Derivatives of luminescent metal-polypyridyl complexes with pendant adenine or thymine groups: Building blocks for supramolecular assemblies based on hydrogen bonding

Alkylation of adenine or thymine with 5-bromomethyl-2,2'-bipyridine afforded bipya and bipyt, in which a 2,2'-bipyridyl (bipy) is attached to the N-9 position of adenine or the N-1 position of thymine via a CH2 spacer. Attachment of the bipy site of bipya to [Ru(bipy)(2)Cl-2] or [Ru(dbbipy)(2)Cl-2] [dbbipy = 4,4'-bis(tert-butyl)-2,2'-bipyridine] gave the complexes [Ru(bipy)(2)(bipya)] [PF6](2) and [Ru(dbbipy)(2)(bipya)][PF6](2) (Ru-Ade) respectively, in which an adenine fragment is pendant from the {Ru(bipy)(3)}(2+) core. Attachment of the bipy site of bipyt to [Os(dbbipy)(2)Cl-2] and [Re(CO)(5)Cl] afforded [Os(dbbipy)(bipyt)][PF6](2) (Os-Thy) and [Re(bipyt)(CO)(3)Cl] (Re-Thy) respectively, in which the {Os(bipy)(3)}(2+) and {Re(bipy)(CO)(3)Cl} cores have pendant thymine groups. Recrystallisation of [Ru(bipy)(2)(bipya)][PF6](2) from wet MeCN resulted in partial protonation to give [{Ru(bipy)(2)(bipya)} {Ru(bipy)(2)(Hbipya)}][PF6](5) . 4MeCN in which [Ru(bipy)(2)(bipya)](2+) and protonated [Ru(bipy)(2)(Hbipya)](3+) complex cations are associated by a Watson-Crick type hydrogen-bonding interaction between the adenine groups across an inversion centre. Similarly, in [Os(dbbipy)(2)(bipyt)][PF6](2) . Me(2)CO there are two [Os(dbbipy)(2)(bipyt)](2+) complex cations associated via a centrosymmetric thymine-thymine hydrogen-bonding interaction across an inversion centre. In contrast, in [Ru(dbbipy)(2)(bipya)][PF6](2) . 2MeCN the [Ru(dbbipy)(2)(bipya)](2+) complex cations are associated via a Hoogsteen-type hydrogen-bonding interaction to give a one-dimensional 'ribbon-like' chain of hydrogen bonds. The electrochemical, UV/VIS spectroscopic and luminescence properties of the complexes are very similar to those of the parent unsubstituted complexes, indicating that the adenine or thymine substituents do not perturb the desirable properties of the complex cores. By monitoring the chemical shift of the thymine NH proton, NMR titrations allowed estimation of the association constants of the complementary Ru-Ade/Os-Thy pair in CD3CN and CD2Cl2 as 60 and 123 dm(3) mol(-1) respectively, and that of the Ru-Ade/Re-Thy pair in CD3CN as 17.9 dm(3) mol(-1). At the very low concentrations used for luminescence studies, these association constants are much too low to allow significant formation of hydrogen-bonded associates in mixtures of complementary complexes such as Ru-Ade/Os-Thy and Ru-Ade/Re-Thy. The requirements for observing energy-transfer across hydrogen-bonded bridges in associates of this type are discussed.