title: Structural bases of oxygen-sensitivity in Fe(II) complexes with tripodal ligands. Steric effects, Lewis acidity and the role of ancillary ligands.
abstract: The complexation of Fe(SO(3)CF(3))(2) to the series of fluoro α-substituted tris-(2-aminomethylpyridyl)amine tripods F(1-3)TPA yields the triflato F(1-3)TPAFe(SO(3)CF(3))(2) complexes which have firstly been characterized in solution. As expected, bis-acetonitrile charged species are present in CH(3)CN, and neutral bis-triflato complexes in CH(2)Cl(2). The X-ray diffraction analyses of F(1)TPAFe(SO(3)CF(3))(2) and F(2)TPAFe(SO(3)CF(3))(2) crystallized from CH(2)Cl(2) solutions show that their structure in solution is retained in the solid state, with coordination of both triflate ions and the κ(4) coordination mode of the tripod in each complex. The solid state structure of the [F(2)TPAFe(NCMe)(SO(3)CF(3))](SO(3)CF(3)) complex obtained from crystallization in acetonitrile of the bis-triflato precursor is also reported. The presence of a bound triflate in the solid state is unexpected and interpreted as the result of solid-state stabilization by a metal center which displays some Lewis acidity character because of its coordination to an electron-deficient tripod. The fourth compound whose solid state structure is reported is [F(2)TPAFe(H(2)O)(2)](SO(3)CF(3))(2), fortuitously obtained after the bis-triflato precursor was handled under aerobic conditions. In CH(3)CN, all complexes are oxygen stable. The gain in stability of the bis-acetonitrile adducts is certainly responsible for the lack of reactivity of all complexes in this solvent. In CH(2)Cl(2), the parent TPAFe(SO(3)CF(3))(2) complex reacts with O(2) to yield a compound belonging to the well-known class of μ-oxo diferric compounds. Whereas F(1)TPAFe(SO(3)CF(3))(2) is poorly reactive, F(2)TPAFe(SO(3)CF(3))(2) and F(3)TPAFe(SO(3)CF(3))(2) turn out to be completely inert. This strongly contrasts with the behavior of the known F(1-3)TPAFeCl(2) complexes for which an increased reactivity is observed upon ligand substitution. In CH(2)Cl(2), conductimetry measurements indicate extremely weak (if any!) dissociation of the ancillary ligands in all complexes. Comparative analysis of the structures reveals relatively invariant structural parameters within the series of Fe(SO(3)CF(3))(2) complexes, whereas FeCl(2) complexes display important metal to ligand elongations upon tripod substitution. The reactivity increase upon fluorination of the ligand in the FeCl(2) complexes is interpreted as resulting from sterically-induced pyridine flexibility. The opposite situation with Fe(SO(3)CF(3))(2) complexes is due to the lock of the coordination polyhedron in the absence of important steric stress, especially when the metal center becomes electron-deficient.
authors.affiliation: Laboratoire de Chimie Biomimétique des Métaux de Transition, UMR CNRS n° 7177, Institut de Chimie de Strasbourg et Université de Strasbourg, Bâtiment Le Bel, 4 rue Blaise Pascal, CS 90032, F-67081 Strasbourg cedex, France.
authors.forename: Laila
authors.initials: L
authors.lastname: Benhamou
authors.forename: Aurore
authors.initials: A
authors.lastname: Thibon
authors.forename: Lydia
authors.initials: L
authors.lastname: Brelot
authors.forename: Mohammed
authors.initials: M
authors.lastname: Lachkar
authors.forename: Dominique
authors.initials: D
authors.lastname: Mandon
mesh_terms: D003460:Crystallization; D005296:Ferrous Compounds / Q000737:chemistry*; D058116:Lewis Acids / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D005296:Ferrous Compounds; D058116:Lewis Acids; D008024:Ligands; D010100:Oxygen