Synthesis and characterization of two new photochromic organic–inorganic hybrid materials based on isopolyoxomolybdate: (HDBU)3(NH4)[β-Mo8O26]·H2O and (HDBU)4[δ-Mo8O26]

Two new organic–inorganic hybrid compounds of molybdenum(VI) (HDBU)3(NH4)[β-Mo8O26]·H2O 1 and (HDBU)4[δ-Mo8O26] 2 have been synthesized and characterized by elemental analyses, FT-IR and UV–vis spectroscopies, and single-crystal X-ray diffraction (at 293 and 100 K, respectively). Compound 1 is obtained at room temperature by adding 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) on aqueous molybdate solution at pH=5.7. Compound 2 is obtained hydrothermally from 1 (110 °C, 24 h, autogenous pressure). Compound 1 crystallizes in the triclinic crystal system, space group image, with a=11.2492(9), b=14.1907(10), c=16.7498(10) Å, α=80.993(4), β=72.272(6), γ=71.696(7)°, V=2412.5(3) Å3 at T=20 °C and Z=2. The refinement of the structure leads to a residual factor R=0.0697 for 10234 independent observed reflections [I/σ(I)⩾2] and 623 parameters. Compound 2 crystallizes in the triclinic crystal system, space group image, with a=11.3158(9), b=11.3773(8), c=13.2884(17) Å, α=92.110(8), β=112.127(9), γ=117.581(7)°, V=1357.4(3) Å3 at T=−173 °C and Z=2. The refinement of the structure leads to a residual factor R=0.0226 for 11812 independent observed reflections [I/σ(I)⩾2] and 354 parameters. The structure of 1 consists of β-[Mo8O26]4− anions with HDBU+ and NH4+ cations connected to the mineral parts by hydrogen bonds and the structure of 2 contains δ-[Mo8O26]4− anions with HDBU+ counter-cations. Both the thermal and chemical isomerizations of the two [Mo8O26]4− isomers are highlighted and discussed. Photochromic behaviors of compounds 1 and 2 are also reported.