SYNTHETIC AND FLUXIONALITY STUDIES OF SMALL ORGANOTRANSITION METAL CLUSTERS

Abstract

<p>A series of chiral organotransition metal clusters of the type (PhC≡CCO₂CHMe₂)MM' where M = CpNi and M' = CpMo(CO)₂, Co(CO)₃ or CpNi have been treated with Fe₂(CO)₉ to give clusters of the general formula (RC≡CR')MM'Fe(CO)₃. These clusters are predicted to adopt a square pyramidal geometry and this has been confirmed by X-ray crystallographic studies. In the case of M' = CpNi or Co(CO)₃ the Fe(CO)₃ moiety caps the basal plane whereas when M' = CpMo(CO)₂, it is this moiety which serves as the capping vertex. The former arrangement leads to all the metals pbeying the eighteen electron rule while in the latter the Fe and Mo atoms possess electron counts of 17 and 19 respectively.</p> <p>These trimetallic-alkyne clusters have been shown to exhibit fluxional behaviour. This behaviour has been explained in terms of an alkyne rotation relative to the triangle of metals. When M' = CpNi, the cluster has been shown to undergo racemisation while when M' = CpMo(CO)₂ or Co(CO)₃, the clusters exhibit interconversion of diastereomers thus demonstrating the fluxional process to be intramolecular. The mechanism of isomerisation of these M₃C₂ clusters is compared to those predicted by theoretical calculations and is shown to be related to that of the analogous C₅H₅⁺ cation.</p> <p>A series of synthetic manipulations carried out on the cluster system Co₃(CO)₉CR where R = CO₂CHMe₂, led to a variety of mixed metal clusters being obtained. Treatment with Cp₂Ni, [CpNi(CO)₂] or [CpMo(CO)₃]₂ resulted in replacement or modification of one, two or all three of the original Co(CO)₃ vertices. The problems of employing Cp₂Ni as a reagent in cluster synthesis are demonstrated. Reaction of these tetrahedral M₃CR clusters with Fe₂(CO)₉ yielded not only the expected square pyramidal systems but in one case a molecule adopting a closo trigonal bipyramidal structure. This latter geometry has been confirmed by X-ray crystallography. The concept of isolability has been used to compare the structure of this molecule with the previously known Fe₄CR clusters of similar geometry. Qualitative molecular orbital arguments are presented to rationalise the substitution and addition of metal vertices to the tetrahedral clusters.</p> <p>The geometry of the acylium cation Co₃(CO)₉CCO⁺ has been examined by ¹³C NMR and evidence is outlined for a tilted configuration of the CCO ligand. A stable hexafluorophosphate salt of CpMoCo₂(CO)₈CCO⁺ has been obtained by addition of HPF₆ to a solution of CpMoCO₂(CO)₈COO₂CHMe₂in propionic anhydride. The presence of the acylium cation has been confirmed by its treatment with various nucleophiles resulting in formation of the appropriate derivatives.</p>