Conformational studies of 2,3-diacyl-5-nitrocyclopentadienes: delocalized systems with very short intramolecular O ? H ? O hydrogen bonds. Crystal and molecular structures of 2,3-diacetyl- and 2,3-dibenzoyl-5-nitrocyclopentadiene

Abstract

Crystals of (IVa) 2,3-diacetyl-5-nitrocyclopentadiene and of the dibenzoyl analogue (IVb) are monoclinic, space group P21/c, with Z= 4; unit-cell dimensions: (IVa)a= 5.115(1), b= 10.780(2), c= 16.126(2)A, β= 90.38(2)°; (IVb)a= 14.241(2), b= 7.441(1), c= 19.433(2)A, β= 131.49(1)°. Both structures were solved from diffractometer data by direct methods and refined by least-squares calculations to R 0.044[1 226 observed reflections, (IVa)] and 0.039 [1 690 observed reflections, (IVb)]. In both molecules, in the solid state, the enolic hydrogen is midway between the two oxygen atoms and forms a short symmetrical intramolecular O ⋯ H ⋯ O hydrogen bond [O ⋯ O 2.433(2)(IVb) and 2.446(2)A(IVa)]. This results in a symmetrically delocalized structure. There is a small amount of conjugation between the nitro-groups and cyclopentadiene rings. The molecular packing may give rise to weak charge-transfer effects. In (IVb) the molecules are stacked in infinite columns parallel to b, interplanar separations 3.18 and 3.33 A; in (IVa) there are discrete pairs of molecules, interplanar spacing 3.32 A.