Triclinic La₇Zn₂P₁₁ with P^3-, P₂^4-, and P₃^5- units: a combined study by ³¹P solid-state NMR spectroscopy and single crystal X-ray diffraction

The ternary polyphosphide La7Zn2P11 was synthesized from the elements by using a salt flux or via a ceramic method in sealed quartz ampoules. The obtained samples were investigated by X-ray powder and single crystal diffraction: own type, P, a = 775.33(13), b = 827.45(13), c = 1502.8(3) pm, alpha = 82.111(3), beta = 77.034(3), gamma = 89.996(3)degrees, wR(2) = 0.1553, 5852 F-2 values and 183 variables. This peculiar structure is characterized by the simultaneous presence of three distinct anionic phosphide species, namely P3-, P-2(4-), and P-3(5-) units. La7Zn2P11 is an electron precise Zintl phase: (7La(3+))(21+)(2Zn(2+))(4+)(4P(3-))(12-)(2P(2)(4-))(8-)(P-3(5-)). The P-P single bond distances range from 219.2 to 223.0 pm. The zinc sites show tetrahedral phosphorus coordination by three P3- and one P-2(4-) species. The tetrahedra are condensed to chains via common corners. The P-3(5-) units with P-P-P angles of 113.7 degrees have exclusively lanthanum coordination. P-31 solid-state NMR was used to probe the phosphorus local environments, connectivities and spatial proximities. The eleven crystallographically distinct phosphorus atoms were assigned with the help of two-dimensional homonuclear dipolar correlation experiments. Even though the application of 2D measurements on such phosphorus-based polyanionic compounds is exceedingly challenging because of the wide dispersion of chemical shifts, the fast irreversible decay of the transverse magnetization, and slow spin-lattice relaxation, a complete assignment is possible using radiofrequency-driven dipolar recoupling (RFDR), J-RESOLVED and total-through-bond correlation with R-sequence (R-TOBSY) techniques.