Azonia spiro polyaza macrocycles containing biphenyl subunits as anion and cation receptors

The reaction of N-Boc triprotected cyclam with bis(chloromethyl)biphenyl followed by the corresponding deprotection of the nitrogen atoms allows the preparation of receptor 3 containing an azonia spiro subunit. This receptor shows slightly increased basicity than cyclam, in particular for the formation of the appropriate triply charged species as a consequence of the reduced capacity of the structure present in 3 to stabilize the species with lower protonation degrees through the formation of intramolecular hydrogen bonds. The properties of 3 as a receptor for Cu2+ and Zn2+ and the anions derived from PO43−(Pi),P2O74−(PPi),P3O105−(TPP) and ATP have been studied by pH-metric titrations carried out in aqueous solution. While Cu2+ forms a CuL2+ complex and two hydroxylated species of moderate stability, Zn2+ forms only hydroxylated complexes. The association constants obtained for the 1:1 A:L anion complexes denote significant stability for Pi, PPi and ATP; the stability of the H5LA complex found in the case of TPP being lower. 1H NMR spectra for the ATP:L carried out at pD=5.8 show features attributable to the occurrence of intermolecular π-π stacking between the biphenyl unit of 3 and the adenine ring of the nucleotide. DFT calculations have been carried out to rationalize some of the results found, in particular the remarkable different basicity between receptor 3 and cyclam.

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