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 (t1/2 < 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) (t1/2 < 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.