The effect of carboxylate and N,N′-ditopic ligand lengths on the structures of copper and zinc coordination polymers

Abstract

A series of one-dimensional coordination polymers with parallel chains has been prepared by linking together M₂(O₂CR)₄ paddle wheel units, where M is Cu or Zn and RCO₂⁻ is biphenyl-4-carboxylate or 4-iodobenzoate, with pyrazine (pyz), 2-aminopyrazine (apyz) or 1,4-diazabicyclo[2.2.2]octane (dabco) bridging ligands. The longer length of the substituent in biphenyl-4-carboxylate (bpc) allows for greater separation of the chains in [Cu₂(bpc)₄(pyz)]·3.8BzOH 1 than has been previously observed in related benzoate compounds, leading to the formation of larger channels within the structure. In [Cu₂(bpc)₄(dabco)]·BzOH 2 and [Zn₂(bpc)₄(dabco)]·2DMF 3, neighbouring chains are offset relative to each other, enabling them to pack more efficiently and reducing the channel width from that seen in 1. For the 4-iodobenzoate (ibz) compounds [Cu₂(ibz)₄(pyz)] 4, [Cu₂(ibz)₄(apyz)]·2BzOH 5 and [Zn₂(ibz)₄(dabco)]·3.25DMF 6, the chains are closer together than in the bpc compounds, and again when the neighbouring chains are offset with respect to each other, they are able to pack more efficiently. When similar reactions were carried out using 4,4′-bipyridyl (4,4′-bipy) as the bridging ligand, the products were very different. [Cu(bpc)₂(4,4′-bipy)(BzOH)₂]·2BzOH 7, [Zn(bpc)₂(4,4′-bipy)] 8 and [Zn(ibz)₂(4,4′-bipy)] 9 all have structures in which single metal centres, as opposed to M₂(O₂CR)₄ dimers, are bridged into chains by the ditopic linker ligand, and the chains pack efficiently without forming channels.