Crystalline structure of a new strontium-lasalocid complex: aqua bis-(lasalocid)-strontium(II) containing nine-coordinate Sr

The title complex, [Sr(C 68 H 106 O 16 )(H 2 O)], forms a pseudo-centrosymmetric monomer complex, in which one aquo (or water) ligand bridges three O centres of the first lasalocid anionic ligand (Lasa 2) and in which this ligand is tridentate and bonded to just one Sr atom, and the other lasalocid ligand (Lasa 1) is pentadentate and bridges the Sr centre within the monomer. The monomeric unit is stabilized by intramolecular C ⎯ H O and O ⎯ H…O type-hydrogen bonding interactions.


Introduction
Lasalocid A salt of sodium is one of the most commonly used veterinary antibiotics, where it has found wide spread application as an anticoccidial and to improve feed efficiency.The mechanism of action of lasalocid is clearly attributed to its ionophoric properties, because it has been reported to determine the influx of Na + in the cell of Gram positive and anaerobic bacteria, causing swelling, vacuolization, and death.At the origin of these processes, there is the property of forming lipophilic metal complexes, which can penetrate membranes and disrupt cation equilibria. 1,2he molecular basis of this action are still debated; more specifically which of the oxygen atoms are directly involved in cation coordination.To date, this problem has been the object of many investigations almost invariably taking advantage of the concerted use of several experimental and computational techniques, which demonstrate both the relevance of the problem and its intrinsic difficulty. 3he identity of the various complexes formed according to the nature of the cation, to the solvent, and to the solution composition has been initially faced with optical spectroscopy and circular dichroism (CD), possibly using lanthanides as probes. 4X-ray diffraction data became available for several cations, among others Na + , 5 and Ba 2+ . 6Often, it has been observed that aggregates of different stoichiometry can take place, leading to the formation of sandwiches, where the cation occupies a cavity between two ligand molecules. 3Molecular dynamic calculations have been reported both in vacuo, 7 and in solvent. 8Finally, there has recently appeared a series of papers on polyoxaalkyllasalocid esters/cation complexes making use of multinuclear NMR, IR, ESI-MS, and semiempirical methods. 9t has been proposed that antibacterial and fungicidal activity and also antitumor and anti-HIV-integrase inhibition of antibiotics lie in their ability to chelate the essential metals, which the microorganisms need in their metabolism.9e-g

Results and Discussion
We report the synthesis and structure of the first compound of a series of strontium(II) complexes that is readily prepared as its pure stochioisomer ligand/metal (2/1).This new strontium(II) complex, deriving from a lasalocid/ strontium coordination, is obtained through a simple and economical synthetic method.The reaction of lasalocid acid (1) (Figure 1), with strontium salt under ordinary atmosphere, with controlling pH (pH>8), provides a stable coordination product (Figure 2), the aquo-strontium-bis-lasalocid, which is characterised by Xray (Figure 3).The title complex, [Sr(C 68 H 106 O 16 )(H 2 O)], crystallizes with nine-coordinated Sr atom threedimensionally interconnected into a framework structure.The crystallographically dependent water molecule is located on an axial site.These two lasalocid ligands (Lasa 1 and Lasa 2) are tied together by a three-dimensional hydrogen-bonding network.These monomeric units are not stacked by any Van der Waals forces between the coordination spheres.
Since the coordination properties of the two molecules of lasalocid ligands are different, it is concluded that the two lasalocid ligands are not simultaneously coordinated.To have a clear idea about the coordinative aspect of the two ligands, the view for the complex [Sr(Lasa) 2 (OH 2 )]) has been added here as shown in Figure 2.

Conclusions
In conclusion, we describe here what is to our knowledge the first solid-state structure obtained with a strontium-lasalocid complex.The tetra-butyl ammonium salt of lasalocid is able to form a stable 2:1 (ligand : metal) complex with divalent cations such as strontium(II).The structure of this complex is completely different with the respective complexes with other divalent cations ARKAT USA, Inc.
(Mg 2+ and Ca 2+ ). 15A preliminary biological activity of the Sr-Lasalocid ester on fungus F. oxysporum was studied in vitro at Oujda.It is shown that this compound is biologically active.This result is very important for future agricultural applications and deserves to be extended to a wide series of transition metals [Ru(II), Co(II), Fe(II)].

Experimental Section Preparation of complex (I).
A solution of tetramethylammonium-lasalocid free salt (6.64 g, 2 mmole, prepared in 20 mL of CHCl 3 ) was stirred with 0.1 M aqueous SrCl 2 (0.793g, 1 mmole, prepared in 50 mL of H 2 O).The mixture was stirred at 20 °C for 4 h under argon in the dark.The organic layer was then dried over anhydrous Na 2 SO 4 , filtered and evaporated.The solid residue was dissolved in MeOH and the solvent was left to evaporate at 20 °C for 1 week in the dark.Yellow crystals obtained proved suitable for X-ray analysis (65% Yield).
Crystal structure analysis.The crystal structure of the title compound, C 68 H 108 O 17 Sr, has been determined at room temperature.Diffraction data were collected using a Bruker SMART APEX-II CCD diffractometer system, using graphite-monochromated MoK α radiation.The crystallographic details are given in Table 1.The structure was solved by direct methods by using SIR-97 program and refined by least-squares on F obs 2 and by using SHELXL-97 programs.
All other H atoms were located in calculated positions and treated as riding on their parent atoms, with C-H = 0.96 (CH 3 ), 0.97 (CH 2 ) or 0.98 Å (CH), and with U iso (H) = 1.5U eq (CH 3 ) or 1.2U eq (CH 2 , CH).A displacement ellipsoid plot with the atomic numbering scheme of the title compound is shown in figure 3; with selected bond lengths and bond angles, and the details of the hydrogen-bonding geometry in Tables 2 and 3 3).

Figure 3 .
Figure 3.An ORTEP view of the title compound, with the atom-numbering scheme and 10% probability displacement ellipsoids.H atoms have been omitted for clarity.Displacement ellipsoids are drawn at the 10% probability level.
, respectively.The average Sr-O [2.653(2) Å] distance, consistent with usual distances for nine-coordinated Sr II ions [Sr-O 2.564(3) -2.967(3) Å]. 13 Within the ligands, other geometric parameters (C-O and C-C distances, and O-C-O and O-C-C angles) all lie in the expected ranges. 14The crystal structure is stabilized by the intramolecular O-H O and C-H O type-hydrogen bonding interactions (Table

Table 1 .
Crystal and experimental data

Table 3 .
Hydrogen-bonding geometry (Å, °).CCDC 664624 contains the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.