High-pressure synthesis carbonyl complexes

Low Oxidation State Chromium Compounds. Cr(0) compounds are TT-bonded complexes that require electron-rich donor species such as CO and C H to stabilize the low oxidation state. A direct synthesis of Cr(CO)g, from the metal and CO, is not possible. Normally, the preparation requires an anhydrous Cr(III) salt, a reducing agent, an arene compound, carbon monoxide that may or may not be under high pressure, and an inert atmosphere (see Carbonyls). 

The majority of these contain CO ligands. The neutral homoleptic carbonyls have not been isolated, but anionic M(CO)6 as well as highly reduced M(CO) species are known. The original synthesis of the -1 species required elevated temperatures and high pressure but recently two simple, atmospheric pressure methods have been developed. They involve reduction of pentahalides in pyridine with Zn/Mg or in dimethoxyethane with sodium naphthalenide under an atmosphere of CO. These yellow salts contain discrete M(CO)6 anions. The facile syntheses of the octahedral hexacarbonyl anions allowed systematic exploration of the previously difficult to access area of low-valent complexes of Nb and Ta. Since M(CO)s are rather inert towards displacement of CO the substitution products of general formula M(CO)6- L have to be obtained by other routes, for example, by reduction of MX(CO)6- L compounds. The monosubstituted derivatives are conveniently prepared via the following method87 ... 

In C, chemistry, the Rh carbonyl complexes are homogeneous catalyst precursors for the conversion of synthesis gas to ethylene glycol under high pressure and high temperature (eq (28)) [35]. 

In contrast, metal clusters have several active centers or can form multi-electron systems. Metal clusters such as Rh (CO)i6, Rh4(CO)i2, It4(CO)i2, Ru3(CO)i2, and more complex structures have been successfully tested in carbonylation reactions. Rhodium clusters catalyze the conversion of synthesis gas to ethylene glycol, albeit at very high pressures up to now. 

In general high pressure and temperature are required for these reactions to occur. However there are significant examples of reactions catalysed at atmospheric pressure, in part icular for the synthesis of isocyanates (4.2.5.). In the majority of cases the most important steps of these reactions are supposed to be the deoxygenation of the nitro function by carbon monoxide iving a nitrene residue bound to the metal centre, followed by insertion of carbon monoxide into the metal-nitrene bond. This is a likely hyphotesis since nitrene complexes can be obtained by stoichiometric reactions of nitro compounds with metal carbonyls. Conversion of the imido metal complex to the observed... 

The existence of [Rh2( 0)8] under normal conditions has not been confirmed, although its synthesis from dispersed metal under pressure (28 MPa) and at 473 K had been reported. However, it was shown that [Rh2( 0)8] exists in solutions only at high pressures and at low temperatures.[Rh2(CO)8] and [Ir2( 0)8] were also prepared by reactions of atomic Rh and Ir with carbon monoxide in 0 matrices. During condensation at 10 K, [Rh( 0)4] or [Ir( 0)4] are formed first, and subsequently at 50 K react to give [M2( 0)8] compounds. At 223 K, the dinuclear carbonyls are transformed into [M4( 0)i2]. This means that with respect to tetranuclear compounds, the dinuclear complexes are thermodynamically unstable. [Rh2( 0)8] in solutions as well as in the matrix and [Ir2( 0)8] in the matrix have bridging carbonyl groups (Table 2.26). 

In a high pressure reaction, a carbene complex of Mn adds CO affording a ketene complex, which reacts further under Hg pressure, equation (25). The reaction may have parallels with intermediate steps in the Fischer-Tropsch hydrocarbon synthesis, and represents the first carbonylation reaction of a co-ordinated carbene. 

The direct reductive amination (DRA) is a useful method for the synthesis of amino derivatives from carbonyl compounds, amines, and H2. Precious-metal (Ru [130-132], Rh [133-137], Ir [138-142], Pd [143]) catalyzed reactions are well known to date. The first Fe-catalyzed DRA reaction was reported by Bhanage and coworkers in 2008 (Scheme 42) [144]. Although the reaction conditions are not mild (high temperature, moderate H2 pressure), the hydrogenation of imines and/or enam-ines, which are generated by reaction of organic carbonyl compounds with amines, produces various substituted aryl and/or alkyl amines. A dihydrogen or dihydride iron complex was proposed as a reactive intermediate within the catalytic cycle. 

Not only fragmentation but also cluster formation can be observed. For example, polynuclear Pt carbonyls can be formed from Pt-t-phosphine complexes under CO pressure . A review on the synthesis of high nuclearity clusters, including reactions of CO with various complexes has appeared . 

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