Studies have been carried out on the synthesis and reactivity of rhodium compounds containing metal carbon multiple bonds. Some years ago, it was found that the coordinatively unsaturated species (RhC1 (PiPr3)2) smoothly reacts with 1-alkynes RCCH to give the first alkyne complexes trans-(RhC1 (RCCH)(PiPR3)2) which rearrange to form the vinylidene isomers trans-(RhC1 (CCHR)(PiPr3)2). In order to understand the mechanism of this process and to explain the lability of the square planer rhodium compounds, the carbon-13 nuclear magnetic resonance (NMR) spectra of a whole series of complexes of the general type trans-(RhC1 (RCCR')(PR3)2) were investigated. The data obtained provided good evidence that the alkyne ligand is coordinated as a 2 Pi-electron (and not a 4 Pi-electron) donor. The complexes trans (RhC1 (RCCH)(PiPr3)2) are thus really 16-electron species and able to rearrange to the vinylidene isomers via an intramolecular oxidative addition.
The main conclusion of this work is that binuclear rhodium complexes are accessible in which 2 metal centres are connected through an extended Pi-electron system composed of cumulated and conjugated carbon carbon double bonds. One of the next goals will be to investigate the electrochemical properties of these complexes primarily to find out whether they can serve as chemical electron reservoirs.
The second major objective of the work was the investigation of the recently discovered rhodium catalyzed synthesis of trisubstituted olefins from ethene derivatives and diazoalkanes. During a systematic variation of the coordination sphere around the catalytically active rhodium centre it was found that monoolefinic ligands provided the highest reactivity. With (RhC1 (C2H4)2)2 as catalyst turnover numbers of 500 to 700 have now been obtained. The new method of carbon carbon bond formation can also be applied to diazoketones and to olefins containing a functional group and may thus become a useful tool in organic synthesis. Fur ther studies will be concerned with:
elucidation the mechanism of the catalytic reaction;
the preparation of compounds which may be related in structure to the supposed reaction intermediates;
changing the metal centre and investigate the behaviour of other 4d transition metal complexes, mainly of ruthenium and palladium, (ie, the neighbours of rhodium in the periodic table).
Funding SchemeCSC - Cost-sharing contracts