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

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.