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Fig. 23. The EPR signals from crystalline DNIC-MT (a) with subsequent dissolving with methanol: partial (b) and complete (c) dissolving. Recordings at ambient temperature. (From Ref. [40].)

The parameters of the latter (position in magnetic field and width) coincided with that of crystalline MT-DNIC recorded at ambient temperature (Fig. 24, curve a). The contribution of this signal to total EPR spectra (Fig. 25, curve b) estimated by double integral method exceeded 90%.

This result demonstrates that polycrystalline solids of paramagnetic MT-DNIC have a narrow symmetric singlet EPR spectrum and that this spectrum is not sensitive to changes in the temperature. It indicates the presence of strong magnetic interaction between the complexes in the solid. It is evident that tight packing of DNICs in the crystal should favor electronic coupling and exchange interactions between the adjacent centers. The latter could average the anisotropy of g-factor to giso resulting in the observation of isotropic EPR signal at g = 2.031 either at ambient or low temperature.

Fig. 24. The EPR signals from crystalline DNIC-MT complex dissolved in methanol (curves a,c) or dimethylsulfoxide (b,d). Recordings at ambient temperature (curves a,b) or 77 K (curves c,d). (From Ref. [40].)

Fig. 24. The EPR signals from crystalline DNIC-MT complex dissolved in methanol (curves a,c) or dimethylsulfoxide (b,d). Recordings at ambient temperature (curves a,b) or 77 K (curves c,d). (From Ref. [40].)

In solid crystals, the EPR spectrum does not resolve the anisotropy of the Zeeman g-factor. Therefore, we cannot make a comparison with the g-values as found in solution. This does not allow us to spot any differences in the electronic configuration and ligand fields of MT-DNIC in crystalline or in dissolved states. The only subtle difference appears in the averaged g-factor being 2.031 and 2.035 for crystalline and dissolved states, respectively.

In principle, a d9 configuration of the iron atom is possible in DNIC with thiol-containing ligands, which can arise due to transfer of two electrons on the iron from two thiol ligands (metal spin crossover effect [112]). This results in {(RS^Fe-1 (NO+)2}+ structure (d7 ^ d9 transition), i.e. the complex becomes a three-spin system, involving two thiyl radicals and iron on d9 configuration. The total spin of this system in the ground state is S = 1 /2. Such electronic configuration can be realized in DNICs with a tetrahedral structure (Scheme 10A), which is typical for these complexes in the crystalline state. However, such a structure is hardly typical for DNICs with thiol-containing ligands in the solution. The point is that in tetrahedral DNICs with d9 electronic configuration, the unpaired electron should be localized on one of dxy, dxz or dzy orbitals (Scheme 10A), while in DNICs with thiol-containing ligands it is localized on dz2 iron orbitals (see above). This difference suggests that the electronic structure of DNIC changes significantly upon transition from the crystalline to dissolved state. Diffraction experiments have clearly shown that the complex has tetrahedral structure in the crystalline state. In contrast, the EPR experiments favor a square-plane structure for

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