Kinetics and mechanism of ligand substitution in β-diketone complexes of iron(III). Solvolysis controlling the substitution process in alcohol media

• Published in: Inorganica Chimica Acta Vol. 342; pp. 97 - 106
• Main Authors: Gumbel, Gerhard, Elias, Horst
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 10.01.2003
• Subjects: Iron(III) complexes; Kinetics; Ligand substitution; Mechanism; β-diketones; β-diketones; Ligand substitution; Iron(III) complexes; Kinetics; Mechanism
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(02)01143-X

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) 2S 2 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 k 1, k 2, and k 3 are independent of the concentration of the entering ligand HO ∩N and follow the order k 1> k 2> k 3, with k 1/ k 2≈10 and k 1/ k 3≈10 2. For a given system Fe(O ∩O) 3/HO ∩N/ROH, the size of k 1, k 2, and k 3 correlates with the solvent polarity parameter E T(30). Rate constant k 1 describes the solvolytic dissociation of the complexes Fe(O ∩O) 3 and rate constant k 3 the solvent-initiated splitting of the binuclear complexes [Fe(O ∩O) 2(RO)] 2. Rate constant k 2 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, k 1 ranges from 0.04 to 2 s −1, k 2 from 0.007 to 0.2 s −1, and k 3 from 0.002 to 0.01 s −1 at 298 K. The mechanism of the ligand substitution processes is discussed. The kinetics of ligand substitution in tris β-di-ketone iron(III) complexes, Fe(O ∩O) 3, by 8-hydroxyquinoline was studied in alcohol media. Complexes Fe(O ∩O) 3 solvolyze in alcohols ROH to form species such as Fe(O ∩O) 2S 2 and [Fe(O ∩O) 2(RO)] 2 (S=ROH). The reaction of Fe(O ∩O) 3 with HO ∩N is triphasic, with ligand-independent first-order rate constants k 1, k 2, and k 3, following the order k 1> k 2> k 3 with k 1/ k 2≈10 and k 1/ k 3≈10 2. The rate constants are mechanistically assigned.