Hydrogen bonding in phenylhydrazone derivatives of benzophenone: crystal and molecular structures of benzophenone (2-nitrophenyl)hydrazone, containing an intramolecular NO2⋯ NH ⋯π bifurcated hydrogen bond, and benzophenone (4-nitrophenyl)hydrazone, containing an intramolecular NH ⋯π hydrogen bond. Comments on hydrogen bond vagaries in various types of phenylhydrazone

Abstract

Crystal structures have been determined of the two title compounds. The (2-nitrophenyl)hydrazone (2) is monoclinic, space group P2₁/a, Z= 4, with a= 12.074(5), b= 12.771(8), c= 11.998(8)Å, β= 118.0°. The (4-nitrophenyl) hydrazone (3) is orthorhombic, space group Pcab, Z= 8, with a= 8.468(7), b= 13.98(1), c= 26.82(1)Å. Respectively, 1 244 and 1 181 independent reflections above background were collected and the structures were refined to R 0.074 and 0.068. The two structures show remarkably different conformations. In (2), the hydrazone N(7)–H(7) group is involved in an intramolecular hydrogen bond to the ortho-nitro group, and there is also a possible H(7)⋯π intraction. In (3) there is a definite H(7)⋯π interaction with the C–C bond of a phenyl group, with H(7)⋯ C distances of 2.43 and 2.26 Å. The difference between the two structures is probably due to the fact that in (2) the hydrogen bond requirements of the N(7)–H(7) group are satisfied by the ortho-nitro group, and that in consequence there is no need for much subsidiary overlap with the π-electrons. In (3) there is no ortho-group and therefore the hydrogen bond can only be formed with the C–C bond.

Evidence for these N–H ⋯ C–C interactions is also obtained from the i.r. spectra of a series of compounds. Frequency difference values [ν_NH(crystals)–ν_NH(CHCl₃)] of 14–15 cm^–1 are observed for (3) and similar compounds, providing a clear indication of an intramolecular NH ⋯π bond in the crystals which does not survive in solution.