Formic acid metal complexes

Formamidine, N,N -di-2-anthraquinonyl-metal complexes, 275 Formamidine, JV,iV -diaryl-metal complexes, 275 Formamidine, N,N -dibenzyl-metal complexes, 276 Formamidine, N,N -diisopropyl-metal complexes, 276 Formic acid metal complexes, 436 Fulminates... 

The reduction of iminium salts can be achieved by a variety of methods. Some of the methods have been studied primarily on quaternary salts of aromatic bases, but the results can be extrapolated to simple iminium salts in most cases. The reagents available for reduction of iminium salts are sodium amalgam (52), sodium hydrosulfite (5i), potassium borohydride (54,55), sodium borohydride (56,57), lithium aluminum hydride (5 ), formic acid (59-63), H, and platinum oxide (47). The scope and mechanism of reduction of nitrogen heterocycles with complex metal hydrides has been recently reviewed (5,64), and will be presented here only briefly. 

General procedures for the synthesis of ligands and metal complexes are shown in Scheme 1. For file synthesis of 2a acenaphthenequinone (0.38 g, 2.1 mmol) and a,a-bis(4-amino-3,5-dimethylphenyl)-toluene(Kccess) were dissolved in 50 mL of CH3OH in a round-bottom flask. Five drops of formic acid were added, and the sealed solution was stirred at 50 C overnight. After filtration, the red solid was washed with hot methanol and dried to give 1.2 g of red powder in 50 % yield. 

As already mentioned before the elaitrochemical reduction of CO2 at a metal electrode leads only to the formation of formic acid. Recently it has been reported by Ogura et al. (see and literature cited therein), however, that at a Pt-electrode coated by a layer of Everitt s salt (ES), K2Fe(II)[Fe(II) (CNg)], CO2 is selectively reduced to methanol in the presence of metal complexes as homogeneous catalysts and a primary alcohol. The overall reaction is given by... 

Approach (a) is normally the easiest to control, and is used in the application of levelling acid and 1 1 metal-complex dyes to wool or nylon, and of the reactive, sulphur or vat dyes to cellulosic fibres. The agents traditionally used are the stronger acids and alkalis such as sulphuric, hydrochloric and formic acids, sodium carbonate and sodium hydroxide. In... 

Levelling acid dyes and particularly 1 1 metal-complex types generally require an exceptionally low pH in order to promote exhaustion and levelling up to 3% o.w.f. sulphuric acid is most commonly used for levelling acid dyes, although hydrochloric, formic and phosphoric acids are also effective. In the case of conventional 1 1 metal-complex dyes it is essential to use a sufficient excess of acid over and above the typical 4% o.w.f. sulphuric acid normally absorbed by the wool, otherwise there may be a tendency towards tippy dyeings and lower wet fastness. The actual excess required depends on applied depth and liquor ratio [2] typical recommendations are given in Table 12.2. 

Hiratsuka et al102 used water-soluble tetrasulfonated Co and Ni phthalocyanines (M-TSP) as homogeneous catalysts for C02 reduction to formic acid at an amalgamated platinum electrode. The current-potential and capacitance-potential curves showed that the reduction potential of C02 was reduced by ca. 0.2 to 0.4 V at 1 mA/cm2 in Clark-Lubs buffer solutions in the presence of catalysts compared to catalyst-free solutions. The authors suggested that a two-step mechanism for C02 reduction in which a C02-M-TSP complex was formed at ca. —0.8 V versus SCE, the first reduction wave of M-TSP, and then the reduction of C02-M-TSP took place at ca. -1.2 V versus SCE, the second reduction wave. Recently, metal phthalocyanines deposited on carbon electrodes have been used127 for electroreduction of C02 in aqueous solutions. The catalytic activity of the catalysts depended on the central metal ions and the relative order Co2+ > Ni2+ Fe2+ = Cu2+ > Cr3+, Sn2+ was obtained. On electrolysis at a potential between -1.2 and -1.4V (versus SCE), formic acid was the product with a current efficiency of ca. 60% in solutions of pH greater than 5, while at lower pH... 

However, since the goal of this work was the synthesis of alcohols from olefins via hydrohydroxymethylation (75, 76), little attention was given to developing a shift-catalyst per se. Pettit has recently reexamined some of this work and shown that, by careful control of the pH of the reaction mixture, systems based on either Fe(CO)5 or Cr(CO)6 can be developed for the production of either formic acid or methanol from carbon monoxide and water (77, 78). Each of these latter systems involves the formation of metal hydride complexes consequently, molecular hydrogen is also produced according to the shift reaction [Eq. (16)]. 

The possible intermediacy of the formate ion (eqs. 6 and 18) in the WGSR has been considered (2,6,10), but its involvement has not been clearly demonstrated. The Group VI metal carbonyl complexes are effective in the decomposition of formic acid (as sodium formate), as shown in Table VII. Some heterogeneity is observed in those reactions carried out under nitrogen pressure, but in no case was CO detected. The similarity in rates for WGSR... 

Carbon dioxide is known to readily insert into a metal-hydride bond to give a metal formate [57, 58] this forms the first step in insertion mechanisms of C02 hydrogenation (Scheme 17.2). Both this insertion step and the return path from the formate complex to the hydride, generating formic acid, have a number of possible variations. 

In this first section, we will describe the experimental knowledge on the hydrogenation of C02 to formic acid and the early theoretical works on this topic, which are concerned with the structure and formation of transition-metal C02 complexes. 

As briefly discussed in section 1.1, and shown in Figure 1, the accepted mechanism for the catalytic cycle of hydrogenation of C02 to formic add starts with the insertion of C02 into a metal-hydride bond. Then, there are two possible continuations. The first possibility is the reductive elimination of formic acid followed by the oxidative addition of dihydrogen molecule to the metal center. The second possible path goes through the a-bond metathesis of a metal formate complex with a dihydrogen molecule. In this section, we will review theoretical investigations on each of these elementary processes, with the exception of oxidative addition of H2 to the metal center, which has already been discussed in many reviews. 

Reductive Elimination of Formic Acid from Transition-Metal Formate Complexes... 

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

The other reaction path to obtain formic acid from the transition metal formate complex is metathesis with a dihydrogen molecule. This reaction course has been proposed experimentally, but no clear evidence has been reported so far. Energetics of this reaction from different complexes and with a variety of methods are collected in Table 4. 

Asymmetric transfer hydrogenation of imines catalyzed by chiral arene-Ru complexes achieves high enantioselectivity (Figure 1.34). Formic acid in aprotic dipolar solvent should be used as a hydride source. The reaction proceeds through the metal-ligand bifunctional mechanism as shown in the carbonyl reduction (Figure 1.24). 

Since the early 2000s, different sources of CO have been explored and applied to carbonylation reactions for laboratory organic synthesis. For example, the use a stoichiometric amount of metal-carbonyl complexes, thermolysis of formic acid at high temperature, and the use of aldehydes via decarbonylation have been investigated. For the use of metal-carbonyl complexes and formaldehyde as carbonyl source, it has been shown that microwave irradiation greatly accelerates the process. ... 

Trost exploited the annulation of electron rich phenols and alkynoates to obtain coumarins in the presence of transition metal complexes. Ethyl propiolate and 3,4,5-trimethoxyphenol were coupled in formic acid in the presence of a palladium complex and sodium acetate to give 5,6,7-trimethoxycoumarin via a net C-H insertion in acceptable yield (4.42.). The coupling, characteristic of electron rich phenols, was also catalyzed by other transition metals, such as platinum or silver.56... 

Homogeneous Hydrogenation. The mild reaction conditions used in the homogeneous hydrogenation of C02 catalyzed by transition metal complexes allows the partial hydrogenation of C02 to yield formic acid and derivatives ... 

Water-soluble transition metal complexes, which are effective catalysts in other hydrogenation processes, were found to be effective catalysts in C02 hydrogenation. The first report disclosed the application of Rh complexes with water-soluble phosphine ligands in water-amine mixtures to afford formic acid.122 Water-soluble Ru-phosphine121,123,124 and Rh-phosphine123 124 complexes were used in aqueous solution to hydrogenate C02 or HCO3 to formic acid or HCOO-, respectively. 

A large number of metal complexes have been proved active in electrochemical reduction of C02. Among these, certain Re, Ni, and Ru complexes, mostly the type [Ru(bpy)2(CO)L]"+ (bpy = bipyridyne, L = CO or H, n=l,2), have attracted much attention because of their characteristic reactivity and high efficiency.129 131 The catalytic cycles have been elucidated.132 These complexes, however, cannot be used in aqueous solution because of the competing hydrogen evolution. Re complexes, in turn, when incorporated into Nation membrane, proved to be efficient in formic acid production.133... 

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