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 cm"1 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, µ = 4.0 M, is 1.4 ± 0.2 M"1. 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-A,S)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, = 0.072 (8) Á. 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

When extracellular gadolinium-based agents are used for infarct size measurement, imaging parameters and timing are important because the kinetics of both normal and irreversibly injured myocardium must be considered. Manganese-based agents are highly specific and less sensitive to timing for infarct size determination, but further studies are required to determine if they are feasible for human use.

Kinetics of the reaction of H202 with the 11 thiolato complexes [(H20)5CrSCH2CH2NH2]3+, [(H20)5CrSC6H4NH3]3+, [(H20)4Cr(SCH2C00)]+, [(en)2M(SCH2CH2NH2)]2+, [(en)2M(SCH2COO)]+, [(en)2M(SC(CH3)2COO)]+, [(en)2Cr-(SCH2CH2COO)]+, and [(en)2Co(SCH(CH3)COO)]+, where = Co and Cr, have been investigated in aqueous media.All cobalt(III) complexes undergo 2-equiv oxidation to isolable S-bonded sulfenic acid complexes, while all chromium(III) complexes undergo Cr-S bond fission via a presumed unstable sulfenato-chromium(III) intermediate. The mechanism of these reactions is taken to be nucleophilic attack by coordinated sulfur on the 0-0 peroxide bond. With respect to this process, coordinated thiols are potent nucleophiles, about equivalent to thiosulfate. When bonded to cobalt(III), thiols are significantly better nucleophiles than when bonded to chromium(III), presumably due to greater back-bonding in the cobalt system. The rate law takes the form rate = k2[Nuc][H202], and typical rate and activation parameters are k2 = 1.13 (2) M-1 s-\ AH* = 9.7 (2) kcal/mol, and AS* = -26(1) eu (for [(en)2Cr(SCH2CH2NH2)]2+, 25 °C, µ = [HC104] = 1.00 M). The reactivity patterns for H202 oxidation of analogous chromium and cobalt complexes are very similar but are different from the reactivity pattern exhibited for normal aquation of thiolato-chromium(III) complexes. All oxidation reactions show a slight acid dependence of the generalized form k2 = a + 6[H+], typical for oxidation by H202 and H302+.The ratio b/a ranges from 1.5 to 3.0 M-1, these small values being consistent with attack by cationic nucleophiles. The rate of H202 oxidation of a coordinated thiol is relatively insensitive to the nature of the thiolato complex (thiolato chelate ring size or steric requirements, ancillary ligands, etc.). However, the rate of H202 oxidation of a coordinated sulfur atom is very sensitive to the steric requirements of the sulfur atom, two-coordinate sulfur being oxidized more than 101 23 times faster than three-coordinate sulfur.

Proton T2 relaxation and its biexponential components have been determined in rabbit skeletal muscle in the presence and absence of GdDTPA. The effect of GdDTPA, which distributes only in the extracellular space, was greatest in the longer-relaxing T2 component (T22). A 27% reduction in T22 was measured for slow-twitch (red) muscle and 17% for fast-twitch (white) muscle, consistent with the larger extracellular space of the former. Magnetic resonance images demonstrated apparent contrast between red and white rabbit muscles. This contrast was instantaneously enhanced by administration of GdDTPA and returned to near normal levels after approximately 30 min. These functional changes in tissue contrast are consistent with differences in blood perfusion and biological water compartmentation between fast- and slow-twitch skeletal muscles.