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 [CuL^3m][MeOSO₃]₂ {L^3m = [HNC(OMe)NHC(NH₂)NCH₂]₂} and [CuL^3e][EtOSO₃]₂ {L^3e = [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 [CuL^3m][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 [CuL^3e][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, [CuL^3e][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.