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

Abstract

Single crystals of [NpO2(PO(C6H5)3)4]ClO4 were grown and the structure of this compound was studied (CAD4 diffractometer, MoKα; triclinic unit cell: a = 9.140(2), b = 13.528(3), c = 13.918(3) A, α = 97.14(3)°, β = 108.17(3)°, γ = 91.75(3)°, space group P-1, Z = 1, V = 1618.0 A3, dcalc = 1.521 g cm-3; R1 = 0.0245 for 5342 observed reflections with F0 > 4σ(F0), wR2 = 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 ClO4- 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) A. The equatorial coordination plane of the bipyramid is formed by oxygen atoms of four triphenylphosphine oxide (TPPO) molecules. The Np-Oeq bond lengths vary from 2.434(2) to 2.442(2) A (average 2.438 A). The Oeq-Np-Oeq 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 A; 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 etaas(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.