Trans- and Cis-Water Reactivities in d⁶ Octahedral Ruthenium(II) Pentaaqua Complexes:  Experimental and Density Functional Theory Studies^1,2

Abstract

The hexaaqua complex of ruthenium(II) represents an ideal starting material for the synthesis of isostructural compounds with a [Ru(H(2)O-ax)(H(2)O-eq)(4)L](2+) general formula. We have studied a series of complexes, where L = H(2)O, MeCN, Me(2)SO, H(2)C=CH(2), CO, and F(2)C=CH(2). 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 degrees '(+2/+3), increases with the pi-accepting capabilities of the ligands. For L = N(2), 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)-N(2) bond. The water exchange reactions have been followed by (17)O 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(H(2)O)(5)(H(2)C=CH(2))](2+) (DeltaV(ax)() and DeltaV(eq)() = +6.5 +/- 0.5 and +6.1 +/- 0.2 cm(3) 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: F(2)C=CH(2) congruent with CO < Me(2)SO < N(2) < H(2)C=CH(2) < MeCN < H(2)O. The increase of the lability is accompanied by a decrease of the E degrees ' values, but no change was found in the calculated Ru-H(2)O(eq) bond lengths. The increasing trans-effect series, established from the lability of the axial water molecule, is the following: N(2) << MeCN < H(2)O < CO < Me(2)SO < H(2)C=CH(2) < F(2)C=CH(2). A variation of the Ru-H(2)O(ax) bond lengths is observed in the calculated structures. However, the best correlation is found between the lability and the calculated Ru-H(2)O(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 pi-accepting capability of the ligand. For L = N(2), the calculations have shown that the Ru(II)-N(2) bond is weak. Consequently, the water exchange reaction proceeds through a different mechanism, where first the N(2) 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(H(2)O)(5)N(2)](2+) complex.

Improvement in radiolabelling proteins with the99mTc-tricarbonyl-core [99mTc(CO)3]+, by thiol-derivatization with iminothiolane: application to γ-globulins and annexin V

“…Alberto Ligand exchange is expected to be based on dissociative or interchange dissociative mechanism of the more labile water ligands, with a bifunctional ligand attachable to biomolecules. 3 The replacement of one water molecule by a monodentate ligand provides good stability but the use of bi-or tridentate ligands improves it. 4 It has been shown that various N-containing ligands, such as histidine, histamine, imidazole and Schiff bases are able to react with the organometallic aquaion [Tc(H 2 O) 3 (CO) 3 ] + to give stable complexes.…”

“…3 The replacement of one water molecule by a monodentate ligand provides good stability but the use of bi-or tridentate ligands improves it. 4 It has been shown that various N-containing ligands, such as histidine, histamine, imidazole and Schiff bases are able to react with the organometallic aquaion [Tc(H 2 O) 3 (CO) 3 ] + to give stable complexes. 5,6 Investigations by Egli et al 7 showing that histidine was the most potent ligand among other amino acids, have led to the direct radiolabelling of peptides or proteins.…”

Aspects of Aqueous Ruthenium(II) Chemistry

“…Table 2). Therefore, all ligands produce a stronger electron-withdrawing effect than H 2 O and, consequently, increase the redox potential of the metal center [11]. Chemical shift for axial and equatorial H 2 O molecules are clearly contrasted and span from Results obtained by DFT calculations allow us to better understand these results.…”

“…Therefore, geometric, kinetic, and electronic parameters are strongly correlated and allow predictions about the reactivity of complexes that are, at the moment, not fully characterized. [11].…”

Application of High Pressure in the Elucidation of Inorganic and Bioinorganic Reaction Mechanisms

“…The volumes of activation increase from þ 0.4 cm 3 /mol for the solvento cation to þ 2.4 and þ 11.1 cm 3 /mol, respectively, for the arene and cyclopentadiene species; increases that could be interpreted as interchange, interchange-dissociative, and dissociative mechanisms. The kinetic effect of replacing one or more solvent molecule by p-arene ligands or other organic ligands on solvent exchange of remaining coordinated solvent has more recently been addressed for among others mixed p-arene, bpy compounds of Ru(II), 211 and in addition for monocarbonyl-pentaaqua, 212 DMSO-pentaaqua, and substituted alkene-pentaaqua complexes 213 of Ru(II). In each case, the rate constant for water exchange on both the equatorial and axial positions could be extracted from the data.…”