Novel Inorganic Hydrogels Based on the Polymerization of Cyanometalate Transition Metal Complexes With [PdCl 4 ] 2- : A
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ANDREW B. BOCARSLY, GIREESH KUMAR, AND MARIJA HEIBEL Department of Chemistry, Princeton University, Princeton, NJ 08550
ABSTRACT The reaction of a wide variety of cyanometalate complexes of the general form [M(CN)x]n- (where M= a transition metal ion) with square planar [PdCLI] 2 - in aqueous solution leads to the formation of linear polymers. Polymerization occurs via substitution of chloride ligands on the Pd(l) centers, by the nitrogen end of the cyanide ligand to generate extended bridging cyanide structures. Upon generation at room temperature polymer solutions of this type under go a sol-gel transition to generate robust hydrogels having water content in excess of 95%. In the case of the cyanocobaltate/tetrachloropalladate gel, pyrolysis at 900'C produces ferromagnetic Pd/Co metallic alloys having novel morphological character. Materials formed with a hydrogel having a 2:1 Pd to Co stoichiometry are found to be "sponge-like". When placed in water, the metallic matrix swells becoming pliable and holding up to seven equivalents of water per metal site. The conductivity and magnetic properties of this material are maintained in the swollen state. Sintering of the Pd/Co hydrogel in air generates the layered oxide, PbCoO2 having a delafossite structure.
INTRODUCTION Within the realm of inorganic chemistry, the formation of hydrogel structures is typically limited to metal oxides and their related hydroxides. While this provides obvious opportunities with respect to the hydrogel production and processing of oxide based ceramics such as silica, titania, and vanadium oxide, it limits sol-gel processing in general to oxides of specific chemical stoichiometry as defined by the parent hydrogel'. In theory, the synthesis of solid state materials in general, and ceramics in particular would benefit from the existence of inorganic hydrogels based on non-oxide, transition metal coordination complexes in which ligand substitution either in the gel phase or upon thermal processing of the gel was utilized to establish the final chemical composition of the solid state product. Of course, in order to employ this synthetic strategy stable, non-oxide, gels containing coordination polymers must be available. Typical inorganic coordination complexes adopt either an octahedral or tetrahedral geometry. Such platonic geometries are ideal either for the dissolution of the complex in solution or the formation of ordered solids depending on the ligands and solvents employed. This geometric consideration argues against the existence of coordination polymers which undergo gelation in aqueous solution, since this process can be considered to lie along the crystallization coordinate. However, if sufficient packing strain is built into the coordination polymer via the intermixing of different complex geometries, a situation which is not favorable to solid state formation can be established. We have recently applied this strategy to the generalized synthesis of transition metal complex hydrogels 2 via the reaction of the square planar complex
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