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Keywords:

  • boron cluster;
  • calorimetry;
  • carborane;
  • cyclodextrin;
  • molecular recognition;
  • platinum

Abstract

The synthesis and characterisation of a novel isomeric family of closo-carborane-containing PtII complexes ((R/S)-(14)⋅2 NO3) are reported. Related complexes (5⋅NO3 and 6⋅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 host–guest β-cyclodextrin (β-CD) adducts ((R/S)-(14)β-CD⋅2 NO3) were also prepared and fully characterised. HR-ESI-MS experiments confirmed the presence of the host–guest adducts, and 2D-1H{11B} ROESY NMR studies showed that the boron clusters enter the β-CD from the side of the wider annulus. Isothermal titration calorimetry (ITC) experiments revealed enthalpically driven 1:1 and higher-order supramolecular interactions between β-CD and (R/S)-(14)⋅2 NO3 in aqueous solution. A comparison of the predominate 1:1 binding mode established that the affinity of β-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 β-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 β-CD, and it provides fascinating insights into the structural features influencing the thermodynamics of this phenomenon.