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

A Ru ^II -pentadentate polypyridyl complex [Ru ^II (κ-N ⁵ -bpy2PYMe)Cl] ⁺ (1 ⁺ , bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [Ru ^II (κ-N ⁵ -bpy2PYMe)(H ₂ O)] ²⁺ (2 ²⁺ ) 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-1 ²⁺ generates a [Ru ^III (κ-N ⁴ -bpy2PYMe)Cl ₂ ] ⁺ 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-1 ²⁺ is in direct equilibrium with trans-1 ²⁺ , with absence of the κ-N ⁴ -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-N ⁵ -bpy2PYMe)(H ₂ O)] ²⁺ isomer (trans-2 ²⁺ ), 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-N ⁵ -bpy(bpyMe)PYMe)Cl] ⁺ (3 ⁺ ) and [Ru ^II (κ-N ⁵ -bpy(bpyMe)PYMe)(H ₂ O)] ²⁺ (4 ²⁺ ) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [Ru ^II (PY5Me ₂ )X] ⁿ⁺ system (X = Cl, n = 1 (5 ⁺ ); X = H ₂ O, n = 2 (6 ²⁺ )), which also contains a κ-N ⁵ -Ru ^II coordination environment, and to the newly synthesized [Ru ^II (PY4Im)X] ⁿ⁺ complexes (X = Cl, n = 1 (7 ⁺ ); X = H ₂ O, n = 2 (8 ²⁺ )), which possess an electron-rich κ-N ⁴ C-Ru ^II site due to the replacement of a pyridyl group by an imidazolic carbene.