A square lattice topology coordination network that exhibits highly selectiveC_(2)H_(2)/CO_(2)separation performance

C_(2)H_(2)/CO_(2)separation is an industrially important process that remains challenging because of the similar physicochemical properties of C_(2)H_(2)and CO_(2).We herein report that the new square lattice(sql)coordination network[Cu(bipy‐xylene)2(NO3)2]n,sql‐16‐Cu‐NO3,16=bipy‐xylene=4,4′‐(2,5‐dimethyl‐1,4‐phenylene)dipyridine,exists in at least three forms,as‐synthesised(α),activated(α′)and hydrated(β).The activated phase,sql‐16‐Cu‐NO3‐α′,is an ultramicroporous material that exhibits high selectivity towards C_(2)H_(2)over CO_(2)as revealed by dynamic gas breakthrough experiments(1:1,C_(2)H_(2)/CO_(2))that afforded 99.87%pure CO_(2)in the effluent stream.The separation selectivity at 298 K and 1 bar,78,is the third best value yet reported for C_(2)H_(2)selective physisorbents whereas the mid‐loading performance sets a new benchmark.The performance of sql‐16‐Cu‐NO3‐α′is attributed to a new type of C_(2)H_(2)binding site in which CH···ONO2 interactions enable moderately strong sorbent‐sorbate binding(Qst(C_(2)H_(2))=38.6 kJ/mol)at low loading.Conversely,weak CO_(2)binding(Qst(CO_(2))=25.6 kJ/mol)at low loading means that(ΔQst)AC[Qst(C_(2)H_(2))–Qst(CO_(2))]is 13 kJ/mol at low coverage and 11.4 kJ/mol at mid‐loading.Analysis of in situ powder X‐ray diffraction and modelling experiments provide insight into the sorption properties and high C_(2)H_(2)/CO_(2)separation performance of sql‐16‐Cu‐NO3‐α′.