Rhenium compounds carbonyl clusters

Although less fully documented than osmium cluster chemistry, rhenium cluster chemistry has been subjected to many structural studies, including those on approximately 20 neutral or anionic carbonyls, particularly carbonyl hydrides [Rev(CO). H ] of nuclearities x = 2 to 6 (Fig. 7). In addition, some ten or more rhenium carbonyl carbides [Rev(CO)vH C] have been shown to contain a core carbon atom, usually occupying a central octahedral site. These systems offer scope not only to explore for rhenium the trends we have already shown for osmium, but also to study the effect on metal-metal distances (and so enthalpies) of such core carbon atoms, which formally donate all four of their valence shell electrons to the cluster bonding. To our knowledge only one rhenium carbonyl cluster compound, Re2(CO)io, has been subjected to calorimetric study to determine its enthalpy of formation. ... 

Metal carbonyl clusters are also numerous. In addition to dinuclear carbonyls such as M2(CO)io (M = Mn, Tc, Re), many hydrido clusters and ionic species exist which contain three or more metal atoms [M3H3(CO)i2] (M = Mn, Tc, Re), Mn3(CO)i (linear compound, 1)4 ), and Re3(/i-H)(CO)i4 (the Re —Re —Re angle is 90°). Rhenium forms a number of tetranuclear compounds. Some of them have a butterfly structure while others adopt a triangle of rhenium atoms with one Re attached to it Re4(/i3-H)4(CO)i2, [(Re4( -H)4(CO)i3] , [Re4( -H)6(CO)i4], and [Re4(CO)i6]. Rhenium also forms mixed metal carbonyls possessing several metal atoms (Table 3.14). There are numerous clusters of the group 7 metals containing cyclopentadienyl groups, ... 

Besides the use of vanadium-based catalysts, a wide variety of other catalyst compositions were reported. A recent review focussed on FeSbO based catalysts promoted by appropriate additives as suitable for the ammoxidation of alkyl-substituted aromatics and hetero aromatic compounds. A unique preparation method of a fluid-bed catalyst is presented using nitric acid oxidation of antimony trioxide catalyzed with iron ions. The catalysts thus prepared have superior catalytic and physical properties. [78]. In addition, some unique compositions were reported by different research groups. For instance, new ammoxidation catalysts based on rhenium carbonyl cluster complexes containing antimony and bismuth ligands were reported by Adam et al. [79]. Single-site multifunctional catalysts based on [Cu RUj C ] nanocluster anchored to inner walls of mesoporous silica were also used in the ammoxidation of 3P [80]. 

Pyrolysis reactions of mononuclear carbonyls and low-nuclearity cluster compounds have been used extensively in the syntheses of HNCC of osmium (54, 72,80,95,108), ruthenium (18,20,29), and, more recently, rhenium (2-4). The reactions have been carried out either in inert solvents or, to facilitate the ejection of CO or other volatile ligands, in the solid state under vacuum. Condensation processes under pyrolytic conditions are rarely specific and, as such, lead to the formation of a wide range of products. In order to obtain optimum yields of a particular HNCC, the reaction conditions must be carefully screened. Solution reactions offer advantages such as the ability to monitor the progress of the reaction using IR spectroscopy. As they often give... 

Both compounds contain the carbide atom in an octahedral cavity. The hydride cluster is a simple octahedron with three terminal carbonyls on each rhenium atom. The heptanuclear cluster can instead be considered as the same octahedron in which the two hydrides are replaced by a [Re ( 0)3] fragment leading to a monocapped octahedral cluster. Both structures have seven skeletal dectron pairs. 

---