Crystal Structures of the Homologues Diethyl and Dimethyl (10H-Indeno[1,2-b]quinoxalin-11-yl)phosphonate: Use of Non-Spherical Scattering Factors

The two title compounds, 4a C19H19N2O3P and 4b C17H15N2O3P, are not isotypic. The tetracyclic ring systems are essentially planar. The phosphonate groups are similarly oriented. Compound 4a crystallizes in P1 with Z′ = 1; molecules are linked in inversion-symmetric pairs by classical hydrogen bonds N—H...O=P, forming rings of graph set R22(12). Compound 4b crystallizes in P21/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 (2θmax ca 105° using Mo Kα radiation).