A diamagnetic iron complex and its twisted sister - structural evidence on partial spin state change in a crystalline iron complex

Abstract

<p> We report here the syntheses of a diamagnetic Fe complex [Fe(HL)<small>₂</small>] (<strong>1</strong>), prepared by reacting a redox non-innocent ligand precursor <em>N</em>,<em>N</em>′-bis(3,5-di-<em>tert</em>-butyl-2-hydroxy-phenyl)-1,2-phenylenediamine (H<small>₄</small>L) with FeCl<small>₃</small>, and its phenoxazine derivative [Fe(L′)<small>₂</small>] (<strong>2</strong>), which was obtained <em>via</em> intra-ligand cyclisation of the parent complex. Magnetic measurements, accompanied by spectroscopic, structural and computational analyses show that <strong>1</strong> can be viewed as a rather unusual Fe(<small>III</small>) complex with a diamagnetic ground state in the studied temperature range due to a strong antiferromagnetic coupling between the low-spin Fe(<small>III</small>) ion and a radical ligand. For a paramagnetic high-spin Fe(<small>II</small>) complex <strong>2</strong> it was found that, when crystalline, it undergoes a thermally induced process where 25% of the molecules in the material change to a diamagnetic low-spin ground state below 100 K. Single crystal X-ray studies conducted at 95 K afforded detailed structural evidence for this partial change of spin state of <strong>2</strong> showing the existence of crystallographically distinct molecules in a 3 : 1 ratio which exist in high- and low-spin states, respectively. Also, the magnetic behaviour of <strong>2</strong> was found to be related with the crystallinity of the material as demonstrated by near-IR radiation to unpaired electrons conversion ability of amorphous sample of <strong>2</strong>. </p>