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 phosp horus chemical shift tensors and P-31-P-31 spin-spin coupling interactions for bisphosphine molybdenum complexes where only one phosphorus of the bisp hosphine ligand is coordinated to the metal. The bisphosphine ligand of eac h complex was either tetraphenyldiphosphine, tpdp, or bis(diphenylphosphino )methane, dppm. For one compound, (OC)(5)Mo(eta(1)-dppm), single crystals w ere examined by P-31 NMR and X-ray diffraction. For the metal-bound phospho rus, the most shielded direction of the chemical shift tensor is near the M o-P bond axis. For the non-coordinated phosphorus, the most shielded direct ion is oriented in the direction of the formal electron lone pair on phosph orus. For the other compounds, stationary powder samples were examined usin g dipolar-chemical shift and 2D spin-echo techniques. Powder samples were a lso examined under slow magic-angle spinning, variable-angle spinning, and rotational-resonance conditions. Analysis of these P-31 NMR spectra suggest ed chemical shift tensor orientations analogous to those measured for the s ingle crystal; however, in the case of powders only the relative orientatio ns of the two chemical shift tensors with respect to the P-P axis can be de termined. Our investigations indicate that spectra from powder samples shou ld be analyzed at two or more applied magnetic fields. The 2D spin-echo exp eriment proved to be invaluable for obtaining the effective dipolar P,P cou pling constant, R, and the relative signs of R and the indirect spin-spin c oupling constant, J. Values of (n)J(P-31,P-31) measured in the solid state often differ significantly from the conformationally averaged values obtain ed in solution. The tpdp derivatives appear to have a reduced R, whereas fo r the dppm systems, R agrees very well with the value calculated from the k nown P,P separations.