Synthesis and co-ordination chemistry of two penta-azamacrocycles containing a 1,4-piperazine backbone. Crystal structure determinations of 6,12-dioxo-1,5,13,17,22-penta-azatricyclo[15.2.2.17,11]docosa-7(22),8,10-triene and 1,5,13,17,22-penta-azatricyclo[15.2.2.17,11]docosa-7(22),8,10-trienenickel(II) perchlorate

Abstract

Two closely related pyridine-containing penta-azamacrocycles with a 1,4-piperazine backbone have been prepared, and the co-ordination chemistry of one of them {1,5,13,17,22-pentaazatricyclo[15.2.2.1^7,11]docosa-7(22),8,10-triene, L²} with nickel(II), copper(II), and zinc(II) investigated. L² was obtained in low yield from the related diamide macrocycle 6,12-dioxo-1,5,13,17,22-penta-azatricyclo[15.2.2.1^7,11]docosa-7(22),8,10-triene, L¹, by reduction with BH₃·thf (thf = tetrahydrofuran). The crystal structure of L¹ shows that the 1,4-piperazine backbone adopts a chair conformation, with the lone pairs of the two 1,4-piperazine N atoms pointing in opposite directions, unfavourable for chelation. Complexes of L¹ could not be isolated. Complexes of L² with Ni²⁺, Cu²⁺, and Zn²⁺ were readily obtained, and the crystal structure of [Ni(L²)][ClO₄]₂ shows the 1,4-piperazine is in the boat conformation required for chelation. The Ni²⁺ is co-ordinated to all five N atoms of the macrocycle in a symmetric, but distorted, trigonal-bipyramidal geometry, with the pyridine ring at right angles to the plane of the carbon framework of the 1,4-piperazine backbone. The pyridine and two 1,4-piperazine N atoms are equatorial, and the other two N atoms axial; the Ni–N bond lengths are in the ranges 2.16–2.18 Å(axial) and 2.02–2.08 Å(equatorial), with the Ni–pyridine–N bond length being the shortest. The ¹³C n.m.r. spectrum of the Zn²⁺ complex is consistent with the same symmetric structure as the Ni²⁺ complex.