SYNTHESIS, CHARACTERIZATION, AND REACTIVITY OF COORDINATED SULFENIC ACIDS

Several bis(ethylenediamine)cobalt(III) complexes containing chelated, S-bonded sulfenic acid ligands have been synthesized and characterized. [(en)2Co(S(0)CH2CH2NH2)]2+ and [(en)2Co(S(0)CH2COO)]+ are prototypes of this class of complexes. The general preparative route to sulfenato complexes involves stoichiometric oxidation of the corresponding thiolato complex with a 2-equiv oxidant, e.g., H202, in aqueous solution. Subsequent oxidation converts the sulfenato complex to the corresponding sulfinato complex. The ligand field strength and the electronegativity of the ligating sulfur atom increase monotonically along the thiolato-sulfenato-sulfinato series. The S-bonded sulfenato ligand exhibits a characteristic S-0 stretch in the region 950-1000 cnT1 and a characteristic charge-transfer absorption at ca. 360 nm. Reversible protonation of the sulfenato ligand occurs in aqueous HC104 solutions; the protonation constant of [(en)2Co(S(0)CH2CH2NH2)]2+ at 25 °C, iu = 4.0 M, is 1.4 ± 0.2 M1. Sulfenato-cobalt(III) complexes slowly decompose in aqueous acidic media to yield a variety of products including cobalt(II). The single-crystal X-ray structure analysis of (2-sulfenatoethylamine-Ar,5)bis(ethylenediamine)cobalt(III) thiocyanate, refined to a conventional R factor of 0.030, is reported. The cobalt(III) center is approximately octahedrally coordinated (five N atoms and one S atom) while the sulfenato sulfur atom is three-coordinate (Co, C, and O). The Co-N bond trans to the sulfenato sulfur atom is significantly longer than the average of the cis Co-N bonds, A = 0.072 (8) A. The rates of H202 oxidation of [(en)2CoS(0)CH2CH2NH2)]2+ in aqueous perchloric acid media are reported as a function of [H202], [H+], and temperature. The mechanism of this reaction is discussed in terms of nucleophilic attack by the coordinated sulfur on the 0-0 bond of H202, the coordinated sulfur atom having about the same nucleophilicity as the sulfur atom of SCN. The observed acid catalysis presumably reflects the greater reactivity of H302+ as a nucleophilic substrate. This acid catalysis is significantly less effective in this system than in analogous H202 oxidations, implying that within the acid-catalyzed transition state there is an interaction between the proton and the sulfenato moiety which decreases the overall efficacy of this path. © 1979, American Chemical Society. All rights reserved.