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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2012-12, Vol.41 (47), p.14369-1438
• Main Authors: Benhamou, Laila, Thibon, Aurore, Brelot, Lydia, Lachkar, Mohammed, Mandon, Dominique
• Format: Article
• Language: English
• Subjects: Crystallization; Displays; Ferrous Compounds - chemistry; Invariants; Lewis Acids - chemistry; Ligands; Models, Molecular; Oxygen - chemistry; Precursors; Pyridines; Solid state; Stereoisomerism; Tripods
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c2dt31558k

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 co