(Tetrakis(2-pyridylmethyl)ethylenediamine)iron(II) Perchlorate, the First Rapidly Interconverting Ferrous Spin-Crossover Complex

The preparation and characterization of the first Fe^II spin-crossover complex that interconverts between highand low-spin states at a rate faster than the ⁵⁷Fe Mössbauer time scale is reported. [Fe(tpen)](ClO₄)₂·²/₃H₂O crystallizes in the monoclinic space group C2/c, which at 298 K has a unit cell of a = 40.87 (2) Å, b = 9.497 (4) Å, c = 23.946 (9) Å, and β-108.42 (4)° with Z-12 and at 358 K the unit cell is characterized by a = 41.00 (2) Å, b = 9.517 (5) Å, c = 24.21 (1) Å, and β-109.46 (4)° with Z = 12. The hexadentate ligand tpen is tetrakis(2-pyridylmethyl)ethylenediamine. The refinements were carried out with 3110 (2.58σ) and 2221 (2.58σ) observed reflections at 298 and 358 K, respectively, to give R = 0.073 and R_w = 0.076 at 298 K and R = 0.082 and R_w = 0.082 at 358 K. At both temperatures there are two crystallographically different [Fe(tpen)]²⁺ cations. One of these two cation sites has a greater high-spin content, as evidenced by Fe-ligand atom bond lengths and trigonal distortions which are greater than those found at the other cation site. The Fe-N bond lengths and trigonal distortion increase for both cations as the temperature is increased from 298 to 358 K. Solid-state magnetic susceptibility data show that the critical temperature, T_c where there are equal amounts of highand low-spin complexes, is 7_C = 365 K. Faraday balance data for this same perchlorate salt in DMF solution give T_c = 363 K. The similarity of these solid- and solution-state T_c values and the susceptibility data taken for the pure solid and solid solutions in the isostructural Zn²⁺ complex definitively show that the spin-crossover cations in [Fe(tpen)](ClO₄)₂·²/₃H₂O experience no appreciable intermolecular interactions. Each cation acts independently in a high-/low-spin equilibrium. The ⁵⁷Fe Mössbauer spectrum exhibits only one quadrupole-split doublet for each cation up to the highest temperature (350 K) investigated. Thus, this is the first Fe¹¹ spin-crossover complex that interconverts in the solid state between highand low-spin states at a rate that is faster than the Mossbauer time scale. A careful analysis of the changes in the structure of the [Fe(tpen)]²⁺ cation as a function of temperature together with angular overlap calculations suggest that it is the increase in Fe-N bond lengths together with an increase in the trigonal distortion that leads to the fast rate of spin-state interconversion in [Fe(tpen)]²⁺. The steric constraints introduced by the hexadentate ligand lead to a relatively large trigonal distortion lowering the energy of triplet excited states (³T₁ and/or ³T₂). This then leads to greater spin-orbit interaction of the ¹A low-spin state with components of the ⁵T₂ high-spin state, and a greater rate of interconversion results. Additional evidence supporting the presence of fluxional distortions of [Fe(tpen)]²⁺ along a trigonal twisting coordinate is presented in the form of variable-temperature ¹H NMR data. In solution [Fe(tpen)]²⁺ exhibits a very fast rate (>600 s^-1) of enantiomerization. Finally, the preparation and properties (T_c > 400 K) of [Fe(tpen)](ClO₄)₂ are given. This non-hydrated complex crystallizes in the monoclinic space group P2₁/c, which at 298 K has a unit cell characterized by a = 17.865 (3) Å, b-9.878 (1) Å, c = 17.213 (4) Å, and β = 110.01 (2)° with Z = 4. This structure was refined with 3031 (2.58σ) observed reflections to give R = 0.049 and R_w = 0.053. The trigonal twist found for the cation is in keeping with magnetic susceptibility data indicating that this nonhyrated complex is totally low spin at 298 K.