Formation and structural chemistry of the unusual cyanide-bridged dinuclear species [Ru2(NN)2(CN)7]3− (NN = 2,2′-bipyridine or 1,10-phenanthroline)

Crystallisation of simple cyanoruthenate complex anions [Ru(NN)(CN)4]2− (NN = 2,2′-bipyridine or 1,10-phenanthroline) in the presence of Lewis-acidic cations such as Ln(III) or guanidinium cations results, in addition to the expected [Ru(NN)(CN)4]2− salts, in the formation of small amounts of salts of the dinuclear species [Ru2(NN)2(CN)7]3−. These cyanide-bridged anions have arisen from the combination of two monomer units [Ru(NN)(CN)4]2− following the loss of one cyanide, presumably as HCN. The crystal structures of [Nd(H2O)5.5][Ru2(bipy)2(CN)7] · 11H2O and [Pr(H2O)6][Ru2(phen)2(CN)7] · 9H2O show that the cyanoruthenate anions form Ru–CN–Ln bridges to the Ln(III) cations, resulting in infinite coordination polymers consisting of fused Ru2Ln2(μ-CN)4 squares and Ru4Ln2(μ-CN)6 hexagons, which alternate to form a one-dimensional chain. In [CH6N3]3[Ru2(bipy)2(CN)7] · 2H2O in contrast the discrete complex anions are involved in an extensive network of hydrogen-bonding involving terminal cyanide ligands, water molecules, and guanidinium cations. In the [Ru2(NN)2(CN)7]3− anions themselves the two NN ligands are approximately eclipsed, lying on the same side of the central Ru–CN–Ru axis, such that their peripheries are in close contact. Consequently, when NN = 4,4′-tBu2-2,2′-bipyridine the steric bulk of the t-butyl groups prevents the formation of the dinuclear anions, and the only product is the simple salt of the monomer, [CH6N3]2[Ru(tBu2bipy)(CN)4] · 2H2O. We demonstrated by electrospray mass spectrometry that the dinuclear by-product [Ru2(phen)2(CN)7]3− could be formed in significant amounts during the synthesis of monomeric [Ru(phen)(CN)4]2− if the reaction time was too long or the medium too acidic. In the solid state the luminescence properties of [Ru2(bipy)2(CN)7]3− (as its guanidinium salt) are comparable to those of monomeric [Ru(bipy)(CN)4]2−, with a 3MLCT emission at 581 nm.