Complementarity of anion-mediated hydrogen-bonding and alkyl substitution in the construction of two-dimensional rhombic (4,4) grids by bis(N-alkylamidino-O-alkylurea)copper(ii) nitrates and tetrafluoroborates

Abstract

A series of co-ordination complexes containing bis(N-alkylamidino-O-alkylurea)copper(II) cations, [CuL₂]²⁺, and either nitrate or tetrafluoroborate anions has been synthesised. Single crystal X-ray diffraction analysis of five products has revealed the presence of elegant supramolecular hydrogen-bonded networks in which the [CuL₂]²⁺ cations act as four-connecting centres and the anions as two-connecting centres to form 2-D rhombic (4,4) grids. To generate this architecture, the pendant alkyl groups of the N-alkylamidino-O-alkylurea ligands have to be able to fit in the cavity formed within the grid. Ideal fits (complementarity) occur for [Cu(L^me)₂]·2BF₄ (L^me = N-methylamidino-O-ethylurea) and [Cu(L^em)₂]·2NO₃ (L^em = N-ethylamidino-O-methylurea) where the pendant alkyl groups are coplanar with the remainder of the cation. An enforced fit is indicated in [Cu(L^ee)₂]·2X (L^ee = N-methylamidino-O-ethylurea; X = NO₃ or BF₄) and [Cu(L^2m)₂(ONO₂)][Cu(L^2m)₂(OHMe)]·3NO₃·Et₂O (L^2m = N-benzylamidino-O-methylurea) by some or all of the pendant groups being bent out of the plane of the cation. The four-fold connectivity of the [CuL₂]²⁺ cations arises from the presence of four pairs of N–H donors, set at ca. 90° to each other, which form hydrogen bonds to four anions. The two-fold connectivity of the nitrate and tetrafluoroborate anions arises from two oxygens/fluorines acting as acceptors in an R²₂(8) motif and one oxygen/fluorine acting as an acceptor in an R¹₂(6) motif. The resultant hydrogen-bonding assemblies are similar for both anions and are considered to be viable supramolecular synthons.