This dissertation presents three topics dealing with the influence of the ligand environment of pentadentate ligands on geometry, spin state, and reactivity of iron complexes. On the one hand, the isolated influence of the donor atoms on the ligand environment and thus on the structure of the complexes could be clearly demonstrated. For this purpose, the ligand TPSEN (N, N-bis(pyridine-2-ylmethyl)-2-[(pyridine-2-ylmethyl)thio]ethane-1-amine) was prepared and its iron(II) complex [Fe(TPSEN)(MeCN)](OTf)2 structurally described. The comparison of the complexes of TPSEN with those of the topologically similar ligands known by literature BnTPEN (N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) and TPOEN (N,N-bis(pyridine-2-ylmethyl)-2-(pyridine-2-ylmethoxy)ethane-1-amine) showed the influence of the variable donors (N, O, S) on the ligand field strength and thus on the structural and electronic properties as well as the reactivity of the iron complexes. Whereas the existence of high-valent iron complexes of TPSEN could only be shown indirectly, [Fe(BnTPEN)(MeCN)]2+ (a pure ls compound) can stabilize a relatively long-lived oxoiron species. An iron(III)-hydroperoxide species is accessible to both ligands. The variation of the co ligand (OTf, MeCN, py) in the iron(II) complexes with TPSEN also showed a large influence on its properties of the complex consistent with the assumptions of the ligand field theory. While [Fe(TPSEN)(OTf)]+ is a pure hs species, [Fe(TPSEN)(MeCN)]2+ and [Fe(TPSEN)(py)]2+ are SCO systems with different turnover temperatures (t1/2). Furthermore, the influence of the arrangement of the donor atoms in three-dimensional space on the structure, configurational and spin-state preferences of the metal complexes could be shown by ligand-mediated distortion. For this purpose, the iron(II) complexes of two structurally similar N5 ligands L03 (6-methyl-6-(pyridine-2-yl)-1,4-bis(pyridine-2-ylmethyl)-1,4-diazepane) and L04 (N1,N3,2-trimethyl-2-(pyridine-2-yl)-N1,N3-bis(pyridine-2-ylmethyl)propane-1,3-diamine) with identical donor set have been studied in the solid state and in solution. While the complexes of the open-chain ligand L04 show a nearly ideal octahedral geometry, the rigidity of L03 forces a strongly distorted coordination polyhedron which is between octahedron and a trigonal prism. The reason is the trans-extension of the Fe–N bonds which can not be compensated by the rigid ligand skeleton. This ligand-induced distortion significantly reduces the ligand field strength of the complexes, which leads to a clear preference for the electronic hs state, the formation of configurational isomers and a high kinetic lability in terms of ligand exchange and oxidation of the central ion. In addition, the oxo-iron(IV) complex [Fe(L04)(O)]2+ could be investigated as a biomimetic non-heme model complex in terms of formation efficiency and rate, stability and consecutive reactions, and as a model system for the iron-mediated O–O bond formation within the Photosystem II. The reaction of [Fe(L04)(MeCN)]2+ with an excess of mCPBA and without addition of another substrate showed the formation of dioxygen and the partial recovery of the starting complex. This reactivity is unprecedented for non-heme iron compounds.
|School:||Technische Universitaet Berlin (Germany)|
|Source:||DAI-C 81/3(E), Dissertation Abstracts International|
|Subjects:||Analytical chemistry, Biochemistry|
|Keywords:||Pentadentate ligands , TPSEN , Oxoirons|
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