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.