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In the title thiophene derivative, C13H18N2O3S, the dihedral angles between the thiophene ring and the [(dimethylamino)methylidene]amino side chain (r.m.s. deviation = 0.009 Å) and the –CO2 ester group are 3.01 (16) and 59.9 (3)°, respectively. In the crystal, inversion dimers linked by pairs of C—H...O hydrogen bonds generate R22(16) loops. The dimers are linked by another weak C—H...O interaction, forming chains along [001]. In addition, weak C—H...π interactions are observed, which link the chains into (001) layers.

The title molecular salt, C21H25N2O5S+·Cl−, crystallizes with two ion pairs in the asymmetric unit. The cations have similar conformations (r.m.s. overlay fit = 0.40 Å), with one of them showing disorder of the terminal methyl group of the ester in a 0.72 (2):0.28 (2) ratio. In the first cation, the 3,4-dimethoxy-substituted phenyl ring subtends a dihedral angle of 88.38 (7)° with the pyrimidine ring and 6.79 (8)° with the thiazole ring. The equivalent data for the second cation are 89.97 (3) and 6.42 (7)°, respectively. The pyrimidine ring adopts a sofa conformation in each cation. In the crystal, the components are linked by N—H...Cl hydrogen bonds, generating isolated ion pairs. The ion pairs are are linked by C—H...O interactions, generating a three-dimensional network. In addition, a weak C—H...π interaction is observed.

In the title compound, C17H18N2O3S2, the pyrimidine ring adopts a shallow sofa conformation, with the C atom bearing the axially-oriented thiophene ring as the flap [deviation = 0.439 (3) Å]. The plane of the thiophene ring lies almost normal to the pyrimidine ring, making a dihedral angle of 79.36 (19)°. In the crystal, pairs of very weak C—H...O hydrogen bonds link the molecules related by twofold rotation axes, forming R22(18) rings, which are in turn linked by another C—H...O interaction, forming chains of rings along [010]. In addition, weak C—H...π(thiophene) interactions link the chains into layers parallel to [001] and π–π interactions with a centroid–centroid distance of 3.772 (10) Å connect these layers into a three-dimensional network.

The asymmetric unit of the title compound, C15H21N3OSi, contains two molecules with similar conformations (r.m.s. overlay fit for the 20 non-H atoms = 0.163 Å). The dihedral angles between the planes of the 1,2,3-triazole and 2,4-dimethylbenzene rings are 27.0 (3) and 19.5 (3)°. In the crystal, molecules are linked by very weak C—H...O and C—H...N hydrogen bonds to generate [100] chains. The chains are cross-linked by C—H...π interactions.