BASE HYDROLYSIS IN HOMOMETALLIC DINUCLEAR CHROMIUM(III) COMPLEXES BRIDGED BY HYDROXIDE AND CARBOXYLATE

Base hydrolysis of dinuclear complexes [Cr-2(mu-OH)(mu-RCO(2))(en)(4)](4+) and [(nta)Cr(mu-OH)(mu-RCO(2))Cr(en)(2)](+) (en = ethane-1,2-diamine, nta = nitrilotriacetate; R = H, Me, Et, Pr-n. CH2 Cl, CH2 ClCH2, MeOCH(2) or Ph) was studied by using the UV/VIS absorption and H-2 NMR spectral changes and chromatography. The reactions were found to occur in two stages: the first-stage rates (k(1)) varied with R, but the second-stage ones (k(2)) 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 (p*) and steric (delta) parameters gave a critical clue to establishing the difference in bond-cleavage sites. The values were p* = 2.29 +/- 0.20 and delta = 1.41 +/- 0.13 for [Cr-2(mu-OH)(mu-RCO(2))(en)(4)](4+) and p* = 0.01 +/- 0.11 and delta = 0.32 +/- 0.08 for [(nta)Cr(mu-OH)(mu-RCO(2))Cr(en)(2)](+). The carboxylato bridge-cleavage reaction is influenced by both the inductive and steric effects of the substituents in [Cr-2(mu-OH)(mu-RCO(2)) (en)(4)](4+), and both effects contribute to smaller extents for [(nta)Cr(mu-OH)(mu-RCO(2))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.