Peroxosolvates: Formation criteria, H₂O₂ hydrogen bonding, and isomorphism with the corresponding hydrates

The Cambridge Structural Database has been used to investigate the detailed environment of H₂O₂ molecules and hydrogen-bond patterns within "true" perox-osolvates in which the H₂O₂ molecules do not interact directly with the metal atoms. A study of 65 crystal structures and over 260 hydrogen bonds reveals that H₂O₂ always forms two H-bonds as proton donors and up to four H-bonds as a proton acceptor, but the latter can be absent altogether. The necessary features of peroxosolvate coformers are clarified. (1) Coformers should not participate in redox reactions with H₂O₂ and should not catalyze its decomposition. (2) Coformers should be Brønsted bases or exhibit amphoteric properties. The efficiency of the proposed criteria for peroxosolvate formation is illustrated by the synthesis and characterization of several new crystals. Conditions preventing the H₂O₂/H₂O isomorphous substitution are essential for peroxosolvate stability: (1) Every H₂O₂ in the peroxosolvate has to participate in five or six hydrogen bonds. (2) The distance between the two proton acceptors forming H-bonds with the H₂O₂ molecule should be longer than the distance defined by the nature of the acceptor atoms.