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, CH3O, CN, NO).: The S=1/2⇆S=3/2 spin equilibrium of [FeLPr(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)(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) reversible arrow S (3)/(2) in the solid state. Complexes [Zn(L-Pr)](1), 4.CH3OH, 5.0.5toluene.CH2Cl2, 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 = PR3, CO, SR2) have been studied by EPR and Mossbauer spectroscopies and DFT calculations (pyS(4) = 2,6-bis(2-mercaptophenylthio-methyl)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 (Fell-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) reversible arrow S = (3)/(2) equilibrium is then that of valence tautomers rather than that of a simple high spin reversible arrow low spin crossover.