η 3 ‐Allyl‐ und η 3 ‐Benzyl‐Rhodiumkomplexe: Synthese, Strukturdynamik und Reaktionen mit Carbonsäuren

η 3 ‐Allyl and η 3 ‐Benzyl Rhodium Complexes: Synthesis, Structure Dynamic, and Reactions with Carboxylic Acids The (η 3 ‐allyl)rhodium complexes [Rh(η 3 ‐2‐RC 3 H 4 )(P i Pr 3 ) 2 ] ( 3–5 ) have been prepared in 70–90% yield from the in situ generated chlororhodium precursor [RhCl(P i Pr 3 ) 2 ] ( 2 ) and 2‐RC 3 H 4 MgX. On a similar route, the corresponding ethene(phosphane) derivative [Rh(η 3 ‐2‐MeC 3 H 4 )(P i Pr 3 )] ( 9 ) has been obtained. Compound 9 smoothly reacts with P i Pr 3 and PMe 3 to give 4 (R = Me) and [Rh(η 3 ‐2‐MeC 3 H 4 )(PMe 3 )‐(P i Pr 3 )] ( 10 ), respectively. In contrast to the η 3 ‐allyl complexes which are configurationally stable, the η 3 ‐benzyl analogues [Rh(η 3 ‐CH 2 C 6 H 4 R) (P i Pr 3 ) 2 ] ( 11, 12 ), prepared from 2 and 4‐RC 6 H 4 CH 2 MgCl, are highly fluctional in solution. At room temperature, an antarafacial (π‐δ‐π) as well as a suprafacial rearrangement occurs, the first one of which is frozen out at 263 K. On cooling to 193 K, the faster process (equally designated as a metallotropic shift) is also slowed down, and the rigid structure of 11 and 12 is observed. The Δ G * values for the antara and suprafacial rearrangements of 11 and 12 which have been determined from the 1 H‐ and 31 P‐NMR spectra at variable temperature are 60.0 ± 1.5 and 39.5 ± 1.0 kJ/mol, respectively. The η 3 ‐benzyl compound 11 reacts even at –78°C with CO to give the monocarbonylrhodium(I) complex trans ‐[Rh(η 1 ‐CH 2 C 6 H 5 )(CO)(P i Pr 3 ) 2 ] ( 13 ). Treatment of 3,4 , or 11 with RCO 2 H (R = CF 3 , CH 3 , C 6 H 5 , C 6 H 4 ‐4‐OMe, C 6 H 4 ‐4‐NO 2 ) affords the monomeric η 2 ‐carboxylatorhodium(I) compounds [Rh(η 2 ‐O 2 CR) (P i Pr 3 ) 2 ] ( 14–18 ) almost quantitatively. If the reaction of 3 or 4 with CF 3 CO 2 H is performed at −20°C in pentane, the octahedral (η 3 ‐allyl)hydrido complexes [Rh(η 3 ‐2‐RC 3 H 4 )(H)(η 1 ‐O 2 CCF 3 )(P i Pr 3 ) 2 ] ( 19 , 20 ) are isolated. The crystal structures of 12 (at 223 K) and of 15 (at 298 K) have been determined by X‐ray diffraction studies. They confirm that the η 3 ‐benzyl ligand is highly unsymmetrically and the acetato ligand completely symmetrically bound to the [Rh(P i Pr 3 ) 2 ] unit.