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Li M, Bonnet D, Bill E, Neese F, Weyhermuller T, Blum N, Sellmann D, Wieghardt K: Tuning the electronic structure of octahedral iron complexes [FeL (X)] (L = 1-alkyl-4,7-bis (4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane, X = Cl, CH (3) O, CN, NO). Inorg Chem. 2002 Jul 1;41(13):3444-56.
The S = 1/2 <==> 3/2 Spin equilibrium of [FeL (Pr)(NO)].. Two new pentadentate, pendent arm macrocyclic ligands of the type 1-alkyl-4,7-bis (4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane where alkyl represents an isopropyl, (L (Pr))(2-), or an ethyl group, (L (Et))(2-), have been synthesized. It is shown that they bind strongly to ferric ions generating six-coordinate species of the type [Fe (L (alk)) X]. The ground state of these complexes is governed by the nature of the sixth ligand, X: [Fe (III)(L (Et)) Cl] (2) possesses an S = 5/2 ground state as do [Fe (III)(L (Et))(OCH (3))] (3) and [Fe (III)(L (Pr))(OCH (3))] (4). In contrast, the cyano complexes [Fe (III)(L (Et))(CN)] (5) and [Fe (III)(L (Pr))(CN)] (6) are low spin ferric species (S = 1/2). The octahedral [FeNO](7) nitrosyl complex [Fe (L (Pr))(NO)] (7) displays spin equilibrium behavior S = 1/2 <==> S = (3)/(2) in the solid state. Complexes [Zn (L (Pr))] (1), 4.CH (3) OH, 5.0.5toluene.CH (2) Cl (2), and 7.2.5CH (2) Cl (2) have been structurally characterized by low-temperature (100 K) X-ray crystallography. All iron complexes have been carefully studied by zero- and applied-field Mossbauer spectroscopy. In addition, Sellmann's complexes [Fe (pyS (4))(NO)](0/1+) and [Fe (pyS (4)) X] (X = PR (3), CO, SR (2)) have been studied by EPR and Mossbauer spectroscopies and DFT calculations (pyS (4) = 2,6-bis (2-mercaptophenylthiomethyl) pyridine (2-)). It is concluded that the electronic structure of 7 with an S = 1/2 ground state is low spin ferrous (S (Fe) = 0) with a coordinated neutral NO radical (Fe (II)-NO) whereas the S = 3/2 state corresponds to a high spin ferric (S (Fe) = 5/2) antiferromagnetically coupled to an NO (-) anion (S = 1). The S = 1/2 <==> S = 3/2 equilibrium is then that of valence tautomers rather than that of a simple high spin <==> low spin crossover. |
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