INTRA-MOLECULAR ASSOCIATIVE ASSISTANCE IN THE LABILIZATION OF CHROMIUM(III) COMPLEXES - COMPARISON OF THE ACID AQUATION OF 2,4-PENTANEDIONE FROM CR(HEDTA)(ACAC)- AND CR(EDDA)(ACAC)

The H3O+-assisted aquation of Hacac from Cr(hedta)(acac)- and Cr(edda)(acac) has been studied. An accelerated rate of about 103 relative to normal dissociative paths of Cr(III) complexes is observed for the Cr(hedta)(acac)- case. The pendant group effect for labilizing Cr(III), as proposed by Ogino and by Sykes, is proposed to account for the 103 enhancement. H3O+ scavenging of the distorted intermediates produced by pendant group association at Cr(III) is found to be much more rapid than the reverse reaction to the ground state. The Cr(hedta)(acac)- aquation occurs via two pendant arm-assisted pathways: (A) for Cr(hedtaH)(acac) and (B) for Cr(hedta)(acac)-. The following kinetic parameters are obtained at µ=0.20 (NaClO4): ΔHA‡=21.34 ± 1.30 kcal/mol, ΔSA‡=-0.89 ± 1.15 eu; ΔHB‡=16.68 ± 2.50 kcal/mol, ΔSB‡=-21.39 ± 8.20 eu. The relative rates to achieve the distorted intermediates (k2/k1) is 51 in favor of assistance from the anionic carboxylate, the (B) path. The association constant for formation of Cr(hedta)(acac)- from Cr(hedta)(H2O) and acac- was evaluated by competition with H3O+ to be (5.87 ± 0.24) × 105 M-1 (µ=0.20, T=25.0 °C). Bidentate acac-exhibits the normal chelate effect on the stability constant relative to monodentate anions, X-, of ca. 2.9 × 104. The acid aquation of Cr(edda)(acac) proceeds by the normal dissociative pathway, scavenged by H3O+, in competition with return of the distorted intermediate to the ground state. At n=1.00 (NaClO4), 25.0 °C, k5=2.99 × 10-5 s-1 for the distortion step with k-5/k6, reversion to ground state vs. H3O+ scavenging, of 0.19. Comparisons are made to previous studies concerning the acid aquation of Cr(acac)3. © 1979, American Chemical Society. All rights reserved.