Ligand substitution reactions
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The complex [Rh(acetone)2(C8H14)2]PF6 (2), which is prepared from [RhCl(C8H14)2]2 and AgPF6 in acetone, reacts with PiPr3 to afford the PF6 salt 3 of the cation [Rh(acetone)2(PiPr3)2]+ containing the ketone and phosphane ligands in cis dispositions. Treatment of 3 with MeC≡CPh leads to the displacement of one acetone ligand and the formation of the corresponding π-alkyne complex 4. In contrast, from 3 and terminal alkynes such as HC≡CC6H4-4-Me or HC≡CCPh2OH the cationic vinylidenerhodium(I) compounds 5 and 6 are obtained. The latter, with C=CHCPh2OH as the ligand, is rather labile and even at room temperature eliminates water to give the cationic four-coordinate rhodium allenylidene 7. The molecular structure of this compound has been determined by X-ray crystallography. In the presence of pyridine or ammonia, the acetone ligand of 5 and 7 is readily displaced and the substitution products 8-10 are formed almost quantitatively. Anions such as acetate or hydroxide also displace the ketone unit of 7 and yield the neutral allenylidenerhodium(I) complexes trans-[RhX(=C=C=CPh2)(PiPr3)2] with × = OAc (11) and OH (12).
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