Tetra-dentate ligands

Zaleski and coworkers47 later expanded upon this line of work by using tetra-dentate enediyne ligands, in which the reactivity could be modulated by metal complexation (Scheme 12). 

An analysis of the most significant homogeneous catalytic systems reported in the literature reveals a structural variety for Pd which is not found for Pt. In fact, although in most cases Pd is incorporated into the (pre)catalyst as divalent ion, active Pd(0) -catalysts have also been reported. By contrast, Pt(0) -catalysts are a rarity. Moreover, Pd complexes containing mono-, di-, tri-, and even tetra-dentate ligands have found application as hydrogenation catalysts, and often their activity and selectivity is governed by the steric and electronic features of... 

Table 15.17 Hydrogenation of dimethyl oxalate using bi-, tri-, and tetra-dentate ligands.3 ...

With the knowledge that the presence of Ti(OPr )4 promotes the alkylation of diethylzinc to benzaldehyde, Ho et al.118 demonstrated that the chiral tetra-dentate sulfonamide ligand 125 catalyzes the addition of diethylzinc to aldehyde in the presence of Ti(OPr )4 with good yield and enantioselectivity (Scheme 2-48). 

Complexes with tri- and tetra-dentate open-chain ligands containing tertiary phosphines and arsines,... 

Table 70 Tri- and Tetra-dentate Tripodal Ligands Cited in Section 50.5.4.6 Structures, Formulas...

The sulfide group forms a large number of complexes where it is in chelation with a different heteroatom. Among the common heteroatoms are N, P and As. These complexes are too numerous to list here, but individual complexes can be found from Table 9 or from refs. 1224 and 1667. It is also possible to synthesize compounds which will form bi-, tri- and tetra-dentate complexes to platinum(II), where sulfur, selenium and tellurium.are the only atoms which coordinate to the metal. A review of complexes formed from ligands of the type RS(CH2) SR has been recently published.1734 This article outlines the synthesis, reactions and spectroscopy of these complexes, and allows the complexes of platinum to be placed in context with those of other transition metals. 

Octamethyltetrabenzoporphine is a dark-blue powder that, although much more soluble than tetrabenzoporphine, has a low solubility in most common organic solvents. It is, however, very soluble in trifluoroacetic acid, giving a green solution of the diprotonated macrocycle. This molecule is a square-planar tetra-dentate ligand. It reacts with a variety of metal salts to form porphine-metal... 

The presence of this and other materials presents potential problems in industrial processing, and there is great interest in characterization of the chemical nature of the vanadium species present. EPR is most widely used in these studies since the metal is in the oxo-vanadium(IV) state. Optical spectroscopy in the visible region can also be used on extracts since oxo-vanadium(IV) porphyrins, which absorb at around 572 nm and 534 nm 116,119), can readily be detected. However, it has been shown (120-122) that the total amount of vanadium present in crude oils and tar sand bitumen is higher than can be accounted for by the presence of oxo-vanadium(IV) porphyrins. It has therefore been suggested that the vanadyl may be bound to a range of different tetra-dentate ligands in crude oils (120-123). 

Figure 5. The tetra-dentate ligand tren coordinates metal ions according to a trigonal-bipyramidal geometry. The fifth coordination site is available for a solvent molecule or for a coordinating anion.

Although square planar structures are generally assured by the use of cyclic tetra dentate ligands, the use of simpler heterocyclic ring systems... 

With monoorganotin derivatives, some of the oxygen-containing tri- and tetra-dentate ligands shown in Figure 2.5.4 have been employed for the formation of interesting cages, ladders, and clusters. ... 

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