New members of the Ru(diimine)(CN)₄^2- family: Structural, electrochemical and photophysical properties

A series of complexes of the type K₂Ru(NN)(CN)₄ has been prepared, in which NN is a diimine ligand, and were investigated for both their structural and photophysical properties. The ligands used (and the abbreviations for the resulting complexes) are 3-(2-pyridyl)pyrazole (Ru-pypz), 2,2′-bipyrimidine (Ru-bpym), 5,5′-dimethyl-2,2′-bipyridine (Ru-dmb), 1-ethyl-2-(2-pyridyl)benzimidazole (Ru-pbe), bidentate 2,2′:6′,2‴-terpyridine (Ru-tpy). The known complexes with NN = 2,2′-bipyridine (Ru-bpy) and 1,10-phenathroline (Ru-phen) were also included in this work. A series of crystallographic studies showed that the Ru(NN)(CN)₄^2- complex anions form a range of elaborate coordination networks when crystallised with either K⁺ or Ln ³⁺ cations. The K⁺ salts are characterised by a combination of near-linear Ru-CN-K bridges, with the cyanides coordinating to K⁺ in the usual 'end-on' mode, and unusual side-on π-type coordination of cyanide ligands to K⁺ ions. With Ln³⁺ cations in contrast only Ru-CN-Ln near-linear bridges occurred, affording 1-dimensional helical or diamondoid chains, and 2-dimensional sheets constituted from linked metallamacrocyclic rings. All of the K₂Ru(NN)(CN) ₄ complexes show a reversible Ru(ii)/Ru(iii) couple (ca. +0.9 V vs. Ag/AgCl in water), the exception being Ru-tpy whose oxidation is completely irreversible. Luminescence studies in water showed the presence of ³MLCT-based emission in all cases apart from Ru-bpym with lifetimes of tens/hundreds of nanoseconds. Time-resolved infrared studies showed that in the ³MLCT excited state the principal C-N stretching vibration shifts to positive energy by ca. 50 cm^-1 as a consequence of the transient oxidation of the metal centre to Ru(iii) and the reduction in back-bonding to the cyanide ligands; measurement of transient decay rates allowed measurements of ³MLCT lifetimes for those complexes which could not be characterised by luminescence spectroscopy. A few complexes were also examined in different solvents (MeCN, dmf) and showed much weaker emission and shorter excited-state lifetimes in these solvents compared to water.