Luminescent Piano-Stool Complexes Incorporating 1-(4-Cyanophenyl)imidazole: Synthesis, Spectral, and Structural Studies

Three novel luminescent piano-stool arene ruthenium complexes of general formula [(η⁶-arene)RuCl₂(CPI)] (η⁶-arene = benzene, 1, p-cymene, 2, and hexamethylbenzene, 3; CPI=1-(4-cyanophenyl)imidazole were prepared. The molecular structures of 2 and 3 were determined crystallographically. Reaction of 1−3 with EPh₃ (E = P, As, or Sb) and N−N donor bases such as 2,2‘-bipyridine and 1,10-phenanthroline afforded cationic mononuclear complexes of general formula [(η⁶-arene)RuCl(CPI)(EPh₃)]⁺ (η⁶-arene = C₆H₆, E = P (1a), E = As (1b), E = Sb (1c); η⁶-arene = C₁₀H₁₄, E = P (2a), E = As (2b), E = Sb (2c); η⁶-arene = C₆Me₆, E = P (3a), E = As (3b), E = Sb (3c)) and [(η⁶-arene)Ru(N−N)(CPI)]²⁺ (η⁶-arene = C₆H₆, N−N = bipy (1d), N−N = phen (1e); η⁶-arene = C₁₀H₁₄, N−N = bipy (2d), N−N = phen (2e); η⁶-arene = C₆Me₆, N−N = bipy (3d), N−N = phen (3e)). Molecular structures of 1a and 2a were also confirmed by X-ray crystallography. Structural studies of the complexes 2, 3, 1a, and 2a supported coordination of CPI through the imidazole nitrogen and the presence of a pendant nitrile group. Structural data also revealed stabilization of crystal packing in the complexes 2, 3, and 2a by C−H···X (X = Cl, F) type inter- and intramolecular interactions and in complex 1a by π−π stacking. Moreover, neutral homonuclear bimetallic complexes 2f,g were prepared by using complex 2 as a metallo-ligand, where CPI acts as a bridge between two metal centers. Emission spectra of the mononuclear complexes [(η⁶-arene)RuCl₂(CPI)] and its derivatives exhibited intense luminescence when excited in the metal to ligand charge-transfer band.