Dinitrosyl

The dinitrosyl [Cr(NO)2(Et2dtc)2] was isolated as an air-stable solid by Malatesta (15,16), and by Connelly and Dahl (122), from the following reaction. 

IR-spectral data indicated that the dinitrosyl has a cis conformation (123). 

In recent years, several model complexes have been synthesized and studied to understand the properties of these complexes, for example, the influence of S- or N-ligands or NO-releasing abilities [119]. It is not always easy to determine the electronic character of the NO-ligands in nitrosyliron complexes thus, forms of NO [120], neutral NO, or NO [121] have been postulated depending on each complex. Similarly, it is difficult to determine the oxidation state of Fe therefore, these complexes are categorized in the Enemark-Feltham notation [122], where the number of rf-electrons of Fe is indicated. In studies on the nitrosylation pathway of thiolate complexes, Liaw et al. could show that the nitrosylation of complexes [Fe(SR)4] (R = Ph, Et) led to the formation of air- and light-sensitive mono-nitrosyl complexes [Fe(NO)(SR)3] in which tetrathiolate iron(+3) complexes were reduced to Fe(+2) under formation of (SR)2. Further nitrosylation by NO yields the dinitrosyl complexes [(SR)2Fe(NO)2], while nitrosylation by NO forms the neutral complex [Fe(NO)2(SR)2] and subsequently Roussin s red ester [Fe2(p-SR)2(NO)4] under reductive elimination forming (SR)2. Thus, nitrosylation of biomimetic oxidized- and reduced-form rubredoxin was mimicked [121]. Lip-pard et al. showed that dinuclear Fe-clusters are susceptible to disassembly in the presence of NO [123]. 

Based on previous studies [15, 22-25], the band at 1941 cm-i is assigned to Co2+(NO), and the pair of bands at 1894 and 1815 cm-i, to Co2+(NO)2- The shoulders at 1874 and 1799 cm may be due to a second dinitrosyl species. While little is known about the location and coordination of the Co 2+ in ZSM-5, it is likely that cobalt ions are associated with both [Si-0-Al]- and [Al-0-Si-0-AI]2- structures in the zeolite. In the former case, the cobalt cations are assumed to be present as Co2+(OH-) cations and in the latter case as Co2+ cations. The presence of cobalt cations in different environments could account for the appearance of two sets of dinitrosyl bands. The band at 2132 cm-> is present not only on Co-ZSM-5 but also on H-ZSM-5 and Na-ZSM-5, and has been observed by several authors on Cu-ZSM-5 [26-28]. 

Temperatures in excess of 150 °C are required to desorb NO from Co-ZSM-5 (see Fig. 2). The ratio of the intensity of either of the dinitrosyl peaks to the mononitrosyl peak decreases with increasing temperature suggesting that NO in dinitrosyls is less strongly bound than that... 

A series of spectra taken during TPR of a mixture of NO and O2 are presented in Figure 6. Bands are observed for both mon- and dinitrosyls, together with bands characteristic of NO2 and NO3- species. As the temperature rises, the ratio of nitrosyl to NO2/NO3 bands increases, consistent with what is expected on the basis of equilibrium considerations for the reaction NO + 1/2 O2 = NO2 [35]. 

Figure 7 shows spectra recorded during a TPR experiment in which a mixture of NO, O2, and CH4 are passed over the catalyst. At room temperature several new bands are present. These are located at 2189, 1878, and 1747 cm-. The peak at 2189 cm- is most likely due to N02 [36, 37], since this band is observed upon adsorption of NO2 at room temperature (see Figure 8). The band at 1747 cm- is assigned to N2O4 [38], and the feature at 1878 cm- is probably due to N2O3 [30, 39]. Elevating the temperature removes all three of these bands. The NO2/NO3 bands are quite intense at room temperature relative to the mono- and dinitrosyl nitrosyl bands. As the temperature rises, the ratio of nitrosyl to NO2/NO3 band intensities increases in a manner similar to that seen in Figure 6. Above 350 °C, the intensities of the NO2 and NO3 bands are smaller than those observed in the absence of CH4, a pattern identical to that already noted in the comparison of Figures 2 and 3. When the temperature is raised to 450 °C, the only features remaining are weak bands located at 2264, 1934, and 1635 cm-1. The first two bands are attributed to A13+-NCO and C02+-NO, respectively, and the third is due to adsorbed H2O.

Since the formation of NO2 can occur homogeneously, it was of interest to establish whether adsorbed NO could be oxidized. NO was adsorbed at 225 C, after which the infrared cell was purged with He and subsequently a stream of 10.1% O2 in He was allowed to flow over the catalyst. Prior to the introduction of the 02-containing stream, the only features evident were those for mono- and dinitrosyls. In the presence of O2 at 225 °C, the intensities of the bands for both mono- and dinitrosyl species attenuated and new features appeared at 1628 and 1518 cm-, corresponding to nitrate and nitrito species, respectively. A similar experiment carried out in the absence of O2, showed only a small decrease in the intensity of the nitrosyl bands due to NO desorption and the absence of bands for nitrate and nitrito species during a 30 min purge in He at 225 °C. 

The principal mechanistic events include N-N bond formation stage, where the coordinated NO reactant is transformed into the NzO semi-product via dinitrosyl ( M-(NO)2]Z) or dinitrogen dioxide ( M-N202 Z) intermediates, depending on the nature of TMI (vide infra). Simultaneously, the primary (M Z active sites are converted into the secondary [M-0]Z active sites involved in the dioxygen formation cycle [5], The mononitrosyl complexes are usually postulated to be the key intermediate species of this step [2,5,41], whereas the mechanistic role of dinitrosyls and dinitrogen dioxide is more indistinct as yet. 

Depending on the difference in adsorption energies (see Section 5.4) dinitrosyl complexes are formed either concomitantly or subsequently with the mononitrosyl complexes. Those processes have been widely investigated for selected TMIs and can be followed easily by IR technique [57], The appearance of a characteristic doublet due to the collective antisymmetric and symmetric vibrations of the M(NO)2 moiety growing at the expanse of the NO valence band is usually taken as a confirmation of the dinitrosyl formation. As discussed below in more detail, they play important role in the inner-sphere route of the N—N bond making (see Section 6.2.1). 

Table 2.5. Comparison of molecular properties of the dinitrosyl complexes 171 M(N0)2 " of selected TMIs encaged within ZSM-5 zeolite...

Surface nitrosyl complexes of TMI have been thoroughly investigated by the computational spectroscopy [22,23,32,33,36,49], and their molecular structure has been ascertained by a remarkable agreement between the theory and experiment of both vibrational (oscillation frequencies and intensities) and magnetic (g and A tensors) parameters. The calculated pNO values for the examined mononitrosyls along with the experimental frequencies are listed in Table 2.6. Analogous collation of the IR data for dinitrosyl species is shown in Table 2.7. 

Although the spectroscopic parameters prove to be diagnostic for simple finger-print identification of the corresponding species, more attentive analysis of the data contained in Tables 2.7 and 2.8 indicate that there is no correlation between the pNO values and the M—N—O bond angles, for both the mono- and the dinitrosyl complexes. It is then incorrect to attempt assignments of the MNO geometries based on the observed N—O... 

Table 2.7. Comparison of DFT calculated and experimental stretching frequencies for the selected dinitrosyl complexes...

Figure 2.13. Optimized geometries (DMol, VWN/DNP) of three selected dinitrosyl complexes, (a) 1Cu(NO)2 12M7, (b) 1Ni(NO)2 10silT5, and (c) 2Co(NO)2 9Z6. All bond lengths are given in A, and angles, in degrees (after [71,75]).

As shown in Table 2.7, the calculated intensity of the antisymmetric band is five times greater for the repulso form, whereas for the attracto conformer the intensities of both bands are more equilibrated. Comparison of the calculated (Figure 2.14) and experimental IR spectra (/sym/7asym = 0.58) shows that the copper dinitrosyls encaged in ZSM-5 exhibit the attracto conformation. 

Formation of the mono- and dinitrosyl complexes is a thermodynamically favorable process, which distinctly depends on the electronic configuration of the metal center. The adsorption energy, defined as A=. Eaddukt - ( mzsm-5+ no)> is shown in the form of a histogram in Figure 2.16. Formation of mononitrosyl complexes is exothermic... 

Figure 2.16. Calculated (BP/DNP) energies of the formation of the mononitrosyl, dinitrosyl, and dinitrogen dioxygen complexes of selected TMI encaged in ZSM-5 zeolite.

Generally, this tendency is in line with the changes in the M—NO bond lengths and the bond orders (Tables 2.4 and 2.5). However, there is a remarkable variance for the d5 configuration and between the Fe11 and Fe111 centers, already noted elsewhere [68], The energy of the formation of the dinitrosyl complexes (i.e., adsorption of the second NO molecule) is with the exception of Cr+ distinctly smaller (5 20 kcal/mol) that those... 

The specific goal of the mechanistic studies of DcNOx reaction is to identify the key intermediates involved in the N—N and 0—0 bonds making, discriminate them from spectator species and ascertain the sequence and conditions of their appearance. To clarify the role of the mono- and dinitrosyl complexes as intermediates or spectators of the principal mechanistic reaction steps, it is necessary to develop a more in-depth insight into the structure-reactivity relationships for both adducts, and to understand the possible ways of attaching the second NO molecule to the mononitrosyl complex. 

The dinitrosyl complex exhibits the repulso conformation with both Ni—N—O moieties bent outwardly, with the angle /J = 125(1)° and the ON—Ni—NO angle 6 = 97°... 

Figure 2.20. Transformation of silica supported dinitrosyl complexes of nickel(II) leading to formation of nitrogen dioxide and its final stabilization on the support. The picture shows the molecular structure and the spin density contours calculated with BP/DNP method of the involved species, and evolution of the X-band EPR spectra of the NiN02 Si02 complex due to spillover of the ligand (adopted from [71]).

Figure 2.21. Changes in (a) total energy and (b) partial charge on the terminal oxygen atoms of the NO ligands, calculated for the stepwise decrease of the 0—0 distance in the attracto conformation of the copper(I) dinitrosyl complex (after [75]).

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