While conventional topochemistry is coupled with cation redox, it was proposed that anionic sp bands are also subject to redox reactions during (de)intercalation processes in highly covalent chalcogenides like CuxCr2Se4, 6 TiS3, LixFeS2 8 and VS4. That concept is gaining ground in battery research aiming to maximise cumulative electrochemical capacity from both cation and anion redox. Anion redox enables extension of chemical space by topochemistry and is expected to often involve formation or cleavage of anion-anion covalent bonds, for example in the recent reports of Fe 3+ (S 2-)(S2 2-)0.5 and Mg3V2S8, which were respectively derived by Li deintercalation from Li2FeS2 to form S-S bonds and Mg intercalation into V 4+ (S2 2-)2 to break them. These structural transformations are drastic despite their reversibility and low reaction temperatures. The recent proof-of-concept work of Cario and co-workers demonstrated that deintercalation of Cu from LaOCuS led to conversion of its antifluorite (Cu2S2) slabs into 2D arrays of (S2) dimers in LaOS. Here, we present multistep deintercalation of Cu from Sr2MnO2Cu1.5Ch2 (Ch = S, Se), leading to collapse of antifluorite-type (Cu1.5Ch2) 2.5-slabs into 2D arrays of chalcogenide dimers while retaining structural integrity of their intergrown (Sr2MnO2) 2.5+ slabs. To achieve such a drastic structure transformation, we designed a synthetic pathway focusing on (1) chemoselectivity and (2) the use of reactive synthetic intermediates. Previous use of solvated I2 as oxidant successfully deintercalated ~10% of the Cu 15 from Sr2MnO2Cu1.5S2 enabling control of magnetic exchange pathways between Mn ion moments. Further Cu deintercalation attempted at above 0 °C or using stronger oxidants such as Br2 and NO2BF4 led to overall decomposition, so to circumvent such limitations, oxidants must be chemoselective toward removal of Cu while unreactive to the rest of the oxychalcogenide framework under the reaction conditions. Synthetic intermediates may circumvent large activation barriers, which are often encountered during topochemical anion redox. For example, the topochemical conversion of LaOCuS to LaOS required a reaction temperature of 200 °C14 while conversion of CeOAg0.8S to CeSO, involving Ce 3+/4+ redox instead of S 2-/-redox, proceeded at ambient temperature . Kinetic barriers during anion redox are known to be an adverse factor causing voltage hysteresis in secondary battery systems. Organic syntheses routinely evade such barriered reactions by using energetic intermediates (e.g. via lithiation) . In solid-state chemistry, Kageyama and co-workers proposed introduction of "labile" H -anions in synthetic intermediates enabling access to heavily nitrided or fluorinated BaTiO3 20 at far lower temperatures or pressures than conventional methods.