Running in the Family: Molecular Factors controlling Spin Crossover of Iron(II) Complexes with Schiff‐base like Ligands

Abstract

Tailoring of spin state energetics of transition metal complexes and even the correct prediction of the resulting spin state is still a challenging task, both for the experimentalist and the theoretician. Apart from the complexity in the solid state imposed by packing effects, molecular factors of the spin state ordering are required to be identified and quantified on equal rights. In this work we experimentally record the spin states and SCO energies within an eight-member substitution-series of N 4 O 2 ligated iron(II) complexes both in the solid state (SQUID magnetometry and single-crystal X-ray crystallography) and in solution (VT-NMR). The experimental survey is complemented by exhaustive theoretical modelling of the molecular and electronic structure of the open-chain N 4 O 2 family and its macrocyclic N 6 congeners through density-functional theory methods. Ligand topology is identified as the leading factor defining ground-state multiplicity of the corresponding iron(II) complexes. Invariably the low-spin state is sterically trapped in the macrocycles, whereas subtle substitution effects allow for a molecular fine tuning of the spin state in the open-chain ligands. Factorization of computed relative SCO energies holds promise for directed design of future SCO systems.

Asymmetric Design of Spin-Crossover Complexes to Increase the Volatility for Surface Deposition

“…Herein, we apply such a strategy to a homoleptic SCO complex based on the combination of an Fe II ion with tridentate Schiff-base type ligands. The ligands of Schiff-base type are well established to provide an appropriate ligand field strength for the occurrence of SCO in Fe II complexes. − We demonstrate that the complex [Fe­( t Bu 2 qsal) 2 ] (Scheme ), where t Bu 2 qsal – is a deprotonated form of 2,4-di­( tert -butyl)-6-((quinoline-8-ylimino)­methyl)­phenol ( t Bu 2 qsalH), undergoes an abrupt spin transition centered at 123 K with a 12 K thermal hysteresis. The complex also exhibits light-induced spin-state switching at low temperatures.…”

Sterically Encumbered Coordination Sites. Iron(II) Complexes of Jäger‐type ligands with a Terphenyl Backbone

“…[31] Viewing from the front, the chelate resembles a boat or saddle conformation, whereas a side view reveals a significant pyramidalization of the coordinating nitrogen atoms with a N-distance from the CÀ FeÀ C plane of about 0.15 Å. For comparison, other systems exhibit minor pyramidalization [15,19] 5). This translates into an approximately C 2 symmetric orientation which leaves the N 2 O 2 chelate close to planar.…”

“…It is not possible to reliably predict the transition curve (gradual, abrupt, hysteresis) and whether SCO occurs at all. Contrary to SCO in solution, where the transition usually obeys Boltzmann statistics and can be correlated with DFT modelling, [14,15] the complete understanding of the interplay of molecular properties and intermolecular interactions in the solid-state remains difficult. Consensus, however, exists about the need of intermolecular interactions; so, the synergy between long-and short-range contacts such as hydrogen bonding and linkage of metal centers via bridging ligands seems to be the key towards cooperative spin transitions which are abrupt or even show thermal bistability (hysteresis).…”

An Iron(II) Spin Crossover Complex with a Maleonitrile Schiff base‐like Ligand and Scan Rate‐dependent Hysteresis above Room Temperature

“…Based on reliable structures of HS and LS formulations, the SCO energies of the complexes [FeL(HIm) 2 ] and [FeL(Py) 2 ] were addressed within an exact-exchange scan utilizing derivatives of the well-established B3LYP hybrid functional; this approach proved very efficient in predicting the SCO energy ordering within families of complexes. [19,20] Plotted in Figure 9 are the apparent SCO energies, Δ SCO E = E(HS)-E(LS) as a function of the admixture of exact Fock-exchange a 0 in the functional. The plots are highly linear with large negative slopes, as is a typical feature of iron(II) complexes.…”

“…Although the T 1/2" and T 1/2# values are slightly higher compared to SQUID and DSC measurements, they are still in good agreement considering the different measurement technique. [10,11,15,19] Imidazole (HIm) in the axial position renders the complex S2 in the SI). We find close agreement of predicted and experimental metrics (within Δd < 2 pm), when the LS data are considered.…”