Iron(II) Spin Crossover Complexes Based on a Redox Active Equatorial Schiff-Base-Like Ligand

Abstract

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.

Spin States of 1D Iron(II) Coordination Polymers with Redox Active TTF(py)2 as Bridging Ligand

“…This setting has been found suitable to match the metrics of iron(II) complexes in a number of previous cases. [ 13 ] The optimized structure of [(py)L1Fe(TTF(py) 2 )FeL1(py)] in the all‐low spin electron configuration corresponding to singlet multiplicity with S = 0 closely matches the experimental metrics of C2a (Table 5).…”

“…The room temperature color and the impression at low temperature are consistent with SCO as they closely match results reported and analyzed recently for a related system. [ 13 ] At 100 K the single crystal C2a has the same color as the cooled filtrate as can be seen in Figure 7. Unfortunately, it was not possible for us to trace the color changes using UV/Vis spectroscopy by cooling a solution as the axial ligand dissociates at the required concentrations.…”

“…Indeed cyclovoltammetric studies, given in Figure 10, reveal an anodic shift of the iron(II/III) couple by ca. 100 mV if axially decorated with TTF(py) 2 across the entire series 1 and 2 ; the iron(II/III) couple in [L2Fe(py) 2 ] resides at E 1/2 = –0.395 V, [ 13 ] whereas the values of 1 and 2 crowd at E 1/2 ≈ –0.28 V.…”

“…A promising candidate to realize a linkage between conductivity, redox activity and spin crossover is tetrathiafulvalene (TTF) that is known for its excellent electron‐donating properties. [ 10,13–15 ] The diverse chemistry of TTF began 1973 by discovering the first organic metal TTF‐TCNQ (TCNQ = tetracyanoquino‐dimethane), [ 16 ] making use of the reversible oxidation of TTF to the stable radical cation (TTF • + ) and the dication (TTF 2+ ). [ 17 ] While mononuclear complexes incorporating TTF moieties have been known for a while, it was only very recently that incorporation into iron(II)‐based coordination polymers with ligand‐appended TTF was reported.…”

“…[ 17 ] While mononuclear complexes incorporating TTF moieties have been known for a while, it was only very recently that incorporation into iron(II)‐based coordination polymers with ligand‐appended TTF was reported. [ 13,14 ] In both studies the Schiff base‐like equatorial ligand system established by Weber and Jäger et al is used, known for its variety of spin transitions. [ 3,18,19,20 ] As a large number of interesting iron(II) coordination polymers with descendants of bidentate pyridine‐containing ligands is known, [ 3,18,21 ] the ligand 2,6(7)‐bis(4‐pyridyl)‐1,4,5,8‐tetrathiafulvalene (TTF(py) 2 ) reported 2007 by Han et al, is an interesting aspirant for the synthesis of coordination polymers combining SCO behavior and redox active properties.…”