The R21(6) hydrogen-bonded synthon in neutral urea and metal-bound halide systems

The recurrence of the R21(6) motif between urea and metal bound halides is explored through studies in the CSD, including examples of the closely related thiourea and guanidine-based structures. A series of compounds containing the isomeric pyridyl–urea ligands 1 and 2 attached to trans-dihalide metal units have been prepared and their X-ray crystal structures determined. A hydrogen-bonding motif between the metal-bound halide ligands and the urea groups on the pyridyl ligands is observed in most cases which takes the form of an R21(6) ring. Three copper(II) complexes, [Cu(2)2Cl2]·2MeOH 3, [Cu(2)2Br2(MeOH)] 4 and [Cu(2)2Cl2(EtOH)] 5 all retain this supramolecular synthon despite a changing coordination environment and alteration of the halide ligands. A series of solvates of the complex [Pd(1)2Cl2] (6–8) shows that the R21(6) synthon does not occur in the presence of strongly hydrogen-bond accepting solvents such as DMF (6) and MeCN (7) but is observed in the presence of the hydrogen bond donor solvent methanol (8). The motif also occurs within an octahedral cadmium system [Cd(1)4Cl2]·2H2O·2MeCN 9. In all cases in which the R21(6) synthon is observed chain-like structures form between the metal complexes. Two contrasting structures based around tetrahedral zinc(II) centres [Zn(1)2Cl2] (10) and [Zn(1)2I2]·0.5MeOH (11) do not display the bifurcated synthon, showing instead an R22(8) motif in the case of 10 or no ring-type halide interaction in 11.