Carbonyl clusters group

M ,(CO) Metal carbonyl clusters with bridging CO groups M-C(0) M... 

The dehydration of primary amides with hydrosilane catalyzed by iron carbonyl clusters, such as [Et3NH][HFe3(CO)n] and Fe2(CO)9, was achieved by Seller and coworkers in 2009 (Scheme 43) [145]. This reaction shows good functional group tolerance (e.g., such as aromatic, heteroaromatic, and aliphatic substrates). 

Supported metal carbonyl clusters are alternatively formed from mononuclear metal complexes by surface-mediated synthesis [5,13] examples are [HIr4(CO)ii] formed from Ir(CO)2(acac) on MgO and Rh CCOlie formed from Rh(CO)2(acac) on y-Al203 [5,12,13]. These syntheses are carried out in the presence of gas-phase CO and in the absence of solvents. Synthesis of metal carbonyl clusters on oxide supports apparently often involves hydroxyl groups or water on the support surface analogous chemistry occurs in solution [ 14]. A synthesis from a mononuclear metal complex precursor is usually characterized by a yield less than that attained as a result of simple adsorption of a preformed metal cluster, and consequently the latter precursors are preferred when the goal is a high yield of the cluster on the support an exception is made when the clusters do not fit into the pores of the support (e.g., a zeolite), and a smaller precursor is needed. 

Main-group cobalt carbonyl clusters are well known, with neutral clusters such as Sn Co(CO)4 4 and anionic clusters such as [Bi Co(CO)4 4] reported.87 Synthesis proceeds for the former as follows (Equation (3)) ... 

The dominant role of copper catalysts has been challenged by the introduction of powerful group VIII metal catalysts. From a systematic screening, palladium(II) and rhodium(II) derivatives, especially the respective carboxylates62)63)64-, have emerged as catalysts of choice. In addition, rhodium and ruthenium carbonyl clusters, Rh COJjg 65> and Ru3(CO)12 e6), seem to work well. Tables 3 and 4 present a comparison of the efficiency of different catalysts in cyclopropanation reactions with ethyl diazoacetate under standardized conditions. 

The idea (50, 5/) of dual coordination of CO implies the presence of two coordination centers in a Fischer-Tropsch catalyst system, i.e., a carbonyl carbon coordinating center, Ma, and a carbonyl oxygen coordinating center, M6 (14). It is this concept which has led at least two groups to examine transition metal carbonyl cluster compounds as homogeneous Fischer-Tropsch catalysts. 

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. 

Deviations of Carbonyl Clusters from Main-Group Analogs... 

The tetranuclear derivatives are classified as derivatives of the three principal carbonyl cluster units observed, H4M4(CO)12, H2M4(CO)13, and MjCCOlxs, with a final group involving bonding to platinum or gold. 

Just as for group 5, 6, and 7 ( -CsF MCU species, Fehlner has shown that BH3-THF or Li[BH4] react with group 8 and 9 cyclopentadienyl metal halides to result in metallaborane clusters, many of them having a metal boron ratio of 1 3 and 1 4, and much of the synthetic chemistry and reactivity shows close connections with the earlier transition metals. The main difference between the early and later transition metallaboranes that result is that the latter are generally electron precise cluster species, while as has been shown, the former often adopt condensed structures. Indeed, as has been pointed out by King, many of the later transition metallaborane clusters that result from these syntheses have structures closely related to binary boranes and, in some cases, metal carbonyl clusters such as H2Os6(CO)18.159... 

The carbonyl cluster Rh,5(CO)i,5 was initially stable as such on the completely dehydroxylated alumina surface. But as soon as hydroxyl groups were generated (e.g., by adding traces of water) it decomposed to give various surface transformations. First, the cluster structure was dismpted, with breakage of the core cluster frame, into (Al-0-)(Al-0H)Rh (C0)2, Rh > monoatomic species sigma and n-bonded to the oxygens atoms of the alumina surface, with formation of molecular... 

Many further examples could be added to those already cited to support the close relationship between carbonyl clusters and borane clusters, particularly for species based on an octahedron. These other clo80 octahedral clusters include anions such as Co0(CO)i6 (34), [Fe6(CO)ieC] - (35, 40), [Rhe(CO)i5l]- (6), [Nie(CO)i2] - (31), [Co4Ni2(CO)i4] (5), and [Rhi2(CO)8o], in which two Rhg octahedra are linked by a metal-metal bond and two bridging carbonyl groups (2). 

The role of bridging groups in metal carbonyl cluster compounds is still not completely understood but has been discussed qualitatively by... 

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