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

[NHCu(OH)MoO] 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(NH)]MoO. [Cu(NH)]MoO was subjected to moisture of open air at room temperature. Light blue orthorhombic [Cu(NH)(HO)]MoO 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, [NHCu(OH)MoO], ( 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(NH)(HO)]MoO intermediate transforms into NHCu(OH)(HO)MoO, which loses two waters and yields [NHCu(OH)MoO]. Upon heating, both [Cu(NH)]MoO and [Cu(NH)(HO)]MoO decompose, losing three NH and three HO ligands, respectively, and stable [Cu(NH)MoO] is formed from both. The latter can partially be hydrated in boiling water into [NHCu(OH)MoO. This compound can also be prepared in pure form by boiling the saturated aqueous solution of [Cu(NH)]MoO. All properties of [NHCu(OH)MoO] match those of the active photocatalyst described earlier in the literature under the formulas (NH)[Cu(MoO)] and (NH)Cu(NH)MoO.