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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, (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)(OCH₃)] (3) and [Fe^III(L^Pr)(OCH₃)] (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·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 (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.