Anion recognition by conservation of hydrogen-bonding patterns in salts of copper(II) co-ordinated by tetradentate bis(amidino-O-alkylurea) ligands †

Abstract

Refluxing 1,2-bis(2-cyanoguanidino)ethane (L1) in methanol or ethanol containing copper(II) sulfate resulted in the formation of [CuL3m][MeOSO3]2 {L3m = [HNC(OMe)NHC(NH2)NCH2]2} and [CuL3e][EtOSO3]2 {L3e = [HNC(OEt)NHC(NH2)NCH2]2}. Structural analysis of these complexes has revealed hydrogen-bonded supramolecular architectures constructed from [CuLi]2+ cations with square planar CuN4 chromophores and [ROSO3]− anions. The basic supramolecular synthon is a 1-D chain of alternating cations and anions, a motif which is also found with Cl− and [BF2(OMe)2]− anions. The [ROSO3]− anion is an ideal fit for the space between two cations; its hydrophobic O-alkyl group is surrounded by the O-alkyl groups of the cations and its hydrophilic SO3− moiety forms N–H⋯O hydrogen bonds with their amino and imino functionalities. In [CuL3m][MeOSO3]2 the chains, which are linear, are hydrogen bonded through a second methyl sulfate anion to give a 2-D sheet motif. The sheets are bound together to form a 3-D framework through very weak Cu⋯O co-ordinative interactions which complete a tetragonally elongated octahedral geometry for the copper(II) centre. In [CuL3e][EtOSO3]2 the chains, which are sinusoidal, are hydrogen bonded through a second ethyl sulfate anion to give a bilayered arrangement. The bilayers aggregate into a 3-D framework by a combination of hydrogen bonding and weak Cu⋯O co-ordinative interactions which result in a square pyramidal co-ordination geometry for the copper(II) centre. Despite the fit between [CuLi]2+ cations and alkyl sulfate anions in the chain, when a mixture of anions (chloride and ethyl sulfate) is present in the crystallisation medium, the preferred chain supramolecular synthon of the product, [CuL3e][EtOSO3]Cl·2H2O, is that containing chloride. The ethyl sulfate anions are involved, together with water molecules, in a complex hydrogen-bonding network which connects the chains into 2-D sheets, which are assembled, in turn, into a 3-D network structure by a combination of hydrogen bonding networks and exceedingly weak Cu⋯Cl co-ordinative interactions, which give rise to a square pyramidal copper(II) co-ordination geometry.