Spin Crossover in Fe(II) and Co(II) Complexes with the Same Click-Derived Tripodal Ligand

Abstract

The complexes [Fe(tbta)2](BF4)2·2EtOH (1), [Fe(tbta)2](BF4)2·2CH3CN (2), [Fe(tbta)2](BF4)2·2CHCl3 (3), and [Fe(tbta)2](BF4)2 (4) were synthesized from the respective metal salts and the click-derived tripodal ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (tbta). Structural characterization of these complexes (at 100 or 133 K) revealed Fe-N bond lengths for the solvent containing compounds 1-3 that are typical of a high spin (HS) Fe(II) complex. In contrast, the solvent-free compound 4 show Fe-N bond lengths that are characteristic of a low spin (LS) Fe(II) state. The Fe center in all complexes is bound to two triazole and one amine N atom from each tbta ligand, with the third triazole arm remaining uncoordinated. The benzyl substituents of the uncoordinated triazole arms and the triazole rings engage in strong intermolecular and intramolecular noncovalent interactions. These interactions are missing in the solvent containing molecules 1, 2, and 3, where the solvent molecules occupy positions that hinder these noncovalent interactions. The solvent-free complex (4) displays spin crossover (SCO) with a spin transition temperature T1/2 near room temperature, as revealed by superconducting quantum interference device (SQUID) magnetometric and Mössbauer spectroscopic measurements. The complexes 1, 2, and 3 remain HS throughout the investigated temperature range. Different torsion angles at the metal centers, which are influenced by the noncovalent interactions, are likely responsible for the differences in the magnetic behavior of these complexes. The corresponding solvent-free Co(II) complex (6) is also LS at lower temperatures and displays SCO with a temperature T1/2 near room temperature. Theoretical calculations at molecular and periodic DFT-D3 levels for 1-4 qualitatively reproduce the experimental findings, and corroborate the importance of intermolecular and intramolecular noncovalent interactions for the magnetic properties of these complexes. The present work thus represents rare examples of SCO complexes where the use of identical ligand sets produces SCO in Fe(II) as well as Co(II) complexes.

Metal Complexes of Click-Derived Triazoles and Mesoionic Carbenes: Electron Transfer, Photochemistry, Magnetic Bistability, and Catalysis

“…While amine anchored triazole containing tripodal ligands have been the most investigated, there have also been reports of other types of tripodal ligands like L11 based on a rigid backbone, or ligands L12 that are based on a carbon anchor (Figure ). The ligand tbta was shown to form complexes of the form [ M (tbta) 2 ] 2+ with iron(II), 45 2+ , cobalt(II), 46 2+ , and nickel(II), 47 2+ (Figure ) . In all these complexes the tbta ligands bind to the central metal atoms through the central amine nitrogen atom and two of the triazole N‐donors.…”

“…In all these complexes the tbta ligands bind to the central metal atoms through the central amine nitrogen atom and two of the triazole N‐donors. The third triazole arm remains uncoordinated, and in each of these complexes the phenyl ring from the benzyl substituents of the uncoordinated triazole rings form intramolecular C–H ··· π interactions (Figure ) . These interactions have strong consequences for the magnetic properties of these metal complexes (see below).…”

“…Thus, whereas an 1:2 ratio of a cobalt(II) salt and tbta led to the formation of the aforementioned complex 46 2+ , a 1:1 ratio led to the formation of the complex [Co(tbta)(CH 3 CN) 3 ] 2+ ( 48 2+ ). [49b] Intriguingly, the tbta ligand in 48 2+ binds through all the three triazole N‐donors with the central amine donor remaining uncoordinated. Hence, in the complexes 46 2+ and 48 2+ , which can be switched back and forth in solution (Figure and Figure ), the ligand tbta shows an ambivalent coordination behavior.…”

Hexakis (propargyl-1H-tetrazole) Iron(II) X2 [X = BF4, ClO4]—Spin Switchable Complexes with Functionalization Potential and the Myth of the Explosive SCO Compound

“…The latter can especially be regarded as highly suitable for this purpose, as similar examples of post-functionalization of complexes are known in the literature [31][32][33].…”

Hybrid polystyrene based electrospun fibers with spin-crossover properties

“…The area of spin crossover (SCO) attracts interest both on a fundamental level and in terms of potential applications for compounds displaying this phenomenon . A few families of Co(II) and Fe(II) complexes have become paramount for the advance of this topic, for instance linear polymeric chains of Fe(II) ions bridged by triazol ligands have been extensively studied over the past decades . These metallo‐organic polymers exhibit the SCO phenomenon; a thermally induced transition between two electronic states: a low‐spin (LS) and high‐spin (HS) state that are dia‐ and paramagnetic, respectively.…”