One of the major metabolites of dimethylthiourea, dimethylaminoiminomethanesulfinic acid (DMAIMSA), was synthesized by controlled oxidation of dimethylthiourea using hydrogen peroxide. The crystal structure was determined by X-ray crystallography. It is a zwitterionic species in its solid form, with a positive charge delocalized around an sp2-hybridized carbon center and two nitrogen atoms. It crystallizes in the triclinic P[Formula: see text] space group. The CS bond, at 1.880(2) Å, is much longer than a typical CS single bond length of 1.79 Å. It is also longer than the one observed in thiourea trioxide, a comparable sulfonic acid. This CS bond is stable in acidic conditions and is easily cleaved in basic conditions or in the presence of suitable nucleophiles that can attack the positively disposed carbon center. DMAIMSA is highly reactive and easily decomposes in basic conditions to yield dithionite in the presence of oxygen, whereas in strictly anaerobic conditions it gives a mixture of sulfite and sulfide. The precursor to dithionite, SO2·, is formed from the one-electron oxidation of the sulfoxylate anion, SO22, which results from an initial heterolytic cleavage of the CS bond in DMAIMSA. The sulfur center is oxidized, even by mild oxidants such as aqueous iodine, to sulfate. Key words: thiourea metabolites, synthesis, structure, reactivity.
The oxidation of hydroxymethanesulfinic acid, HMSA, by acidic iodate has been studied by spectrophotometric
techniques. The reaction presents clock reaction characteristics in which in excess iodate conditions there is
an initial quiescent period that is followed by a rapid production of iodine. The induction period before
formation of iodine is inversely proportional to the iodate concentrations and the concentrations of acid to
the second order. Iodide ions have a strong catalytic effect on the rate of the reaction by reducing the duration
of the induction period. The stoichiometry of the reaction is dependent on the ratio of oxidant to reductant.
In excess HMSA conditions the stoichiometry was deduced to be 3HOCH2SO2H + 2IO3
- → 3HCHO +
3SO4
2- + 2I- + 6H+ whereas in excess iodate and after prolonged standing the stoichiometry is 6IO3
- +
5HOCH2SO2H → 5SO4
2- + 5HCOOH + 3I2 + 4H+ + 3H2O. The mechanism is dominated by the standard
oxyiodine kinetics that involve the initial formation of the reactive oxyiodine species HIO2 and HOI: IO3
-
+ 2H+ + I- ⇌ HIO2 + HOI. Further reactions will then occur between the organosulfur species with HOI.
The direct reaction of aqueous iodine with HMSA is fast enough to be considered diffusion controlled, with
a stoichiometry of 2I2 + HOCH2SO2H + 2H2O → HCHO + SO4
2- + 4I- + 6H+. The facile nature of this
reaction implies that HMSA and iodine cannot coexist in the reaction medium and that the end of the induction
period coincides with a complete consumption of HMSA by iodate. The first 2-electron oxidation of HMSA
yields a stable bisulfite addition compound, hydroxymethanesulfonic acid, HMSOA. Further oxidation of
HMSOA is very slow, with the pathway involving the initial dissociation of HMSOA to HSO3
- and HCHO.
Although HSO3
- is rapidly oxidized to SO4
2-, HCHO is only slowly oxidized to HCOOH.
Reaction of sodium hydrotris(methimazolyl)borate (NaTm(Me)) with cobalt halides leads to the formation of paramagnetic pseudotetrahedral [Co(Tm(Me))X] (X = Cl, Br, I), of which the bromide has been crystallographically characterized. Mass spectrometry reveals the presence of higher molecular weight fragments [Co(Tm(Me))(2)](+) and [Co(2)(Tm(Me))(2)X](+) in solution. Aerial oxidation in donor solvents (e.g. MeCN) leads to formation of the [Co(Tm(Me))(2)](+) cation, which has been crystallographically characterized as the BF(4)(-), ClO(4)(-), Br(-), and I(-), salts. Attempts to prepare the mixed sandwich complex, [Co(Cp)(Tm(Me))](+), resulted in ligand decomposition to yield [Co(mtH)(3)I]I (mtH = 1-methylimidazole-2-thione), but with the more electron donating methylcyclopentadienyl (Cp(Me)) ligand, [Co(Cp(Me))(Tm(Me))]I was isolated and characterized. Electrochemical measurements reveal that the cobalt(III) Tm(Me) complexes are consistently more difficult to reduce than their Tp and Cp congeners.
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