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

As the workhorses for many applications, neutral dimeric mu(2)-X-bridged diphosphine rhodium complexes of the type [{Rh(diphosphine)(mu(2)-X)}(2)] (X=Cl, OH) are usually prepared in situ by the addition of diphosphine ligands to the rhodium complex [{Rh(diolefin)(mu(2)-X)}(2)] (diolefin=cyclooctadiene (cod) or norbornadiene (nbd)) or [{Rh(monoolefin)(2)(mu(2)-Cl)}(2)] (monoolefin=cyclooctene (coe) or ethylene (C2H4)). 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-PtBu2), 1,1'-bis(diisopropylphosphino)ferrocene (DiPPF), 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 P-31 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)(mu(2)-X)}(2)] are formed, for example, [(diolefin)Rh(mu(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)(mu(2)-X)}(2)] complexes.