Charge‐Transfer Phase Transition of a Cyanide‐Bridged Fe^II/Fe^III Coordination Polymer

Abstract

Heterometallic Prussian blue analogues are known to exhibit thermally induced charge transfer, resulting in switching of optical and magnetic properties. However, charge‐transfer phase transitions have not been reported for the simplest FeFe cyanide‐bridged systems. A mixed‐valence FeII/FeIII cyanide‐bridged coordination polymer, {[Fe(Tp)(CN)3]2Fe(bpe)⋅5 H2O}n, which demonstrates a thermally induced charge‐transfer phase transition, is described. As a result of the charge transfer during this phase transition, the high‐spin state of the whole system does not change to a low‐spin state. This result is in contrast to FeCo cyanide‐bridged systems that exhibit charge‐transfer‐induced spin transitions.

Thermally Induced Reversible Metal-to-Metal Charge Transfer in Mixed-Valence {Fe III 4 Fe II 4 } Cubes

“…18 Such spectral variation clearly confirmed the occurrence of charge transfer between the {Fe II LS -CN-Fe III HS } and {Fe III LS -CN-Fe II HS } configurations. 30 Upon further cooling, the variation, however, is not recovered, likely due to the bad desolvation at above room temperature. Similar IR activities were also found for 2, where the cyanide stretching absorption at 2074 cm -1 almost vanished with the new band at 2156 cm -1 being visible when the temperature increased from 190 to 298 K. Average distance (Å) Variable-temperature magnetic susceptibility data for the fresh samples of 1 and 2 were measured under an applied direct current (dc) field of 1 kOe with a sweeping rate of 2 K/min (Figure 3).…”

“…At 270 K, the spectra of 2 showed three unbalanced absorptions, which are perfectly superimposed with two quadrupole doublets with the paraments of δ = -0.052 mm s -1 , ΔE Q = 0.71 mm s -1 and δ = 0.83 mm s -1 , ΔE Q = 1.18 mm s -1 , characteristic of Fe III LS and Fe II HS , respectively. 27,30 The peak area ratio of the two doublets is 50. , respectively. Accordingly, the other set of three accessible redox waves at E 1/2 = -0.08, -0.29, -0.54 for 1, and -0.07, -0.25 (E pc ), -0.45 V for 2 correspond to the one-electron Fe III/II redox processes of the [(Tp R )Fe III (CN) 3 ]units (Supporting Information Figures S7 and S8).…”

“…26 Especially, series of complexes containing {Fe III -CN-Fe II } motifs have been reported to exhibit the typical spin-crossover (SCO) behaviour, [27][28][29] HS } states (Scheme 1). 30 Interestingly, the same group and co-workers observed the fast-fluctuating valence phenomenon in a nanosized {Fe 14 } cluster complex. 31 Furthermore, Liu et al demonstrated that heat treatment of a mixed-valence [Fe 13 ] cluster induced an irreversible MMCT event, which eventually regulated the original SCO property.…”

MMCT energy change in cyanidometal-bridged trinuclear complexes by changing the ligand electron donating ability

“…22 The electron density of D and A ligands can be changed by adjusting the auxiliary ligand on the metal centers or by altering the MMCT using external physical stimuli. 7,30,31 The introduction of electron-donating or -withdrawing substituents in the redox centers to adjust the energy levels of terminal metal centers is necessary to facilitate the study of electron transfer processes of MV compounds. 32 Cyanidometal-bridged MV compounds play an important role in the study of MMCT due to their good electron transfer coordination ability.…”