Polymeric metal complexes with

Pyrazine (L39) and substituted pyrazines have long been known to act as exo-didentate ligands to linearly bridge metal ions, generating oligomeric and polymeric metal complexes with infinite chain and... 

The incorporation of a rigid metal complex in a polymer chain reduces the solubility and processibility as is known for aromatic polyamides or polyesters. Polymeric metal complexes with aliphatic alkylene moieties between the chelate units or bulky groups as substituents are easier to handle. Cross-linked polymeric metal complexes are, of course, more difficult to analyze. 

Polymeric Metal Complexes with Noncyclic Organic Ligands... 

Polymeric Metal Complexes with Cyclic Organic Ligands 239... 

Polymeric metal complexes with polyester macroligands have been generated by initiation from hydroxyl functionalized ligand and metal complex reagents. Ligand initiators lead to macroligands which can be combined with metal precursors in coordination reactions to produce PMCs (51). Metalloinitiators, on the other hand, produce PMCs directly (2). 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

The use of y-ray induced radical pol5unerization proved to be a successful alternative for the radical co-polymer-ization of metal complexes with ligands containing acrylic C—C double bonds [100-102,129,130]. In particular, the palladium(II) complex cw-[PdCl2(ICPA)2] (1, Scheme 4) was co-polymerized in DMF solution with DMA and MBAA (cross-linker, 4% mol), with no degradation of the metal center [100,101]. 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

More insight into the reasons for magnetism of derivatives may be obtained by a closer look at other recently discovered ferromagnetic Cjo materials, which include complexes of cobaltocene with a CgQ-derivative [156, 157], but also metal complexes with europium [175-177] or cerium [178-180] or pure C q in a polymeric modification [181-183]. 

This article deals with the polymer-metal complexes (Schemes 1 —5), because they have the following merits in comparison with other polymeric metal complexes, (i) Metal ion and ligand site can be chosen for study without restrictions, (ii) It is not difficult to control the molecular weight of a polymer complex and to modify the structure of a polymer ligand, (iii) The polymer complex is soluble in both aqueous and nonaqueous solvent, (iv) It is possible to change the ratio of the organic polymer part to the inorganic metal complex part. This explains why the polymer often affects the behavior of the metal complex. 

It is suggested here that the greater insolubility of the humic fraction of coal may be the result, in part, of polymerization by complexing with metals, particularly aluminum and silicon. These elements are suggested because of their presumed greater availability. The minor elements also play a part, however, only in relation to their availability compared with major elements such as aluminum and silicon. 

Table 3. lg K values of metal complexes with picolinic acid and its polymeric analogs (dioxane -water)... 

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