All chemicals and solvents used for the syntheses were of reagent grade and were used without further purification. The radical ligand and M(hfac)₂∙H₂O (M = Ni(1), Zn(2)) have been performed according to the literature methods [11] [12]. All reactions were conducted in aerobic conditions. Elemental analyses for carbon, hydrogen, and nitrogen were carried out on a Model 240 Perkin -Elmer elemental analyzer. Infrared spectra were taken on a Bruker Tensor 27 Fourier transform infrared spectroscopy in the region 4000 - 400 cm⁻¹, using KBr pellets. Ultraviolet-visible spectra were recorded on a Cary 50 UV-Vis spectrophotometer in stated solvent.

Ni(hfac)₂·2H₂O (0.1 mmol) was refluxed at 80˚C for 30 minutes in 10 mL n-heptane. Then the reactant was cooled to 60˚C, 6 mL of dichloromethane solution containing 0.1 mmol o-quinoxaline imino nitroxide (IM-o-QN) was added, and stirring was continued for 30 minutes. Then, it was cooled to the room temperature, and filtered. One week later, block red-orange crystals suitable for single crystal diffraction had been obtained. Anal. Calcd. (%) for C₂₅H₁₉F₁₂N₄O₅Ni (742.13): C, 40.45; N, 7.55; H, 2.58; Found: C, 40.51; N, 7.49; H, 2.71%. FTIR (KBr, cm⁻¹): 1639 (s), 1530 (s), 1476 (m), 1352 (m), 1253 (s), 1204 (m), 1138 (s), 1096 (s), 799 (s), 670 (s). UV-Vis (nm): 243 and 313 nm.

The complex 2 was obtained in the similar manner using Zn(hfac)₂∙2H₂O instead of Ni(hfac)₂·2H₂O. A few days later, block red-orange crystals suitable for single crystal diffraction had been obtained. Anal. Calcd. (%) for C₂₅H₁₉F₁₂N₄ O₅Zn (748.83): C, 40.01; N, 7.48; H, 2.56; Found: C, 40.06; N, 7.46; H, 2.49%. FTIR (KBr, cm⁻¹): 1640 (s), 1529 (s), 1488 (m), 1355 (m), 1255 (s), 1196 (m), 1135 (s), 1094 (s), 796 (s), 664 (s). UV-Vis (nm): 242 and 307 nm.

The diffraction data were collected at room temperature with Mo-Kα (λ = 0.71073 Å) radiation using a Bruker CCD APEX-Ⅱ diffractometer. The structures were solved by direct methods and refined by full-matrix least-squares methods on F² by using the SHELXL-97 program package [13]. Empirical absorption corrections from φ and ω scan were applied. Metal atoms in each compound were located from the E-maps.

All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were added theoretically. The crystal parameters, data collection procedure, and refinement results for the two complexes 1 and 2 are summarized in detail in Table 1. Selected bond lengths and angles for the two complexes are listed in Table 2.

In IR spectra of the two complexes, the characteristic strong bands around 1253, 1204, 1138 cm⁻¹ for the complex 1 and 1256, 1196, 1135 cm⁻¹ for complex 2 can be assigned to the stretching vibrations of the ν(C-F) of the hfac. The strong absorptions around 1640 cm⁻¹ for the two complexes are ascribed to the ν(C-O) of the hfac, indicating that the hexafuoroacetone is coordinated to the metal by enol form. The N-O stretching vibrations at 1352 cm⁻¹ (for 1) or 1355 cm⁻¹ (for 2) of the imino radical ligands, which indicate the existence of uncoordinated N-O group of IM-o-QN radical ligands. In addition, peaks around 799/670cm⁻¹ (for 1) and 796/664cm⁻¹ (for 2) can be attributed to aromatic benzene.

The UV-Vis spectra for the two complexes are measured in methyl alcohol solvent. The absorptions around 313 nm for 1 and 307 nm for 2 can be attributed to π → π* of the ONCN conjugate groups of the quinoxalinyl-substituted imino nitroxide radicals and the absorptions around 243 nm for 1 and 242 nm for 2 should be attributed to π → π* of quinoxalinyl group.

TGA (Thermogravimetric analysis) of the two complexes are conducted under N₂ atmosphere in the temperature range of 40˚C - 800˚C at the heating rate of 10˚C/min. The thermal decomposition curves of the complex 1 and 2 are as shown in Figure 3 and Figure 4, respectively. As can be seen from the Figure 3, the thermal decomposition process of the complex 1 is mainly concentrated in 187˚C - 400˚C, which can be roughly divided into two parts. In the first stage, the complex 1 decomposes and significant weight loss is 79.65% at 167˚C - 312˚C, which corresponds to loss of two hafc molecules and a few imino nitroxide section. In the second stage, the fragment for the imino nitroxide ligand further decomposes. The weight loss is about 12.14%. Considered from the ratio, the last residue should be the nitride of nickel.

Seen from Figure 4, the thermal decomposition process of the complex 2 is mainly concentrated in 168˚C - 450˚C, which can be also roughly divided into two parts. In the first stage, the complex 2 decomposes and significant weight loss is 69.98% at 168˚C - 365˚C which corresponds to loss of two hafc molecules and NO of the imino nitroxide section. In the second stage, the fragment for the imino nitroxide ligand further decomposes. The weight loss is about 13.29%. Thinking to the ratio, the last residue should be the nitride of zinc.