Phosphines multidentate ligands

Alternatively, bi- or multidentate ligands can also be used for support. As an additional benefit, the latter offer greater stability for the coordinatively bound metal center against leaching through ligand dissociation and substitution reactions. The first, somewhat remarkable, approach to this is shown in Figure 42.11, based on numerous examples of the support of bidentate phosphines on polymers [1-5]. 

Phosphines and arsines containing more than one olefinic group were neglected as possible multidentate ligands until Hall prepared the tris (orfAo-vinylphenyl) derivatives of phosphorus, arsenic and antimony (tvpp, tvpa and tvps) and their platinum(II) (48) and rhodium(I) (49) complexes. 

The many synthetic methods available for preparing NHCs have allowed for their ready incorporation into an increasingly diverse set of multidentate ligands. The topic of chelating ligands containing NHCs has been covered in several reviews.Heteroleptic variations have included phosphines, amines, imines, pyridines, oxazolines, amides, alkoxides, aryloxides, thioethers, and other donors. 

When carbon chain poly(phosphines) act as multidentate ligands they form chelate rings containing carbon, phosphorus and a metal atom. They can be made by reactions analogous to those employed for monodentate derivatives above. In reaction (8.137) a linear complex is first formed, but on raising the temperature a cyclic derivative is obtained. 

A second strategy in this area involves the use of multidentate ligands which can datively bind to metal centers. Thus, DME, TMEDA, and phosphines such as... 

Pt(en)(N03)2] and [Pt(OTf)2L2] (L = mono- or 1/2 bidentate tertiary phosphine) or dinuclear complexes of the type [Pt2(OTf)2(/i-monodentate tertiary phosphine cr-aryl = 4, -biphenyl, / -terphenyL 4,4 -benzophenone, etc.) other structural motifs employing platinum(II) have also been reported.2 0 The addition of bridging, multidentate N-donor ligands of various shapes and sizes to the labile complexes in a suitable solvent system has afforded several classes of discrete, plat-inum(II)-containing polygons, polyhedra, and catenanes. 

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