Probing Homogeneous Catalysts and Precatalysts in Solution by Exchange-Mediated Overhauser Dynamic Nuclear Polarization NMR

Abstract

Triphenylphosphine (PPh₃) is a ubiquitous ligand in organometallic chemistry that has been shown to give enhanced ³¹P NMR signals at high magnetic field via a scalar-dominated Overhauser effect dynamic nuclear polarization (OE DNP). However, PPh₃ can only be polarized via DNP in the free form, while the coordinated form is DNP-inactive. Here, we demonstrate the possibility of enhancing the ³¹P NMR signals of coordinated PPh₃ in metal complexes in solution at room temperature by combining Overhauser effect DNP and chemical exchange between the free and coordinated PPh₃ forms. With this method, we successfully obtain ³¹P DNP enhancements of up to 2 orders of magnitude for the PPh₃ ligands in Rh(I), Ru(II), Pd(II), and Pt(II) complexes, and we show that the DNP enhancements can be used to determine the activation energy of the ligand exchange reaction.