Phosphorus-31 Solid-State NMR Studies of Homonuclear Spin Pairs in Molybdenum Phosphine Complexes:  Single-Crystal, Dipolar-Chemical Shift, Rotational-Resonance, and 2D Spin−Echo NMR Experiments

Several solid-state NMR techniques have been employed to characterize phosphorus chemical shift tensors and 31P−31P spin−spin coupling interactions for bisphosphine molybdenum complexes where only one phosphorus of the bisphosphine ligand is coordinated to the metal. The bisphosphine ligand of each complex was either tetraphenyldiphosphine, tpdp, or bis(diphenylphosphino)methane, dppm. For one compound, (OC)5Mo(η1-dppm), single crystals were examined by 31P NMR and X-ray diffraction. For the metal-bound phosphorus, the most shielded direction of the chemical shift tensor is near the Mo−P bond axis. For the non-coordinated phosphorus, the most shielded direction is oriented in the direction of the formal electron lone pair on phosphorus. For the other compounds, stationary powder samples were examined using dipolar-chemical shift and 2D spin−echo techniques. Powder samples were also examined under slow magic-angle spinning, variable-angle spinning, and rotational-resonance conditions. Analysis of these 31P NMR spectra suggested chemical shift tensor orientations analogous to those measured for the single crystal; however, in the case of powders only the relative orientations of the two chemical shift tensors with respect to the P−P axis can be determined. Our investigations indicate that spectra from powder samples should be analyzed at two or more applied magnetic fields. The 2D spin−echo experiment proved to be invaluable for obtaining the effective dipolar P,P coupling constant, R, and the relative signs of R and the indirect spin−spin coupling constant, J. Values of nJ(31P,31P) measured in the solid state often differ significantly from the conformationally averaged values obtained in solution. The tpdp derivatives appear to have a reduced R, whereas for the dppm systems, R agrees very well with the value calculated from the known P,P separations.