Triclinic La7Zn2P11 with P3−, P24−, and P35− units: a combined study by 31P solid-state NMR spectroscopy and single crystal X-ray diffraction

Abstract

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, α = 82.111(3), β = 77.034(3), γ = 89.996(3)°, wR₂ = 0.1553, 5852 F² values and 183 variables. This peculiar structure is characterized by the simultaneous presence of three distinct anionic phosphide species, namely P³⁻, P₂⁴⁻, and P₃⁵⁻ units. La₇Zn₂P₁₁ is an electron precise Zintl phase: (7La³⁺)²¹⁺(2Zn²⁺)⁴⁺(4P³⁻)¹²⁻(2P₂⁴⁻)⁸⁻(P₃⁵⁻). 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 P₂⁴⁻ species. The tetrahedra are condensed to chains via common corners. The P₃⁵⁻ units with P–P–P angles of 113.7° have exclusively lanthanum coordination. ³¹P 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.