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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.