Porphyrins, metal coordination polymers

Tetra(4-pyridyl) porphyrin (TPyP) metalloligands also were explored as potential MOF constituents. It is of more than passing interest that TPyPs are often capable of self-association via bonding of the pyridyl nitrogens to the coordina-tively unsaturated central metal of another porphyrin molecule. Several research groups have made an active study of such stmctures (111-117). However, because these coordination polymers tend to be formed from a single molecular component, rather than having the metalloporphyrin bound to a secondary metal center or SBU, they will be omitted from further discussion here. 

Because coordinatively bound complexes are quite easy to prepare, numerous papers on this subject have been published. Soluble complexes are prepared by dissolving stoichiometric amounts of polymer and metal complex in an organic solvent or water. To obtain solid materials, the solvent is removed or a film is cast. Cross-linked insoluble polymers are suspended in a solvent and a solution of a metal complex is added. The equilibrium of the coordinatively polymer-bound metal complex with the unbound one in solution is not favorable (Eq. 5-6), so an excess of strong low molecular weight donor base can destroy the polymer coordinative bond. Even the heme in myoglobin can be cleaved to give the apomyoglobin, as described in Section 2.3.1, and another porphyrin derivative subsequently coordinatively bound. 

A. The framework lattice occupies just 42% of the total crystal volume. Porphyrins gives access to coordination polymers, which are shaped by metal-ligand interactions and by a variation of substituents. 

A second motif encountered in tetrapyridylporphyiin systems is typified by inclusion compounds with wet methanol and water that produce three-dimensional coordination polymers. Ttv/ni-pyridyl substituents on a Zn(TPyP) were ob.served to axially ligate the metal centers of adjacent porphyrin moieties generating a polymeric chain in one dimension. Cross-linking in a second dimension occurs when the original porphyrin molecule is coordinated by two pyridyl moieties from two additional porphyrin molecules... 

Bedioui, R, M. Voisin, J. Devynck, and C. Bied-Charreton (1991). Electrochemistry of conducting polypyrrole films containing cobalt porphyrin. 2. New developments and inclusion of metallic aggregates in the coordination polymer. J. Electroanal. Chem. 297, 257-269. 

All of these examples show that the use of metal coordination at the periphery of the porphyrin chromophores can be extremely helpful in modular approaches targeting multiporphyrinic assemblies. As we will see later, the formation of coordination polymers has already found applications in the preparation of materials. 

The Stability constant for the metal/ligand (monomer) complexation should be large to afford high molecular weight polymers in isotropic solution. Moreover, the complexation should be readily reversible (kinetically labile) to impart dynamic properties to the polymers. Two recent examples of soluble, reversible coordination polymers are the self-assembled porphyrins of Michelsen and Hunter [122] and the high molecular weight copper(II) coordination polymers of Leize, Lehn, and coworkers [123],... 

The first examples of porphyrin coordination polymers that have been characterized according to our definition in Sect. 3.1 were reported in 1991 by Fleischer and Shachter [72]. Upon metalation of 5-pyridyl-10,15,20-triphenyl-porphyrin with a Zn " ion, a self-complementary building block was obtained which readily assembles to polymer 19 as confirmed by concentration dependent UV/vis spectroscopy and NMR studies. The structure of the polymer 19 in the solid state was established by X-ray analysis. Recent reinvestigations of this system suggest that in solution tetrameric squares prevail, and polymer formation may take place at higher concentration (> 1 M) as predicted by computer simulation [12]. 

Two more examples of coordination polymers have recently been introduced by this group applying Co + and Ga metal ions in a porphyrin framework analogous to the dimer 11. hi both examples the porphyrin units are strongly aggregated which makes these systems potentially interesting for electronic and optoelectronic applications [ 13,76]. 

This chapter describes the synthesis and properties of a number of classes of polymers containing metal coordination complexes in their structures. These polymers are prepared by polymerization reactions of metal-containing monomers and through metal coordination reactions. Schiff base-containing polymers (5) were one of the earUest classes of coordination polymers examined. Polymers incorporating macro-cycUc porphyrins and phthalocyanines (7) in their backbones and sidechains are known to exhibit interesting optical and electrical properties. The best-studied classes of metal-containing polymers contain bipyridyl and other related units coordinated to metal ions (8). 

ICP-MS = inductively coupled plasma mass spectrometry MIM = mechanically interlocked molecule MOF = metal organic framework MORF = metal-organic rotaxane framework PRF = polyrotaxane framework RCP = rotaxane coordination polymer TCPP = tetra 4-carboxy)-phenyl-porphyrin TGA = thermogravimetric analysis. 

Porous Coordination Polymer Nanoparticles and Macrostructures Mesoporous Metal-Organic Frameworks Porphyrinic Metal-Organic Frameworks Postsynthetic Modification of Metal-Organic Frameworks Open Metal Sites in Metal-Organic-Frameworks. 

---