Dinuclear spin-crossover compounds

Table 2 Thermodynamic parameters obtained from DSC curves for the dinuclear spin-crossover compounds.

Bipyrimidine-Bridged Dinuclear Iron(II) Spin Crossover Compounds... 

J.J.A. Kolnaar, M.l. de Heer, H. Kooijman, A.L. Spek, G. Schmitt, V. Ksenofontov, P. Giitlich, J.G. Haasnoot, J. Reedijk, Synthesis, structure and properties of a mixed mononuclear/dinuclear iron(ll) spin crossover compound with the ligand 4-(p-tolyl)-1,2,4-triazole. Eur. J. Inorg. Chem. 5, 881-886 (1999)... 

Abstract This review reports on the study of the interplay between magnetic coupling and spin transition in 2,2 -bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. The coexistence of both phenomena has been observed in [Fe(bpym)(NCS)2]2(bpym), [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) (bpym = 2,2 -bipyrimidine, bt = 2,2 -bithiazoline) by the action of external physical perturbations such as heat, pressure or electromagnetic radiation. The competition between magnetic exchange and spin crossover has been studied in [Fe(bpym)(NCS)2]2(bpym) at 0.63 GPa. LIESST experiments carried out on [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) at 4.2 K have shown that it is possible to generate dinuclear molecules with different spin states in this class of compounds. A special feature of the spin crossover process in the dinuclear compounds studied so far is the plateau in the spin transition curve. Up to now, it has not been possible to explore with a microscopic physical method the nature of the species... 

As mentioned above, the particular characteristics of the spin crossover process in dinuclear compounds is the appearance of a plateau in the spin transition curve. From the analysis of the results of the pressure experiments, it is inferred that the plateau results from successive ST in the two metal centres, leading first to the formation of relatively stable [HS-LS] pairs and then, above a critical pressure, to the formation of [LS-LS] pairs on further lowering of the temperature. The intermolecular interactions between [HS-LS] pairs leads to domains that contribute to the stability of the... 

The special feature of the spin crossover process in all bpym-bridged dinuclear compounds studied so far is the occurrence of a plateau in the spin transition curve. A reasonable assumption to account for this observation is that a thermal spin transition takes place successively in the two metal centres. However, it cannot be excluded that spin transition takes place simultaneously in the dinuclear units leading directly from [HS—HS] pairs to [LS-LS] pairs with decreasing temperature. Therefore, two possible conversion pathways for [HS—HS] pairs with decreasing temperature may be proposed [HS—HS]<->[HS—LS]<->[LS—LS] or [HS-HS] [LS-LS]. The differentiation of the existence of the [LS—LS], [HS—LS], and [HS—HS] spin pairs is not trivial and has recently been solved experimentally by utilisation of magnetisation versus magnetic field measurements as a macroscopic tool [9], and by Mossbauer spectroscopy in an applied magnetic field as a microscopic tool [11]. 

Abstract This review deals with spin crossover effects in small polynuclear clusters, particularly dinuclear species, and in extended network molecular materials, some of which have interpenetrated network structures. Fe(II)Fe(II) species are the main focus but Co(II)Co(II) compounds are included. The sections on dinuclear compounds include short background reviews on (i) synergism of SCO and spin-spin magnetic exchange (ii) cooperativity (memory effects) in polynuclear compounds, and (iii) the design of dinu-... 

Dinuclear Fe(III) compounds were also obtained using substituted salten derivatives together with 4,4 -bipyridine as bridging ligand [200]. The 3-OMe-salten tetraphenylborate compound seems to show the onset of spin crossover at about 270 K, the 5-OMe-salten material is probably a purely high spin compound, whereas the 5-Cl-salten derivative exhibits gradual spin crossover behaviour. 

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). 

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