Structural bases of oxygen-sensitivity in Fe(II) complexes with tripodal ligands. Steric effects, Lewis acidity and the role of ancillary ligands

The complexation of Fe(SO3CF3)(2) to the series of fluoro alpha-substituted tris-(2-aminomethylpyridyl)amine tripods F(1-3)TPA yields the triflato F(1-3)TPAFe(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 F(1)TPAFe(SO3CF3)(2) and F(2)TPAFe(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. 4 coordination mode of the tripod in each complex. The solid state structure of the [F(2)TPAFe(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 [F(2)TPAFe(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 O-2 to yield a compound belonging to the well-known class of mu-oxo diferric compounds. Whereas F(1)TPAFe(SO3CF3)(2) is poorly reactive, F(2)TPAFe(SO3CF3)(2) and F(3)TPAFe(SO3CF3)(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 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.