Halogen bonding in crystal structure of bis(1,4,7,10-tetraoxacyclododecane-κ⁴O,O′,O′′,O′′′)cesium triiodide, C₁₆H₃₂CsI₃O₈

Source of material

Iodine (>=99.0%), caesium iodide, 1,4,7,10-tetraoxacyclododecan (12-crown-4) and ethanol were received from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). All the used reagents were of analytical grade and were used as purchased without further purification. The Raman spectra were measured using a Jobin Yvon U 100 spectrometer from Spectra-Physics (Argon laser at 514.5 nm). The title compound was synthesized by dissolving 0.16 g (0.63 mmol) of CsI and 0.16 g (0.63 mmol) I₂ in 10 mL ethanol/10 ml methanol mixture at room temperature. Then 0.2 mL (1.26 mmol) 12-crown-4 is added under continuous stirring. The clear solution gives after 4 days at room temperature reddish-brown crystals of [Cs(12-crown-4)₂]I₃.

Experimental details

A reddish-brown single crystal was selected from the mother liquor and transferred to the Kappa CCD diffractometer [2]. The measurement was done at 293 K. The structure solution, refinement and further calculations were done with the programs SHELXL [3], [4]. The data collection was undertaken several years ago using a point detector. Unfortunately, the measured section produced a completeness of only ∼90%. Nevertheless during the refinement no correlation effects were detected.

Comment

Polyiodides are interesting compounds with a rich chemistry and many applications, especially as antimicrobial agents [5]. Previously, polyiodides and iodine were incorporated into cellulose membranes [6], polymer foams [7], silicon membranes [8] and wound dressing applications in form of nanoparticles [9] due to their microbicidal actions. Only few examples of polyiodides with 12-crown-4 and alkalimetal cations have been reported previously [10], [11]. In these compounds, the crown ether and the metal cation stabilize the polyiodide structures by forming sandwich structures [10], [11]. The long time stability of a polyiodide is important for its use as antimicrobial agent to increase its long term effectiveness, durability and reduce the iodine sublimation [6]. Triiodides seem to be the most interesting class of polyiodides due to their stability. There are hundreds of examples of triiodides in the literature [12]. Halogen bonding is a major factor of stability in polyiodides within three-center-systems [13] and their resulting antimicrobial activity [14].

The asymmetric unit of the title structure contains two crystallographically independent, isolated, symmetrical, linear triiodide anions I₃⁻ with crystallographic inversion symmetry. The bond lengths and angles within these structures are in expected ranges and are another example of a three-center-system [I-I-I]⁻ with halogen bonding like the previously reported triiodies in our group [14]. This three-center-system is an indicator for possible antimicrobial activities. As reported before [14], the complex compound interacts with the cell membrane of the microorganism due to electrostatic interactions and is deformed. This results in gradual free molecular iodine release from the triiodide-unit. Iodine directly attacks the pathogens by destroying their cell membrane and causing protein oxidation [5], [9], [14]. There is only one strong Raman stretching vibration at 108 cm⁻¹ for the triiodide, which is available in our previous compounds [14] and in the very recently reported cyclic I₁₀⁻² anion [15]. The later showed another strong line at 172 cm⁻¹ due to weak connections to two neighboring halogen bond donors. Usually, covalent I₂ is detected at around 180 cm⁻¹ [13], [15]. All geometric parameters of the cationic cesium bis(12-crown-4) complex are in the expected ranges [16].