21626-24-4Relevant academic research and scientific papers
Redox interconversions and aqueous solution properties of the cuboidal complexes [Mo4S4(edta)2]m- (m = 4, 3, 2)
Dimmock, Paul W.,McGinnis, Joseph,Ooi, Bee-Lean,Sykes, A. Geoffrey
, p. 1085 - 1089 (2008/10/08)
Redox interconversions of the recently prepared Mo4S4 complexes present in the 4+, 5+, and 6+ states, and in this case with N,N,N′,N′-ethylenediaminetetraacetate (edta) as ligand, [Mo4S4(edta)2]m- (m = 4, 3, and 2, respectively), have been studied at 25°C, I = 1.00 M (LiClO4). The most readily obtained complex is green [Mo4S4(edta)2]3-, the EPR spectrum of which (in H2O at 10 K) confirms one unpaired electron, gav value 2.48. From cyclic voltammetry in 0.5 M LiClO4, reduction potentials (vs NHE) are [Mo4S4(edta)2]3-/4- (-0.046 V) and [Mo4S4(edta)2]2-/3- (0.65 V). The [Mo4S4(edta)2]4- ion is air-sensitive, and in the presence of H+ and ClO4- is oxidized to [Mo4S4(edta)2]3-. Reactions studied were at pH >3 to limit also dechelation of the edta, except in the case of the fast [Fe(H2O)6]3+ oxidation of [Mo4S4(edta)2]3-, when it was necessary to work at higher [H+] in the range 0.075-0.30 M. The kinetics of the 1:1 cross-reactions with [Co(edta)]- (0.37 V; 5.4 × 103 M-1 s-1) and [Fe(edta)]- (0.12 V; 2.4 × 106 M-1 s-1) as oxidants for [Mo4S4(edta)2]4- give a self-exchange rate constant for the [Mo4S4(edta)2]4-/3- couple of 1.5 × 107 M-1 s-1. Similarly with [Co(dipic)2]- (0.747 V; 17.8 M-1 s-1) as the oxidant, a self-exchange rate constant of 7.7 × 105 M-1 s-1 is obtained for the [Mo4S4(edta)2]3-/2- couple. The reaction with [Fe(H2O)6]3+ as oxidant (0.77 V; 6.7 × 104 M-1 s-1 at 10 °C) gives no dependence on [H+] and is too fast to be other than outer-sphere. Both self-exchange reactions are clearly very favorable processes. The comproportionation reaction of [Mo4S4(edta)2]4- with [Mo4S4(edta)2]2- is too fast to measure directly by stopped-flow UV-vis spectrophotometry. A rate constant of 2.4 × 1010 M-1 s-1 calculated from the self-exchange values is close to or at the diffusion-controlled limit for reactants of radius ～7.4 A?. The two-electron oxidant [PtCl6]2- (0.726 V) oxidizes [Mo4S4(edta)2]4- in one-electron steps. Interestingly, the self-exchange rate constant for [Mo4S4(edta)2]4-/3- is ～104 times more favorable than that for the [Mo4S4(H2O)12]4+/5+ exchange.
