NO rs

Scheme 9.

The cofacial stacking implies significant overlap between the axial dz2 orbitals of the adjacent iron atoms. The overlap causes antiferromagnetic spin-pairing that leaves the dimer diamagnetic [43,44].

We now consider the crystalline state of paramagnetic DNIC with thiol-containing lig-ands. As aforementioned diffraction experiments have revealed that the crystalline solid corresponds to iron in a tetrahedral crystal field [37,41,90,100-109]. In a tetrahedral crystal field, the degeneracy of the five d-levels of iron is increased as shown in Scheme 10. In this configuration, the three ^xz,yz,xy orbitals point towards the ligands and are shifted upward in energy. The ^x2-y2 and ^¿2 orbitals have little overlap with the ligands and remain unaffected. Scheme 10 shows the occupancy level of paramagnetic DNIC complexes in {Fe(NO)2}9 and {Fe(NO)2}7 states, respectively.

Scheme 10. Proposed electronic structures for DNIC in crystalline state with tetrahedral ligand field. In this configuration, Hund's rule makes that the {3d7} complex is high spin S = 3/2. For {3d9} complex S = 1/2.

This tetrahedral structure for the solid differs from the square-planar geometry in solution.

In principle, EPR spectroscopy should be able to distinguish between the spin state of DNIC in the molecular crystal and in solution. However, no EPR spectra seem to have been published for the crystalline state [102,105-107,109]. It could be suggested that the paramagnetism of the complexes is unobservable in the solid due to antiferromagnetic coupling or spin-spin interactions. Recently, our groups tried to perform the comparative investigation by using DNIC with 3-mercaptotriazole (1H—1,2,4-triazole-3-thiol, MT) This MT-DNIC complex can be observed with EPR both in crystalline state and in the solution. Good solvents are methanol or DMSO [40]. The complex [Fe(SC2H3N3)2(NO)2]1/2H2O was synthesized by the reaction of 3-mercaptotriazole with [Na2Fe2(S2O3)2(NO)4]2H2O and Na2S2O3.5H2O with molar ratio of 10:1:2 in water-alkaline solution, with subsequent re-crystallization of the complex from CH3OH [41]. The X-ray analysis confirmed tetrahedral structure of MT-DNIC as presented in Fig. 22.

At ambient temperature, the EPR spectrum of solid MT-DNIC had the form of a single Lorentzian at g = 2.031 and half-width 1.7 mT (Fig. 23, curve a). The sample was polycrys-talline since rotation of the sample in the magnetic field did not affect the intensity or the lineshape of the spectrum, which appears as an isotropic powder spectrum. Upon dissolving the crystals in methanol or DMSO, the spectrum narrowed to a singlet line with the center

Fig. 22. A fragment of the crystal structure of DNIC with 3-mercaptotriazole (DNIC-MT) with atom indexes (from X-ray analysis [41]). (With permission.)

Fig. 22. A fragment of the crystal structure of DNIC with 3-mercaptotriazole (DNIC-MT) with atom indexes (from X-ray analysis [41]). (With permission.)

at g = 2.030 and half-width 0.7 mT (Fig. 23, curves b,c). Double integration confirmed that the number of spins in the solid and dissolved polycrystallites were equal within experimental accuracy (ca 5%). Recordings were made at modulation amplitude of 0.5 mT.

At ambient temperature in methanol, the EPR spectrum was characterized by an isotropic weakly resolved quintet HFS when recordings were made at modulation amplitude of 0.05 mT (Fig. 24, curve a). HFS resolution increased drastically when DMSO was used as a solvent (Fig. 24, curve b). At 77 K, the solution of MT-DNIC in methanol showed a S = 1/2 complex with axial Zeeman coupling characterized by an anisotropic EPR signal of the shape similar to that of EPR signals from frozen solutions of DNIC with various thiol-containing ligands. The difference was in lower amplitude of the peak at g|| for MT-DNIC (Fig. 24, curve c).

It should be remarked that EPR spectra from MT-DNIC in DMSO (Fig. 24, curves b,d) were indistinguishable from EPR spectra if MT-DNIC was synthesized by exposing a mixture of Fe2+ and 3-mercaptotriazole (1:20) in DMSO to gaseous NO.

The spectra depend sensitively on temperature. At 77 K, the EPR signal of a polycrystalline MT-DNIC appears as a superposition of two signals. The first has an axially symmetric narrow shape with half-width 1.7 mT. The second appears as a broader powder spectrum with a peak at g = 2.045 (Fig. 25, curve b).

It was suggested that the latter could be attributed to a small fraction of complexes being dissolved in pockets of residual solvent (methanol) used for synthesis. When the signal characteristic of this fraction (MT-DNIC dissolved in methanol) (Fig. 24, curve c or Fig. 25, curve a) was subtracted from the signal presented in Fig. 25, curve b the isotropic EPR signal was obtained (Fig. 25, curve c).

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