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···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···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···NC(Ru) interactions are therefore clearly of value as synthons in crystal engineering applications. In CH₂Cl₂ solution, spectroscopic titrations between [Ru(4,4′-^tBu₂-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 (>10⁷ 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···NC(Ru) hydrogen bonds or C−X···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′-^tBu₂-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′-^tBu₂-bipy)(CN)₄]²⁻ and the various N-methyl-pyridinium cations in solution results in a clear blue-shift of the ¹MLCT 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 ³MLCT luminescence from the [Ru(4,4′-^tBu₂-bipy)(CN)₄]²⁻ center, however, does not show the usual associated increase in intensity associated with this blue shift in the ¹MLCT absorptions, most likely because of electron-transfer quenching by the N-methyl-pyridinium cations in the assemblies.