KINETIC-STUDIES OF (MU-OXO)(MU-CARBOXYLATO)BIS((TRIS(2-PYRIDYLMETHYL)AMINE)CHROMIUM(III)) DIMER ACID AND BASE HYDROLYSIS REACTIONS

Kinetic studies of [(tmpa)Cr(mu-O)(mu-RCO(2))Cr(tmpa)](3+) aquation and hydroxide-assisted hydrolysis reactions 1 and 2 are reported (tmpa = tris(2-pyridylmethyl)amine; R = H, CH3, CH2Cl, CHCl2, C(CH3)(3), CPh(3), 1-adamantyl, Ph-4-X (X = H, OH, NMe(2), OCH3, CH3, CF3, NO2 F, Cl)): [(tmpa)Cr(mu-O)(mu-RCO(2))Cr(tmpa)](3+) + H2O --> [(tmpa)Cr(mu-OH)(2)Cr(tmpa)](4+) + RCO(2)(.) (1); [(tmpa)Cr(mu-O)(mu-RCO(2))Cr(tmpa)](3+) + 20H(-) + H2O --> 2[Cr(tmpa)(OH)(2)](+) + RCO(2)(.) (2). A linear Hammett plot of acid-independent aquation rate constants (k(aq)) gives rho = + 0.30, while the corresponding plot of log(k(aq)) vs -pK(a)(RCOOH) for R = H, CH3, CH2Cl and CHCl2 complexes exhibits a slope of + 0.10; reactivities of carboxylate-bridged complexes with sterically-demanding R groups (CPh(3), C(CH3)(3) 1-adamantyl) are smaller than anticipated on the basis of RCO(2)(.) basicity alone. For R = H, CH3, CPh(3), and Ph at 60 degrees C, and I = 0.1 M, k(aq) = 1.52 x 10(-3) s(-1) (Delta H-double dagger = 83 kJ mol(-1), Delta S-double dagger = -50 J mol(-1) K-1); 1.08 x 10(-3) s(-1) (Delta H-double dagger = 77 kJ mol(-1), Delta S-double dagger = -71 J mol(-1) K-1); 3.3 x 10(-4) s(-1) (Delta H-double dagger = 96 kJ mol(-1), Delta S-double dagger = -25 J mol(-1) K-1), and 1.64 x 10(-3) s(-1) (Delta H-double dagger = 80 kJ mol(-1), Delta S-double dagger = -59 J mol(-1) K-1), respectively. The proposed aquation mechanism features rate-limiting bridging carboxylate ring-opening to give [(RCO(2))(tmpa)Cr(mu-O)Cr(tmpa)(H2O)](3+) as a short-lived intermediate. For base-assisted hydrolysis at 60 degrees C and I = 1.0 M, observed pseudo-first-order rate constants follow the relationship k(obsd) = k(0) + k(OH)[OH-] when R = H (k(0) = 2.8 x 10(-3) s(-1), Delta H-double dagger = 83 kJ mol(-1), Delta S-double dagger = -46 J mol(-1) K-1; k(OH) = 6.84 x 10(-2) M(-1) s(-1), Delta H-double dagger = 86 kJ mol(-1), Delta S-double dagger = -8 J mol(-1) K-1). All other [(tmpa)(2)Cr-2(mu-O)(mu-RCO(2))](3+) species exhibit rate saturation described by k(obsd) = k(0) + k(b)Q(p)[OH-]/(1 + Q(p)[OH-]), where the minor k(0) term typically is not resolved at 60 degrees C; for R = CH3, k(b) = 6.2 x 10(-3) s(-1) (Delta H-double dagger = 115 kJ mol(-1), Delta S-double dagger = + 59 J mol(-1) K-1) and Q(p) = 1.5 X 10(2) M(-1) (Delta H degrees = + 65 kJ mol(-1), Delta S degrees = + 238 J mol(-1) K-1). The proposed base hydrolysis mechanism for [(tmpa)(2)Cr-2(mu-O)(mu-RCO(2))](3+) (R not equal H) requires preequilibrium displacement of a tmpa pyridyl arm by OH-, followed by rate-limiting ring-opening of the bridging RCO(2)(.) ligand assisted by the solvent or migratory intracomplex nucleophilic attack from the hydroxide ligand.