High-Pressure Investigations under CO/H2 of Rhodium Complexes Containing Hemispherical Diphosphites

The two rhodium complexes [Rh(acac)(LR)] (LR=(S,S)‐5,11,17,23‐tetra‐tert‐butyl‐25,27‐di(OR)‐26,28‐bis(1,1′‐binaphthyl‐2,2′‐dioxyphosphanyloxy)calix[4]arene; 6: R=benzyl, 7: R=fluorenyl), each based on a hemispherical chelator forming a pocket about the metal centre upon chelation, are active in the hydroformylation of 1‐octene and styrene. As expected for this family of diphosphanes, both complexes resulted in remarkably high selectivity towards the linear aldehyde in the hydroformylation of 1‐octene (l/b≈15 for both complexes). Linear aldehyde selectivity was also observed when using styrene, but surprisingly only 6 displayed a marked preference for the linear product (l/b=12.4 (6) vs. 1.9 (7)). A detailed study of the structure of the complexes under CO or CO/H2 in toluene was performed by high‐pressure NMR (HP NMR) and FT‐IR (HP‐IR) spectroscopies. The spectroscopic data revealed that treatment of 6 with CO gave [Rh(acac)(CO)(η1‐Lbenzyl)] (8), in which the diphosphite behaves as a unidentate ligand. Subsequent addition of H2 to the solution resulted in a well‐defined chelate complex with the formula [RhH(CO)2(Lbenzyl)] (9). Unlike 6, treatment of complex 7 with CO only led to ligand dissociation and concomitant formation of [Rh(acac)(CO)2], but upon addition of H2 a chelate complex analogous to 9 was formed quantitatively. In both [RhH(CO)2(LR)] complexes the diphosphite adopts the bis‐equatorial coordination mode, a structural feature known to favour the formation of linear aldehydes. As revealed by variable‐temperature NMR spectroscopy, these complexes show the typical fluxionality of trigonal bipyramidal [RhH(CO)2(diphosphane)] complexes. The lower linear selectivity of 7 versus 6 in the hydroformylation of styrene was assigned to steric effects, due to the pocket in which the catalysis takes place being less adapted to accommodate CO or larger olefins and, therefore, possibly leading to facile ligand decoordination. This interpretation was corroborated by an X‐ray structure determination carried out for 7. Rhodium in confinement: As revealed by high‐pressure NMR/IR spectroscopic studies, the reaction of rhodium complexes containing hemispherical diphosphites with CO/H2 leads to trigonal bipyramidal intermediates in which the phosphorus atoms exclusively occupy equatorial sites (see figure). The resulting metal confinement selectively drives olefin hydroformylation reactions to form aldehydes that best fit the cavity.