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The imidazolium trans-tetrachloro(dimethylsulfoxide)imidazoleruthenate(III) complex [ImH][Ru(III)Cl(4)(DMSO)(Im)], NAMI-A, has shown an interesting antimetastatic activity. Since Ru(III) complexes are coordinatively more inert than the corresponding Ru(II) derivatives, an "activation by reduction" mechanism has been proposed to explain the biological activity of NAMI-A, thus acting as a pro-drug. We report here an electrochemical study on NAMI-A in aqueous solutions which emphasizes the structural and chemical consequences accompanying the easy Ru(III)/Ru(II) electron transfer (e.g., axial imidazole/water exchange in acidic solution in the short timescale of cyclic voltammetry followed by equatorial chloride/water exchange in the longer timescale of macroelectrolysis).

Deuterium-labeling experiments on the sequential reactions of the previously reported electron-deficient complexes Os3(CO)9(μ3-η2-C9H4NRR‘)(μ-H)(R = R‘ = H, 1a; R = 4-Me, R‘ = H, 1b; R = H, R‘ = 6-CH3, 1c) with X-/X+ (X = H or D) reveal that initial attack of H- is at the 5-position of the quinoline ring and that the reduction of the C(5)−C(6) double bond to yield Os3(CO)9(μ3-η3-C9H6RR‘N)(μ-H) (2a − c) is not stereoselective. Related experiments with 2a − c reveal that hydride attack at the 7-position is followed by protonation at the metal core to yield Os3(CO)9(μ3-η2-C9H7RR‘N)(μ-H)2 (3a − c). The conversion of 2a to 3a is also achieved by reaction with H2 at 75 °C and 100 psi. When this reaction is carried out with excess D2, deuterium incorporation is observed at C(7) and at the metal core, suggesting a concerted, irreversible hydrogen addition or a radical chain reaction. The related 46-electron cluster Os3(CO)9(μ3-η2-C9H8N)(μ-H) (5) containing a CN bond in a partially reduced heterocyclic ring, as well as the three-center two-electron bond at C(8), undergoes H- attack at C(2) and not at C(5), as for 1a − c, followed by protonation at the metal core to yield Os3(CO)9(μ3-η2-C9H9N)(μ-H)2 (4). Photolysis or thermolysis of the previously reported Os3(CO)9(μ-η2-(4-Me)C9H5N)(μ-H)(P(OEt)3) (6b) does not yield the phosphite-substituted 46-electron clusters related to 1a − c but leads only to nonspecific decomposition. Partially selective incorporation of 13CO into 1a − c is observed to yield the corresponding decacarbonyl derivatives, and the pattern of 13CO incorporation helps to elucidate the interconversion of the nona- and decacarbonyl derivatives. The electrochemical behavior of 1a, the dynamical behavior of 2b, and the solid-state structures of 2b, 3a, 5, and 6b are reported.

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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.