Base hydrolysis in homometallic dinuclear chromium(III) complexes bridged by hydroxide and carboxylate

Abstract

Base hydrolysis of dinuclear complexes [Cr₂(µ-OH)(µ-RCO₂)(en)₄]⁴⁺ and [(nta)Cr(µ-OH)(µ-RCO₂)Cr(en)₂]⁺(en = ethane-1,2-diamine, nta = nitrilotriacetate; R = H, Me, Et, Prⁿ, CH₂Cl, CH₂ClCH₂, MeOCH₂ or Ph) was studied by using the UV/VIS absorption and ²H NMR spectral changes and chromatography. The reactions were found to occur in two stages: the first-stage rates (k₁) varied with R, but the second-stage ones (k₂) were almost the same throughout the series. Analysis of the kinetic data for the two types of complexes disclosed the effects of the substituent groups R and/or the non-bridging ligands on the carboxylato bridge-cleavage reaction mechanism. Examination of the substituent effects in terms of Taft's equation with the electronic (ρ*) and steric (δ) parameters gave a critical clue to establishing the difference in bond-cleavage sites. The values were ρ*= 2.29 ± 0.20 and δ= 1.41 ± 0.13 for [Cr₂(µ-OH)(µ-RCO₂)(en)₄]⁴⁺ and ρ*= 0.01 ± 0.11 and δ= 0.32 ± 0.08 for [(nta)Cr(µ-OH)(µ-RCO₂)Cr(en)₂]⁺. The carboxylato bridge-cleavage reaction is influenced by both the inductive and steric effects of the substituents in [Cr₂(µ-OH)(µ-RCO₂)(en)₄]⁴⁺, and both effects contribute to smaller extents for [(nta)Cr(µ-OH)(µ-RCO₂)Cr(en)₂]⁺; the former complexes undergo cleavage at the carboxylic acyl C–O bond and the latter at the Cr–O bond in the Cr(en)₂ site.