Ligand site environment, influence

The ferrocene moiety is not just an innocent steric element to create a three-dimensional chiral catalyst environment. Instead, the Fe center can influence a catalytic asymmetric process by electronic interaction with the catalytic site, if the latter is directly coimected to the sandwich core. This interaction is often comparable to the stabilization of a-ferrocenylcarbocations 3 (see Sect. 1) making use of the electron-donating character of the Cp2Fe moiety, but can also be reversed by the formation of feirocenium systems thereby increasing the acidity of a directly attached Lewis acid. Alternative applications in asymmetric catalysis, for which the interaction of the Fe center and the catalytic center is less distinct, have recently been summarized in excellent extensive reviews and are outside the scope of this chapter [48, 49], Moreover, related complexes in which one Cp ring has been replaced with an ri -arene ligand, and which have, for example, been utilized as catalysts for nitrate or nitrite reduction in water [50], are not covered in this chapter. 

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

Fig. 12. Schematic views of bis-histidyl ferri-, ferro-, and CO-ferro-heme-hemopexin. Unlike myoglobin with one open distal site, heme bound to hemopexin is coordinated to two strong field ligands, either of which a priori may be displaced by CO. This may well produce coupled changes in protein conformation like the Perutz mechanism for 02-binding by hemoglobin (143). The environment of heme bound to hemopexin and to the N-domain may be influenced by changes in the interactions of porphyrin-ring orbitals with those of aromatic residues in the heme binding site upon reduction and subsequent CO binding.

Furthermore, ruthenium-carbene complexes are highly tunable, well-defined, single-site, homogeneous catalysts. These characteristics provide the ability to access all catalytically active sites and thus to influence catalyst initiation, propagation, and stability properties. The relatively simple ligand environment of Ru-2, Ru-4, and... 

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