Crystal structures of the homologues diethyl and dimethyl (10 <i>H</i> -indeno[1,2- <i>b</i> ]quinoxalin-11-yl)phosphonate: use of non-spherical scattering factors

The two title compounds, 4a C 19 H 19 N 2 O 3 P and 4b C 17 H 15 N 2 O 3 P, are not isotypic. The tetracyclic ring systems are essentially planar. The phosphonate groups are similarly oriented. Compound 4a crystallizes in P 1 with Z ′ = 1; molecules are linked in inversion-symmetric pairs by classical hydrogen bonds N—H...O=P, forming rings of graph set R 2 2 (12). Compound 4b crystallizes in P 2 1 / c with Z ′ = 2; the two independent molecules are linked by a hydrogen-bond system analogous to that of 4a , but the ring systems subtend an interplanar angle of 55.84 (1)°. For 4a , the hydrogen bonding combines with ring stacking in pairs to form a ribbon structure parallel to the b axis. For 4b , the classical hydrogen bonding combines with two ‘weak' hydrogen bonds C—H...O to form a layer structure parallel to the ac plane. Non-spherical atom scattering factors were employed (using the program NoSpherA2 ) to avoid the problem of badly fitting reflections in high-resolution data (2θ max ca 105° using Mo K α radiation).