Cascade complexes of an octaaza cryptand: co-ordinated azide with linear M–NNN–M geometry
The octaamino cryptand, L2, obtained by tetrahydroborate reduction of the [2 + 3] condensation product of tris(2-ethylamino)amine with terephthalaldehyde, acts as a host for pairs of protons or first-series transition-metal cations. An X-ray crystallographic structure determination of [H4L2]4+ reveals a pair of protons held at opposite ends of the molecule by strong intramolecular hydrogen bonding of each proton to two of the amino nitrogen atoms. Pairs of transition-metal cations co-ordinated to the amino N-donors accommodate mono- and tri-atomic bridging ligands, such as OH– or imidazolate, generating weak to moderate antiferromagnetic interaction. Dicopper(II)µ-azido complexes have been prepared where the combination of a large zero-field splitting in the ESR spectrum and a near Curie-law dependence of magnetic susceptibility on temperature seems to suggest a triplet ground state. Together with other unusual spectroscopic properties of the bridging azide group, this is believed to derive from a near-linear alignment of the M–NNN–M assembly.