Metal–heptaiodide interactions in cyclomaltoheptaose (β-cyclodextrin) polyiodide complexes as detected via Raman spectroscopy

The Raman spectra of the cyclomaltoheptaose (β-cyclodextrin, β-CD) polyiodide complexes (β-CD)2·NaI7·12H2O, (β-CD)2·RbI7·18H2O, (β-CD)2·SrI7·17H2O, (β-CD)2·BiI7·17H2O and (β-CD)2·VI7·14H2O (named β-M, M stands for the corresponding metal) are investigated in the temperature range of 30–140 °C. At room temperature all systems show an initial strong band at 178 cm−1 that reveals similar intramolecular distances of the disordered I2 units (∼2.72 Å). During the heating process β-Na and β-Rb display a gradual shift of this band to the final single frequency of 166 cm−1. In the case of β-Sr and β-Bi, the band at 178 cm−1 is shifted to the final single frequencies of 170 and 172 cm−1, respectively. These band shifts imply a disorder–order transition of the I2 units whose I–I distance becomes elongated via a symmetric charge-transfer interaction I2I2←I3-→I2I2. The different final frequencies correspond to different bond lengthening of the disordered I2 units during their transformation into well-ordered ones. In the Raman spectra of β-V, the initial band at 178 cm−1 is not shifted to a single band but to a double one of frequencies 173 and 165 cm−1, indicating a disorder–order transition of the I2 molecules via a non-symmetric charge-transfer interaction I2I2←I3-→I2. The above spectral data show that the ability of I3- to donate electron density to the attached I2 units is determined by the relative position of the different metal ions and their ionic potential q/r. The combination of the present results with those obtained from our previous investigations reveals that cations with an ionic potential that is lower than ∼1.50 (Cs+, Rb+, Na+, K+ and Ba2+) do not affect the Lewis base character of I3-. However, when the ionic potential of the cation is greater than ∼1.50 (Li+, Sr2+, Cd2+, Bi3+ and V3+), the Mn+⋯I3- interactions become significant. In the case of a face-on position of the metal (Sr2+, Bi3+) relative to I3-, the charge-transfer interaction is symmetric. On the contrary, when the metal (Li+, Cd2+, V3+) presents a side-on position relative to I3-, the charge-transfer interaction is non-symmetric.

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