Crystal and Molecular Structure of a New Hexamolybdate-Cyclophosphazene Complex

The reaction between molybdenum trioxide and hexakis(dimethylamino)cyclotriphosphazene, [NP(NMe₂)₂]₃, in water yields yellow-green crystals of the salt [HN₃P₃(NMe₂)₆⁺]₂[Mo₆O₁₉^2-] (I). Crystals of I are monoclinic with space group P2₁/C anda= 13.571 (13), b = 10.966 (11), c = 21.066 (19) A, β= 108.36 (6)°, Z = 2. A three-dimensional X-ray structure was determined by Patterson and Fourier methods with refinement by least-squares techniques to a conventional R factor of 0.029. The structure is characterized by the presence of puckered, ring-protonated cyclotriphosphazene structures in which long P-N skeletal bonds (1.663, 1.675 (5) A) and short skeletal bonds (1.560-1.599 (5) A) are found in the same ring. The N-P-N skeletal angles are in the range of 109.6-114.5 (3)° and the P-N-P angles are in the range of 126-129 (3)°. The Mo₆O₁₉^2- ion consists of a slightly distorted cage of six molybdenum atoms located octahedrally around a central oxygen atom, with twelve oxygen atoms disposed in Mo-O-Mo units and one terminal oxygen attached to each molybdenum. The terminal Mo-O bond distances have a range of 1.676-1.678 (5) A, while the bridge Mo-O bonds have lengths within the range of 1.855-2.005 (4) A. The central Mo-O bond lengths are in the range of 2.312-2.324 (4) A. This constitutes the first X-ray structural identification of the Mo₆O₁₉^2- system.