Reduction of the metal center

Radiolytic studies in aqueous solutions demonstrated reduction of Fe P to Fe P, of Co P to Co P to Co P/ of Mn P to Mn P/ and in 2-propanol reduction of Rh P to Rh P via an unstable Rh P/ Trivalent metalloporphyrins generally contain a halide ion and/or a solvent molecule as axial ligands. Reduction of these complexes to form the divalent metal center often is accompanied by release of the axial ligands. 

When radiolytic reduction of Mn and Co porphyrins was carried out in frozen MTHF at 77 K, the anionic axial ligands could not diffuse away and thus the optical and ESR spectra of the reduced complexes were quite different from those of the equilibrated reduced products. On the other hand, when the complexes were dissolved in alcohol, the halide axial ligand was exchanged with a solvent molecule and when these solutions were frozen and irradiated, the reduced species were identical with the equilibrated stable products.  

Porphyrin complexes of certain metal ions have reduction potentials for the ligand and the metal center that are sufficiently close to lead to uncertainties in the site of reduction. The outcome of the reaction can be determined from the optical absorption spectrum of the product (transient or stable). The site of reduction is dependent on the electron affinity of the ligand and, therefore, complexes of pyridylporphyrins or TMPyP are more likely to be reduced at the ligand than complexes of TPP or OEP. This was found to be the situation with Ni -porphyrins, where several complexes were reduced to Ni P but the presence of a single we o-pyridyl group was sufficient to direct the reduction toward the ligand to form Ni P . However, all one-electron reduction products were unstable in water and decayed to form the Ni -chlorin or phlorin. 

The site of reduction of Cr -porphyrins was found to be dependent also on the medium, because the medium can be involved in axial ligation. For the 

The effect of the axial ligand is very strong in the case of CN and shifts the site of reduction for a Co -porphyrin. Thus, the complex (CN)2Co TM4PyP is the only example in which a Co -porphyrin is found to be reduced at the ligand rather than at the metal. 

---

The dispersion of Pt(0) inside the functionalized resins was carried out by two main routes. The first is based on impregnation of the resin with a mesitylene solution of Pt nanoclusters (Solvated Pt Atoms) obtained via MVS. The second procedure, called Chemical Incorporation and Reduction (CIR), implies the immobilization of convenient molecular Pt precursors (i.e. [Pt(NH3)4]Cl2) in the pre-swollen resins, followed by chemical reduction of the metal center. Among the Pt catalysts obtained by the CIR procedure only Pt/CF3 exhibits a high conversion of the... 

Several other polypyridyl metal complexes have been proposed as electrocatalysts for C02 reduction.100-108 For some of them HCOO- appears as the dominant product of reduction. It has been shown for instance that the complexes [Rhin(bpy)2Cl2]+ or [Rh n(bpy)2(CF3S03)2]+ catalyze the formation of HCOO- in MeCN (at —1.55 V vs. SCE) with a current efficiency of up to 80%.100,103 The electrocatalytic process occurs via the initially electrogenerated species [RhI(bpy)2]+, formed by two-electron reduction of the metal center, which is then reduced twice more (Rlr/Rn Rh°/Rh q. The source of protons is apparently the supporting electrolyte cation, Bu4N+ via the Hoffmann degradation (Equation (34)). 

Some nitrosyl-Mo1 complexes of the form [Mo(Tp )(NO)Cl(py-R)] (where py-R is a substituted pyridine) also undergo moderate NIR electrochromism on reversible reduction to the Mo° state. In these complexes reduction of the metal center results in appearance of a Mo° —> py(7r ) MLCT... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Green et al. observed that the tantalum complex 101 is converted into the bicyclic complex 109 after prolonged exposure to sodium and trimethyl-phosphine.930 Two phosphine ligands of 101 are easily displaced by butadiene, affording the complex 110, while treatment of 109 with dihydrogen leads to the reduction of the metallic center. In both cases, the metallacycle remains intact.930... 

MoNC framework to MoCN with formal reduction of the metal center (from 18-electron) Mo to (16-electron) Mo"... 

In acid solution, cobalt(III) ammines are unstable with respect to reduction of the metal center to cobalt(II). The potential for the reduction shown in reaction (11) is about 1.8 V, but most of the energy difference is due to the protonation of the ammonias released by cobalt(II) which is a labile d1 system. 

The mechanism depicted in Scheme 2 involves two main steps. Rupture of the first metal-nitrogen bond accompanied by coordination of a water ligand at the metal center is followed by reversible deprotonation and intramolecular reduction of the metal center. Under the experimental conditions wherein the concentration of base is much larger than the concentration of tris(diimine) complex, and, applying the steady-state approximation to the concentration of the intermediate species with the monodentate diimine ligand, Eq. (6) can be derived as... 

Another type of redox reaction is the formation of disulfide with a two-electron reduction of the metal center M" (equation 3).115 The disulfide formation may be regarded as a limiting case of the sulfur-sulfur interactions which occur in thiolato complexes. 

The existence of additional occupied states of Mo character, located above the O 2sp derived valence region, is relevant for the interpretation of experimental photoemission spectra of molybdenum oxide surfaces. According to the results of the cluster studies additional photoemission intensity above the valence band region may be indicative of chemical reduction of the metal centers, leading to lower oxidation states, where the effect can be introduced by oxygen vacancies or by different chemical composition of the oxide. This has been verified in UPS experiments on differently prepared MoOsCOlO) surfaces in comparison with measurements of other single and mixed valency molybdenum oxide samples [212]. 

In both syntheses reduction of the metal center occurs as a side reaction. In the case of... 

Evans and coworkers have reported that cationic copper(II)-bis(oxazoline) complexes derived from ferf-leucine are effective Lewis acids for a wide range of enantioselective Diels-Alder reactions. While initial investigations employed cy-clopentadiene as the diene and triflate catalyst 31a (Scheme 24) as the Lewis acid [82], subsequent studies revealed that the reaction rate is strongly dependent on the counterion X [83]. The hexafluoroantimonate catalyst 31b is approximately 20 times more reactive than 31a and is typically more stereoselective. The heightened reactivity and selectivity conferred by catalyst 31b allows access to more substituted adducts in uniformly high enantioselectivity. The active catalyst is easily prepared and robust exposure to air is not deleterious and the reactions may be conducted in the presence of free hydroxy groups. However, reduction of the metal center can be problematic with electron-rich dienes this side reaction may be controlled by a judicious choice of temperature. 

The former corresponds to reduction of the metal center by CO with the two paths evolving CO2 showing the stoichiometric equivalence of a reduced metal plus proton with a metal hydride. The latter reaction corresponds to proton reduction. Clearly, (a) and (b) are influenced by the relative redox ability of the metal, which in turn is affected by the ligand environment of the complex. Proton concentration will also have an effect low pH favors reaction (b), while high pH promotes equation (a) through nucleophilic attack by OH rather than by HjO on coordinated CO to yield the M-COOH species penultimate to CO2 formation. 

Nitric oxide displays a wonderful diversity of ligand reactions. It is important to distinguish two classes of these reactions reductive nitrosation and reductive nitrosylation. Reductive nitrosation refers to the addition of NO to a bound amide ligand with concomitant reduction of the metal center as in... 

Reductive nitrosylation, on the other hand, can refer to the addition of NO to a metal center Mox with formal reduction of the metal center to yield Mred(NO +), but in the context of ligand reactions reductive nitrosylation refers to the net reactions of NO with metal-bound NO and the ensuing events. Reductive nitrosation of coordinated amines to form nitrosamines occurs through the conjugate base of the amine, and this process has been reported for reactions of NO with [Ni(tacn)2]3 +, 198 with methyl-amine coordinated to a macrocyclic Ni(III) complex,199 with triglycyl complexes of Fe(III), Ni(III), and Cu(III),200 and with Cu(II) macrocyclic complexes.201 Reductive nitrosation of [Ru(NH3)6]3+ produces [Ru(NH3)5N2]2 + with base-catalyzed kinetics the coordinated N2 is produced by hydrolysis after the nitrosation step.170... 

NO2 adds to oxo complexes such as Cr02 +, 205 [(TMPS)FeIVO],20 and MbI e,vO to form nitrato complexes. 203 It adds reversibly to CrOO2 + to form a rather unreactive peroxynitrato complex.208 These reactions lead to the reduction of the metal center and thus can be considered as good examples of the inner-sphere electron transfer mechanism. 

---