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

Hsiu Rong Chang, James K. McCusker, Scott R. Wilson, James K. McCusker, David N. Hendrickson, Hans Toftlund, Alfred X. Trautwein, Heiner Winkler

102 Citations (Scopus)


The preparation and characterization of the first FeII 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 Rw = 0.076 at 298 K and R = 0.082 and Rw = 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, Tc where there are equal amounts of highand low-spin complexes, is 7C = 365 K. Faraday balance data for this same perchlorate salt in DMF solution give Tc = 363 K. The similarity of these solid- and solution-state Tc 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 (Tc > 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 Rw = 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.

Original languageEnglish
JournalJournal of the American Chemical Society
Issue number19
Pages (from-to)6814-6827
Number of pages14
Publication statusPublished - 01.09.1990


Dive into the research topics of '(Tetrakis(2-pyridylmethyl)ethylenediamine)iron(II) Perchlorate, the First Rapidly Interconverting Ferrous Spin-Crossover Complex'. Together they form a unique fingerprint.

Cite this