Unexpected Sequential NH 3 /H 2 O Solid/Gas Phase Ligand Exchange and Quasi-Intramolecular Self-Protonation Yield [NH 4 Cu(OH)MoO 4 ], a Photocatalyst Misidentified before as (NH 4 ) 2 Cu(MoO 4 ) 2 .

[NH4 Cu(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 )(H2 O)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, [NH4 Cu(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 )(H2 O)3 ]MoO4 intermediate transforms into NH4 Cu(OH)(H2 O)2 MoO4 , which loses two waters and yields [NH4 Cu(OH)MoO4 ]. Upon heating, both [Cu(NH3 )4 ]MoO4 and [Cu(NH3 )(H2 O)3 ]MoO4 decompose, losing three NH3 and three H2 O ligands, respectively, and stable [Cu(NH3 )MoO4 ] is formed from both. The latter can partially be hydrated in boiling water into [NH4 Cu(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 [NH4 Cu(OH)MoO4 ] match those of the active photocatalyst described earlier in the literature under the formulas (NH4 )2 [Cu(MoO4 )2 ] and (NH4 )2 Cu4 (NH3 )3 Mo5 O20 .