Probing the stereo-electronic properties of cationic rhodium complexes bearing chiral diphosphine ligands by 103Rh NMR.

(103)Rh NMR represents a powerful tool to assess the global electronic and steric contribution of diphosphine ligands on Rh(COD)(diphosphine) complexes. In the case of DIOP, BINAP and MeDUPHOS, this approach proved to be more informative than classical CO-stretching frequency measurements. After validation, this method has been extended to a set of seven diphosphines. (103)Rh NMR measurements on [Rh(COD)(diphosphine)]PF(6) lead to the following order of donor properties: dppe > MeBPE > MeDUPHOS > dppb > DIOP > BINAP > Tol-BINAP. This trend has been validated by DFT in the case of DIOP, BINAP and MeDUPHOS. In conjunction, (31)P NMR chemical shift has been shown to reflect the ring constraints of the Rh-diphosphine scaffold. This contribution is a step towards a mechanistic investigation of the catalytic hydrogenation of unsaturated substrates by (103)Rh NMR and DFT.