Donor-functionalized chelating ligands

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

The chelate ligand in dionato complex 3 is planar and it is particularly stabilized by 71-delocalization. In addition to this thermodynamic stability, the iron center has 17 valence electrons in an octahedron, hence its coordination sphere is kinetically labile. By ligand exchange, the acceptor 41a is coordinated at a vacant site to form species 44 (Scheme 8.18). The function of the center metal is not only to hold the acceptor in proximity to the donor. Additionally, the acceptor is activated by Lewis acidity of the center metal. Subsequently, the nucleophilic carbon atom of the dionato ligand is... 

Stability. Ligands with two or more donor groups afford monocyclic or polycyclic-cyclic metal chelates that result in enhancement of the complex stability relative to two or more mono functional nucleophiles. The increasing tendency to form complexes with increasing number of donor groups per ligand, known as the chelate effect, was discovered by Schwarzenbach [22] and is discussed in [23],... 

The heteroleptic B-type ligand sphere offers more steric flexibility. However, chelation by simple donor-functionalities can yield strongly-bonded ligand-metal moieties which are kinetically rather stable, as is shown, for example, in benzamidinate [179] or siloxide complexes [180]. 

Internally tailoring of the alkoxide ligand with additional coordination sites leads to the class of functionalized ligands. When required, the alkoxide ligands can support these coordination sites using the effect of chelation. The idea donor functionalization of alkoxide ligands is developed in Scheme 9. Such modification of the tritox ligand as expressed in Fig. 9 represents a simple case of functionalization (monovalent, Do = OEt, n = 1). 

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