A Brønsted‐Ligand‐Based Iron Complex as a Molecular Switch with Five Accessible States

Abstract

A mononuclear FeII complex, prepared with a Brønsted diacid ligand, H2L (H2L=2‐[5‐phenyl‐1H‐pyrazole‐3‐yl] 6‐benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [FeII(H2L)2](BF4)2 (1A), exhibits abrupt spin transition at T1/2=258 K, and treatment with base yields a deprotonated analogue [FeII(HL)2] (1B), which shows gradual SCO above 350 K. A range of FeIII analogues were also characterized. [FeIII(HL)(H2L)](BF4)Cl (1C) has an S=5/2 spin state, while the deprotonated complexes [FeIII(L)(HL)], (1D), and (TEA)[FeIII(L)2], (1E) exist in the low‐spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid‐state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.

Proton‐Induced Spin State Switching in an FeIII Complex

“…In the case of copper the challenge is further compounded by the significant difference in coordination preferences of the two redox forms, and this is also an issue in spin state cycling between the spin singlet and triplet forms of Ni II . Analyte‐induced spin state switching in Fe II has therefore been more intensively studied and many ions including halides, [12a–c] polyatomic anions, [12c–e] and protons [12f–n] have thus far been detected. This has been achieved either by recognition of the analyte at ligand sites on the periphery of the complex in a ligand‐driven spin state switching effect (LD‐SSS) [12b–l] or by disruption of the coordination sphere in a coordination‐induced spin state switching (CISSS) event, [12a, m, n] Figure 1.…”

“…A major drawback in using ferrous spin state sensors, however, is their tendency to oxidize under ambient conditions [12l] . Moreover the idealized binary on/off switching between spin singlet and quintet states of Fe II is rarely achieved in practice due to varying spin equilibria ratios across a measured analyte range.…”