Crystal structure and antimicrobial properties of (1,4,7,10-tetraoxacyclododecane-κ4O,O′,O′′,O′′′)cesium(I) pentaiodide, C16H32CsI5O8

Abstract C16H32CsI5O8, monoclinic, I2/a (no. 15), a = 16.386(2) Å, b = 11.7050(10) Å, c = 19.005(3) Å, β = 108.008(14)°, V = 3466.6(8) Å3, Z = 4, Rgt(F) = 0.0437, wRref(F2) = 0.1085, T = 293(2) K.


Experimental details
A black single crystal was chosen from the mother liquor and measured at 293 K on an IPDS diffractometer [2]. The structure solution, refinement and other calculations were done with the programs SHELXL [3,4]. All antimicrobial studies were performed as described in our previous work [5].

Comment
Iodine has important applications as antimicrobial agent because of its well known biocidal properties [6,7]. Iodine and related compounds in form of nanoparticles are already used in wound dressing applications [8]. The drawbacks of iodine use in wound care are sublimation and skin irritation [6,8].
The iodine content can be stabilized by designing polyiodides complexed by crown ethers as previously reported in  [9,10]. Triiodides with halogen bonding are usually stable compounds with many potential applications [11][12][13]. Triiodides with halogen bonding can be used as antimicrobial agents because they are stable and gradually release free iodine [5]. In these compounds, the released free iodine interacts with cell membranes and leads to protein oxidation [5,6,8].
In our title compound [Cs(12-crown-4) 2 ]I5, the asymmetric unit contains symmetrical, linear triiodide anions I 3 − with crystallographic symmetry of a 2-fold axis. Two iodine molecules (I3-I4) are weakly connected to these symmetric triiodide (I2-I1-I2′) halogen bond donor. The resulting pentaiodide structure is a zig-zag chain. The bond lengths and angles within this structure is in expected ranges [16], which is also true for the cationic Cs(I) complex [9] and are further examples of a three-center-system [I-I-I] − with halogen bonding as our previously reported triiodides [5,9,10]. This compound is the ideal candidate to act as an antimicrobial agent. Therefore we investigated its inhibitory effects on common microorganisms.
These results support our hypothesis, that polyiodides can be used as antimicrobial agents. Once the complex compound interacts with the microbial cell membranes by electrostatic interactions, the title compound decomposes and results in controlled free molecular iodine release [5].