Tri- and Higher Nuclearity Complexes

Oxidation of one of the hydroxyl groups of a 1,2 glycol to form an a-hydroxy carboxylic acid increases the complexity of the vanadate chemistry. Mono, di, tri and higher nuclearity compounds are formed with a-hydroxy carboxylic acids. Except for the dimeric products, the structures adopted by these compounds are not known with any degree of certainty. 

Several factors affect the nature of the products in a reaction between a transition metal cluster and an alkyne or alkene. In this section, the various synthetic routes to alkyne or alkene-substituted clusters will be presented, and these will be used to analyze the changes in reactivity of the cluster systems when one or more of the important reaction parameters is altered. In order to simplify the discussion, tri-, tetra-, and higher nuclearity clusters will be treated separately. Finally, in this section, there is a brief description of the chemistry of alkylidyne-substituted clusters since synthetic routes to alkyne-containing complexes may involve these species. 

There are several examples of well-characterized tri- and tetranu-clear hydroxo-bridged complexes of chromium(III) and cobalt(III). Penta- and hexanuclear aqua chromium(III) complexes have been prepared in solution, but their structure and properties are unknown. Oligomers of nuclearity higher than four have not been reported for cobalt(IIl), with the exception of some hetero-bridged heteronuclear species (193, 194). There appear to be no reports of rhodium(III) or iridium(III) complexes of nuclearity higher than two. 

The M 114.02 clusters were more active than the Mn30 clusters and the Mn clusters also catalyzed t-butyl hydroperoxide decomposition ( 1%) to acetone and methanol, which prevented reliable analysis of methane activation results. We could not compare the dinuclear manganese complexes to their tri and tetra analogues because of relative solubility differences however, we were able to do this with several iron di and tetra clusters and found that the Fe402 clusters were more active with the hydrocarbons studied. Therefore, higher nuclearity or a variety of ligands may provide the shape selectivity we seek fa ultimate methane activation with Mn and Fe clusters. 

Although polynuclear alkyne completes are often prepared by reaction of the alkyne with a suitable metal cluster fragment, heteropolynuclear complexes 69 (Scheme 4-38) have been obtained also by isolobal metal fragment substitution, as noted previously [26]. Higher-nuclearity alkyne complexes also can be produced by the addition of various metal carbonyl fragments to a lower-nuclearity alkyne complex [119], A novel entry to heterobi- (and tri-)metallic neutral p-propargyl complexes (e.g., Fe/Mo) via protonation of trinuclear p-Ti, T1 -o-propargyl derivatives 70 was recently described by Wojcicki and coworkers [120,121]. 

The use ofpoly-NHC allows to easily obtain polymetallic systems mainly di-iii-ii and tri-nuclear complexes have been reported, although some examples of higher nuclearity species have been also presented." ... 

---