Divalent ligands

Thus it has so far proved possible to isolate stable derivatives of monomeric metaphosphoric acid and of metathio- and metaselenophosphoric acid, which, understandably, generally bear tert-butyl and/or trimethylsilyl substituents u. Specifically, we know aminobisiminophosphoranes (3, Z = NR2, X = Y = NR)2,3,4), aminoiminothio (or seleno)phosphoranes (5, Z = NR2, X = NR, Y = S or Se)5), and aminoiminomethylenephosphoranes (1, R = NR2, X = NR)6>. Conspicuously, no stable phosphorus(V) three-coordinate compounds have been synthesized with oxygen as divalent ligand. 

Phthalocyanine is a divalent ligand with a geometry appropriate for forming a four-coordinated square-planar complex and was thought ideal for generating an Au(II) species. The action of AuBr on molten 1,3-diiminoisoindoline in the absence of solvent indeed yields a neutral gold phthalocyanine (63). The EPR spectra of the complex in 1-chloronaphthalene at 77 K clearly showed the presence of Au(II), a d9 ion. The g value is 2.065, comparable to 2.042 for copper and 2.093 for silver phthalocyanine (64, 80). 

Arylalkynes can also form, with Pt, Pd and Ir, complexes such as that28 shown in Fig. 23-15, in which they can be considered to serve as bidentate, divalent ligands. In these the C—C stretching frequency is lowered considerably, to the range 1750-1770 cm-1, indicative of a C—C double bond. The C—C bond length of 1.32 A is consistent with this view, as is the relatively large distortion from linearity. 

P. V. Murphy, H. Bradley, M. Tosin, N. Pitt, G. M. Fitzpatrick, and W. K. Glass, Development of carbohydrate-based scaffolds for restricted presentation of recognition groups. Extension to divalent ligands and implications for the structure of dimerized receptors, J. Org. Chem., 68 (2003) 5692-5704. 

Figure 17 Binding of a divalent ligand AA to a tetravalent receptor B4 with a free inter-...

Figure 18 Binding of a divalent ligand AA to a divalent receptor model BB for the evaluation of the reference EM value. The statistical factor is obtained by considering that each species has cr = 2 for the presence of a single twofold symmetry axis.

Fesik showed how NMR can be used as a tool for the rational design of high-affmity divalent ligands. Two small molecules (M2 and M9) were... 

Figure 7 (A) A divalent ligand for the protein FK506 binding protein (FKBP). (B) The reference system is the complex formed between FKBP and the two fragments, M2 and M9, which independently bind In the two different pockets filled by the divalent composite molecule. FKBP structure is from PDB (1FKS).

FarreraJ-A, Hidalgo-Femandez P, HanninkJM, et al. Divalent ligand for intramolecular complex formation to streptavidin. Org Biomol Chem. 2005 3 2393-2395. 

Chelate cooperative model, 60—61 Chelate cooperativity, 42—43, 51—55, 78 binding of divalent asymmetric ligand, 51 f binding of divalent ligand, 53 factor, 54, 54f—55f Chelate effect, 51—55 Chlorophyll a/b (Chi a/b), 232 Chromophore, 221—222 Cluster, 204—205 COCOC chain, 30-31 Coherences, 230—231 Computational studies of environmental effects... 

IdU. See 5-Iodo-2 -deoxyuridine (IdU) Inline metathesis, DLs from, 31—34 Iminium ions, 134—135 Interannular cooperativity, 42—43, 56—59 binding of divalent ligand, 56f Interannular-chelate cooperative model, 62 Intermolecular process, 4f Internal symmetry number, 44—45 Intramolecular process, 3, 4f Intramolecular reactions, 78—81, 91 2 -Iodo-2 -deoxyuridine, 136, 138 5-Iodo-2 -deoxyuridine (IdU), 125, 125f, 136... 

The chemistry of carbonyl compounds with transition metal compounds is very rich. The paper by J. Bryan and M. Mayer [22] deserves special attention. It reports on a new type of oxidative addition reaction in which a carbonyl double bond is cleaved to form divalent ligands. The final result is an oxoalkylidene complex (Sch. 6). Such complexes are discussed in a mechanistic study of OM mediated by high-valent group 6 catalysts [23]. Three bimolecular chain termination steps, resulting in olefin, carbonyl compound and singlet oxygen, are proposed. 

In the case for a bivalent interaction, Lees proposed an extension of this model by assuming that the bivalent association constant can be expressed as three related association constants K, K2, and K-iP Each equilibrium constant is derived on the basis of certain approximations such as the number of possible permutations assumed in receptor-ligand site association, monovalent binding (each monomer of the bivalent ligand is capable of binding to one receptor), and effective concentration of bound divalent ligand. An overall divalent association constant, which provides an estimate of the enhancement factor of divalent association, is obtained as a product of the three equilibrium constants as shown in (5). The refinement in p that is afforded from Lees treatment is shown in (6). 

Soon after Guldi et al. published on their system, Stod-dart and coworkers published a report wherein dynamic chemistry was used to form self-assembled oligomers that were found to be capable of binding and, possibly even, sorting SWNTs by size. Their system is composed of 5,15-fran.y-pyridyl metalloporphyrin, 13, which can act as a linear divalent ligand and cw-protected Pd or Pt complexes (Figure 13). While neither the Pd/Pt ligand nor the porphyrin displayed an ability to solubilize SWNTs on its own, they were found to completely solubilize SWNTs in aqueous acetonitrile solutions when used in conjunction with one another. 

With this result in hand. Bundle and coworkers tethered divalent ligand 38 to a central glucose core to create multivalent binder 39, affectionately named STARFISH. This scaffold presents a total of 10 trisaccharide ligands. 

Dendrimers up to a G3 generation (octavalent) were synthesized based on the 3,5-di-(2-aminoethoxy)-benzoic acid branching unit and tested in an SPR competition assay with immobilized asialofetuin. Strong multivalency effects were observed, up to a factor of 4t)0, which reached the Hmit of the assay [47]. Even more spectacular enhancements were obtained with a divalent ligand, prepared by tethering two units of 42 onto a functionalized caHx[4]arene. A 3800-fold (1900-fold per sugar mimic) affinity eiihancement was measured by fluorescence spectroscopy, thus reaching the same potency of GMl-os [59]. 

Similar structural effects also dominate the interaction of polyelectrolytes with divalent ligands, for example, of polycarboxylic acids with divalent cations. Investigations by Zwick [13] and Michaeli [14] had shown that only some 80% of the fixed charges can be neutralized and that this could be accounted for by the statistics of divalent ligand placement on a linear lattice. 

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