Synthesis,Crystal Structure,and Spectral Characteristics of the Np(V) Complex with Triphenylphosphine Oxide [NpO2(PO(C6H5)3)4]ClO4

Single crystals of NpO₂(PO(C₆H₅)₃)₄]ClO₄ were grown and the structure of this compound was studied (CAD4 diffractometer, MoK ; triclinic unit cell: a = 9.140(2), b = 13.528(3), c = 13.918(3) Å, = 97.14(3)°, = 108.17(3)°, = 91.75(3)°, space group P-1, Z = 1, V = 1618.0 ų, d _{c a l c} = 1.521 g cm^-3; R ₁ = 0.0245 for 5342 observed reflections with F ₀ > 4(F ₀), wR ₂ = 0.0677 for 5601 unique reflections, 419 refined parameters). NpO₂(PO(C₆H₅)₃)₄]ClO₄ consists of complex cations NpO₂(PO(C₆H₅)₃)₄]⁺ and disordered ClO_4- anions. The Np(V) atom has tetragonal-bipyramidal oxygen surrounding with coordination number (CN) 6. The NpO₂ group is linear, the O = Np = O bond angle is 180.0°, and the Np = O bond length is 1.797(2) Å. The equatorial coordination plane of the bipyramid is formed by oxygen atoms of four triphenylphosphine oxide (TPPO) molecules. The Np-O_eq bond lengths vary from 2.434(2) to 2.442(2) Å (average 2.438 Å). The O_eq-Np-O_eq bond angle is close to the right angle (88.98°). The average bond lengths in TPPO ligands are P-O 1.498, P-C 1.798, and C-C 1.376 Å; the average bond angles are O-P-O 111.12°, C-P-C 107.78°, P-C-C 120.2, and C-C-C 120.0°. The electronic absorption spectra (Shimadzu UV-3100, 900-1050 nm) and IR spectra (Specord-M80, 400-4000 cm^{- 1}) of crystalline NpO₂(PO(C₆H₅)₃)₄] were measured in an NaCl matrix. The absence of the main characteristic absorption band of the NpO₂ ⁺ dioxocation (980 nm) in the electronic absorption spectrum suggests centrosymmetrical equatorial surrounding of the Np(V) atom. The narrow absorption band ta_{a s}(NpO₂ ⁺) in the IR spectra in the region of 860 cm^-1 suggests the absence of cation-cation interaction, and its position is consistent with a short Np-O bond length in neptunyl(V) groups.