Influence of CF₃ Substituents on the Spin Crossover Behavior of Iron(II) Coordination Polymers with Schiff Base‐like Ligands

Abstract

A Schiff base‐like ligand bearing CF3 substituents was synthesized and converted to iron(II) coordination polymers [{FeL(Lax)}n] using five different bridging ligands Lax. The structure of the coordination polymers was investigated using powder X‐ray diffraction and single‐crystal X‐ray diffraction in the case of [{FeL(bipy)}n]. The later revealed an untypical ABAB pattern of alternating equatorial ligands rotated by 180° with regard to each other along the chain. The temperature‐dependent magnetic behavior was investigated with a SQUID magnetometer and the spin states at room temperature were confirmed by 57Fe‐Mössbauer spectroscopy. Three out of five coordination polymers show spin crossover behavior in the temperature range between 50 and 400 K with different kind of curve progressions (abrupt, gradual, step‐wise). The other two coordination polymers are either fully highspin or fully low spin.

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

“…[9,10] The proto-ligands H 2 5-H 2 8 were synthesized in good yields from o-phenylenediamine and the respective keto-enol ethers, as summarized in Scheme 1. [3,[11][12][13] The 1 H NMR resonance pattern of the proto-ligands is in agreement with a predominant formulation as the ketoenamine tautomer. [13] It is noted that the complementary *salophen* ligand family rather justifies an enol-imine description.…”

“…While poor crystal quality so far interfered with structure analysis of [Fe8(py) 2 ], it is noted that the molecular structure of its coordination polymer with 4,4'-bipyridine, {[Fe8(bipy)]} n , could be recently resolved. [3] The coordination metrics were fully in accord with the LS state structures assembled in Table 2.…”

“…Iron(II) acetate, [24] [Fe5(MeOH) 2 ]/H 2 5, [11] H 2 6, [12] H 2 7 [13] and H 2 8 [3] were synthesized as described in literature. Methanol was purified as described in literature.…”

“…Based on an invariant N 2 O 2 2À planar ligating platform (Scheme 1), the derived polymers {[Fe8(py∪py)]} n show remarkably variable SCO in the solid state, when the axial bridging bipyridine ligand py∪py is varied. [3] While 4,4'-bipyridine as a bridge supports a LS polymer up to 400 K, the equally linear bridge 1,2-bis(4-pyridyl)ethyne traps the HS state even at lowest temperatures. Clearly, such massive differences cannot be associated with the very minor (if any) variations in the ligand-field imposed by the axial ligands; they are governed by packing effects, which are currently beyond control, not to say rational design.…”

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

“…[17,[32][33][34] Recently, an ABAB coordination polymer with [Fe(L)] units tilted by 180°was found, having À CF3 groups at the chelate cycle. [35] The FeÀ N ax bond length (ca. 2.2 Å) is comparable to similar bond lengths in other reported coordination polymers, [32,36] as is the torsion angle between the pyridyl rings with 20.3°.…”