Effects of temperature and pressure on the molecular and electronic structure of N,N′-ethylenebis(salicylideneiminato)nitrosyliron, Fe(NO)(salen)

Abstract

The crystal and molecular structures of the two magnetically distinct forms of Fe(NO)(salen), C 16H 14N 3O 3Fe, have been determined by single crystal X-ray diffraction at 23 °C and at −175 °C. The complex crystallizes in the orthorhombic space group Pna2 1 with 4 molecules per unit cell. The unit cell dimensions are a = 26.377(2), b = 8.598(5) and c = 6.951(5) Å at 23 °C and a = 26.18(2), b = 8.52(1) and c = 6.783(6) Å at −175 °C. Full-matrix least-squares refinement of the 23 °C structure based on the 840 reflections with F 2 o ⩾ 3σ(F 2 o) gave a conventional R-factor of 0.038. The structure consists of discrete Fe(NO)(salen) molecules with tetragonal pyramidal coordination about the iron atom. The disordered nitrosyl group occupies the axial coordination site in a bent geometry (average FeNO angle 147°). Full-matrix least-squares refinement of the −175°C structure based on the 406 reflections with F 2 o ⩾ 2σ(F 2 o) gave a conventional R-factor of 0.087. This form of Fe(NO)(salen) also exhibits tetragonal pyramidal coordination geometry with a bent disordered nitrosyl group in the apical position. Marginally significant structural changes are observed: 1) the FeNO angle is 127°; 2) the Fe atom is 0.1 Å closer to the mean coordination plane of the salen ligand are more nearly coplanar. These structural changes are consistent with a spin state of Fe(NO)(salen) from S = 3 2 to S = 1 2 upon cooling. The significant differences in th conformations of the salen ligand at the two temperatures are attributed to crystal packing. The infrared spectra of Fe(NO)(salen) at room temperature for various pressures ranging from ambient pressure up to 37 kbar are reported. The spectra suggest that Fe(NO)(salen) is converted to the S = 1 2 state at high pressure.