Synthesis and Isomeric Analysis of Ru^II Complexes Bearing Pentadentate Scaffolds

Abstract

A Ru^II-pentadentate polypyridyl complex [Ru^II(κ-N⁵-bpy2PYMe)Cl]⁺ (1⁺, bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [Ru^II(κ-N⁵-bpy2PYMe)(H₂O)]²⁺ (2²⁺) were synthesized and characterized by experimental and computational methods. In MeOH, 1⁺ exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, trans-1²⁺ generates a [Ru^III(κ-N⁴-bpy2PYMe)Cl₂]⁺ intermediate after the exchange of a pyridyl ligand by a Cl^- counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru(II). On the contrary, cis-1²⁺ is in direct equilibrium with trans-1²⁺, with absence of the κ-N⁴-bis-chlorido Ru^III-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[Ru^II(κ-N⁵-bpy2PYMe)(H₂O)]²⁺ isomer (trans-2²⁺), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [Ru^II(κ-N⁵-bpy(bpyMe)PYMe)Cl]⁺ (3⁺) and [Ru^II(κ-N⁵-bpy(bpyMe)PYMe)(H₂O)]²⁺ (4²⁺) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [Ru^II(PY5Me₂)X]ⁿ⁺ system (X = Cl, n = 1 (5⁺); X = H₂O, n = 2 (6²⁺)), which also contains a κ-N⁵-Ru^II coordination environment, and to the newly synthesized [Ru^II(PY4Im)X]ⁿ⁺ complexes (X = Cl, n = 1 (7⁺); X = H₂O, n = 2 (8²⁺)), which possess an electron-rich κ-N⁴C-Ru^II site due to the replacement of a pyridyl group by an imidazolic carbene.