Nitrate ligands, reduction

In contrast to the facile reduction of aqueous V(III) (—0.26 V versus NHE) [23, 24], coordination of anionic polydentate ligands decreases the reduction potential dramatically. The reduction of the seven-coordinate capped-octahedral [23] [V(EDTA)(H20)] complex = —1.440 V versus Cp2Fe/H20) has been studied extensively [25,26]. The redox reaction shows moderately slow electron-transfer kinetics, but is independent of pH in the range from 5.0 to 9.0, with no follow-up reactions, a feature that reflects the substitutional inertness of both oxidation states. In the presence of nitrate ion, reduction of [V(EDTA) (H20)] results in electrocatalytic regeneration of this V(III) complex. The mechanism was found to consist of two second-order pathways - a major pathway due to oxidation of V(II) by nitrate, and a minor pathway which is second order in nitrate. This mechanism is different from the comproportionation observed during... 

Copper(I) nitrate complexes with organophosphines, arsines, and stibines can be readily prepared in good yield in alcohol solution by such ligand reduction reactions.9 In the cases of the arsines and stibines, addition of copper powder... 

Aromatic amines are found in biologically active natural products, common pharmaceuticals, dyestuffs, materials with conductive and emissive properties, and ligands for transition-metal-catalyzed reactions. For these reasons much effort has been spent for more than a century on methods to prepare aromatic amines. The synthetic methods to obtain these materials range from classical methods, such as nitration and reduction of arenes, direct displacement of the halogens in haloarenes at high temperatures, or copper-mediated chemistry, as well as modem transition-metal-catalyzed processes and improved copper-catalyzed processes. The following sections describe each of these synthetic routes to aromatic amines, including information on the scope and mechanism of most of these routes to anilines and aniline derivatives. 

Nitrate Reductase.—Reduction of nitrate by molybdenum(v) complexes of the type [M0OCI3L] and [MoOClL 2l where L and L are neutral or anionic bidentate ligands, proceeds by rate-determining loss of Cl in DMF at 25 °C ... 

The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst environment. Instead, the Fe center can influence a catalytic asymmetric process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. This interaction is often comparable to the stabilization of a-ferrocenylcarbocations 3 (see Sect. 1) making use of the electron-donating character of the Cp2Fe moiety, but can also be reversed by the formation of feirocenium systems thereby increasing the acidity of a directly attached Lewis acid. Alternative applications in asymmetric catalysis, for which the interaction of the Fe center and the catalytic center is less distinct, have recently been summarized in excellent extensive reviews and are outside the scope of this chapter [48, 49], Moreover, related complexes in which one Cp ring has been replaced with an ri -arene ligand, and which have, for example, been utilized as catalysts for nitrate or nitrite reduction in water [50], are not covered in this chapter. 

Gomez Arrayas R, Adrio J, Carretero JC (2006) Recent applications of chiral ferrocene ligands in asymmetric catalysis. Angew Chem Int Ed 45 7674—7715 Dai LX, Hou XL (2010) Chiral ferrocenes in asymmetric catalysis. Wiley-VCH, Weinheim Rigaut S, Delville MH, Losada J, Astrac D (2002) Water-soluble mono- and star-shaped hexanuclear functional organoiron catalysts for nitrate and nitrite reduction in water syntheses and electroanalytical study. Inorg Chim Acta 334 225-242... 

The monovalent Co chemistry of amines is sparse. No structurally characterized example of low-valent Co complexed exclusively to amines is known. At low potentials and in non-aqueous solutions, Co1 amines have been identified electrochemically, but usually in the presence of co-ligands that stabilize the reduced complex. At low potential, the putative monovalent [Co(cyclam)]+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) in NaOH solution catalyzes the reduction of both nitrate and nitrite to give mixtures of hydroxylamine and ammonia.100 Mixed N-donor systems bearing 7r-acceptor imine ligands in addition to amines are well known, but these examples are discussed separately in Section 6.1.2.1.3. 

As one traverses through the lanthanide series, there is a reduction in the cation size as the atomic number increases. This results in small differences in the strength of interactions of the ligand with the lanthanide ions. These trends are reflected in the IR spectra of these complexes in a few cases. Cousins and Hart (203) have observed an increase in Pp Q with decreasing lanthanide ion radius for the complexes of TPPO with lanthanide nitrates. This observation has been attributed to an increase in the Ln—O bond strength with an increase in the atomic number of the lanthanide ion. 

Many polarographic studies of the reduction of cobalt(II) to form an amalgam at a dropping mercury electrode have been reported, but most of the work has focused on systems involving complexes with ligands other than water [1, 2]. In one of the few investigations of the behavior of Co(H20)6 + (in aqueous 0.1 M potassium nitrate) [3], the following information was deduced ... 

This mechanism can be illustrated by the reaction of ferrous ions with hydrogen peroxide (42), the reduction of organic peroxides by cuprous ions (63), as well as by the reduction of perchlorate ions by Ti(III) (35), V(II) (58), Eu(II) (71), The oxidation of chromous ions by bromate and nitrate ions may also be classified in this category. In the latter cases, an oxygen transfer from the ligand to the metal ion has been demonstrated (8), As analogous cases one may cite the oxidation of Cr(H20)6+2 by azide ions (15) (where it has been demonstrated that the Cr—N bond is partially retained after oxidation), and the oxidation of Cr(H20)6+2 by 0-iodo-benzoic acid (6, 8), where an iodine transfer was shown to take place. 

In conclusion, the ionic bond as driving force for the formation of a heterobidentate bisphosphine seems a useful tool. Sulfonation of one ligand, and nitration followed by reduction to amine, may afford a relatively easy entry into this chemistry. Several... 

For the synthesis of bidentate ligands, supramolecular approaches have led to a renaissance in homogeneous catalyst discovery (Chapters 2, 4, 8, 9,10), and in a few cases even monodentate ligands have been modified in a supramolecular fashion (Chapter 8, Section 8.2). Combinations of monodentate ligands can be left to chance and in several instances this has led to successful, new catalysts [96]. Such heterocombinations can form spontaneously for steric or electronic reasons or the reactivity of the combinations can be different such that on certain occasions highly enantioselective catalysts are obtained. There are many ways to synthesize the desired heterocombinations selectively and the ionic modification outlined in Section 10.4 is only one of them since nitration (followed by reduction to amines) and sulfonation are robust methods, the ionic route may prove useful. Hydrogen bonding between different donor-acceptors (Chapters 2 and 8), Lewis add-base interactions (Chapter... 

As with xanthine oxidase, the sulfido ligand of the active form of aldehyde oxidoreductase is readily replaced by an oxido ligand to yield a cofactor with a structure that resembles that of oxidized sulfite oxidase and assimilatoiy nitrate reductase. Both x-ray and EXAFS data are available for the bis(oxido) form, and, with the exception of the oxido replaced sulfido ligand, few changes are obvious in the overall structure of the oxidized form of the desulfo cofactor. Upon reduction of the enzyme the oxido ligand is presumably reduced to hydroxido, an observation that is supported by EPR data for the Mov state. 

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