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Abstract:
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, (LPr)2-, or an ethyl group, (LEt)2-, have been synthesized. It is shown that they bind strongly to ferric ions generating six-coordinate species of the type [Fe(Lalk)X]. The ground state of these complexes is governed by the nature of the sixth ligand, X: [FeIII(LEt)Cl] (2) possesses an S = 5/2 ground state as do [FeIII(LEt)(OCH3)] (3) and [FeIII(LPr)(OCH3)] (4). In contrast, the cyano complexes [FeIII(LEt)(CN)] (5) and [FeIII(LPr)(CN)] (6) are low spin ferric species (S = 1/2). The octahedral {FeNO}7 nitrosyl complex [Fe(LPr)(NO)] (7) displays spin equilibrium behavior S = 1/2 ⇌ S = 3/2 in the solid state. Complexes [Zn(LPr)] (1), 4·CH3OH, 5·0.5toluene·CH2Cl2, and 7·2.5CH2Cl2 have been structurally characterized by low-temperature (100 K) X-ray crystallography. All iron complexes have been carefully studied by zero- and applied-field Mössbauer spectroscopy. In addition, Sellmann's complexes [Fe(pyS4)(NO)]0/1+ and [Fe(pyS4)X] (X = PR3, CO, SR2) have been studied by EPR and Mössbauer spectroscopies and DFT calculations (pyS4 = 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 (SFe = 0) with a coordinated neutral NO radical (FeII−NO) whereas the S = 3/2 state corresponds to a high spin ferric (SFe = 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.
