In Situ Synthesis of Neutral Dinuclear Rhodium Diphosphine Complexes [{Rh(diphosphine)(μ₂‐X)}₂]: Systematic Investigations

Abstract

Abstract As the workhorses for many applications, neutral dimeric μ 2 ‐X‐bridged diphosphine rhodium complexes of the type [{Rh(diphosphine)(μ 2 ‐X)} 2 ] (X=Cl, OH) are usually prepared in situ by the addition of diphosphine ligands to the rhodium complex [{Rh(diolefin)(μ 2 ‐X)} 2 ] (diolefin=cyclooctadiene (cod) or norbornadiene (nbd)) or [{Rh(monoolefin) 2 (μ 2 ‐Cl)} 2 ] (monoolefin=cyclooctene (coe) or ethylene (C 2 H 4 )). The in situ procedure has been investigated for the diphosphines 2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl (BINAP), 5,5′‐bis(diphenylphosphino)‐4,4′‐bi‐1,3‐benzodioxole (SEGPHOS), 5,5′‐bis[di(3,5‐xylyl)phosphino]‐4,4′‐bi‐1,3‐benzodioxole (DM‐SEGPHOS), 5,5′‐bis[di(3,5‐di‐ tert ‐butyl‐4‐methoxyphenyl)phosphino]‐4,4′‐bi‐1,3‐benzodioxole (DTBM‐SEGPHOS), 2,2′‐bis(diphenylphosphino)‐1,1′‐dicyclopentane (BICP), 1‐[2‐(diphenylphosphino)ferrocenyl]ethyldi‐ tert ‐butylphosphine (PPF‐P t Bu 2 ), 1,1′‐bis(diisopropylphosphino)ferrocene (D i PPF), 1,2‐bis(diphenylphosphino)ethane (DPPE), 1,2‐bis( o ‐methoxyphenylphosphino)ethane (DIPAMP), 4,5‐bis(diphenylphosphinomethyl)‐2,2‐dimethyl‐1,3‐dioxalane (DIOP), 1,2‐bis(2,5‐dimethylphospholano)benzene (Me‐DuPHOS), 1,4‐bis(diphenylphosphino)butane (DPPB), and 1,3‐bis(diphenylphosphino)propane (DPPP); the resulting complexes have been characterized by 31 P NMR spectroscopy and, in most cases, also by X‐ray analysis. Depending on the diphosphine ligand, the solvent, the temperature, and the rhodium precursor, species other than the desired one [{Rh(diphosphine)(μ 2 ‐X)} 2 ] are formed, for example, [(diolefin)Rh(μ 2 ‐Cl) 2 Rh(diphosphine)], [Rh(diphosphine)(diolefin)] + , [Rh(diphosphine) 2 ] + , and [Rh(diphosphine)(diolefin)(Cl)]. The results clearly show that the in situ method commonly applied for precatalyst preparation cannot be regarded as an optimal strategy for the formation of such neutral [{Rh(diphosphine)(μ 2 ‐X)} 2 ] complexes.