• Towards accurate estimates of the spin-state energetics of spin-crossover complexes within density functional theory: a comparative case study of cobalt(ii) complexes
    A. Vargas, I. Krivokapic, A. Hauser and L.M. Lawson Daku
    Physical Chemistry Chemical Physics, 15 (11) (2013), p3752-3763
    DOI:10.1039/c3cp44336a | unige:26498 | Abstract | Article HTML | Article PDF
 
We report a detailed DFT study of the energetic and structural properties of the spin-crossover Co(II) complex [Co(tpy)2]2+ (tpy = 2,2′:6′,2′′-terpyridine) in the low-spin (LS) and the high-spin (HS) states, using several generalized gradient approximation and hybrid functionals. In either spin-state, the results obtained with the functionals are consistent with one another and in good agreement with available experimental data. Although the different functionals correctly predict the LS state as the electronic ground state of [Co(tpy)2]2+, they give estimates of the HS–LS zero-point energy difference ΔE0HL (tpy)  which strongly depend on the functional used. This dependency on the functional was also reported for the DFT estimates of the zero-point energy difference ΔE0HL (bpy)  in the HS complex [Co(bpy)3]2+ (bpy = 2,2′-bipyridine) [A. Vargas, A. Hauser and L. M. Lawson Daku, J. Chem. Theory Comput., 2009, 5, 97]. The comparison of the ΔE0HL (tpy)  and ΔE0HL (bpy)  estimates showed that all functionals correctly predict an increase of the zero-point energy difference upon the bpy → tpy ligand substitution, which furthermore weakly depends on the functionals, amounting to (ΔE0HL)bpy->tpy  ≈ +2670 cm-1 . From these results and basic thermodynamic considerations, we establish that, despite their limitations, current DFT methods can be applied to the accurate determination of the spin-state energetics of complexes of a transition metal ion, or of these complexes in different environments, provided that the spin-state energetics is accurately known in one case. Thus, making use of the availability of a highly accurate ab initio estimate of the HS–LS energy difference in the complex [Co(NCH)6]2+ [L. M. Lawson Daku, F. Aquilante, T. W. Robinson and A. Hauser, J. Chem. Theory Comput., 2012, 8, 4216], we obtain for [Co(tpy)2]2+ and [Co(bpy)3]2+best estimates of ΔE0HL (bpy) ≈ -2800 cm-1  and ΔE0HL (tpy) ≈ 0 cm-1 , in good agreement with the known magnetic behaviour of the two complexes.
  
  • Low-Spin→High-Spin Relaxation Dynamics in the Highly Diluted Spin-Crossover System [FexZn1−x(bbtr)3](ClO4)2
    I. Krivokapic, P. Chakraborty, C. Enachescu, R. Bronisz and A. Hauser
    Inorganic Chemistry, 50 (5) (2011), p1856-1861
    DOI:10.1021/ic102269y | unige:14980 | Abstract | Article HTML | Article PDF
Whereas the neat polymeric iron(II) compound [Fe(bbtr)3](ClO4)2, bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [FexZn1−x(bbtr)3](ClO4)2x = 0.02 and 0.1, stay predominantly in the 5T2 high-spin state down to cryogenic temperatures. However, the 1A1 low-spin state can be populated as metastable state via irradiation into the spin-allowed 5T25E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spinhigh-spin relaxation is 1840(20) cm−1.
  • Significant variation of the singlet-quintet intersystem crossing rate constant in an iron(II) high-spin complex as a function of temperature
    I. Krivokapic, P. Chakraborty, R. Bronisz, C. Enachescu and A. Hauser
    Angewandte Chemie, 49 (45) (2010), p8509-8512
    DOI:10.1002/anie.201004500 | unige:14713
In the dilute mixed-crystal system [Zn1−xFex(bbtr)3](ClO4)2, x=2 % (bbtr=1,4-di(1,2,3-triazol-1-yl)butane), the iron(II) centers are predominantly in the high-spin state. The low-spin state can be populated as a metastable state by irradiation with near-IR light; the rate constant of the low-spin→high-spin relaxation spans 14 orders of magnitude between 40 and 220 K
  • The interaction between the spin transition and a crystallographic phase transition in the spin-crossover compound [Fe(bbtr)3](ClO4)2: Nucleation, formation of domains and fluctuations
    I. Krivokapic, C. Enachescu, R. Bronisz and A. Hauser
    Inorganica Chimica Acta, 361 (12-13) (2008), p3616-3622
    DOI:10.1016/j.ica.2008.03.064 | unige:3576 | Abstract | Article PDF
 
The thermal and the light-induced spin transition in [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)) as well as the high-spin → low-spin relaxation following the light-induced population of the high-spin state below the thermal transition temperature are discussed in relation to the accompanying crystallographic phase transition. The experimental data have exclusively been obtained using optical single crystal absorption spectroscopy.
  • Spin transition and relaxation dynamics coupled to a crystallographic phase transition in a polymeric iron(II) spin-crossover system
    I. Krivokapic, C. Enachescu, R. Bronisz and A. Hauser
    Chemical Physics Letters, 455 (4-6) (2008), p192-196
    DOI:10.1016/j.cplett.2008.02.088 | unige:74 | Article HTML | Article PDF
The spin-crossover compound [Fe(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane) forms a polymeric hexagonal sheet structure. It shows an abrupt thermal spin transition with 13 K wide hysteresis around 105 K, as evidenced by single crystal optical spectroscopy. The transition temperature for the thermal high-spin→low-spin transition on cooling as well as the relaxation kinetics just below Tc depend upon the history of the sample. This is typical for a nucleation and growth mechanism and domain formation. In contrast, the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is governed by the intersystem crossing process.
  
The spin transition of the [Co(terpy)2]2+ complex (terpy = 2,2′:6′,2″-terpyridine) is analysed based on experimental data from optical spectroscopy and magnetic susceptibility measurements. The single crystal absorption spectrum of [Co(terpy)2](ClO4)2 shows an asymmetric absorption band at 14 400 cm−1 with an intensity typical for a spin-allowed d–d transition and a temperature behaviour typical for a thermal spin transition. The single crystal absorption spectra of suggest that in this compound, the complex is essentially in the high-spin state at all temperatures. However, the increase in intensity observed in the region of the low-spin MLCT transition with increasing temperature implies an unusual partial thermal population of the low-spin state of up to about 10% at room temperature. Finally, high-spin → low-spin relaxation curves following pulsed laser excitation for [Co(terpy)2](ClO4)2 dispersed in KBr discs, and as a comparison for the closely related [Co(4-terpyridone)2](ClO4)2 spin-crossover compound are given.
Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)3]2+ and [Co(terpy)2]2+ (bpy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy.

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