Two-step spin crossover

Some compounds exhibit a spin crossover of the form that the fraction xHs of molecules in the HS state increases with temperature in two steps a plateau of a few K exists between these steps. This behaviour can be explained by considering two sublattices (A and B) containing the same number of molecules [26], The Ising-like Hamiltonians corresponding to the respective lattices, in the mean field approach, are defined as follows 

In the above Hamiltonians, JA and JB are the intra-sublattice interaction parameters JAB is the inter-sublattice interaction parameter. Their positive values mean a ferromagnetic interaction whereas negative values refer to antiferromagnetic ones. The corresponding eigenvalues are 

The thermal evolution of the conversion parameters [aA) and (crB) is very complex and fully analysed in [26]. There are several critical temperatures when the mean spin values alter their sign  

The two-step transition is obtained when the intra-sublattice interactions are ferromagnetic (JA, JB 0) and the inter-sublattice interactions are antiferromagnetic (JAB 0). The two-step character of the conversion is provided by Tg = TB for 17 1 7B and the plateau width is (Tg — Tg). 

The thermal evolution of the HS state conversion has been modelled for a typical set of parameters JA = 7B 0 (the ferromagnetic intra-sublattice interaction) and JAB 0 (the antiferromagnetic inter-sublattice interaction)—see Fig. 9.17. It can be seen that the first (low-temperature) step is more abrupt than the second (high-temperature) one which is smoother. 

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Fig. 9.17. Two-step spin crossover in a mononuclear system (a) conversion curve, (b) Arrhenius plot Aq/k = 300 K, gfis/gis = 5 JA/k = JB/k = 50K, JAB/k = -50K (solid), -80 K (long dashed), — 100K (medium dashed) and —120 K (short dashed).

Two-step spin crossover and spin crossover in dinuclear compounds has roots in a proper combination of the inter-sublattice interaction parameter (which is ferromagnetic) and the intra-sublattice one (antiferromagnetic). 

Figure 1.18 Two-step spin crossover in the interpenetrated square grid framework [Fe (bpe)2(NCS)2] (guest) (SCOF-4 shown in the background viewed down the ID pores). The switching in this porous system proceeds from fully high-spin (right) to fully low-spin (left) via a chequerboard-type arrangement of high-spin and low-spin Fe nodes (centre). Reprinted with permission from G.J. Haider, K.W. Chapman, S.M. Neville, B. Moubaraki, K.S. Murray, J.F. Letard and C.J. Kepert, /. Am. Chem. Soc., 130,17552. Copyright (2008) American Chemical Society...

On the basis of their results the authors concluded that the crossover from the neutral to oxidised form of poly pyrrole, and vice versa, requires the number of spins to pass through a maximum, i.e. the creation and anihilation of bipolarons involves the passage through the polaron state. They expressed this as a two-step redox reaction ... 

In 1977 Driessen and van der Voort identified an extremely abrupt spin crossover with Tm of 213 K for [Fe(isoxazole)6](Cl04)2 [71]. Although various spectroscopic techniques have been employed to study this spin transition, the structural features of this compound at the time could not be determined, due to its extreme sensitivity to decomposition [71]. The same applies to the tetrafluoroborate salt that also displays a spin crossover, but in this instance a two-step transition was observed [71]. 

Keywords Spin crossover Dinuclear complexes Two-step transition Plateau Magnetic field Mossbauer spectroscopy... 

Reedijk and co-workers have studied the iron(II) spin-state crossover in [Fe(teec)6]X2, where X is BF4, CIO4, or PFg, and teec is the monodentate l-(2-chloroethyl)tetrazole ligand. Depending upon the anion and the rate of precipitation, these complexes exhibit differing, partial or complete, spin-state crossover behavior with gradual or sharp, one- or two-step, transitions both with and without hysteresis. 

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