Remarkably selective recognition of iodobenzene derivatives by a macrocyclic bis-Pt(II) metallohost

We designed and synthesized self-assembled bis-Pt(II) dimer 1⋅4 BF(4) with quino[8,7-b][1,10]phenanthroline as an extended π-face contact area, which acts as the first artificial receptor with high affinity toward iodinated aromatic compounds significantly based on noncovalent iodine⋅⋅⋅aromatic-plane interactions in a "side-on" fashion. Despite their structural similarity to a previously reported metallohost 2(4+) that bears 2,2':6',2''-terpyridine units, a dramatic change in selectivity toward substituted benzene derivatives was observed for 1(4+). (1)H NMR spectroscopic titration revealed a high affinity of 1(4+) towards haloarenes, with exceptionally large association constants for 2-iodophenol (K(a) = 16,000  M(-1)) and 1,2-diiodobenzene (K(a) = 21,000  M(-1)), which are 93- and 140-fold higher, respectively, than the values obtained for 2(4+). In addition, 1(4+) showed a remarkably high affinity and selectivity toward 2,6-diiodophenol (K(a) = 35,000  M(-1)), which is an important substructure of the thyroid hormone T(4). X-ray crystallography and theoretical calculations strongly suggest that "side-on" iodine⋅⋅⋅aromatic-plane interactions and π-π stacking contribute to the strong 1,2-diiodobenzene and 2,6-diiodophenol binding. The results obtained here give unique and valuable insight into the nature of halogen atom interactions in their "side-on" region with an electropositive aromatic plane, which may provide useful guidance for designing artificial receptors for iodinated biomolecules.