Noncyclic ligands

Complexes containing noncyclic ligands with N,N, N",N "-donor sets... 

Haymore (4) has pointed out that the noncyclic ligand has many degrees of freedom with respect to rotations about single bonds, whereas the cyclic analog has fewer since the ends are connected. Thus, the cyclic chelate should be greatly favored from an entropy consideration. If correct, this explanation allows one to postulate that the poor metal complexing properties observed for non-cyclic ethers is a result of small... 

Stability constants of rare earths with a variety of macrocyclic ligands are presented in Tables 3.13 to 3.19 (see Appendix) and the associated references are given at the bottom of the tables. As is the case with noncyclic ligands, stability constants are determined by several different techniques. One should be careful when comparing the values, since different conditions and techniques might have been applied. Stability constants for the complexes of cations with macrocyclic compounds are often influenced by the relative sizes of the cations and the cavities of the macrocycles, and hence the macrocycles have specific selectivities to various cations. It is naturally expected that macrocyclic compounds also exhibit unique selectivities to lanthanides. 

It appears that a modified mechanism operates when tr .s-(o-tolyl)phosphine is used as the ligand,133 and this phosphine has been found to form a palladacycle. Much more stable than noncyclic Pd(0) complexes, this compound is also more reactive toward oxidative addition. As with the other mechanisms, various halide adducts or halide-bridged compounds may enter into the overall mechanism. 

Similarly, secondary phosphines and a,a -dibromo-m-xylene usually react to give diprotonated ditertiary diphosphines [44-48]. However, in the case of the fluorous secondary phosphine 12-Rfs (Scheme 2, bottom), dialkylation occurred to give the metacyclophane 13-Rfs. As detailed elsewhere [40], extensive efforts to adjust the stoichiometry to favor noncyclized products failed. Fortunately, the reduction of 13-Rfs with IiAlH4 gave some of the target ligand lO-Rfs. 

Complexation studies were carried out by electrospray ionization mass spectrometry (ESI-MS) with three lanthanides (La, Eu, and Yr) at a concentration of 10 4 M, while the ratio of concentration of CPw2 versus calixarene (r = [calix]org/[Ln]aq) was varied between 0.1 and 10.165 The behavior of the three lanthanides is the same the 1 1 Ln calix-CMPO complex is always predominant in the range of concentration studied the 2 1 Ln calix-CMPO appears in the case of metal excess with a maximum for r = 0.5, while the percent of 1 2 Ln calix-CMPO increases as the concentration of ligand increases, but does not exceed 10% for r = 10. These studies show that two factors play a key role in the selectivity of CMPO derivatives, firstly the presence of phenyl groups on the phosphorus atom, which confers the selectivity to CMPO ligands, secondly the calixarene structure, which amplifies this selectivity due to its preorganization. Indeed the noncyclic derivative (Os2), in spite of the presence of phenyl units, does not display noticeable selectivity. 

Comparative studies between crown ethers and the noncyclic glymes (linear polyethers) suggest that glyme complexes are favored — the chelate effect is thus more important than the macro-cyclic effect. The more flexible glymes may accommodate water molecules and other small ligands (e.g., anions) more easily. Solubility effects may play a part too " ... 

Polymeric Metal Complexes with Noncyclic Organic Ligands... 

In copper(I)-catalyzed cyclopropanations of cyclic or noncyclic alkenes 25 with diazoacetates, two classes of bimacrocyclic 1,10-phenanthrolines 2 and 3 have been checked (see Fig. Surprisingly, bath ligands are... 

Weber, E. Voegtle. F. Crystalline 1 1 complexes of alkali metals with noncyclized neutral ligands. Tetrahedron Lett. 1975. 29. 2415-2418. 

Scheme 2 Lanthanide complexes with noncyclic polydentate ligands...

Furthermore, lanthanides form stable complexes with polydentate chelators like DTPA, which exhibit a noncyclic structure. Two structures are depicted in Scheme 2. The following examples are only representatives for the variety of analyte molecules that can be determined by these kind of lanthanide complexes. Structure 9 employs a quinolyl ligand both as chromophore and acceptor for Zn ". The emission of the europium ion is strongly enhanced upon binding of Zn " and showing distinct selectivity over other biologically relevant metal cations in aqueous solution at neutral pH [29]. The luminescence of the chelate 10 is efficiently quenched by Cu " ions in aqueous medium [30]. The presence of Fe ", Co ", Ni ", Cr ", and Mn " interferes with the determination of Cu, although to a relatively small extent, whereas the ions Zn ", Cd ", Hg, and Pb do not interact with probe 10. 

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