Operando High-Pressure NMR and IR Study of the Hydroformylation of 1-Hexene by 1,1'-Bis-(Diarylphosphino)metallocene-Modified Rhodium(I) Catalysts

Some rhodium(I) complexes of the general formula [Rh(P-P)(COD)]X were synthesized and characterized by multinuclear NMR spectroscopy (COD ) cyclocta-1,5-diene; P-P ) 1,1¢-bis(diphenylphosphino)ferrocene, dppf, X ) BPh4, PF6; P-P ) 1,1¢-bis(diphenylphosphino) ruthenocene, dppr, X ) BPh4; P-P ) 1,1¢-bis(diphenylphosphino)osmocene, dppo, X ) BPh4, PF6; P-P ) 1,1¢-bis(diphenylphosphino)octamethylferrocene, dppomf, X ) BAr¢4; P-P ) (1,1¢-bis(di(o-isopropylphenyl)phosphino)ferrocene, o-iPr-dppf, X ) BAr¢4). These complexes were employed as catalyst precursors for the hydroformylation of 1-hexene in THF either in standard autoclaves or in high-pressure (HP) NMR tubes and IR cells. All catalysts exhibited good activity (TOFs ranging from 700 to 1000 mol aldehyde (mol cat)-1 h-1) and moderate regioselectivity in n-heptanal (67-74%). Irrespective of the rhodium precursor, the HP-NMR experiments under catalytic conditions showed the formation of kinetic dicarbonyl products at room temperature, which were independently prepared by reaction of the COD precursors with 1 bar CO in THF. Square-planar dicarbonyl complexes containing two cis carbonyl groups were obtained with the dppf and dppomf ligands, while the precursors with the dppr, dppo, and o-iPr-dppf ligands gave trigonal-bipyramidal dicarbonyl complexes with the equatorial positions occupied by two carbonyl groups and by the metallocene metal atom. The complexes [Rh(CO)2(dppf)]PF6 and [Rh(CO)2(dppo)]PF6 were isolated in the solid state and characterized by single-crystal X-ray analysis. On increasing gradually the temperature of the HP-NMR hydroformylation experiments, the dppf, dppr, and dppo dicarbonyl complexes disappeared. Formed in their place were neutral fivecoordinate hydride(dicarbonyl) complexes RhH(CO)2(P-P) that exist in solution as two rapidly equilibrating geometric isomers. The reaction of the o-iPr-dppf precursor with syngas at 60 °C gave a trigonal-bipyramidal dicarbonyl complex with a dative Fe-Rh bond, while the dppomf complex decomposed to various CO-containing rhodium complexes. Unlike HPNMR spectroscopy, HP-IR spectroscopy showed no kinetic product at any stage of the catalytic reactions. Also, HP-IR spectroscopy allowed us to distinguish the ee and ea geometric isomers of the hydride(dicarbonyl) resting states with dppf, dppr, and dppo. Irrespective of the temperature, o-iPr-dppf formed a stable dicarbonyl complex as a result of the catalytic reaction, while the dppomf dicarbonyl was unstable under hydroformylation conditions, converting into phosphine-free carbonyl Rh compounds.