Solid-State NMR Connectivity Studies in Dipolarly Coupled Inorganic Networks: Crystal Structure and Site Assignments for the Lithium Polyphosphide LiP5

The crystal structure of LiP5 has been refined from single-crystal X-ray data to a residual of R=0.020 for 2357 structure factors and 56 variable parameters. The structure as determined in the acentric space group Pna21 by H. G. von Schnering and W. Wichelhaus [Naturwissenschaften 59, 78 (1972)] is confirmed. However, the lithium position has been determined with considerably greater accuracy. The polyanionic network of the phosphorus atoms can also be refined in the corresponding centrosymmetric group Pnam, where two adjacent lithium positions with partial occupancy are found. The phosphorus connectivities within this network are studied by means of advanced solid-state NMR techniques. Crystalline polyphosphides represent a particular challenge for such experiments owing to the presence of strong homonuclear 31P–31P dipole–dipole couplings within the network of phosphorus polyanions. To meet this challenge a powerful strategy has been designed by combining magic-angle spinning with 31P–31P double-quantum-filtered two-dimensional exchange, rotational resonance, and heteronuclear cross-polarization spectroscopies. Based on this information, the NMR results are discussed in connection with the local environments of the phosphorus sites.