5-Aminotetrazole induces spin crossover in iron(III) pentadentate Schiff base complexes: experimental and theoretical investigations

A series of novel mononuclear iron(III) complexes [Fe(saldpt)(atz)] (1) and [Fe(Rsalpet)(atz)] (2a-d) involving 5-aminotetrazole (Hatz) and pentadentate Schiff base ligands (H(2)saldpt = N,N'-bis(2-hydroxybenzyliden)-1,7-diamino-4-azaheptane, H(2)salpet = N,N'-bis(2-hydroxybenzyliden)-1,6-diamino-3-azahexane, H(2)5Cl-salpet = N,N'-bis(5-chloro-2-hydroxybenzylidene)-1,6-diamino-3-azahexane, H(2)5Br-salpet = N,N'bis(5-bromo-2-hydroxybenzylidene)-1,6-diamino-3-azahexane and H(2)3,5Br-salpet = N,N'-bis(3,5-dibromo-2-hydroxybenzylidene)-1,6-diamino-3-azahexane) was prepared and characterized by elemental analysis, FTIR spectroscopy, mass spectrometry, single crystal X-ray analysis, magnetic measurements and Fe-57 Mossbauer spectroscopy. In contrast to the high-spin magnetic properties of [Fe(saldpt)(atz)] (1), the complexes [Fe(Rsalpet)(atz)] (2a-d) undergo spin crossover with critical temperatures above 416 K. Furthermore, DFT calculations with the B3LYP functional were employed to investigate the effect of the conductor-like screening model (COSMO) and van der Waals corrections (VDW) as well as relativistic effects with the scalar relativistic second-order Douglas-Kroll-Hess Hamiltonian (DKH2) on low-spin and high-spin molecular geometries and thus on spin crossover parameters (enthalpy, entropy and critical temperature).