[Fe(nsal₂trien)]SCN, a New Two-Step Iron(III) Spin Crossover Compound, with Symmetry Breaking Spin-State Transition and an Intermediate Ordered State

We report the synthesis of the iron­(III) complex of the hexadentate Schiff base ligand nsal₂trien obtained from the condensation of triethylenetetramine and 2 equiv. of 2-hydroxy-1-naphthaldehyde. The study of the salt [Fe­(nsal₂trien)]­SCN (<b>1</b>) by magnetic susceptibility measurements and Mössbauer spectroscopy reveals a rather unique behavior that displays thermally induced spin crossover (SCO) with two well-separated steps at 250 (gradual transition) and 142 K (steep transition). Single crystal X-ray structures were obtained at 294, 150, and 50 K, for the high spin (HS), intermediate (Int), and low spin (LS) phases. The HS and LS phases are isostructural, and based on a single Fe^III site (either HS or LS) an unusual symmetry break occurs in the transition to the Int ordered phase, where the unit cell includes two distinct Fe^III sites and is based on a repetition of the [HS–LS] motif. The two-step SCO behavior of <b>1</b> must result from the existence of structural constraints preventing the full conversion HS ↔ LS in a single step

Dithiophene-TTF Salts; New Ladder Structures and Spin-Ladder Behavior

(α-DT-TTF)₂[Au­(<i>i</i>-mnt)₂] and (α-DT-TTF)₂[Co­(mnt)₂] are two new salts of the donor α-dithiophene-tetrathiafulvalene with stable diamagnetic anions, both presenting a ladder structure of the donors organized in paired segregated stacks. The first one is isostructural with previously reported closely related compounds and presents a magnetic spin-ladder behavior with <i>J</i>_∥= 83.5 K and <i>J</i>_⊥ = 110.3 K as estimated from spin susceptibility data in single crystals. (α-DT-TTF)₂[Co­(mnt)₂] presents a new structural type with a different arrangement of pairs of donor stacks, alternating with stacks of dimerized [Co­(mnt)₂] anions which are however arranged in an uncorrelated fashion perpendicular to the stacking axis. Due to the strong coupling between the disordered anion chains and the donor chains, this compound does not present a magnetic spin-ladder behavior. The low temperature superstructure of (DT-TTF)₂[Cu­(mnt)₂] below the transition at 235 K, previously known to be associated with a lattice doubling along the stacking axis, was solved by synchrotron radiation diffraction in small single crystals. It is found that this dimerization is due to donor charge localization with the spin carriers being associated with fully oxidized donor species alternating with neutral donors