Influence of Terpyridine as π-Acceptor Ligand on the Kinetics and Mechanism of the Reaction of NO with Ruthenium(III) Complexes

Abstract

The kinetics of the unusually fast reaction of cis- and trans-[Ru(terpy)(NH3)2Cl]2+ (with respect to NH3; terpy=2,2':6',2"-terpyridine) with NO was studied in acidic aqueous solution. The multistep reaction pathway observed for both isomers includes a rapid and reversible formation of an intermediate Ru(III)-NO complex in the first reaction step, for which the rate and activation parameters are in good agreement with an associative substitution behavior of the Ru(III) center (cis isomer, k1=618 +/- 2 M(-1) s(-1), DeltaH(++) = 38 +/- 3 kJ mol(-1), DeltaS(++) = -63 +/- 8 J K(-1) mol(-1), DeltaV(++) = -17.5 +/- 0.8 cm3 mol(-1); k -1 = 0.097 +/- 0.001 s(-1), DeltaH(++) = 27 +/- 8 kJ mol(-1), DeltaS(++) = -173 +/- 28 J K(-1) mol(-1), DeltaV(++) = -17.6 +/- 0.5 cm3 mol(-1); trans isomer, k1 = 1637 +/- 11 M(-1) s(-1), DeltaH(++) = 34 +/- 3 kJ mol(-1), DeltaS(++) = -69 +/-11 J K(-1) mol(-1), DeltaV(++) = -20 +/- 2 cm3 mol(-1); k(-1)=0.47 +/- 0.08 s(-1), DeltaH(++)=39 +/- 5 kJ mol(-1), DeltaS(++) = -121 +/-18 J K(-1) mol(-1), DeltaV(++) = -18.5 +/- 0.4 cm3 mol(-1) at 25 degrees C). The subsequent electron transfer step to form Ru(II)-NO+ occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis isomer the reduction of the Ru(III) center is induced by the coordination of an additional NO molecule (cis isomer, k2=51.3 +/- 0.3 M(-1) s(-1), DeltaH(++) = 46 +/- 2 kJ mol(-1), DeltaS(++) = -69 +/- 5 J K(-1) mol(-1), DeltaV(++) = -22.6 +/- 0.2 cm3 mol(-1) at 45 degrees C). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms of a dissociative substitution mechanism (cis isomer, DeltaV(++) = +23.5 +/- 1.2 cm3 mol(-1); trans isomer, DeltaV(++) = +20.9 +/- 0.4 cm3 mol(-1) at 55 degrees C) that produces two different reaction products, viz. [Ru(terpy)(NH3)(H2O)NO]3+ (product of the cis isomer) and trans-[Ru(terpy)(NH3)2(H2O)]2+. The pi-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern.

Three Redox States of Metallonitrosyls in Aqueous Solution

“…Timely work from van Eldik's group dealt with the reaction of NO with the cis-and trans-isomers of [Ru III (trpy)(NH 3 ) 2 (Cl)] 2+ (87). The mechanistic picture was complex, with faster reactions than for the related Ru IIIpentammine systems, and consistent with the more oxidant character induced by the highly acceptor trpy ligand.…”

Synthesis, characterization and cytotoxic activities of the [RuCl2(NO)(dppp)(L)]PF6 complexes

“…pÃNO) [50]. These bands are not observed in the precursor and are prob- (1) (2) ably obscured by the intraligands processes. Using higher concentrations for the measurements, it was possible to observe a band in the visible region, around 440 nm, with e values < 200 M À1 cm À1 , also present in the precursor.…”

“…Until now, most progress in this area has been achieved from studies using transition metal complexes. Ruthenium complexes are by far the most studied and their relation with nitric oxide acting as scavengers [2][3][4][5][6][7] or as NO releasers has been well explored in the literatures [8][9][10][11][12]. It is worth to point out that ruthenium compounds normally exhibit fewer general toxicity problems than other metal-based drugs [13], as documented by clinical experiences with a sort of ruthenium complexes [14][15][16][17][18] including some containing coordinated nitric oxide [10].…”

Reactivity of Inorganic Radicals in Aqueous Solution

“…169 The reactions of NO with cis-and trans-[Ru III (terpy)(NH 3 ) 2 Cl] 2 þ are multistep processes, and the first step is the reversible displacement of Cl À by NO; surprisingly, it is argued that the second step is the conversion of Ru III --NO to Ru II --NO þ . 172 In further steps, the Ru(terpy) complexes release nitrite and ammonia. NO binds rapidly and strongly to various Ru III polyaminocarboxylates such as Ru(EDTA).…”