Synthesis and structure of copper(II) complexes of two new poly(2-pyridylalkyl)diamine ligands

The new pentapyridyldiamine ligand, L1, which incorporates two bis(2-pyridylethyl)amine donor domains held together by a 2,6-dimethylenepyridine linker, is readily prepared. In the presence of metal salts, L1 is unstable due to facile elimination of vinyl pyridine. Complexes of L1 are therefore difficult to isolate. Nonetheless, a novel copper dimer [Cu2(L1)(μ-OH)(CH3CN)](ClO4)3 has been isolated in small quantities along with the interesting monomer [Cu(L2)](ClO4)2, in which L2 is the tetrapyridyldiamine ligand derived from the decomposition of L1 by loss of one pyridylethyl `armʹ. The crystal structures of the two complexes are reported: the [Cu2(L1)(μ-OH)(CH3CN)]2+ cation exhibits a μ-hydroxo-bridged dicopper(II) core and a coordinated acetonitrile molecule, akin to a putative intermediate in nitrile hydrolysis, and the chiral [Cu(L2)]2+ cation is revealed to have a five-coordinate copper(II) centre that is stabilised by an intramolecular hydrogen-bond between the 2° amine group and a pendant pyridylethyl `armʹ.