Halogen Bonded Supramolecular Assemblies of [Ru(bipy)(CN)4]2- Anions and N-Methyl-Halopyridinium Cations in the Solid State and in Solution

The interactions between the [Ru(bipy)(CN)(4)](2-) anion and N-methyl-halopyridinium cations have been examined in both the solid state and in solution. In the solid state, crystal structures of [Ru(bipy)(CN)(4)](2-) salts containing iodinated cations (N-methyl-3-iodopyridinium and N-methyl-3,5-diiodopyridinium) show clear C-I center dot center dot center dot NC(Ru) halogen bonds between the externally directed cyanide lone pairs of the anion and the iodine atoms of the cation which dominates the structures. In contrast the analogous brominated cations (N-methyl-3-bromopyridinium and N-methyl-3,5-dibromopyridinium) do not exhibit C-Br center dot center dot center dot NC(Ru) interactions in the solid state, with the cyanide groups instead involved in hydrogen bonding, principally to lattice water molecules. The charge-assisted C-I center dot center dot center dot NC(Ru) interactions are therefore clearly of value as synthons in crystal engineering applications. In CH2Cl2 solution, spectroscopic titrations between [Ru(4,4'-Bu-t(2)-bipy)(CN)(4)](2-) and both N-methyl-3-iodopyridinium and N-methyl-3-bromopyridinium cations show clear evidence for formation of distinct 1:1, 3:2, and then 2:1 cation/anion adducts with high association constants (>10(7) M-1 for the first 1:1 association constant). However the presence of identical results using the non-halogenated cation N-methyl-pyridinium indicates that this strong cation/anion association in CH2Cl2 is dominated by electrostatic effects: either C-H center dot center dot center dot NC(Ru) hydrogen bonds or C-X center dot center dot center dot NC(Ru) halogen bonds could be involved in the ion pairs but it is the charge-assistance that makes the association strong. This is confirmed by a titration between [Ru(4,4'-Bu-t(2)-bipy)(CN)(4)](2-) and the neutral halogen-bond acceptor C6F5I for which the first association constant is very low (ca. 6 M-1). The formation of adducts between [Ru(4,4'-Bu-t(2)-bipy)(CN)(4)](2-) and the various N-methyl-pyridinium cations in solution results in a clear blue-shift of the (MLCT)-M-1 absorption maxima associated with the Ru(II) unit, a characteristic consequence of interaction of the cyanide lone pairs with a Lewis-acidic site on the cation. The (MLCT)-M-3 luminescence from the [Ru(4,4'-Bu-t(2)-bipy)(CN)(4)](2-) center, however, does not show the usual associated increase in intensity associated with this blue shift in the (MLCT)-M-1 absorptions, most likely because of electron-transfer quenching by the N-methyl-pyridinium cations in the assemblies.