Spin Crossover in Fe(II) and Co(II) Complexes with the Same Click- Derived Tripodal Ligand David Schweinfurth, † Serhiy Demeshko, ‡ Stephan Hohloch, † Marc Steinmetz, § Jan Gerit Brandenburg, § Sebastian Dechert, ‡ Franc Meyer, ‡ Stefan Grimme,* ,§ and Biprajit Sarkar* ,† † Institut fü r Chemie und Biochemie, Anorganische Chemie, Fabeckstraße 34-36, D-14195, Berlin, Germany ‡ Institut fü r Anorganische Chemie, Georg-August-Universitä t Gö ttingen, Tammannstraße 4, D-37077, Gö ttingen, Germany § Mulliken Center for Theoretical Chemistry, Institut fü r Physikalische und Theoretische Chemie, Universitä t Bonn, Beringstraße 4, D-53115, Bonn, Germany * S Supporting Information ABSTRACT: The complexes [Fe(tbta) 2 ](BF 4 ) 2 ·2EtOH (1), [Fe(tbta) 2 ](BF 4 ) 2 ·2CH 3 CN (2), [Fe(tbta) 2 ](BF 4 ) 2 ·2CHCl 3 (3), and [Fe(tbta) 2 ](BF 4 ) 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 T 1/2 near room temperature, as revealed by superconducting quantum interference device (SQUID) magnetometric and Mö ssbauer 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 T 1/2 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. ■ INTRODUCTION Octahedral metal complexes with a d 4 −d 7 electronic conﬁg- uration can, in principle, exist in the high spin (HS) or low spin (LS) states. 1 The stabilization of one spin state or the other is dependent on the interplay between the ligand ﬁeld stabilization energy (LFSE) and the spin pairing energy. Magnetic bistability is observed for systems where these two energy terms are comparable in magnitude, and such systems can be switched between the two spin states by using external perturbations such as temperature; this is a phenomenon that is known as spin crossover (SCO). 2−4 Octahedral Fe(II) complexes 3 and Co(II) complexes 5 are the ones that have been investigated the most, with respect to their SCO properties. In this context, it is well-established that ligands with diﬀerent ligand ﬁeld strengths are required for generating SCO behavior in Fe(II) and Co(II) complexes. SCO compounds have fascinated both physicists and chemists over Received: September 1, 2013 Article pubs.acs.org/IC © XXXX American Chemical Society A dx.doi.org/10.1021/ic500264k | Inorg. Chem. XXXX, XXX, XXX−XXX