Dinitrosyl–iron complexes with thiol-containing ligands: Spatial and electronic structures

Abstract Parameters of the EPR signals of monomeric dinitrosyl–iron complexes with 1 H -1,2,4-triazole-3-thiol (DNIC–MT), obtained by treating MT + ferrous iron in DMSO solution with gaseous NO, have been compared with those of the crystalline monomeric DNIC–MT with tetrahedral structure. Dissolved DNIC–MT were characterized by the isotropic EPR signal centered at g  = 2.03 with half-width of 0.7 mT and quintet hyperfine structure when recorded at ambient temperature or the anisotropic EPR signal with g ⊥  = 2.045, g ∥  = 2.014 from frozen solution at 77к. DNIC–MT in crystalline state showed the structure-less symmetrical singlet EPR signal centered at g  = 2.03 and half-width of 1.7 mT at both room and liquid nitrogen temperature. The Lorentz shape of this signal indicates the strong exchange interaction between these complexes in the DNIC–MT crystal. Being dissolved in DMSO the crystalline sample of DNIC–MT demonstrated the EPR signal typical for DNIC–MT, obtained by treating MT + ferrous iron in DMSO solution with gaseous NO. Low spin ( S  = 1/2) d 9 electron configuration of DNIC–MT with tetrahedral structure (formula {(MT-S · ) 2 Fe −1 (NO + ) 2 } + ) was suggested to be responsible for the signal of DNIC–MT in crystalline state. Dissolving of the crystals of DNIC–MT may result in the change of their spatial and electronic structure, namely, tetrahedral structure of the complexes characterized by low spin d 9 electronic configuration transforms into a plane-square structure with d 7 electronic configuration and low spin S  = 1/2 state (formula {(MT- S − ) 2 Fe + (NO + ) 2 } + ). The latter was suggested to be characteristic of other DNICs with various thiol-containing ligands in the solutions. The proposed mechanism of these DNICs formation from ferrous iron, thiol and NO shows that the process could be accompanied by the ionization of NO molecules to NO + and NO − ions in the complexes. Detailed analysis of the shape of the EPR signals of these complexes provided additional information about the exchange interaction typical for DNIC–MT in crystals.