Unexpected Sequential NH₃/H₂O Solid/Gas Phase Ligand Exchange and Quasi-Intramolecular Self-Protonation Yield [NH₄Cu(OH)MoO₄], a Photocatalyst Misidentified before as (NH₄)₂Cu(MoO₄)₂

Abstract

[NH4Cu­(OH)­MoO4] as active photocatalyst in the decomposition of Congo Red when irradiated by UV or visible light has been prepared in an unusual ammonia/water ligand exchange reaction of [tetraamminecopper­(II)] molybdate, [Cu­(NH3)4]­MoO4. [Cu­(NH3)4]­MoO4 was subjected to moisture of open air at room temperature. Light blue orthorhombic [Cu­(NH3)­(H2O)3]­MoO4 was formed in 2 days as a result of an unexpected solid/gas phase ammonia–water ligand exchange reaction. This complex does not lose its last ammonia ligand on further standing in open air; however, a slow quasi-intramolecular (self)-protonation reaction takes place in 2–4 weeks, producing a yellowish-green microcrystalline material, which has been identified as a new compound, [NH4Cu­(OH)­MoO4], (a = 10,5306 Å, b = 6.0871 Å, c = 8.0148 Å, β = 64,153°, C2, Z = 4). Mechanisms are proposed for both the sequential ligand exchange and the self-protonation reactions supported by ab initio quantum-chemical calculations and deuteration experiments as well. The [Cu­(NH3)­(H2O)3]­MoO4 intermediate transforms into NH4Cu­(OH)­(H2O)2MoO4, which loses two waters and yields [NH4Cu­(OH)­MoO4]. Upon heating, both [Cu­(NH3)4]­MoO4 and [Cu­(NH3)­(H2O)3]­MoO4 decompose, losing three NH3 and three H2O ligands, respectively, and stable [Cu­(NH3)­MoO4] is formed from both. The latter can partially be hydrated in boiling water into [NH4Cu­(OH)­MoO4. This compound can also be prepared in pure form by boiling the saturated aqueous solution of [Cu­(NH3)4]­MoO4. All properties of [NH4Cu­(OH)­MoO4] match those of the active photocatalyst described earlier in the literature under the formulas (NH4)2[Cu­(MoO4)2] and (NH4)2Cu4(NH3)3Mo5O20.

Thermal analysis of solvatomorphic decakis (dimethylammonium) dihydrogendodecatungstate hydrates

“…Since in our compounds there are strongly reducing cations and oxidizing anions, one can expect a thermally induced redox reaction between them. The occurrence of such kind of redox reactions within one compound having oxidizing and reducing components is well documented in the literature [11][12][13][14][15][16][17]. Since in inert atmosphere the decomposition steps are endothermic, one can conclude that the energy required for elimination of water and dimethylamine as well as the disruption of the lattice exceeds the exothermicity of the redox interaction.…”

(Me2NH2)10[H2-Dodecatungstate] polymorphs: dodecatungstate cages embedded in a variable dimethylammonium cation + water of crystallization matrix

“…From this we learned that the O-H stretch, H-O-H bend and the N-H stretch frequencies are really shied towards lower wave numbers with respect to their gas-phase positions, indicating that the hydrogen atoms of these bonds are donated into hydrogen bonds. [35][36][37] The peaks are also smeared out and their separability remains limited even aer complete deuteration. From this one can conclude that there are many kinds of N-H/O and O-H/(O,N,C) hydrogen bonds with different strength.…”

Investigation into the synthesis conditions of CuMoO4 by an in situ method and its photocatalytic properties under visible light irradiation

“…Similarly, the other three bands at 796-937 cm À1 (weak in the Raman spectrum, strong in the FTIR spectrum) belong to the three components of the Mo-O asymmetric stretching mode (F2, triple degenerated). 17 The Raman spectrum was used to further examine the structural properties of the prepared photocatalysts. The Raman spectrum of CuMoO 4 is shown in Fig.…”

“…The room-temperature triclinic (alpha) polymorph was studied. In the thermal analysis results, a small signal of a phase transition could be seen on the DSC curve around 570 C, 17 which belonged to the room-temperature (a) / hightemperature (b) phase transition of CuMoO 4 .…”