MOLECULAR RECOGNITION AND REACTIVITY OF RUTHENIUM(II) BIPYRIDINE BARBITURIC-ACID GUESTS IN THE PRESENCE OF COMPLEMENTARY HOSTS - RUTHENIUM(II) PROMOTED ENOLIZATION OF BARBITURIC-ACIDS IN GUEST-HOST COMPLEXES

The binding of the host H1 6-pivalamidopyrid-2-yl)-3,5-pyridinedicarbo xamide) to three different ruthenium polypyridine complexes with an at tached barbituric acid and barbital moieties (RuG1, RuG2, RuG3) (where G1 = -bipyridylidene]-2,4,6-(1H,3H,5H)-pyrimidinetrione G2 = ipyridyl ]methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione, and G3 = 5-ethyl, 5-[4-(4'- methyl)-2,2'-bipyridyl] methyl-2,4,6-(1H,3H,5H)-pyrimidinetrione) and Ru = (4,4'-di-tert-butyl-bpy)(2)Ru (bpy = 2,2'-bipyridine) has been st udied in chlorinated solvents by NMR and fluorescence titrations. Sign ificant binding was only observed between H1 and the RuG2 series, whil e steric hindrance significantly diminished binding between H1 and RuG 1 or RuG3. The high binding constant for RuG2 was related to the prese nce of the enolate form of the barbituric acid guest which forms stron g H-bonds with the complementary host H1. For the organic barbituric a cid and barbital guests, the keto and enol bind only weakly to H1 (K s imilar to 10(2) M-1); binding is further increased in the presence of base to generate the enolate. In contrast, formation of the RuG2 enola te occurs upon binding to H1 without any additional base. The rutheniu m polypyridine cation (compared to the organic barbituric acid derivat ives) facilitates ionization of the enol to enolate thus producing a b etter complementary H-bonding site between the guest and host. Molecul ar mechanics calculations confirmed the experimental observatons that the enolate has the highest binding constant to the Host H1, while the corresponding enol form has the weakest binding.