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-cyclo- hepta[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.