Tris(bipyridine)iron(II) chloride

Tris(bipyridine)iron(II) chloride is the chloride salt of the coordination complex tris(bipyridine)iron(II), (often shortened to [Fe(bipy₃]²⁺). It is a red, water soluble solid. It is one of the most studied transition metal complexes of 2,2'-bipyridine.

Structure

Tris(bipyridine)iron(II) center features an octahedral Fe(II) center bound to three bipyridine ligands. As confirmed many times by x-ray crystallography, the dication is octahedral, the Fe(II) center occupying a chiral pocket. The cation has idealized D₃ symmetry, and as such both Δ and Λ are observed. The Fe-N distances are near 196 picometers, consistent with low spin complex.

The complex has been isolated as salts with many anions. The chloride salt is readily soluble in aqueous solution, and the hexafluorophosphate salt is soluble in organic solvents such as nitriles.

Synthesis and reactions

The sulfate salt is produced by combining ferrous sulfate with excess bipy in aqueous solution. This result illustrates the preference of Fe(II) for bipyridine vs water. The potentials for |[Fe(bipy)₃]^2+/3+ and [Fe(phen)₃]^2+/3+ are very similar.

Addition of cyanide to an aqueous solution of precipitates .

[Fe(bipy)₃]²⁺ salts can also be prepared from Iron(II) tetrafluoroborate in the solid state via mechanochemistry.

Electronic absorption spectrum

The electronic absorption spectrum of [Fe(bipy)₃]²⁺ features a metal-to-ligand charge-transfer (MLCT) band at 540&nbsp;nm (ε&nbsp;=&nbsp;8,000‑14,000&nbsp;M^{−1}cm^{−1}), a weak shoulder due to the spin-forbidden ³MLCT transition at 640&nbsp;nm (ε&nbsp;<&nbsp;50&nbsp;M^{−1}cm^{−1}), a higher lying MLCT band at 350&nbsp;nm (ε&nbsp;=&nbsp;6,000‑10,500&nbsp;M^{−1}cm^{−1}), and a bipy π‑π* transition at 290&nbsp;nm (ε&nbsp;=&nbsp;40,000‑70,000&nbsp;M^{−1}cm^{−1}).

Photoinduced Dynamics

In contrast to tris(bipyridine)ruthenium(II), this iron complex is not a useful photosensitizer because its excited states relax too rapidly, a consequence of the primogenic effect.

[Fe(bipy)₃]²⁺ is a model system for photoinduced spin crossover/Light Induced Excited Spin State Trapping (LIESST). Upon photoexcitation of its ¹MLCT band, the molecule undergoes intersystem crossing to the high-spin ⁵d–d state in less than 100 fs. The lifetime of the high-spin state is 650 ps. The large changes in electronic structure, spin state, and Fe–N bond length in the high-spin state lead to strong transient signals in the time-resolved X-ray absorption spectra (XAS) and X-ray emission spectra (XES). Time-resolved Fe K-edge EXAFS has revealed a 0.203&nbsp;±&nbsp;0.008&nbsp;Å increase in Fe–N bond length in the high-spin state. The large transient signals and ultrafast formation of the high-spin state has led to the application of [Fe(bipy)₃]²⁺ as a reference compound used at many synchrotron beamlines to find temporal overlap of the laser pump and X-ray probe pulses and to measure the instrument response function (IRF). The time resolution of these experiments is typically limited by the ~70 ps duration of the X-ray pulses, so the formation of the high-spin state is effectively instantaneous and the rise of the transient signal is IRF-limited. Since the advent of X-ray free-electron laser (XFEL) and high harmonic generation sources capable of producing sub-picosecond pulses of X-rays, the ultrafast spin-crossover dynamics of [Fe(bipy)₃]²⁺ and related complexes have been a popular target for femtosecond X-ray spectroscopy experiments. The improved time resolution of these experiments enabled detection of the short-lived intermediate ³d–d state, whose lifetime was measured with Fe Kβ XES to be 58 fs in [Fe(bipy)₃]²⁺ and with Fe M_2,3-edge XANES to be 39 fs in tris(o-phenanthroline)iron(II).

Related complexes

• Tris(o-phenanthroline)iron(II)

References