Core carbon coordination

The NMR resonances due to the core carbon atoms in f c iCC ())id and the other metal carbonyl carbides shown in Figure 4.11 tend to lie well downfield. As these core carbon atoms serve as the sites at which carbonyl ligands may coordinate, they can alternatively be regarded as sequestered electron-deficient carbonium ions rather than as carbanionic carbides. 

The data in Table 4.3 correspond to a radius for the octahedrally coordinated carbon atom that Ues in the range 0.59-0.69 A. We noted earlier that the radius of the core carbon in osmium, rhenium, and rhodium clusters lie in the range 0.59-0.62 A. It appears likely that the enthalpy change ZE(M-C), needed to cleave the six M-C bonds in these molecular carbonyl clusters, will lie in the same range (239-306 kcal moT i.e., 38-51 kcal mol per MC link) that we have now calculated for the similarly coordinated carbon atoms in these extended lattice binary carbides MC or M2C. 

These observations on the structurally simple carbides of the early transition metals show how the strength of binding of core carbon atoms in molecular metal carbonyl clusters can in principle be estimated by comparison with metal carbides for which structural and theoretical data are available, and leads us to hope that examination of the wider body of transition metal carbides will provide relationships between the length and strength of bonds between metal atoms and octahe-drally coordinated carbon atoms that can be applied to specific molecular metal carbonyl clusters containing core carbon atoms. 

Nucleophiles can also substitute at the internal carbon position. Reaction of m-benziporphyrin with silver tetrafluoroborate in pyridine under reflux conditions yielded a pyridine-appended macrocyle, with a C-N bond at the core of the ring. The pyridine then forms a second bond at the ortho position with an adjacent meso carbon (70). Recently, we noticed a directly analogous reaction upon the metalation of dicarbahemiporphyrazine with Cu(II) in pyridine (33). In this macrocycle, one of the two core carbon positions is substituted with a pyridine. In both cases, the formation of the C-N bond is believed to result from reductive elimination at the metal. Initial metalation results in the formation of a M-C bond, and the metal is subsequently coordinated by a solvent pyridine. The activation of the C-H bond at the core results in a high valent metal center (Ag(III) and Cu(III) respectively) which can then reductively eliminate to form a monovalent metal center and a C-N bond. 

The sensitivity of the internal carbon to reaction would be a significant complicating factor for carbaporphyrins in protein active sites. Dioxygen binding and activation, for example, would readily result in oxidation at the core carbon position. In addition, the susceptibility of the internal carbon to nucleophiles could affect the coordination chemistry of the side chains of cysteine and histidine, frequently bound at the axial positions of heme sites. While novel inorganic structures do result from adducts formed upon substitution at the internal carbon, directly analogous chemistry to that seen in normal porphyrins is much less likely due to its susceptibility. 

A cluster with five Au atoms exists in Au5(dppm)3[(Ph2P)2CH]KN03)2. This cluster has a AU4 core, with an additional Au incorporated in the phosphine ligand, linearly coordinated between a phosphorus and a carbon from the methanido bridge (IV) ... 

A wide range of carbon, nitrogen, and oxygen nucleophiles react with allylic esters in the presence of iridium catalysts to form branched allylic substitution products. The bulk of the recent literature on iridium-catalyzed allylic substitution has focused on catalysts derived from [Ir(COD)Cl]2 and phosphoramidite ligands. These complexes catalyze the formation of enantiomerically enriched allylic amines, allylic ethers, and (3-branched y-8 unsaturated carbonyl compounds. The latest generation and most commonly used of these catalysts (Scheme 1) consists of a cyclometalated iridium-phosphoramidite core chelated by 1,5-cyclooctadiene. A fifth coordination site is occupied in catalyst precursors by an additional -phosphoramidite or ethylene. The phosphoramidite that is used to generate the metalacyclic core typically contains one BlNOLate and one bis-arylethylamino group on phosphorus. 

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