Synthesis and Supramolecular Studies of Chiral Boronated Platinum(II) Complexes: Insights into the Molecular Recognition of Carboranes by β-Cyclodextrin

The synthesis and characterisation of a novel isomeric family of closo-carborane-containing PtII complexes ((R/S)-(14)center dot 2?NO3) are reported. Related complexes (5 center dot NO3 and 6 center dot NO3) that contain the 7,8-nido-carborane cluster were obtained from the selective deboronation of the 1,2-closo-carborane analogues. The corresponding water-soluble supramolecular 1:1 hostguest beta-cyclodextrin (beta-CD) adducts ((R/S)-(14)center dot beta-CD center dot 2?NO3) were also prepared and fully characterised. HR-ESI-MS experiments confirmed the presence of the hostguest adducts, and 2D-1H{11B} ROESY NMR studies showed that the boron clusters enter the beta-CD from the side of the wider annulus. Isothermal titration calorimetry (ITC) experiments revealed enthalpically driven 1:1 and higher-order supramolecular interactions between beta-CD and (R/S)-(14)center dot 2?NO3 in aqueous solution. A comparison of the predominate 1:1 binding mode established that the affinity of beta-CD for the guest molecule is mainly influenced by the pyridyl ring substitution pattern and chirality of the host, whilst the nature of the closo-carborane isomer also plays some role, with the most favourable structural features for beta-CD binding being the presence of the 4-pyridyl ring, 1,12-closo-carborane, and an S configuration. The results reported here represent the first comprehensive calorimetric study of the supramolecular interactions between closo-carborane compounds and beta-CD, and it provides fascinating insights into the structural features influencing the thermodynamics of this phenomenon.