Trans- and Cis-Water Reactivities in d⁶ Octahedral Ruthenium(II) Pentaaqua Complexes: Experimental and Density Functional Theory Studies

The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H₂O-ax)(H₂O-eq)₄L]²⁺ general formula. We have studied a series of complexes, where L = H₂O, MeCN, Me₂SO, H₂C=CH₂, CO, and F₂C=CH₂. We have evaluated the effect of L on the cyclic voltammetric response, on the rate and mechanism of exchange reaction of the water molecules, and on the structures calculated with the density functional theory (DFT). As expected, the formal redox potential, E°′(+2/+3), increases with the π-accepting capabilities of the ligands. For L = N₂, the oxidation to Ru(III) is followed by a fast substitution of dinitrogen by a solvent molecule, revealing the poor stability of the Ru(III)-N₂ bond. The water exchange reactions have been followed by ¹⁷O NMR spectroscopy. The variable-pressure and variable-temperature kinetic studies made on selected examples are all in accordance with a dissociative activation mode for exchange. The positive activation volumes obtained for the axial and equatorial water exchange reactions on [Ru(H₂O)₅(H₂C=CH₂)]²⁺ (ΔV_ax^‡ and ΔV_eq^‡ = +6.5 ± 0.5 and +6.1 ± 0.2 cm³ mol^-1) are the strongest evidence of this conclusion. The increasing cis-effect series was established according to the lability of the equatorial water molecules and is as follows: F₂C=CH₂ ≅ CO < Me₂SO < N₂ < H₂C=CH₂ < MeCN < H₂O. The increase of the lability is accompanied by a decrease of the E°′ values, but no change was found in the calculated Ru-H₂O_eq bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N₂ ≪ MeCN < H₂O < CO < Me₂SO < H₂C=CH₂ < F₂C=CH₂. A variation of the Ru-H₂O_ax bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru-H₂O_ax bond energies. It appears, also, that a decrease of the electronic density along the Ru-O_ax bond and the increase of the lability can be related to an increase of the π-accepting capability of the ligand. For L = N₂, the calculations have shown that the Ru(II)-N₂ bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N2 ligand is substituted by one water molecule to produce the hexaaqua complex of Ru(II). The water exchange takes place on this compound before re-formation of the [Ru(H₂O)₅N₂]²⁺ complex.