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M-C02 complex

C02 is rather unreactive32 and only three examples of M—C02 complexes have been characterized33 33 (see Chapter 15.6). [Pg.584]

Raman and UV-visible spectroscopy, but no precise characterization was made. A report was made in 1981 where the IR spectrum of Cu atoms deposited with C02 at 80 K was interpreted in terms of the formation of a -coordinated complex between C02 and zerovalent copper [32]. Almond et al. [33] prepared a (C02) M(CO)5 molecule (M = Cr, W), that led to the formation of CO and oxometal carbonyl under UV irradiation. The first complete study of the reactivity of C02 with the first row of transition metals was made by Mascetti et al. [34, 35]. Here, it was shown that the late transition metal atoms (Fe, Co, Ni, and Cu) formed one-to-one M(C02) complexes, where C02 was bonded in a side-on (Ni), end-on (Cu), or C-coordinated (Fe, Co) manner, while the earlier metal atoms (Ti, V, and Cr) spontaneously inserted into a CO bond to yield oxocarbonyl species OM(CO) or 0M(C0)(C02). Normal coordinate analysis showed that the force constants of CO bonds were significantly decreased by 50%, compared to free C02, and that the OCO angle was bent between 120 and 150°. [Pg.61]

The first step in the activation of carbon dioxide by transition metal compounds is the formation of a M-C02 complex, since it is through coordination that the electronic structure of this molecule, and hence its reactivity, can be substantially modified. Transition metal complexes containing carbon dioxide in its intact form have received considable attention in the last decade (Inone et al, 1982), mainly with the aim of finding model systems for the activation of C02 and subsequent transformation into organic chemicals of comercial interest (Aresta et al, 1987). Despite considerable and intensive work in this area, the number of structurally characterized carbon dioxide complexes is stilt very limited, and they have been found to contain side-on (Alvarez et al, 1986), 72 -coordinated and, l2 C-coordinated (Calabrese et al, 1983) C02. [Pg.99]

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... [Pg.368]

A somewhat related problem is the nature of the bridged carbonyl group between two metal centers. Obvious correlations with organic ketonic behavior in general provide difficulties. In general nucleophilic attack by OR- (R = H or Me) does appear to occur at the carbon center. For R = Me, stable M- C02R complexes may often be isolated, but for R = H, transfer of hydrogen to the metal with elimination of C02 occurs readily, to yield the hydridocarbonyl. [Pg.269]

Recent work shows that a phenoxide-C02 complex is formed competitively with direct carboxylation. This does not go on to form products, but rather decomposes to phenoxide and carbon dioxide. (Y. Kosugi, Y. Imaoka, F. Gotoh, M. A. Rahim, Y. Matsui, K. Sakanishi, Organic and Biomolecular Chemistry 2003,1, 817)... [Pg.374]

Figure 11 Stability constants for complexes of polyammonium macrocycles with dicarboxylate substrates 02C(CH2)m-C02-. a = [24]N6 6HC1 (72a) O = [32]N6 6HC1 (72b) - [38]N6 6HC1 (72c)... Figure 11 Stability constants for complexes of polyammonium macrocycles with dicarboxylate substrates 02C(CH2)m-C02-. a = [24]N6 6HC1 (72a) O = [32]N6 6HC1 (72b) - [38]N6 6HC1 (72c)...
Reaction 6 has been documented for H2Rh(02C0H)(PCy3)2 (16).) Other IR bands at 1580(s) and 1235(m) cm"1 that result from the solution C02 reactions could arise from a C02 complex (54), a formate complex formed via C02 insertion into Rh-H (16), or even a bicarbonate formed via Reaction 7, which has been documented again for a rhodium/PCy3 system (16). [Pg.159]

It appears, then, that the most-often observed reactions of a metal atom with C02 in a low-temperature matrix are the formation of a metal complex, and/or the insertion into one CO bond of C02. These studies, which were conducted at low temperature on naked metal atoms, could not reproduce the reactions obtained with real metal complexes containing ligands, which can in turn influence further reactions with C02 (some examples are provided in Sections 4.3 and 4.4). However, with the assistance of theoretical calculations, the studies have allowed the identification of general trends in the Periodic Table, as well as a description of the different C02 bonding modes through the vibrational analysis of isolated M(C02) moieties. [Pg.64]

Two different metal-C02 complex intermediates have been proposed for the production of CO-metallocarboxylates and metal formates. The difference between the two species is based on the site of protonation, at the carbon atom in metallocarboxylates and at one of the oxygen atoms in metal formates. Carbon-protonation has not been observed experimentally, while oxygen-protonation is well known [9]. Isomerization can occur between metallocarboxylates and metal formates, and loss of a hydroxide group from the metal formate species yields the M-CO complex. Similarly, the direct reaction of metal complexes with free, dissolved C02 has also been described. In this mechanism, the metal complex reacts with an oxide acceptor, such as C02, generating the metal-CO complex and C032- [9],... [Pg.293]

Much attention has been devoted to the activation of CO2 with metal complexes. A variety of metal complexes have been used for the reduction of CO2, in which metal-T) -C02 complexes function as precursors for CO2/CO conversion. In protic media, CO2/CO conversion on metal complexes generally takes place through acid-base equilibria among [M-ril-C02]"+, [M-ril-C(0)OH](n+l)+, and [M-CO](n+2)+ (eq l).l In the absence of proton donors,... [Pg.459]

Highly reduced T l-C02 complexes, [W(CO)5(T 1-C02)]2 , [CpFe(CO)2(Tl 1-C02)] and [Ru(bpy)2(qu)( nl-C02)] react with CO2 to produce W(CO)6, [CpFe(CO)3]- - and [Ru(bpy)2(qu)(CO)]2+, respectively (eq 2). 2 To promote smooth CO2/CO conversion in aprotic media, characters of M-CO2 bond should be revealed. The knowledge on the bond character, however, is quite limited due to the lack of well characterized metal complexes with T) I-CO2 ligands. This paper describes the interconversion of Ru-CO and Ru-COj complexes through a reversible oxide transfer. [Pg.459]

Insertion of C02 into M—H bonds results only in metal formate complexes, M OCHO, not M—COOH.13,21 M 11 insertions have received much attention because they are thought to be intermediate in catalytic hydrogenations.12,13 Calculations indicate only weak M—O interaction in the transition state leading to formate a stable C02 complex may not precede the formate.28... [Pg.599]

Metal-T)HC)-C02 Complexes Equilibrium of CO2 and CO on Metals A. Decarboxylation of M-COOH and M-CO2 Electro- and Photochemical Reduction of CO2... [Pg.409]

Veillard [19] covers a similar range of molecules but from the Hartree-Fock and post-HF view. The discussion is organised more in terms of molecular properties. Thus, he deals with metal carbonyls, carbides, cyanides, C02 complexes, alkyls, carbenes, carbynes, alkenes, alkynes and metallocenes under the headings of electronic states, electronic spectra, optimised geometries, binding energies, Ionisation Potentials and Electron Affinities, nature of M-L bonding and other properties (e.g. vibrational spectra, dipole moments and electron distributions). [Pg.27]

Stable and highly selective complexes with organic dicarboxylate anions, 02C-(CH2)m-C02. These macrocycles display linear molecular recognition based on ditopic binding between two triammonium units of the macrocycle and the two terminal C02 groups of the dicarboxylate anion. The most stable... [Pg.14]

On the basis of the above studies, it can be ruled out that metal systems interact with CO2 in low temperature matrices with formation of metal complexes, which may or may not be the precursors of the 0=M(C0) species, the product of a formal insertion of the M-atom into one of the two C=0 bonds of CO2. Although naked metal atoms cannot be compared to LnM systems, the ensemble of the two types of metal systems allow the identification of general trends in the periodic table for what concerns the description of the modes of bonding of CO2 in M(C02) moieties and can depict a reaction mechanism for their further conversion into 0=M (CO) species. [Pg.58]


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See also in sourсe #XX -- [ Pg.93 ]




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