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Kinetics and mechanism of ligand substitution in β-diketone complexes of iron(III). Solvolysis controlling the substitution process in alcohol media
Gumbel, Gerhard,Elias, Horst
, p. 97 - 106 (2003)
Conventional and stopped-flow spectrophotometry was used to study the kinetics of ligand substitution in a number of tris β-diketone iron(III) complexes, Fe(O∩O)3, by 8-hydroxyquinoline (=HO∩N) in alcohol media (O∩O-=anion of the β-diketones pentane-2,4-dione, 2,6-dimethylheptane-3,5-dione, 2,2,6,6-tetramethylheptane-3,5-dione, 1-phenylbutane-1,3-dione, 1,3-diphenylpropane-2,3-dione, and 1-(2-thienyl)-4,4,4-trifluorobutane-1,3-dione). As shown by spectrophotometry, the solutions of complexes Fe(O∩O)3 in alcohols ROH are subject to solvolytic dissociation, leading to solvento species Fe(O∩O)2S2 and to binuclear complexes [Fe(O∩O)2(RO)]2 (S=ROH and RO-, respectively). The reaction of complexes Fe(O∩O)3 with HO∩N in alcohol media, leading to Fe(O∩N)3, is triphasic. The corresponding first-order rate constants k1, k2, and k3 are independent of the concentration of the entering ligand HO∩N and follow the order k1>k2>k3, with k1/k2≈10 and k1/k3≈102. For a given system Fe(O∩O)3/HO∩N/ROH, the size of k1, k2, and k3 correlates with the solvent polarity parameter ET(30). Rate constant k1 describes the solvolytic dissociation of the complexes Fe(O∩O)3 and rate constant k3 the solvent-initiated splitting of the binuclear complexes [Fe(O∩O)2(RO)]2. Rate constant k2 is assigned to the solvolytic dissociation of the intermediate complex Fe(O∩O)2(O∩N). Depending on the nature of the coordinated β-diketone and solvent ROH, k1 ranges from 0.04 to 2 s-1, k2 from 0.007 to 0.2 s-1, and k3 from 0.002 to 0.01 s-1 at 298 K. The mechanism of the ligand substitution processes is discussed.
