Carbonyl clusters activation

Carbonylation diaryltrazenes, 30 34 reaction, 19 111, 129 Carbonyl clusters activation, 30 144-146 cluster... 

There are many related compounds, including rhodium carbonyl cluster anions, which are present in the solutions cataly2ing ethylene glycol formation and which may be the catalyticaHy active species or in equiUbrium with them (38). 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

These types of clusters represent some of the more modest sizes and geometries detected in homo- and hetero-metal carbonyl clusters. From dimetallic up to pentadecametallic clusters have been defined by crystal structures, and assembly of the metal centers in these clusters adopt a number of well-defined arrangements.83 Redox activity in these polymetallic clusters is anticipated and has been observed. Routes to large carbonyl polymetal clusters have been reviewed 83,84... 

Transition metal compounds in various form such as metal carbonyls 0), carbonyl clusters (2), Pt(II) chloride/tin chloride (3) PtLn (L=PR3) (4), etc. have been proposed as homogeneous catalysts for the water gas shift (wgs) reaction (eq. 1). Some of them are reportedly active at relatively low temperature (<150°)... 

It is proposed that the rate dependence on concentration and pressure involves cluster dissociation and that the monomeric species, Ru(CO), is responsible for the high activity of this system. Dissociation is well known for ruthenium carbonyl clusters (25,27-31). Piacenti and co-workers (31) have demonstrated that at temperatures above 80° and CO pressures greater than 150 psi, monomeric ruthenium carbonyl is observed in significant quantities due to the equilibrium,... 

Mononuclear ruthenium complexes were found to be superior to carbonyl clusters during a comprehensive comparison of a variety of catalysts in the reduction of acetone [49]. Without solvent, most catalysts were highly selective, although the activity was quite low. The addition of water to the system vastly increased yields, in agreement with Schrock and Osborrfs observations into rhodium-catalyzed hydrogenations (Table 15.9) [41],... 

In this body of catalysts, the metal cluster is said to be formed around the carbonyl precursor. According to SEM and TEM imaging, it appears that the carbonyl clusters are on the order of 1 pm in diameter when supported on carbon.192 Analysis with FTIR has shown that the carbonyl is present.189 190 198-200 203 Non-noble metals have also been studied along side the noble-metals in this group of catalysts. Table 4 lists the non-noble metal carbonyl catalysts studied.189-192 198-200 The non-noble metal carbonyl catalysts studied produced mixed results for the ORR activity. 

The metal carbonyl clusters correspond to situations intermediate between metals and simple mononuclear or binuclear carbonyls. Their existence must be connected either with a delicate thermodynamic balance or with remarkably high activation energies. The last hypothesis is valid for species such as Rh6(CO)j6 which is kineti-cally inert, but in general we are inclined to believe that thermodynamic control is the more significant, especially since reactions, such as that shown in Eq. (1), can be carried out in both directions using mild conditions. 

Iridium carbonyl clusters of several nuclearities (2, 4 and 6) have been prepared by a controlled carbonylation of [lr(CO)2(acac)] complex adsorbed in the cages of a NaY zeolite. Then, decarbonylation of the clusters gave rise to lr2, lr4 and Ir frames. Studies of the dependence of the catalytic activity on the size of the iridium frames in NaY zeolites show that there is no simple explanation for the variation in catalytic performance in ethene hydrogenation with cluster size [208]. 

Finally, it is worth noting that the water-soluble Ru carbonyl cluster [Ru3(CO)6L 3] where L = Ph2P (3-S03Na)CgH4 has been prepared and shown to be an active hydrogenation and hydroformylation catalyst. ... 

In many instances, the formation of inactive dimers from active, monomeric catalytic species is observed during catalysis. When weak or unstable ligands are used, even larger rhodium carbonyl clusters like Rh4(CO)i2 and Rh5(CO)i5 can be observed [42-44]. The formation of dimers is often a reversible equilibrium (Scheme 6.2). This only leads to a reduction in the amount of catalyst available and does not kill the catalyst. One of the first examples was the formation of the so-called orange dimer from HRh(PPh3)3CO, already reported by Wilkinson [45]... 

Activation, of carbonyl clusters, 30 144-146 Activation analysis, charged particles and, 1 339-341... 

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