[Ru2II(ttha)(H2O)2]2− is a rapid No scavenger (ttha6− = triethylenetetraminehexaacetate)

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Abstract

The reaction of NO(aq.) with [Ru2II (ttha)(H2O)2]2− = (A) and [Ru2II(ttha) (bpy) (H2O)]2− = (B), (ttha6− = triethylenetetraminehexaacetate; bpy = 2.2′-bipyridine) was monitored by electrochemical methods (cyclic voltammetry, differential pulse polarography). Each of two sites of [Ru2II(ttha)] may be represented by [LRu] for convenience. Waves for the [LRuII (NO·)] → −1e−[LRuII (NO+)], 2[LRuII (NO2)] → −2e−[LRuIII(NO3)] + [LRuIII (NO+)], and [LRuII (NO+)] → −1e−[LRuIII(NO+)] oxidations were identified at −0.06, +0.98, and 1.14 V vs normal hydrogen electrode, respectively. The protonated form of [LRuII (NO·)], [LRuII(NOH)], was detected as a separate RuII/III wave at +0.10 V. The pKa of [LRuII (NOH)] is 1.80. The rate of substitution of NO on (A) is 22.7 M−1s−1 at 22°C indicating a normal rate of neutral ligand substitution on RuII-polyaminopolycarboxylates via dissociative intermediates. The identical waves for the nitrosylated (B) indicate that the nitrosyls of (A), [(RuII(NO)·))2(ttha)]2− = (C), are terminally coordinated rather than a single bridged nitrosyl. The nitrosyl (C) does not react with H2, precluding a catalytic scavenging of NO by (A) followed by H2 reduction for environmental control purposes. However, the nitrosyl (C) is robust and dissociates very slowly under Ar purging. Thus the parent complex [Ru2(ttha)(H2O)2]2− and related mononuclear RuII-polyaminopolycarboxylates such as [Ru(hedta)(H2O)] have several features that lend them toward uses as antisepsis agents for the control of septic shock. [Ru2III(ttha) (H2O)2] also reacts directly with NO, forming [(RuII(NO+))2(ttha)] which exhibits the same waves as [Ru2II(ttha)(NO)2]2− since the RuII(NO+) units readily reduce electrochemically at glassy carbon to the RuII(NO·) complex (C) below −0.06 V. It was observed that the [LRuIII(NO+)] catalyzes the electrochemical oxidation of NO to NO+ and, hence HNO2 at 1.14 V at glassy carbon.

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