Abstract The work in this report deals with the syntheses, characterization, molecular and supramolecular structures of eight compounds of the composition [{CuIILOEt–en⊂(H3O)}(o-chlorobenzoate)] (1), [{CuIILOEt–en⊂(H2O)}2(fumaric acid)] (2), [{NiIILOEt–en⊂(H2O)}2(fumaric acid)] (3), [{CuIILOEt–en⊂(H2O)}2(succinic acid)] (4), [{NiIILOEt–en⊂(H2O)}2(succinic acid)] (5), [{NiIILOEt–en⊂(H2O)}2(terephthalic acid)] (6), [{CuIILOEt–en⊂(H2O)}(1,3,5-benzenetricarboxylic acid)]·MeOH (7) and [{NiIILOEt–en⊂(H2O)}(1,3,5-benzenetricarboxylic acid)]·MeOH (8), where H2LOEt–en is N,N′-ethylenebis(3-ethoxysalicylaldimine). These compounds are obtained on reacting the mononuclear host–guest compound [CuIILOEt–en⊂(H2O)]/[NiIILOEt–en⊂(H2O)], which contains a trapped water molecule, with the corresponding mono/di/tricarboxylic acid. One water molecule in 2–8 and one oxonium ion in 1 is trapped in each O(phenoxo)2O(ethoxy)2 compartment. Both the carboxylic acid groups of the dicarboxylic acids are involved in interacting with the trapped water molecule, forming the supramolecular dimers 2–6, while one of the three carboxylic acid groups of 1,3,5-benzenetricarboxylic acid is involved in O–H⋯O(trapped water) hydrogen bond, forming the 1:1 adducts 7 and 8. Compound 1 contains o-chlorobenzoate and the trapped species herein is the oxonium ion (H3O+). [{CuIILOEt–en⊂(H3O)]+ and o-chlorobenzoate fragments in 1 are self-assembled because of an O–H(oxonium ion)⋯O(carboxylate) hydrogen bond. The above mentioned adducts are further self-assembled to generate 1-D/2-D/3-D supramolecular structures because of hydrogen bonds/π⋯π interactions/NiII⋯NiII interactions/halogen bonds.