In this work, two iron(II) coordination compounds with a N 2 O 2 coordinating Schiff base-like ligand bearing a redox active tetrathiafulvalene (TTF) unit and pyridine or trans-1,2-bis(4-pyridylethylene) as an axial ligand are synthesized. Crystals suitable for single X-ray structure analysis were obtained for the new ligand. The complexes were characterized by magnetic susceptibility measurements, T-dependent UV−vis spectroscopy, and cyclic voltammetry. Both complexes display spin transition behavior below room temperature with T 1/2 values of 146 and 156 K. The mononuclear iron(II) complex [Fe TTF L(py) 2 ] is relatively stable up to 400 K compared to similar complexes, showing no loss of axial ligands upon heating. Temperature dependent Mossbauer spectroscopy was conducted for the coordination polymer {[Fe TTF L(bpee)]} n to get more information regarding the origin of the stepwise spin crossover (SCO) behavior observed in the magnetic measurements. The change of the spin state is accompanied by a change of the optical properties, which can be monitored by VT−UV−vis spectroscopy for the mononuclear complex and has been analyzed in theoretical studies. The redox behavior of the iron(II) complexes reveals three reversible redox steps which are located at the iron center and at the TTF unit of the ligand. Oxidation of the TTF unit induces characteristic changes in the UV−vis spectrum that can be followed by spectroelectrochemical UV−vis spectroscopy. Addressing the potential of the ironcentered redox process results in similar changes in the UV−vis spectrum, which indicates an electronic coupling of the redox active unit with the metal center under certain circumstances.

A 1-D chain CP with both SCO and redox activity has been successfully prepared by the combination of a bis-pyridyl functionalised TTF core and a Schiff base-like N2O2 ligand.

Two new ironIJII) 1D coordination polymers with spin crossover behaviour were synthesised using 3,3′azopyridine as bridging axial ligand and N 2 O 2 coordinating Schiff base-like equatorial ligands. The X-ray structures of both complexes were solved revealing very different packing patterns for the two complexes. Magnetic measurements reveal a spin transition with hysteresis and kinetic trapping effects at lower temperatures for one complex and a spin transition above room temperature for the second complex.

In this manuscript, we report the solvent‐dependent synthesis of 1D coordination polymers derived from two planar N2O2‐coordinate iron(II) complexes FeL1 and FeL2, which incorporate TTF(py)2 as a bridging bis‐monodentate ligand. The obtained 1D polymers were characterized through elemental analysis, Mössbauer spectroscopy, single crystal structure analysis for 2a·2 DMF, magnetic susceptibility measurements, X‐ray powder diffraction, cyclic voltammetry and diffuse reflectance spectroscopy, supplemented by DFT computation. The results revealed additive electronic properties of the sub‐units FeL and TTF(py)2 with only minor mutual influence. Intriguingly however, the solvent‐of‐synthesis is found to be a steering factor of the magnetic spin crossover properties of the resulting materials, yielding divergent behavior if obtained from DMF, MeCN or EtOH. This becomes strikingly evident for the magnetic properties of the DMF‐derived polymer which is found trapped in the low‐spin state in the single crystal 2a· 2 DMF, but shows a gradual spin crossover if all solvent is removed.

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.

Three new unique mononuclear iron(II) pincer complexes were synthesized using 1,2-bis(pyridin-2-ylethynyl)benzene as axially coordinating pincer ligand and N2O2 coordinating Schiff base-like equatorial ligands. Magnetic susceptibility measurements reveal that all three complexes remain in the high spin state throughout the entire temperature range investigated. Reasons for this are restraining sterical interactions revealed in the single crystal x-ray structure analysis and extended DFT-computational studies of one of the pincer complexes. Those interactions also lead to the formation of unexpected side products during the synthesis such as a complex with two ethanol molecules as axial ligand, whose x-ray structure was determined.

No abstract

In this study, a synthesis route of tri(quinolin-8yl)amine (L), a recent member of the tetradentate tris(2pyridylmethyl)amine (TPA) ligand family, is reported. With the neutral ligand L bound to an iron(II) center in κ 4 mode, two cisoriented coordination sites remain vacant. These can be occupied by coligands such as counterions and solvent molecules. How sensitive this equilibrium can be is most evident if both triflate anions and acetonitrile molecules are available. All three combinations�bis(triflato), bis(acetonitrile), and mixed coligand species�could be characterized by single-crystal X-ray diffraction (SCXRD), which is unique so far for this class of ligand. While at room temperature, the three compounds tend to crystallize concomitantly, the equilibrium can be shifted in favor of the bis(acetonitrile) species by lowering the crystallization temperature. Removed from their mother liquor, the latter is very sensitive to evaporation of the residual solvent, which was observed by powder X-ray diffraction (PXRD) and Mossbauer spectroscopy. The solution behavior of the triflate and acetonitrile species was studied in detail using time-and temperature-resolved UV/vis spectroscopy, Mossbauer spectroscopy of frozen solution, NMR spectroscopy, and magnetic susceptibility measurements. The results indicate a bis(acetonitrile) species in acetonitrile showing a temperature-dependent spin-switching behavior between highand low-spin. In dichloromethane, the results reveal a high-spin bis(triflato) species. In pursuit of understanding the coordination environment equilibria of the [Fe(L)] 2+ complex, a series of compounds with different coligands was prepared and analyzed with SCXRD. The crystal structures indicate that the spin state can be controlled by changing the coordination environment�all of the {N 6 }-coordinated complexes display geometries expected for low-spin species, while any other donor atom in the coligand position induces a shift to the high-spin state. This fundamental study sheds light on the coligand competition of triflate and acetonitrile, and the high number of crystal structures allows further insights into the influence of different coligands on the geometry and spin state of the complexes.