Oligonuclear complexes

Di- and oligonuclear complexes with open-chain alkoxo- or phenolato-based dinucleating ligands 427... 

In this chapter we will present a series of polynuclear nickel(II) complexes, placing our attention mainly on those complexes for which the largest number of experimental observations are available. We will first review dimers and later oligonuclear complexes and extended systems in the order of increasing number of constituent metal ions. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

Further studies on the supramolecular coordination chemistry of copper(II) focused on the synthesis of oligonuclear complexes by self-assembly with the pentadentate ligand (L3)2 with 2,6-pyridinyl-spacer [70]. To this end, H2L3 (4) was treated with calcium hydride and copper(II) acetate to give the metallocoronate [(Cac Cu2(L3)2 )(OAc)2] (5 Scheme 2). In the crystal, 5 is present as a dinuclear copper(II) coronate in which a calcium ion is encapsulated in the center two acetates act as counterions. 

Further hydrolysis leads to binuclear and oligonuclear complex ions. The situation is similar with [Fe(H20) ] ". Only [Cr(H20)5] is kinetically stable at neutral pH. In the so-called basic aluminum chloride solutions at pH 4—8, the cation [Alj304(0H)24(H20),2] is formed [81]. It has a spherical Keggin-hke structure with a central tetrahedrally coordinated Al " " ion, analogous to the structure of the phosphomolybdate anion [PMoj204q] . 

High metal content and binding with both essential and toxic metals through thiolate bonds and oligonuclear complexes (clusters). 

Helicates are defined as metallo-supramolecular coordination compounds in which linear ligands wrap around several metal ions in a helical fashion. The helicate term was introduced by J. M. Lehn in 1987, and from that point the chemistry of helicates became a prominent subdiscipline of supramolec-ular chemistry. Initially, helicates were considered simple model compounds explaining mechanistic aspects of self-assembly. Recently, special properties of the oligonuclear complexes became more important for developing novel materials based on the helicate motif. ... 

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