallel to (010). Fig 3 shows the resulting planar array: entries 1 to 11 in Table 3 describe bonds which take part in the substructure cohesion; in particular, the O2W—H2WB···O12i one is intradimeric (see Fig. 1) and the R(8)22 H-bonding (Sm1—O—H···O)2 loop it generates in the Sm1 dimer ("A" in Fig. 2, pale gray) mimics the covalent (Sm2—O—C—O)2 loop ("B" in Fig 2, cyan) in the second dimer (Sm2) (For graph set nomenclature see Bernstein et al.8). The intricate H-bonding scheme results in a tight mesh of H-bonding rings building up near, or around, the four relevant inversion centres in the structure, viz. those halving both dimers, ("A" and "B" in Fig. 2) and those at the centres of the hydrophobic regions ("C" and "D"). These latter "C" and "D" centres lodge the largest, centrosymmetric R(22)88, R(18)66 rings in the structure, but in spite of the large number of donors and acceptors there are no closed H-bonding loops surveying only light atoms (C,H,O): all of the generated rings include at least a couple of Sm atoms and, accordingly, they effectively provide to interdimeric linkage.

 
Figure 2. A simplified packing view drawn along [010] showing the intricate H-bonding netwrok giving raise to the 2D (010) structure (butenoato CH=CH—CH3 tails removed, for clarity). Grey and cyan loops are discussed in the text.

 
Figure 3. A simplified packing view drawn along [001] showing the way in which planar arrays are connected with each other (butenoato CH=CH—CH3 tails removed, for clarity).

These strongly bound planar arrays are in turn weakly connected to each other via a single bridging water molecule: the (depleted) O7W. In this bridge, the molecule acts as acceptor in the O6W—H6WB-O7W bond (Table 2, last entry) and as donor to a bond having O5E as acceptor, no directly detectable since the corresponding H7W atoms could not be found in the difference map; however, the interaction is evidenced by the short distance between oxygens (O7W···O5W[x,-1+y,z]= 2.830 (7)Å). Fig 3 shows the way in which the planes are linked along [010] by way of these two interactions. It is perhaps worth mentioning that O7W and, accordingly, this latter water···water interplanar interaction, run undetected in the previous (room temperature) Gd structure determination by Rizzi and coworkers1.

In addition to H-bonding interactions there are a number of C=C···C=C π contacts between inversion related butenoate anions, depicted in Fig. 4, where the complete, H-removed dimers have been represented. The most significant interactions, with Cg..Cg < 3.60Å, are of an intra-plane nature, while the weaker one represented in the third place provides to the inter-planar linkage.

 

Figure 4. Same view as in Fig 3, now with full butenoato anions, showing the C= C···C=C π interactions.

ACKNOWLEDGEMENTS

The authors acknowledge FONDECYT (Grant 1110154) for financial support.

 

REFERENCES

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7- Atria, A. M., Baggio, R., Garland, M. T., Munoz, J. C. & Pena, O. Inorg. Chim. Acta, 357, 1997-2006. (2004).

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9- Bruker SMART, V5.624. Data Collection Software. Siemens Analytical X-ray Instruments Inc., Madison, Wisconsin, USA. (2001).

10- Bruker (SAINT, V6.22A (Including SADABS). Data Reduction Software. Siemens. Analytical X-ray Instruments Inc., Madison, Wisconsin, USA. (2002).

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