Dinitrosyl complex

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]. 

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...

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]).

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... 

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]).

Since the copper dinitrosyl complexes are essentially unstable at the temperatures above 223 K, their involvement in the NO decomposition over CuZSM-5 as intermediates of the N—N bond formation step is unlikely, despite some earlier claims [72,76], in accordance with the structure-reactivity analysis discussed above. 

Molybdenum dinitrosyl complexes with the general formula Mo(NO)2(CHR) (0R )2(A1C12)2 have been found to be active in a variety of metathesis reactions [110]. New alkylidenes could be identified. Variations such as Mo(NO)2(CHMe) (RC02)2 also are known [111]. Complexes of this type are believed to be more reduced than typical d° species discussed here, although they appear to be much more active as metathesis catalysts than typical Fischer-type carbene complexes. 

How does nitric oxide mediate /8-cell dysfunction and destruction The observation of an IL-1/3-induced iron-dithio-dinitrosyl complex in islets suggests... 

But, because there was also a first-order term, reduction via a dinitrosyl complex may not be compulsory. It is doubtful that cytochromes could participate in NO reduction via dinitrosyl complexes, because of strong axial coordination of Fe by at least one protein ligand. It is of course possible that the nonheme iron of nitric oxide reductase is the actual site of reduction of NO. 

Treatment of [ MoX2(NO)2 ] with sodium dialkyl dithiocarbamates (Nadtc) affords the dinitrosyl complexes cis-[Mo(NO)2(S2CNR2)2] (R = Me or Et). These compounds may also be prepared by treatment of [Mo(NO)2(MeCN)4][PF6]2 with Nadtc and the complex [Mo(NO)2(acac)2] is obtained using Na[acac].26,27... 

A further class of dinitrosyl complex is that containing dithiolene and related ligands. Because of electronic delocalization, there are problems of assignment of oxidation state in... 

An important use of dinitrosyl complexes of molybdenum has been as catalyst precursors in the alkene disproportionation reaction. Thus [MoCl2(NO)2L2] (L = PPh3 or py) together with AlCl2Et forms an active homogeneous catalyst for alkene disproportionation. The catalysis appears to require loss of NO from the metal and the field has been reviewed.19,31... 

When [Fe4S3(NO)7] reacts with RS in DMF solution, then, for a range of substituents R, the products are (23) [Fe(NO)2(SR)2] , as formed from [Fe2(SR)2(NO)4], together with [Fe(NO)(SR)3] in this latter series (Table IV), although the A(14N) value depends upon R, no hyperfine coupling to the a-hydrogen atoms in R was resolved, so that all the spectra of [Fe(NO)(SR)3] comprise three lines only. The mono-nitrosyl complexes are fairly short lived, and within 2 days only the dinitrosyl complexes were detectable. Since the formal oxidation states of iron in [Fe(NO)2(SR)2] and [Fe(NO)(SR)3] are Fe(-I) and Fe(I), respectively, it is likely that [Fe(NO)2(SR)2] arises from the basal iron atoms in [Fe4S3(NO)7] , and that [Fe(NO)(SR)3] arises from the apical iron [Eq. (17)]. 

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