LINKAGE ISOMERIC PENTAAMMINECOBALT(III) COMPLEXES OF METHANESULFINAMIDE

The selective syntheses and reactivities of the linkage isomeric pentaamminecobalt(III) complexes containing N- and O-bound methanesulfinamide (MeSONH2) are described. The N-bonded isomer is isolated as the deprotonated form [(NH3)5Co-NHSOCH3]2+, which is stable above pH ca. 4. It protonates in more acidic media, forming the reactive intermediate [(NH3)5Co-NH2SOCH3]3+. The acidity constant was determined both kinetically (pK′a = 2.91 ± 0.03, 1.0 M KCl; pK′a = 2.44 ± 0.06, 1.0 M NaClO4, 25 °C) and spectrophotometrically (pK′a = 2.83 ± 0.04, 1.0 M KCl, 25 °C). The site of protonation was established by 1H NMR spectra in Me2SO-d6; the NH2 protons are diastereotopic and appear as two separate signals because of the presence of the chiral sulfur center, and neither is in rapid exchange with free H+. This protonated N-bonded isomer rearranges rapidly in solution, yielding [(NH3)5Co-OS(NH2)CH3]3+ (95%) and [(NH3)5Co-OH2]3+ (5%); Jt(obsd) = (1.30 ± 0.03) × 10-2 s-1, I = 1.0 M (KCl), 25 °C. The rearrangement rate is comparable in Me2SO; k(obsd) = (1.47 ± 0.02) × 10-2 s-1, 25 °C. The O-bonded isomer is less reactive, undergoing complete but slow solvolysis (25 °C, ks = 1.0 × 10-5 s-1, 0.1-2.0 M HClO4, I = 1 or 2 M) to [(NH3)5Co(solvent)]3+ (and free NH2SOCH3). The disappearance of the O-bonded sulfinamide complex is appreciably acid catalyzed but only in the presence of Cl-, and here the reaction is mainly ligand hydrolysis, producing NH4+ and the O-bonded sulfinate complex [(NH3)5CoOS(CH3)0]2+, which has been synthesized independently. The sulfinamide NH2 group on the O-bonded isomer is rapidly nitrosated (NO+) under milder acidic conditions to give the O-bonded sulfinate complex, in aqueous solution above pH ca. 3 an O- to N-linkage isomerization competes with hydrolysis; this competition is more effective via the spontaneous (pH 6.2; 24% isomerization, 76% aquation) compared with the base-catalyzed route (0.1 M OH-, 8.5% isomerization). From these data kON could be obtained, and kON coupled with kNO enabled the calculation of the isomeric equilibrium constant K′NO (=kNO/kON = 4450). This result indicates the greater stability of the O-bonded form over the (protonated) N-bound form in aqueous acid, consistent with results for analogous [(NH3)5Co(NH2COR)]3+ systems (e.g., R = OH, H, aryl, alkyl, NR′R″, OR′); however, in neutral to basic solution, the calculated relative stability constant shows that the N-bound form is more stable, and this is because it is selectively deprotonated. © 1990, American Chemical Society. All rights reserved.