Synthesis and Substitution Stereochemistry of trans-Chlorobis(Ethylenediamine)-(Trifluoromethanesulfonato)Cobalt(III) Trifluoromethanesulfonate

Abstract

The achiral green salt trans-[Co(en)₂(O₃SCF₃) Cl ] CF₃SO₃ has been synthesized by controlled solvolysis of Λ- cis -or trans-[Co(en)₂Cl₂] Cl in CF₃SO₃H.Kinetic studies and proton n.m.r . and electronic absorption spectral data indicate that cis /trans isomerization of Co(en)2(O3SCF3) Cl⁺ in triflic acid is extremely rapid, and that the equilibrium strongly favours the trans geometry. The loss of the first Cl ^- from Co(en)₂Cl₂₊ was too fast to measure (t_1/2 < 1 s, -20°C, CF₃SO₃H), but the second Cl ^- was released at least 10⁵-fold more slowly: k(3.5±0.1)x10^-4 s^-1, 25°C; ΔH 73.1±1.7kJ mol^-1, ΔS - 66±5 J mol^-1 K^-1 in the range 30-60°C. Reasons for this enormous rate difference are discussed. Solvolysis of CF₃SO₃^- from trans- Co(en)₂(O₃SCF₃) Cl ⁺ was immeasurably rapid in H₂O, dimethyl sulfoxide (Me₂SO) and dimethylformamide (Me₂NCHO) (t_1/2 < 1 s, 25°C), but it was detectable in MeCN : k 5.0×10^-2 s^-1, 25°C. The steric course of substitution for these solvents has been determined spectrophotometrically , and by n.m.r . spectroscopy: 0.1 M HClO₄, 40.5% cis - and 59.5% trans-Co(en)₂(OH₂)Cl²⁺; Me₂SO, 77% cis - and 23% trans- Co(en)₂(OSMe₂)Cl²⁺; Me₂NCHO, 73% cis - and 27% trans-Co(en)₂(OCHNMe₂)Cl²⁺; MeCN , 53% cis - and 47% trans-Co(en)₂(NCMe₂)Cl²⁺ (25°C). These results are discussed in the context of a wider body of data for the stereochemistry of substitution of trans- and cis -Co(en)₂ClXⁿ⁺ in H₂O and non-aqueous solution, where a clear leaving group dependence emerges for the trans but not the as isomers. Finally, trans- [Co(en)₂(O₃SCF₃) Cl ] CF₃SO₃ provides a useful synthetic route to cis- and trans-Co(en)₂ClXⁿ⁺ complexes.