Reduction of Ru^VI≡N to Ru^III—NH₃ by Cysteine in Aqueous Solution

The reduction of metal nitride to ammonia is a key step in biological and chemical nitrogen fixation. We report herein the facile reduction of a ruthenium(VI) nitrido complex [(L)Ru^VI(N)(OH₂)]⁺ (1, L = N,N′-bis(salicylidene)-o-cyclohexyldiamine dianion) to [(L)Ru^III(NH₃)(OH₂)]⁺ by _L-cysteine (Cys), an ubiquitous biological reductant, in aqueous solution. At pH 1.0-5.3, the reaction has the following stoichiometry: [(L)Ru^VI(N)(OH₂)]⁺ + 3HSCH₂CH(NH₃)CO₂ → [(L)Ru^III(NH₃)(OH₂)]⁺ + 1.5(SCH₂CH(NH₃)CO₂)₂. Kinetic studies show that at pH 1 the reaction consists of two phases, while at pH 5 there are three distinct phases. For all phases the rate law is rate = k₂[1][Cys]. Studies on the effects of acidity indicate that both HSCH₂CH(NH₃⁺)CO₂ ^- and ^-SCH₂CH(NH₃ ⁺)CO₂ ^- are kinetically active species. At pH 1, the reaction is proposed to go through [(L)-Ru^IV(NHSCH₂CHNH₃CO₂H)(OH₂)]²⁺ (2a), [(L)Ru^III(NH₂SCH₂CHNH₃CO₂H)(OH₂)]²⁺ (3), and [(L)Ru^IV(NH₂)(OH₂)]⁺ (4) intermediates. On the other hand, at pH around 5, the proposed intermediates are [(L)Ru^IV(NHSCH₂CHNH₃CO₂)-(OH₂)]⁺ (2b) and [(L)Ru^IV(NH₂)(OH₂)]⁺ (4). The intermediate ruthenium(IV) sulfilamido species, [(L)-Ru^IV(NHSCH₂CHNH₃CO₂H)(OH₂)]²⁺ (2a) and the final ruthenium(III) ammine species, [(L)Ru^III(NH₃)(MeOH)]⁺ (5) (where H₂O was replaced by MeOH) have been isolated and characterized by various spectroscopic methods.