Ethylene-bridged ligands

The 3,5-bis(trifluoromethyl)pyrazolate analog [Ir(cod)(/x-3,5-(CF3)2pz)]2 does not enter into oxidative addition with iodine, methyl iodide, or acetylenes. The mixture of pyrazolate and 3,5-bis(trifluoromethyl)pyrazolate gives [(rj -codllrf/x-pz)(/L-3,5-(CF3)2pz)Ir(rj -cod)], which reacts with bis(trifluoromethyl)acetylene in a peculiar manner [83JCS(CC)580], producing 145, where 3,5-bis(trifluoromethyl) pyrazolate is replaced by the ethylene bridge and the rj -coordination mode of one of the cod ligands is converted into the rj -allylic mode. 

Isomers are, of course, possible when the equatorial ligand lacks a plane of symmetry, as in the corrinoids [see (HI)]. All the acetamide side chains project to one side of the corrin ring, which we shall call the upper side, and all the propionamide side chains and the nucleotide side chain to the lower side. Isomers are then theoretically possible whenever the two axial ligands are different, and their existence has been shown experimentally for corrinoids where one axial ligand is CN , Me, or Et and the other is H2O or is absent [for further details see Section 8.2 of ref. (136)]. Salen and BAE also show minor deviations from planarity due to the bending of the two halves (mentioned above in Section II,B,2) and to torsion about the C—C bonds in the ethylene bridge (see references in Table I), but these are not expected to give rise to separable isomers. 

Zinc(II) dithiophosphates and their adducts, e.g. Zn[S2P(OR)2]2 L [R = Pr, Cy L = 4,4 -bipy, Zra ,s-l,2-6z,s(4-pyridyl)ethylene, l,2-Zw(4-pyridyl)ethane]22 and Zn[S2P(OPr )2]2(4,4 -bipy)23 are self-asssembled in the solid state through bridging ligands to form supramolecular chain-like arrays. The metal is four-coordinate tetrahedral. 

Figure 7.19 Several ligands and gadolinium(III) complexes used in MRI imaging. (A) DTPA ligand (B) [Gd(DTPA)(H20)]2, Magnevist (C) MS-264-L, ethylene-bridge-substituted DTPA (D) BOPTA, acetate-substituted DTPA (E) [Gd(B0PTA)(H20)]2, MultiHance (F) AngioMARK . (Adapted with permission from Chart 1 of Caravan, P. Ellison, J. J. McMurry, T. J. Lauffer, R. B. Chem. Rev., 1999, 99(9), 2293-2352. Copyright 1999, American Chemical Society.)...

For noncrowded complexes, the influence of the ethylene bridge conformation (and hence of the chirality of the tertiary carbon atom of the bridge) on the stereospecific behavior is poor (compare calculations for ligands 38 and 39, in Table 1.1). This is in qualitative agreement with measured stereospecificities of corresponding catalytic systems (39% vs. 34% of mmmm pentads, respectively).63... 

When phenylene- and ethylene-bridged bis(oxazolones) are used as ligands, dinuclear palladium(II) and platinum(ll) 333, 334, and 335 complexes are obtained (Fig. 7.35)." ° In some cases, the close proximity of the ortho phenyl-H-atom to the metal rendered these complexes suitable precursors for ortho metalation. 

Figure 7.35. Dinuclear palladium(II) and platinum(ll) complexes with phenylene- and ethylene-bridged bis(oxazolone) ligands.

In this type of hetero-Diels-Alder reaction ethylene-bridged bissulfoximines 55 were also applied as ligands, giving bicyclic products with up to 99% ee [59, 60]. It is noteworthy that these results compared very well with those obtained using the well-established Cu(II)/BOX-based systems, which were investigated by j0r-gensen and afforded cycloaddition products with up to 97% ee [61]. 

Later, Ewen reported low-temperature polymerization of propylene with a Cp2Ti(C6H5)2-MAO system leading to an isotactic polymer (139, 140). With ethylene-bridged bisindenyls as ligands of Ti (141), two diastereomers are possible (Scheme 64). The chiral Ti complex (ra-... 

Most of the complexes [NiL] given by the potentially pentadentate ligands (IX) and (X) (Table 97) are five-coordinate high-spin and possess an essentially trigonal bipyramidal structure (320).2320-2322 In contrast with the aforementioned complexes, those formed by the ligands (VIII) which possess the less flexible ethylene bridging chain are essentially square planar.2313,2318... 

The deformations induced by steric interactions of the ethylene bridge substituents with apical ligands can produce distortions from a square pyramidal geometry towards a trigonal bipyramid. 

---