Selective recognition of metal complexes by macrocyclic ethers: further observations on the macrocycle size dependence and the first-sphere ligand composition dependence of recognition thermodynamics

Additional studies of solution phase recognition of Ru(NH₃)_x(pyridine)_y²⁺, Ru(NH₃)_x(2,2′-bipyridine)_y²⁺ and Ru(NH₃)₄(1,10-phenantroline)²⁺ species by dibenzo crown ethers are reported. The factors most closely examined were crown size, ammine ligand content and ancillary ligand composition. The overall study confirms that recognition or association derives primarily from H-bond formation (ammine hydrogen/ether oxygen. Evidently opposing these interactions, however, are crown conformational rearrangements. Consequently, straight-forward correlatins between association strength and potential number of H-bond interactions are found only in selected cases. Based on comparisons of association constants for (bis) pyridine, bipyridine and phenanthroline ligand-containing species with dibenzo crowns, evidence is also found for favorable polypyridine/benzene interactions. NMR (NOE) measurements indicate that the preferred association geometrics in solution are those that make each of the benzenes of the crown coplanar (or nearly coplanar) with the ligated polypyridine.