Crystal structures of three homologues with increasing ring size: 2-methoxy-4-(thiophen-2-yl)-5,6,7,8-tetrahydroquinoline-3-carbonitrile, 2-methoxy-4-(thiophen-2-yl)-6,7,8,9-tetrahydro-5H-cyclohepta[b]pyridine-3-carbonitrile and 2-methoxy-4-(thiophen-2-yl)-5,6,7,8,9,10-hexahydrocycloocta[b]pyridine-3-carbonitrile

The title compounds, C₁₅H₁₄N₂OS (1a), C₁₆H₁₆N₂OS (1b), and C₁₇H₁₈N₂OS (1c), form a homologous series in which the size of the saturated ring increases from six- to eight-membered (with four, five and six methyl­ene groups respectively). For 1b and 1c, the central (CH₂)_n moieties are all displaced to the same side of their ring, and the CH₂—CH₂—CH₂ angles are much wider than the standard sp³ value; a database search indicates that these are general features of such ring systems. For 1a, the thio­phene ring lies with the sulfur atom on the opposite side of the C_thio­phene—C_pyridine bond to the cyano group, in contrast to 1b and 1c. For each compound, the packing may be described in terms of two `weak' C—H⋯N hydrogen bonds, which link the mol­ecules to form one-dimensional (1a, 1c) or three-dimensional (1b) assemblies.