Exchanging Conformations of a Hydroformylation Catalyst Structurally Characterized Using Two-Dimensional Vibrational Spectroscopy

Catalytic transition-metal complexes often occur in several conformations that exchange rapidly (<ms) in solution so that their spatial structures are difficult to characterize with conventional methods. Here, we determine specific bond angles in the two rapidly exchanging solution conformations of the hydroformylation catalyst (xantphos)Rh(CO)(2)H using two-dimensional vibrational spectroscopy, a method that can be applied to any catalyst provided that the exchange between its conformers occurs on a time scale of a few picoseconds or slower. We find that, in one of the conformations, the OC-Rh-CO angle deviates significantly from the canonical value in a trigonal-bipyramidal structure. On the basis of complementary density functional calculations, we ascribe this effect to attractive van der Waals interaction between the CO and the xantphos ligand.