Nitrosyl complex catalysts

In a related observation, reported by Tanaka et al. (81), the copper(II) complex Cu(tpa)2+ (tpa = tris[(2-pyridyl)methyl] amine) was shown to serve as a catalyst for the electrochemical reduction of nitrite to N20 and traces of NO in aqueous solution. NO and/or a copper nitrosyl complex would appear to be the likely intermediates in this process (81a). 

Complexes of Other Metals. Having studied in detail catalysts derived from molybdenum nitrosyl complexes, it was interesting to investigate the effect on catalytic activity of substituting other transition metals in both nitrosyl derivatives and in other related complexes. 

The active catalyst is maintained only under NO and CO gas mixtures. Under CO alone the carbonyl catalyst precursor [RhCl2(CO)2] is reformed, and under NO alone a nitrosyl complex is formed which also functions as a... 

The pH dependence of the rate of formation of a nitrosyl complex shows that nitrous acid is the reactive intermediate in the reaction when the pH is in the range of 2-8. The catalysts are not deactivated during repeat cycles between their oxidized and reduced states. The catalyzed reduction appears to depend on the ability of the multiply reduced heteropolyanions to deliver electrons to the NO group bound to the iron center. 

Although this catalytic reaction appeared to be of synthetic interest, it has since then neither been applied in synthesis nor further developed. This might be attributed in part to problems with reproducibility and catalyst stability under the reaction conditions, although the Hieber complex was used in a stoichiometric manner for the preparation of a variety of 7i-allyl-Fe complexes. These latter compounds served as starting materials for a plethora of subsequent reactions [34]. The results obtained by Nakanishi and coworkers on the stability and reactivity of n-allyl-Fe-nitrosyl complexes proved such intermediates to be reactive towards a variety of nucleophiles however, the Fe complexes formed upon nucleophilic substitution were catalytically inactive. Hence, in order to maintain the catalytic activity, the formation of intermediate 7i-allyl-Fe complexes had to be circumvented. About 3 years ago we started our research in this field and envisioned the use of a monodentate ligand to be a suitable way to stabilize the proposed catalytically active G-allyl complex. The replacement of one CO by a non-volatile basic ligand was thought to prevent the formation of the catalytically inactive 7t-allyl-Fe complex (Scheme 7.21). 

In the presence of metal catalysts nitrosations using nitric oxide proceed rapidly and it is clear that a very powerful nitrosating species is produced. Rate measurements on the reaction of diethylamine with nitric oxide in the presence of Cu(II) salts indicated that a copper-nitrosyl complex was that species (Brackman and Smit, 1965). Many metal-nitrosyl complexes are now known... 

The rhodium nitrosyl complex is a deep-red crystalline solid which is moderately stable in the air but decomposes rapidly in solution in the presence of oxygen. It is soluble in chloroform, dichloromethane, and benzene and is insoluble in ether, methanol, and ethanol. The compound is a homogeneous hydrogenation catalyst for unsaturated organic compounds. > ... 

The ability of Pd-H-ZSM-5 catalysts to form Pd(I) nitrosyl species was related to their specific behavior of selectively reducing NO to N2 (25). This statement finds support in the curve of NO conversion versus Pd content (Figure 7A). Indeed, for reaction temperatures less than 500°C, NO conversion clearly increases with Pd content, in a manner similar to the amount of Pd nitrosyl complexes versus Pd content. Above 500°C, volcano shape curves are observed and NO conversion decreases for Pd content higher than 0.5 wt.-%. This can be easily explained by the simultaneous total conversion of CH4. The absence of reductant in the feed is expected to decrease the rate of NO reduction. This implies that CH4 participates to two distinct reactions, SCR reaction and methane combustion by O2, which compete at high temperatures. This competition is confirmed by the selectivity results, which indicates that the combustion is strongly favored above 500°C. The question arises to know whether these two reactions are catalyzed by the same types of sites. 

Table 1 summarizes the most important surface complexes formed when NO and CO are adsorbed on noble metal catalysts. According to the literature NO and CO are adsorbed as nitrites, nitrates and carbonates on alumina [2]. The most important surface complexes for CO and NO adsorption on rhodium are a gem-dicarbonyl (Rh(CO)2) and a linear Rh-NO complex [1]. However, tricarbonyl and bridged Rhx-CO complexes have been proposed to be formed and different kinds of linear Rh nitrosyl complexes are possible [1-4]. The adsorption of CO on R and Pd catalysts depends much on the oxidation stage of R and Pd [5]. CO adsorption on R forms mostly linear and bridged carbonyls [6-11]. NO is adsorbed linearly on R [12]. In the case of Pd the most common surface complexes are linear carbonyls [13], strong multilaterally-bonded carbonyls, bridged carbonyls [5,14,15] and triply-bonded CO [5]. Isocyanate, nitrous oxide or nitrogen dioxide are proposed to be coimected to the reaction mechanism of the NO-CO reactions [2,16-19]. 

Finally, it may be noted that the band at 1805 cm of the mono-nitrosyl complexes, Cu (NO), is more intense and broader for the CuAPSO-34 catalyst and these are further evidence that Cu sites in this sample are more abundant and more heterogeneously distributed. 

The ability of the nitrosyl ligand to behave as an electron pair reservoir has also been considered to play an important part in certain catalytically active systems. The vacant site provided by isomerization of the ligand could enable an unsaturated organic molecule to enter the transition metal s coordination sphere, thus forming an active intermediate. Examples of catalysis by nitrosyl complexes include the hydrogenation of alkenes by Rh(NO)L3 species and the dimerization of dienes in the presence of Fe(CO)2(NO)2 or Fe(n-C3Hs)(CO)2NO. Certain molybdenum dinitrosyl complexes, such as MoCljfNOljfPPhjlj, have also been found to provide very efficient alkene dismutation catalysts. ... 

Allylic sulfonylations can also be accomplished by using the iron nitrosyl complex 31 as catalysts [98]. In these cases, 2-methoxylethanol was used as a co-solvent to DMF due to the poor solubility of the sodium sulfinate substrate to be applied as a nucleophile. The effect of the phosphine ligand was also investigated. Tuning efforts revealed an outstanding performance of P(p-MeOAr)3 as co-catalyst. These efforts rendered a suitable protocol for efficient and regioselective allylic sulfonylation allowing C-S bond formations, as depicted in Scheme 37. 

Moreover, selectivity could be also enhaneed if a specific interaction between the immobilized catalyst and the substrate in solution were to occur. A significant example is that of the voltam-metric detection of NO in the rat brain from a carbon fiber microelectrode modified with a [(H20)Fe PWii039] -containing poly(N-methylpyrrole) film and a Nation outer layer [150]. The good selectivity of this sensor was attributed not only to the Nafion membrane, which constituted an efficient electrostatic barrier against anionic interferents, but also to the formation of a metal-nitrosyl complex between the heteropolyanion and NO. The in vivo NO measurements were validated by injecting the rat with an NO-synthase inhibitor, which led gradually to the disappearance of the NO oxidation peak (Fig. 7). 

The metathesis reaction offers some opportunities for the synthesis of highly pure cycloolefins, as well. A first efficient method consists of the metathesis cycli-zation of linear a, co-dienes in the presence of tungsten- or molybdenum-based catalysts. Ethene or other low alkenes formed as by-products are easily removed from the reaction mixture, enabling convenient separation of the cycloalkene. A pertinent example is the synthesis of cyclohexene, in high yield, from 1,7-octadiene with molybdenum nitrosyl complexes [23] ... 

The carbonyl complex [Ru(EDTAH)(CO)] has been reported to be a very good catalyst for reactions like hydroformylation of alkenes, carbonylation of ammonia and ammines as well as a very active catalyst for the water gas shift reaction. The nitrosyl [Ru(EDTA)(NO)] is an oxygen-transfer agent for the oxidation of hex-l-ene to hexan-2-one, and cyclohexane to the corresponding epoxide. 

---