Substitution and Oxidative Addition Reactions of the Monoolefin Complex Rh(acac)(cyclooctene)(PCy₃) Including the X-ray Structure Analyses of Rh(acac)(PCy₃)₂ and [Rh(acac){(<i>E</i>)-CHCHCy}(PCy₃)₂]BF₄

Abstract

The olefin complex Rh(acac)(cyclooctene)(PCy3) (2), which is formed from Rh(acac)(cyclooctene)2 (1) and PCy3 in nearly quantitative yield, reacts with CO and alkynes RC⋮CR by ligand displacement to give Rh(acac)(CO)(PCy3) (3) and Rh(acac)(η2-RC⋮CR)(PCy3) [R = CO2Me (4), Ph (5)], respectively. The bis(phosphine) compound Rh(acac)(PCy3)2 (6) cannot be prepared directly from 2 and excess PCy3 but via Rh(acac)(η2-HC⋮CCO2Me)(PCy3) (7) as intermediate. The X-ray crystal structure analysis of 6 reveals that the rhodium is coordinated in a distorted square-planar manner with O−Rh−O and P−Rh−P bond angles of 85.9(1) and 105.63(4)°. Compound 2 reacts with H2 in the presence of PCy3 to yield Rh(acac)H2(PCy3)2 (8) and with HC⋮CR/PCy3 to give Rh(acac)H(C⋮CR)(PCy3)2 [R = Ph (9), Cy (10), SiMe3 (11)]. On treatment of 10 and 11 with HBF4·OEt2, the cationic alkenylrhodium(III) derivatives [Rh(acac){(E)-CHCHCy}(PCy3)2]BF4 (12) and [Rh(acac)(CHCH2)(PCy3)2]BF4 (13) are obtained. Labeling experiments using DBF4·OEt2 illustrate that the deuterium is found at the β-C carbon atom of the alkenyl ligand. Both 12 and [Rh(acac){(E)-CHCDCy}(PCy3)2]BF4 (12-d1) react with NEt3 to regenerate 10. The structure of 12 was determined by X-ray analysis. The coordination geometry around the metal center can be rationalized as a square pyramid with the alkenyl group in the apical position.