Strategies in the development of synthetic model compounds for dinuclear and polynuclear metal sites in metalloproteins

Abstract

Many of the known metalloproteins and metalloenzymes contain more than one metal ion at the active site. The metal ions can be relatively close together (with a bridging ligand in between them), or separated over more than 5-6 Å. This paper will only deal with the first category. The ligand-bridged dinuclear or polynuclear centers can either be homopolyatomic (such as the copper dimers in hemocyanin [1] and the iron tetramers [2] in ferredoxin), or heteropolyatomic (such as the CuZn site in bovine syperoxide dismutase [3], the Fe and Mo-containing cofactor in nitrogenase [4], and the CuFe site in cytochrome c oxides [5]). In some cases, such as in the ironsulfur proteins, the polynuclear coordination entity can be extracted from the proteins, without decomposition [6]. In developing strategies for the synthesis of low-molecular weight analogs for these dinuclear and polynuclear metal sites, two approaches can be followed, i.e.: 1. Use of small ligands that are well known to form bridges between two (or more) metal ions. Examples are OH − (for Cu dimers) [7], S 2− (for ironsulfur clusters) [8] and deprotonated imidazole (for copper dimers) [9]. Under these circumstances dinuclear or polynuclear metal units can only be formed in certain cases, and generalisations are difficult to make. 2. Use of binucleating (or polynucleating) chelating ligands to hold two (or more) metal ions bound to the same ligand [10]. In addition small bridging ligands, such as OH −, may be present. These systems may or may not contain a ligand atom bridging two or more metal ions. Asymmetric binucleating chelating ligands can be designed to synthesize asymmetric dimers, which may even hold two different metal ions [11]. Steric constraints and/or intramolecular interactions ( e.g. hydrogen bonding) can be used to generate a certain coordination geometry, or to regulate metal–ligand distances [12]. The most successful results have been obtained so far, when method (b) was used. However, many clusters have also been prepared according to method (a). The present paper will mainly deal with nitrogen (amine, imidazole) and oxygen (phenolate, alcoholate) as ligand donor atoms in binucleating and polynucleating chelating ligands. Special attention will be given to ligand system accessible via relatively simple synthetic methods. The formation of the dinuclear and polynuclear coordination compounds, their molecular structures, and their chemical and spectroscopic properties will be discussed in detail for a few selected systems.