A complete series of stepwise oxidation of [Co(2-pyridinethiolato)(en)₂]²⁺. Characterization of the 2-pyridinesulfenato-N,S and -N,O, 2-pyridinesulfinato-N,S and -N,O, and 2-pyridinesulfonato-N,O complexes

The H₂O₂ oxidation of [Co(pyt)(en)₂]²⁺ [1, pyt = 2-pyridinethiolate(1-) ion, en = ethylene- diamine] afforded the orange [Co(Pyse-N,S)(en)₂]²⁺ [2, pyse = 2-pyridinesulfenate(1-) ion] complex. Irradiation of this orange complex in the solid state yielded a green complex. The molecular structure of the green complex perchlorate was determined by X-ray diffraction to be [Co(pyse-N,O)(en)₂](ClO₄)₂ (4); linkage isomerization took place upon photolysis. The crystal data and final R value are: triclinic, P1, a = 9.374(4) Å, b = 11.248(4) Å, c = 9.238(8) Å, α = 95.26(4)°, β = 95.14(5)°, γ = 75.75(3)°, V = 938(2) ų, Z = 2 and R = 0.040 for 2757 unique reflections. The pyse-N,O complex has an anomalous feature in the UV-vis spectrum. It has an extra band (16 400 cm^-1) on the low-energy side of the first d-d absorption band (22 200 cm^-1). This band can be rationalized as a splitting component of the first d-d band generated by the anisotropic interaction of the antibonding π* orbital of the coordinating oxygen with the t_2g orbitals on the cobalt center. Treatment of [Co(pyt)(en)₂]²⁺ with excess H₂O₂ afforded the yellow-orange sulfinato-N,S complex, [Co(pysi-N,S)(en)₂]²⁺ [3, pysi = 2-pyridinesulfinate(1-) ion]. The molecular structure of the pysi-N,S complex was determined by X-ray diffraction to be [Co(pysi-N,S)(en)₂](ClO₄)₂ (3), which cocrystallized with the pysi- N,O complex (5). The crystal data and final R value for the cocrystal (3/5) are: monoclinic, C2, a = 28.559(8) Aπ, b = 6.960(2) Aπ, c = 10.406(3) Aπ, β = 99.90 (2)°, V = 2038(2) Aπ³, Z = 4 and R = 0.040 for 2391 unique reflections. The pysi-N,S complex was unstable and was easily converted photochemically or thermally to the linkage-isomerized orange pysi-N,O complex (5). The sulfur atom becomes chiral (R and S) upon linkage isomerization, and two kinds of racemic salt, Λ(R)Δ(S) and Λ(S)Δ(R), were separated by fractional crystallization of the complex perchlorate. The molecular structure of the less soluble isomer (Λ(S)Δ(R)-5) of the perchlorate was determined by X-ray diffraction to be Λ(S)Δ(R)-[Co (pysi- N,O)(en)₂](ClO₄)₂. The crystal data and final R value are: triclinic, P1, a = 9.384(4) Å, b = 11.807(8) Å, c = 8.640(5) Å, α = 98.54(5)°, β = 94.40(4)°, γ = 96.65(5)°, V = 936(2) Å₃, Z = 2 and R = 0.038 for 3061 unique reflections. The two diastereomers of Λ-[Co(pysi-N,O)(en)₂]²⁺ were separated by SP-Sephadex column chromatography, and one of them (Λ(R)-5) was structurally characterized by X-ray crystallography. Λ(R)-[Co(pysi-N,O) (en)₂](ClO₄)₂, which crystallizes as orthorhombic in space group P2₁2₁2₁ with Z = 4, has the cell parameters a = 9.896(2) Å, b = 20.758(6) Å, c = 9.494(4) Å, V = 1950(2) ų. The final R value was 0.041 for 1543 unique reflections. Kinetic studies of reversible isomerization (epimerization) between Λ(R)- and Λ(S)-[Co(pysi-N,O)(en)₂]²⁺ were performed in aqueous solutions at 60°C. The rate constants were k_{Λ(R)→Λ(S)} = 5.5 × 10^-6 s^-1 and k_{Λ(S)→Λ(R)} = 1.7 × 10^-6 s^-1. The orange-red [Co(pyso-N,O)(en)₂]²⁺ [6, pyso = 2-pyridinesulfonate(1-) ion] complex was prepared by the reaction of cis-[Co{OS(O)₂CF₃}₂(en)²] ⁺ with the 2-pyridinesulfonate ligand or by the oxidation of [Co(pyse-N,O)(en)₂]²⁺ (4) with H₂O₂. The molecular structure of the trifluoromethanesulfonate salt of complex 6 was determined by X-ray diffraction. [Co(pyso-N,O)(en)₂](CF₃SO₃)₂ crystallizes as monoclinic in space group P2₁/a with Z = 4. The cell parameters are a = 19.079(8) Å, b = 16.699(6) Å, c = 7.462(4) Å, β =100.51(4)°, V = 2338(2) ų. The final R value was 0.050 for 3966 unique reflections. The pyso ligand has the strongest ligand field strength among the ligands studied here. A series of sulfur oxidation from [Co(pyt)(en)₂]²⁺ to [Co(pyso-N,O)(en)₂]²⁺ is established.