Reaction Chemistry of the syn-[Mo₂O₂(μ-S)₂(S₂)(DMF)₃] Complex with Cyanide and Catalytic Thiocyanate Formation

Abstract

Removal of cyanide as nontoxic thiocyanate under physiological conditions may serve as a catalytic detoxification route in vivo. Aqueous catalytic reaction conditions were explored where at the conditions employed the reaction proceeded to exhaustion in 1 h. The complex, syn-[Mo 2 O 2 (μ-S) 2 (S 2 )(DMF) 3 ] 1, participates in a ligand exchange reaction of the dimethylformamide ligands and cyanide. Simultaneous sulfur abstraction reaction from the terminal disulfide group forms thiocyanate and terminal sulfido ligand. Respective reaction rates for the two reactions appear competitive where different products were isolated solely based on change of reaction temperature. The approach to determine the number of cyanide ligands participating in the ligand exchange reaction by varying the stoichiometry and reaction temperature led to identification and isolation of tetranuclear complexes 2 and 5 and dinuclear complexes 3, 4, and 6. A synthesized and fully characterized thiocyanate analog of 6 (7) supports spectroscopic characterization of 6. The tetranuclear anion, [Mo 4 O 4 (μ-S) 6 (CN) 4 ] 4− , 2, was crystallized from a reaction at ambient temperature. The dinuclear anion, [Mo 2 O 2 (μ-S) 2 (S)(CN) 3 ] 3− , 3, was crystallized from similar reaction conditions at lower temperature. The reaction yield of thiocyanate obtained at pH of 7.4 and at 9.2 as a function of time, for several ratios of cyanide, favors the sulfur abstraction reaction at elevated pH. The sulfur abstraction reaction is the first step in a proposed mechanism of the reaction of cyanide and thiosulfate to form thiocyanate and sulfite by 1.

Water-Soluble α-Amino Acid Complexes of Molybdenum as Potential Antidotes for Cyanide Poisoning: Synthesis and Catalytic Studies of Threonine, Methionine, Serine, and Leucine Complexes

“…A complex with sulfate bridging two molybdenum atoms has been reported, demonstrating that it is a possible coordination mode for thiosulfate . The literature has a single example of a metal catalyzing this reaction, but it has been explored with the rhodanese enzyme in vitro . − …”

“…Up to 75% of the available cyanide was quantified as thiocyanate after a 2 h reaction time for 1 – 4 , and extended sampling did not increase the yields significantly. The remaining 25% and the apparent slowing of the reaction could be caused by catalyst deactivation . The deactivation of the catalyst could be brought about by cyanide–thiocyanate coordination or the formation of a tetranuclear species through a dimerization reaction of two sulfido complexes.…”

“…A complex with sulfate bridging two molybdenum atoms has been reported, demonstrating that it is a possible coordination mode for thiosulfate. 30 The literature has a single example of a metal catalyzing this reaction, 85 but it has been explored with the rhodanese enzyme in vitro. 86−88 Figure 7 reveals about a 10% variance in the catalytic activity of 1−4, which differ only in the R groups of the amino acid ligands.…”

“…The remaining 25% and the apparent slowing of the reaction could be caused by catalyst deactivation. 85 The deactivation of the catalyst could be brought about by cyanide−thiocyanate coordination or the formation of a tetranuclear species through a dimerization reaction of two sulfido complexes.…”

“…The terminal sulfido group on molybdenum reacts in stoichiometric reactions with cyanide to form thiocyanate in a reaction commonly employed to model the reactivity and inactivation of molybdenum hydroxylases as well as other molybdoenzymes. ,− Molybdenum–cyano complexes were reviewed recently, and the reaction chemistry of molybdenum sulfur compounds with cyanide was reported earlier. , Most recently, binuclear molybdenum complexes with the “Mo­(O)­(η 2 -S 2 )” moiety in [Mo 2 O 2 (μ-S) 2 (S 2 )­(DMF) 3 ] were shown to react with CN  to form SCN  in stoichiometric amounts . The complex catalyzes the reaction of thiosulfate and cyanide, forming sulfite and thiocyanate.…”