Synthesis, structure, and electrochemical characterization of a mixed-ligand diruthenium(iii,ii) complex with an unusual arrangement of the bridging ligands

Abstract

A mixed-ligand metal–metal bonded diruthenium complex having the formula Ru₂(2,4,6-(CH₃)₃ap)₃(O₂CCH₃)Cl where ap is the anilinopyridinate anion was synthesized from the reaction of Ru₂(O₂CCH₃)₄Cl and H(2,4,6-(CH₃)₃ap), after which the isolated product was structurally, spectroscopically and electrochemically characterized. The crystal structure reveals an unusual arrangement of the bridging ligands around the dimetal unit where one ruthenium atom is coordinated to one anilino and two pyridyl nitrogen atoms while the other ruthenium atom is coordinated to one pyridyl and two anilino nitrogen atoms. To our knowledge, Ru₂(2,4,6-(CH₃)₃ap)₃(O₂CCH₃)Cl is the only example of a mixed-ligand diruthenium complex of the type [Ru₂L₃(O₂CCH₃)]⁺, where L is an unsymmetrical anionic bridging ligand that has been structurally characterized with a “(2,1)” geometric conformation of the bridging ligands, all others being “(3,0)”. The initial Ru₂⁵⁺ compound in CH₂Cl₂ or CH₃CN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) undergoes up to four one-electron redox processes involving the dimetal unit. The Ru₂^5+/4+ and Ru₂^5+/6+ processes were characterized under N₂ using thin-layer UV-visible spectroelectrochemistry and this data is compared to UV-visible spectral changes obtained during similar electrode reactions for related diruthenium compounds having the formula Ru₂L₄Cl or Ru₂L₃(O₂CCH₃)Cl where L is an anionic bridging ligand. Ru₂(2,4,6-(CH₃)₃ap)₃(O₂CCH₃)Cl was also examined by UV-visible and FTIR spectroelectrochemistry under a CO atmosphere and two singly reduced Ru₂⁴⁺ species, [Ru₂(2,4,6-(CH₃)₃ap)₃(O₂CCH₃)(CO)Cl]⁻ and Ru₂(2,4,6-(CH₃)₃ap)₃(O₂CCH₃)(CO) were in situ generated for further characterization. The CO-bound complexes could be further reduced and exhibited additional reductions to their Ru₂³⁺ and Ru₂²⁺ oxidation states.