Kinetics of the reactions between [S2MoS2Cu(SC6H4R-4)]2− (R = MeO, H, Cl or NO2) and CN−: substitution mechanism at a 3-coordinate CuI site

Abstract

The kinetics of the reaction between [S₂MoS₂Cu(SC₆H₄R-4)]²⁻ (R = MeO, H, Cl or NO₂) and CN⁻ to form [S₂MoS₂CuCN]²⁻ have been studied in MeCN using stopped-flow spectrophotometry. In all cases, the rate law is of the form, Rate = {k^R₁ + k₂^R[CN⁻]}[S₂MoS₂Cu(SC₆H₄R-4)²⁻]. It is proposed that both k^R₁ and k^R₂ correspond to associative substitution mechanisms. The k^R₂ pathway involves attack by CN⁻ at the copper site followed by dissociation of the thiolate. The k^R₁ pathway involves attack of the solvent (MeCN) at the copper site, followed by dissociation of the thiolate to form [S₂MoS₂Cu(NCMe)]⁻. Subsequent rapid substitution of the coordinated solvent by cyanide produces [S₂MoS₂CuCN]²⁻. The evidence that both the k^R₁ and k^R₂ pathways involve associative mechanisms are: (i) the 4-R-substituent on the thiolate ligand has a similar effect on both k^R₁ and k^R₂, with electron-withdrawing 4-R-substituents facilitating substitution; (ii) both the k^R₁ and k^R₂ pathways are associated with similar activation parameters (for k₁^H: ΔH^‡ = 5.5 ± 0.5 kcal mol⁻¹, ΔS^‡ = −23.9 ± 2.0 cal deg⁻¹ mol⁻¹; for k₂^H: ΔH^‡ = 2.3 ± 0.5 kcal mol⁻¹, ΔS^‡ = − 23.9 ± 2.0 cal deg⁻¹ mol⁻¹) and (iii) addition of C₆H₅S⁻ results in a similar increase in both k^R₁ and k^R₂.