Switching the Spin-Crossover Phenomenon by Ligand Design on Imidazole–Diazineiron(II) Complexes

Abstract

The iron­(II) complexes of two structural isomers of 2-(1H-imidazol-2-yl)­diazine reveal how ligand design can be a successful strategy to control the electronic and magnetic properties of complexes by fine-tuning their ligand field. The two isomers only differ in the position of a single diazinic nitrogen atom, having either a pyrazine (Z) or a pyrimidine (M) moiety. However, [Fe­(M)3]­(ClO4)2 is a spin-crossover complex with a spin transition at 241 K, whereas [Fe­(Z)3]­(ClO4)2 has a stable magnetic behavior between 2 and 300 K. This is corroborated by temperature-dependent Mössbauer spectra showing the presence of a quintet and a singlet state in equilibrium. The temperature-dependent single-crystal X-ray diffraction results relate the spin-crossover observed in [Fe­(M)3]­(ClO4)2 to changes in the bond distances and angles of the coordination sphere of iron­(II), hinting at a stronger σ donation of ligand Z in comparison to ligand M. The UV/vis spectra of both complexes are solved by means of the multiconfigurational wave-function-based method CASPT2 and confirm their different spin multiplicities at room temperature, as observed in the Mössbauer spectra. Calculations show larger stabilization of the singlet state in [Fe­(Z)3]2+ than in [Fe­(M)3]2+, stemming from the slightly stronger ligand field of the former (506 cm–1 in the singlet). This relatively weak effect is indeed capable of changing the spin multiplicity of the complexes and causes the appearance of the spin transition in the M complex.

Analysis of solvent-accessible voids and proton-coupled electron transfer of 2,6-bis(1H-imidazol-2-yl)pyridine and its hydrochloride

“…The effect of water as an implicit solvent was assessed using the C-PCM (Barone & Cossi, 1998) method with the standard implementation in the software. This choice of functional, basis set and implicit solvation is well known to give reliable results for protonation and tautomeric equilibria in solution and it has been used over the years by our group (Abbehausen et al, 2012;Bibi et al, 2018b;Timm et al, 2008;Bonacin et al, 2007). In all calculations, the RIJCOSX approximation (Izsá k & Neese, 2011) and auxiliary functions (Weigend, 2006) were employed with convergence criteria of 1.0 Â 10 À8 Eh for the energies and 1.0 Â 10 À4 Eh/a 0 for the gradients in geometry optimizations.…”

Ab Initio Methods in First‐Row Transition Metal Chemistry

“…The results of standard CASPT2 also lead to a proposal of the so‐called CASPT2/CC method, which treats valence correlation at the CASPT2 level and semicore correlation at the CCSD(T) level [106] . The performance of CASPT2/CC was further demonstrated by Pierloot and co‐workers when studying two imidazole–diazineiron(II) complexes [158] and two di‐Fe II complexes with polydentate thiadiazole or oxadiazole ligands [45] . In both cases, the error of CASPT2/CC is expected to be a few kcal/mol.…”

OctaDist: a tool for calculating distortion parameters in spin crossover and coordination complexes

“…5,6 These studies have provided new principles for the design, not only of SCO compounds, but also iron-based catalysts where the reactivity is often spinstate dependent. 7,8 Spin crossover involves switching between low-spin (LS) and high-spin (HS) states and is determined by the free energy difference and depends on the oxidation state of the central metal ion and the ligand donors. In the LS state, the bonding t 2g orbitals are preferentially occupied while in the HS state some electrons are promoted in to the antibonding e g orbitals.…”