Schiff base coordination polymers

FIGURE 13. Schiff base coordination polymers prepared by O connell. 

The first soluble Schiff base coordination polymer was reported by Archer et al. in 1985,31 which was prepared by condensation of 1,2,4,5-tetra-amino benzene (TAB) with tetrakis(salicyladehydato)zirconium(IV) (Zr(sal)4) in dry dimethyl sulfoxide (Fig. 15). The polymer has a number-average molecular weight of up to 4.9 X 104 based on inherent viscosity, gel permeation chromatography, and elemental analyses and shows no decomposition at temperature <500°C. 

Another case in which the thermal stability of the coordination polymer is reported to be lower than that of the monomeric coordination compound is worth mentioning. Marvel and Tarkoy (18) examined the thermal stability of the nonpolymeric Zn-salicylaldehyde-o-phenylenedi-amine compound (X), which they reported was stable when heated in air at 250°C., losing less than 5% by weight after 24 hours. The related Schiff base coordination polymer (XI) was less stable and decreased in weight by 12% after only 3 hours in air at 250°C. 

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). 

One method of solving the kinetics dilemma is well known in coordination chemistry that is, start with a labile metal ion and render it inert during the course of the synthetic reaction. We have accomplished this in the case of zirconium(IV) by starting with tetrakis(salicylaldehydo)zirconium(IV), which is quite labile, and polymerization with 1,2,4,5-tetraaminobenzene in a Schiff-base condensation reaction in situ (6). The polymeric product contains a "double-headed" quadridentate ligand, which is much more inert to substitution. However, 1,2,4,5-tetraaminobenzene has become very expensive. Therefore, the synthesis of a zirconium polymer with 3,3, 4,4 -tetraaminobiphenyl (commercially 3,3 -diami nobenzidine) with zirconium salicylaldehyde, Zr(sal)4 (7) has been undertaken as shown below ... 

Infrared and thermal studies also add to our knowledge of these polymers. For example, the aromatic C-0 stretch of the Schiff-base ligands is shifted above 1300 cm-1 when coordinated, and coordinated carboxylate shows no CO stretch above 1600 cm-1 when bidentate. The... 

Of importance, these Schiff base metal complexes that contain cis coordination sites allow binding of a substrate (monomer) and the growing polymer chain or can capture a bidentate substrate (growing carbonate polymer chain). Indeed, the use of... 

In a more recent report, a novel dinuclear N,N,0-donor Schiff base-calcium complex [(DAIP)2Ca]2 where DAIP = 2-[2(-dimethylamino-ethylimino)methyl] phenol 76 (Fig. 12) was shown to initiate ROP of L-lactide in a controlled fashion in the presence of benzyl alcohol giving a 96% conversion within 30-60 min at room temperature, yielding polymers with high molecular weight and low PDIs [89]. Taking into account the dimeric molecular structures of hexa- and hepta-coordinated Ca" in the solid state and the kinetic results gained from this study (first order dependency on [LA] a first order dependency on [BnOH] with d[LA]/ df = prop[LA] [BnOH] and a p op constant of 1.95 s ), a coordinatio-... 

Eujii Y, Matsutani K, Kikuchi K. Formation of a specific coordination cavity for a chiral amino-acid by template synthesis of a polymer Schiff-base cobalt(III) complex. Chem Commun 1985 415-417. 

Mercury, like zinc, appears in some chain-like structures, but apparently only in combination with other metals such as As, Sb, Nb, and Ta. Some of these have the characteristics of metallic conductors.15 Rather high molecular weights have been obtained in the preparation of 6-coordinate cobalt (III) chelate polymers with acetylacetonato and leucinato ligands, 7-coordinate dioxouranium-(VI) dicarboxylate polymers, and 8-coordinate zirconium (IV) polymers with Schiff-base ligands.77... 

A material with nitrogen-coordinated Ru was obtained from a silica-linked 2-(phenylazo)pyridine ligand. Results for cyclobutanol oxidation with 02 and the sacrificial oxidant isobutyraldehyde indicate that one- and two-electron oxidations occur simultaneously. The stability of the catalyst is not always guaranteed, probably because acids may be formed in oxidations of alcohols (284). Leaching problems are also encountered with a polymer-bound Ru Schiff base complex, used in oxidation of benzyl alcohol (285). 

The addition of diethylzinc to aldehydes produces secondary alcohols. This process can be stereoselectively catalyzed by chiral amino alcohols that form Schiff-base zinc complexes with the aldehyde and the metal. With the aim of simplifying the work-up of these reactions and to use continuous-flow processes, the polymer-supported amino alcohols 115 and 116 were synthesized (Scheme 21) [91]. The polymers were obtained by co-polymerization of the chiral monomer 117 and styrene 58 in the presence of divinylbenzene (118) or cross-tinldng agent 119 containing a flexible oxyethylene chain. The latter was used to ensure sufficient flexibility within the cross-linked network of the polymer and to further activate the nucleophile by coordination of the oxyethylene chain to the metal. 

E. Tsushida, H. Nishide, M. Ohyanagi, H. Kawakami, Facilitate transport of molecule oxygen in the membranes of polymer — coordinated cobalt Schiff-base complexes, Macromolecules 20 (1987) 1907-1912. 

As with other polymers, the metal complex may be part of the polymer backbone, or attached to the backbone by a conjugated linker. Investigations into the conjugated polymers have been assisted by studies of model complexes.44 Polythiophenes incorporating organometallic complexes are discussed first, followed by those incorporating other coordination complexes with bipyridine, terpyridine, Schiff base, and other ligands. 

---