Crystalline complexes of N,N'-ditritylurea (DTU) and N-tritylurea (NTU) hosts with molecular guests

This paper reports crystalline complexes of the new hosts N,N'-ditritylurea (DTU) and N-tritylurea (NTU) with various uncharged molecular guests. The crystal structures of the following complexes were elucidated by single crystal X-ray diffraction analysis at 115^oK: (I) 1:1 DTU-propanamide - space group C2/c, a=15.839Å, b=9.088Å, c=24.584Å, β=111.05^o, Z=4; (II) 1:1 DTU-ethyl N-acetylglycinate - space group P1, a=9.010Å, b=10.800Å, c=19.810 Å, α=105.29^o β=94.33^o, γ=93.03^o, Z=2; (III) 2:1 NTU-N, N-dimethylformamide - space group Cc, a=29.614Å, b=8.906Å, c=16.127Å, β=121.04^o, Z=4. The three crystal structures are stabilized mainly by a cooperative effect of hydrogen bonding between amide fragments displaced along the shortest axis of each crystal. This interaction occurs between host and guest in complexes I and II, and between host and host in complex III. The latter also represents a cage-type clathrate in which the guest molecules are accommodated in voids between the hydrophobic fragments of four neighboring NTU hosts. On the other hand, complexes of DTU are characterized by a more specific interaction between the two components, each guest molecule being inserted between two adjacent hosts (related by translation) and strongly bound to them via hydrogen bridges. These results illustrate a useful concept in the design of molecular species which can be potential hosts upon crystallization with neutral molecular guests.