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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 Pt(II) complexes ((R/S)-(1-4)⋅2 NO(3)) are reported. Related complexes (5⋅NO(3) and 6⋅NO(3)) 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)-(1-4)⋅β-CD⋅2 NO(3)) were also prepared and fully characterised. HR-ESI-MS experiments confirmed the presence of the host-guest adducts, and 2D-(1) H{(11)B} 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)-(1-4)⋅2 NO(3) 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.