Impact of Substituent Variation on the Presence of Thermal Spin Crossover in a Series of Mononuclear Iron(III) Schiff Base Complexes with Terminal Pseudohalido Co-ligands.

A series of novel iron(III) complexes of the general formula [Fe(L)X] (where L is a dianion of pentadentate Schiff base ligand N,N'-bis({2-hydroxy-3,5-dimethylphenyl}phenyl)methylidene-1,6-diamino-3-azapentane=H2 L1 for 1 and 2; N,N'-bis({2-hydroxy-3-ethoxyphenyl}methylidene)-1,6-diamino-3-azapentane=H2 L2 for 3 and 3⋅C3 H6 O) and X is terminal pseudohalido ligand (X=N3 for 1, X=NCS for 2, and X=NCSe for 3 and 3⋅C3 H6 O) were synthesized and thoroughly characterized. Magnetic measurements revealed the above room temperature spin crossover for isomorphic complexes 1 and 2 (T1/2 =441 K and T1/2 =435 K, respectively), whereas the solvent-free complex 3 showed a half complete spin crossover (T1/2 =250 K), which was detected by variable temperature crystallography as well. On the other hand, solvated complex 3⋅C3 H6 O exhibited permanent high spin state behaviour and either recrystallization or in situ thermal desolvation converts 3⋅C3 H6 O to solvent-free and spin-crossover-active form 3. Magnetic properties of all the reported complexes were also supported by EPR spectroscopy experiments and in addition, DFT and ab initio calculations were employed for the evaluation of the g-factor and zero field splitting parameters.