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(SO₃CF₃)₂ to the series of fluoro α-substituted tris-(2-aminomethylpyridyl)amine tripods F_1–3TPA yields the triflato F_1–3TPAFe(SO₃CF₃)₂ complexes which have firstly been characterized in solution. As expected, bis-acetonitrile charged species are present in CH₃CN, and neutral bis-triflato complexes in CH₂Cl₂. The X-ray diffraction analyses of F₁TPAFe(SO₃CF₃)₂ and F₂TPAFe(SO₃CF₃)₂ crystallized from CH₂Cl₂ solutions show that their structure in solution is retained in the solid state, with coordination of both triflate ions and the κ⁴ coordination mode of the tripod in each complex. The solid state structure of the [F₂TPAFe(NCMe)(SO₃CF₃)](SO₃CF₃) 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 [F₂TPAFe(H₂O)₂](SO₃CF₃)₂, fortuitously obtained after the bis-triflato precursor was handled under aerobic conditions. In CH₃CN, 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 CH₂Cl₂, the parent TPAFe(SO₃CF₃)₂ complex reacts with O₂ to yield a compound belonging to the well-known class of μ-oxo diferric compounds. Whereas F₁TPAFe(SO₃CF₃)₂ is poorly reactive, F₂TPAFe(SO₃CF₃)₂ and F₃TPAFe(SO₃CF₃)₂ turn out to be completely inert. This strongly contrasts with the behavior of the known F_1–3TPAFeCl₂ complexes for which an increased reactivity is observed upon ligand substitution. In CH₂Cl₂, 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(SO₃CF₃)₂ complexes, whereas FeCl₂ complexes display important metal to ligand elongations upon tripod substitution. The reactivity increase upon fluorination of the ligand in the FeCl₂ complexes is interpreted as resulting from sterically-induced pyridine flexibility. The opposite situation with Fe(SO₃CF₃)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.